Accepted Manuscripts

Peng Wang and Hongtao Wang
J. Appl. Mech   doi: 10.1115/1.4037683
Massively parallel molecular dynamics (MD) simulations have been performed to understand the plastic deformation of metals. However, the intricate interplay between the deformation mechanisms and the various material properties is largely unknown in alloy systems for the limited available interatomic potentials. We adopt the meta atom method proposed by Wang et al., which unifies MD simulations of both pure metals and alloys in the framework of the embedded atom method (EAM). Owing to the universality of EAM for metallic systems, meta atom potentials can fit properties of different classes of alloys. Meta-atom potentials for both aluminum bronzes and hypothetic face-centered-cubic metals have been formulated to study the parametric dependence of deformation mechanisms, which captures the essence of competitions between dislocation motion and twinning or cleavage. Moreover, the solid-solution strengthening effect can be simply accounted by introducing a scaling factor in meta atom method. As the computational power enlarges, this method can extend the capability of massively parallel MD simulations in understanding the mechanical behaviors of alloys. The calculation of macroscopic measurable quantities for engineering oriented alloys is expected to be possible in this way, shedding light on constructing materials with specific mechanical properties.
Kai Wei, Yong Peng, Weibin Wen, Yongmao Pei and Daining Fang
J. Appl. Mech   doi: 10.1115/1.4037589
Current studies on tailoring the coefficient of thermal expansion (CTE) of materials focused on either exploring the composition of the bulk material or the design of composites which strongly depend on a few negative CTE materials or fibers. In this work, an approach to achieve a wide range of tailorable CTEs through a dual-constituent triangular lattice material is studied. Theoretical analyses explicitly reveal that through rational arrangement of commonly available positive CTE constituents, tailorable CTEs including negative, zero and large positive CTEs can be easily achieved. We experimentally demonstrate this approach through CTE measurements of the specimens, which were exclusively fabricated from commonly alloys. The triangular lattice material fabricated from positive CTE alloys is shown to yield large positive (41.6 ppm/°C), near-zero (1.9 ppm/°C), and negative (-32.9 ppm/°C) CTEs. An analysis of the collapse strength and stiffness ensures the robust mechanical properties. Moreover, hierarchal triangular lattice material is proposed, and with certain constituents, wide range of tailorable CTEs can be easily obtained through the rationally hierarchal structure design. The triangular lattice material presented here integrates tailorable CTEs, lightweight characteristic, and robust mechanical properties, and is very promising for engineering applications where precise control of thermally induced expansion is in urgently needed.
Sami F. Masri, John P. Caffrey and H Li
J. Appl. Mech   doi: 10.1115/1.4037551
Explicit, closed-form, exact analytical expressions are derived for the covariance kernels of a MDOF system with arbitrary amounts of viscous damping (not necessarily proportional-type), that is equipped with one or more auxiliary mass damper-inerters placed at arbitrary location(s) within the system. The ``inerter" is a device that imparts additional inertia to the vibration damper, hence magnifying its effectiveness without a significant damper mass addition. The MDOF system is subjected to nonstationary stochastic excitation consisting of modulated white noise. Results of the analysis are used to determine the dependence of the time-varying mean-square response of the primary MDOF system on the key system parameters such as: primary system damping, auxiliary damper mass ratio, location of the damper-inerter, inerter mass ratio, inerter node choices, tuning of the coupling between the damper-inerter and the primary system, and the excitation envelope function. Results of the analysis are used to determine the dependence of the peak transient mean-square response of the system on the damper/inerter tuning parameters, and the shape of the deterministic intensity function. It is shown that, under favorable dynamic environments, a properly designed auxiliary damper, encompassing an inerter with a sizable mass ratio, can significantly attenuate the response of the primary system to broad band excitations; however, the dimensionless ``rise-time" of the nonstationary excitation substantially reduces the effectiveness of such a class of devices (even when optimally tuned) in attenuating the peak dynamic response of the primary system.
TOPICS: Transients (Dynamics), Excitation, Dampers, Damping, Dynamic response, Shapes, White noise, Inertia (Mechanics), Vibration dampers
Qian Deng
J. Appl. Mech   doi: 10.1115/1.4037552
The flexoelectric effect is an electromechanical phenomenon that is universally present in all dielectrics and exhibits a strong size-dependency. Through a judicious exploitation of scale effects and symmetry, flexoelectricity has been used to design novel types of structures and materials including piezoelectric materials without using piezoelectric. Flexoelectricity links electric polarization with strain gradients and is rather difficult to estimate experimentally. One well-acknowledged approach is to fabricate truncated pyramids and/or cones and examine their electrical response. A theoretical model is then used to relate the measured experimental response to estimate the flexoelectric properties. In this work, we revisit the typical model that is used in the literature and solve the problem of a truncated cone under compression or tension. We obtained closed-form analytical solutions to this problem and examine the size and shape effects of flexoelectric response of the aforementioned structure. In particular, we emphasize the regime in which the existing models are likely to incur significant error.
TOPICS: Electromechanical effects, Shapes, Tension, Strain gradient, Errors, Polarization (Waves), Dielectric materials, Piezoelectric materials, Polarization (Electricity), Polarization (Light), Design, Compression
Gabriel Secheli, Andrew D. Viquerat and Guglielmo Aglietti
J. Appl. Mech   doi: 10.1115/1.4037503
Thin metal-polymer laminates make excellent materials for use in inflatable space structures. By inflating a stowed envelope using pressurised gas, and by increasing the internal pressure slightly beyond the yield point of the metal films, the shell rigidizes in the deployed shape. Structures constructed with such materials retain the deployed geometry once the inflation gas has either leaked away, or it has been intentionally vented. For flight, these structures must be initially folded and stowed. This paper presents a numerical method for predicting the force required to achieve a given fold radius in a three - ply metal-polymer-metal laminate and to obtain the resultant springback. A coupon of the laminate is modelled as a cantilever that under the action of an increasing tip force. Fully elastic, elastic-plastic, relaxation and springback stages are included in the model. The results show good agreement when compared with experimental data at large curvatures.
TOPICS: Polymers, Metals, Laminates, Packaging, Pressure, Relaxation (Physics), Space frame structures, Numerical analysis, Cantilevers, Geometry, Pressurized gas, Shapes, Shells, Yield point, Flight
Richard M. Christensen
J. Appl. Mech   doi: 10.1115/1.4037412
An applications methodology is synthesized from the research based development of failure theory for orthotropic fiber composite laminates. In effect this work continues and completes Ref. [1]. This failure theory applies for the condition of fiber dominated behavior, appropriate to carbon fiber - polymeric matrix composites and similar such systems. A lamination theory for stiffness and a separate lamination theory for strength are the outcomes derived here. Final forms are given for 0, 90, ±45 type orthotropic laminates, with the four lamina orientation volume fractions to be specified in any particular application of interest. Many examples are given of using the new methodology in specific design cases.
TOPICS: Composite materials, Fibers, Laminates, Failure, Lamination, Stiffness, Carbon fibers, Design, Performance

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