Research Papers

J. Appl. Mech. 2019;86(9):091001-091001-12. doi:10.1115/1.4043519.

This paper presents an analysis of void growth and coalescence in isotropic, elastoplastic materials exhibiting sigmoidal hardening using unit cell calculations and micromechanics-based damage modeling. Axisymmetric finite element unit cell calculations are carried out under tensile loading with constant nominal stress triaxiality conditions. These calculations reveal the characteristic role of material hardening in the evolution of the effective response of the porous solid. The local heterogeneous flow hardening around the void plays an important role, which manifests in the stress–strain response, porosity evolution, void aspect ratio evolution, and the coalescence characteristics that are qualitatively different from those of a conventional power-law hardening porous solid. A homogenization-based damage model based on the micromechanics of void growth and coalescence is presented with two simple, heuristic modifications that account for this effect. The model is calibrated to a small number of unit cell results with initially spherical voids, and its efficacy is demonstrated for a range of porosity fractions, hardening characteristics, and void aspect ratios.

Commentary by Dr. Valentin Fuster
J. Appl. Mech. 2019;86(9):091002-091002-10. doi:10.1115/1.4043792.

This paper presents a new energy dissipation system composed of multistable cosine-curved domes (CCD) connected in series. The system exhibits multiple consecutive snap-through and snap-back buckling behavior with a hysteretic response. The response of the CCDs is within the elastic regime and hence the system's original configuration is fully recoverable. Numerical studies and experimental tests were conducted on the geometric properties of the individual CCD units and their number in the system to examine the force–displacement and energy dissipation characteristics. Finite element analysis (FEA) was performed to simulate the response of the system to develop a multilinear analytical model for the hysteretic response that considers the nonlinear behavior of the system. The model was used to study the energy dissipation characteristics of the system. Experimental tests on 3D printed specimens were conducted to analyze the system and validate numerical results. Results show that the energy dissipation mainly depends on the number and the apex height-to-thickness ratio of the CCD units. The developed multilinear analytical model yields conservative yet accurate values for the dissipated energy of the system. The proposed system offered reliable high energy dissipation with a maximum loss factor value of 0.14 for a monostable (self-recoverable) system and higher for a bistable system.

Commentary by Dr. Valentin Fuster
J. Appl. Mech. 2019;86(9):091003-091003-6. doi:10.1115/1.4043890.

The interlayer attraction force between concentric carbon nanotubes (CNTs) plays an important role in CNT-based nanodevices. However, the precise measurement of the interlayer attraction force remains to date a challenge. Although theoretical investigations have identified the dependence of the interlayer attraction force on the tube radius, no explicit relation for such dependence has been established so far. Here, based on an analytical model, we find that the interlayer attraction force between two telescoping concentric CNTs is proportional to the mean (but not the inner nor the outer) radius of the contacting two tubes and consequently propose an explicit expression that relates the interlayer attraction force with the mean radius as well as the interlayer spacing. We also implement the effect of temperature in the present expression based on the linear dependence of the attraction force on temperature. The present expression can be compared with the existing theoretical and experimental results, offering an efficient way to evaluate the interlayer attraction force in the nanodevices composed of concentric CNTs.

Commentary by Dr. Valentin Fuster
J. Appl. Mech. 2019;86(9):091004-091004-10. doi:10.1115/1.4043830.

In the past decades, various novel functions (i.e., negative Poisson's ratio, zero thermal expansion) have been obtained by tailoring the microstructures of the cellular structures. Among all the microstructures, the horseshoe topology shows a J-shaped stress–strain curve, which is quite different from the conventional materials. It can be inferred that the 2D lattice structure with horseshoe microstructure will also exhibit excellent out-of-plane impact resistance since the spider silk also exhibits the J-shaped stress–strain curve. In this paper, the out-of-plane sphere impact of 2D truss lattice structure is conducted using finite element method (FEM) simulation. The point has been made that, by replacing the direct-line beam to horseshoe curved beam, the out-of-plane impact resistance has been greatly improved. The most curved beam structure is found to have the best out-of-plane performs with the maximum energy absorption and the minimum passing through velocity.

Commentary by Dr. Valentin Fuster
J. Appl. Mech. 2019;86(9):091005-091005-11. doi:10.1115/1.4043885.

We perform atomistic simulations of dislocation nucleation in two-dimensional (2D) and three-dimensional (3D) defect-free hexagonal crystals during nanoindentation with circular (2D) or spherical (3D) indenters. The incipient embryo structure in the critical eigenmode of the mesoregions is analyzed to study homogeneous dislocation nucleation. The critical eigenmode or dislocation embryo is found to be localized along a line (or plane in 3D) of atoms with a lateral extent, ξ, at some depth, Y, below the surface. The lowest energy eigenmode for mesoregions of varying radius, rmeso, centered on the localized region of the critical eigenmode is computed. The energy of the lowest eigenmode, λmeso, decays very rapidly with increasing rmeso and λmeso ≈ 0 for rmesoξ. The analysis of a mesoscale region in the material can reveal the presence of incipient instability even for rmesoξ but gives reasonable estimate for the energy and spatial extent of the critical mode only for rmesoξ. When the mesoregion is not centered at the localized region, we show that the mesoregion should contain a critical part of the embryo (and not only the center of embryo) to reveal instability. This scenario indicates that homogeneous dislocation nucleation is a quasilocal phenomenon. Also, the critical eigenmode for the mesoscale region reveals instability much sooner than the full system eigenmode. We use mesoscale analysis to verify the scaling laws shown previously by Garg and Maloney in 2D [2016, “Universal Scaling Laws for Homogeneous Dissociation Nucleation During Nano-Indentation,” J. Mech. Phys. Solids, 95, pp. 742–754.] for the size, ξ, and depth from the surface, Y*, of the dislocation embryo with respect to indenter radius, R, in full 3D simulations.

Commentary by Dr. Valentin Fuster
J. Appl. Mech. 2019;86(9):091006-091006-15. doi:10.1115/1.4043887.

At micro- and nanoscales, the gas pressure load is generally simulated by the thermal motion of gas molecules. However, the pressure load can hardly be produced or controlled accurately, because the effects of the wall thickness and the atomic weight of the gas molecules are not taken into account. In this paper, we propose a universal gas molecules model for simulating the pressure load accurately at micro- and nanoscales, named mock gas molecules model. Six scale-independent parameters are established in this model, thus the model is applicable at both micro- and nanoscales. To present the validity and accuracy of the model, the proposed model is applied into the coarse-grained molecular dynamics simulation of graphene blister, and the simulation results agree well with experimental observations from the graphene blister test, indicating that the model can produce and control the pressure load accurately. Furthermore, the model can be easily implemented into many simulators for problems about the solid–gas interaction, especially for membrane gas systems.

Commentary by Dr. Valentin Fuster
J. Appl. Mech. 2019;86(9):091007-091007-8. doi:10.1115/1.4043888.

Staggered architectures widely seen in load-bearing biological materials provide not only excellent supporting functions resisting static loading but also brilliant protecting functions attenuating the dynamic impact. However, there are very few efforts to unveil the relationship between staggered architectures and damping properties within load-bearing biological and bioinspired materials, while its static counterpart has been intensively studied over the past decades. Here, based on the Floquet theory, we developed a new generic method to evaluate the dynamic modulus of the composites with various staggered architectures. Comparisons with the finite element method results showed that the new method can give more accurate predictions than previous methods based on the tension-shear chain model. Moreover, the new method is more generic and applicable for two- and three-dimensional arbitrarily staggered architectures. This method provides a useful tool to understand the relationship between micro-architecture and damping property in natural load-bearing biological materials and to facilitate the architectural design of high-damping bioinspired composites.

Commentary by Dr. Valentin Fuster
J. Appl. Mech. 2019;86(9):091008-091008-8. doi:10.1115/1.4043889.

This work investigates the mode I and II interlaminar fracturing behavior of laminated composites and the related size effects. Fracture tests on geometrically scaled double cantilever beam (DCB) and end notch flexure (ENF) specimens were conducted. The results show a significant difference between the mode I and mode II fracturing behaviors. The strength of the DCB specimens scales according to the linear elastic fracture mechanics (LEFM), whereas ENF specimens show a different behavior. For ENF tests, small specimens exhibit a pronounced pseudoductility. In contrast, larger specimens behave in a more brittle way, with the size effect on nominal strength closer to that predicted by LEFM. This transition from quasi-ductile to brittle behavior is associated with the size of the fracture process zone (FPZ), which is not negligible compared with the specimen size. For the size range investigated in this study, the nonlinear effects of the FPZ can lead to an underestimation of the fracture energy by as much as 55%. Both the mode I and II test data can be captured very accurately by the Bažant’s type II size effect law (SEL).

Commentary by Dr. Valentin Fuster
J. Appl. Mech. 2019;86(9):091009-091009 -11. doi:10.1115/1.4043911.

A method to passively align bonded components without direct mechanical contact has been developed. This method uses the pressure field generated by the squeeze flow between the parts during the bonding process to increase the parallelism of planar components. A computational fluid dynamic (CFD) model has been developed to study the squeeze flow phenomenon and to determine generated efforts. Based on these calculations, an assembly stage standing on a flexure pinned linkage has been developed. This assembly stage had two purposes. The first was to show the possibility of passive mechanical alignment using a squeeze flow. The second was to measure efforts to confirm the CFD model. These measurements have led to a refined CFD model taking into account the non-Newtonian behavior of the fluid at high shear rates. This technique was initially developed for the assembly of a fiber-optic-to-silicon-chip-interface. Other potential applications could be wafer bonding, bonding of multiple wafer stacks, or 3D integrated circuits.

Commentary by Dr. Valentin Fuster
J. Appl. Mech. 2019;86(9):091010-091010-8. doi:10.1115/1.4044017.

As fiber-reinforced polymer matrix composites are often cured from stress-free high temperature, when subjected to ambient temperature, both the mismatch of the coefficient of linear thermal expansion between the fiber and the matrix and the dependence of material properties on temperature will influence the interfacial behavior. Thus, it is necessary to provide an insight into the mechanism of temperature effects on the thermomechanical properties and behaviors along the interface. In this work, we conducted microbond tests of the glass fiber–epoxy material system at controlled testing temperature (Tt). A modified interface model is formulated and implemented to study the interfacial decohesion and frictional sliding behavior of microbond tests at different Tt. With proper cohesive parameters obtained, the model can predict temperature-dependent interfacial behaviors in fiber-reinforced composites. Both the slope of the peak force as well as the measured force at the stage of frictional sliding decrease with Tt in a wide range of the length of microdroplet-embedded fiber (le). The interfacial shear strength (IFSS) keeps almost constant at Tt ≤ 40 °C and decreases with le when temperature is above 40 °C. The average frictional stress (τfAverage) along the interface increases with le when temperature is below 80 °C but is almost constant when temperature is above or equal to 80 °C. Overall, in the same range of le, τfAverage is greater when Tt is at low temperature.

Commentary by Dr. Valentin Fuster
J. Appl. Mech. 2019;86(9):091011-091011-5. doi:10.1115/1.4044016.

The problem of special interest is the nature of the mode of failure in uniaxial compression at the brittle limit. This problem is known by observation to undergo a splitting mode of failure. The present work gives a full theoretical treatment and proof for this mode of failure. The general failure theory of Christensen for isotropic materials provides the basis for the derivation. The solution demonstrates the depth of technical capability that is required from the failure theory to treat such a classically difficult problem.

Topics: Brittleness , Failure
Commentary by Dr. Valentin Fuster
J. Appl. Mech. 2019;86(9):091012-091012-6. doi:10.1115/1.4044140.

Scavenging mechanical energy from the deformation of roadways using piezoelectric energy transformers has been intensively explored and exhibits a promising potential for engineering applications. We propose here a new packaging method that exploits MC nylon and epoxy resin as the main protective materials for the piezoelectric energy harvesting (PEH) device. Wheel tracking tests are performed, and an electromechanical model is developed to double evaluate the efficiency of the PEH device. Results indicate that reducing the embedded depth of the piezoelectric chips may enhance the output power of the PEH device. A simple scaling law is established to show that the normalized output power of the energy harvesting system relies on two combined parameters, i.e., the normalized electrical resistive load and normalized embedded depth. It suggests that the output power of the system may be maximized by properly selecting the geometrical, material, and circuit parameters in a combined manner. This strategy might also provide a useful guideline for optimization of piezoelectric energy harvesting system in practical roadway applications.

Commentary by Dr. Valentin Fuster

Technical Brief

J. Appl. Mech. 2019;86(9):094501-094501-6. doi:10.1115/1.4043886.

Protecting concrete structures from high energetic dynamic events such as blasts and impact is a major concern, in both civil- and military-related applications. Most conventional techniques fail to counter the unpredictable nature of dynamic loads, as well as the complex response of structures due to stress wave propagation. Hence, this paper explores the possibility of using a functionally graded—according to impedance—metallic composite system as a protective mechanism to a concrete structure. An analytical framework was developed using matlab, based on elastic and shock wave propagation theories, especially incorporating multiple interactions within the composite system, as well as reflections of free surfaces. A numerical analysis was carried out using the advanced finite element code LS-DYNA. The main objective of this paper was to compare the performance of the composite system against the conventional monolithic system.

Commentary by Dr. Valentin Fuster
J. Appl. Mech. 2019;86(9):094502-094502-1. doi:10.1115/1.4043744.

Engineering students occasionally wonder: why not 12mc2? While most have no need for Einstein’s special theory of relativity, this theory nevertheless offers them a shining example of the power of mathematical deduction and beautiful simplicity. At its 100th anniversary in 2005, I gave my students a one-page pencil note explaining this glorious gem of a theory. The note became popular, and here, upon invitation, I give a simplified derivation, trying to waste no word and achieve utmost brevity, a boost to clarity.

Commentary by Dr. Valentin Fuster

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