An Open-Source ABAQUS Plug-In for Delamination Analysis of 3D Printed Composites

This article presents the development and implementation of the Delamination Plug-in, an open-source tool for modeling delamination tests in the ABAQUS software. Specifically designed for stochastic modeling of 3D printed composites, the plug-in combines the benefits of the graphical user interface (GUI) and the programming of commercial finite element (FE) software. The Delamination Plug-in offers an effortless alternative to the time-consuming analytical modeling and GUI work involved in delamination tests and includes algorithms for several tests, such as the double cantilever beam, end-loaded split, end-notched flexure, and modified end-loaded split tests, solved using the virtual crack closure technique and the cohesive zone method. It enables the user to develop simulations for both simple symmetric laminates and generally layered laminates with additional thermal stresses. The applicability of the tool is demonstrated through its use in two distinct delamination problems, one for conventional and one for 3D printed composite laminates, and its results are compared to analytical models and experimental data from the open literature. The results demonstrate that the Delamination Plug-in is efficient and applicable for such materials. This establishes the tool as an important means of automating delamination analysis and for the development and testing of 3D printed composites, making it a valuable tool for both researchers and industry professionals.

Measuring and Predicting the Effects of Residual Stresses from Full-Field Data in Laser-Directed Energy Deposition

This article presents a novel approach for assessing the effects of residual stresses in laser-directed energy deposition (L-DED). The approach focuses on exploiting the potential of rapidly growing tools such as machine learning and polynomial chaos expansion for handling full-field data for measurements and predictions. In particular, the thermal expansion coefficient of thin-wall L-DED steel specimens is measured and then used to predict the displacement fields around the drilling hole in incremental hole-drilling tests. The incremental hole-drilling test is performed on cubic L-DED steel specimens and the displacement fields are visualized using a 3D micro-digital image correlation setup. A good agreement is achieved between predictions and experimental measurements.

The Influence of Different Lay-Up Parameters on the Fatigue Response of Carbon/Epoxy Laminates under Internal Multiaxial Stress States

The inherent anisotropy of composites complicates their damage response. The influence of multiaxiality, particularly in carbon-based composites, is not thoroughly understood due to obstacles related to damage monitoring during loading. In this study, the response of different carbon/epoxy laminates under fatigue is examined through dedicated in situ microscopic observations. By varying the orientation of off-axis layers, the impact of multiaxiality on the mechanical and damage response is evaluated. Furthermore, balanced and unbalanced laminates are compared, considering the limited information for the latter. The influence of the number of off-axis layers is finally assessed leading to important conclusions about optimal fatigue response. The fatigue response is evaluated in all cases considering both the mechanical properties and the damage characteristics. Significant conclusions are drawn, especially for the benefits of unbalanced laminates and the impact of shear stresses, allowing for the utilization of the obtained data as important input for the establishment of reliable fatigue damage models.

Elastic Wave Monitoring of Cementitious Mixtures Including Internal Curing Mechanisms

The mitigation of autogenous shrinkage in cementitious materials by internal curing has been widely studied. By the inclusion of water reservoirs, in form of saturated lightweight aggregates or superabsorbent polymers, additional water is provided to the hydrating matrix. The onset of water release is of high importance and determines the efficiency of the internal curing mechanism. However, the monitoring of it poses problems as it is a process that takes place in the microstructure. Using acoustic emission (AE) sensors, the internal curing process is monitored, revealing its initiation and intensity, as well as the duration. In addition, AE is able to capture the water evaporation from saturated specimens. By ultrasonic testing, differences in the hydration kinetics are observed imposed by the different methods of internal curing. The results presented in this paper show the sensitivity of combined AE and ultrasound experiments to various fundamental mechanisms taking place inside cementitious materials and demonstrate the ability of acoustic emission to evaluate internal curing in a non-destructive and easily implementable way.

Chasing the Bubble: Ultrasonic Dispersion and Attenuation from Cement with Superabsorbent Polymers to Shampoo

This study aims to experimentally investigate the ultrasonic behavior of fresh cement focusing on the contribution of the entrapped air bubbles. Frequency dispersion and attenuation carry delicate information that is not possible to gather by traditional ultrasonic pulse velocity. This is measured by simple indicators that quantify the frequency dependence of propagation velocity of longitudinal waves through fresh cementitious media. It seems that dispersion shows much stronger sensitivity to the microstructural processes, since the presence of superabsorbent polymers in mortar induces a large difference in dispersion parameters when compared to reference cement mortar, while only marginal difference in threshold-based pulse velocity. To reach this aim, references are taken from, and comparisons are made to other liquids in order first in order to validate the reliability of the methodology and to better understand the contribution of the cavities in the obtained dispersion and attenuation curves. Ultrasonic dispersion assessment of cementitious media has the potential to bring a lot of information on the microstructure of materials, as well as the ongoing processes.

The Contribution of Elastic Wave NDT to the Characterization of Modern Cementitious Media

To mitigate autogenous shrinkage in cementitious materials and simultaneously preserve the material's mechanical performance, superabsorbent polymers and nanosilica are included in the mixture design. The use of the specific additives influences both the hydration process and the hardened microstructure, while autogenous healing of cracks can be stimulated. These three stages are monitored by means of non-destructive testing, showing the sensitivity of elastic waves to the occurring phenomena. Whereas the action of the superabsorbent polymers was evidenced by acoustic emission, the use of ultrasound revealed the differences in the developed microstructure and the self-healing of cracks by a comparison with more commonly performed mechanical tests. The ability of NDT to determine these various features renders it a promising measuring method for future characterization of innovative cementitious materials.

A Novel Design of Autonomously Healed Concrete: Towards a Vascular Healing Network

Concrete is prone to crack formation in the tensile zone, which is why steel reinforcement is introduced in these zones. However, small cracks could still arise, which give liquids and gasses access to the reinforcement causing it to corrode. Self-healing concrete repairs and seals these small (300 µm) cracks, preventing the development of corrosion. In this study, a vascular system, carrying the healing agent, is developed. It consists of tubes connected to a 3D printed distribution piece. This distribution piece has four outlets that are connected to the tubes and has one inlet, which is accessible from outside. Several materials were considered for the tubes, i.e., polymethylmethacrylate, starch, inorganic phosphate cement and alumina. Three-point-bending and four-point-bending tests proved that self-healing and multiple self-healing is possible with this developed vascular system.

Shear stress sensing with Bragg grating-based sensors in microstructured optical fibers

We demonstrate shear stress sensing with a Bragg grating-based microstructured optical fiber sensor embedded in a single lap adhesive joint. We achieved an unprecedented shear stress sensitivity of 59.8 pm/MPa when the joint is loaded in tension. This corresponds to a shear strain sensitivity of 0.01 pm/µε. We verified these results with 2D and 3D finite element modeling. A comparative FEM study with conventional highly birefringent side-hole and bow-tie fibers shows that our dedicated fiber design yields a fourfold sensitivity improvement.

Investigation of contact acoustic nonlinearity in delaminations by shearographic imaging, laser doppler vibrometric scanning and finite difference modeling

Full-field dynamic shearography and laser Doppler vibrometric scanning are used to investigate the local contact acoustic nonlinear generation of delamination-induced effects on the vibration of a harmonically excited composite plate containing an artificial defect. Nonlinear elastic behavior caused by the stress-dependent boundary conditions at the delamination interfaces of a circular defect is also simulated by a 3-D second-order, finite-difference, staggered-grid model (displacement-stress formulation). Both the experimental and simulated data reveal an asymmetric motion of the layer above the delamination, which acts as a membrane vibrating with enhanced displacement amplitude around a finite offset displacement. The spectrum of the membrane motion is enriched with clapping-induced harmonics of the excitation frequency. In case of a sufficiently thin and soft membrane, the simulations reveal clear modal behavior at sub-harmonic frequencies caused by inelastic clapping.