缺口件表象疲劳极限的定量化分析

测定了淬火、高温回火35CrMo钢光滑试样和三种缺口试样的三点弯曲疲劳极限,并利用ANSYS有限元软件计算了缺口试样加载时缺口截面上的应力分布.用“疲劳源形成的微细观过程理论”对试验结果进行了分析,认为,疲劳源的形成虽然发生在个别薄弱晶粒内部,是其中位错往返运动及交互作用的结果,但还必须满足一定的形变协调条件和概率条件,因而必须形成由相当多晶粒组成的“细观屈服区”.对试验结果的分析表明,如取“细观屈服区”临界尺寸(xW)等于11个奥氏体晶粒平均直径,则计算得到的缺口试样的表象疲劳极限,与实测值相比,其综合误差最小.据此计算的三种缺口试样的表象疲劳极限,其误差都小于5％.该xW可认为是材料的特征参量.     

Analysis of cylindrical shells of nonuniform thickness using two-dimensional photoelastic models

An experimental method is described whereby symmetrically loaded cylinders of nonuniform thickness are analyzed using two-dimensional photoelastic models mounted on elastic foundations. The technique is most conveniently applied to ring-stiffened or notched cylinders. The particular model studied simulated a notched cylindrical pressure vessel which had been previously studied with three-dimensional photoelasticity. The stress-concentration factors at the base of the notch, found using both methods, showed excellent agreement. An analysis was also performed which allows estimation of the error involved when a beam-on-elastic-foundation model does not rigorously simulate a cylinder.     

Stress analysis of notched anisotropic finite plates under mechanical and hygrothermal loads

Summary Notch-induced stress concentrations in anisotropic composite materials depend on their directional material properties, especially for uniaxially reinforced composites with high-modulus fibres. The design of notched high-performance composites requires therefore a special proof of their notched strength, which includes the structural parameters of the fibre/matrix combination, fibre orientation and layer arrangement. The assessment of the effects of the finite outer boundary is of practical importance when dimensioning critical notched regions. An anisotropic plate with finite dimensions and a hole in its center will be used here to model stress concentrations. The calculation is based on conformal mappings combined with complex-valued stress functions. The outer boundary is described using point-matching and the least-squares method. The solutions are conducive to the assessment of the essential influencing factors of material properties, geometry and loads. Notched finite plates made of fibre/matrix composites, mainly carbon-fibre reinforced polymers, will be presented as illustrations. Received 29 June 1998; accepted for publication 22 October 1998     

缺口件疲劳寿命分布预测的有效应力法

本文提出了一种由光滑件疲劳寿命试验数据预测缺口件疲劳寿命分布的有效应力法。该方法中缺口件的裂纹可能萌生表面被分解成一个个微元,整个表面可看成是这些微元组成的一个串联模型,按照串联概率失效模型,缺口件的疲劳强度失效概率就可以由各微元的疲劳强度失效概率计算得到,其中微元的疲劳强度失效概率是由光滑件的疲劳强度失效概率通过最弱环节理论计算得到的。在缺口件的疲劳强度失效概率表达式中,引入了有效应力的概念,用它查取光滑件的疲劳寿命试验数据就可以直接得到缺口件的疲劳寿命分布。该方法可以同时考虑到应力梯度和试件尺寸对缺口件疲劳寿命分布的影响。进行了材料LY12CZ的带中心孔缺口件的寿命算例分析,预测结果和试验结果吻合良好,表明该方法是有效的。     

Proposition d'un paramètre énergétique de rupture pour les matériaux dissipatifs

We suggest an energetic fracture parameter for non elastic materials. This one is presented in a domain integral, and founded on the release rate of the total mechanical energy received by a notched solid, using a domain derivation method. This parameter is proposed for a large class of loading and materials described by internal variables. It is shown that this parameter is reduced to the Rice–Cherepanov integral when the material is either elastic or elastoplastic and submitted to proportional loading.     

Effect of two drille holes on the elastic and elastic-plastic strain distribution around a Charpy V-notch

Two small holes (0.0292 in.), appropriately drilled near the root of a Charpy V-notch, have been shown to reduce markedly the Charpy V-notch transition temperature of various steels. In the present study, three experimental techniques were used to define the effect of two holes on the mechanics of deformation and fracture of notched bars loaded in three- and four-point bending: (1) two-dimensional photoelastic stress analyses were performed on models of both the standard Charpy and drilled geometries; (2) a sensitive dislocation etch-pitting technique was used to observe directly the plastic-strain fields developed in V-notch samples of Fe?3% Si alloy loaded in slow bending; and (3) the Charpy striker was instrumented to record load-time curves during impact-bending and thereby determine the dynamic fracture strength of notched and drilled mildsteel samples. It was determined that two holes donot significantly reduce the elastic stress-concentration factor although they cause considerable redistribution of the local shear stresses around the notch. Consequently, the elastic-plastic state develops quite differently in the presence of two holes, and hole drilling can increase the load-carrying capacity of notched mild-steel bars by more than 100 percent even when bars fail by brittle cleavage prior to general yielding. The implications of these results with respect to other forms of “stress-relieving notches” are discussed.     

Fracture in single crystal NiTi

This paper presents an investigation of the pseudoelastic transformation of two crystallographic orientations of a single crystal NiTi shape memory alloy (SMA). Both uniaxial tension and notched tension samples are considered with the tensile axes along the [1 0 0] and [1 1 1] directions. The phase transformation is observed using optical techniques in situ. For the uniaxial tension samples, martensite plates and Lüders-band structures are observed. For the notched samples, transformation structures in the [1 0 0] sample appeared predominantly on the sides of the notch and crack tip with stable crack propagation. In the [1 1 1] notched samples transformation occurs directly ahead of the notch and unstable crack propagation is observed. An available work criterion is used to predict the location of the transformation, with good agreement to the experimental observations. The different fracture behavior of the two notched sample orientations is explained utilizing the available work calculations.     

Computations of modes I and II stress intensity factors of sharp notched plates under in-plane shear and bending loading by the fractal-like finite element method

The fractal-like finite element method (FFEM) is used to compute the stress intensity factors (SIFs) for different configurations of cracked/notched plates subject to in-plane shear and bending loading conditions. In the FFEM, the large number of unknown variables in the singular region around a notch tip is reduced to a small set of generalised co-ordinates by performing a fractal transformation using global interpolation functions. The use of exact analytical solutions of the displacement field around a notch tip as the global interpolation functions reduces the computational cost significantly and neither post-processing technique to extract SIFs nor special singular elements to model the singular region are required. The results of numerical examples of various configurations of cracked/notched plates are presented and validated via published data. Also, new results for cracked/notched plate problems are presented. These results demonstrate the accuracy and efficiency of the FFEM to compute the SIFs for notch problems under in-plane shear and bending loading conditions.     

多轴载荷下缺口试件疲劳寿命预测研究

基于临界平面法,在分析光滑薄壁圆管试件疲劳寿命预测模型的基础上,借助有限元应力应变分析,进一步将模型推广应用到了缺口试件的多轴疲劳寿命预测中,并利用坐标变换原理,明确了临界平面及有效循环变量的确定方法.在存在平均应力的情况下,分析了平均应力对疲劳寿命的影响,并对所建模型进行了平均应力修正.     

缺口复合材料层合板疲劳性能的试验研究

对带各种缺口的复合材料层合板的疲劳性能进行了研究。设计了不同类型的制品试件,分别进行静态和疲劳试验。试验数据表明,缺口对复合材料的静强度和静刚度的影响不大,缺口试样的疲劳寿命带高于缺口试样,与金属材料相比,复合材料对缺口不敏感。采用条状试样获得的力学性能不参代表复合材料的真实性能,实现表明自由边的缺口对结构提高疲劳寿命是有利的。文中还对疲劳过程中复合材料的刚度变化进行了研究,对文中所研究的层合板,     

多轴载荷下缺口试件疲劳寿命预测研究

基于临界平面法,在分析光滑薄壁圆管试件疲劳寿命预测模型的基础上,借助有限元应力应变分析,进一步将模型推广应用到了缺口试件的多轴疲劳寿命预测中,并利用坐标变换原理,明确了临界平面及有效循环变量的确定方法.在存在平均应力的情况下,分析了平均应力对疲劳寿命的影响,并对所建模型进行了平均应力修正.     

An analysis of ductile rupture in notched bars

Ductile fracture in axisymmetric and plane strain notched tensile specimens is analyzed numerically, based on a set of elastic-plastic constitutive relations that account for the nucleation and growth of microvoids. Final material failure by void coalescence is incorporated into the constitutive model via the dependence of the yield function on the void volume fraction. In the analyses the material has no voids initially; but as the voids nucleate and grow, the resultant dilatancy and pressure sensitivity of the macroscopic plastic flow influence the solution significantly. Considering both a blunt notch geometry and a sharp notch geometry in the computations permits a study of the relative roles of high strain and high triaxiality on failure. Comparison is made with published experimental results for notched tensile specimens of high-strength steels. All axisymmetric specimens analyzed fail at the center of the notched section, whereas failure initiation at the surface is found in plane strain specimens with sharp notches, in agreement with the experiments. The results for different specimens are used to investigate the circumstances under which fracture initiation can be represented by a single failure locus in a plot of stress triaxiality vs effective plastic strain.     

Viscoplastic behaviour of a medium density polyethylene (MDPE): Constitutive equations based on double nonlinear deformation model

The viscoplastic behaviour of a medium density ethylene–butene copolymer (MDPE) is investigated by using samples cut out from thick-walled MDPE pipe. Extensive experimental work has been performed to characterise the nonlinear time-dependent response of such semi-crystalline thermoplastic material. Tests were carried out at 60 °C, on smooth tensile, full axisymmetrically notched creep tensile (FNCT) and double edge notched tensile (DENT) specimens.     

Why does necking ignore notches in dynamic tension?

Recent experimental work has revealed that notched tensile specimens, subjected to dynamic loading, may fail by growing a neck outside of the notched region. This apparent lack of sensitivity to a classical stress concentration case was reported but not explained or modeled.The present paper combines experimental and numerical work to address this issue. Specifically, it is shown that the dynamic tensile failure locus is dictated by both the applied velocity boundary condition and the material mechanical properties, specifically strain-rate sensitivity and strain-rate hardening.It is shown that at sufficiently high impact velocities, the flows stress in the notch vicinity becomes quite higher than in the rest of the specimen, so that while the former resists deformation, it transfers the load to the latter. The result will be the formation of a local neck and failure away from the notch.This effect is shown to be active when the material properties are perturbed only at the local level, as in the case of machining of the notch, which in itself may again be sufficient to stabilize the structure under local failure until a neck forms elsewhere.While the physical observations are quite counterintuitive with respect to the engineering views of stress concentrator's effect, the present work rationalizes those observations and also provides information for the designers of dynamically tensioned structures that may contain notches or similar flaws.     

Microscale deformation of a tempered martensite ferritic steel: Modelling and experimental study of grain and sub-grain interactions

In this paper, a finite-element modelling framework is presented with explicit representation of polycrystalline microstructure for a tempered martensite ferritic steel. A miniature notched specimen was manufactured from P91 steel with a 20,000 h service history and tested at room temperature under three point bending. Deformation at the microscale is quantified by electron back scattered diffraction (EBSD) before and after mechanical loading. A representative volume element was developed, based on the initial EBSD scan, and a crystal plasticity model used to account for slip-based inelastic deformation in the material. The model showed excellent correlation with the experimental data when the relevant comparisons were made.     

High temperature sensitivity of notched AISI 304L stainless steel tests

Experiments were designed to determine the failure characteristics of AISI 304L stainless steel under different stress triaxialities and temperatures up to 70% of melt. The data show that as temperature increases the displacement to failure of notched tensile specimens increases. The complex interaction of deformation mechanisms, such as twinning and dynamic recrystallization, appears to negate the damage accumulation at higher temperatures. Microstructural analyses and finite element simulations indicate that voids nucleate, grow, and coalesce more rapidly as temperature and triaxiality increase. Finite element simulations were performed to analyze temperature dependence on the Cocks–Ashby void growth model. The finite element simulations qualitatively show a double-knee that was observed in the notched experimental specimens after loading. The combined experimental–numerical study indicates that failure can be defined at several points in the notch tests when: (1) macrovoids starts to form, (2) the load drop-off occurs, and (3) total perforation of the specimen occurs. These three points occur simultaneously in ambient conditions but occur at different displacements at higher temperatures.     

16MnR钢缺口试样解理断裂行为研究

研究了低合金热轧钢16MnR缺口试样在$-196\,{^\circ}$C和$-130\,{^\circ}$C的解理断裂机 理. 拉伸试验、单、双缺口四点弯曲实验、断口形貌观察以及有限元分析结果表明, 缺口试 样发生解理断裂时均起裂于夹杂物粒子, 一种位于缺口根部前端(IC型), 另一种位于距缺口 根部较远的条形裂纹前端(SIC型); 且随温度升高, 起裂源的类型从$-196\,{^\circ}$C下的IC 型转变为$-130\,{^\circ}$C下的SIC型. 微裂纹均形核于夹杂物, 最终的断裂由铁素体晶粒尺 寸的微裂纹扩展控制. 缺口试样IC型解理断裂遵循裂纹形核条 件$\varepsilon_{\rm p} \ge \varepsilon_{\rm pc}$和裂纹扩展条件$\sigma_{yy} \ge \sigma_{f}$, 而SIC型解理断裂条件则演化为$\varepsilon_{\rm p}+\varepsilon_{\rm ps} \ge \varepsilon_{\rm pc}$和$\sigma_{yy} +\sigma_{yy{\rm s}} \ge \sigma_{f}$.     

Design features and results from fatigue-reliability-research machines

The design, fabrication, development, operation, calibration and results from reversed bending combined with steady-torque fatigue-research machines are presented. Fifteen-centimeter-long, notched, SAE 4340 steel specimens are subjected to various combinations of these stresses and cycled to failure. Failure occurs when the crack in the notch passes through the specimen automatically shutting down the test machine. These cycles-to-failure data are statistically analyzed to develop a probabilistic S-N diagram. These diagrams have many uses including the design of rotating components having minimum size and weight for a specified number of cycles and reliability.     

Fracture analysis of V-notched components – Effects of first non-singular stress term

The effect of the first non-singular stress term on the fracture behavior of notched structures was investigated under symmetric geometry and loading conditions. According to the Williams series expansion, for a large domain of notch angles the non-singular stress terms of sharp notches are functions of complex eigenvalues and their corresponding complex coefficients. Hence, a new representation of stress field near the notch tip was developed in which the higher order terms are expressed as several explicit functions of real and imaginary parts of both the complex eigenvalues and their complex coefficients. A critical stress criterion was then applied to the new stress formulations to assess the influence of the first non-singular stress term on the apparent fracture toughness. Several finite element analyses were also performed on two laboratory specimens in order to show the effects of first non-singular term on the near field stress distribution of notched specimens. The results demonstrated that neglecting the first non-singular stress term could lead to significant errors in predicting the apparent fracture toughness of notched components.