Showing papers on "Deformation (engineering) published in 2018"

Deformation and flow of amorphous solids: Insights from elastoplastic models

Abstract: The deformation and flow of disordered solids, such as metallic glasses and concentrated emulsions, involves swift localized rearrangements of particles that induce a long-range deformation field. To describe these heterogeneous processes, elastoplastic models handle the material as a collection of 'mesoscopic' blocks alternating between an elastic behavior and plastic relaxation, when they are too loaded. Plastic relaxation events redistribute stresses in the system in a very anisotropic way. We review not only the physical insight provided by these models into practical issues such as strain localization, creep and steady-state rheology, but also the fundamental questions that they address with respect to criticality at the yielding point and the statistics of avalanches of plastic events. Furthermore, we discuss connections with concurrent mean-field approaches and with related problems such as the plasticity of crystals and the depinning of an elastic line.

Deformation behaviors and cyclic strength assessment of AZ31B magnesium alloy based on steady ratcheting effect

Abstract: In this paper, deformation behaviors and microstructure evolution of a hot-rolled AZ31B magnesium alloy under cyclic loadings are investigated. The relationship between plastic deformation and microstructure evolution and the crack formation mechanisms are discussed. Under a high cyclic stress (90–140 MPa), steady ratcheting effect occurred in the material and the development of ratcheting strain went through three stages: 1) Stage I - initial rapid increase stage; 2) Stage II - steady stage; and 3) Stage III - final abrupt increase stage. Under a low cyclic stress (≤ 90 MPa), inconspicuous ratcheting effect was found in the material, indicating a light damage in the material. When the cyclic stress is below 30 MPa, no ratcheting effect is found and only elastic deformation occurs in the material. The formation of cracks in Stages I & II is mainly due to the activation of the basal slip system. The mean geometrically necessary dislocations (GND) are calculated to analyze the relationship between the basal slip and the ratcheting effect during the cyclic loading. Finally, a new approach is proposed to estimate the AZ31B magnesium alloy’s cyclic strength (at 107 cycles) according to the cyclic stress at which steady ratcheting effect starts to occur in the material.

Room-temperature ductile inorganic semiconductor.

Abstract: Ductility is common in metals and metal-based alloys, but is rarely observed in inorganic semiconductors and ceramic insulators. In particular, room-temperature ductile inorganic semiconductors were not known until now. Here, we report an inorganic α-Ag2S semiconductor that exhibits extraordinary metal-like ductility with high plastic deformation strains at room temperature. Analysis of the chemical bonding reveals systems of planes with relatively weak atomic interactions in the crystal structure. In combination with irregularly distributed silver–silver and sulfur–silver bonds due to the silver diffusion, they suppress the cleavage of the material, and thus result in unprecedented ductility. This work opens up the possibility of searching for ductile inorganic semiconductors/ceramics for flexible electronic devices.

A comprehensive study on microstructure and tensile behaviour of a selectively laser melted stainless steel

Abstract: 316L stainless steel samples have been prepared by selective laser melting (SLM) using a pulsed laser mode and different laser powers and scanning patterns. The as-fabricated samples were found to be dominated by clusters of nano-sized γ needles or cells. TEM imaging shows that these needles contain a high population of dislocations while TEM-EDX analysis reveals high chemical homogeneity throughout the as-fabricated samples as evidenced by the fact that there is even no micro-/nano-segregation at interfaces between neighbouring γ needles. The good chemical homogeneity is attributed to the extremely high cooling rate after SLM (>106 °C/s) and the formation of Si- and Mn-oxides that distribute randomly in the current samples. The laser-processed samples show both superior strength and ductility as compared with conventionally manufactured counterparts. TEM examination on the deformed specimens reveals a significantly high density of dislocations and a great number of twinning within nano-needles, suggesting that the plastic deformation has been governed by both gliding of dislocations and twinning deformation, which is believed to be responsible for the simultaneous acquisition of superior strength and ductility. Finally, laser power shows a much more dominant role than laser scanning pattern in porosity and grain size development for the SLM-processed 316L stainless steel samples.

A new grain orientation spread approach to analyze the dynamic recrystallization behavior of a cast-homogenized Mg-Zn-Zr alloy using electron backscattered diffraction

Abstract: A new grain orientation spread approach (GOS ≤ 5°) was proposed to study DRX of a Mg-Zn-Zr alloy during hot deformation. DRXed grains possessed random texture while the deformed grains contributed to basal texture. The alloy exhibited rapid DRX at low deformation strains followed by near-saturated behavior as strain increased.

Experimental and numerical studies on the impact response of damage-tolerant hybrid unidirectional/woven carbon-fibre reinforced composite laminates

Abstract: A woven five-harness satin (5HS) weave with AS4 carbon fibres, and unidirectional high strength IMS60 carbon fibres were used to manufacture hybrid laminates, using resin infusion, to assess their performance in low velocity impact tests. Load/energy-time curves and load-displacement curves were extracted from the experimental data, and non-destructive C-scanning was performed on all pre- and post-impacted specimens to quantify the extent of damage incurred. A finite element-based computational damage model was developed to predict the material response of these hybrid unidirectional/woven laminates. The intralaminar damage model formulation, by necessity, consists of two sub-models, a unidirectional constitutive model and a woven constitutive model. The built-in surface-based cohesive behaviour in Abaqus/Explicit was used to define the interlaminar damage model for capturing delamination. The reliability of this model was validated using in-house experimental data obtained from standard drop-weight impact tests. The simulated reaction-force and absorbed energy showed excellent agreement with experiment results. The post-impact delamination and permanent indentation deformation were also accurately captured. The accuracy of the damage model facilitated a quantitative comparison between the performance of a hybrid unidirectional/woven (U/W) laminates and a pure unidirectional (PU) carbon-fibre reinforced composite laminates of equivalent lay-up. The hybrid laminates were shown to yield better impact resistance.

Spatial heterogeneity as the structure feature for structure-property relationship of metallic glasses.

Abstract: The mechanical properties of crystalline materials can be quantitatively described by crystal defects of solute atoms, dislocations, twins, and grain boundaries with the models of solid solution strengthening, Taylor strain hardening and Hall–Petch grain boundary strengthening. However, for metallic glasses, a well-defined structure feature which dominates the mechanical properties of the disordered materials is still missing. Here, we report that nanoscale spatial heterogeneity is the inherent structural feature of metallic glasses. It has an intrinsic correlation with the strength and deformation behavior. The strength and Young’s modulus of metallic glasses can be defined by the function of the square root reciprocal of the characteristic length of the spatial heterogeneity. Moreover, the stretching exponent of time-dependent strain relaxation can be quantitatively described by the characteristic length. Our study provides compelling evidence that the spatial heterogeneity is a feasible structural indicator for portraying mechanical properties of metallic glasses. Directly relating the mechanical properties of metallic glasses to their atomic structure remains a challenge. Here, the authors use high resolution microscopy to show many mechanical properties of metallic glasses depend on a single structural parameter, the characteristic length of spatial heterogeneity.

Strong grain neighbour effects in polycrystals.

Abstract: Anisotropy in single-crystal properties of polycrystals controls both the overall response of the aggregates and patterning of local stress/strain distributions, the extremes of which govern failure processes. Improving the understanding of grain–grain interactions has important consequences for in-service performance limits. Three-dimensional synchrotron X-ray diffraction was used to study the evolution of grain-resolved stresses over many contiguous grains in Zr and Ti polycrystals deformed in situ. In a significant fraction of grains, the stress along the loading axis was found to decrease during tensile plastic flow just beyond the macroscopic yield point; this is in the absence of deformation twinning and is a surprising behaviour. It is shown that this phenomenon is controlled by the crystallographic orientation of the grain and its immediate neighbours, particularly those adjacent along the loading axis. Understanding how individual crystals share load inside a polycrystal is crucial to improve component lifetime, but remains difficult to measure. Here, the authors show that the crystal orientation of a grain and that of its neighbours can surprisingly cause stress relaxation in zirconium and titanium under load.

Influence of scan strategy and molten pool configuration on microstructures and tensile properties of selective laser melting additive manufactured aluminum based parts

Abstract: Selective laser melting additive manufacturing of the AlSi12 material parts through the re-melting of the previously solidified layer using the continuous two layers 90° rotate scan strategy was conducted. The influence of the re-melting behavior and scan strategy on the formation of the “track-track” and “layer-layer” molten pool boundaries (MPBs), dimensional accuracy, microstructure feature, tensile properties, microscopic sliding behavior and the fracture mechanism as loaded a tensile force has been studied. It showed that the defects, such as the part distortion, delamination and cracks, were significantly eliminated with the deformation rate less than 1%. The microstructure of a homogeneous distribution of the Si phase, no apparent grain orientation on both sides of the MPBs, was produced in the as-fabricated part, promoting the efficient transition of the load stress. Cracks preferentially initiate at the “track-track” MPBs when the tensile stress increases to a certain value, resulting in the formation of the cleavage steps along the tensile loading direction. The cracks propagate along the “layer-layer” MPBs, generating the fine dimples. The mechanical behavior of the SLM-processed AlSi12 parts can be significantly enhanced with the ultimate tensile strength, yield strength and elongation of 476.3 MPa, 315.5 MPa and 6.7%, respectively.

Mechanical Properties of Optimized Diamond Lattice Structure for Bone Scaffolds Fabricated via Selective Laser Melting

Abstract: Developments in selective laser melting (SLM) have enabled the fabrication of periodic cellular lattice structures characterized by suitable properties matching the bone tissue well and by fluid permeability from interconnected structures. These multifunctional performances are significantly affected by cell topology and constitutive properties of applied materials. In this respect, a diamond unit cell was designed in particular volume fractions corresponding to the host bone tissue and optimized with a smooth surface at nodes leading to fewer stress concentrations. There were 33 porous titanium samples with different volume fractions, from 1.28 to 18.6%, manufactured using SLM. All of them were performed under compressive load to determine the deformation and failure mechanisms, accompanied by an in-situ approach using digital image correlation (DIC) to reveal stress–strain evolution. The results showed that lattice structures manufactured by SLM exhibited comparable properties to those of trabecular bone, avoiding the effects of stress-shielding and increasing longevity of implants. The curvature of optimized surface can play a role in regulating the relationship between density and mechanical properties. Owing to the release of stress concentration from optimized surface, the failure mechanism of porous titanium has been changed from the pattern of bottom-up collapse by layer (or cell row) to that of the diagonal (45°) shear band, resulting in the significant enhancement of the structural strength.