Fatigue Crack Initiation Potential from Defects in terms of Local Stress Analysis

The competition of surface and subsurface crack initiation induced failure is critical to understand very high cycle fatigue（VHCF） behavior, which necessitates the elucidation of the underlying mechanisms for the transition of crack initiation from surface to interior defects. Crack initiation potential in materials containing defects is investigated numerically by focusing on defect types, size, shape, location, and residual stress influences. Results show that the crack initiation potency is higher in case of serious property mismatching between matrix and defects, and higher strength materials are more sensitive to soft inclusions（elastic modulus lower than the matrix）. The stress localization around inclusions are correlated to interior crack initiation mechanisms in the VHCF regime such as inclusion-matrix debonding at soft inclusions and inclusion-cracking for hard inclusions（elastic modulus higher than the matrix）. It is easier to emanate cracks from the subsurface pores with the depth 0.7 times as large as their diameter. There exists an inclusion size independent region for crack incubation, outside which crack initiation will transfer from the subsurface soft inclusion to the interior larger one. As for elliptical inclusions, reducing the short-axis length can decrease the crack nucleation potential and promote the interior crack formation, whereas the long-axis length controls the site of peak stress concentration. The compressive residual stress at surface is helpful to shift crack initiation from surface to interior inclusions. Some relaxation of residual stress can not change the inherent crack initiation from interior inclusions in the VHCF regime. The work reveals the crack initiation potential and the transition among various defects under the influences of both intrinsic and extrinsic factors in the VHCF regime, and is helpful to understand the failure mechanism of materials containing defects under long-term cyclic loadings.

Extended Kantorovich method for local stresses in composite laminates upon polynomial stress functions

The extended Kantorovich method is employed to study the local stress concentrations at the vicinity of free edges in symmetrically layered composite laminates subjected to uniaxial tensile load upon polynomial stress functions. The stress fields are initially assumed by means of the Lekhnitskii stress functions under the plane strain state. Applying the principle of complementary virtual work,the coupled ordinary differential equations are obtained in which the solutions can be obtained by solving a generalized eigenvalue problem. Then an iterative procedure is established to achieve convergent stress distributions. It should be noted that the stress function based extended Kantorovich method can satisfy both the traction-free and free edge stress boundary conditions during the iterative processes. The stress components near the free edges and in the interior regions are calculated and compared with those obtained results by finite element method（FEM）. The convergent stresses have good agreements with those results obtained by three dimensional（3D） FEM. For generality, various layup configurations are considered for the numerical analysis. The results show that the proposed polynomial stress function based extended Kantorovich method is accurate and efficient in predicting the local stresses in composite laminates and computationally much more efficient than the 3D FEM.

Local Stress Field in Wafer Thinning Simulations with Phase Space Averaging

From an ingot to a wafer then to a die,wafer thinning plays an important role in the semiconductor industry.To reveal the material removal mechanism of semiconductor at nanoscale,molecular dynamics has been widely used to investigate the grinding process.However,most simulation analyses were conducted with a single phase space trajectory,which is stochastic and subjective.In this paper,the stress field in wafer thinning simulations of 4H-SiC was obtained from 50 trajectories with spatial averaging and phase space averaging.The spatial averaging was conducted on a uniform spatial grid for each trajectory.A variable named mask was assigned to the spatial point to reconstruct the shape of the substrate.Different spatial averaging parameters were applied and compared.The result shows that the summation of Voronoi volumes of the atoms in the averaging domain is more appropriate for spatial averaging.The phase space averaging was conducted with multiple trajectories after spatial averaging.The stress field converges with increasing the number of trajectories.The maximum and average relative difference(absolute value)of Mises stress was used as the convergence criterion.The obtained hydrostatic stress in the compression zone is close to the phase transition pressure of 4H-SiC from first principle calculations.

Conditional Generative Adversarial Network Enabled Localized Stress Recovery of Periodic Composites

Structural damage in heterogeneousmaterials typically originates frommicrostructures where stress concentration occurs.Therefore,evaluating the magnitude and location of localized stress distributions within microstructures under external loading is crucial.Repeating unit cells(RUCs)are commonly used to represent microstructural details and homogenize the effective response of composites.This work develops a machine learning-based micromechanics tool to accurately predict the stress distributions of extracted RUCs.The locally exact homogenization theory efficiently generates the microstructural stresses of RUCs with a wide range of parameters,including volume fraction,fiber/matrix property ratio,fiber shapes,and loading direction.Subsequently,the conditional generative adversarial network(cGAN)is employed and constructed as a surrogate model to establish the statistical correlation between these parameters and the corresponding localized stresses.The stresses predicted by cGAN are validated against the remaining true data not used for training,showing good agreement.This work demonstrates that the cGAN-based micromechanics tool effectively captures the local responses of composite RUCs.It can be used for predicting potential crack initiations starting from microstructures and evaluating the effective behavior of periodic composites.

Coupling damage and reliability model of low-cycle fatigue and high energy impact based on the local stress–strain approach

Fatigue induced products generally bear fatigue loads accompanied by impact processes,which reduces their reliable life rapidly. This paper introduces a reliability assessment model based on a local stress–strain approach considering both low-cycle fatigue and high energy impact loads.Two coupling relationships between fatigue and impact are given with effects of an impact process on fatigue damage and effects of fatigue damage on impact performance. The analysis of the former modifies the fatigue parameters and the Manson–Coffin equation for fatigue life based on material theories. On the other hand, the latter proposes the coupling variables and the difference of fracture toughness caused by accumulative fatigue damage. To form an overall reliability model including both fatigue failure and impact failure, a competing risk model is developed. A case study of an actuator cylinder is given to validate this method.

WAVE LOCALIZATION IN RANDOMLY DISORDERED PERIODIC PIEZOELECTRIC RODS WITH INITIAL STRESS

The elastic wave localization in disordered periodic piezoelectric rods with initial stress is studied using the transfer matrix and Lyapunov exponent method. The electric field is approximated as quasi-static. The effects of the initial stress on the band gap characteristics are investigated. The numerical calculations of localization factors and localization lengths are performed. It can be observed from the results that the band structures can be tuned by exerting the suitable initial stress. For different values of the piezoelectric rod length and the elastic constant, the band structures and the localization phenomena are very different. Larger disorder degree can lead to more obvious localization phenomenon.

Effect of local wound infiltration with ropivacaine on postoperative pain relief and stress response reduction after open hepatectomy

AIM To prospectively evaluate the effect of local wound infiltration with ropivacaine on postoperative pain relief and stress response reduction after open hepatectomy.METHODS A total of 56 patients undergoing open hepatectomy were randomly divided into two groups:a ropivacaine group(wound infiltration with ropivacaine solution)and a control group(infiltration with isotonic saline solution).A visual analog scale(VAS)at rest and on movement was used to measure postoperative pain for the first 48 h after surgery.Mean arterial pressure(MAP),heart rate(HR),time to bowel recovery,length of hospitalization after surgery,cumulative sufentanil consumption,and incidence of nausea and vomiting were compared between the two groups.Surgical stress hormones(epinephrine,norepinephrine,and cortisol)were detected using enzyme-linked immunosorbent assay,and the results were compared. RESULTS VAS scores both at rest and on movement at 24 h and48 h were similar between the two groups.Significantly lower VAS scores were detected at 0,6,and 12 h in the ropivacaine group compared with the control group(P<0.05 for all).MAP was significantly lower at 6,12,and 24 h(P<0.05 for all);HR was significantly lower at 0,6,12,and 24 h(P<0.05 for all);time to bowel recovery and length of hospitalization after surgery(P<0.05 for both)were significantly shortened;and cumulative sufentanil consumption was significantly lower at 6,12,24,and 36 h(P<0.05 for all)in the ropivacaine group than in the control group,although the incidence of nausea and vomiting showed no significant difference between the two groups.The levels of epinephrine,norepinephrine,and cortisol were significantly lower in the ropivacaine group than in the control group at 24 and 48 h(P<0.01 for all). CONCLUSION Local wound infiltration with ropivacaine after open hepatectomy can improve postoperative pain relief,reduce surgical stress response,and accelerate postoperative recovery.

Localized corrosion of LY12CZ under stress in chloride media

Electrochemical methods were applied to investigate the corrosion behaviour of LY12CZ under applied stress in 3% sodium chloride aqueous solution. The experimental results indicate that LY12CZ shows two breakdown potentials on its polarization curve: one is related to the dissolution of the intermetallic particles and the other is related to the cracking of oxidation film and the dissolution of the matrix. When the stress is applied, it varies both breakdown potentials and makes the intermetallic particles and matrix dissolute at lower potentials. The relationship between the variation of the two breakdown potentials and applied stress was summarized. At the same time the influence of stress on the pitting and intergranular corrosion sensitivity of LY12CZ was also investigated. From the results it can be concluded that the stress can significantly affect the localized corrosion behaviour of LY12CZ in 3% NaCl solution.

ANALYSIS OF THE LOCALIZATION OF DAMAGE AND THE COMPLETE STRESS-STRAIN RELATION FOR MESOSCOPIC HETEROGENEOUS BRITTLE ROCK SUBJECTED TO COMPRESSIVE LOADS

A micromechanics-based model is established. The model takes the interaction among sliding cracks into account, and it is able to quantify the effect of various parameters on the localization condition of damage and deformation for brittle rock subjected to compressive loads. The closed-form explicit expression for the complete stress-strain relation of rock containing microcracks subjected to compressive loads was obtained. It is showed that the complete stress-strain relation includes linear elasticity,nonlinear hardening,rapid stress drop and strain softening.The behavior of rapid stress drop and strain softening is due to localization of deformation and damage. Theoretical predictions have shown to be consistent with the experimental results.

NUMERICAL ANALYSIS OF RESIDUAL STRESSES IN TITANIUM ALLOY DURING ELECTRON BEAM LOCAL POST-WELD HEAT TREATMENT

The distributions of temperature and residual stresses in thin plates of BT20titanium alloy are numerically analyzed by three-dimensional finite element software duringelectron beam welding and electron beam local post-weld heat treatment (EBLPWHT). Combined withnumerical calculating results, the effects of different EBLPWHT mode and parameters, including heattreating position, heating width and heating time, on the distribution of welding residual stressesare analyzed. The results show that, the residual tensile stresses in weld center can be largelydecreased when the weld is heat treated at back preface of the plate. The numerical results alsoindicated that the magnitude of the residual longitudinal stresses of the weld and the zone vicinityof the weld is decreased, and the range of the residual longitudinal stresses is increased alongwith the increase of heating width and heating time.

ANALYSIS OF THE LOCALIZATION OF DAMAGE AND THE COMPLETE (STRESS-STRAIN) RELATION FOR MESOSCOPIC HETEROGENEOUS ROCK UNDER UNIAXIAL TENSILE LOADING

The mechanical behavior of rock under uniaxial tensile loading is different from that of rock under compressive loads. A micromechanics-based model was proposed for mesoscopic heterogeneous brittle rock undergoing irreversible changes of their microscopic structures due to microcrack growth. The complete stress-strain relation including linear elasticity, nonlinear hardening,rapid stress drop and strain softening was obtained. The influence of all microcracks with different sizes and orientations were introduced into the constitutive relation by using the probability density function describing the distribution of orientations and the probability density function describing the distribution of sizes. The influence of Weibull distribution describing the distribution of orientations and Rayleigh function describing the distribution of sizes on the constitutive relation were researched. Theoretical predictions have shown to be consistent with the experimental results.

Application of the local approach to the fatigue assessment for welded joints

The fatigue behavior of welded structures is currently determined by means of recommendations defined in terms of S-N curve corresponding to the detail classes of welded joints without taking account of the actual geometry of the weld. A new fatigue strength assessment method based on Dang Van multiaxial fatigue limit criterion was introduced, which is named the local approach and presented by Institut de Soudure recently. The local approach has advantages in taking welding residual stresses and the geometry of the weld toe and weld root into consideration. The application of the local approach to the fatigue strength assessment of low carbon steel Q235B welded joints was studied. The fatigue tests and finite element analysis results show that the local approach parameters recommended by Institut de Soudure were incorrectly for low carbon steel Q235B welded joints. With aluminum alloy welded joints being used widely, the parameters of the local approach used for aluminum alloy welded joints were obtained and verified on bases of the fatigue tests and finite element analysis.

The nonlocal solution of two parallel cracks in functionally graded materials subjected to harmonic anti-plane shear waves

In this paper, the dynamic interaction of two parallel cracks in functionally graded materials （FGMs） is investigated by means of the non-local theory. To make the analysis tractable, the shear modulus and the material density are assumed to vary exponentially with the coordinate vertical to the crack. To reduce mathematical difficulties, a one-dimensional non-local kernel is used instead of a twodimensional one for the dynamic problem to obtain stress fields near the crack tips. By use of the Fourier transform, the problem can be solved with the help of two pairs of dual integral equations, in which the unknown variables are the jumps of displacements across the crack surfaces. To solve the dual integral equations, the jumps of displacements across the crack surfaces are expanded in a series of Jacobi polynomials. Unlike the classical elasticity solutions, it is found that no stress singularity is present at the crack tips. The non-local elastic solutions yield a finite hoop stress at the crack tips. The present result provides theoretical references helpful for evaluating relevant strength and preventing material failure of FGMs with initial cracks. The magnitude of the finite stress field depends on relevant parameters, such as the crack length, the distance between two parallel cracks, the parameter describing the FGMs, the frequency of the incident waves and the lattice parameter of materials.

Cloning,Localization and Expression Analysis of ZmHsf-like Gene in Zea mays

Using homology cloning method, a heat shock transcription factor （Hsf） like gene, ZmHsf-like, was cloned from maize （Zea mays） leaves. Sequence analyses showed that the open reading frame （ORF） of the gene ZmHsf-like is 1 404 bp long, encoding 467 amino acids. The sequence of amino acids encoded by ZmHsf-like contains the most conserved and typical DNA-binding domain of Hsf family. By bombardment into onion epidermis, we ifrstly found that the ZmHsf-like was subcellular-located in nucleus. NucPred analysis revealed there is a classic NLS of KKRR peptide in protein. Real-time PCR showed that ZmHsf-like gene expressed in leaves, stems and roots of maize seedlings under normal growth conditions, and the highest expression level was in roots, lower in leaves and the lowest in stems. The ZmHsf-like gene expression could be up-regulated by heat shock, PEG, ABA, and H2O2 in different degrees, among which the heat shock and ABA worked more efifciently. Obvious differences of the peak value and its corresponding time point of ZmHsf-like gene expression were observed among treatments. Experiments with inhibitor further suggested that the up-regulation ZmHsf-like gene expression of heat shock was H2O2-dependent while the induction of ZmHsf-like with PEG did not depend on the existence of H2O2. These results pointed out that ZmHsf-like gene probably regulates responsive reactions to abiotic stresses especially heat shock and drought through different signal transduction pathways.

Helical Wire Stress Analysis of Unbonded Flexible Riser Under Irregular Response

A helical wire is a critical component of an unbounded flexible riser prone to fatigue failure. The helical wire has been the focus of much research work in recent years because of the complex multilayer construction of the flexible riser. The present study establishes an analytical model for the axisymmetric and bending analyses of an unbonded flexible riser. The interlayer contact under axisymmetric loads in this model is modeled by setting radial dummy springs between adjacent layers. The contact pressure is constant during the bending response and applied to determine the slipping friction force per unit helical wire. The model tracks the axial stress around the angular position at each time step to calculate the axial force gradient, then compares the axial force gradient with the slipping friction force to judge the helical wire slipping region, which would be applied to determine the bending stiffness for the next time step. The proposed model is verified against the experimental data in the literature. The bending moment-curvature relationship under irregular response is also qualitatively discussed. The stress at the critical point of the helical wire is investigated based on the model by considering the local flexure. The results indicate that the present model can well simulate the bending stiffness variation during irregular response, which has significant effect on the stress of helical wire.

Transferability of Charpy Absorbed Energy to Fracture Toughness Based on Weibull Stress Criterion

The relationship between Charpy absorbed energy and the fracture toughness by means of the (crack tip opening displacement (CTOD)) method was analyzed based on the Weibull stress criterion. The Charpy absorbed energy and the fracture toughness were measured for the SN490B steel under the ductile-brittle transition temperature region. For the instrumented Charpy impact test, the curves between the loading point displacement and the load against time were recorded. The critical Weibull stress was taken as a fracture controlled parameter, and it could not be affected by the specimen configuration and the loading pattern based on the local approach. The parameters controlled brittle fracture are obtained from the Charpy absorbed energy results, then the fracture toughness for the compact tension (CT) specimen is predicted. It is found that the results predicted are in good agreement with the experimental. The fracture toughness could be evaluated by the Charpy absorbed energy, because the local approach gives a good description for the brittle fracture even though the Charpy impact specimen or the CT specimen is used for the given material.

锦屏一级水电站库区中小地震震源机制解与局部应力场特征

利用四川及邻区数字地震台网2013—2018年的波形资料,基于CAP和HASH方法反演得到锦屏一级水电站(以下简称锦屏水库)库区及周边M_(L)≥2.5地震的震源机制解,根据DRSSI方法计算得到库区局部应力场信息,结合该区域的地质资料,从震源机制参数和局部应力场变化等方面讨论了水库加卸载对该区域地震活动的影响.取得的主要认识如下:(1)库区地震活动类型以走滑型为主,不同加卸载阶段震源机制类型存在一定差异,库区蓄水前期震源机制类型呈现多样性,蓄水后期与区域背景场趋于一致;(2)库区应力场最大主应力方向以NNW-SSE向为主,分时段结果显示,蓄水后期局部应力场最大主应力方向由NNW向逐渐发生偏转至NW向;(3)库区周边的地质构造和岩性条件有利于库水的渗透,蓄水后重力载荷和流体渗透的共同作用导致岩体断层面库仑应力的变化,从而呈现出库区蓄水前后中小地震活跃程度的显著差异特征.

Expression, purification, and subcellular localization of phospholipase C in Dunaliella salina

Plants possess effective mechanisms to respond quickly to the external environment. Rapid activation of phosphatidylinositol-specific phospholipase C (PLC) enzymes occurs after a stimulus. The PLC in Dunaliella salina plays important roles in growth and stress responses. However, the molecular basis of PLC action in D. salina remains little understood. To gain insight into the potential biological functions of this enzyme, we cloned a phospholipase C gene from D. salina in a previous study, named DsPLC (GenBank No. KF573428). Here, we present the prokaryotic expression, purification, and characterization of the DsPLC gene. The entire coding region of DsPLC was inserted into an expression vector pET32a, and the DsPLC gene was successfully expressed in Escherichia coli. The DsPLC protein was purified and identified using a polyclonal antibody and western blotting. Expressing DsPLC fused with a green fluorescent protein (GFP) in onion showed that DsPLC-GFP was localized to the intracellular membrane. Quantitative real-time PCR analysis revealed that the relative expression of the DsPLC gene was induced significantly by 3.0-mol/L NaCl at 4 h. Our results support the importance of PLC enzymes in plant defense signaling. This study provides a basis for further functional studies of the DsPLC gene and for additional analysis of the potential roles of PLC enzymes in response to abiotic stress.

Analysis of damage localization for ductile metal in process of shear band propagation

Distribution of localized damage in shear band can’ t be predicted theoretically based on classical elastoplastic theory. The average damage variable in shear band was considered to be a non-local variable. Based on non-local theory, an analytical expression for the localized damage in strain-softening region of shear band in the process of shear band propagation was presented using boundary condition and symmetry of local damage variable, etc. The results show that dynamic shear softening modulus, dynamic shear strength and shear elastic modulus influence the distribution of the localized damage in shear band. Internal length of ductile metal only governs the thickness of shear band. In the strain-softening region of shear band, the local damage variable along shear band’s tangential and normal directions is non-linear and highly non-uniform. The non-uniformities in the normal and tangential directions of shear band stem from the interactions and interplaying among microstructures and the non-uniform distribution of shear stress, respectively. At the tail of the strain-softening region, the maximum value of local damage variable reaches 1. This means that material at this position fractures completely. At the tip of shear band and upper as well as lower boundaries, no damage occurs. Local damage variable increases as dynamic shear softening modulus decreases or shear elastic modulus increases, leading to difficulty in identification or detection of damage for less ductile metal material at higher strain rates.

STRESS RELAXATION AND RELIABILITY EVALUATION OF SODA-LIME GLASS

Stress relaxation of glass is a dualism effect, it often lead to strength degradation in strengthened glass, but on the other hand, it improves the reliability and stress-uniformity of glasses. In this work, stress relaxation of soda-lime glass was investigated using three-point bending tests at 400-560℃ which is near the brittle to ductile transition temperature, for enhancing the safety of glass productions and exploring the most economic anneal process. The experimental results show that the speed of stress relaxation increases but the ultimate stress decreases with increasing temperature. The stress uniformity of the glass samples before and after anneal was examined using spherical indentation at arranged testing points. It indicates that the scatter of the local strength measured by the Hertzian indentation is smaller in the anneal glass than in initial specimen, so that the estimated Weibull modulus for the anneal specimen is higher. Furthermore, the strength evaluation by Hertzian indentation and statistical analysis was presented.