Assessing the Viscoelasticity of Photopolymer Nanowires Using a Three-Parameter Solid Model for Bending Recovery Motion

Photopolymer nanowires prepared by two-photon polymerization direct laser writing (TPP-DLW) are the building blocks of many microstructure systems. These nanowires possess viscoelastic characteristics that define their deformations under applied forces when operated in a dynamic regime. A simple mechanical model was previously used to describe the bending recovery motion of deflected nanowire cantilevers in Newtonian liquids. The inverse problem is targeted in this work; the experimental observations are used to determine the nanowire physical characteristics. Most importantly, based on the linear three-parameter solid model, we derive explicit formulas to calculate the viscoelastic material parameters. It is shown that the effective elastic modulus of the studied nanowires is two orders of magnitude lower than measured for the bulk material. Additionally, we report on a notable effect of the surrounding aqueous glucose solution on the elasticity and the intrinsic viscosity of the studied nanowires made of Ormocomp.

Uniaxial Compression Mechanical Properties of Foam Nickel/Iron-Epoxy Interpenetrating Phase Composites

The damage process and failure mechanisms were analyzed by a series of quasi-static compressive experiments of seven materials including pure epoxy (EP), three different PPI (pores per linear inch) foam nickel-iron, and three different PPI foam nickel/iron-epoxy interpenetrating phase composites (IPC). Plotting the stress–strain curves of different materials, their change rules are discussed, then the effective elastic modulus and yield limit of the materials are provided, and the energy absorption properties of different materials are analyzed by the stress–strain curves. It was found that the effective elastic modulus and specific stiffness of the three IPC materials were higher than pure foam nickel-iron. The brittleness of epoxy can be obviously changed by selecting a suitable PPI foam nickel-iron composited with it. The unit volume energy absorption rate of foam nickel/iron-epoxy was significantly higher than pure epoxy and pure foam nickel-iron. It was also found that the energy absorption rate decreased with the increase in PPI. The stress relaxation rate decreased first and then increased with the increase in PPI. The creep behavior of the three composites was obvious in the creep elastic stage, and the creep rate increased with the increase in PPI. The creep rate decreased with the increase in PPI in the creep transition stage.

Nanoindentation of Nanoglasses Tested by Molecular Dynamics Simulations: Influence of Structural Relaxation and Chemical Segregation on the Mechanical Response

We present molecular dynamics simulations of nanoindentation in order to investigate the effects of segregation and structural relaxation on the mechanical properties of Cu64Zr36 nanoglasses prepared by particle consolidation and long-time annealing. Our analysis of load-displacement curves shows that the effective elastic modulus of nanoglasses is lower than that of their homogeneous metallic glass counterpart. This is mainly because of the defective short-range order present in the glass-glass interface, but to a lesser extend due to chemical inhomogeneities. Structural relaxation obtained by long-time annealing (500 ns) at 0.8 Tg leads to a shift from a homogeneous deformation to a mix of homogeneous deformation and shear bands. The obtained hardness values of annealed nanoglass are comparable to those of homogenous glass samples, but significantly higher as compared to juvenile as-prepared nanoglass samples. The results are discussed in the context of recent nanonindentation experiments.

Characteristics of dynamic crack propagation in a weak snowpack layer over its entire life cycle

<p>For a slab avalanche to release, a weak layer buried below a cohesive snow slab is required, and the system of weak layer and slab must support crack propagation over large distances. This process, called “dynamic crack propagation”, is highly relevant for avalanche release, and computational models are nowadays able to model crack propagation over increasingly larger scales. Field measurements on dynamic crack propagation are however very scarce, although these are required to validate models. We therefore performed a series of flat field PST experiments up to ten meters long over a period of 10 weeks. During this time, PST results evolved from crack arrest to full propagation and back to crack arrest – reflecting the life cycle of the weak layer. All PST experiments were analyzed using digital image correlation to derive high-resolution displacement fields to compute dynamic crack propagation metrics, including crack length and speed as well as touchdown distance, the distance from the crack tip to the trailing point where the slab comes into contact with the substratum. Comparing the displacement fields during sawing to a mechanical model, we estimated the effective elastic modulus of slab and weak layer as well as the specific fracture energy of the weak layer. Our results show how dynamic crack propagation characteristics change over the life cycle of a weak layer and how these measures relate to snowpack properties such as load and effective elastic modulus of the slab. We found that crack speed was highest for PSTs resulting in full propagation and that the touchdown length increased with increasing elastic modulus of the slab. Our dataset provides unique insight into the dynamics of crack propagation, and provides valuable data to validate models used to study sustained crack propagation.</p>

Meso-Simulation and Experimental Research on the Mechanical Behavior of an Energetic Explosive

This study establishes a model for polymer-bonded explosives (PBX) using Digimat-FE. The model identifies the relationship between the material’s effective elastic modulus and the explosive particle volume fraction, shape and gradation, and porosity, as well as other factors. Further, finite element analysis of the stress distribution of the PBX composite material is performed, and the mathematical models between the ultrasonic attenuation coefficient, particle volume fraction, and ultrasonic frequency are established. Finally, an efficient ultrasonic nondestructive testing system is designed to determine the stress distribution and fine crack groups in the material. Experimental results indicate that the relative error of stress detection is within 15%, which meets the requirements of engineering applications.

Experimental and Numerical Analysis of the Mechanical Properties of a Pretreated Shape Memory Alloy Wire in a Self-Centering Steel Brace

As a stimulus-sensitive material, the difference in composition, fabrication process, and influencing factors will have a great effect on the mechanical properties of a superelastic Ni-Ti shape memory alloy (SMA) wire, so the seismic performance of the self-centering steel brace with SMA wires may not be accurately obtained. In this paper, the cyclic tensile tests of a kind of SMA wire with a 1 mm diameter and special element composition were tested under multi-working conditions, which were pretreated by first tensioning to the 0.06 strain amplitude for 40 cycles, so the mechanical properties of the pretreated SMA wires can be simulated in detail. The accuracy of the numerical results with the improved model of Graesser’s theory was verified by a comparison to the experimental results. The experimental results show that the number of cycles has no significant effect on the mechanical properties of SMA wires after a certain number of cyclic tensile training. With the loading rate increasing, the pinch effect of the hysteresis curves will be enlarged, while the effective elastic modulus and slope of the transformation stresses in the process of loading and unloading are also increased, and the maximum energy dissipation capacity of the SMA wires appears at a loading rate of 0.675 mm/s. Moreover, with the initial strain increasing, the slope of the transformation stresses in the process of loading is increased, while the effective elastic modulus and slope of the transformation stresses in the process of unloading are decreased, and the maximum energy dissipation capacity appears at the initial strain of 0.0075. In addition, a good agreement between the test and numerical results is obtained by comparing with the hysteresis curves and energy dissipation values, so the numerical model is useful to predict the stress–strain relations at different stages. The test and numerical results will also provide a basis for the design of corresponding self-centering steel dampers.