Challenges in Mechanics of Time-Dependent Materials & Mechanics of Biological Systems and Materials, Volume 2

This study evaluates the effect of the carbon black (CB) on the strain distribution in the vicinity of a crack in natural rubber (NR). In addition, the difference in crack growth behavior due to difference of the amount of CB is also evaluated. For this purpose, the crack growth behavior is observed on rubber test specimens with various of the amounts of CB. Then, the images recorded with a high-speed camera are analyzed by using digital image correlation (DIC). From this experiment, the relationship between tearing energy and crack growth rate is obtained for NR. It is revealed only the specimen filled the most CB is not broken. This result suggests that the dissipation process at the crack tip differs depending on the amount of CB.

Photocured elastomers are widely used as 3D-printed structural materials due to their outstanding extensibility. However, their fracture behavior under different loading rates remains poorly characterized. Here, we conducted extensive experimental tests to investigate the rate-dependent mechanical behavior of a typical photocured elastomer – TangoPlus FLX 930. We applied thermal analysis to analyze its constituents and uniaxial tension tests to characterize the mechanical behavior of the elastomer from quasi-static to moderate strain rates and showed an increase in both fracture strain and stress of TangoPlus with increasing applied strain rate. To further investigate its time-dependent fracture behavior, we conducted relaxation tests under different strain and strain rates.

A direct consequence of time-temperature superposition is that the observed glass transition temperature of polymers increases with strain rate. It also increases with heating rate, as has been observed in our experiments and by other authors. Our research has established a clear relationship between heating rates in thermal experiments and strain rates in mechanical experiments, by considering thermal expansion and mechanical strain to be equivalent quantities. The time-temperature superposition principle is most commonly used in the context of mechanical measurements, where the observation timescale is related to the strain rate, or oscillation frequency of the experiment. Thermal experiments access significantly larger timescales than mechanical experiments; therefore, they can extend the timescale range over which time-temperature superposition methods are validated. We use input from thermal expansion and calorimetric measurements to validate models used to predict the glass transition temperature observed via mechanical measurements, across an extended range of timescales.

Increasing antibiotic resistance in bacteria is a critical issue that often leads to infections or other morbidities. Mechanical properties of the bacterial cell wall, such as thickness or elastic modulus, may contribute to the ability of a bacterial cell to resist antibiotics. Techniques like atomic force microscopy (AFM) are used to quantify bacterial cell mechanical properties and image cell structures at nanoscale resolutions. An additional benefit of AFM is the ability to probe samples submerged in liquids, meaning that live bacteria can be imaged or evaluated in environments that more accurately simulate in vivo conditions as compared to other methods like electron microscopy.

However, because AFM measurements are highly sensitive to small perturbations in the deflection of the tip of a sensor probe brought into contact with the specimen, immobilization of bacteria prior to measurement is essential for accurate measurements. Traditional chemical fixatives crosslink the molecules within the bacterial cell wall, which prevent the bacteria from locomotion. While effective for imaging, chemical crosslinkers are known to affect the measured stiffness of eukaryotic cells and also may affect the measured stiffness of the bacterial cell wall. Alternative immobilization methods include Cell-Tak™, an adhesive derived from marine mussels that does not interact with the bacterial wall and filters with known pore sizes which entrap bacteria. Previous studies have examined the effect of these immobilization methods on successful imaging of bacteria but have not addressed differences in measured modulus. This study compares the effects of immobilization methods including chemical fixatives, mechanical entrapment in filters, and Cell-Tak™ on the stiffness of the bacterial cell wall as measured by force spectroscopy.

Studying the mechanical properties of aortic tissue is important in determining future medical action regarding both healthy and diseased conditions. Although uniaxial and bi-axial experimental testing has been previously conducted on porcine aortas, shear testing is still being preliminarily explored. We studied the response of porcine descending thoracic aorta under cyclic torsional shear at five different compressive strains (5%, 10%, 15%, 20%, and 25%). From preliminary experimental results, we found that varying the normal compressive strain has a significant effect on the shear response. It can be concluded that porcine aorta should be treated as a viscoelastic material.

Cerebral nitric oxide (NO), a small diffusive molecule, plays a critical role in brain’s functionality. More precisely, NO acts as a neuro-glial-vascular messenger that aids various chemo-mechanical communications among brain cells, blood, and cerebral vasculature. A disequilibrium in the NO bioavailability and/or a delay in (or lack of) interactions of NO with other molecules due to, for instance, inflammation can lead to the onset and progression of cerebrovascular diseases. Mathematical models of cerebral NO biotransport can provide essential insights into mechanisms of cerebrovascular diseases that may lead to the development of better preventive, diagnostic, and therapeutic methods. In this chapter, a two-dimensional space-fractional reaction-diffusion equation is proposed to model cerebral NO biotransport. The equation uses spatial fractional order derivatives to describe NO anomalous diffusion caused by the experimentally observed entrapping of NO in circulating endothelial microparticles. Cerebrovascular diseases cause an increase in the amount of endothelial microparticles and thus may lead to an enhanced anomalous diffusion of NO. The model includes the NO production by synthesis in neurons and by shear-induced mechanotransduction in the endothelial cells, and the loss of NO due to its interactions with superoxide and hemoglobin. The blood-endothelium mechanical interactions contribute to the shear-induced production of NO. Perturbation series solutions for the coupled blood-endothelium mechanics are adapted from literature for two cases: impermeable and permeable endothelium. Thus, the viscous dissipation at the blood-endothelium interface can be calculated analytically. The model generalizes a published one-dimensional model of cerebral NO anomalous diffusion in which the blood flow effect on the vascular wall was modeled as a mere oscillatory boundary condition. Numerical simulations suggest that for NO anomalous diffusion of fractional order 0.85 and in the presence of endothelial permeability and blood flow, the NO concentration at the endothelium increases in time which agrees with studies of stroke.

Granular materials are utilized in various applications, including civil engineering, energy, and defense. They exhibit very different mechanical behavior under different loading conditions. Glass beads are often used as a model granular material in simulations due to their regular geometries. In this study, we focus on understanding the incipient failure of confined glass beads assembly and a glass bead/epoxy composite under quasi-static compression using in situ X-ray micro-computed tomography (μCT). An μCT system with an integrated mechanical loading frame provides in situ volumetric images in quasi-static compression. Glass beads of various diameters (2, 3, 4, and 5 mm) are placed inside a hollow cylinder, creating an assembly consisting of glass beads with different combinations of volume fraction and particle contacts. In addition, glass bead/epoxy composite cylindrical specimens are also prepared and compressed in a separate set of experiments. The deformed configurations of such glass bead assembly and the glass bead/epoxy composite specimens at different strain levels are captured by the μCT volumetric images. The incipient failure and the damage behavior of glass beads, under the rigid confinement as well as within the epoxy binder in quasi-static compression, are visualized and assessed. This work provides experimental results for the validation of mesoscale simulations.

The fracture properties of polymers are complex to estimate because they strongly depend on the viscoplastic behavior of the polymer. This is especially true for semicrystalline thermoplastic polymers used for the transport or storage of fluids under pressure. The methodology presented in this work allows the construction of a part of the kinetic law of fracture of polymers, the one that reveals the brittleness of the polymer. The fracture energy of the material is estimated by taking into account the cracking regime and thus the potential inertial effects induced during the rapid propagation of cracks in the case of brittle behavior. Polyamide-11, a polymer used for the manufacture of hydrogen tank liners, is studied. A decrease in fracture energy is observed between the initiation resistance and the fast propagation resistance. This is referred to as polymer embrittlement. Electron microscopy analysis of fracture surfaces in three different zones (initiation, propagation, arrest) has revealed the mechanisms responsible for the ductility of the polymer in quasi-static regime and its brittleness in dynamic regime.

Pressure-sensitive adhesives (PSAs) are commonly used in applications where the adhesive is required to withstand impact events, such as the consumer electronic device, aerospace, and automotive industries. Due to the viscoelastic mechanical behavior of PSAs, it is necessary to characterize these materials using strain rates and temperatures that are relevant to the intended application. This often proves challenging as the application design typically entails complex loading and deformation modes that are difficult and costly to measure directly or replicate in a controlled setting. In the present work, coupon level test methods which isolate through-thickness tension and overlap shear failure modes are used to investigate rate- and temperature-dependent failure properties of an acrylic foam tape. The time-temperature superposition principle was utilized to develop failure master curves to characterize failure properties at rates which could not be measured directly. This approach was validated by comparing master curves created using quasi-static and impact test equipment and varying test speed by approximately three orders of magnitude.

In this research, the aluminium alloy AA6063-T6 was investigated for evaluation of its failure parameters at room and high temperatures. The weight percentage of different elements in AA6063-T6 was also obtained using spectroscopy. The quasi-static tests at different strain rates from room temperature to higher temperatures were performed on a universal testing machine. Notched tensile specimens of notch radii 1 mm, 2 mm, and 3 mm were used to find the effect of stress triaxialities. The tensile tests at high strain rates are performed using a tensile Hopkinson pressure bar setup. The different temperatures considered during quasi-static conditions were 25 °C, 50 °C, 100 °C, 150 °C, and 200 °C. It was found that flow stresses were increased with strain rates, whereas flow stresses were decreased at higher temperatures. Using experimental results, the Johnson-Cook failure model parameters were evaluated. The Johnson-Cook failure model parameters are used as input parameters for finite element simulation.

Adhesion of bacteria to oral implant surfaces can lead to oral infections, and the prevention of strong biofilm adherence to implant surfaces can assist in the prevention of these infections like peri-implantitis. In prior studies, single species biofilm adhesion has been quantitatively measured via the laser spallation technique. However, colonizing oral biofilms rarely consists of a single bacteria species. Multiple early colonizer species, including several strains of Streptococci, dominate initial oral biofilm formation. This study aims to characterize the adhesion of a multi-species oral biofilm consisting of S. oralis, S. sanguinis, and S. gordonii on titanium, a common implant material, using the laser spallation technique. Previous work has established these specific Streptococci strains as a multi-species periodontal biofilm model. This study is the first to provide a quantitative adhesion measurement of this multi-species model onto a dental implant surface. First, adhesion strength of the multi-species model is compared to adhesion strength of the single-species streptococci constituents. Fluorescent staining and imaging by fluorescent microscopy are used to identify individual bacteria species within the biofilm. The multi-species biofilm presented in this study provides a more representative model of in vivo early biofilms and provides a more accurate metric for understanding biocompatibility on implant surfaces.

Time-resolved impact studies were conducted to bridge the gap between material property testing and full-scale ballistic experiments. The motivation is to characterize the projectile-target interaction at early time scales, which includes both the target and projectile response. Prior work has focused on demonstrating the utility of in situ diagnostic techniques for investigating the physics of failure of high-strength metallic and ceramic targets; however, less work has been done to experimentally quantify the projectile deformation. Digital image correlation has been identified as a promising technique to provide real-time, full-field characterization of the dynamic failure behavior during ballistic impact. In instances where the material response is multiaxial, DIC provides one of the only practical means of collecting full-field 3D data. Stereo DIC involves applying a speckle pattern to the target surface and tracking the deformation using two synchronized cameras. This allowed for the 3D reconstruction of deformation histories. The focus of the current study is to understand the limitations and uncertainty of ultra-high-speed DIC for measuring high-rate deformation. Copper long rods were launched into ceramic anvils at a range of velocities (250-500 m/s) to characterize the axial strain and strain rate. A methodology is presented for implementing ultra-high-speed DIC in addition to concurrent multiflash X-ray imaging.

We utilize an in situ/embedded digital image correlation (DIC) technique to evaluate the effect of natural aging on interface delamination strength on an idealized model system. The system consists of a single ~500–650 μm glass bead inclusion at the center of a Sylgard 184 dog-bone tensile sample along with an embedded DIC speckle pattern at the midplane of the sample. The speckle pattern enables the measurement of the evolving strain field in the region surrounding the embedded glass bead, while the sample undergoes a tensile load to failure. During the tensile loading of the sample, the applied stresses at the particle-binder interface force a debonding event to occur before the ultimate tensile failure of the sample. The measured strain field at the moment of the forced debonding event is then utilized to characterize the localized stress required to induce a failure at the interface. Through repetition of the experiment, the debonding behavior can be described in a statistically meaningful fashion. This approach is then extended to capture the effect of natural aging on the delamination behavior through a time period of approximately 24 days.