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Abstract This study investigates the failure mechanisms of cross-ply laminates subjected to longitudinal compressive loading. A sequence of failure initiation and propagation process is observed based on optical microscopy images of failed specimens. Specifically, the failure process in the cross-ply laminates involves a combination of four failure mechanisms: fiber kinking, delamination, matrix cracking, and fiber/matrix splitting. We find that the kink-bands in the middle 0° plies of the cross-ply laminates most often show a wedge shape and the angle of matrix cracks in the 90° plies is slightly larger than that of pure 90° plies. The reason has been attributed to the constraining effects by the adjacent plies. The next focus of this study is to propose hybrid micro-macro models to more systematically study the failure mechanisms of the cross-ply laminates. We show that these models accurately predict the combined failure modes of the cross-ply laminates and enable us to closely investigate the interactions of different failure mechanisms. More importantly, the hybrid models achieve great computational efficiency compared to full-scale microstructural models. The combined experimental and computational analyses presented here provide a new level of understanding of the kink-band morphology and damage mechanisms in the cross-ply laminates. [ABSTRACT FROM AUTHOR]

Abstract Failure prediction for carbon fiber reinforced polymer (CFRP) composites has been a longstanding challenge. In this study, we address this challenge by first applying a well-established computational micromechanics model based on representative volume element to predict the failure envelopes of unidirectional (UD) CFRP composites. Then, these failure envelopes are compared with the classical failure criteria. We have evaluated the performances of these failure criteria and identified the aspects for further improvement in their accuracies for the UD CFRP composites studied herein. Based on the failure mechanisms from computational analyses and the comparisons between predicted failure envelopes and classical failure criteria, a new set of homogenized failure criteria is proposed. The newly proposed failure criteria show significant improvement according to our computational and experimental results. Furthermore, we have compared the proposed failure criteria with existing experimental data and computational results available in the literature for different types of composites. Good agreements are generally observed. [ABSTRACT FROM AUTHOR]

Abstract This study investigates the failure mechanisms of notched unidirectional (UD) carbon fiber reinforced polymer (CFRP) laminates subjected to longitudinal compressive loading. A sequence of failure initiation and propagation is observed based on optical microscopy images of specimens during failure. The micrographs reveal that the main failure mechanism in UD laminates is fiber kinking failure. The influence of manufacturing defects and voids on fiber kinking mechanisms is also analyzed. The results show that the voids in the resin-rich area lead to kink-band splitting, while in some cases the voids cause the kink-band to deflect in a new direction depending on the location of the voids. The next focus of this study is to propose a computational micromechanics model considering local fiber waviness to study the fiber kinking failure mechanism. The computational results on kink-band formation and stress-strain response show good consistency with our experimental analysis. Finally, the failure envelopes of σ 11 -σ 22 and σ 11 - τ 12 subjected to combined loading conditions are obtained from the computational model, which provide essential inputs for future theoretical failure criteria. [ABSTRACT FROM AUTHOR]

A multi-scale computational analysis based on representative volume element (RVE) modeling and molecular dynamics (MD) simulations is developed to investigate the microscopic failure mechanisms of unidirectional (UD) carbon fiber reinforced polymer (CFRP) composites. The average properties of the 200-nm thickness interphase region between fiber and matrix are characterized through MD simulations and an analytical gradient model. The results demonstrate that the interphase region has higher Young’s modulus and strength, compared to the bulk matrix. This stiffened interphase region influences the composite response significantly. Specifically, the traditional two-phase model with zero-thickness interface fails to capture the stress-strain behavior compared to the experimental data. However, by adding the interphase region to a modified RVE model, the accuracy of simulation results will be improved significantly. Furthermore, a coupled experimental-computational micromechanics approach is adopted to calibrate and validate the cohesive parameters of the interface. By including the cohesive interface, our modified RVE model accurately captures the failure strength of the composites. Finally, different failure mechanisms for specimens are investigated using our multi-scale computational framework. The results show that the failure modes of UD CFRP composites are very complex and multiple failure mechanisms co-exist depending on the loading conditions, agreeing well with our experimental analyses. [ABSTRACT FROM AUTHOR]

The partial pseudoelastic stress-strain behavior and the underlying microstructure evolution of Ni-Mn-Ga ferromagnetic shape memory alloy under different values of constant (assisted) magnetic field H and demagnetization factor N are studied by phase-field simulations. The results show that both H and N field (component of N parallel to H ) have significant influence on the partial strain recovery in the compressive stress loading-unloading cycle, while N stress (component of N parallel to the stress) only affects the formation of magnetic domains in the stress-preferred variant and has no influence on the strain recovery behavior. The recovered strain can be increased by raising H or reducing N field . Moreover, the range of H for the partial strain recovery increases with N field . It is shown that Ni-Mn-Ga exhibits the partial pseudoelasticity at moderate/high H under a large N field rather than just at low H as observed in existing experiments, demonstrating the significant influence of N field on the strain recovery behavior. The work reveals that both H and N field affect the strain recovery behavior through the effective magnetic field * H field (component of the internal magnetic field parallel to H ) during the stress cycle. * H field decreases with the twin boundary motion during the reverse martensite reorientation and eventually becomes insufficient to trigger the twin boundary motion, which is responsible for the partial strain recovery. [ABSTRACT FROM AUTHOR]

The grain size (GS) dependence of fracture toughness ( K IC ) and static crack-growth resistance ( K R ) of superelastic NiTi with average GS from 10 to 1500 nm are investigated. The measurements of strain and temperature fields at the crack-tip region are synchronized with the force–displacement curves under mode-I crack opening tests. It is found that with GS reduction down to nanoscale, the K IC and the size of crack-tip phase transformation zone monotonically decrease and the K R changes from a rising to a flat R-curve. The roles of intrinsic and extrinsic toughening mechanisms in the GS dependence of the fracture process are discussed. [ABSTRACT FROM AUTHOR]

In situ X-ray diffraction during loading and unloading is used to investigate the effects of grain size (GS) on the stress-induced nanoscale phase transition (PT) mechanism in polycrystalline superelastic NiTi. The average GS studied (10–1500 nm) spans the range in which significant changes of macroscopic thermomechanical properties (due to GS reduction) have been observed. It is shown that when the GS ⩾ 68 nm, the evolution of the diffraction profiles (DPs) during loading and unloading exhibits well-defined distinct diffraction peaks with significant changes in their diffracted intensities corresponding to the nucleation and growth mechanism of B19′ martensite (high strain) phase. However, when GS < 68 nm, the evolution of DPs gradually degenerates from the multiple peaks mode to a continuous and reversible single peak shift mode. Measurements of the lattice parameters and the corresponding components of lattice strains show that such drastic changes in characteristics of DPs indicate a gradual change in the PT mechanism from traditional nucleation and growth mode in coarse-grained polycrystals to a continuous lattice deformation inside the nano-sized grains. Moreover, the middle eigenvalue ( λ 2 ) of the transformation matrix gradually approaches 1 with GS reduction, almost fulfilling the proposed λ 2 = 1 condition in the literature for the vanishing of hysteresis in SMAs. The results provide lattice level scenarios for the understanding of GS effects on the change of PT type from first-order to continuous PT which brings significant changes in the macroscopic thermomechanical behavior and properties of the polycrystalline superelastic NiTi. [ABSTRACT FROM AUTHOR]

This paper performs experimental and analytical investigations on jump phenomena of thermomechanical responses of a nonlinear torsional vibration system with a phase transformable NiTi Shape Memory Alloy (SMA) wire as a nonlinear spring. By synchronizing the measurement of rotational angle and temperature of the NiTi wire in the torsional vibration system under external sinusoidal excitation, their evolutions in the transient and steady states are acquired. By monotonically increasing/decreasing the excitation frequency with small intervals around the primary resonance frequency, Frequency Response Curves (FRCs) of these two physical quantities are obtained and their jumps on the FRCs are captured in the frequency domain. To understand and quantify such jump phenomena, an effective Duffing oscillation model is introduced to simulate the mechanical behaviour of NiTi SMA in the dynamic system and to determine the FRCs and the characteristic jump frequencies. A lumped heat transfer analysis is then performed to establish the relationship between the temperature oscillation and the rotational angle. It is shown that the jump of the rotational angle is accompanied by a simultaneous jump of the temperature oscillation of the NiTi wire in torsional vibration. Particularly, the coupling effect between the thermal response and mechanical response can be significant and must be considered in the study of a nonlinear dynamic system with NiTi SMA component. [ABSTRACT FROM AUTHOR]

Highlights: [•] We build a continuum model to describe rate-dependent filament phase transition. [•] Our FEM simulation captured the key features of rate-dependent responses. [•] Under rapid loading, filament phase transition will not happen. [•] Interfacial and kinetic properties can be described by non-local model. [Copyright &y& Elsevier]

• Presents a novel computationally-efficient reconstruction of 3D Representative Volume Element (RVE) model of second-phase particle-strengthened AA7075-O sheet with real distribution of particles based on FIB-SEM techniques • Three types of second-phase particles (i.e., irregularly-shaped iron-rich particle Al 3 Fe, elliptically-shaped particle MgZn 2 , and needle-like particle CuAl 2) are determined using SEM-based EDS and EBSD techniques • Presents a systematic study of the overall elastic response by using 3D-microstructrure based RVE Model with periodic boundary condition (PBC) and kinematically uniform boundary condition (KUBC) • A comparison of RVE results with that of modified rule of mixtures (MROMs) is presented. MROMs predictions of elastic properties generally well match with the trends of RVE-based computational results, which implies the validity of simpler MROMs approach AA7075 is of special interest amongst aluminum alloys for structural applications in automotive and aerospace industries because of its higher strength. Due to advances in experimental and numerical analysis tools, there is much interest amongst researchers in developing analytical and computational methods to calculate the effective material properties of AA7075 alloy based of its complex microstructure. This paper presents a novel computationally-efficient reconstruction of three-dimensional microstructure of representative volume element model with real distribution of particles using focused ion beam equipped with scanning electron microscopy techniques and specialized computer software to investigate the mechanical properties of AA7075-O sheet. As part of material microstructure characterization for computational analysis, particle identification and chemical characterization of the AA7075-O sheet are also investigated by scanning electron microscopy-based energy dispersive X-ray spectroscopy and electron back-scatter diffraction analysis. The results show that there are three main types of particles that exist in AA7075-O sheet, i.e., irregularly-shaped iron-rich particle Al 3 Fe, elliptically-shaped particle MgZn 2 , and needle-like particle CuAl 2. The spatial distribution of these particles in AA7075-O sheet from a set of scanning electron microscopy images are utilized to reconstruct RVE models in a finite element analysis environment and analyzed with periodic boundary condition and kinematically uniform boundary condition. Also, a comparison of RVE results with that of modified rule of mixtures is presented. It is shown that the particular boundary conditions have almost no influence over the effective elastic properties when the RVE size is 10 um × 10 um × 10 um and volume fraction of particle is large. Also, the modified rule of mixtures predictions of elastic properties generally well match with the trends of RVE-based computational results, which implies the validity of simpler modified rule of mixtures approach. [Display omitted] [ABSTRACT FROM AUTHOR]

More than 400 pentatricopeptide repeat ( PPR) genes have been found in higher plants, but most of them have not been functionally analyzed and their origins are still obscure. In this study, we performed phylogenetic genomewide comparisons of the PPR gene family in indica and japonica rice to explore the expansion mechanisms of these genes in higher plants. The functions of PPR genes in plant CMS/ Rf systems are also discussed. The results indicate that (1) unequal crossing over participated in the expansion of the newly evolved PPR genes in indica and japonica rice genomes, (2) CMS/ Rf systems are different in monocots and dicots, (3) the BT-type CMS/ Rf system exists in both indica and japonica rice, and (4) both the PPR gene family and the BT-type CMS/ Rf system may have existed before the divergence of indica and japonica rice. [ABSTRACT FROM AUTHOR]

The mechanical response of spherical indentation of superelastic shape memory alloys (SMAs) was theoretically studied in this paper. Firstly, the friction effect was examined. It was found that the friction influence is negligibly small. Secondly, the influence of the elasticity of the indenter was investigated. Numerical results indicate that this influence can not be neglected as long as the indentation depth is not very small. After that, this paper focused on the effect of transformation volume contraction. Our results show that the transformation volume contraction due to forward martensitic transformation can reduce the maximum indentation force and the spherical indentation hardness. These research results enhance our understanding of the spherical indentation responses, including the hardness of the smart material SMAs. [ABSTRACT FROM AUTHOR]

Abstract: Phase transformational shakedown of a structure refers to a status that plastic strains cease developing after a finite number of loading cycles, and subsequently the structure undergoes only elastic deformation and alternating phase transformations with limited magnitudes. Due to the intrinsic complexity in the constitutive relations of shape memory alloys (SMA), there is as yet a lack of effective methods for modeling the mechanical responses of SMA structures, especially when they develop both phase transformation and plastic deformation. This paper is devoted to present an algorithm for analyzing shakedown of SMA structures subjected to cyclic or varying loads within specified domains. Based on the phase transformation and plastic yield criteria of von Mises-type and their associated flow rules, a simplified three-dimensional phenomenological constitutive model is first formulated accounting for different regimes of elastic–plastic deformation and phase transformation. Different responses possible for SMA bodies exposed to varying loads are discussed. The classical Melan shakedown theorem is extended to determine a lower bound of loads for transformational shakedown of SMA bodies without necessity of a step-by-step analysis along the loading history. Finally, a simple example is given to illustrate the application of the present theory as well as some basic features of shakedown of SMA structures. It is interesting to find that phase transformation may either increase or decrease the load-bearing capacity of a structure, depending upon its constitutive relations, geometries and the loading mode. [Copyright &y& Elsevier]

Abstract: Dimensional analysis and the finite element method are applied in this paper to study spherical indentation of superelastic shape memory alloys. The scaling relationships derived from dimensional analysis bridge the indentation response and the mechanical properties of a superelastic shape memory alloy. Several key variables of a superelastic indentation curve are revealed and examined. We prove that the bifurcation force in a superelastic indentation curve only relies on the forward transformation stress and the elastic properties of the initial austenite; and the return force in a superelastic indentation curve only relies on the reverse transformation stress and the elastic properties of the initial austenite. Furthermore, the dimensionless functions to determine the bifurcation force and the return force are proved to be identical. These results not only enhance our understanding of spherical indentation of superelastic shape memory alloys, but also provide the theoretical basis for developing a practicable method to calibrate the mechanical properties of a superelastic material from the spherical indentation test. [Copyright &y& Elsevier]

Abstract: Using dimensional analysis and the finite element method, the spherical indentation hardness of shape memory alloys (SMAs) is investigated. The scaling relationship between the hardness and the mechanical properties of a SMA, such as the forward transformation stress, the maximum transformation strain magnitude, has been derived. Numerical results demonstrated that the hardness increases with the indentation depth but there is no three-fold relationship between the hardness and the forward transformation stress. Increasing the maximum transformation strain magnitude would reduce the hardness of the material. These research results enhance our understanding of the hardness from the spherical indentation of SMAs. [Copyright &y& Elsevier]

Abstract: The excellent microwear performance of nano-grained superelastic nickel titanium (NiTi) polycrystalline shape memory alloy (SMA) is reported in this paper. The microwear test was conducted at temperatures ranging from 22 to 120°C by a Hysitron triboindenter. The results showed that the NiTi SMA has superior microwear resistance compared to traditional tribo-materials such as stainless steel AISI 304 and that the material exhibits unusual hardness dependence of wear within certain temperature regimes. With the increase in temperature from 22 to 120°C, wear resistance was found to decrease anomalously with an increase in hardness. Further investigation and analysis confirmed that the stress-induced phase transition during contact and wear play an essential role in the material''s high wear resistance. It is demonstrated through contact mechanics analysis that the increase of hardness with temperature was mainly due to the increase in the phase transition stress. The observed applied threshold load that corresponds to the onset of the plastic deformation in the contact area was strongly influenced by the phase transition process at the tip region. For the investigated superelastic NiTi, the temperature-dependent interplay between reversible phase transition and irreversible plastic yielding plays a key role in the temperature dependence of the wear performance and is responsible for the observed apparent unusual hardness–wear relationships. [Copyright &y& Elsevier]

The effect of one-step overstressed compressive plastic deformation on functional fatigue of polycrystalline NiTi is investigated. Cuboidal micropillars are subjected to one-step plastic deformation at 1.8 GPa with residual strain of 3.5% and then cyclically compressed at 1 GPa for 106 cycles. Compared with as-received micropillars, the total residual strain, transition stress and hysteresis loop area of the plastically-deformed micropillar are reduced by 74%, 52% and 67%. Microstructure analysis reveals that the observed changes originate from saturated dislocation structures and residual nanosized martensite in the one-step plastic deformation. The former suppresses further dislocation formation in the subsequent cyclic phase transition and thus enhances the cyclic stability, while the latter reduces the overall transition stress and the hysteresis dissipation through the created residual stress and the direct martensite growth. The proposed method is simple and effective in reducing the functional fatigue and enhancing the cooling performance of NiTi for elastocaloric refrigeration. [Display omitted] [ABSTRACT FROM AUTHOR]

In this study, integrated experimental tests and computational modeling are proposed to investigate the failure mechanisms of open-hole cross-ply carbon fiber reinforced polymer (CFRP) laminated composites. In particular, we propose two effective methods, which include width-tapered double cantilever beam (WTDCB) and fixed-ratio mixed-mode end load split (FRMMELS) tests, to obtain the experimental data more reliably. We then calibrate the traction-separation laws of cohesive zone model (CZM) used among laminas of the composites by leveraging these two methods. The experimental results of fracture energy, i.e. G Ic and G Tc , obtained from WTDCB and FRMMELS tests are generally insensitive to the crack length thus requiring no effort to accurately measure the crack tip. Moreover, FRMMELS sample contains a fixed mixed-mode ratio of G IIc / G Tc depending on the width taper ratio. Examining comparisons between experimental results of FRMMELS tests and failure surface of B–K failure criterion predicted from a curve fitting, good agreement between the predictions and experimental data has been found, indicating that FRMMELS tests are an effective method to determine mixed-mode fracture criterion. In addition, a coupled experimental-computational modeling of WTDCB, edge notched flexure, and FRMMELS tests are adopted to calibrate and validate the interfacial strengths. Finally, failure mechanisms of open-hole cross-ply CFRP laminates under flexural loading have been studied systematically using experimental and multi-scale computational analyses based on the developed CZM model. The initiation and propagation of delamination, the failure of laminated layers as well as load-displacement curves predicted from computational analyses are in good agreement with what we have observed experimentally. [ABSTRACT FROM AUTHOR]

In this work, multi-scale finite element analyses based on three-dimensional (3D) hybrid macro/micro-scale computational models subjected to various loading conditions are carried out to examine the in-situ effect imposed by the neighboring plies on the failure initiation and propagation of cross-ply laminates. A detailed comparative study on crack suppression mechanisms due to the effect of embedded laminar thickness and adjacent ply orientation is presented. Furthermore, we compare the results of in-situ transverse failure strain and strength between the computational models and analytical predictions. Good agreements are generally observed, indicating the constructed computational models are highly accurate to quantify the in-situ effect. Subsequently, empirical formulas for calculating the in-situ strengths as a function of embedded ply thickness and different ply angle between embedded and adjacent plies are developed, during which several material parameters are obtained using a reverse fitting method. Finally, a new set of failure criteria for σ 22 - τ 12 , σ 22 - τ 23 , and σ 11 - τ 12 accounting for the in-situ strengths are proposed to predict laminated composites failure under multi-axial stress states. This study demonstrates an effective and efficient computational technique towards the accurate prediction of the failure behaviors and strengths of cross-ply laminates by including the in-situ effects. [ABSTRACT FROM AUTHOR]

The mechanical behaviors and damage evolutions of carbon/epoxy woven fabric composites with three different geometries, i.e., one plain weave and two twill weave patterns with different areal densities, are studied under tensile loading. The effects of weave patterns on mechanical properties are investigated by monotonic and cyclic tension tests. Remarkable variations in stress–strain curve, Poisson's ratio, residual strain and strain map exist in the three composites. Crimp ratio is found to be a critical factor to govern the mechanical properties. With smaller crimp ratio, a quasi-linear stress–strain curve with higher elastic modulus and strength is observed. The stress–strain curves of composites with higher crimp ratio contain transition stages with significant tangent modulus degradation. Elastic modulus, strength and damage initiation are all correlated with the crimp ratio linearly regardless of the fabric pattern. Dramatic nonlinear evolution in Poisson's ratio occurs in the composite with higher crimp ratio. Cyclic tension results indicate that the residual strain is a more appropriate damage indicator than the unloading elastic modulus. Microstructure examination shows that damage developments are essentially related to the fabric geometry, and result in various mechanical behaviors. This study provides important insights into the geometry-deformation mechanism-mechanical property relationship of the woven composites. [ABSTRACT FROM AUTHOR]

A bottom-up multi-scale modeling approach is used to develop an Integrated Computational Materials Engineering (ICME) framework for carbon fiber reinforced polymer (CFRP) composites, which has the potential to reduce development to deployment lead time for structural applications in lightweight vehicles. In this work, we develop and integrate computational models comprising of four size scales to fully describe and characterize three types of CFRP composites. In detail, the properties of the interphase region are determined by an analytical gradient model and molecular dynamics analysis at the nano-scale, which is then incorporated into micro-scale unidirectional (UD) representative volume element (RVE) models to characterize the failure strengths and envelopes of UD CFRP composites. Then, the results are leveraged to propose an elasto-plastic-damage constitutive law for UD composites to study the fiber tows of woven composites as well as the chips of sheet molding compound (SMC) composites. Subsequently, the failure mechanisms and failure strengths of woven and SMC composites are predicted by the meso-scale RVE models. Finally, building upon the models and results from lower scales, we show that a homogenized macro-scale model can capture the mechanical performance of a hat-section-shaped part under four-point bending. Along with the model integration, we will also demonstrate that the computational results are in good agreement with experiments conducted at different scales. The present study illustrates the potential and significance of integrated multi-scale computational modeling tools that can virtually evaluate the performance of CFRP composites and provide design guidance for CFRP composites used in structural applications. Image 1 [ABSTRACT FROM AUTHOR]

• Efficient RVE generation method for woven composite with consistent mesh and realistic geometry • Elasto-plastic-damage model with tension-compression asymmetry for yarn material • Significant different behaviors including stress-strain curve and Poisson's ratio in tension and compression. • Different damage evolutions affect the yarn deformation and thus the mechanical behaviors. The mechanical properties and damage behaviors of carbon/epoxy woven fabric composite under in-plane tension and compression are studied at the meso-scale level through experiment and simulation. An efficient representative volume element (RVE) modeling method with consistent mesh, high yarn volume fraction and realistic geometry is proposed. The material constitutive laws with plasticity, tension-compression asymmetry and damage evolution are established for the three components - yarn, matrix and interface, respectively. Significantly different mechanical properties and damage evolutions are observed depending on loading conditions and initial geometry characteristics. It shows a non-linear stress-strain curve with clear transition region and intensive damage in tension, while a quasi-linear behavior up to facture is observed in compression with little damage prior to final fracture. Moreover, compared to the constant Poisson's ratio with straining in compression, a dramatic increase in Poisson's ratio appears in tension. Simulation shows damage mechanisms including transverse damage, matrix damage and delamination, which all play critical roles in the property evolution. In particular, the rapid damage accumulation after elastic deformation destroys the strong bonds and causes the easy deformation of transverse yarns which results in the transition region and large Poisson's ratio in tension. All the mechanical behaviors and damage evolutions are well captured and explained with the current RVE model. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Abstract: The indentation slope curve from a spherical indentation on elastic-plastic materials is examined. By comparing it with that of an linear elastic material of the same elastic properties, we found that the start point of plastic yielding for an elastic-plastic material can be easily located from the indentation slope curve. Based on this analysis, a simple but effective method is proposed to measure the plastic yield stress of very small samples from a spherical nano-indentation slope curve. [Copyright &y& Elsevier]

The mechanical properties and damage evolutions of carbon/epoxy woven fabric composites with three different fabric architectures, including one plain weave and two twill weave patterns, are experimentally investigated under multiaxial stress states. In particular, the effects of weave patterns are investigated by monotonic and cyclic off-axis tension tests. Both elastic modulus and strength degrade remarkably with increasing off-axis loading angle, while Poisson's ratio is much higher than that measured from on-axis tests and increases with loading strain gradually. Different fabric architectures show limited effects on the modulus and strength under multiaxial stress states, and they are well predicted by transformation equation and Tsai-Wu failure criteria, respectively. However, significantly different failure behaviors are observed in three fabric composites, and microstructure observation shows that fabric architecture affects the stress concentration and the damage development. Smaller crimp ratio and compacted structure postpone the damage development but result in more abrupt failure under multiaxial stress states. • Fabric architecture's effects on properties of woven composite are analyzed under multiaxial stress states. • With resin being the main loading carrier, significant different behaviors are observed in off-axis tension. • Different fabric architectures have limited effects on mechanical properties, especially the elastic modulus. • Fabric architecture determines the stress concentration and results in remarkable variations in damage development. [ABSTRACT FROM AUTHOR]

• Dynamic crushing characteristics of UD CFRP composites are investigated under two loading types, i.e. dynamic three-point bending and axial crushing. • Crushing processes with different layup orientations and velocities are compared by high speed camera and ultrasonic scan. • A finite element model is established to analyze the contributions of different mechanisms to energy absorption at different conditions. The dynamic crushing characteristics of unidirectional carbon fiber reinforced plastic composites under two loading types, dynamic three-point bending and axial crushing, are investigated by experiment and finite element simulation. Hat section samples with two different layup orientations are tested at various impact velocities to investigate its effects. The experiment results show that delamination plays a critical role in dynamic bending deformation. Different layup orientations lead to similar crushing bending behaviors, but remarkable variations in delamination pattern evolutions. In contrast, layup orientation exerts more significant effects on failure modes and energy absorption in axial crushing. A finite element model with multiple layers of thick shell element and cohesive element is established, and simulations are performed for both dynamic three-point bending and axial crushing. The model can capture the crushing behaviors in the two loading types reasonably well. The damage evolution and energy absorption through various mechanisms at different loading conditions are discussed. [ABSTRACT FROM AUTHOR]

Deformation behaviors of the gradient nanostructured superelastic NiTi shape memory alloy strip with grain size ranging from 20 to 175 nm along the tensile axis are investigated. Compared with the commercial NiTi of uniform grain size, the gradient nanostructure leads to the increase of transformation stress as well as the morphology change, from localized deformation to homogeneous one. However, the behaviors in local different grain size regions are independent and maintain the grain-size dependent characteristics. [ABSTRACT FROM AUTHOR]

The spatiotemporal evolution of macro-domain patterns in a NiTi thin strip during stress-induced non-isothermal phase transformation (PT) are investigated through 2D FEM simulations using nonconvex nonlocal continuum model and 1D theoretical analysis. The FEM simulations shows that, with the increase of applied stretching rate (nominal strain rate), the way of PT in a thin strip (30 mm × 2.5 mm × 0.5 mm) changes from the sequential nucleation and growth mode in the low rates range (≤ 0.03 / s) to the emergence of periodic patterns in the medium rates range (0.03/ s ~3.0/ s) and eventually to stable homogeneous deformation mode in the high rates range (≥ 3.0 / s). The domain spacing is governed by a power-law scaling to the strain rate, where the exponent changes from −1/2 (low rates range) to −1/6 (medium rates range). The 1D theoretical analysis further demonstrates that the rate-dependent patterns are essentially governed by a nondimensional internal material parameter A - 0 and a nondimensional external driving parameter k -. A - 0 represents the ratio of adiabatic hardening to isothermal softening in PT of the material, while k - controls the time scale competition of heat release and heat conduction, through which the thermodynamic condition of PT changes from near isothermal (k - → ∞) to near adiabatic (k - → 0) with the increase of the strain rate. It is shown that, as long as the material has the properties of isothermal softening and adiabatic hardening (i.e., A - 0 > 1), the conventional nucleation-growth paradigm of PT will eventually break down for small k - (or large stain rate). This is due to the fact that the increase of the energy barrier caused by the thermal and the interfacial effects can convexify the energy landscape of the system near the adiabatic condition and suppress the nucleation events at the macroscopic level. [ABSTRACT FROM AUTHOR]

The grain size (GS) dependence of fatigue crack growth in nanocrystalline superelastic NiTi shape memory alloys (SMAs) with the average GS of 10, 30, 60 and 235 nm is investigated. Synchronized measurements of the fatigue crack growth, load-displacement response and temperature field of the compact tension specimens are performed. It is found that the fatigue crack growth rate (d a /d N) is monotonically decreased by 10 times when the GS is increased from 10 to 235 nm. The significant GS effect on the fatigue crack growth is also confirmed by the direct observation of fatigue striation spacing on the postmortem fracture surface. It is shown that the increase of the GS significantly enhances the phase transition and plasticity of the material and therefore enhances the crack-tip shielding (K sh) which reduces the effective driving force (Δ K eff) of the fatigue crack growth. Subtracting K sh due to phase transition and plastic deformation from the external applied K app , the effective crack-tip fatigue driving force Δ K eff is obtained. The d a /d N -Δ K eff curves for different GS specimens then almost collapse onto a single curve which may represent an intrinsic fatigue property of the NiTi in the absence of the shielding effects. Therefore the observed apparent GS dependence of the fatigue crack growth resistance in the superelastic NiTi SMAs is mainly caused by the shielding effect from the GS dependent phase transition and plastic deformation. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Materials with unusual large negative and positive thermal expansions are attractive for applications in high-precision actuation and thermal expansion compensation. So far, such unusual property has been reported in a variety of framework materials and polymers but it remains challenging to obtain in metals. Here, we report a colossal linear thermal expansion in single-phase nanocrystalline NiTi ferroelastic alloy under uniaxial compressive stress. By microscale isothermal severe plastic deformation, we fabricated fully austenitic and martensitic NiTi nanocrystalline micropillars that respectively possess high yield stresses of 2.65 GPa and 2.23 GPa and strong negative, and positive temperature dependence of Young′s modulus (d E /d T). We demonstrate that due to the large d E /d T , the coefficient of linear thermal expansion (CTE) of the austenitic and martensitic NiTi micropillars can be tailored to values of +106×10−6 K–1 and -88×10−6 K–1 by applying 2.5 GPa and 2.0 GPa compressive stress. Such tailored CTEs are approximately 10 times larger than the typical value (∼10×10−6 K–1) of metallic materials. Our work provides a mechanical route to obtain colossal positive and negative thermal expansion in single-phase nanostructured ferroelastic alloys at microscale. [Display omitted] [ABSTRACT FROM AUTHOR]

Nonlinear dynamics of vibration systems containing NiTi shape memory alloy (SMA) bars has long been obscured by the lack of an analytical solution, like the analytical solution for duffing equation. The problem results from the nonlinear and hysteretic restoring force of the SMA bar. Here we use a piecewise linear hysteretic model to describe the force-displacement relation of the SMA bar and use the averaging method to solve the equation of motion. We thus obtain an approximate analytical solution of the steady-state response of an SMA mass-spring system. The analytical solution can describe both stable and unstable behaviors of the vibration system and therefore offer a comprehensive understanding of the nonlinear responses. It is shown that the phase transition induced softening nonlinearity bends the frequency response curve (FRC) to the left, while the subsequent rehardening of martensite further bends the FRC to the right, leading to multi-valued regions and jump phenomena. The hysteresis is found to have little influence on the bending but it can significantly suppress the response amplitude. Comparison of the analytical results with experimental data validates the piecewise linear hysteretic model and the analytical solutions. This work provides a theoretical tool for design and vibration control of SMA mass-spring systems. [ABSTRACT FROM AUTHOR]

Abstract Superelastic NiTi shape memory alloy (SMA) cuboidal micropillars (1 μm × 1 μm × 3 μm) with average grain sizes (GS) of 26 nm, 127 nm and 421 nm are compressed by flat-tip nanoindentation. Increasing GS from 26 nm to 127 nm enhances the transformation by promoting nucleation and growth of martensite. Nevertheless, by further increasing GS to 421 nm, the overall transformation is significantly suppressed, leading to a non-monotonic variation in transformation stress, recoverable strain and hysteresis loop area with GS. Such strong effects of GS on microscale phase transformation behavior of SMA are quantitatively elucidated by a simple three-length-scale model in this paper. Graphical abstract Unlabelled Image [ABSTRACT FROM AUTHOR]

Abstract We investigate the cyclic compressive fatigue behavior of pseudoelastic polycrystalline NiTi shape memory alloy (SMA) under maximum applied stresses of 800, 950 and 1200 MPa. A fatigue life of over 20 million cycles under 800 MPa is recorded with an average coefficient of performance (COP) of 17.6. Rock-like splitting and chipping through nucleation and slow stable growth of compression-parallel cracks driven by very small stress intensity factor are the major fatigue failure modes of the material. The high fatigue life and cooling efficiency of NiTi SMA under cyclic compressive phase transformation provide a solid mechanical base for its application in solid-state cooling technology. [ABSTRACT FROM AUTHOR]

Microscale plastic deformation mechanisms of nanocrystalline NiTi micropillars with an average grain size (GS) of 65 nm are investigated by uniaxial compression. Cuboidal NiTi micropillars with uniform cross-sectional area along the height are manufactured by focused ion beam. It is found that the plastic deformation of the micropillars under compression is realized by localized shear band formation with large strain bursts, leading to significant amorphization and GS reduction from 65 nm to as small as 11 nm in the shear band. The experiment indicates that microfabrication by severe plastic deformation could be a promising manufacturing technology. [ABSTRACT FROM AUTHOR]

In this paper, grain size (GS) dependence of Young’s modulus and hardness for nanocrystalline NiTi shape memory alloy is investigated by experiments. Amorphous NiTi with nanocrystalline debris is fabricated via cold-rolling and polycrystalline NiTi with average GS from 10 nm to 120 nm is obtained by subsequent annealing. Young’s modulus and hardness of nanocrystalline NiTi are quantified by macroscopic isothermal tension and microscopic nanoindentation. It is shown that Young’s modulus of nanocrystalline NiTi first decreases (for GS < 62 nm) and then increases (for GS > 62 nm) with GS in the nano-scale region. The non-monotonic GS dependence of Young’s modulus originates from the combined effects of grain size and volume fractions of austenite, martensite and amorphous phase in the material. It is also shown that with the increase of GS up to 120 nm, hardness of nanocrystalline NiTi monotonically decreases due to the reduced nominal phase transition stress and plastic yielding stress. Such GS dependence of hardness can be utilized for rapid determination of GS in nanocrystalline NiTi via nanoindentation hardness test. [ABSTRACT FROM AUTHOR]

Stress hysteresis (H) and temperature dependence of phase transition stress (dσ/dT) are the two signatures of first-order phase transition in shape memory alloys. We studied the effects of grain size on these two properties in polycrystalline superelastic NiTi with the average grain size from 10 nm to 1500 nm. We identified a critical grain size (∼60 nm) below which both H and dσ/dT rapidly decrease, leading to vanishing hysteresis and breakdown of Clausius-Clapeyron equation. The physics behind such grain size effects are the dominance of interfacial energy in the energetics of the polycrystal and the lack of two-phase coexistence at nano-scales. [ABSTRACT FROM AUTHOR]

The strength of polycrystal increases as the grain diameter l decreases, i.e. the Hall–Petch behaviour. This trend reverses at about 3 < l < 15 nm, i.e. the inverse-Hall–Petch behaviour. How the grain size affects material's strength at l < 3 nm (~12 particles) remains unclear. Here our simulations use mixtures of soft and hard particles so that compression can continuously reduce l to merely a few particles, resulting in ultrafine-grained solids termed as glass-crystal composites. Beyond the conventional Hall–Petch strengthening and inverse-Hall–Petch softening, we observe a power-law strengthening at l < 14 particles as a result of the blockage of shear-banding by crystalline grains. Amorphous and crystalline regions accommodate shear strains via bond-breaking and collective rotation, respectively. Moreover, a polycrystal–glass transition occurs at l = 14 particles featured with peaks of various quantities, which deepens the understanding on softening–strengthening transition. Hall-Petch behaviour dictates that with a reduction in grain size the strength of a polycrystalline material increases. Here, the authors model glass-crystal composites of soft and hard particles uncovering a power-law strengthening behaviour beyond the Hall-Petch and inverse-Hall-Petch regimes when grain size is reduced to below 3 nm. [ABSTRACT FROM AUTHOR]

In this article, the elastocaloric effect of a commercial superelastic NiTi shape memory alloy (SMA) tube (with an outer diameter of 5 mm and wall thickness of 1 mm) to be applied in a compressive cooler was measured and analyzed. The elastocaloric effect of the tube was measured vs the applied strain and strain rate. The largest temperature changes of 21 K during loading and 16 K during unloading were measured at an applied strain of 3.30% and strain rate of 0.33 s−1. In the fatigue testing of the sample, only 0.20% of the residual strain accumulated after a runout of 1 × 106 sinusoidal force-controlled loading–unloading cycles at a maximum compressive stress of 1100 MPa and frequency of 20 Hz. Numerical results of the cooling characteristics of a compressive device using a single NiTi tube with the above-mentioned cross section and an aspect ratio of 60:1 as the refrigerant showed that the device could produce a total cooling power of up to 20 W and a coefficient of performance of up to 6.5. The results of this article demonstrate that superelastic NiTi SMA tubes of suitable wall thickness and aspect ratios are good candidates to be applied in a compressive elastocaloric cooler. [ABSTRACT FROM AUTHOR]

The measurement of residual stress is a challenging issue in industrial fields. In this work, focused ion beam (FIB) and digital image correlation (DIC) are combined together to measure the two-dimensional residual stress in nanocrystalline NiTi plates processed with pre-strain laser shock peening (LSP). A four-point bending experiment verifies the accuracy of this measurement method. The pre-strain LSP-treated surfaces are found to have significant compressive residual stress along the pre-strain direction and tensile residual stress along the vertical to pre-strain direction, which verifies pre-strain LSP as an efficient approach to create gradient residual stress layers in nanocrystalline NiTi plates. The FIB-DIC method has also proven to be an attractive tool to measure the two-dimensional residual stress in phase transition nanocrystalline materials. [ABSTRACT FROM AUTHOR]

Near-infrared (NIR) spectral analysis, which has the advantages of rapidness, nondestruction and high-efficiency, is widely used in the detection of feed, food and mineral. In terms of qualitative identification, it can also be used for the discriminant analysis of medicines. Long short-term memory (LSTM) neural network, bidirectional long short-term memory (BiLSTM) neural network and gated recurrent unit (GRU) network are variants of the recurrent neural network (RNN). The potential relationship between nonlinear features learned from the sequence by these variants is used to complete the missions in fields such as natural language processing, signal classification and video analysis. Since the effect of these variants in drug identification is still to be studied, this paper constructs a multiclassifier of these three variants, using compound α -keto acid tablets produced by four manufacturers and repaglinide tablets produced by five manufacturers as the research object. Then, the paper analyzes the impacts of seven different pre-processed methods on the drug NIR data by constructing different layers of LSTM, BiLSTM and GRU networks and compares different classification model indicators and training time of each model. When the spectrum data are pre-processed by z -score normalization, the GRU-3 model has the best accuracy in all models. The BiLSTM models are better for analyzing high coincidence data. The method proposed in this paper can be further extended to other NIR spectroscopy data sets. [ABSTRACT FROM AUTHOR]

We fabricate three polycrystalline superelastic NiTi samples with average grain sizes of 110, 60, 35 nm and one crystalline-amorphous NiTi nanocomposite sample to investigate their elastocaloric cooling performance under compression. It is found that grain refinement and partial amorphization can notably improve coefficient of performance (COP) and cyclic stability with decrease in adiabatic temperature change (ΔT). The decrease in ΔT is caused by the decrease in austenite and reduction in transformation in small grains. The increase in COP results from the change from discontinuous to continuous phase transformation. The enhancement in cyclic stability stems from suppression of dislocations. Of the four samples, the 35 nm-grain-size one demonstrates the best elastocaloric cooling performance where ΔT reaches 19 °C and COP increases from 7 to 19 in 104 cycles under 1200 MPa. The study shows that the elastocaloric cooling performance of NiTi can be enhanced via grain refinement through controlled thermomechanical processing. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract: Nanofretting refers to cyclic movements of contact interfaces with the relative displacement amplitude at the nanometer scale, where the contact area and normal load are usually much smaller than those in fretting. Nanofretting widely exists in microelctromechanical systems (MEMS) and may become a key tribological concern besides microwear and adhesion. With a triboindenter, the nanofretting behaviors of a nickel titanium (NiTi) shape memory alloy are studied under various normal loads (1–10mN) and tangential displacement amplitudes (2–500nm) by using a spherical diamond tip. Similar to fretting, the nanofretting of NiTi/diamond pair can also be divided into different regimes upon various shapes of tangential force–displacement curves. The dependence of nanofretting regime on the normal load and the displacement amplitude can be summarized in a running condition nanofretting map. However, due to the surface and size effects, nanofretting operates at some different conditions, such as improved mechanical properties of materials at the nanometer scale, small apparent contact area and single-asperity contact behavior. Consequently, different from fretting, nanofretting was found to exhibit several unique behaviors: (i) the maximum tangential force in one cycle is almost unchanged during a nanofretting test, which is different from a fretting test where the maximum tangential force increases rapidly in the first dozens of cycles; (ii) the tangential stiffness in nanofretting is three orders magnitude smaller than that in fretting; (iii) the friction coefficient in nanofretting is much lower than that in fretting in slip regime; (iv) no obvious damage was observed after 50 cycles of nanofretting under a normal load of 10mN. [Copyright &y& Elsevier]

Abstract: Fretting wear tests of a NiTi shape memory alloy plate against a GCr15 steel ball (NiTi/GCr15) were performed on a horizontal servo-hydraulic fretting test machine at various temperatures. As a comparison, fretting wear tests of a GCr15 steel plate against a GCr15 steel ball were also conducted under the same experimental conditions. The fretting behavior of GCr15 and NiTi alloy was studied by analyzing both the frictional logs (variation of the tangential force F t and displacement D with the number of fretting cycles N) and fretting scar features. The experiments showed that NiTi exhibits much better fretting behavior than GCr15 steel even with a relatively low hardness at room temperature. With an increase in temperature from 22 to 200°C, while the fretting behavior of GCr15 is almost temperature independent, the range of the partial slip regime of NiTi/GCr15 becomes small and the fretting wear resistance of NiTi is found to anomalously decrease with an increase in nominal hardness. It was demonstrated that the unique temperature dependent phase transition properties of NiTi play a key role in its fretting behavior. The stress-induced phase transition not only greatly improves the elastic accommodation ability of NiTi through the large recoverable deformation, but also increases its fretting wear resistance through the transformation shielding effect. With the increasing temperature, the elastic accommodation ability of NiTi decreases not only because of the increase of austenite elastic modulus and phase transition stress but also the plastic deformation of austenite, which in turn causes a small range of the partial slip regime in the NiTi/GCr15 pair. As the temperature increases, the decrease in fretting wear resistance is attributed to the decrease in the elastic accommodation ability of NiTi and the plastic yield of austenite before phase transition at high temperatures. [Copyright &y& Elsevier]

Discusses RNA editing in plant mitochondria and the formation of functional proteins. Association of RNA editing and cytoplasmic male sterility (CMS), which is linked to mitochondrial genome mutations; Materials and methods used in studying CMS rice; Editing sites found in transcripts of atp6 and coxil by comparing sequences of cDNAs and DNAs; Implications of RNA editing for hydrophobicity.

Focuses on the discovery of mitochondrial chimeric-gene associated with cytoplasmic male sterility of HL-rice. Construction of the mitochondrial genome libraries of HL-type sterile line and maintainer line; Use of the mitochondrial gene atp6 to screen libraries; Presence of two copies of atp6 gene in sterile line and one in maintainer line.

With a better balance among good mechanical performance, high freedom of design, and low material and manufacturing cost, chopped carbon fiber chip reinforced sheet molding compound (SMC) composites show great potential in different engineering applications. In this paper, bending fatigue behaviors of SMC composites considering the heterogeneous fiber orientation distributions have been thoroughly investigated utilizing both experimental and computational methods. First, four-point bending fatigue tests are performed with designed SMC composites, and the local modulus is adopted as a metric to represent the local fiber orientation of two opposing sides. Interestingly, SMC composites with and without large discrepancy in local modulus of opposing sides show different fatigue behaviors. Interrupted tests are conducted to explore the bending fatigue failure mechanism, and the damage processes of valid specimens are also closely examined. We find that the fatigue failure of SMC composites under four-point bending is governed by crack propagation instead of crack initiation. Because of this, the heterogeneous local fiber orientations of both sides of the specimen influence fatigue life. The microstructure of the lower side shows a direct influence while that of the upper side also exhibiting influence which becomes more prominent for high cycle fatigue cases. Furthermore, a hybrid micro–macro computational model is proposed to efficiently study the cyclic bending behavior of SMC composites. The region of interest is reconstructed with a modified random sequential absorption algorithm to conserve all the microstructural details including the heterogeneous fiber orientation, while the rest of the regions are modeled as homogenized macro-scale continua. Combined with a framework to capture the progressive fatigue damage under cyclic bending, the bending fatigue behaviors of SMC composites are accurately captured by the hybrid computational model comparing with our experimental analysis. [ABSTRACT FROM AUTHOR]