Glass Fiber-Epoxy Composites with Boron Nitride Nanotubes for Enhancing Interlaminar Properties in Structures

Hybrid composite materials are a class of materials where more than one type of reinforcement is integrated into a matrix to achieve superior properties. This typically involves nanoparticle fillers employed within traditional advanced composites with fiber reinforcements such as carbon or glass. The current study builds on previous investigations of boron nitride nanotube (BNNT) hybrid composites, specifically glass fiber (GF)-epoxy/BNNT composite laminates. GF is an effective and affordable primary reinforcement fiber in many applications, and boron nitride nanotubes (BNNTs) exhibit impressive mechanical properties comparable to carbon nanotubes (CNTs) with distinct functional properties, such as electrical insulation, which is desirable in manufacturing insulating composites when combined with GF. GF-epoxy/BNNT composite laminates, incorporating BNNT materials with different loadings (1 and 2 wt %) and purity, were manufactured using a hand layup technique and prepared for three-point bending, modified Charpy, dynamic mechanical analysis (DMA), and fracture toughness (mode I and mode II) measurements. A comprehensive microscopy study was also performed using scanning electron microscopy (SEM) showing prominent failure mechanism, nanotube dispersion, and their mode of reinforcement in different loading scenarios. Enhanced properties, including a 43% increase in mode II fracture toughness, were observed in hybrid composites with 1 wt % BNNT compared to the GF composites with neat epoxy, and the reinforcement mechanisms were discussed.

Intermediate Strain Rate Material Characterization with Digital Image Correlation

The mechanical response of a material under dynamic load is typically different than its behavior under static conditions; therefore, the common quasistatic equipment and procedures used for material characterization are not applicable for materials under dynamic loads. The dynamic response of a material depends on its deformation rate and is broadly categorized into high (i.e., greater than 200/s), intermediate (i.e., 10-200/s) and low strain rate regimes (i.e., below 10/s). Each of these regimes calls for specific facilities and testing protocols to ensure the reliability of the acquired data. Due to the limited access to high-speed servo-hydraulic facilities and validated testing protocols, there is a noticeable gap in the results at the intermediate strain rate. The current manuscript presents a validated protocol for the characterization of different materials at these intermediate strain rates. Strain gauge instrumentation and digital image correlation protocols are also included as complimentary modules to extract the utmost level of detailed data from every single test. Examples of raw data, obtained from a variety of materials and test setups (e.g., tensile and shear) is presented and the analysis procedure used to process the output data is described. Finally, the challenges of dynamic characterization using the current protocol, along with the limitations of the facility and methods of overcoming potential problems are discussed.

Silane crosslinking of electrospun poly (lactic acid)/nanocrystalline cellulose bionanocomposite

Biodegradable nanofibrous mats fabricated by electrospinning are commonly used in tissue engineering, however, lack of essential mechanical properties of such nanofibers is a challenging issue. In this work, vinyltrimethoxysilane (VTMS) was grafted onto poly (lactic acid) (PLA) and the silane grafted PLA was subsequently applied in electrospinning process. Electrospun nanofibrous mats based on PLA/nanocrystalline cellulose (NCC) and PLA-g-silane/NCC nanocomposites were fabricated and immersed in hot water (70°C) for crosslinking of silane grafted PLA. It was found that introducing NCC to the samples cause to reduction in fiber diameter and the other hand the silane crosslinking of PLA increase the mean fiber diameter. DSC thermograms also revealed that silane grafting caused a reduction in mobility of polymer segments, and consequently reduction of crystallinity. On the contrary, the NCC in the PLA-g-silane samples effectively influenced the crystal nucleation, while in the PLA nanofibers the nucleation was lower. The impact of NCC on tensile strength enhancement of samples was notable. The results suggested that the chemical crosslinking remarkably improves the mechanical properties of PLA nanofibers. Furthermore, biocompatibility of such modified nanofibers was also evaluated through cytotoxicity results, therefore the modified PLA nanocomposite can be considered as a practical candidate for hard tissue engineering applications.

Characterization of a hierarchical network of hyaluronic acid/gelatin composite for use as a smart injectable biomaterial

Hybrid HA/Ge hydrogel particles are embedded in a secondary HA network to improve their structural integrity. The internal microstructure of the particles is imaged through TEM. CSLM is used to identify the location of the Ge molecules in the microgels. Through indentation tests, the Young's modulus of the individual particles is found to be 22 ± 2.5 kPa. The overall shear modulus of the composite is 75 ± 15 Pa at 1 Hz. The mechanical properties of the substrate are found to be viable for cell adhesion. The particles' diameter at pH = 8 is twice that at pH = 5. The pH sensitivity is found to be appropriate for smart drug delivery. Based on their mechanical and structural properties, HA-Ge hierarchical materials may be well suited for use as injectable biomaterials for tissue reconstruction.

An interaction stress analysis of nanoscale elastic asperity contacts

A new contact mechanics model is presented and experimentally examined at the nanoscale. The current work addresses the well-established field of contact mechanics, but at the nanoscale where interaction stresses seem to be effective. The new model combines the classic Hertz theory with the new interaction stress concept to provide the stress field in contact bodies with adhesion. Hence, it benefits from the simplicity of non-adhesive models, while offering the same applicability as more complicated models. In order to examine the model, a set of atomic force microscopy experiments were performed on substrates made from single-walled carbon nanotube buckypaper. The stress field in the substrate was obtained by superposition of the Hertzian stress field and the interaction stress field, and then compared to other contact models. Finally, the effect of indentation depth on the stress field was studied for the interaction model as well as for the Hertz, Derjaguin-Muller-Toporov, and Johnson-Kendall-Roberts models. Thus, the amount of error introduced by using the Hertz theory to model contacts with adhesion was found for different indentation depths. It was observed that in the absence of interaction stress data, the Hertz theory predictions led to smaller errors compared to other contact-with-adhesion models.

Interaction stresses in carbon nanotube-polymer nanocomposites

A new technique of atomic force microscopy interaction measurement is used to obtain the three-dimensional stress field in nanocomposites made of single-walled carbon nanotubes (SWNT) and poly(methyl methacrylate) (PMMA) matrix. This original approach expands the current capability of AFM from imaging and force mapping to three-dimensional stress field measurements. Latest developments in the field have been limited to three-dimensional imaging at the surface only, and one value (adhesion) force mapping. The current work provides the interaction stress results for a PMMA-SWNT nanocomposite and shows a maximum estimated load transfer of less than 7 MPa (the maximum attraction stress estimated). This value is obtained for an unfunctionalized nanocomposite and hence the interaction stress is mainly based on van der Waals interactions. This means that for this system, carbon nanotubes behave similar to an elastic-fully plastic material with a yield stress of less than 7 MPa. This phenomenon illustrates why carbon nanotubes may not show their strong mechanical properties (yield strength of above 10 GPa) in polymeric nanocomposites.