Influence of Defect Number, Distribution Continuity and Orientation on Tensile Strengths of the CNT-Based Networks: A Molecular Dynamics Study

Networks based on carbon nanotube (CNT) have been widely utilized to fabricate flexible electronic devices, but defects inevitably exist in these structures. In this study, we investigate the influence of the CNT-unit defects on the mechanical properties of a honeycomb CNT-based network, super carbon nanotube (SCNT), through molecular dynamics simulations. Results show that tensile strengths of the defective SCNTs are affected by the defect number, distribution continuity and orientation. Single-defect brings 0 ~ 25% reduction of the tensile strength with the dependency on defect position and the reduction is over 50% when the defect number increases to three. The distribution continuity induces up to 20% differences of tensile strengths for SCNTs with the same defect number. A smaller arranging angle of defects to the tensile direction leads to a higher tensile strength. Defective SCNTs possess various modes of stress concentration with different concentration degrees under the combined effect of defect number, arranging direction and continuity, for which the underlying mechanism can be explained by the effective crack length of the fracture mechanics. Fundamentally, the force transmission mode of the SCNT controls the influence of defects and the cases that breaking more force transmission paths cause larger decreases of tensile strengths. Defects are non-negligible factors of the mechanical properties of CNT-based networks and understanding the influence of defects on CNT-based networks is valuable to achieve the proper design of CNT-based electronic devices with better performances. Graphical Abstract

Modeling the Hydrogen Redistribution at the Grain Boundary of Misoriented Bicrystals in Austenite Stainless Steel

Hydrogen embrittlement, as one of the major concerns for austenitic stainless steel, is closely linked to the diffusion of hydrogen through the grain boundary of materials. The phenomenon is still not well understood yet, especially the full interaction between hydrogen diffusion and the misorientation of the grains. This work aimed at the development of a robust numerical strategy to model the full coupling of the hydrogen diffusion and the anisotropic behavior of crystals in 316 stainless steel. A constitutive model, which allows easy incorporation of crystal orientation, various loading conditions, and arbitrary model geometries, was established by using the finite element package ABAQUS. The study focuses on three different bicrystal models composed of misoriented crystals, and the results indicate that the redistribution of hydrogen is significant closely to the grain boundary, and the redistribution is driven by the hydrostatic pressure caused by the misorientation of two neighboring grains. A higher elastic modulus ratio along the tensile direction will lead to a higher hydrogen concentration difference in the two grains equidistant from the grain boundary. The hydrogen concentration shows a high value in the crystal along the direction with stiff elastic modulus. Moreover, there exists a large hydrogen concentration gradient in a narrow region very close to the grain boundary to balance the concentration difference of the neighboring grains.

Microstructure, Mechanical Properties, and Martensitic Transformation in NiTi Shape Memory Alloy Fabricated Using Electron Beam Additive Manufacturing Technique

The electron beam additive manufacturing (EBAM) method was applied in order to fabricate rectangular-shaped NiTi component. The process was performed using an electron beam welding system using wire feeder inside the vacuum chamber. NiTi wire containing 50.97 at.% Ni and showing martensitic transformation near room temperature was used. It allowed to obtain a good quality material consisting of columnar grains elongated into the built direction growing directly from the NiTi substrate, which is related to the epitaxial grain growth mechanism. As manufactured material showed martensitic and reverse transformations diffused over the temperature range from −10 to 44 °C, the applied aging at 500° C moved the transformation to higher temperatures and transformation peaks became sharper. The highest recoverable strain of about 3.5% was obtained in the as-deposited sample deformed along the deposition direction. In the case of deformation of the alloy aged at 500 °C for 2h, the formation of martensite occurs at significantly lower stress; however, at about 2.5% the stress begins to increase gradually and only a small shape recovery was observed due to a higher martensitic transformation temperature. In situ SEM tensile deformation in the direction perpendicular to deposition direction showed that the martensite began to appear at the surface of the sample and at the grain boundaries due to heterogeneous nucleation. In situ studies allowed to determine the following crystallographic relationships between B2 and B19’ martensite: (100)B2||(100)B19’ and (100) B2 || (011)B19’; (011)B2|| (001)B19’ and $${(011)}_{\mathrm{B}2}||{\left(11\bar{1 }\right)}_{\mathrm{B}1{9}^{\mathrm{^{\prime}}}}$$ ( 011 ) B 2 | | 11 1 ¯ B 1 9 ′ . Samples aged at 500 °C exhibited fully austenitic microstructure; however, with increasing degree of deformation, the formation of martensite was observed. The majority of needles were tilted about 45° with respect to the tensile direction, and the presence of type I (11 $$\bar{1 }$$ 1 ¯ ) invariant twin boundaries was observed at higher degrees of deformation.

Anisotropy in tensile behavior of an extruded Mg-4.50Zn-1.13Ca (wt.%) alloy

The present study investigated anisotropy in tensile behavior of an extruded Mg-4.50Zn-1.13Ca (wt.%) alloy through tensile testing along different tilt angles relative to the extrusion direction. Results showed that the as-extruded Mg-4.50Zn-1.13Ca (wt.%) alloy exhibited anisotropy in tensile behavior due to the formation of basal texture. Basal slip, prismatic slip and tensile twinning were the dominant deformation modes depending on the tensile direction. Prismatic slip was the dominant deformation mode for samples with small tilt angle (θ = 0° and 22.5°). Basal slip was activated when the tilt angle was increased, which also resulted in the decrease of yield strength. Tensile twinning was responsible for the yielding of the samples with high tilt angles (θ = 67.5° and 90°). The ductility was significantly reduced at high tilt angle, which was mainly attributed to the appearance of tensile twinning during tensile deformation.

Cumulative Shear Damage Mechanism to Short Fiber Type C/SiC

A cumulative damage mechanism for short fiber type C/SiC during shear loading–unloading testing was examined and quantified using Iosipescu specimens parallel in the in-plane and through-thickness plane, and by using modified fracture and damage mechanics theory referring to measured damage characteristics (crack length, number and angle). A nonlinear stress–strain relation was found for both specimens. Decrease in the apparent modulus was confirmed with increased peak stress, although permanent strain increased. Inelastic strain of the decomposed tensile direction derived from shear stress was greater than that of the compressive one. Cracks propagated perpendicularly to the tensile direction, intruding on the boundary of the transverse fibers and connecting to other cracks. The theoretical damage mechanics model succeeded to predict the stress–strain relation, proposing that the shear mechanical properties are predictable by measuring the damage characteristics.

Effects of Aging and Infill Pattern on Mechanical Properties of Hemp Reinforced PLA Composite Produced by Fused Filament Fabrication (FFF)

Reinforcing natural fibers to polymer matrices provides an increase in mechanical properties. In addition, bio composite materials contribute to the sustainable ecosystem with its ease of recyclability. The effect of accelerated aging on the mechanical properties of PLA matrix bio composite specimens has been observed in previous research, but the effect of accelerated aging on the mechanical properties and the resulting mass loss of the material produced with fused filament fabrication (FFF) has been discussed for the first time in this study. Aging was applied to the bio composite consisted of 10% hemp and PLA matrix produced at a constant rate, parallel to the tensile direction and cross (+/– 45°) angle, and the results were examined as tensile stress and mass loss. The aging effect has been observed even from the first week. Specimens with parallel printing to tensile direction showed a lower tensile performance than cross printing one, since the structure in the laminates is quite durable, the adhesion performance in the laminate or through thickness direction has been low. Natural fibers are found so highly hygroscopic that chemical treatments will improve the interface and increase the mechanical properties.

An Experimental Study of the Tension-Compression Asymmetry of Extruded Ti-6.5Al-2Zr-1Mo-1V under Quasi-Static Conditions at High Temperature

The tension-compression asymmetry (TCA) behavior of an extruded titanium alloy at high temperatures has been investigated experimentally in this study. Uniaxial tensile and compressive tests were conducted from 923 to 1023 K with various strain rates under quasi-static conditions. The corresponding yield stress and asymmetric strain hardening behavior were obtained and analyzed. In addition, the microstructure at different temperatures and stress states indicates that the extruded TA15 profile exhibits a significant yield stress asymmetry at different testing temperatures. The flow stress and yield stress during tension are greater than compression. The yield stress asymmetry decreases with the increase in temperature. The alloy also exhibits TCA behavior on the strain hardening rate. Its mechanical response during compression is more sensitive than tension. A dynamic recrystallization phenomenon is observed instead of twin generated in tension and compression under high-temperature quasi-static conditions. The grains are elongated along the tensile direction and deformed by about 45° along the compressive load axis. Finally, the TCA of Ti-6.5Al-2Zr-1Mo-1V (TA15) alloy is due to slip displacement. The tensile deformation activates basal <a>, prismatic <a> and pyramidal <c + a> slip modes, while the compressive deformation activates only prismatic <a> and pyramidal <c + a> slip modes.

Lattice modeling for the influence of geometrical patterns of 3D spacer fabric on tensile behavior of concrete canvas

Tensile behavior of concrete canvas (CC) mainly depends on the geometric patterns of 3D spacer fabric. A lattice model is proposed to model the three-dimensional structure of CC to investigate the influence of geometric patterns of 3D spacer fabric on the tensile behavior of CC. The stress intensity factor is also applied into the lattice model to study the crack development of CC subjected to tensile load. The simulation results are compared to the experiments to verify the model. Finally, the influence of geometric pattern of outer layer and spacer yarns on tensile behavior of CC are simulated based on our proposed lattice model. The results indicate that the tensile strength of CC increases as the loop unit size of outer surface decreases or the amount of spacer yarns increases; the tensile strength of CC with rhombus loop unit of outer surface layer is higher than that of CC with rectangle loop unit. The tensile strength of CC significantly increases with the increasing inclination angle of spacer yarns in 3D spacer fabric. Furthermore, CC specimens subjected to uni-axial tensile exhibit a multi-cracking behavior, the average crack spacing of specimen decreases with the decreasing inclination angle of spacer yarns in tensile direction.

Optimal Muscular Arrangement Using Genetic Algorithm for Musculoskeletal Potential Method with Muscle Viscosity

Muscle contractions (or equivalent mechanical elements) are responsible for joint movement in systems with musculoskeletal structure. Because muscles can only transmit force in the tensile direction in such systems, the internal force exists between the muscles. By utilizing the potential field generated by the internal force, the musculoskeletal potential method makes it possible to control the position without complex real-time calculations or sensory feedback by entering step-inputs of the balanced internal force at the target posture. However, the conditions of convergence to the target posture strongly depend on muscular arrangement. Previous studies have elucidated the mathematical conditions of the muscular arrangement; however, they provide sufficient conditions that must be satisfied by the muscular arrangement to converge to the target posture, which do not necessarily lead to optimal muscular arrangement conditions. This study proposes a method to determine the optimal muscular arrangement of a two-joint six-muscle system, wherein muscle viscosity is considered, that uses a genetic algorithm and an evaluation function considering the motion response time. The effect of the obtained muscular arrangement is verified in a simulation.