Dominant Deformation Mode Research Articles

Low Velocity Impact of Flat and Doubly Curved Polycarbonate Panels

Three-dimensional finite transient deformations of polycarbonate (PC) panels impacted at low velocity by a hemispherical-nosed rigid cylinder have been studied by using the commercial finite element software ls-dyna with a thermo–elasto–viscoplastic material model for the PC incorporated in it as a user defined subroutine. The implementation of the subroutine has been verified by comparing analytical and numerical solutions of simple initial-boundary-value problems. The mathematical model of the low velocity impact problem has been validated by comparing the computed and the experimental results for the maximum deflection and time histories of the centroidal deflection. It is found that the initial slope of the reaction force between the impactor and the panel versus the indentation for a curved panel can be nearly 20 times that for the flat panel of the same thickness as the curved panel. For the impact velocities considered, it is found that the maximum effective plastic strain in the PC shell near the center of impact and the dominant deformation mode there strongly depend on the panel curvature, the panel thickness, and the impact speed. Effects of the panel curvature, the panel thickness, and the impact speed on stresses and strains developed in a panel are delineated. This information should help designers of impact resistant transparent panels such as an airplane canopy, automobile windshield, and goggles. However, damage initiation and propagation, and the final indentation induced in the clamped panels have not been computed.

In-situ neutron diffraction studies on high-temperature deformation behavior in a CoCrFeMnNi high entropy alloy

An equiatomic, face-centered-cubic, CoCrFeMnNi high-entropy alloy was tensile tested at 800 K and 1,000 K with a nominal strain rate of 6.7 × 10−6 s−1. In addition to the macroscopic (bulk) behavior, mesoscopic (lattice) phenomena during the pseudo-creep deformation were investigated simultaneously using in-situ neutron diffraction. The evolutions of lattice strains, peak widths, and intensities of several hkl reflections suggest that the dominant deformation mode is the dislocation glide at 800 K and diffusion-controlled dislocation creep at 1,000 K.

Effects of frictional material properties on thermoelastic instability deformation modes

This paper deals with the effects of friction material properties on thermoelastic instability (TEI) and the associated dominant deformation mode. Both analytical and finite element models are constructed for determining the variation of the critical speed and the dominant TEI mode. Some important factors, e.g. the nonconductive friction material as well as the symmetric and anti-symmetric TEI deformation modes, are investigated. It has been found that the dominant TEI deformation mode is strongly dependent on the properties of frictional material. The results show that the symmetric mode can be dominant in the condition when the friction material has either a sufficiently large elastic modulus or a sufficiently small thermal conductivity. These research findings are useful for vehicle clutch and brake designs.

Fabrication, pore structure and compressive behavior of anisotropic porous titanium for human trabecular bone implant applications

Porous titanium with average pore size of 100–650μm and porosity of 30–70% was fabricated by diffusion bonding of titanium meshes. Pore structure was characterized by Micro-CT scan and SEM. Compressive behavior of porous titanium in the out-of-plane direction was studied. The effect of porosity and pore size on the compressive properties was also discussed based on the deformation mode. The results reveal that the fabrication process can control the porosity precisely. The average pore size of porous titanium can be tailored by adjusting the pore size of titanium meshes. The fabricated porous titanium possesses an anisotropic structure with square pores in the in-plane direction and elongated pores in the out-of-plane direction. The compressive Young׳s modulus and yield stress are in the range of 1–7.5GPa and 10–110MPa, respectively. The dominant compressive deformation mode is buckling of mesh wires, but some uncoordinated buckling is present in porous titanium with lower porosity. Relationship between compressive properties and porosity conforms well to the Gibson–Ashby model. The effect of pore size on compressive properties is fundamentally ascribed to the aspect ratio of titanium meshes. Porous titanium with 60–70% porosity has potential for trabecular bone implant applications.

Shape memory polymer filled honeycomb model and experimental validation

An analytical model predicting the in-plane Young’s and shear moduli of a shape memory polymer filled honeycomb composite is presented. By modeling the composite as a series of rigidly attached beams, the mechanical advantage of the load distributed on each beam by the infill is accounted for. The model is compared to currently available analytical models as well as experimental data. The model correlates extremely well with experimental data for empty honeycomb and when the polymer is above its glass transition temperature. Below the glass transition temperature, rule of mixtures is shown to be more accurate as bending is no longer the dominant mode of deformation. The model is also derived for directions other than the typical x and y allowing interpolation of the stiffness of the composite in any direction.

Temperature Dependence of Low Cycle Fatigue Behavior in AZ31 Magnesium Alloy

In situ neutron diffraction has been used to investigate the deformation behaviour of extruded magnesium at room temperature, -100°C and 150°C. Significant differences in the active twinning/slip systems, attributed to the strong crystallographic texture and temperature dependence, were observed. Twining is the dominant deformation mode during the cyclic loading at room temperature and at cryogenic temperature. In contrast, at high temperature, additional slip planes were activated and reduction of twinning activities was observed and the deformation mode is slip-dominated.

Enhancing yield strength by suppressing detwinning in a rolled Mg–3Al–1Zn alloy with {101¯2} twins

Abstract A remarkable improvement in the tensile yield strength of rolled Mg–3Al–1Zn alloy with pre-existing { 10 1 ¯ 2 } twins was achieved by inducing double twinning through compression deformation. The resulting formation of { 10 1 ¯ 2 }–{ 10 1 ¯ 2 } double twins inside pre-existing { 10 1 ¯ 2 } twin bands effectively suppressed the detwinning of pre-existing twins during subsequent deformation. This in turn led to a change in the dominant deformation mode from detwinning to slip, significantly increasing the yield strength. The mechanisms behind this behavior are considered to be a combination of the redistribution of residual tensile stress within pre-existing twins, refinement of pre-existing twins by double twins, a change in texture due to double twinning and a loss of coherency at twin boundaries.

Stochastic size-dependent slip-twinning competition in hexagonal close packed single crystals

In this paper, we present an analytical model to investigate stochastic size effects on the critical stress for the activation of non-basal slip and contraction twinning (CT) in miniaturized hexagonal-close packed (HCP) single crystals. The analysis is based on the basic model developed by Parthasarathy et al (Scr. Mater. 56 313–6), but extends it to account for the mechanism-based description of pyramidal slip and CT in HCP metals. The results demonstrate that for a given initial dislocation density, a probabilistic competition exists among CT nucleation, CT growth and pyramidal slip. Based on the stochastic model, we construct a mechanism map that provides regimes of dominant deformation modes that govern the macroscopic yield stress in HCP single crystals.

Microstructure- and surface orientation-dependent mechanical behaviors of Ag nanowires under bending

The bending mechanical behaviors of Ag nanowires with different microstructures and surface orientations are investigated via molecular dynamics simulations. Our results show the fivefold twin boundary strongly hinders the dislocation motion, leading to the dislocation pile-ups at twin boundaries which is different from the results from tensile tests in which the fivefold twin boundary does not act as the substantive barriers for dislocation propagation and the dislocations can easily pass through twin boundary. The fivefold twin boundary strengthens the nanowires by strain hardening. In single crystalline nanowires, the dislocation nucleation sites and dislocation characteristics are determined by the loading directions and surface orientations. The different dislocation characteristics lead to different ductility when the same nanowire is loaded along different directions. The first order effect of surface orientation on the dominant deformation mode has not been observed in the present work. The stress gradient plays an important role on the different results observed under tension and bending.

Computer simulation of the strong surface effects on deformation behaviors of metallic glass nanowires

Size effects on deformation behaviors of metallic glass (MG) nanowires are found to be associated with their surface imperfections from phase-field modeling. Necking instead of shear banding is the dominant deformation mode in MG nanowire with diameter d

Investigating internal architecture effect in plastic deformation and failure for TPMS-based scaffolds using simulation methods and experimental procedure

Rapid prototyping (RP) has been a promising technique for producing tissue engineering scaffolds which mimic the behavior of host tissue as properly as possible. Biodegradability, agreeable feasibility of cell growth, and migration parallel to mechanical properties, such as strength and energy absorption, have to be considered in design procedure. In order to study the effect of internal architecture on the plastic deformation and failure pattern, the architecture of triply periodic minimal surfaces which have been observed in nature were used. P and D surfaces at 30% and 60% of volume fractions were modeled with 3∗3∗ 3 unit cells and imported to Objet EDEN 260 3-D printer. Models were printed by VeroBlue FullCure 840 photopolymer resin. Mechanical compression test was performed to investigate the compressive behavior of scaffolds. Deformation procedure and stress–strain curves were simulated by FEA and exhibited good agreement with the experimental observation. Current approaches for predicting dominant deformation mode under compression containing Maxwell's criteria and scaling laws were also investigated to achieve an understanding of the relationships between deformation pattern and mechanical properties of porous structures. It was observed that effect of stress concentration in TPMS-based scaffolds resultant by heterogeneous mass distribution, particularly at lower volume fractions, led to a different behavior from that of typical cellular materials. As a result, although more parameters are considered for determining dominant deformation in scaling laws, two mentioned approaches could not exclusively be used to compare the mechanical response of cellular materials at the same volume fraction.

Density dependent mechanical properties and structures of a freeze dried biopharmaceutical excipient – Sucrose

Knowledge of the mechanical behaviour of freeze dried biopharmaceutical products is essential for designing of products with physical robustness that will not to crack, crumble or collapse during processing or transportation. The compressive mechanical deformation behaviour for freeze-dried sucrose cakes has been experimentally studied from a relative density (ρf/ρs) of 0.01–0.30 using a novel in-vial indentation test. Cakes exhibited more open like structures at lower densities and more closed structures at higher densities with some faces being present at all densities, as confirmed by SEM. The reduced elastic modulus Ef/Es=0.0044(ρf/ρs)1 for all cake densities, indicating that face stretching was the dominant deformation mode assuming Gibson and Ashby’s closed cell model. This linear scaling for the reduced elastic modulus is in line with various theoretical treatments based on tetrakaidecahedral cells and other experimental studies. Consistently, the wall thickness to cell diameter ratio scaled ρf/ρs with a power constant of 1.05. The maximum crushing stress was given by σmax=3800(ρf/ρs)1.48 which agrees with a strut bending failure stress, assuming Gibson and Ashby’s open cell model. Overall, the freeze dried cakes behaved as neither classic closed cell nor open cell materials, with their compressive elastic moduli reflecting a closed cell elastic response whilst their failure stresses reflecting an open cell failure mode. It was concluded that the mechanical response of freeze dried cellular materials depends upon their complex cellular structures and morphologies, and they cannot be rationalised using simple limiting case models of open or closed cell solids.

Nanocrystalline NiTi shape memory alloys with huge superelasticity and high mechanical damping

Superelasticity and mechanical damping are important functional properties of nickel–titanium (NiTi) shape memory alloys (SMAs). Owing to the generation and accumulation of dislocation during loading, the recovery strain of commercial NiTi SMAs is usually smaller than 10%, which limits their ability to dissipate energy. In this paper, the superelasticity and mechanical damping of a nanocrystalline NiTi SMA was studied. The results show that the nanocrystalline NiTi SMA alloy possessed a large recovery strain of about 14%, greater than that of the commercial NiTi SMAs, and a high level of absorbed energy, or toughness, of 111 MJ m−3, which is higher than the highest value (about 81 MJ m−3) of all SMAs reported so far. The transmission electron microscopy (TEM) studies suggest that few full dislocations were generated in the nanocrystalline NiTi alloy during loading. Instead, the dominant deformation modes after stress induced martensitic transformation were elastic deformation and detwinning. The detwinning process decreased the twin boundary energy, which stabilised the martensitic phase.

Plastic deformation mechanism of polycrystalline copper foil shocked with femtosecond laser

Plastic deformation mechanism of polycrystalline copper foil shocked with femtosecond (fs) laser has been characterized through optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Experiments of ns laser shocking copper (Cu) and fs laser shocking aluminum (Al) were also conducted for comparison. Dislocations arranged in multiple forms, profuse twins and stacking faults (SFs) coexist in the fs laser shocked copper. At small strain condition, dislocation slip is the dominant deformation mode and small amount of SFs act as complementary mechanism. With strain increasing, profuse twins and SFs form to accommodate the plastic deformation. Furthermore, new formed SFs incline to locate around the old ones because the dislocation densities there are more higher. So there is a high probability for new SFs overlapping on old ones to form twins, or connecting old ones to lengthen them, which eventually produce the phenomena that twins connect with each other or twins connect with SFs. Strain greatly influences the dislocation density. Twins and SFs are more dependent on strain rate and shock pressure. Medium stacking fault energy (SFE) of copper helps to extend partial dislocations and provides sources for forming SFs and twins.

Temperature dependent deformation mechanisms in pure amorphous silicon

High temperature nanoindentation has been performed on pure ion-implanted amorphous silicon (unrelaxed a-Si) and structurally relaxed a-Si to investigate the temperature dependence of mechanical deformation, including pressure-induced phase transformations. Along with the indentation load-depth curves, ex situ measurements such as Raman micro-spectroscopy and cross-sectional transmission electron microscopy analysis on the residual indents reveal the mode of deformation under the indenter. While unrelaxed a-Si deforms entirely via plastic flow up to 200 °C, a clear transition in the mode of deformation is observed in relaxed a-Si with increasing temperature. Up to 100 °C, pressure-induced phase transformation and the observation of either crystalline (r8/bc8) end phases or pressure-induced a-Si occurs in relaxed a-Si. However, with further increase of temperature, plastic flow rather than phase transformation is the dominant mode of deformation. It is believed that the elevated temperature and pressure together induce bond softening and “defect” formation in structurally relaxed a-Si, leading to the inhibition of phase transformation due to pressure-releasing plastic flow under the indenter.

Microscopic mechanism of plastic deformation in a polycrystalline Co–Cr–Mo alloy with a single hcp phase

A Co–Cr–Mo alloy with a single ε (hexagonal close-packed, hcp) phase exhibits excellent tensile properties with a 0.2% proof stress of 630MPa, an ultimate tensile stress of 1072MPa and an elongation to fracture of 38.3%. The dominant deformation modes are basal 〈a〉 slip and prismatic 〈a〉 slip, and the apparent respective critical resolved shear stresses at room temperature are calculated to be 184 and 211MPa. This simultaneous activation of both 〈a〉 slips can be explained in terms of the lattice constant ratio c/a of 1.610. There is a tendency for the geometrically necessary dislocations (GNDs) to accumulate at grain boundaries, and the magnitude of this GND accumulation at a particular boundary is dependent on its character. Numerical analysis using a dislocation-model-based strain gradient crystal plasticity calculation makes it possible to characterize the distributions of dislocation density, local stress and local strain in the polycrystalline ε Co–Cr–Mo alloy, and the calculation is largely consistent with the experimental results. This simulation reveals that the activity of the prismatic 〈a〉 slip in addition to the basal 〈a〉 slip contributes to the stress relaxation at the boundary. For this reason, excellent tensile ductility is obtained in the polycrystalline ε Co–Cr–Mo alloy.

Ductility improvement by twinning and twin–slip interaction in a Mg-Y alloy

An extruded Mg-3.0Y alloy with non-basal texture of 〈42¯2¯3〉 component was fabricated by common extrusion and exhibited a high elongation of ∼33%. The deformation modes and microstructure evolution of the extruded Mg-3.0Y alloy during the tensile test at room temperature were investigated to explore the reasons for the high ductility by transmission electron microscopy (TEM) and electron backscattered diffraction (EBSD). The results suggested that texture changed from 〈42¯2¯3〉 to 〈101¯0〉 component during the tensile deformation, which is attributed the slip and twinning activity. Basal slip and extension twinning are the dominant deformation modes for the high ductility. Meanwhile, the activation of non-basal slip, contraction twinning and twin–slip interaction also contributes to the good ductility of Mg-3.0Y alloy.

Deformation behavior and texture evolution in an extruded Mg–1Gd alloy during uniaxial compression

This paper reports the deformation behavior and texture evolution during uniaxial compression in an extruded Mg–1Gd alloy with a typical 〈2−1−11〉 fiber texture using the electron backscattering diffraction (EBSD) technique. Extension twinning and basal slip are the dominant deformation modes in the early stage of compressive deformation, which are responsible for the remarkable change of texture components. With increasing strain, the texture is changed from the 〈2−1−11〉 into the 〈0001〉 fiber texture component mainly due to the gradual rotation of c-axes towards the compressive direction caused by basal slip.

Compliant cellular structures: Application to a passive morphing airfoil

Cellular materials are known to have two properties; structures and mechanisms. Therefore, one may design structures with cellular materials while controlling stiffness and flexibility depending on the struts’ connectivity. The objectives of this study are to investigate in-plane flexible properties of bending dominated cellular materials under macroscopic deformation and to secure a method to design a passive morphing airfoil with flexible cellular cores. The airfoil with three cellular cores (chiral, regular and re-entrant hexagonal honeycombs) is investigated under a static load through the deformation gradient of the cellular cores under an aerostatic load. The structural performance of the airfoil with the designed compliant cellular cores is validated through the fluid–structure interaction through which a structural finite element analysis is combined with fluid statics. Considering the deformation of the airfoil with flexible cellular cores under an aerostatic load, shear is the dominant deformation mode of the cores of the airfoil. The re-entrant hexagonal honeycomb core shows the highest flexibility in shear and causes a lower stress in local cell walls in shear than the other cellular cores when the cellular mesostructures are designed to have the same shear modulus. This implies that the re-entrant hexagonal honeycomb core has the potential to be used as a structure with a passive morphing airfoil.

Mechanisms of anisotropic friction in nanotwinned Cu revealed by atomistic simulations

The nature of nanocrystalline materials determines that their deformation at the grain level relies on the orientation of individual grains. In this work, we investigate the anisotropic response of nanotwinned Cu to frictional contacts during nanoscratching by means of molecular dynamics simulations. Nanotwinned Cu samples containing embedded twin boundaries parallel, inclined and perpendicular to scratching surfaces are adopted to address the effects of crystallographic orientation and inclination angle of aligned twin boundaries cutting the scratching surface. The transition in deformation mechanisms, the evolution of friction coefficients and the friction-induced microstructural changes are analyzed in detail and are related to the loading conditions and the twinned microstructures of the materials. Furthermore, the effect of twin spacing on the frictional behavior of Cu samples is studied. Our simulation results show that the crystallographic orientation strongly influences the frictional response in different ways for samples with different twin spacing, because the dominant deformation mode varies upon scratching regions of different orientations. A critical inclination angle of 26.6° gives the lowest yield strength and the highest friction coefficient, at which the plasticity is dominated by twin boundary migration and detwinning. It is demonstrated that the anisotropic frictional response of nanotwinned Cu originates from the heterogeneous localized deformation, which is strongly influenced by crystallographic orientation, twin boundary orientation and loading condition.