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The Journal of Materials Science awards the annual Robert W. Cahn Prize to recognize the best paper published in the journal each year. The winner of the 2023 Cahn Prize is a research paper titled "Molecular modelling of graphene nanoribbons on the effect of porosity and oxidation on the mechanical and thermal properties." The paper presents molecular simulation models of graphene nanoribbons and examines the impact of porosity and oxidation on their mechanical and thermal conductivity properties. The paper is well-written and provides valuable insights for researchers in this field. Two other papers were selected as runners-up for the prize. All the monthly finalists for the Cahn Prize can be downloaded for free on the journal's website. [Extracted from the article]

The editorial comments on the multi-scale predictive modelling of the composite materials' behavior. Transition of composite materials from nano to micron to meter-size is examined keeping in view the accuracy of computation and fabrication methods. The article probes the physical behavior of complex material systems under stress in terms of size, time and environment.

Due to the machining-induced residual stress while processing, the machining distortion of thin-walled 7050 aluminum alloy parts occurs easily, which greatly affects the pass rate of the products. Herein, electromagnetic coupling treatment (EMCT) was proposed to relieve the residual stress of 7050 aluminum alloy. It was found that EMCT had a significant effect on residual stress relief (13.3–89.7%). The energy input by EMCT promoted the movement of dislocation, resulting in the uniform distribution of microplastic strain, the release of stored elastic strain energy, and the residual stress relief, ultimately. A numerical simulation method to visualize the evolution of the electromagnetic field, the stress, and the strain in the materials was proposed to explain the causes of microplastic strain. Specimen #5 with the optimal EMCT processing parameter (1.5 T, 1.5 V) was considered to be the most effective for both residual stress relief and mechanical property enhancement, in which the residual stress decreased by 85.1% and the tensile strength increased by 8.3%. In this paper, it is proposed that EMCT can be used as an effective method to reduce the residual stress of 7050 aluminum alloy. Moreover, it has certain technical reference value for the study of residual stress relief of other thin-walled part materials. [ABSTRACT FROM AUTHOR]

During high-temperature applications, tensile stability is significantly important and urgent for the security of components. In this paper, the high-temperature tensile tests under different creep strain were performed and analyzed through scanning electron microscope (SEM) and transmission electron microscope (TEM) of TC11 titanium alloy. It is found that the creep strain, dynamic α precipitation and deformation modes have significant influence on the high-temperature tensile behavior of TC11 titanium alloy. An abnormal characteristic of high-temperature tensile stress plateau was observed with the increasing of the creep strain. Moreover, an increment of 100 MPa for yield strength was achieved when the creep strain increased from 0.075 to 0.21%. For low creep strain, sample was controlled by the dislocation slip. While at high creep strain, it promotes the precipitation of nano-sized α particles, and dislocation slip hardly transfers across the α/β interface. Then, dislocation tangles, deformation twins and stacking faults become dominated, which results in an obvious stress plateau in the stress–strain curve. [ABSTRACT FROM AUTHOR]

Some key structural parts made of welding titanium alloys usually serve in hydrogen environment in aerospace, ship building and chemical industries. To investigate the hydrogen embrittlement (HE) behaviors of the welded titanium alloy joints and the impact factors, the TC4 titanium alloy joints welded by laser welding method were first treated by ultrasonic surface rolling process (USRP). Then, slow-rate tensile tests were conducted under electrochemical hydrogen charging condition to compare the mechanical properties of the TC4 laser welded joints before and after USRP. On this basis, the HE behaviors and mechanisms of the USRP-treated TC4 laser welded joint and the USRP-untreated one were discussed in detail. The results show that USRP could significantly improve the anti-hydrogen embrittlement behaviors of the TC4 laser welded joint, which could be attributed to the grain refinement and residual compressive stress in the deformation layer induced by USRP. The results of this paper provide a theoretical basis for the design of anti-hydrogen embrittlement metal materials. [ABSTRACT FROM AUTHOR]

Haynes 282 is a γ ′ precipitation-strengthened nickel-based superalloy known for its exceptional high-temperature creep resistance and excellent fabricability. Recent advancements in powder-bed fusion-based additive manufacturing (PBF-AM) have enabled the fabrication of Haynes 282 with accurate, site-specific control of the grain orientation and morphology at the microscale. This ability opens up new avenues for microstructure design, to improve the material's fatigue crack resistance and service life. This paper investigates the fatigue crack growth behavior of hybrid microstructure Haynes 282 fabricated via PBF-AM. Previous experiments revealed a higher crack propagation rate in the coarse columnar-grained microstructural regions when compared against fine-grained areas. Here, a strain gradient crystal plasticity model was adapted to study the fracture-related mechanical fields at the crack tip in the two microstructures. The simulation results showed a consistent influence of grain structure and texture on crack propagation, as was seen in the experiment. The model analysis revealed higher crack propagation driving force along crack direction in the coarse-grained sharply textured microstructure and higher driving force for crack kinking in fine-grained more diffusely textured microstructure, which is ascribed to the combined effect of yield stress, hardening rate, texture and grain morphology. The presented modeling approach will facilitate the development of the AM-based accurate microstructure design by deepening the fundamental study in AM-specific microstructure-properties relations. [ABSTRACT FROM AUTHOR]

In the last years, a large number of new biocompatible materials for 3D printers have emerged. Due to their recent appearance and rapid growth, there is little information about their mechanical properties. The design and manufacturing of oral appliances made with 3D printing technologies require knowledge of the mechanical properties of the biocompatible material used to achieve optimal performance for each application. This paper focuses on analysing the mechanical behaviour of a wide range of biocompatible materials using different additive manufacturing technologies. To this end, tensile and bending tests on different types of recent biocompatible materials used with 3D printers were conducted to evaluate the influence of the material, 3D printing technology, and printing orientation on the fragile/ductile behaviour of the manufactured devices. A test bench was used to perform tensile tests according to ASTM D638 and bending tests according to ISO 178. The specimens were manufactured with nine different materials and five manufacturing technologies. Furthermore, specimens were created with different printing technologies, biocompatible materials, and printing orientations. The maximum allowable stress, rupture stress, flexural modulus, and deformation in each of the tested specimens were recorded. Results suggest that specimens manufactured with Stereolithography (SLA) and milling (polymethyl methacrylate PMMA) achieved high maximum allowable and rupture stress values. It was also observed that Polyjet printing and Selective Laser Sintering technologies led to load–displacement curves with low maximum stress and high deformation values. Specimens manufactured with Digital Light Processing technology showed intermediate and homogeneous performance. Finally, it was observed that the printing direction significantly influences the mechanical properties of the manufactured specimens in some cases. [ABSTRACT FROM AUTHOR]

It this paper, the comprehensive design process was carried out to fabricate selective, molecularly imprinted polymer (MIP). The material was used as a sorbent in the optimized analytical method, aimed at verifying the hypothesis that the conditions of an analytical process could convert 4-hydroxyphenylacetic acid to 4-hydro-3-nitrophenylacetic acid, comprising a series of novel studies. The design stage consisted of the analysis of specificity of the MIP, synthesized from the cross-linker and used five various functional monomers independently, in the presence of four different templates. The MIP from 1-vinylimidazole, imprinted by 4-hydroxyphenylacetic acid, revealed the highest specificity in relation to 4-hydro-3-nitrophenylacetic acid, with an affinity factor equal to 3, and the highest selectivity from a group of structurally similar and biologically important biomolecules. The theoretical analysis revealed that electrostatic interaction between the analyte and the polymer matrix enhanced selectivity. The physicochemical characterization showed the specific surface area of the MIP as being equal to 368.6 m2 g−1, and the presence of nitrogen atoms at the level of 6.80% wt., confirming the monomer residue in the material structure. The MIP was applied in the solid phase extraction protocol, allowing for the analysis of 4-hydroxy-3-nitrophenylacetic acid in a human urine sample. Finally, the conversion of 4-hydroxyphenylacetic acid in human urine in nitrate and nitrite salts at low pH conditions revealed an almost twofold increase in 4-hydro-3-nitrophenylacetic acid to 775 ± 81 ng L−1. The results also confirmed the applicability of the new MIP sorbent for the purpose of analysis of low levels of analyte, present in the complex sample. [ABSTRACT FROM AUTHOR]

A methodology that accurately simulates the brittle behavior of epoxy polymers initiating at the molecular level due to bond elongation and subsequent bond dissociation is presented in this paper. The system investigated in this study comprises a combination of crystalline carbon nanotubes (CNTs) dispersed in epoxy polymer molecules. Molecular dynamics (MD) simulations are performed with an appropriate bond order-based force field to capture deformation-induced bond dissociation between atoms within the simulation volume. During deformation, the thermal vibration of molecules causes the elongated bonds to re-equilibrate; thus, the effect of mechanical deformation on bond elongation and scission cannot be captured effectively. This issue is overcome by deforming the simulation volume at zero temperature-a technique adopted from the concept of quasi-continuum and demonstrated successfully in the authors' previous work. Results showed that a combination of MD deformation tests with ultra-high strain rates at near-zero temperatures provides a computationally efficient alternative for the study of bond dissociation phenomenon in amorphous epoxy polymer. In this paper, the ultra-high strain rate deformation approach is extended to the CNT-epoxy system at various CNT weight fractions and the corresponding bond disassociation energy extracted from the simulation volume is used as input to a low-fidelity continuum damage mechanics (CDM) model to demonstrate the bridging of length scales and to study matrix failure at the microscale. The material parameters for the classical CDM model are directly obtained from physics-based atomistic simulations, thus improving the accuracy of the multiscale approach. [ABSTRACT FROM AUTHOR]

The tension-tension fatigue behavior and damage mechanism of basalt fiber-reinforced epoxy polymer (BFRP) composites at different stress ratios are studied in this paper. The fatigue experiments were performed under stress ratios, R = σmin/σmax of 0.1 and 0.5, while the lifetime and the stiffness degradation were monitored and analyzed to investigate the effect of stress ratios. The damage propagation during fatigue loading was periodically monitored by using an in situ scanning electron microscope (SEM). The results show that the fatigue life decreases and the fatigue life degradation rate increases with the decrease of stress ratio for examined BFRP composites. The stiffness degradation is also sensitive to different stress ratios, showing a greater stiffness loss before failure at lower stress ratio. From the SEM images, it is indicated that the micro-damage mode shifts from interface debonding and matrix cracking into fiber breaking with decreasing stress ratios. [ABSTRACT FROM AUTHOR]

InGaN/GaN multi-quantum well light-emitting diodes (LEDs) are conventionally grown on a sapphire substrate due to a lack of compatible substrates with a high compressive strain. This is a result of the relatively large lattice, and thermal expansion coefficient mismatches between GaN and sapphire. The compressive strain is considered to be a major obstacle to further improve next-generation high-performance GaN-based LEDs. In this paper, we have designed, electroplated, and tested an efficient substrate using a patterned copper (Cu) layer on the backside of sapphire to relax the compressive strain in a GaN epilayer. The patterned Cu layer has a significant function in that it supports the GaN/sapphire LEDs with an external tensile stress. The external tensile stress is capable of compensating for the compressive strain in the GaN/sapphire LEDs by controlling the curvature of the wafer bowing. This patterned Cu layer, when applied to the GaN/sapphire LEDs, suppresses the compressive strain by up to 0.28 GPa. The GaN-based LEDs on this innovative and effective sapphire/Cu substrate offer improved optical and electrical performance. [ABSTRACT FROM AUTHOR]

Some mean-field models are currently available for discontinuous dynamic recrystallization (DDRX). They offer an affordable way to predict the variation of averages altered by DDRX as flow stress, grain size and recrystallized fraction. They predict also a grain-size distribution, but it appears unrealistic, at least if a deterministic equation governs the migration of grain boundaries. The present paper exposes an extension to mean-field models which introduces some topological and stochastic features in the migration equation. This extension aims to bridge the gap between mean- and full-field models. It allows predicting realistic distributions of grain size by keeping the simplicity of mean-field approaches. The results obtained are consistent with experimental data on an 18% Cr-11% Ni stainless steel. [ABSTRACT FROM AUTHOR]

Polymer composite materials are being increasingly used in primary load-bearing structures in several advanced industrial fields such as aerospace vessels, railway wagons and mega-scaled wind turbines where detection of subcritical damage initiation can significantly reduce safety issues and maintenance costs. It is therefore crucial to inspect these composite structures in order to assess their structural health and to ensure their integrity. Non-destructive testing techniques (NDT) are used for this purpose, making it possible to monitor mechanical damage of composite materials under in situ or ex situ service conditions. This paper reviews the capabilities of the most common NDT techniques used to inspect the integrity of composite materials. Each technique has a detection potential and cannot allow a full diagnosis of the mechanical damage state of the material. Thus, depending on the occurring damage mechanism and the conditions of use, one technique will be preferred over another, or several techniques should be combined to improve the diagnosis of the damage state of the structures. [ABSTRACT FROM AUTHOR]

Stress rupture is an important failure phenomenon in composite overwrapped pressure vessels, which is highly unpredictable other than on a statistical basis. Even then, there are several statistical models, with varying bases in composite micromechanics and molecular failure mechanisms. Choosing among these models is not trivial, even when micromechanical details of the failure process are reasonably well appreciated, and one has available a reasonably large database of strength and lifetime data. As a result, there is little in the way of guidance to choose the most appropriate model. One important issue is that accurate predictions are desired at relatively low service loads compared to the strength, and low probabilities of failure that are far less, e.g., 10-6, than can be directly confirmed using the data itself. In essence, one needs a robust and accurate statistical model free of inconsistencies associated with such low stress levels and probabilities of failure. This paper performs an in-depth comparison of several current models, which have varying physical bases. The models compared differ in the number of parameters to be estimated from data. The results of this study, however, show that over a broad range of parameter values these models give surprisingly similar failure probability predictions. While practitioners may have a preference for one model over another, the basis for such a choice is not easily established, given the fidelity of typical data. [ABSTRACT FROM AUTHOR]

Hydrogen embrittlement is a complex phenomenon, involving several lengthand timescales, that affects a large class of metals. It can significantly reduce the ductility and load-bearing capacity and cause cracking and catastrophic brittle failures at stresses below the yield stress of susceptible materials. Despite a large research effort in attempting to understand the mechanisms of failure and in developing potential mitigating solutions, hydrogen embrittlement mechanisms are still not completely understood. There are controversial opinions in the literature regarding the underlying mechanisms and related experimental evidence supporting each of these theories. The aim of this paper is to provide a detailed review up to the current state of the art on the effect of hydrogen on the degradation of metals, with a particular focus on steels. Here, we describe the effect of hydrogen in steels from the atomistic to the continuum scale by reporting theoretical evidence supported by quantum calculation and modern experimental characterisation methods, macroscopic effects that influence the mechanical properties of steels and established damaging mechanisms for the embrittlement of steels. Furthermore, we give an insight into current approaches and new mitigation strategies used to design new steels resistant to hydrogen embrittlement. [ABSTRACT FROM AUTHOR]

The use of misfit strain to improve the electronic performance of semiconductor films is a common strategy in modern electronic and photonic device fabrication. However, pursuing a favorable higher strain could lead to mechanical instability, on which systematic and quantitative understandings are yet to be achieved. In this paper, we investigate the anisotropy effects on strain-induced thin-film surface roughening by phase field modeling coupled with elasticity. We find that compared with films grown along {111} and {100} surfaces, the instability of {110} film occurs at a much lower strain. Our simulations capture the evolution of interface morphology and stress distribution during the roughening process. Similar characterizations are performed for heteroepitaxial growth from a surface pit. Finally, from 3D simulations, we show that the surface roughening pattern on {110} film exhibits a clear in-plane orientation preference, consistent with experimental observations. [ABSTRACT FROM AUTHOR]

This article continues the work on dipolar thermoelastic materials, which are a special case of multipolar continuum mechanics. This theory allows a double-porous structure: a macro-porosity related to pores in the material and a microporosity, which shows fissures in the porous skeleton. This paper constructs a mathematical model for dipolar materials, which have a double-porosity structure by considering a fractional order Duhamel-Neumann stress-strain relation. The heat conduction is described by Cattaneo's equations. The results are the constitutive equations of the linear theory of thermoelasticity with fractional order strain. The equations are valid for anisotropic materials and are called the Duhamel-Neumann equations with fractional order. Finally, the isotropic case is considered under the conditions of plane strain in order to perform some numerical simulations for samples of porous copper. [ABSTRACT FROM AUTHOR]

SrAlO:(Eu,Dy) is one of the most promising mechanoluminescence materials that have potential applications in stress sensing, lighting, imaging and energy conversion. However, the ML intensity decays with the afterglow time of SrAlO:(Eu,Dy), which hampers its application in the real world. Here, the mechanoluminescence property of SrAlO:(Eu,Dy) was investigated by impact of a load. A method was proposed to overcome the drawback of the mechanoluminescence decay behavior associated with the afterglow time. During the measurement of mechanoluminescence, continuous UV irradiation on SrAlO:(Eu,Dy) can effectively realize steady-state mechanoluminescence that is independent of the afterglow time. The underlying mechanism is discussed in this paper. [ABSTRACT FROM AUTHOR]

Metamagnetic shape memory alloys are a unique class of materials capable of large magnetic field-induced strain due to reverse martensitic phase transformation. A precondition for large shape change is martensite deformation, which heavily depends on microstructure. Elucidation of microstructure is therefore indispensable for strain control and deformation mechanics in such systems. The current paper reports on a self-accommodated martensitic microstructure in metamagnetic NiMnSn single crystal. The microstructure here is hierarchically organised at three distinct levels. On a large scale, martensite plate colonies, distinguished by intercolony boundaries, group individual martensitic plates. Plates are separated by interplate boundaries and deviate by 2.2° from an ideal twin relation. On the lower scale, plates are composed of subplate twins. Conjugation boundaries separating two pairs of twins arise in relation to a subplate microstructure. Modulation boundaries separating two variants with perpendicular modulation directions and with parallel c-axes also appear. Mechanical training frees larger plates from fine subplate microtwins bringing macro-lamellae into twin relation, what then permits further detwinning until a single variant state. [ABSTRACT FROM AUTHOR]

The objectives of this paper are to present the results of experimental study and analysis on the behaviour and the strength of reinforced geopolymer concrete slender columns. The experimental work involved testing of twelve columns under axial load and uniaxial bending in single curvature mode. The compressive strength of concrete for the first group of six columns was about 40 MPa, whereas concrete with a compressive strength of about 60 MPa was used in the other six columns. The other variables of the test program were longitudinal reinforcement ratio and load eccentricity. The test results gathered included the load carrying capacity, the load-deflection characteristics, and the failure modes of the columns. The analytical work involved the calculation of ultimate strength of test columns using the methods currently available in the literature. A simplified stability analysis is used to calculate the strength of columns. In addition, the design provisions contained in the Australian Standard AS3600 and the American Concrete Institute Building Code ACI318-02 are used to calculate the strength of geopolymer concrete columns. This paper demonstrates that the design provisions contained in the current standards and codes can be used to design reinforced fly ash-based geopolymer concrete columns. [ABSTRACT FROM AUTHOR]

The author is involved in a wide-ranging research programme, the objective being to extend the fracture mechanics methodology for sharp cracks to blunt flaws, so as to take credit for the blunt flaw geometry. The approach is based on the cohesive process zone representation of the micro-mechanistic processes that are associated with fracture. An earlier paper has derived a blunt flaw fracture initiation relation which gives the critical elastic flaw-tip peak stress σpcr (a “signifier” of a critical condition in the process zone) in terms of the process zone material parameters, subject to the proviso that the process zone size s is small compared with the flaw depth (length) and any characteristic dimension other than the flaw root radius ρ. The relation has been derived using a “two-extremes” procedure, whereby the separate σpcr solutions for small and large s/ρ are blended together to give an all-embracing relation that is valid for all s/ρ. A key feature of the relation is that σpcr essentially depends on only one geometrical parameter: the flaw root radius ρ. Though the relation has evolved from a consideration of the characteristics of one model, i.e. that of an elliptical flaw in an infinite solid that is subjected to an applied tensile stress, it is anticipated that the relation can be applied equally well for a wide range of geometrical configurations involving different flaw shapes. It is against this background that the present paper demonstrates that the relation also applies to the behaviour of an intrusion type flaw in the surface of a semi-infinite solid subjected to an applied tensile stress. [ABSTRACT FROM AUTHOR]

A new method is developed in this paper to deal with the thermomechanical response of continuous fiber-reinforced composites. Treating the matrix as an elastic-perfectly plastic solid, the analytical formulae of the deformations and stresses of the matrix are obtained from the plasticity theory, axisymmetric equilibrium equation, and stress–strain and strain–displacement relations. The fiber is taken to be an anisotropic, elastic material, and the formulae calculating its deformations and stresses are also presented. The boundary conditions and the consistence of deformations and stresses between the fiber and matrix, and between elastic and plastic regions of the matrix are employed to determine the unknown constants in the analytical formulae. With the developed method, the thermomechanical stress distributions in composites reinforced with circumferentially orthotropic, radially orthotropic and transversely isotropic fibers are investigated, and how the elastic-perfectly plastic property and different materials of the matrix affect the thermomechanical response of the composites is discussed. For the thermomechanical loads and composite systems given in this paper, the elastic-perfectly plastic property of the matrix can reduce the compressive stresses in the fiber, and the tensile circumferential and axial stresses in the matrix. A strong matrix can raise the compressive stresses in the fiber, and the tensile circumferential and axial stresses in the matrix. [ABSTRACT FROM AUTHOR]

This paper assesses the relative accuracy of a number of techniques that are capable of predicting a wide range of creep properties. The techniques studied in this paper include the 4-Θ, 6-θ, CRISPEN, Omega and CDM methodologies. The parameters of these models were estimated from short-term creep property data on 1CrMoV steel and these estimated models were then used to predict the known longer-term creep properties of this steel. It was found that the CDM approach yielded predicted times to failure that were wholly inadequate. The 6-θ approach was best for predicting long term times to failure. The best minimum creep rate predictions came from using either the 4-Θ, or the CRISPEN or the Omega technique. Finally, times to small strains were best predicted using the 6-θ and CRISPEN techniques. [ABSTRACT FROM AUTHOR]

This paper presents an empirical approach to the modeling of fatigue in silicon MEMS structures. The approach is based on the regression analysis of stress-life data obtained from prior fatigue experiments on polysilicon and single crystal silicon. The possible directions for future modeling efforts are identified at the end of the paper, following a discussion on the implications of the current model fatigue life prediction. [ABSTRACT FROM AUTHOR]

The aim of the paper is to present a model of thermal stresses in a layered plane system, and consequently to verify the model validity by comparing calculated thermal stresses with measured thermal stresses. The paper deals thus with thermal stresses in the Fe-3%Si sheet for magnetic applications, having a surface coating of the 2MgO·TiO2, ZrO2, TiO2, Al2O3, 2MgO·SiO2, MgO·Al2O3 oxides. The thermal stresses, originating during a cooling process as a consequence of the difference in thermal expansion coefficients between the Fe-3%Si sheet and the surface coating, degrade hysteresis losses of the Fe-3%Si sheet. Magnetic properties of the presented coating–Fe-3%Si systems, as the magnetic induction and the hysteresis losses before and after coating formation, are presented. The theoretical background, including the model of the thermal stresses in anisotropic and isotropic two-layered plane systems, consequently transformated to the three-layered plane system, is presented. Calculated thermal stresses are compared with those of a tension measurement of the presented coating—Fe-3%Si systems, numerical equality of the calculated and measured thermal stresses is observed. [ABSTRACT FROM AUTHOR]

Strain wave propagation velocity in fibre materials is a primary consideration in the design of body armours for ballistic protection. In this paper, we compare the strain wave propagation (sonic) velocities and moduli of parallel and twisted ballistic fibre strands (yarns) derived from quasi-static tensile test and strain wave pulse test. In parallel multifilament yarns, the individual fibres behave independently from each other, and the yarn sonic velocities and moduli derived from the two test methods match each other very closely. In yarns with a twisted structure by twisting multifilament yarn or by spinning short fibres, fibres in the yarns are compressed against each other and fibre-to-fibre friction plays a significant role. Consequently, the yarn sonic velocities and moduli determined by the strain wave pulse method are significantly greater than that derived from the quasi-static tensile method. [ABSTRACT FROM AUTHOR]

This paper investigates the fatigue properties and deformation behavior of the T6-treated Mg-2.99Nd-0.18Zn-0.38Zr (NZ30K) and an AZ91D-T6 magnesium alloys with the same grain size. Compared with the AZ91-T6 alloy, the NZ30K-T6 alloy shows significant increases in mechanical properties, achieving 17 MPa in YS, 51 MPa in UTS, 2.4 % in elongation, and 15 MPa in FS, respectively. The NZ30K-T6 alloy also shows higher cyclic stress amplitudes in comparison with the AZ91-T6 alloy. This is attributed to increased matrix strength from the higher precipitate strengthening in the NZ30K-T6 alloy. The cyclic stress amplitude of the AZ91-T6 alloy increases with increasing the number of cycles. For the NZ30K-T6 alloy, the cyclic stress amplitude first increases and then decreases during cyclic deformation. Hysteresis loops were symmetrical at all total strain amplitudes due to the lessened extents of texture in these cast magnesium alloys and the absence of twinning-detwinning during cyclic deformation. For both AZ91-T6 and NZ30K-T6 alloys, the cracked slip bands initiate the fatigue cracks and also assist the crack propagation. The Coffin-Manson law and Basquin equation can be used to evaluate the fatigue parameters and predict the LCF lives of both AZ91-T6 and NZ30K-T6 alloys. The multi-scale fatigue (MSF) life models and the same model parameters were also used to predict the HCF lives of these alloys. [ABSTRACT FROM AUTHOR]

Manipulation of spin-flip is important in spintronics applications. In this paper, we report a simple manipulation of three Co-3 d spin states in the CoO spinel. From first principles calculations, we demonstrate that the spin state of Co atoms at octahedral sites (Co) of the CoO spinel can be manipulated through tensile stains. The magnetic moment of the Co changes from the ground non-magnetic (0 µ) state to the intermediate-spin state (2 μ) and high-spin state (4 μ) when 5 % (or higher) uniaxial tensile strain and 3 % (or higher) biaxial tensile strain are applied to the lattice, respectively. Moreover, the mechanism of the spin state transition of the Co-3 d electrons in CoO is discussed, and we show that the occurrence of spin-flip is a result of the competition between the cubic crystal field energy and the exchange correlation energy. These results open a new route to achieve and manipulate triple-spin states in spintronic devices. [ABSTRACT FROM AUTHOR]

Misfit strain relaxation-induced morphology instability is usually observed in epitaxial heterostructures at high temperatures. In this paper, we report that this morphology instability can occur even at room temperature in epitaxial ZnO/AZO (Al-doped ZnO) core-shell nanowires (NWs). As a result, densely distributed ZnO nanodots (NDs) were self-assembled on the NWs. The growth of NDs was slowed down during aging owing to the gradually reduced misfit strain. The final size and shape of the NDs were highly depended on the shell thickness and the doping ratio. It was proved that the morphology stability could be improved by surface passivation, thinning the shell thickness, or lowering the doping ratio. The results may provide instructive suggestions for the reliable design in strain and surface engineering of nanomaterials. [ABSTRACT FROM AUTHOR]

All the five independent elastic properties/constants of micro- and nano-structured hierarchical and self-similar random irregular honeycombs with different degrees of cell regularity are obtained by analysis and computer simulation in this paper. Cell wall bending, stretching, and transverse shearing are the main deformation mechanisms of hierarchical honeycombs. The strain gradient effects at the micro-meter scale, and the surface elasticity and initial stress effects at the nano-meter scale are incorporated into all the deformation mechanisms in the analysis and finite element simulations. The results show that the elastic properties of hierarchical random irregular honeycombs strongly depend on the thickness of the first-order cell walls if it is at the micro-meter scale, and that if the thickness of the first-order cell walls is at the nano-meter scale, the elastic properties of hierarchical random irregular honeycombs are not only size-dependent, but are also tunable and controllable over large ranges. In addition, the geometrical properties of nano-structured hierarchical random irregular honeycombs are also tunable and controllable. [ABSTRACT FROM AUTHOR]

In this paper, the effects of biaxial mean stress, mainly contributed by the weld residual stress, and thermal loading conditions on cracking directions and damage in high cycle thermal fatigue crazing subjected to variable amplitude loadings are investigated by a combined analytical and computational approach. The cracking directions are related to the orientation of the critical plane defined by the maximum damage. Analytical solutions of the critical plane orientation under constant amplitude biaxial tension/compression loadings are first derived and then employed to study the effects of biaxial mean stress on the critical plane orientation. The critical plane orientation appears to strongly depend on the dominant direction defined by the larger maximum stress. The developed analytical solution of the critical plane orientation and the analytical solution of the thermal stress from the literature are employed to study the effects of thermal loading conditions on the critical plane orientation. The critical plane orientation does not seem to significantly depend on the frequency, the amplitude and the mean value of the fluid temperature fluctuations, and the heat transfer film coefficient between the fluid and the pipe wall. The critical plane orientation under variable amplitude loadings is also studied, and an approximate solution is proposed for convenient engineering applications. The critical plane orientations are used to partially explain the observed cracking directions in the high cycle thermal fatigue crazing in the old residual heat removal system of a nuclear power plant. Finally, the effects of biaxial mean stress and thermal loading conditions on the fatigue crack initiation life are discussed. [ABSTRACT FROM AUTHOR]

This paper aims to statistically estimate the dynamic fatigue strength in brittle materials under a wide range of stress rates. First, two probabilistic models were derived on the basis of the slow crack growth (SCG) concept in conjunction with two-parameter Weibull distribution. The first model, Model I, is a conventional probabilistic delayed-fracture model based on a concept wherein the length of the critical crack growth due to SCG is enough larger than the initial crack length. For the second model, Model II, a new probabilistic model is derived on the basis of a concept wherein the critical cracks have widely ranging lengths. Next, a four-point bending test using a wide range of stress rates was performed for soda glass and alumina ceramics. We constructed fracture probability-strength-time diagrams (F-S-T diagrams) with the experimental results of both materials using both models. The F-S-T diagrams described using Model II were in good agreement with plots of the fracture strength and the fracture time of both materials more so than Model I. [ABSTRACT FROM AUTHOR]

This paper develops a modification of the shared frailty model that allows for the distribution of the response and the random effects to be determined by the data (from within the exponential family of distributions). A simulated maximum likelihood and a computationally simpler simulated linear least squares approach to estimating this model are also presented. These generalizations are important for modelling high-temperature deformation because they allow life time predictions to be made with levels of confidence attached to them, for the real world situation where repeated experimental observations are not normally distributed and not independent of each other. An application to a commonly used material in power generation revealed that ignoring these characteristics lead to an underestimate of the minimum safe life when operating at 773 K and stresses above 235 MPa. [ABSTRACT FROM AUTHOR]

Carbon nanotubes (CNTs) are expected to be ideal reinforcements of composite materials used in aircraft and sports industries due to their high modulus and low density. In the present paper, severe plastic deformation by high pressure torsion (HPT) of powders at elevated temperature (473 K) was employed to achieve both powder consolidation and grain refinement of aluminum-matrix nanocomposites reinforced by 5 vol% CNTs. Before the HPT, the powders were ball milled using planetary ball mill in order to achieve molecular level mixing. Aluminum was treated by the same process for a reference. The HPT processed disk were composed of considerably equilibrium grain boundaries with high misorientation angles. The CNT-reinforced ultrafine grained microstructural features resulted in high strength and good ductility. [ABSTRACT FROM AUTHOR]

Future advanced nuclear plants are considered to operate as cogeneration plants for electricity and heat. Metals and alloys will be the main portion of structural materials employed (including fuel claddings). Due to the operating conditions these materials are exposed to damaging conditions like creep, fatigue, irradiation and its combinations. The paper uses the most important alloys: ferritic-martensitic steels, superalloys, oxide dispersion strengthened steels and to some extent titanium aluminides to discuss its responses to these exposure conditions. Extrapolation of stress rupture data, creep strain, swelling, irradiation creep and creep–fatigue interactions are considered. Although the stress rupture- and the creep behavior seem to meet expectations, the long design lives of 60 years are really challenging for extrapolations and particularly questions like negligible creep or occurrence of diffusion creep need special attention. Ferritic matrices (including oxide dispersion strengthened (ODS), steels) have better irradiation swelling behavior than austenites. Presence and size of dispersoids having a strong influence on high-temperature strength bring only insignificant improvements in irradiation creep. A strain-range-separation based approach for creep–fatigue interactions is presented which allows a real prediction of creep–fatigue lives. An assessment of capabilities and limitations of advanced materials modeling tools with respect to damage development is given. [ABSTRACT FROM AUTHOR]

The paper focuses on modeling the strain fatigue lives of three commonly used cable insulation polymers, namely (1) polyvinyl chloride, (2) crosslinked polyethylene, and (3) polyphenylene ether under selected strain and temperature ranges. On the basis of results obtained from their fatigue tests, Coffin–Manson model, mean/maximum strain fatigue model, and a set of new semi-empirical equations were applied to establish the relationship between fatigue lives and strains. The unified strain model, herein we name it the Wei–Wong model, is developed to predict the fatigue lives of three polymers studied and their prediction capability was examined using our experimental data. It was found that the proposed Wei–Wong model can provide a better life prediction compared to the experimental data and other methods in the literature at selected temperatures, namely −40, 25, and 65 °C. [ABSTRACT FROM AUTHOR]

In this paper, the plastic deformation behavior and strain homogeneity of the ZK60 Mg alloy during the multi-pass cyclic extrusion and compression (CEC) was simulated using the finite-element method (FEM) with a view to provide an insight into the mechanics of the process. Physical modeling (PM) experiment with same alloy was carried out to verify the results of the numerical simulations. The results show that two vortex flow regions with opposite flow direction are formed inside the cylindrical workpiece during CEC deformation. Although the deformation is inhomogeneous in both end regions of workpiece, a uniform region of equivalent strain exists, and the extent of uniform deformation increased with the increase in workpiece length. [ABSTRACT FROM AUTHOR]

This paper has clarified the anelasticity of 8Y-FSZ/α-alumina composites wherein the 8Y-FSZ phases are dispersed like islands. The amount of anelastic strain generated and the manner of anelastic deformation were compared to those of monolithic 8Y-FSZ. The anelastic strains of six kinds of 8Y-FSZ/α-alumina, as well as of monolithic 8Y-FSZ and monolithic α-alumina, were measured. The results showed that the anelastic strain was produced even in the composite where 8Y-FSZ phases existed as islands, and that the more the anelastic strain produced, the higher the volume fraction of 8Y-FSZ. In addition, the composition with a fully densified alumina phase had the effect of inhibiting anelastic strain in the 8Y-FSZ phase. [ABSTRACT FROM AUTHOR]

Some basic aspects of electromagnetic radiation (EMR) emission during quasi-static plastic deformation and crack propagation in ASTM B 265 grade 2 titanium sheets are reported in this paper. It has been observed that titanium emits several intermittent EMR signals starting near the yield and continuing up to fracture. The EMR emissions are sensitive to the applied strain rates. The number of EMR emissions shows a parabolic variation with strain rate, exhibiting a maxima. It appears that the temporary pinnings of the dislocations injected from the crack tip, which is essential for the emission of EMR as described in a recent model by the authors, do not adequately occur at high as well as low strain rates. At elevated temperatures, the square of the EMR amplitude is observed to be proportional to $$ \exp {\left( { - {mU} \mathord{\left/ {\vphantom {{mU} {kT}}} \right. \kern-\nulldelimiterspace} {kT}} \right)}, $$ where U is the activation energy for the movement of jogs, k the Boltzmann constant, T the temperature and m ≤ 1.0. [ABSTRACT FROM AUTHOR]

Acid treatment and triethylene-tetramine (TETA) modification of multi-walled carbon nanotubes (MWCNTs) purposing to attain better dispersibility and stronger interfacial bonding between MWCNTs and epoxy matrix have been carried out in this paper. The epoxy and MWCNTs/epoxy composites were produced by cast molding method. Stress–strain curves show that TETA-MWCNTs/epoxy hold the greatest toughness of all samples with 0.5 wt.% nanoparticles. The Young’s modulus of TETA-MWCNTs/epoxy has a significant increase about 38% compared to the neat epoxy, while the Young’s modulus of unmodified MWCNTs/epoxy or acid-modified MWCNTs/epoxy has a bit of decrease. Tensile and impact strength tests reflect that TETA-MWCNTs reinforced epoxy composites have an obvious improvement of tensile strength about 30% and an enhancement of impact strength over 34% compared to the pure epoxy composites with only 0.5 wt.% loading of TETA-MWCNTs. Scanning electron microscopy images of fractured surface of MWCNTs/epoxy indicate homogeneous dispersibility of TETA-MWCNTs and strong interfacial adhesion between the TETA-MWCNTs and the epoxy in the MWCNTs/epoxy composite. [ABSTRACT FROM AUTHOR]

Wood fibres constitute the structural framework of e.g. wood, paper, board and composites, where stiffness and dimensional stability are of importance. An analytical modelling approach has been used for prediction of hygroelastic response, and assessment of the stresses in thick-walled cylinder models of wood fibres. A wood fibre was idealised as a multilayered hollow cylinder made of orthotropic material with helical orientation. The hygroelastic response of the layered assembly due to axisymmetric loading and moisture content changes was obtained by solving the corresponding boundary value problem of elasticity. A simple solution scheme based on the state space approach and the transfer matrix method was employed. This was combined with an analytical ultrastructural homogenisation method, used to link hygroelastic properties of constituent wood polymers to properties of each layer. Predicted hygroelastic response captured experimentally measured behaviour. Fibres that were constrained not to twist showed a stiffer response than fibres allowed twisting under uniaxial loading. It was also shown that the ultrastructure, i.e. the microfibril angle, will control the hygroexpansion in the same way as it controls the compliance of the cell wall. Qualitative failure trends comparable with experimental observations could be established with stress analysis and a simple plane-stress failure criterion. [ABSTRACT FROM AUTHOR]

Part 1 of the paper reviews four categories of protein fibers. (1) Wool and other hairs, which have been used as textiles for thousands of years. (2) Regenerated protein fibers, which were manufactured in the 1950s but did not achieve continued commercial success, and, in the 1990s, fibers from artificially produced spider silk proteins. (3) Hagfish slime threads, as an example of other biological fibers. (4) Silks, from silkworms and spiders. In Part 2, stress–strain curves are compared and discussed in relation to what is known of the structure of the fibers and the mode of formation. Models of the structural mechanics are described. The possibilities for scientific and commercial advances in future are presented in Part 3. [ABSTRACT FROM AUTHOR]

The mechanical behaviour of polymeric foams depends on several parameters such as temperature, material density and strain rate. This last point implies that compression tests on conventional testing machines are not sufficient. Study of the behaviour in practical situations requires special apparatus like fly wheels, drop towers or Hopkinson bars, allowing high compression speeds. The polypropylene foams studied are multi-scale materials; agglomerated beads (2–3 mm in diameter), visible to the naked eye, are composed of microscopic closed cells (a few tens of microns). The response of the material to a shock consists of three regions: an elastic phase, a plastic phase and densification. The plastic phase is of prime interest since a great part of the shock energy is dissipated there. Microtomography was used in order to better understand damage mechanisms during the stress plateau of the plastic phase. The final objective of this work is to determine the strain field of porous materials at several levels of shock. As tomography is not fast enough to directly follow the impact deformation, interrupted impact tests were carried out by controlling the levels of sample deformation. Between each impact step, a microtomographic analysis offers insight on the progressive deformation of the sample. The results of these impact tests completed by a microtomographic visualisation in 2D are presented and commented in this paper. [ABSTRACT FROM AUTHOR]

It is of great interest for economical and security reasons to understand the compressive properties of underground cemented paste backfill. In this paper, the stress–strain behaviours of cemented paste backfill (CPB) subjected to uniaxial compression and conventional triaxial tests are presented and discussed. The effect of CPB basic components, strength, ageing and confining pressure on the deformation behaviour of CPB are evaluated and discussed. The results show that the stress–strain behaviour of CPB is strongly influenced by the confinement, the age and strength of CPB, and its components. The increase in confining pressure leads to a change in the mode of failure, in the stiffness, and an increase in the strength. [ABSTRACT FROM AUTHOR]

In the present study, we investigate dislocation processes in a Ni-base single crystal superalloy (LEK94) after high temperature and low stress creep. Specimens were creep deformed at 240 MPa and 980 °C to a strain of 20%. We use diffraction contrast transmission electron microscopy (TEM) to show that two γ-channel dislocations with different Burgers vectors combine and form a super dislocation that shears the γ′-particle. This type of cutting event has now been observed for three single crystal superalloys with different alloy chemistry (CMSX-4, CMSX-6 and LEK94 in the present work) and we therefore conclude that it represents a generic elementary dislocation process which governs high temperature (around 1000 °C) and low stress (around 240 MPa) creep. The present paper provides microstructural evidence for this type of cutting processes and discusses the results in the light of previous work published in the literature. [ABSTRACT FROM AUTHOR]

The processing, structure and properties of deformed metals and alloys with a structural scale from the micrometer to the nanometer dimensions has been the subject of a recent viewpoint set []. The present paper will focus on deformed metals and alloys with a structural scale from 5 nm to 100 nm, concentrating on materials processed by high pressure torsion (HPT), surface mechanical attrition treatment (SMAT) and sliding. A detailed microstructural characterization has been followed by an analysis of the relationship between structural features and processing parameters. In this analysis, some general approaches have been applied for example scaling of the evolution of the boundary spacing. This analysis is the basis for a brief discussion of the relationship between the microstructural parameters and the strength. [ABSTRACT FROM AUTHOR]

Ultrafine-grained materials are attractive for achieving superplastic elongations provided the grains are reasonably stable at elevated temperatures. Since the strain rate in superplasticity varies inversely with the grain size raised to a power of two, a reduction in grain size to the submicrometer level leads to the occurrence of superplastic flow within the region of high strain rate superplasticity at strain rates >10−2 s−1. This paper tabulates and examines the various reports of superplasticity in ultrafine-grained materials. It is shown that these materials exhibit many characteristics similar to conventional superplastic alloys including strain rates that are consistent with the standard model for superplastic flow and the development of internal cavitation during the flow process. [ABSTRACT FROM AUTHOR]

In this paper, an accurate formula for calculating the thermal residual stress field in a particle-reinforced composite are presented. Numerical examples are given to show r-variations of the thermal residual stresses. The increase in fracture toughness of matrix predicted by the thermal residual stress field is compared well with the experimentally measured increase. [ABSTRACT FROM AUTHOR]

Fifty years ago, in a series of classic creep experiments conducted at the University of California in Berkeley, Harper and Dorn obtained unique experimental data revealing the possibility of a new and heretofore unrecognized flow process occurring in pure aluminum when tested at low stresses and at temperatures very close to the melting temperature. This flow mechanism, subsequently designated Harper–Dorn creep, has been the center of much argument and speculation in the ensuing years. The present paper looks back over the last half-century and charts the various developments in attempts to obtain a more detailed understanding of whether Harper–Dorn creep is (or is not) a viable creep process. Examples are presented for both metals and non-metals. It is concluded that, although it appears Harper–Dorn creep may occur only under restricted conditions associated with high purity materials and low initial dislocation densities, nevertheless there is good evidence supporting the validity of this creep mechanism as a viable and unique flow process. [ABSTRACT FROM AUTHOR]

A high fidelity assessment of accumulative damage of woven fabric composite structures subjected to aggressive loadings is strongly reliant on the accurate characterization of the inherent multi-scale microstructures and the underlying deformation phenomena. Damage in composite sandwich and joint structures is characterized by the coexistence of discrete (delamination) and continuum damage (matrix cracking and intralaminar damage). A purely fracture mechanics-based or a purely continuum damage mechanics-based tool alone cannot effectively characterize the interaction between the discrete and continuum damage and their compounding effect that leads to the final rupture. In this paper, a hybrid discrete and continuum damage model is developed and numerically implemented within the LS-DYNA environment via a user-defined material model. The continuum damage progression and its associated stiffness degradation are predicted based on the constituent stress/strain and their associated failure criteria while the discrete delamination damage is captured via a cohesive interface model. A multi-scale computational framework is established to bridge the response and failure predictions at constituent, ply, and laminated composite level. The calculated constituent stress and strain are used in a mechanism-driven failure criterion to predict the failure mode, failure sequence, and the synergistic interaction that leads to global stiffness degradation and the final rupture. The use of the cohesive interface model can capture the complicated delamination zone without posing the self-similar crack growth condition. The unified depiction of the continuum and discrete damage via the damage mechanics theory provides a rational way to study the coupling effects between the in-plane and the out-of-plane failure modes. The applicability and accuracy of the damage models used in the hybrid dynamic failure prediction tool are demonstrated via its application to a circular plate and a composite hat stiffener subjected to shock and low velocity impact loading. The synergistic interaction between the continuum and discrete damage is explored via its application to a sandwich beam subjected to a low velocity impact. [ABSTRACT FROM AUTHOR]