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Internal strains that develop between grains during creep of an austenitic stainless steel were measured using in situ neutron diffraction. The secondary creep pre-strained test specimens were considered. Measurements were undertaken before, during and post creep deformation at 550 °C. There was no measurable change of internal strains between grains during in situ creep for 4 h at 550 °C. In addition, the effect of increasing/reducing temperatures in a range from 470 to 550 °C on the internal strains was measured and interpreted with respect to contributions from thermal expansion/contraction. No further internal misfit strains between grains were created when specimen crept during the dwell time at 530, 510, 490 and 470 °C. Results are discussed with respect to (i) the general structure of self-consistent models and (ii) the optimised use of neutron sources for creep studies. [ABSTRACT FROM AUTHOR]

Abstract: In situ neutron diffraction combined with the incremental deformation at room temperature has been used to provide a measure of the internal stress and internal resistance generated by prior inelastic deformation at high temperature in an austenitic stainless steel. Interactions between the internal stress and internal resistance are considered explicitly by using the proposed measurement technique. The magnitude of the intergranular internal stress is found to be a function of the total inelastic strain created by prior high temperature deformation. The deviation from linearity observed in the lattice strain response is used to derive the microscopic internal resistance, but a crystal plasticity model is required to infer the absolute value. The macroscopic internal resistance is shown to be consistent with Taylor hardening. A refined internal state concept is proposed based on the Kocks–Mecking model to provide a further step to predict the inelastic deformation. [Copyright &y& Elsevier]

Abstract: Although it has been widely recognized that stress transfer in the nanostructure of biological composites follows a tension–shear chain, with mineral platelets primarily under tension and protein matrix primarily under shear, some of the critical assumptions of this model has not been thoroughly investigated. For example, an implicit assumption of the tension–shear chain model is that the shear stress in the protein layer between two neighboring platelets is uniformly distributed. Here we derive a characteristic length for stress transfer in a unit cell of biocomposites and show that the assumption of uniform shear stress in protein is valid only if the length of the mineral platelets is below a characteristic value. Numerical simulations have also been carried out to confirm that stress transfer in the staggered biocomposites is most efficient when the length of the mineral platelets is equal to the characteristic length. [Copyright &y& Elsevier]

Abstract: A viscous-elastoplastic constitutive equation of a welding stainless steel material incorporating phase transformation was proposed based on the assumption that the welding material is a mixture of several phases, and each of which possesses viscous-elastoplastic property. The heat conduction equation incorporating phase transformation and the transformation kinetics equation were also presented. The numerical algorithm and the finite element procedure related to these equations were developed and applied to the investigations of the temperature change and the distribution of the residual stress in the welding slot area of a Cr21Ni6Mo9 stainless steel pipe. The investigated result of the residual stress was in good agreement with the data measured experimentally by X-ray diffraction. [Copyright &y& Elsevier]

Abstract: The ability to understand and predict fatigue crack growth is central to both the design and the continued operational safety of aircraft, rail, offshore structures, nuclear power plants, and many other engineering assets. However, most current crack growth models are based on the concept of similitude. This paper examines cracking in a range of steels and reveals that the similitude hypothesis is invalid in Region I, where the crack growth rate is low. [Copyright &y& Elsevier]

Abstract: The stress and strain distributions in a ring strain sensor, subjected a pair of pressing loadings at its upper and lower top points, are investigated with Finite Element Method (FEM). The investigated results show that there are maximum stresses/strains and distinct stress/strain concentrations at its upper and lower top points, which are not appropriate locations for sticking strain gauges. The investigated results also show that there are second-larger stresses/strains and without stress/strain concentrations at the left and right top points of the sensor, which are the most appropriate locations for sticking strain gauges. The precision of the strain sensor with the strain gauges at its left and right top points are tested and compared with that of the sensor with the strain gauges at its upper and lower top points. The compared result shows that the test precision of the former is distinctly better than that of the latter, which verifies the validity of the results obtained with FEM. [Copyright &y& Elsevier]

Abstract: The constitutive behavior of metallic materials with numerous microvoids is investigated. The representative volume element of the materials is idealized as an incompressible ellipsoidal cell with a single ellipsoidal void at its center. Through the analysis of the microscopic velocity and strain fields of the void-cell model and making use of a nonclassical elastoplastic constitutive relation, the microscopic stresses of the ellipsoidal void-cell model are obtained, and it is further transformed to macroscopic stresses with the principle of energy coincidence. Combining the evolution of void growth with that of void nucleation, the void evolution rule is obtained. The obtained expression of macroscopic stress and the void evolution rule are embedded in a nonclassical elastoplastic constitutive description obtained from a simple mechanical model incorporating damage. The corresponding numerical algorithm and the finite element approach are developed and applied to the analysis of the elastoplastic response and the porosity of PD3 steel. The computed results show satisfactory agreement with experiments. [Copyright &y& Elsevier]

Recent molecular dynamics simulations of dynamic crack propagation have shown that there is a finite crack opening for a shear crack propagating at a sub-Rayleigh speed, but the crack opening becomes significantly smaller once the crack tip velocity exceeds the shear wave speed. To understand this difference between the crack opening for sub-Rayleigh and intersonic shear cracks, we develop in this paper a finite deformation continuum theory incorporating the linear harmonic potential to describe the deformation of a crack in a solid with triangular lattice structure. Using the asymptotic method developed by Knowles [Eng. Fract. Mech. 15 (1981) 469], we show that even after the geometric nonlinearity of finite deformation is accounted for, the intersonic shear cracks have a vanishing crack opening displacement. [Copyright &y& Elsevier]

In this paper, a microscopic damage model based on an ellipsoidal void for mixed-hardening materials is investigated under a particular coordinate. The change of void shape is considered in this model. The damage evolution law obtained from the model is affected by the void volume fraction, the void shape, and the triaxiality of the stress. The result is used to analyze the closing process of voids. Then, the damage evolution law is embedded into a new incremental elastoplastic constitutive equation for large deformation, and a damage constitutive equation is obtained. A numerical algorithm and finite element program for large elastoplastic deformation of damaged materials is developed, and is then applied to the analysis of the stress and damage distribution near the tip of the notch of the notched bar under uniaxial loading. Calculated results for the distribution of stress and damage show satisfactory agreement with experiment data. [Copyright &y& Elsevier]

Abstract: Stress-induced martensitic transformation of as-sputtered and post-annealed Ti50.1Ni40.8Cu9.1 thin films was investigated using in-situ synchrotron X-ray diffraction (S-XRD) technique. For the as-deposited film, in-situ S-XRD analysis showed a martensitic transformation from parent phase to martensite during initial loading, followed by reorientation of martensite variants via detwinning. This detwinning process induced a strong 〈020〉 fiber texture along the loading direction and a strong 〈002〉 fiber texture perpendicular to the loading direction. For the 650°C annealed film, there is only elastic deformation, followed by a martensitic transformation during deformation. [Copyright &y& Elsevier]

Abstract: Assuming the voids in a pearlitic steel are cylindrical, a cylindrical void-cell model is presented. From the analysis of the void-cell model a void evolution equation is obtained. Defining a new intrinsic time and the softening function related to the void evolution, and introducing them into an endochronic constitutive equation, the constitutive equation involving void evolution is derived. The corresponding numerical algorithm and the finite element approach are offered, which are applied to analyse the stress and porosity distributions of the unnotched and notched cylindrical specimens of a pearlitic steel. The analytical result of the unnotched specimen shows that the porosity of the specimen increases with the increase of its plastic deformation, which is consistent with the experimental data. The analytical result of the notched specimen reveals that the stress of considering void evolution is larger than that of no considering void evolution. Both the stress and porosity reach their maximum at the notch root. The latter agrees with the experimental result. [ABSTRACT FROM AUTHOR]

Aluminum composites reinforced with carbon-based nano-particles or fibers have been widely studied. Yet, the rate dependence of their properties has been barely reported. In the present study, CNTs-reinforced Al composites with CNTs of two aspect ratios were produced by different powder metallurgy methods, followed by spark plasma sintering and hot extrusion. The mechanical properties and the underlying mechanisms of CNTs-reinforced Al composites at various loading rates were studied, with the aim of exploring the role of CNTs on strengthening, esp. on rate-dependent properties. The mechanical experiments revealed that the addition of CNTs not only increased the strength but also the strain rate sensitivity in comparison with pure Al. It was found that, under dynamic loading, the materials showed an increased strength and elongation-to-failure simultaneously, due to improved strain hardening rates. A careful analysis suggested that the long-range back stress produced at the CNTs-Al interfaces and the geometrically necessary dislocations accumulated due to strain gradient along the interface, mainly contributed to the strain hardening of CNTs/Al composites. A novel microstructure-based model using microscopically non-uniform dispersion of CNTs was proposed to account for the mechanical properties with better prediction than the traditional models. The results might shed some light on understanding metal matrix composites reinforced with nano-particles or fibers. [ABSTRACT FROM AUTHOR]

Twinning mechanism of powder metallurgy Ti-Si alloy was investigated by interrupted in-situ tensile tests. Extension twins {10−12}<10-1-1> in the fine-grained Ti-Si alloy were found in the uniform deformation period, but no twinning in the coarse pure Ti. Three deformation twinning nucleation mechanisms were proposed: i) local stress concentration by neighboured slip incompatibility, ii) slip/twin oriented relationship in the parent grain and iii) slip/twin transfer by high Luster-Morris oriented relationship. The interior back-stress state, grains rotation and dislocations pile-up drove the occurrence of detwinning phenomenon. Silicon-facilitation twinning verified the hypothesis that the substitutional Si solutes affected the core structures and thus the mobility of screw dislocations. Enhanced driving force and decreased energy barrier of nucleation in the micro/atomic scale were further proposed in the twinning activation. It was expected to deepen the understanding of twinning/detwinning behaviors and supply direct evidences building immature twinning mechanism. In-depth understanding about the relationship among the processing, mechanical properties and microstructure of Ti alloy was facilitated in the present work. [ABSTRACT FROM AUTHOR]

The key components of railway rolling stock are almost constantly in the multiaxial stress state in service. To evaluate the fatigue strength of the structure, stress parameters should be obtained beforehand. The traditional projection method assumes that the maximum principal stress is the maximum tensile stress, and then the projection process is performed without taking the tension/compression states of principal stresses into account. To address these inadequacies, a method for fatigue parameter determination was proposed based on principal stress projection toward a spherical direction cosine group. This method can determine the direction of maximum tensile stress and preserve the tension/compression attribute of stress during calculating the maximum stress and the minimum stress. Therefore, the physical meaning of present improved method is more apparent than the traditional projection method, and its algorithm is simple and effective. The fatigue strength assessments of a welded bogie frame were performed utilizing the traditional projection method and present method, respectively. Results obtained show that maximum stresses calculated by the improved method can be 19.4% higher than those obtained by the traditional one at some nodes, while the minimum stresses and the stress ratios derived by these two methods can even be of opposite sign. The present method, which takes the tension/compression properties of principal stresses into consideration, is more reasonable, as compared with the traditional projection method, and thus, its fatigue strength assessment results are more credible. [ABSTRACT FROM AUTHOR]

We investigate the strain relaxation and surface morphology of epitaxial SrTiO3 (STO) films grown on (001)O and (110)O planes of orthorhombic NdGaO3 (NGO), and (001) plane of cubic (LaAlO3)0.3(Sr2AlTaO6)0.7 (LSAT) substrates. Although the average lattice mismatches are similar, strikingly regular crosshatched surface patterns can be found on STO/NGO(001)O[(110)O] films, contrary to the uniform surface of STO/LSAT(001). Based on the orientation and thickness dependent patterns and high-resolution x-ray diffractions, we ascribe the crosshatch morphology to the anisotropic strain relaxation with possibly the 60° misfit dislocation formation and lateral surface step flowin STO/NGOfilms, while an isotropic strain relaxation in STO/LSAT. Further, we show that the crosshatched STO/NGO(110)O surface could be utilized as a template to modify the magnetotransport properties of epitaxial La0.6Ca0.4MnO3 films. This study highlights the crucial role of symmetry mismatch in determining the surface morphology of the perovskite oxide films, in addition to their epitaxial strain states, and offers a different route for designing and fabricating functional perovskite-oxide devices. [ABSTRACT FROM AUTHOR]

The secondary compression behavior of compacted GMZ01 bentonite was experimentally investigated by conducting a series of suction-controlled high-pressure oedometer tests. GMZ01 bentonite powder was first exposed to suction controlled conditions at 110 MPa and then statically compacted to a dry density of 1.70 g/cm³. Then, the compacted samples were hydrated to designed suctions using the vapor equilibrium technique and finally submitted to multi-staged loading oedometer tests. Corresponding results were presented in terms of stress-strain-time curves. Based on the test results, the incremental compression index Cc* and secondary compression index Cα were defined. Relationships between Cc*, Cα, vertical stress and suction were analyzed. It is observed that both Cc* and Cα increase linearly with increasing vertical stress, which is different from the convex relationship reported by other researchers in saturated soils. Moreover, Cc* and Cα also increase with decreasing suction. The relationship between Cc* and Cα is linear in the considered stress range and the slope of this linear relationship (Cα/Cc*) increases with decreasing suction. Finally, a logarithm function is established between Cα*/Cc* and suction s, which represents the coupling of suction, creep and stress of GMZ01 bentonite. [ABSTRACT FROM AUTHOR]

Abstract: Gaomiaozi (GMZ) Bent has been recognized as the potential buffer/backfill material for construction of Chinese deep geological repository for disposal of high-level radioactive waste (HLW). For investigation of pressure and suction effects on the thermal volumetric behavior of the highly compacted GMZ01 Bent, a new suction-temperature controlled oedometer was developed. Several suction and pressure controlled cyclic heating and cooling tests have been performed on highly compacted GMZ01 Bent. Suctions ranging from 4 to 110MPa, temperatures from 25 to 80°C and vertical loads from 0.1 to 5MPa were applied. Results show that heating at constant suction and pressure induces either swelling or contraction. At high pressure and low suction, heating tends to induce contraction, while at low pressure and high suction, heating induces expansion. Thermal volumetric strain of GMZ01 Bent tested strongly depends on the overconsolidation ratio (OCR). Two opposite mechanisms are considered to explain the observed phenomena, that is, the volume increase induced by the thermal expansion of the mineralogical components and the adsorbed water and the double-layer thickness at the microstructure level, as well as the volume reduction caused by the macrostructural rearrangements. [Copyright &y& Elsevier]

In order to quantitatively evaluate the damage level in high-performance concrete (HPC) with pozzolanic minerals under constant amplitude cyclic loads, three methods for real-time damage detection are employed in the present work, i.e., dynamic modulus instrument, real-time strain collector, and digital speckle correlative method (DSCM). Six mechanical parameters at different numbers of loading cycles are real-time captured by these three methods. For a maximum applied fatigue stress equal to 70% of the static flexural strength, a cohesive crack is detected on the specimen surface by the DSCM system from 10% of concrete fatigue life. The nucleation and propagation of the cohesive crack is reflected by the change of the strain concentration zone in 2-dimensional strain fields. The experimental results show that the admixtures of Class F Fly Ash (FA) and S95 Ground Granulated Blast-furnace Slag (GGBS) in high proportions increase the strain and cohesive-crack opening displacement as well as remarkably improve the fatigue performance of HPC. [ABSTRACT FROM AUTHOR]