K and J-Integral Variations Due to Residual Stresses in Welded Plates

Due to residual stress in a material, stress intensity factors (SIF) and also J-integral values are not constant and become path dependent parameters. Modified relations are used for calculations to take into account the effects of residual stresses. These relations give path independence values for the J-integrals in spite of existing the residual stresses. Effects of residual stresses due to welding on variations of SIF and J-integral are studied. Results have a good agreement with theoretical values obtained by weight function method. Through and surface cracks are considered on welded plate and distributions of J-integral on the crack front are obtained. Effects of lengths, depths and orientation angles of cracks are investigated due to applying only residual stress and also combination of residual and applied stresses. Redistributions of residual stress are determined for welded plates with through cracks and compared with experimental results in literature.     

Fatigue crack propagation in a directionally-solidified nickel-base alloy

High cycle fatigue crack growth rates have been measured in the cast nickel-base alloy IN 738 LC in directionally-solidified form, at room and high temperature and for crack propagation both parallel and perpendicular to the solidification direction. The resistance of this material to crack propagation has been compared with conventionally-cast material of the same composition. The considerable differences in observed growth rates may be understood in terms of the effects of chemical segregation, crack branching and crystallographic fracture. In particular, the high-temperature cyclic fracture toughness for crack growth perpendicular to the solidification direction is higher than in conventionally-cast material as cracks tend to deviate along the segregated interdendritic regions. However the room temperature threshold stress intensity amplitude is low because fatigue crack growth occurs on definite crystallographic planes.     

Fatigue crack propagation in a directionally-solidified nickel-base alloy

High cycle fatigue crack growth rates have been measured in the cast nickel-base alloy IN 738 LC in directionally-solidified form, at room and high temperature and for crack propagation both parallel and perpendicular to the solidification direction. The resistance of this material to crack propagation has been compared with conventionally-cast material of the same composition. The considerable differences in observed growth rates may be understood in terms of the effects of chemical segregation, crack branching and crystallographic fracture. In particular, the high-temperature cyclic fracture toughness for crack growth perpendicular to the solidification direction is higher than in conventionally-cast material as cracks tend to deviate along the segregated interdendritic regions. However the room temperature threshold stress intensity amplitude is low because fatigue crack growth occurs on definite crystallographic planes.     

冲击荷载下岩石裂纹扩展研究进展

为了给深部资源开采和大型地下空间工程中围岩体的变形机理及稳定性控制提供理论基础,通过查阅大量关于表征岩石裂纹扩展的裂纹扩展模型、应力强度因子和断裂韧性的国内外文献,总结了前人的研究成果。依据现有研究,提出了动荷载作用下岩石裂纹扩展的几点建议：（1）综合考虑弹性力学、断裂力学和损伤力学建立岩石材料从微观断裂到宏观破坏这一演变过程的理论模型,使理论模型更加适应岩石材料的非线性特征;（2）采用分形、自组织和混沌等非线性理论表征动荷载作用下岩石内部以及表面裂纹的扩展演化特征;（3）采用颗粒离散元和有限差分模拟岩石材料裂纹扩展演化特征。     

Analysis of strip edge cracks during cold rolling of thin strip

In this paper,the mechanics of strip edge cracks and its propagation has been studied,and the effects of strip edge drop and stress intensity factor(SIF) on edge crack defections during cold rolling of thin strip have been discussed.An experimental investigation was presented into the effect of strip edge drop on edge cracks during cold rolling of thin strip.The edge crack increases significantly due to more inhomogeneous deformation and work hardening at the strip edge.The effective stress intensity fac...     

Mode I stress intensity factors induced by fracture surface roughness under pure mode III loading: Application to the effect of loading modes on stress corrosion crack growth

A model to estimate the reduction of effective crack tip Mode III stress intensity factors by frictional and asperity interaction of an idealized fracture surface is described. An extension of the model is used to calculate the Mode I stress intensity factors due to the crack tip opening displacement induced by the mismatch of the fracture surface asperities. The results of calculations based on a “reasonable” fracture surface profile are used to analyze experimental studies designed to determine the relative significance of hydrogen embrittlement and crack tip dissolution in stress corrosion crack growth in Al alloys by comparison of Mode I and Mode III stress corrosion cracking (SCC) resistance. It is concluded that a pure Mode III stress state is not possible for cracks with microscopically rough surfaces and that the magnitude of the induced Mode I stress intensity factor is sufficient to cause stress corrosion crack growth.     

复合材料粘结层中具有双裂纹时的热应力

采用拉普拉斯-傅利叶变换和施密特(Schmidt)方法,研究了复合材料因热流在裂纹周围引起的热应力,并对用环氧树脂粘结的陶瓷-钢板进行了数值计算.     

The mechanics of matrix cracking in brittle-matrix fiber composites

Matrix fracture in brittle-matrix fiber composites is analyzed for composites that exhibit multiple matrix cracking prior to fiber failure and have purely frictional bonding between the fibers and matrix. The stress for matrix cracking is evaluated using a stress intensity approach, in which the influence of the fibers that bridge the matrix crack is represented by closure tractions at the crack surfaces. Long and short cracks are distinguished. Long cracks approach a steady-state configuration, for which the stress intensity analysis and a previous energy balance analysis are shown to predict identical dependence of matrix cracking stress on material properties. A numerical solution and an approximate analytical solution are obtained for smaller cracks and used to estimate the range of crack sizes over which the steady-state solution applies.     

Analysis on morphology and stress concentration in continuous casting bloom to learn the formation and propagation of internal cracks induced by soft reduction technology

ABSTRACT

The crack formation and propagation were analysed according to actual internal crack morphology and the finite element model simulation of stress concentration. The results showed that most cracks were distributed along the transverse and the longitudinal section of the bloom. Distinct differences between these two major types of cracks were found in dimension and inclination, which were owing to different local stress concentrations. In the longitudinal section of the bloom, shear stress was concentrated in the brittle temperature region, which led to the formation of initial cracks and subsequently cracks propagation. Meanwhile, the maximum tensile stress occurred at the edge between the brittle temperature region and surrounding material, which resulted in crack formation and propagation along the transverse section of the bloom. This phenomenon was due to the obvious bulging deformation of the solidified shell induced by soft reduction.     

A Higher Order Shear Deformation Model for Bending Analysis of Functionally Graded Plates

In this paper, the static response of simply supported functionally graded plates subjected to a transverse uniform load and resting on an elastic foundation is examined by using a new higher order displacement model. The present theory exactly satisfies the stress boundary conditions on the top and bottom surfaces of the plate. No transverse shear correction factors are needed, because a correct representation of the transverse shear strain is given. The material properties of the plate are assumed to be graded in the thickness direction according to a simple power-law distribution in terms of volume fractions of material constituents. The foundation is modeled as a two-parameter Pasternak-type foundation, or as a Winkler-type one if the second parameter is zero. The equilibrium equations of a functionally graded plate are given based on the new higher order shear deformation theory of plates presented. The effects of stiffness and gradient index of the foundation on the mechanical responses of the plates are discussed. It is established that the elastic foundations significantly affect the mechanical behavior of thick functionally graded plates. The numerical results presented in the paper can serve as benchmarks for future analyses of thick functionally graded plates on elastic foundations.     

Dynamic Instability of Functionally Graded Shells Using Higher-Order Theory

This paper reports the dynamic instability behavior of functionally graded (FG) shells subjected to in-plane periodic load and temperature field using a higher-order shear deformation theory in conjunction with the finite-element approach. Properties of FG materials are assumed to be temperature dependent and graded in the thickness direction according to the power-law distribution in terms of volume fraction of the constituents. Five forms of shells considered in this investigation are singly curved cylindrical, doubly curved spherical, and hyperbolic paraboloid having two principal curvatures, doubly curved hypar having twist curvature only, and doubly curved conoid having one curvature and twist curvature. The boundaries of dynamic instability regions are obtained using Bolotin’s approach. The structural system is considered to be undamped. The correctness of the formulation is established by comparing the writers’ results with those of problems available in the published literature. Effects of material composition and geometrical parameters are studied on the dynamic instability characteristics of the aforementioned five forms of shells having practical applications in many engineering disciplines.     

条形药包爆炸全场应变以及裂纹动态断裂特性研究

为了探究不同起爆位置下条形药包全场应变以及裂纹动态断裂特性,采用爆炸荷载动态焦散线实验系统和数字图像相关方法（DIC）,开展了爆破模型实验研究。研究结果表明:条形药包一端起爆时,起爆点处翼裂纹扩展长度最小,随着炸药爆轰的传播,翼裂纹扩展长度增长;中心起爆时,中心位置翼裂纹扩展长度小于两端位置翼裂纹扩展长度,一端起爆时非起爆端翼裂纹扩展长度最长。无论中心起爆或一端起爆,条形药包中心区域翼裂纹扩展主要为Ⅰ型裂纹,并且中心翼裂纹起裂韧度最大,端部翼裂纹为以Ⅱ型为主的Ⅰ?Ⅱ型复合裂纹。一端起爆时,拉压应变作用范围沿炸药传爆方向传递,且非起爆端拉压应变作用区域大于起爆端,压应变最大值为距起爆点约0.67 ~ 0.83倍的装药长度。中心起爆时,拉压应变的作用过程沿起爆中心向两端呈对称形式传播,中心点位置应变最大。两种起爆方式下都出现端部压应力集中现象。      

Crack deflection: Implications for the growth of long and short fatigue cracks

The influences of crack deflection on the growth rates ofnominally Mode I fatigue cracks are examined. Previous theoretical analyses of stress intensity solutions for kinked elastic cracks are reviewed. Simple elastic deflection models are developed to estimate the growth rates of nonlinear fatigue cracks subjected to various degrees of deflection, by incorporating changes in the effective driving force and in the apparent propagation rates. Experimental data are presented for intermediate-quenched and step-quenched conditions of Fe/2Si/0.1C ferrite-martensite dual phase steel, where variations in crack morphology alone influence considerably the fatigue crack propagation rates and threshold stress intensity range values. Such results are found to be in good quantitative agreement with the deflection model predictions of propagation rates for nonlinear cracks. Experimental information on crack deflection, induced by variable amplitude loading, is also provided for 2020-T651 aluminum alloy. It is demonstrated with the aid of elastic analyses and experiments that crack deflection models offer a physically-appealing rationale for the apparently slower growth rates of long fatigue cracks subjected to constant and variable amplitude loading and for the apparent deceleration and/or arrest of short cracks. The changes in the propagation rates of deflected fatigue cracks are discussed in terms of thelocal mode of crack advance, microstructure, effective driving force, growth mechanisms, mean stress, slip characteristics, and crack closure.     

Effect of orientation on crystallographic cracking in notched nickel-base superalloy single crystal subjected to far-field cyclic compression

The effects of crystallographic orientation on the fatigue crack initiation and growth under far-field cyclic compression are discussed in single crystals of nickel-base superalloy MAR-M200. Results indicate that cracking occurs primarily due to planar slip on the {111}-type planes. Crystallographic cracking can occur on two or more slip planes simultaneously, but through-thickness cracks are not observed. In addition, it has been shown that the threshold stress intensity for crack initiation shows a strong dependence on orientation. The threshold stress intensity for crack growth in cyclic compression is 5 to 10 times the threshold for crack growth in cyclic tension.     

Modeling of IC Debonding of FRP-Strengthened Concrete Flexural Members

The presence of a fiber-reinforced polymer (FRP) strengthening material bonded to the tension face of a reinforced concrete beam will restrict but not prevent the opening of intermediate flexural cracks due to applied loading. Test results indicate that displacements at the toe of flexural cracks create stress concentrations at the interface of the FRP laminate and the beam, leading to the development of localized interface cracks that, typically, propagate, under the effect of the load, to join the original flexural cracks and cause delamination of the FRP system. This type of FRP delamination is commonly termed intermediate crack (IC) debonding. In this paper the analytical models published in the literature are reviewed and it was found that these models do not correlate well with measured experimental results. This paper proposes an analytical model that characterizes the interface shear stress based on two distinct sources: (1) the change in the applied moments along the length of the member and (2) stress concentrations at the intermediate cracks. The proposed model is compared to an experimental database and shown to predict extremely well most of the test results reported by other researchers. A parametric study, performed using the proposed model, indicates that the model varies with several important variables that are not captured by most of the existing models.     

Clean steels for steam turbine rotors—their stress corrosion cracking resistance

This work presents the results of a comprehensive study concerning stress corrosion crack growth rates in steam turbine rotor steels exposed to hot water. The effects of stress intensity, temperature, and dissolved gases in the water have been investigated. Special attention has been given to the influence of impurities and alloying elements in the steel such as P, S, Mn, Si, Mo, and Ni, and to the effect of yield strength and fracture toughness on the growth rates of stress corrosion cracks. The results of this study clearly show that there exists a threshold stress intensity of about 20 MNm^−3/2 above which the invariably intergranular stress corrosion cracks grow at a constant, stress-independent velocity. This plateau stress corrosion crack growth rate isnot affected by the oxygen and carbon dioxide concentration in the water. The temperature and the yield strength of the steel have a strong influence on the growth rate of stress corrosion cracks. In contrast, there isno effect of the steel composition within the range investigated, neither of the impurity elements such as P and S, nor of the major alloying elements such as Mn, Si, Mo, and Ni. Steels with low fracture toughness due to temper embrittlement do not exhibit faster stress corrosion crack growth rates in water than nonembrittled steels. No direct relationship between intergranular temper embrittlement and intergranular stress corrosion crack growth in water can be demonstrated.     

A theory for creep by interfacial flaw growth in ceramics and ceramic composites

The nucleation and growth of flaws along grain boundaries and interfaces are known to cause significant reductions in elastic moduli and to play an important role in determining the deformation characteristics of ceramic materials at elevated temperatures. This paper presents an analysis of the creep behavior of deteriorating elastic solids where the principal mechanism of deformation is the growth of intergranular or interfacial flaws. The changes in elastic moduli induced by the growth of internal damage are used to derive the stress exponent in the power-law creep regime. When the flaws advance at a rate which is proportional to the local normal stress or normal strain, a power-law creep exponent of 2 is predicted for short time, steady-state creep for a population of aligned slit cracks and randomly oriented penny-shaped cracks. For long-time creep, the variation of nonsteady state creep strain rate as a function of the far-field stress and time is explicitly determined. General solutions for creep strain rates are also presented for situations where the microcrack growth rate has a power-law dependence on the local normal stress or stress intensity factor. The predicted dependence of creep strain rate on the far-field stress, the progression of damage and the consequent reduction in elastic moduli, overall creep ductility, and implications pertaining to microstructural and temperature effects on creep are found to be in accord with a wide variety of experimental observations for ceramics and ceramic composites. The temperature, stress and material conditions for which the proposed mechanism is applicable are discussed and a general theory of creep damage in progressively microfracturing elastic brittle solids is developed.     

On the development of crack closure and the threshold condition for short and long fatigue cracks in 7150 aluminum alloy

In an attempt to analyze the behavior of physically “short” cracks, a study has been made of the development, location, and effect of crack closure on the behavior of fatigue cracks arrested at the “long” crack threshold stress intensity range, ΔK _TH , in underaged, peak aged, and overaged microstructures in a 7150 aluminum alloy. By monitoring the change in closure stress intensity,K _cl, during thein situ removal of material left in the wake of arrested threshold cracks, approximately 50 pct of the closure was found to be confined to a region within ∼500 μm of the crack tip. Following wake removal, previously arrested threshold cracks recommenced to propagate at low load ratios even though nominal stress intensity ranges didnot exceed ΔK _TH , representing the behavior of physically short cracks emanating from notches. No such crack extension at ΔK _TH was seen at high load ratios. With subsequent crack extension, crack closure was observed to redevelop leading to a deceleration in growth rates. The development of such closure was found to occur over crack extensions comparable with microstructural dimensions, rather than those associated with local crack tip plasticity. Such results provide further confirmation that the existence of a fatigue threshold and the growth of physically short cracks are controlled primarily by crack closure, and the data are discussed in terms of the micro-mechanisms of closure in precipitation hardened alloy systems. Formerly Graduate Student with the Department of Mechanical Engineering, University of California, Berkeley