Mixed-Mode Stress Intensity Factor Estimation of Inclined Cracks in an Unnotched Round Bar

Journal of Failure Analysis and Prevention - The extent on mixed-mode stress intensity factor (SIF) of inclined multiple cracks in an unnotched bar has been investigated numerically. Opposite...     

Residual Stresses and Stress Intensity Factor Calculations in T-Welded Joints

In welded joint, the residual stresses effect can be considered using the residual stress intensity factor (K _res). In this study, K _res is calculated using the analytic weight function method (WFM) and the polynomial distribution of residual stresses (σ _res). The different residual stress distributions have been used analytically. It is to be emphasized that the current approach is little investigated. This is because the weight function has already been developed to calculate K for a crack that had already existed, and hence to calculate the stress distribution and stress intensity factor over the crack face. Therefore, the current approach calculates K _res with σ _res consideration for the crack which initiates and propagates until failure. The validity to use the proposed weight function has been shown. The results of K _res have been compared with those obtained from FEM.     

泡沫金属夹层板自冲铆接头的疲劳性能及失效机理

对铝合金自冲铆接头及泡沫金属夹层结构自冲铆接头进行疲劳实验,通过三参数经验公式采用S-N曲线拟合法绘制接头的F-N曲线,分析接头的疲劳寿命及泡沫金属夹层对自冲铆接头疲劳性能的影响;采用扫描电子显微镜对接头的疲劳失效断口进行观测,分析其微观失效机理。结果表明:泡沫金属夹层缩短了自冲铆接头的疲劳寿命,不同泡沫金属夹层对自冲铆接头疲劳性能的影响具有差异性,在高疲劳载荷下泡沫铜夹层接头疲劳性能更优。三组接头疲劳失效形式都为下板断裂,在高疲劳载荷下裂纹易在铆扣区域萌生,在中低疲劳载荷下裂纹萌生于下板一侧,沿铆扣区域下侧向板材另一侧扩展。     

Mixed Mode Stress Intensity Factor Determination of Multiple Cracks in Hollow Circular Pipe

Failure of pressure vessels and piping due to high temperature applications occurs due to the formation of fatigue cracks caused by cyclic load. It is well known that, the consequences of collapses of pipes causing enormous disruption of daily life. Thus there is a need to design and manufacture the pipes with precision and care. The major cause of crack nucleation in pipes is due to corrosion and internal fluid pressure. The crack-tip stresses are determined using stress intensity factor (SIF). In the present work an attempt has been made to determine the SIF for multiple cracks in a circular pipe subjected to internal fluid pressure. Two surface cracks of same size were introduced at the inner wall of the tube. The crack depth ratio (a/t) ranging between 0.1 and 0.5 and crack aspect ratio (a/c) of 0.6 and 1.0 was considered. Internal fluid pressure of 100 MPa was applied at the inner surface of the pipe and the corresponding SIF was measured. SIF values were calculated with consideration of mode-II and mode-III fracture in order to predict the exact SIF. As available SIF solutions of cracked pipes are limited to mode-I fracture, present work presents the influence of additional influence of mode-II and mode-III fracture. It is observed that, as crack depth ratio increases, SIF also increases considerably for semi-circular cracks. Higher SIF values were observed at the crack surface region [S/S ₀ = ±1] compared to crack middle [S/S ₀ = 0] region. A crossover in SIF was noted at a crack depth ratio of 0.3. At higher crack depths, SIF values decrease at the crack surface region due to additional influence of mode-II and mode-III fracture. In contrast to semi-circular cracks, SIF values are higher at the crack surface region for semi-elliptic cracks irrespective of the crack depths.     

Brittle Mixed-Mode (I+II) Fracture: Application of the Equivalent Notch Stress Intensity Factor to Cracks Emanating from Notches

In the present paper, crack initiation in mixed-mode (I+II) fracture has been studied using notched circular ring specimens. A new criterion of brittle mixed-mode (I+II) fracture based on the notch tangential stress and the volumetric approach has been developed. The critical value of the equivalent notch stress-intensity factor has been considered as fracture toughness in mixed-mode (I+II) fracture.     

Determination of the Stress Intensity Factor of an elliptical breathing crack in a rotating shaft

The failures due to the propagation of fatigue cracks are one of the most frequent problems in rotating machines. Those failures sometimes are catastrophic and are sufficient to provoke the loss of the complete machine with high risks for people and other equipments. When a cracked shaft rotates, the breathing mechanism appears. The crack passes from an open state to a close state with a transition in which a partial opening or closing of the crack is produced. In this work, a new general expression that gives the Stress Intensity Factor (SIF) along the crack front of an elliptical crack in a rotating shaft in terms of the crack depth ratio, the crack aspect ratio, the relative position on the front and the angle of rotation has been developed for linear elastic materials. By the moment, no expressions of the SIF in term of these variables have been found in the literature. To this end, a quasi-static 3D numerical model of a cracked shaft with straight and elliptical cracks subjected to rotary bending using the Finite Element Method (FEM) has been made. To simulate the rotation of the shaft, different angular positions have been considered. The SIF in mode I along the crack front has been calculated for each angular position of the cracked shaft and for different crack geometries. The expression results have been compared with solutions obtained from the literature. It has been found that they are in good agreement. The model has been applied to other crack geometries with good results. The obtained SIF expression allows studying the dynamic behavior of cracked shafts and can be used to analyze the crack propagation.     

Determination of Stress Intensity Factor for Cracks in Orthotropic Composite Materials using Digital Image Correlation

Abstract: This paper focuses on the application of the digital image correlation (DIC) technique to determine the stress intensity factor (SIF) for cracks in orthotropic composites. DIC is a full‐field technique for measuring the surface displacements of a deforming object and can be applied to any type of material. To determine the SIF from full‐field displacement data, the asymptotic expansion of the crack‐tip displacement field is required. In this paper the expansion of the crack tip displacement field is derived from an existing solution for strain fields. Unidirectional fibre composite panels with an edge crack aligned along the fibre were tested under remote tensile loading and the displacements were recorded using DIC. The SIF was calculated from the experimental data by fitting the theoretical displacement field using the least squares method. The SIF thus determined was in good agreement with theoretical results and therefore demonstrates the applicability of the derived displacement field and DIC technique for studying fracture in composites.     

焊接桥梁结构的断裂驱动力分析

对焊接桥梁结构的典型单元H型构件进行了简化分析 ,同时考虑了焊接残余应力的影响 ,估计了临界裂纹的长度 ,并将构件按设计应力进行了分类 ,计算出各类应力构件的驱动力。临界裂纹长度 2aC=6 0mm ;H型构件中基体的驱动力KBI=1.12 (α +β1)σs πaC,焊缝的驱动力KWI =1.12 (α +β2 )σs πaC     

TA1钛合金单搭自冲铆接头微动磨损机理

对TA1钛合金单搭自冲铆接头进行疲劳实验研究接头失效形式;用扫描电子显微镜和X射线能谱线扫描研究铆钉各部位微动磨损程度的差异和接头微动磨损机理;采用威布尔分布验证数据有效性。结果表明:接头疲劳失效形式主要为上板断裂,高周疲劳均为上板断裂,低周疲劳为上下板混合断裂;微动磨屑包含氧、钛、锌和锡元素,铆钉头部微动磨损程度高于铆钉腿部。微动磨损区出现严重脱层、微动磨屑堆积和微裂纹萌生等现象,随着微动磨损及剪切力共同作用导致接头断口部位出现大量微裂纹并逐步沿深度和宽度方向扩展为宏观裂纹,最终导致接头疲劳失效。     

TA1钛合金单搭自冲铆接头微动磨损机理

对TA1钛合金单搭自冲铆接头进行疲劳实验研究接头失效形式;用扫描电子显微镜和X射线能谱线扫描研究铆钉各部位微动磨损程度的差异和接头微动磨损机理;采用威布尔分布验证数据有效性.结果表明:接头疲劳失效形式主要为上板断裂,高周疲劳均为上板断裂,低周疲劳为上下板混合断裂;微动磨屑包含氧、钛、锌和锡元素,铆钉头部微动磨损程度高于铆钉腿部.微动磨损区出现严重脱层、微动磨屑堆积和微裂纹萌生等现象,随着微动磨损及剪切力共同作用导致接头断口部位出现大量微裂纹并逐步沿深度和宽度方向扩展为宏观裂纹,最终导致接头疲劳失效.     

Stress Intensity Factor of an Edge Crack in Composite Media

A solution to the 2-D problem of a rigid circular inclusion squeezed by two semi-infinite elastic planes is presented. The size of the contact zone and the length of the opened area as a function of the applied stress, material properties and radius of the inclusion are calculated. The results demonstrate a strong non-linearity of the solution.     

Dynamic Stress Intensity Factor of Interfacial Finite Cracks in Orthotropic Materials

The elastodynamic response of an infinite non-homogeneous orthotropic material with an interfacial finite crack under distributed normal and shear impact loads is examined. Solution for the stress intensity factor history around the crack tips is found. Laplace and Fourier transforms are employed to solve the equations of motion leading to a Fredholm integral equation on the Laplace transform domain. The dynamic stress intensity factor history can be computed by numerical Laplace transform inversion of the solution of the Fredholm equation. Numerical values of the dynamic stress intensity factor history for some materials are obtained. Interfacial cracks between two different materials and between two pieces of the same material but different fiber orientation are considered. Bimaterial formulation of a crack problem is shown to converge to the mono-material formulation, derived independently, in the limiting case when both materials are the same.     

Stress Intensity Factor of an Edge Crack in Bonded Orthotropic Materials

The article presents the problem of an edge crack under normal point loading terminating perpendicular to the surface of an orthotropic strip of finite thickness which is bonded to another orthotropic half plane. Expressing the displacements and stresses in plane strain condition in terms of harmonic functions, the problem is reduced to a pair of simultaneous integral equations with Cauchy type singularities, which are finally been solved by the Hilbert transform technique. The analytical expression of stress intensity factor (SIF) at the crack tip for large thickness of the strip is calculated, which corresponds to the weight function of a crack under normal loading. The influences of elastic constants of two different orthotropic materials, distinct arbitrary locations of normal point loading on the crack surface and length of the crack on the dynamic SIF are depicted through graphs.     

New Formula of Experimental Stress Intensity Factor Evaluation by Caustics

A study of the experimental evaluation of the stress intensity factor K _I was undertaken by the method of caustics. A new formula of experimental calculation stress intensity factor was proposed depended to the area of the caustic. A series of experiments were performed in single edge-cracked specimens made of Lexan (PCBA). It was found that the values of the stress intensity factor K _I , which were based on the diameters of the caustics, were varied with the load, the crack length and the caustic shape because the caustic becomes oval in higher loads or higher crack length. This means that the stress intensity values, which are calculated by the two diameters of the caustics (D _t , D _l ), are significantly differed. The accurate stress intensity factor K _I can be experimentally evaluated by the area of the formed caustic. In the present work a new formula for the calculation of the stress intensity factor was established.     

型材中裂纹应力强度因子计算方法

本文应用分区广义变分原理,提出了求解含有裂纹的型材应力强度因子的计算方法。并对角材中含有边缘裂纹和槽形型材中含有中心裂纹的应力强度因子进行了数值计算,计算结果绘成了曲线。     

铝锂合金T型自冲铆接头疲劳特性及失效机理

为研究T型自冲铆接头的疲劳特性,以铝锂合金(AL1420)同种及其与铜合金(H62)和不锈钢(410)异种组合制备的AA、HA和SA三组T型自冲铆接头为研究对象,基于其静力学试验采用正弦波进行拉-拉加载疲劳试验,用二参数威布尔分布对所得疲劳数据的有效性进行验证,采用最小二乘拟合直线得到拟合的F-N曲线,并采用SEM扫描电镜对各组典型疲劳失效断口进行微观分析。结果表明:SA组疲劳寿命对载荷变化最敏感,并且疲劳寿命随相对滑移量的增加而减小;由三组T型接头失效形式可看出接头薄弱环节有由铆钉向板材转移的趋势;从微观分析疲劳断口可知裂纹萌生区多为形貌特征较为平坦的准解理,裂纹瞬断区形貌更加突出,多为沿晶断裂或韧窝形貌。     

Disappearance Conditions of Thermal Stress Singularities Based on Stress Intensity in Two and Three-Phase Bonded Structures

The characteristics of stress singularities near the free edge of the interface of bonded dissimilar materials subjected to change in temperature are investigated. The thermal stresses are represented by the sum of the singular terms of types r^-1 and log r, no singular ones and the particular ones. The functions, K × F (j=1,2,3), associated with the terms except the particular terms contributing to the thermal stresses are described as new stress intensity factors. The K is obtained from the technique of the Mellin transform and the F is calculated by numerical analysis such as finite element analysis. The emphasis here is placed on clarifying material combinations with arbitrary wedge angles and elastic properties yielding K = 0. The results can be used to determine material/geometry which ensures that interface residual stresses will be in compressive. This revised version was published online in July 2006 with corrections to the Cover Date.     

共面三维裂纹应力强度因子干涉的研究

利用奇异有限元理论与方法对三维多裂纹的应力强度因子干涉问题进行了研究,通过构造裂纹前缘位移场的局部解,提出了一种新的裂纹前缘位移场的位移模式,通过最小势能原理和变分原理直接得到裂纹前缘干涉的应力强度因子,较好地解决了含多裂纹有限体的强度分析问题.算例说明了该方法的有效性,其计算结果为多裂纹简化为单裂纹提供了理论依据.