Simulation study on creep deformation of the impeller in lead-bismuth eutectic environment through fluid-solid coupling method

Lead-based reactor is a new type of reactor using liquid lead or lead-bismuth alloy as a coolant. As the core working element of the main pump, the impeller is subjected to a huge load when conveying heavy metal liquids and is highly susceptible to damage. In this study, we used ANSYS and FLUENT software to investigate the stress, deformation, and creep deformation of the nuclear main pump impeller under a liquid lead-bismuth environment by the fluid-solid coupling method. The maximum equivalent force of the impeller was located at the junction of the blade and hub, which was prone to fatigue damage under the action of alternating load. The stress, deformation, and creep characteristics of the impeller blade were observed to generally increase with rotational speed. Particularly, the junction of the blade root and hub exhibited high susceptibility to stress concentration and fatigue damage. At a flow rate of 0.64 m/s and a speed of 690 r/min, the maximum equivalent force was 16.7 MPa, which was lower than the yield strength of 316L stainless steel. Additionally, the maximum deformation was less than 0.63 mm. Over a five-year period, the creep of the impeller ranged from a minimum of 0.228% to a maximum of 0.447%, indicating that the impeller can reliably operate in a liquid lead-bismuth environment for at least five years.

[Anisotropy and viscoelasticity of different corneal regions in rabbit corneal ectasia model]

The mechanical properties of the cornea in corneal ectasia disease undergo a significant reduction, yet the alterations in mechanical properties within distinct corneal regions remain unclear. In this study, we established a rabbit corneal ectasia model by employing collagenase II to degrade the corneal matrix within a central diameter of 6 mm. Optical coherence tomography was employed for the in vivo assessment of corneal morphology (corneal thickness and corneal curvature) one month after operation. Anisotropy and viscoelastic characteristics of corneal tissue were evaluated through biaxial and uniaxial testing, respectively. The results demonstrated a marked decrease in central corneal thickness, with no significant changes observed in corneal curvature. Under different strains, the elastic modulus of the cornea exhibited no significant differences in the up-down and naso-temporal directions between the control and model groups. However, the cornea in the model group displayed a significantly lower elastic modulus compared to the control group. Specifically, the elastic modulus of the central region cornea in the model group was significantly lower than that of the entire cornea within the same group. Moreover, in comparison to the control group, the cornea in the model group exhibited a significant increase in both creep rate and overall deformation rate. The instantaneous modulus and equilibrium modulus were significantly reduced in the model cornea. No significant differences were observed between the entire cornea and the central cornea concerning these parameters. The results indicate that corneal anisotropy remains unchanged in collagenase-induced ectatic cornea. However, a significant reduction in viscoelastic properties is noticed. This study provides valuable insights for investigating changes in corneal mechanical properties within different regions of ectatic corneal disease.

A systematic review of Inconel 939 alloy parts development via additive manufacturing process

IN939 is a modern class of nickel-based superalloys designed for continuous operational sustenance at elevated temperatures owing to their excellent combination of fatigue, creep, and corrosion resistance. This unique performance of IN939 is associated with the composition of this alloy, along with specific post-processing treatments such as solution treatment and aging, giving rise to features such as the presence of γ' residues, as well as the presence of MC and M23C6 carbides. This also includes the absence of the eutectic and incipient melting phases. For this alloy, the primary part development is by the powder bed fusion process using a laser powder bed fusion machine. At the same time, a solo study highlights the use of an EB-PBF machine for the synthesis. The AM development process of these alloys is hindered by machine parameters, which have been found ineffective in isolation to obtain a fully dense structure with desired properties. The purpose of these parameters is to improve their core properties while minimizing defects associated with powder metallurgy routes, such as porosity, detrimental precipitates, grain anisotropy, etc. This study aims to provide an overview of the advancements in research related to IN939, explicitly focusing on the benchmarks achieved through additive manufacturing techniques. We have discussed the work performed in this area, compared the results of different studies, and identified the gaps in current research. By doing so, we aim to provide a comprehensive understanding of the potential of IN939 and its applications in extreme environments.

Electromechanical interactions between cell membrane and nuclear envelope: Beyond the standard Schwan's model of biological cells

We investigate little-appreciated features of the hierarchical core-shell (CS) models of the electrical, mechanical, and electromechanical interactions between the cell membrane (CM) and nuclear envelope (NE). We first consider a simple model of an individual cell based on a coupled resistor-capacitor (Schwan model (SM)) network and show that the CM, when exposed to ac electric fields, acts as a low pass filter while the NE acts as a wide and asymmetric bandpass filter. We provide a simplified calculation for characteristic time associated with the capacitive charging of the NE and parameterize its range of behavior. We furthermore observe several new features dealing with mechanical analogs of the SM based on elementary spring-damper combinations. The chief merit of these models is that they can predict creep compliance responses of an individual cell under static stress and their effective retardation time constants. Next, we use an alternative and a more accurate CS physical model solved by finite element simulations for which geometrical cell reshaping under electromechanical stress (electrodeformation (ED)) is included in a continuum approach with spatial resolution. We show that under an electric field excitation, the elongated nucleus scales differently compared to the electrodeformed cell.

The effect of sterilization and storage on the viscoelastic properties of human tendon allografts - Continued: Storage for 0 to 4 months

The role of donor-derived tendons, also known as allografts, in anterior cruciate ligament replacement surgeries is steadily increasing. Before surgery, temporary storage and, in most cases, sterilization are essential. It is, thus, crucial to determine how these procedures alter the grafts' biomechanical properties. The purpose of this research was to analyze the effect of different sterilization methods (native, frozen, frozen + 21 kGy gamma irradiation, frozen + 21 kGy electron beam irradiation) and storage durations (0 to 4 months) on the deformation and creep of two tendon types (tibialis anterior, peroneus longus). 80 tibialis anterior and 83 peroneus longus tendons from 51 human cadavers were included. The samples were removed, placed in a radio-cryoprotectant solution, then slowly cooled, sterilized and stored at -80 °C. All groups were subject to 60 s static creep test with 250 N load. Deformation during the loading phase, creep during static loading, and the ratio of these two were evaluated. Deformation at the end of the loading phase and creep consistently exhibited significantly smaller values in the tibialis anterior compared to the peroneus longus type, as well as in electron beam-sterilized grafts as opposed to gamma beam-sterilized ones. Prolonged storage periods (within 0 to 4 months) resulted in a notable increase in these values, particularly in deformation. Based on the experimental data, the tibialis anterior tendon type and sterilization by gamma beam irradiation are better choices for anterior cruciate ligament reconstruction than the peroneus longus and sterilization by electron beam. Increased storage time affects negatively the evaluated mechanical properties.

Effects of storage and toothbrush simulation on Martens hardness of CAD/CAM, hand-cast, thermoforming, and 3D-printed splint materials

OBJECTIVES

To investigate Martens hardness parameters of splint materials after storage in liquids and toothbrush simulation.

MATERIALS AND METHODS

Ten specimens per material and group were fabricated (hand-cast CAST, thermoformed TF, CAD/CAM-milled CAM, 3D-printed PS, PL, PK, PV), stored in air, water, coffee, red wine, and cleaning tablets and investigated after fabrication, 24 h, 2- and 4-week storage or toothbrushing. Martens hardness (HM), indentation hardness (HIT), indentation modulus (EIT), the elastic part of indentation work (ηIT), and indentation creep (CIT) were calculated (ISO 14577-1).

STATISTICS

ANOVA, Bonferroni post hoc test, between-subjects effects, Pearson correlation (α = 0.05).

RESULTS

HM varied between 30.8 N/mm2 for PS (water 4 weeks) and 164.0 N/mm2 for CAM (toothbrush). HIT values between 34.9 N/mm2 for PS (water 4 weeks) and 238.9 N/mm2 for CAM (toothbrush) were found. EIT varied between 4.3 kN/mm2 for CAM (toothbrush) and 1.8 kN/mm2 for PK (water 2 weeks). ηIT was found to vary between 16.9% for PS (water 4 weeks) and 42.8% for PL (toothbrush). CIT varied between 2.5% for PL (toothbrush) and 11.4% for PS (water 4 weeks). The highest impact was identified for the material (p ≤ 0.001).

CONCLUSIONS

Storage and toothbrushing influenced Martens parameters. The properties of splints can be influenced by the choice of materials, based on different elastic and viscoelastic parameters. High HM and EIT and low CIT might be beneficial for splint applications.

CLINICAL RELEVANCE

Martens parameters HM, EIT, and CIT might help to evaluate clinically relevant splint properties such as hardness, elasticity, and creep.

In vivo creep and wear performance of vitamin-E-diffused highly crosslinked polyethylene in total hip arthroplasty

INTRODUCTION

An acetabular liner thickness of around 6 mm remains the "gold standard" in total hip arthroplasty. Some surgeons have been recommending the use of the thickest possible liner because contact stress and strain in articulating surfaces decrease with increasing the wall thickness. The purpose of this study was to determine whether in vivo creep and wear performance could be enhanced using a thicker liner over the standard thickness in vitamin-E-diffused highly crosslinked polyethylene (HXLPE).

MATERIALS AND METHODS

One hundred and twenty-two hips were allocated to age-matched, sex-matched, and body mass index-matched two subgroups implanted either with a 6.8- or 8.9-mm-thick vitamin-E-diffused HXLPE liner against 28-mm cobalt-chrome femoral head, and followed-up for 7 years. Linear and volumetric penetration of femoral head into the liners attributed to creep and wear were analyzed for each group.

RESULTS

Compressive creep strain generated at the initial 6 months was significantly larger in the 6.8-mm group (2.6%) than in the 8.9-mm group (2.2%). The linear steady-state wear observed after 2 years was 0.0019 and 0.0015 mm/year, whereas the volumetric steady-state wear was 0.54 and 0.45 mm3/years in the 6.8- and 8.9-mm-thick groups, respectively. Although less strain in the thicker group resulted in a slightly less wear, it did not reach significant differences in the steady-state wear rates between the groups.

CONCLUSION

No clinical significance for using a thicker liner over the standard thickness (6.8 mm → 8.9 mm) was confirmed in the vitamin-E-diffused HXLPE according to the 7-year follow-up. The wear rates for both thicknesses were very low enough to prevent osteolysis, and no mechanical failure was observed at any follow-up interval. Nevertheless, since the significantly higher strain was seen in the thinner liner, further follow-up is needed to compare the longer term wear and the incidence of osteolysis and component fracture.

Effects of Creep on Shield Tunnelling Through Squeezing Ground

The present work aims to improve the reliability of shield jamming and lining damage risk assessment in squeezing ground by analysing the effects of creep on the evolution of rock pressure over time. The study is based on numerical simulations of typical mechanised tunnelling processes, generally consisting of shield advance phases alternating with shorter or longer standstills for lining installation, maintenance, etc. A linear elastic-viscous plastic constitutive model based upon Perzyna's overstress theory is employed, which considers the time-dependency of plastic deformations via a single viscosity parameter. The investigations demonstrate the following: (i) shield loading during advance increases with increasing viscosity under certain conditions, which contradicts the common perception in many existing works that creep is thoroughly favourable for shield jamming; (ii) creep is thoroughly unfavourable for shield loading during long standstills and long-term lining loading, due to the additional viscoplastic ground deformations manifested over time; (iii) the commonly adopted simplifying assumption of continuous excavation with the gross advance rate is adequate only where standstills are very short (e.g., for lining erection during the stop-and-go shield tunnelling process), but otherwise underestimates the shield loading, even in cases of regular inspection and maintenance standstills lasting only a few hours. Two application examples, the Fréjus safety gallery and the Gotthard Base tunnel, demonstrate the need to consider creep and the accuracy of modelling tunnel construction by a semi-discrete approach, where only the very short standstills for lining erection are considered via an average advance rate, but longer standstills are explicitly simulated.

Nonlinear viscoelastic model for time-dependent mechanical characterization of PMMA acrylic bone cements

Acrylic bone cement materials are widely used in prosthetic implants, orthopaedics and others trauma surgery. From the mechanical constitutive behaviour viewpoint, experimental analyses have provided ample evidence that such materials exhibit time-dependent properties. In this context, this work addresses the formulation of a nonlinear viscoelastic model for the behaviour of PMMA bone cements under compressive creep loading. Relying upon experimental data available for four PMMA bone cement types, a nonlinear Burgers-like rheological model is formulated and related parameters calibrated for the mechanical description of the time-dependent behaviour of these materials under isothermal conditions and one dimensional setting. The proposed model reveals relevant in reproducing both instantaneous and delayed properties of studied PMMA bone cements.

Creep and strength characteristics of cemented gangue backfill under coupling effect of load and acid corrosion

Cemented gangue backfill technology is beneficial to the reuse of solid waste and sustainable economic development. However, mine water has a great impact on the strength and deformation of cemented gangue backfill (CGB). In this study, the CGB specimens under load were placed in simulated acid mine water (H2SO4 solution). The changes in deformation, resistivity, and ultrasonic pulse velocity (UPV) of CGB were monitored. On the 360th day, the stress-strain curve and acoustic emission (AE) energy of the specimen during loading were recorded. The degradation mechanism of CGB was discussed by scanning electron microscope (SEM) and X-ray diffraction (XRD). The results showed that the deformation of CGB increased with time. The effect of H2SO4 solution concentration on the deformation was different in the early and late stages. Applying an 80% stress-strength ratio (SSR) reduced the strength and increased the deformation. The UPV and resistivity had different characteristics at different corrosion ages, which could be used for long-term stability monitoring of CGB. The CGB showed the strongest AE energy characteristics near the peak stress. The AE energy decreased with the increase of pH value in the pore compaction stage, and the AE activity of the CGB under 80% SSR was much greater than that of the CGB under 40% SSR. The erosion of the H2SO4 solution on the CGB was inhibited by applying a small load. Excessive load aggravated the erosion deterioration of CGB due to initial plastic damage. The research results can provide a reference for the durability design of CGB.

Experimental data from service-like creep-fatigue experiments on grade P92 steel

This article refers to the research article entitled "Creep-Fatigue of P92 in Service-Like Tests with Combined Stress- and Strain-Controlled Dwell Times" [1]. It presents experimental mechanical data from complex service-like creep-fatigue experiments performed isothermally at 620°C and a low strain amplitude of 0.2 % on tempered martensite-ferritic grade P92 steel. The datasets in text file format provide cyclic deformation (minimum and maximum stresses) and the total (hysteresis) data of all recorded fatigue cycles for three different creep-fatigue experiments: 1) a standard relaxation fatigue (RF) test with symmetrical dwell times of three minutes introduced at minimum and maximum strain, 2) a fully strain-controlled service-like relaxation (SLR) test combining these three-minute peak strain dwells with a 30-minute dwell in between at zero strain, and 3) a partly stress-controlled service-like creep (SLC) test combining the three-minute peak strain dwells with 30-minute dwells at constant stress. Such service-like (SL) tests with additional long-term stress- and strain-controlled dwell times are non-standard, rare, and expensive, making these data very valuable. They may be used to approximate cyclic softening in the technically relevant range, for the design of complex SL experiments, or for detailed analyses of stress-strain hystereses (e.g., for stress or strain partitioning methods, for the determination of hysteresis energies (work), inelastic strain components, etc.). In addition, the latter analyses may supply important input for advanced parametric lifetime modeling of components under creep-fatigue loading or model calibration parameters.

Type 2 diabetes alters the viscoelastic behavior and macromolecular composition of vertebra

Type 2 diabetes (T2D) affects the functional behavior of vertebra bone by altering its structural and mechanical properties. The vertebral bones are responsible to carry the body weight and it remains under prolonged constant load which results to viscoelastic deformation. The effect of T2D on the viscoelastic behavior of vertebral bone is not well explored yet. In this study, the effects of T2D on the creep and stress relaxation behavior of vertebral bone are investigated. Also, this study established a correlation between T2D associated alteration in macromolecular structure and viscoelastic behavior of vertebra. In this study T2D female rat SD model was used. The obtained results demonstrated a significant reduction in the amount of creep strain (p ≤ 0.05) and stress relaxation (p ≤ 0.01) in T2D specimens than the control. Also, the creep rate was found significantly lower in T2D specimens. On the other hand, molecular structural parameters such as mineral-to-matrix ratio (control vs T2D: 2.93 ± 0.78 vs 3.72 ± 0.53; p = 0.02), and non-enzymatic cross link ratio (NE-xL) (control vs T2D: 1.53 ± 0.07 vs 3.84 ± 0.20; p = 0.01) were found significantly altered in T2D specimens. Pearson linear correlation tests show a significant correlation; between creep rate and NE-xL (r = -0.94, p < 0.01), and between stress relaxation and NE-xL (r = -0.946, p < 0.01). Overall this study explored the understanding about the disease associated alteration in viscoelastic response of vertebra and its correlation with macromolecular composition which can help to understand the disease related impaired functioning of the vertebrae body.

Creep deformation of viscoelastic lumbar tissue during sustained submaximal trunk flexion postures

Most in-vivo human experiments exploring creep deformation of viscoelastic lumbar tissue have used a maximum trunk flexion posture to engage the lumbar passive tissues. Recent evidence suggests that static trunk flexion tasks requiring submaximal trunk flexion can lead to gradual changes in the lumbar lordosis and this leads to our hypothesis that maintaining submaximal trunk flexion postures may lead to significant creep deformation of the viscoelastic lumbar tissues. Sixteen participants maintained a trunk flexion posture that was ten degrees less than the trunk flexion posture eliciting flexion-relaxation phenomenon for 12 min with breaks for a maximal trunk flexion protocol every three minutes. Trunk kinematic and extensor EMG measures were captured during the static, submaximal trunk flexion protocol as well as during the maximal trunk flexion protocol to provide evidence of creep development in the lumbar passive tissues. Results revealed that 12-minutes of submaximal trunk flexion led to significant increases in peak lumbar flexion angle (1.3°) and EMG-off lumbar flexion angle for L3/L4 paraspinals (2.9°). During the submaximal trunk flexion protocol, the changes in the lumbar flexion angle at 3-6 min and 6-9 min (average Δ5.4°) were significantly greater than at 0-3 min (Δ2.0°). The contribution of this study is the demonstration that sustained submaximal trunk flexion posture (i.e., constant global system) can lead to creep deformation of the viscoelastic lumbar tissue due to the increased lumbar flexion (i.e., altered local system) and may be attributed to a reduction in lumbar lordosis as the extensor muscles fatigue.

Global void ratio of municipal solid waste for compression indices estimation

Compressibility is one of the important engineering properties of municipal solid waste (MSW) affecting the stability and functionality of a landfill. Although the correlations between MSW properties and compression parameters have been established, they either have low accuracy and small datasets or are only limited to a few specific landfills in a region. In this study, a new method using the initial global void ratio (e₀*) of MSW to estimate the compression indices is developed based on a comprehensive MSW dataset. The dataset consists of 124 sets (91 laboratory and 33 field) of MSW compression results obtained from 44 studies in 13 countries with different income levels and climate conditions. We categorized MSW as a ternary mixture with biodegradable (B), reinforcing (R), and inert (I) fractions, and suggested average specific gravity values (G_s,B = 1.20, G_s,R = 1.07, and G_s,I = 2.64), respectively. The e₀* values were calculated using the initial dry unit weight (γ_d,0) and ternary composition of MSW. The correlations between the e₀* and the immediate compression index, secondary compression index induced by mechanical creep, and secondary compression index induced by bio-compression of MSW were evidently established. The results are applicable to the MSW with B = 0-79.2 %, R = 0-54.0 %, I = 2.8-100.0 %, and γ_d,0 = 2.0-14.2 kN/m³. A simple flowchart was established to estimate the compression indices and strains of MSW disposed on in landfills and dumpsites in countries with different income levels.

Smooth muscle contribution to vaginal viscoelastic response

Smooth muscle cells contribute to the mechanical function of various soft tissues, however, their contribution to the viscoelastic response when subjected to multiaxial loading remains unknown. The vagina is a fibromuscular viscoelastic organ that is exposed to prolonged and increased pressures with daily activities and physiologic processes such as vaginal birth. The vagina changes in geometry over time under prolonged pressure, known as creep. Vaginal smooth muscle cells may contribute to creep. This may be critical for the function of vaginal and other soft tissues that experience fluctuations in their biomechanical environment. Therefore, the objective of this study was to develop methods to evaluate the contribution of smooth muscle to vaginal creep under multiaxial loading using extension - inflation tests. The vaginas from wildtype mice (C57BL/6 × 129SvEv; 3-6 months; n = 10) were stimulated with various concentrations of potassium chloride then subjected to the measured in vivo pressure (7 mmHg) for 100 s. In a different cohort of mice (n = 5), the vagina was stimulated with a single concentration of potassium chloride then subjected to 5 and 15 mmHg. A laser micrometer measured vaginal outer diameter in real-time. Immunofluorescence evaluated the expression of alpha-smooth muscle actin and myosin heavy chain in the vaginal muscularis (n = 6). When smooth muscle contraction was activated, vaginal creep behavior increased compared to the relaxed state. However, increased pressure decreased the active creep response. This study demonstrated that extension - inflation protocols can be used to evaluate smooth muscle contribution to the viscoelastic response of tubular soft tissues.

Linear viscoelasticity of human sclera and posterior ocular tissues during tensile creep

PURPOSE

Despite presumed relevance to ocular diseases, the viscoelastic properties of the posterior human eye have not been evaluated in detail. We performed creep testing to characterize the viscoelastic properties of ocular regions, including the sclera, optic nerve (ON) and ON sheath.

METHODS

We tested 10 pairs of postmortem human eyes of average age 77 ± 17 years, consisting of 5 males and 5 females. Except for the ON that was tested in native shape, tissues were trimmed into rectangles. With physiologic temperature and constant wetting, tissues were rapidly loaded to tensile stress that was maintained by servo feedback as length was monitored for 1,500 sec. Relaxation modulus was computed using Prony series, and Deborah numbers estimated for times scales of physiological eye movements.

RESULTS

Correlation between creep rate and applied stress level was negligible for all tissues, permitting description as linear viscoelastic materials characterized by lumped parameter compliance equations for limiting behaviors. The ON was the most compliant, and anterior sclera least compliant, with similar intermediate values for posterior sclera and ON sheath. Sensitivity analysis demonstrated that linear behavior eventually become dominant after long time. For the range of typical pursuit tracking, all tissues exhibit Debora numbers less than 75, and should be regarded as viscoelastic. With a 6.7 Deborah number, this is especially so for the ON during pursuit and convergence.

CONCLUSIONS

Posterior ocular tissues exhibit creep consistent with linear viscoelasticity necessary for describing biomechanical behavior of the ON, its sheath, and sclera during physiological eye movements and eccentric ocular fixations. Running Head: Tensile Creep of Human Ocular Tissues.

Biomechanical Weakening of Paper and Plant Cell Walls by Bacterial Expansins

Expansins are proteins that loosen plant cell walls but lack enzymatic activity. Here we describe two protocols tailored to measure the biomechanical activity of bacterial expansin. The first assay relies on the weakening of filter paper by expansin. The second assay is based on induction of creep (long-term, irreversible extension) of plant cell wall samples.

Brinell-Hardness data (HBW 2.5/62.5) of aluminum alloy EN AW-2618A after different aging times and temperatures

The article covers data on the Brinell hardness of the forged precipitation-hardened aluminum alloy EN AW-2618A in the initial T61 condition (i. e. slightly underaged) and after isothermal aging for up to 25,000 h at aging temperatures between 160 °C and 350 °C. In addition, the hardness was determined on specimens after creep testing at 190 °C and various stresses. The hardness decreases with increasing aging time due to the microstructural evolution of the hardening precipitates. The drop occurs faster the higher the aging temperature. Aging under creep load additionally accelerates the hardness decrease.

Ambient- and elevated temperature properties of Sc- and Zr-modified Al-6Ni alloys strengthened by Al3Ni microfibers and Al3(Sc, Zr) nanoprecipitates

The eutectic Al-6Ni (wt.%) alloy exhibits excellent strength at ambient and elevated temperature, provided by a high volume fraction of Al₃Ni microfibers formed during solidification. Here, Al-6Ni is micro-alloyed with Sc and Zr (with 0.1Sc+0.2Zr, 0.2Sc+0.4Zr and 0.3Sc+0.2Zr, wt.%), creating two additional populations of primary and secondary Al₃(Sc,Zr) precipitates. The fully eutectic microstructure (α-Al + Al₃Ni) observed in Al-6Ni alloy changes, with Sc and Zr addition to hypoeutectic microstructure with primary α-Al grains nucleated on solidification by primary Al₃(Sc,Zr) precipitates. Upon subsequent aging, fully-coherent Al₃(Sc,Zr) nano-precipitates form in the α-Al matrix between Al₃Ni microfibers, providing substantial precipitation strengthening, which is maintained for up to 1 month at 350 °C. Alloy strength - both at ambient temperature and during creep at 300 °C - can be quantitatively described through a superposition of precipitation strengthening by Al₃(Sc,Zr) nanoprecipitates and load-transfer strengthening by Al₃Ni microfibers.

Nonlinear time-dependent mechanical behavior of mammalian collagen fibrils

The viscoelastic mechanical behavior of collagenous tissues has been studied extensively at the macroscale, yet a thorough quantitative understanding of the time-dependent mechanics of the basic building blocks of tissues, the collagen fibrils, is still missing. In order to address this knowledge gap, stress relaxation and creep tests at various stress (5-35 MPa) and strain (5-20%) levels were performed with individual collagen fibrils (average diameter of fully hydrated fibrils: 253 ± 21 nm) in phosphate buffered saline (PBS). The experimental results showed that the time-dependent mechanical behavior of fully hydrated individual collagen fibrils reconstituted from Type I calf skin collagen, is described by strain-dependent stress relaxation and stress-dependent creep functions in both the heel-toe and the linear regimes of deformation in monotonic stress-strain curves. The adaptive quasilinear viscoelastic (QLV) model, originally developed to capture the nonlinear viscoelastic response of collagenous tissues, provided a very good description of the nonlinear stress relaxation and creep behavior of the collagen fibrils. On the other hand, the nonlinear superposition (NSP) model fitted well the creep but not the stress relaxation data. The time constants and rates extracted from the adaptive QLV and the NSP models, respectively, pointed to a faster rate for stress relaxation than creep. This nonlinear viscoelastic behavior of individual collagen fibrils agrees with prior studies of macroscale collagenous tissues, thus demonstrating consistent time-dependent behavior across length scales and tissue hierarchies. STATEMENT OF SIGNIFICANCE: Pure stress relaxation and creep experiments were conducted for the first time with fully hydrated individual collagen fibrils. It is shown that collagen nanofibrils have a nonlinear time-dependent behavior which agrees with prior studies on macroscale collagenous tissues, thus demonstrating consistent time-dependent behavior across length scales and tissue hierarchies. This new insight into the non-linear viscoelastic behavior of the building blocks of mammalian collagenous tissues may serve as the foundation for improved macroscale tissue models that capture the mechanical behavior across length scales.

A mathematical model for viscoelastic properties of biological soft tissue

A quaternary viscoelastic structure model with two characteristic times is presented to describe the viscoelastic properties of parallel-fibered collagen tissue. The comparison results of model prediction and experimental data of rabbit medial collateral ligaments show that the model could accurately describe viscoelastic behavior such as stress-relaxation, strain-strengthening and creep of bio-soft-tissue within a small scope of errors. To study the biomechanical mechanism of viscoelasticity that biological soft tissue shows, the influence of model parameters on viscoelastic behavior of bio-soft-tissue is analyzed and researched, which indicated that the major influential elements of stress-relaxation in bio-soft-tissue are elastic modulus, relaxation time and strain rate of proteoglycan-rich matrix. The influence of elastic modulus of collagen fibers on stress-relaxation is not significant. However, the nonlinearity of stress-strain curve and viscoelastic behavior of bio-soft-tissue mostly depends on recruitment and reorientation of collagen fibers under external loading.

Strength of interference screw fixation of meniscus prosthesis matches native meniscus attachments

PURPOSE

Meniscal surgery is one of the most common orthopaedic surgical interventions. Total meniscus replacements have been proposed as a solution for patients with irreparable meniscal injuries. Reliable fixation is crucial for the success and functionality of such implants. The aim of this study was to characterise an interference screw fixation system developed for a novel fibre-matrix-reinforced synthetic total meniscus replacement in an ovine cadaveric model.

METHODS

Textile straps were tested in tension to failure (n = 15) and in cyclic tension (70-220 N) for 1000 cycles (n = 5). The textile strap-interference screw fixation system was tested in 4.5 mm-diameter single anterior and double posterior tunnels in North of England Mule ovine tibias aged > 2 years using titanium alloy (Ti6Al4Va) and polyether-ether-ketone (PEEK) screws (n ≥ 5). Straps were preconditioned, dynamically loaded for 1000 cycles in tension (70-220 N), the fixation slippage under cyclic loading was measured, and then pulled to failure.

RESULTS

Strap stiffness was at least 12 times that recorded for human meniscal roots. Strap creep strain at the maximum load (220 N) was 0.005 following 1000 cycles. For all tunnels, pull-out failure resulted from textile strap slippage or bone fracture rather than strap rupture, which demonstrated that the textile strap was comparatively stronger than the interference screw fixation system. Pull-out load (anterior 544 ± 119 N; posterior 889 ± 157 N) was comparable to human meniscal root strength. Fixation slippage was within the acceptable range for anterior cruciate ligament graft reconstruction (anterior 1.9 ± 0.7 mm; posterior 1.9 ± 0.5 mm).

CONCLUSION

These findings show that the textile attachment-interference screw fixation system provides reliable fixation for a novel ovine meniscus implant, supporting progression to in vivo testing. This research provides a baseline for future development of novel human meniscus replacements, in relation to attachment design and fixation methods. The data suggest that surgical techniques familiar from ligament reconstruction may be used for the fixation of clinical meniscal prostheses.

Experimental study on creep properties prediction of reed bales based on SVR and MLP

BACKGROUND

Reed has high lignin content, wide distribution and low cost. It is an ideal raw material for replacing wood in the paper industry. Reeds are rich in resources, but the density of reeds is low, leading to high transportation and storage costs. This paper aims to study the compression process of reeds and the creep behaviour of compressed reeds, and provide theoretical guidance for the reed compressor management, bundling equipment and the stability of compressed reed bales.

RESULTS

We have established a multi-layer perceptron network prediction model for the creep characteristics of reeds, and the prediction rate R² of this model is greater than 0.997. The constitutive equation, constitutive coefficient and creep quaternary model of the reed creep process were established by using the prediction model. The creep behaviour of the reed bale is positively correlated with the initial maximum compressive stress (σ₀). During the creep of the reed, the elastic power and the viscous resistance restrict each other. The results show that the proportion of elastic strain in the initial stage is the largest, and gradually decreases to 99.19% over time. The viscoelastic strain increases rapidly with time, then slowly increases, and finally stabilizes to 0.69%, while the plastic strain accounts for the proportion of the total strain. The specific gravity of the reed increases linearly with the increase of creep time, and finally accounts for 0.39%, indicating that as time increases, the damage of the reed's own structure gradually increases.

CONCLUSIONS

We studied the relationship between the strain and time of the reed and the strain and creep behaviour of the reed bag under different holding forces under constant force. It is proved that the multi-layer perceptron network is better than the support vector machine regression in predicting the characteristics of reed materials. The three stages of elasticity, viscoelasticity and plasticity in the process of reed creep are analysed in detail. This article opens up a new way for using machine learning methods to predict the mechanical properties of materials. The proposed prediction model provides new ideas for the characterization of material characteristics.

Solute-induced strengthening during creep of an aged-hardened Al-Mn-Zr alloy

We examine the precipitation and creep behavior of Al-0.5Mn-0.02Si (at.%) alloys, with and without the L1₂-forming elements Zr and Er (0.09 and 0.05 at.%, respectively), utilizing isochronal aging experiments as well as compressive and tensile creep tests performed between 275 and 400 °C. The Al-0.5Mn-0.09Zr-0.05Er-0.05Si alloy exhibits an unusually high creep resistance in the peak-aged state, which is significantly better than that observed generally in its Mn-free L1₂-strengthened counterparts; for example, the creep threshold stresses at 300 °C are 34-37 MPa, about three times higher than those in a Mn-free Al-0.11Zr-0.005Er-0.02Si alloy. Scanning transmission electron microscopy illustrates that nanoscale Al ₃ (Zr,Er) L1 ₂ -precipitates are formed in the dendritic cores and micron-sized Al(Mn,Fe)Si α-precipitates in the inter-dendritic channels. Moreover, the Al(f.c.c.)-matrix remains supersaturated with randomly distributed Mn solute atoms, as determined by atom-probe tomography and electrical conductivity measurements, for months at creep temperatures. Creep experiments on the Zr- and Er-free Al-0.5Mn-0.02Si solid-solution alloy reveal a small primary creep strain, a high apparent stress exponent, n_a ~9-7, and a threshold-stress-type behavior. After ruling out other possible mechanisms, we provide evidence that the threshold stress in this precipitate-free alloy originates from dislocation/solute elastic interactions leading to a strong drag force exerted on edge dislocations, hindering their ability to climb. The relatively high creep resistance of Al-0.5Mn-0.09Zr-0.05Er-0.05Si is interpreted in terms of the synergy between this solute-induced threshold stress (SITS, from Mn in solid-solution) and the known precipitate-bypass threshold stress (from the L1₂-nanoprecipitates).

The effect of sterilization and storage on the viscoelastic properties of human tendon allografts

Allografts have become increasingly preferred for anterior cruciate ligament replacement purposes. The risk of infections necessitates thorough sterilization procedures, and the allografts usually need to be stored prior to surgery. Classical mechanical tests have been performed with various types of tendons, however, tibialis anterior and peroneus longus tend to suffer the least biomechanical changes after irradiation. Only few results are available of the strain and creep behaviour of tendons, even though this information is necessary to provide suitable allografts. The aim of the present study is to analyze the effect of different tendon types (T-tibialis anterior, P-peroneus longus), sterilization methods (G-gamma irradiation of 21 kGy, E-electron beam irradiation of 21 kGy) and storage times (5 and 6 months) on the creep behavior, which is characterized by the strain at the end of the loading phase and creep deformation after static loading. Static creep tests were performed with 250 N load during 60 s. Deformation at the end of the loading phase of both tendons was significantly smaller after 5 months long storage than that after 6 months long storage. TE5 showed significantly less creep than group TE6, and TE6 significantly greater than PE6. The creep of TE5 was significantly lower than that of TG5. Based on the data, the peroneus longus sterilized by electron beam and stored deep frozen for 5 months is a better choice for anterior cruciate ligament reconstruction than tibialis anterior sterilized by gamma irradiation stored for 6 months.

Morphometric measurements can improve prediction of progressive vertebral deformity following vertebral damage

PURPOSE

A damaged vertebral body can exhibit accelerated 'creep' under constant load, leading to progressive vertebral deformity. However, the risk of this happening is not easy to predict in clinical practice. The present cadaveric study aimed to identify morphometric measurements in a damaged vertebral body that can predict a susceptibility to accelerated creep.

METHODS

A total of 27 vertebral trabeculae samples cored from five cadaveric spines (3 male, 2 female, aged 36 to 73 (mean 57) years) were mechanically tested to establish the relationship between bone damage and residual strain. Compression testing of 28 human spinal motion segments (three vertebrae and intervening soft tissues) dissected from 14 cadaveric spines (10 male, 4 female, aged 67 to 92 (mean 80) years) showed how the rate of creep of a damaged vertebral body increases with increasing "damage intensity" in its trabecular bone. Damage intensity was calculated from vertebral body residual strain following initial compressive overload using the relationship established in the compression test of trabecular bone samples.

RESULTS

Calculations from trabecular bone samples showed a strong nonlinear relationship between residual strain and trabecular bone damage intensity (R² = 0.78, P < 0.001). In damaged vertebral bodies, damage intensity was then related to vertebral creep rate (R² = 0.39, P = 0.001). This procedure enabled accelerated vertebral body creep to be predicted from morphological changes (residual strains) in the damaged vertebra.

CONCLUSION

These findings suggest that morphometric measurements obtained from fractured vertebrae can be used to quantify vertebral damage and hence to predict progressive vertebral deformity.

Post-irradiation surface viscoelastic integrity of photo-polymerized resin-based composites

OBJECTIVE

A class of ultra-rapid-cure resin-based composites (RBCs) exhibited immediate post-irradiation surface viscoelastic integrity using an indentation-creep/recovery procedure. The aim of this study was to determine whether such behavior is more generally characteristic of a wider range of RBCs.

METHODS

Eight representative RBCs were selected based on different clinical categories: three bulkfills (OBF, Filtek One Bulk Fill; VBF, Venus Bulkfill; EBF, Estelite Bulkfill), three conventional non-flowables (XTE, Filtek Supreme XTE; GSO, GrandioSo; HRZ, Harmonize) and conventional flowables (XTF, Filtek Supreme XTE Flow; GSF, GrandioSo Flow). Stainless steel split molds were used to fabricate cylindrical specimens (4mm (dia)×4mm). These were irradiated (1.2W/cm²) for 20s on the top surface. Post-irradiation specimens (n=3), within their molds, were centrally loaded with a flat-ended 1.5mm diameter indenter under 14MPa stress: either immediately (<2min) or after 24h delayed indentation. Stress was maintained for 2h, then - after removal - recovery measurements continued for a further 2h. Indentation depth (%) versus time was measured continuously to an accuracy of <0.1μm. Data were analyzed by One-way ANOVA and Tukey post-hoc tests (α=0.05).

RESULTS

Time-dependent viscoelastic indentation was observed for all RBCs. For immediate indentation, the maximum indentation range was 1.43-4.92%, versus 0.70-2.22% for 24h delayed indentation. Following 2h recovery, the residual indentation range was 0.86-3.58% after immediate indentation, reducing to 0.22-1.27% for delayed indentation. The greatest immediate indentation was shown by VBF followed by XTF and GSF. OBF, HRZ, XTE and GSO had significantly lower indentations (greater hardness). XTE showed a significantly reduced indentation maximum compared to OBF (p<0.05). Indentations delayed until 24h post-irradiation were reduced (p<0.05) for most materials.

SIGNIFICANCE

The indentation-creep methodology effectively characterized resin-based composites within several categories. Viscoelastic properties evaluated by the indentation-creep method confirmed that highly filled RBCs were more resistant to indentation. Indentations were reduced after 24h post-irradiation due to further matrix-network development.

Tensile creep behavior and mechanism of CoCrFeMnNi high entropy alloy

As a potential novel superalloy, the study on the creep of high entropy alloys is of great significance. Equiatomic ratio CoCrFeMnNi with a mass of 6.7 kg was fabricated by vacuum induction levitation melting. Dendrite structure is obtained for as-cast alloy while a homogeneous equiaxed grain structure can be formed after annealing at 1273 K for 6 h. The tensile creep behavior of as-annealed alloy at 773-973 K and 50-340 MPa is systematically studied. Apparent dynamic recovery and recrystallization occur for creep at 973 K while high dislocation density can be observed for creep at 923 K and lower temperatures. Results of EDS analyses on fractured surface, stress exponent and activation energy for steady state creep rate suggest that the creep mechanism should be the dislocation viscous glide dragged by solute Mn at low stresses and low temperatures and be the dislocation climb associated with Mn diffusion at high stresses and high temperatures.

Viscoelastic properties of the human A2 finger pulley

INTRODUCTION

Biomechanical evaluation of the viscoelastic properties tissue deformation, stiffness, and maximum breaking load of the human A2 pulley. We hypothesized that the A2 pulleys of index, middle, and ring fingers exhibit no difference regarding the aforementioned biomechanical parameters.

METHODS

Forty-one A2 pulleys of 14 upper extremities (8 body donors) were assessed. Cyclic and load-to-failure testing were performed. The biomechanical parameters tissue deformation during cyclic and load-to-failure testing, stiffness, and maximum breaking load were determined.

RESULTS

No significant differences between the fingers could be detected regarding the biomechanical parameters. A significant negative correlation could be detected between stiffness and deformation of the pulley. Significant positive correlations could be identified between stiffness and maximum breaking load and between maximum breaking load and deformation of the pulleys.

CONCLUSIONS

Assessment of the viscoelastic properties of the A2 finger pulley promotes precise diagnosis of pulley lesions and will help to optimize treatment.

Machine learning to determine the main factors affecting creep rates in laser powder bed fusion

There is an increasing need for the use of additive manufacturing (AM) to produce improved critical application engineering components. However, the materials manufactured using AM perform well below their traditionally manufactured counterparts, particularly for creep and fatigue. Research has shown that this difference in performance is due to the complex relationships between AM process parameters which affect the material microstructure and consequently the mechanical performance as well. Therefore, it is necessary to understand the impact of different AM build parameters on the mechanical performance of parts. Machine learning (ML) models are able to find hidden relationships in data using iterative statistical analyses and have the potential to develop process-structure-property-performance relationships for manufacturing processes, including AM. The aim of this work is to apply ML techniques to materials testing data in order to understand the effect of AM process parameters on the creep rate of additively built nickel-based superalloy and to predict the creep rate of the material from these process parameters. In this work, the predictive capabilities of ML and its ability to develop process-structure-property relationships are applied to the creep properties of laser powder bed fused alloy 718. The input data for the ML model included the Laser Powder Bed Fusion (LPBF) build parameters used-build orientation, scan strategy and number of lasers-and geometrical material descriptors which were extracted from optical microscope porosity images using image analysis techniques. The ML model was used to predict the minimum creep rate of the Laser Powder Bed Fused alloy 718 samples, which had been creep tested at 650 ∘ C and 600 MPa. The ML model was also used to identify the most relevant material descriptors affecting the minimum creep rate of the material (determined by using an ensemble feature importance framework). The creep rate was accurately predicted with a percentage error of 1.40 % in the best case. The most important material descriptors were found to be part density, number of pores, build orientation and scan strategy. These findings show the applicability and potential of using ML to determine and predict the mechanical properties of materials fabricated via different manufacturing processes, and to find process-structure-property relationships in AM. This increases the readiness of AM for use in critical applications.

The mechanical characterization of blood vessels and their substitutes in the continuous quest for physiological-relevant performances. A critical review

During the last 50 years, novel biomaterials and tissue engineering techniques have been investigated to produce alternative vascular substitutes that recapitulate the unique elastic mechanical features of blood vessels. A large variation in mechanical characterization, including the test type, protocol, and data analysis, is present in literature which complicates the comparison among studies and prevents the blooming and the advancement of this field. In addition, a limited mechanical assessment of the substitute for the intended application is often provided. In this light, this review presents the mechanical environment of blood vessels, discusses their mechanical behavior responsible for the suited blood flow into the body (non-linearity, anisotropy, hysteresis, and compliance), and compares the mechanical properties reported in literature (obtained with compression, tensile, stress-relaxation, creep, dynamic mechanical analysis, burst pressure, and dynamic compliance tests). This perspective highlights that the mechanical properties extracted through conventional tests are not always suitable indicators of the mechanical performance during the working life of a vascular substitute. The available tests can be then strategically used at different stages of the substitute development, prioritizing the simplicity of the method at early stages, and the physiological pertinence at later stages, following as much as possible ISO standards in the field. A consistent mechanical characterization focused on the behavior to which they will be subdued during real life is one key and missing element in the quest for physiological-like mechanical performance of vascular substitutes.

Differences in the biomechanical behaviors of natural teeth and dental implants

OBJECTIVE

The lack of a PDL, which acts as an energy absorber, is a contributor to implants' early failure; however, these discrepancies are not well understood because of limited in vivo research. This study investigated the discrepancy in biomechanical behaviors between natural teeth and dental implants by detecting micro-movements in vivo.

METHODS

We designed a device that could measure precisely mechanical behaviors such as creep, stress relaxation, and hysteresis by using load-control displacement on teeth and implants. We also compared energy dissipation between natural teeth and dental implants by subtracting the area of the hysteresis loop of natural teeth from that of dental implants.

RESULTS

Biphasic curves with an initial phase of rapid response and a subsequent phase of slow response were confirmed in creep and stress relaxation curves for the load-time relationship in natural teeth. By contrast, the behavior of creep or stress relaxation was less prominent when the dental implants were tested. We observed that the periodontal ligament under an axial intrusive load of 300g in a loading rate 3g/s could dissipate the energy of 7.35±1.18×10^-2 mJ, approximately 50 times that of the dental implants (1.47±1.22×10^-3) with statistically significant (p<0.05).

SIGNIFICANCE

We confirmed natural teeth could achieve greater energy dissipation compared to dental implants, which owe to that natural teeth exhibited fluid and viscoelastic properties.

Interrelated hypoalgesia, creep, and muscle fatigue following a repetitive trunk flexion exposure

Repetitive trunk flexion can damage spinal tissues, however its association with low back pain in the workplace may be confounded by factors related to pain sensitivity. Muscle fatigue, exercise-induced hypoalgesia, and creep-induced neuromuscular changes following repetitive trunk flexion may all affect this assumed exposure-pain relationship. This study's purpose was to determine how mechanical pain sensitivity in the low back is affected by a repetitive trunk flexion exposure and identify factors associated with changes in low back pain sensitivity. Pressure pain thresholds, perceptions of sub-threshold stimuli, and muscle fatigue in the trunk and tibia, as well as lumbar spine creep were tracked in 37 young healthy adults before and up to 40 min after a 10-min repetitive trunk flexion exposure. Pressure pain thresholds (p = 0.033), but not perceptions of sub-threshold stimuli (p > 0.102) were associated with approximately a 12.5% reduction in pain sensitivity 10 min after completing the exposure, while creep and local muscle fatigue effects were only observed immediately following the exposure. Creep and fatigue interactions and the corresponding tibial measure co-varied with individual low back pressure pain thresholds. The net hypoalgesic effects of repetitive trunk flexion have the potential to partially mask possibly injurious loads, which could contribute to the severity or incidence of lower back injuries related to these exposures.

Multi-parameter Sub-Hertz Analysis of Viscoelasticity With a Quality Metric for Differentiation of Breast Masses

We applied sub-Hertz analysis of viscoelasticity (SAVE) to differentiate breast masses in pre-biopsy patients. Tissue response during external ramp-and-hold stress was ultrasonically detected. Displacements were used to acquire tissue viscoelastic parameters. The fast instantaneous response and slow creep-like deformations were modeled as the response of a linear standard solid from which viscoelastic parameters were estimated. These parameters were used in a multi-variable classification framework to differentiate malignant from benign masses identified by pathology. When employing all viscoelasticity parameters, SAVE resulted in 71.43% accuracy in differentiating lesions. When combined with ultrasound features and lesion size, accuracy was 82.24%. Adding a quality metric based on uniaxial motion increased the accuracy to 81.25%. When all three were combined with SAVE, accuracy was 91.3%. These results confirm the utility of SAVE as a robust ultrasound-based diagnostic tool for non-invasive differentiation of breast masses when used as stand-alone biomarkers or in conjunction with ultrasonic features.

A predictive model for creep deformation following vertebral compression fractures

Many vertebral compression fractures continue to collapse over time, resulting in spinal deformity and chronic back pain. Currently, there is no adequate screening strategy to identify patients at risk of progressive vertebral collapse. This study developed a mathematical model to describe the quantitative relationship between initial bone damage and progressive ("creep") deformation in human vertebrae. The model uses creep rate before damage, and the degree of vertebral bone damage, to predict creep rate of a fractured vertebra following bone damage. Mechanical testing data were obtained from 27 vertebral trabeculae samples, and 38 motion segments, from 26 human spines. These were analysed to evaluate bone damage intensity, and creep rates before and after damage, in order to estimate the model parameter, p, which represents how bone damage affects the change of creep rate after damage. Results of the model showed that p was 1.38 (R² = 0.72, p < 0.001) for vertebral trabeculae, and 1.48 for motion segments (R² = 0.22, p = 0.003). These values were not significantly different from each other (P > 0.05). Further analyses revealed that p was not significantly influenced by cortical bone damage, endplate damage, disc degeneration, vertebral size, or vertebral areal bone mineral density (aBMD) (P > 0.05). The key determinant of creep deformation following vertebral compression fracture was the degree of trabecular bone damage. The proposed model could be used to identify the measures of bone damage on routine MR images that are associated with creep deformation so that a screening tool can be developed to predict progressive vertebral collapse following compression fracture.

Influence of creep deformation on sub-regional lumbar spine motion during manual lifting

Prolonged or repetitive spine flexion induces creep deformation of posterior spine tissues allowing for increased intervertebral motion beyond 'normal' limits, which may influence sub-regional (intersegmental) spine motion during subsequent manual lifting tasks. Using spine skin-surface kinematics, intersegmental lumbar spine motion was recorded over 20 minutes of prolonged static spine flexion and a subsequent manual lifting task (2 lifts every 3 minutes, 30 minutes total) in 14 participants. Results demonstrated that mid to lower lumbar intersegmental levels (i.e. L₂/L₃ to L₄/L₅) experienced the greatest overall creep deformation and range of motion during both prolonged flexion and manual lifting; however, overall range of motion during manual lifting was unaffected. Additionally, creep deformation did not completely recover within 30 minutes. Future work should continue to investigate the influence of this residual creep, as well as how overall creep deformation impacts spine neuromuscular control and stability, and ultimately the development of low back disorders. Practitioner summary: Mid to lower lumbar spine levels (i.e. L₂/L₃ to L₄/L₅) experienced the greatest creep deformation and range of motion during both prolonged flexion and manual lifting. Repeated lifting following prolonged flexion may limit creep recovery; however, overall lifting kinematic motion remained unchanged.

Studying cell wall mechanics using an automated confocal micro-extensometer

Recently there has been a lot of interest in quantifying mechanical properties and responses to mechanical stress. This type of data can provide insight into how growth is regulated, the processes that enable it to occur and how stresses that build up during development feedback onto development itself. However, quantifying mechanical properties of plant cell walls is difficult as the material is heterogeneous, anisotropic and shows complex time-dependent properties as well as being subject to the complex geometries of plant tissues. It is therefore necessary to have a range of methods to enable the quantification of these properties at different resolutions and time-scales. Here we provide a guide to quantifying mechanical properties in Arabidopsis thaliana hypocotyls using a tensile testing device an automated confocal micro-extensometer (ACME). In contrast to indentation methods, tensile testing provides information on the tissue as a whole and in the plane of the sample. We also detail how to adapt the method to use it for quantifying responses to mechanical stress.

Sensory regulation and mechanical effects of sustained high intensity stretching of the anterior compartment of the thigh

BACKGROUND

Ballet dancers, contortionists, gymnasts, or other sportspeople spend long hours performing stretches while training. Although most studies on stretching consider fascia lengthening to be difficult, athletes manage to lengthen their fascia.

AIM

To assess the relationship between lengthening fascial structures of the anterior compartment of the thigh and the self-reported sensation of discomfort and pain during a sustained and repeated high intensity stretch.

METHODS

Our analysis was based on the data of 7 high school male elite rugby players who completed 11 sessions of stretching (10-min quasi-static stretch of the rectus femoris and fascia lata, at the maximum intensity tolerated), performed twice per week. The measured outcomes included hip range of motion, the length of the structures of the anterior compartment, subjective pain and tension during the stretch, and the level of surface electromyography activity. Values were compared before and after completion of the 11 sessions.

RESULTS

Myofascial length increased by 1 cm. The necessary force applied increased from 124 to 164 N. However, the maximal tolerated stretching intensity did not change significantly (from 205 to 206 N). The increase in length was principally contributed by the rate of fascial creep upon force application, and not by contractile tissue. Subjective levels of tension were related to the stretching force applied and pain was related to the lengthening.

CONCLUSION

Sensations can be used to adjust the intensity and duration of stretching. Soft matter physics provides a new interpretation of fascia lengthening and strengthening during a high intensity stretch.

Data from cyclic tensile tests on sutured organs to evaluate creep behaviour, distraction, and residual thread strength

A number of applications in the surgical practice are based on tensile sutures aimed to keep soft tissues in place and compensate the exit of neuropathies, prolapses or general tissue relaxation. Long-term behaviour of these constructs need to be carefully examined in order to define tensile forces to be applied and to compare different suture anchors. Data here reported refer to equine laryngoplasties, where a suitable loading system has been designed in order to be able to test sutures in-sito, applying known forces ("On-site testing of sutured organs: an experimental set up to cyclically tighten sutures" (Pascoletti et al., 2020 [1])). The loading protocol was made of two steps: in the first step, 3000 loading cycles have been performed; in the following step, a tensile test up to rupture was performed. Cyclic load/displacement curves allow evaluating suture distraction, as a consequence of suture migration and/or soft tissues creep. Tensile curves allow evaluating the residual thread strength and its ultimate displacement. These data can provide a detailed insight of long-term suture behaviour and can be a reference to compare different threads and/or suture anchors.

On-site testing of sutured organs: An experimental set up to cyclically tighten sutures

A number of surgical practices are aimed to compensate for tissue relaxation or weakened/atrophied muscles by means of suture prostheses/thread lifts. The success rate of these procedures is often very good in the short term, while it is quite variable among subjects and techniques in the middle-long term. Middle-long term failures are mostly related to suture distraction, loosening or wear, coming from repeated loading cycles. In this work, an experimental device to perform ex vivo tests on prosthetic sutures has been set up. An equine laryngoplasty has been used as a benchmark, being representative of sutures aimed to compensate for atrophied muscles. The peculiarity of this experimental set up is that the suture is on-site and it has been tightened with known, repeated loads, which do not depend on thread deformation at different load levels. Preliminary tests have been performed applying over 3000 load cycles and finally a tensile test up to rupture. Force/displacement curves obtained with this experimental set up have been reported and parameters useful to classify the biomechanical performance of sutures versus time (mainly its creep behaviour), have been outlined. Results have outlined that the organ-suture system undergoes significant creep over 3000 cycles, and this should be taken into account in order to foresee its long-term behaviour; in addition, the suture anchorage to cartilage should be improved. The experimental set up can be used to perform on-site testing of sutures, taking into account the compliance and creep response at both suture anchorage ends, in order to compare different surgeries and different kinds of thread.

Characterization of compression behaviors of high food waste content (HFWC) MSW and no food waste content (NFWC) MSW in China

In this paper, three sets of laboratory tests were conducted on high-food-waste-content (HFWC-), no-food-waste-content (NFWC-) and decomposed (D-) MSWs to characterize their compression behaviors. The immediate compression ratios C'_c were 0.30, 0.23 and 0.18 for HFWC-MSW, NFWC-MSW and D-MSW respectively, and tended to increase with the increasing food waste content of MSW. The release of intra-particle water contained in food waste contributed over 23.6-29.2% to immediate compression for HFWC-MSW. The mechanical creep ratios C'_sc were 0.02, 0.015 and 0.01 for HFWC-MSW, NFWC-MSW and D-MSW respectively. A prediction model for C'_sc was proposed which incorporated the effects of moisture content, dry unit weight and organic waste content. The bio-compression ratios C'_sbI, C'_sbII and C'_sbIII in response to degradation stage I, II and III were 0.12, 0.10 and 0.02 for HFWC-MSW, and were 0.01, 0.15 and 0.01 for NFWC-MSW. Bio-compression is dominant in stage I and II and mechanical creep is the major contributor in stage III for HFWC-MSW, but to NFWC-MSW, mechanical creep is dominant in stage I and III, and bio-compression takes the main position in stage II. The bio-compression tended to increase linearly with leachate draining rate for HFWC-MSW, and the release of intra-particle water contributed 61.9-65.6% to bio-compression. A new model was proposed that can well capture the highly non-linear behavior of bio-compression for both HFWC-MSW and NFWC-MSW. Based on the above findings, the settlement behavior of HFWC-MSW and NFWC-MSW landfills was compared, and suggestions for technique-efficient and cost-effective design of a NFWC-MSW landfill were discussed.

The bulk compressive creep and recovery behavior of human dentine and resin-based dental materials

OBJECTIVE

To evaluate and compare the viscoelastic properties of dentine and resin-based dental materials by bulk compressive test and the Burgers model.

MATERIALS AND METHODS

Sound dentine, three resin composites as well as a resin-based cement were prepared into cylindrical specimens (n = 8). A bulk compressive creep test was applied with a constant load of 300 N (23.9 MPa) for 2 h, followed by another 2 h recovery. The maximum strain, creep stain, percentage of recovery and permanent set was measured using a linear variable displacement transducer. The viscoelastic properties were characterized via the Burgers model, and the instantaneous elastic, viscous as well as elastic delayed deformation were separated from the total strain. Data were analysed via ANOVA (or Welch's Test) and Tukey (or Games-Howell Test) with a significance level of 0.05.

RESULTS

Sound dentine presented the lowest maximum strain, creep strain, permanent set and the highest percentage of recovery, followed by 3 resin composites with comparable parameters, while the cement showed a significantly higher maximum strain, permanent set and lower percentage of recovery (p < 0.001). The Burgers model presented acceptable fits for characterization viscoelastic processes of both dentine and resin-based dental materials. Viscous and elastic delayed strain of dentine was significantly lower than those for tested materials (p < 0.001) with the highest instantaneous elastic strain percentage. Similar viscous and delayed strain was found among the 4 resin-based materials (p > 0.05).

SIGNIFICANCE

Sound dentine exhibited superior creep stability compared to resin-based dental materials. The viscous deformation in sound dentine could be ignored when loading parallel to dentine tubules.

Theory and Practice in Measuring In-Vitro Extensibility of Growing Plant Cell Walls

This chapter summarizes four extensometer techniques for measuring cell wall extensibility in vitro and discusses how the results of these methods relate to the concept and ideal measurement of cell wall extensibility in the context of plant cell growth. These in-vitro techniques are particularly useful for studies of the molecular basis of cell wall extension. Measurements of breaking strength, elastic compliance and plastic compliance may be informative about changes in cell wall structure, whereas measurements of wall stress relaxation and creep are sensitive to both changes in wall structure and wall-loosening processes, such as those mediated by expansins and some lytic enzymes. A combination of methods is needed to obtain a broader view of cell wall behavior and properties connected with the concept of cell wall extensibility .

Molecular origin of mechano-sorptive creep in cellulosic fibres

Mechano-sorptive creep, i.e. the increased rate of creep that occurs during changing relative humidity, when loading paper or wood, is a phenomenon still not fully understood. This phenomenon was here investigated by examining the changes occurring at the molecular level utilising FTIR spectroscopy. By subjecting the paper to deuterated water, the changes involving both the crystalline hydroxyls as well as those in accessible regions could be examined. During loading, the cellulose molecular chains are stretched taking the load. In addition, during mechano-sorptive creep a further exchange from OH to OD groups occurred. This was interpreted as caused by slippage between cellulose fibrils allowing previously non-accessible hydroxyls to become available for deuterium exchange. Thus, the loosening of the structure, during the changing moisture conditions, is interpreted as what has led to the increased creep and the possibility for new areas of cellulose fibril/fibril aggregates to be exposed to the deuterium exchange.

Creep behavior of Biodegradable Triple-component Nanocomposites Based on PLA/PCL/bioactive Glass for ACL Interference Screws

BACKGROUND

Short-time creep behaviorfor aseries of biodegradable nanocomposites, which areused as implantable devices inthe body, is a crucial factor.The present study aimed to investigate the effect of bioactive glass nanoparticles (BGn) on creep and creep-recovery behaviors of polylactic acid/polycaprolactone (PLA/PCL) blends at different given loads and different applied temperatures.

METHODS

A series of biodegradable nanocomposites consisted of PLA/PCL blends (comprising 80 parts PLA and 20 parts PCL) with different amounts of modified-BGn (m-BGn) fillers were prepared using the evaporated solvent casting technique. Creep and creep-recovery behaviors of all specimens were studied at different valuable stressesof 3 and 6 MPa and different given temperatures of 25 and 37°C.

RESULTS

In all cases, m-BGn improved the creep resistance of the nanocomposites due to the retardation effect during the creep behaviors of the nanocomposite systems. The obtained results in terms of creep and creep-recovery properties determined that the nanocomposites of PLA/PCL/m-BGn can satisfy the required conditions of an appropriate anterior cruciate ligament reconstruction (ACL-R) screw.

CONCLUSION

The obtained results confirmed that the BGn plays an impeding role in the movement of PLA/PCL chains leading to in increase the creep resistance. According to the results, it was determined that the nanocomposites of PLA/PCL and m-BGn can satisfy the required circumstances of a proper ACL-R screw.

Challenges in the use of zinc and its alloys as biodegradable metals: Perspective from biomechanical compatibility

To date, more than fifty articles have been published on the feasibility studies of zinc and its alloys as biodegradable metals. These preliminary in vitro and in vivo studies showed acceptable biodegradability and reasonable biocompatibility in bone and blood microenvironments for the experimental Zn-based biodegradable metals and, for some alloy systems, superior mechanical performance over Mg-based biodegradable metals. For instance, the Zn-Li alloys exhibited higher UTS (UTS), and the Zn-Mn alloys exhibited higher elongation (more than 100%). On the one hand, similar to Mg-based biodegradable metals, insufficient strength and ductility, as well as relatively low fatigue strength, may lead to premature failure of medical devices. On the other hand, owing to the low melting point of the element Zn, several new uncertainties with regard to the mechanical properties of biomedical zinc alloys, including low creep resistance, high susceptibility to natural aging, and static recrystallization (SRX), may lead to device failure during storage at room temperature and usage at body temperature. This paper comprehensively reviews studies on these mechanical aspects of industrial Zn and Zn alloys in the last century and biomedical Zn and Zn alloys in this century. The challenges for the future design of biomedical zinc alloys as biodegradable metals to guarantee 100% mechanical compatibility are pointed out, and this will guide the mechanical property design of Zn-based biodegradable metals. STATEMENT OF SIGNIFICANCE: Previous studies on mechanical properties of industrial Zn and Zn alloys in the last century and biomedical Zn and Zn alloys in this century are comprehensively reviewed herein. The challenges for the future design of zinc-based biodegradable materials considering mechanical compatibility are pointed out. Common considerations such as strength, ductility, and fatigue behaviors are covered together with special attention on several new uncertainties including low creep resistance, high susceptibility to natural aging, and static recrystallization (SRX). These new uncertainties, which are not significantly observed in Mg-based and Fe-based materials, are largely due to the low melting point of the element Zn and may lead to device failure during storage at room temperature and clinical usage at body temperature. Future studies are urgently needed on these topics.

Data of dynamic microscale strain distributions of Ti-6Al-4V alloys in dwell fatigue tests

Dynamic microscale strain distributions with temporal resolution of 1 s in a smooth and a cracked Ti-6Al-4V alloys during one-cycle dwell fatigue tests are illustrated in videos (URL: https://drive.google.com/drive/folders/1pit_VV2apGOpETVfaJAAtL5Xl2CNOiJ3?usp=sharing). The tensile strain distributions were measured by the video sampling moiré method from the 1-μm-pitch grid images in a scanning electron microscope. The strain concentration factors of the smooth and the cracked specimens are 1.96 and 2.65, respectively. The plastic strain increment is 0.0007 during the displacement holding time of 591s in the smoothed specimen at maximum stress of 900 MPa., and 0.0008 during the displacement holding time of 593s in the cracked specimen at maximum stress of 870 MPa. The typical strain results are analyzed in 1-s-resolved strain mapping in Ti-6Al-4V alloys during dwell fatigue in SEM by video sampling moiré [1].

In vivo stress relaxation of human scalp

OBJECTIVE

Conduct a first in vivo study on the large deformation stress relaxation behavior of the human scalp.

METHODS

This study was conducted during Mohs micrographic surgery of the scalp of 14 patients aged 59-90 with wounds initially ranging from 9 to 41 mm wide. The initial wound diameter was measured under zero applied force. Then, the force required to close each wound using a single size 1 nylon suture and a SUTUREGARD suture retention device was measured, after which the suture was then locked in the retention device at fixed displacement. At time points of 300 s, 600 s, and 1800 s, the suture retention device was released, and the wound opening was again recorded at zero force, and the force required to close the wound was recorded.

RESULTS

The average wound closure force relaxed by 44% and 65% after 300 s and 1800 s, respectively. Average wound width decreased 30% and 42%, after 300 s and 1800 s, respectively, due to creep deformation. Furthermore, all wounds relaxed to be below 15 N of closure force after 600 s, which is considered the maximum clinically acceptable force. A relaxation time of ∼270 s and a threshold force for creep of ∼5 N was found.

SIGNIFICANCE

Results of this study provide the first quantitative clinical guidance for efficient scalp closure of large wounds by creep deformation and stress relaxation. Furthermore, the methodology developed here can be used as a basis for future in vivo studies of the stress relaxation and creep deformation of human scalp, which in turn can provide data for the development and validation of constitutive models for scalp deformation.

A Mechanical Brain Damage Framework Used to Model Abnormal Brain Tau Protein Accumulations of National Football League Players

A mechanics-based brain damage framework is used to model the abnormal accumulation of hyperphosphorylated p-tau associated with chronic traumatic encephalopathy within the brains of deceased National Football League (NFL) players studied at Boston University and to provide a framework for understanding the damage mechanisms. p-tau damage is formulated as the multiplicative decomposition of three independently evolving damage internal state variables (ISVs): nucleation related to number density, growth related to the average area, and coalescence related to the nearest neighbor distance. The ISVs evolve under different rates for three well known mechanical boundary conditions, which in themselves introduce three different rates making a total of nine scenarios, that we postulate are related to brain damage progression: (1) monotonic overloads, (2) cyclic fatigue which corresponds to repetitive impacts, and (3) creep which is correlated to damage accumulation over time. Different NFL player positions are described to capture the different types of damage progression. Skill position players, such as quarterbacks, are expected to exhibit a greater p-tau protein accumulation during low cycle fatigue (higher amplitude impacts with a lesser number), and linemen who exhibit a greater p-tau protein accumulation during high cycle fatigue (lower amplitude impacts with a greater number of impacts). This mechanics-based damage framework presents a foundation for developing a multiscale model for traumatic brain injury that combines mechanics with biology.

Viscoelastic stability of pre-cured resin-composite CAD/CAM structures

OBJECTIVES

To study the effect of water storage (3 months) on the creep deformation and recovery of CAD/CAM composite materials to determine their viscoelastic stability.

MATERIALS AND METHODS

Five CAD/CAM composite blocks, with increasing filler loading, and one polymer-infiltrated ceramic network (PICN) were studied. Six specimens of each material were separated into two groups (n=3) according to their storage conditions (24 h dry storage at 23°C versus 3 months storage in 37°C distilled water). A constant static compressive stress of 20 MPa was applied on each specimen via a loading pin for 2 h followed by unloading and monitoring strain recovery for a further period of 2 h. The maximum creep-strain (%) and permanent set (%) were recorded. Data were analysed via two-way ANOVA followed by one-way ANOVA and Bonferroni post hoc tests (<0.05) for comparisons between the materials. Homogeneity of variance was calculated via Levene's statistics.

RESULTS

The maximum creep strain after 24 h dry ranged from 0.45% to 1.09% and increased after 3-month storage in distilled water to between 0.71% and 1.85%. The permanent set after 24 h dry storage ranged from 0.033% to 0.15% and increased after 3-month water storage to between 0.087% and 0.18%. The maximum creep strain also reduced with increasing filler loading.

SIGNIFICANCE

The PICN material exhibited superior dimensional stability to all of the pre-cured resin composite blocks in both storage conditions with deformation being predominantly elastic rather than viscoelastic. Notwithstanding, two of the resin-matrix composite blocks approached the PICN performance, when dry, but less so after water storage.