A geometric model for fatigue crack closure induced by fracture surface roughness

Mechanisms for fatigue crack closure under plane strain conditions have recently been identified at very low (near-threshold) stress intensities in terms of effects of excess corrosion deposits or fracture surface roughness in promoting premature closure of the crack. In the present paper, a geometric model is presented for crack closure induced by fracture surface roughness. This model specifically addresses the contribution from both Mode I and Mode II crack tip displacements in addition to considering the nature of the fracture surface morphology. The implications of this model are briefly discussed in light of the roles of grain size, yield strength, microstructure, and crack size in influencing near-threshold fatigue behavior in engineering alloys.     

岩石中裂隙损伤对超声波衰减影响的实验研究

在分析岩石裂隙损伤声衰减基础上，形成了损伤参量Ｄ与衰减系数α的关系式，建立了表达缺陷尺度与数量分布特征的裂隙集合。运用能量法进行了玄武岩度件损伤声衰减与应力关系的实验研究。结果表明，品质因数下降量△Ｑｐ随应力比率变化的线性ｔｇα１，能够用来反映岩石裂隙损伤繁衍的规律。本文还对工程岩声衰减检测应用进行探索。     

基于响应曲面在高牌号硅钢拉伸试样加工面粗糙度试验研究#br#

文中采用高速动态加工技术,探究对高牌号硅钢拉伸试样加工面粗糙度的影响规律,采用基于曲面响应曲面法设计试验,建立了预测模型,运用方差分析法检验模型,结合高速动态加工理论分析加工面粗糙度的影响因素。试验结果表明：主轴转速及其平方项对加工面粗糙度影响最大,其次是轴向切深,最后是每齿进给量及其与主轴转速的交互作用,建立的加工面预测模型精确度较高,为实际的加工生产提供参考。可根据实际加工条件运用响应曲面设计,结合动态加工理论,应用于其他材料拉伸试样的高效加工。     

Experimental and numerical study on the relationship between creep crack growth properties and fracture mechanisms

Evaluation of creep crack growth properties taking microscopic aspects into account is effective for developing more accurate life prediction of structural components. The present study investigated the relationship between creep crack growth properties and microscopic fracture aspects for austenitic alloy 800H and 316 stainless steel. The growth rate of wedge-type intergranular and transgranular creep crack could be characterized by creep ductility. Creep damages formed ahead of the void-type crack tip accelerated the crack growth rate. Based on these experimental results, a three-dimensional finite element method (FEM) code, which simulates creep crack growth, has been developed. The effect of creep ductility on da/dt vs C* relations could be simulated based on the critical strain criteria. The diffusion of vacancies toward crack tip would accelerate the crack growth under creep conditions. The change of vacancy concentration during creep was computed for a three-dimensional compact-type (CT) specimen model by solving the diffusive equation under multiaxial stress field. The experimental results that crack growth was accelerated by creep damages formed ahead of the crack tip could be successfully simulated.     

The dependence of the fracture toughness of mi steel on temperature and crack velocity

The dependence of the dynamic plane-strain fracture toughness,K _Id, on temperature and crack velocity was measured for propagating cracks in 1020 steel. The dynamics of crack propagation in double-cantilevered specimens was recorded using electroresistivity techniques. The fracture surface energy was found by comparing the crack propagation to solutions of crack motion in wedged-open cantilevered specimens. The_KId behavior was investigated over a range of temperatures from —196° to —50°C and crack velocities of 3 × 10^-3 to 5 × 10^-2 of √E/p. The rate and temperature dependence ofK _Id over the range ofT and υ_c investigated is well described by:1/K _ld ²= υ₀ are experimental constants. A dynamic value ofK _Id was 70 pct ofK _Ic at the same temperature, although in the temperature and crack velocity range investigated the specific fracture surface energy varies by a factor of 6. The temperatureT _T =B/A in(υ _o/υ_c) for which1/K _Id ² = 0 is similar to Charpy impact transition temperature values whenυ _c = 3 × 10^-3√.E/p. If the plane-strain stress condition could be maintained, thenT _T would define a brittle-ductile transition temperature for dynamic plane-strain fracture toughness. The constantsA andB are interpreted by understanding the plastic energy dissipated by a moving crack. Formerly with Brown University, Providence, R. I.     

Fractal analysis on the fatigue fracture surface along the crack propagation direction

The fractal dimension along the crack propagation direction on the fatigue fracture surface of a dual-phase steel was investigated by both vertical section profile method and secondary electron line scanning method. Results from the vertical section profile method showed that during the crack propagation, the fractal dimension of the fracture surface increases with increasing stress intensity factor, however, the secondary electron line scanning method presented somewhat different results of fractal dimension and was proved not feasible in all kinds of fracture surfaces, so this method is not suggested in calculating the fractal dimension of the fracture surface.     

Fatigue crack deflection and fracture surface contact: Micromechanical models

The variation of cyclic crack propagation rates, under thecombined influence of crack kinking (deflection) and fracture surface contact (closure), is estimated from simple linear elastic analyses of tilted cracks. The predictions of the models are consistent with the experimental results of linear and kinked crack advance in high strength aluminum alloys testedin vacuo. Examples of crack deflection in various engineering alloy systems and some generalizations ofaverage deflection parameters based on microstructural and mechanical factors are discussed. The individual contributions to overall growth rates from deflection and closure processes are evaluated for different mechanical and metallurgical conditions. The significance, implications, and limitations of the models are outlined.     

Fatigue crack deflection and fracture surface contact: Micromechanical models

The variation of cyclic crack propagation rates, under thecombined influence of crack kinking (deflection) and fracture surface contact (closure), is estimated from simple linear elastic analyses of tilted cracks. The predictions of the models are consistent with the experimental results of linear and kinked crack advance in high strength aluminum alloys testedin vacuo. Examples of crack deflection in various engineering alloy systems and some generalizations ofaverage deflection parameters based on microstructural and mechanical factors are discussed. The individual contributions to overall growth rates from deflection and closure processes are evaluated for different mechanical and metallurgical conditions. The significance, implications, and limitations of the models are outlined.     

Dependence of dynamic fracture resistance on crack velocity in tungsten: Part 1. single crystals

The dependence of dynamic fracture resistance on crack propagation velocity on (100) in tungsten has been examined. A correlation is obtained between the measured local crack velocity with the surface and subsurface deformations. Based on the experimental results on one pass, two passes, and prestrained, electron beam zone refined single crystals, a discussion is given on the slip modes activated at the crack tip, the contributions to the dynamic fracture resistance from dislocations and surface features, and from the preexisting deformed microstructure. Formerly with the State University of New York at Stony Brook Formerly with the State University of New York at Stony Brook     

Dependence of dynamic fracture resistance on crack velocity in tungsten: Part II. bicrystals and polycrystals

The experimental techniques for crack velocity measurements have been applied to bicrystals of tungsten with twist orientations about «100» and polycrystals. The hesitation of the propagating cleavage crack in the vicinity of the grain boundary is examined. The contributions to energy dissipation from deformation and fracture processes in the grain boundary region as well as the indirect effects of crack deceleration are discussed. These findings have been applied to explain the dynamic fracture resistance and crack arrest in polycrystals.     

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.     

Effect of crack surface geometry on fatigue crack closure

The geometry of crack faces often plays a critical role in reducing crack extension forces when crack closure occurs during fatigue crack growth. Most previous studies of fatigue crack closure are concerned with mechanical measures of closure as related to the crack growth rate; very little attention has been given to the geometry of the crack surfaces. Our objective is to identify those aspects of crack surface geometry that are important in the closure process, to develop quantitative fractographic techniques to estimate such attributes in a statistically significant and robust manner, and to correlate them to the physical process of crack closure. For this purpose, fatigue crack propagation experiments were performed on a Ni-base superalloy and crack growth rates and crack closure loads were measured. Digital image profilometry and software-based analysis techniques were used for statistically reliable and detailed quantitative characterization of fatigue crack profiles. It is shown that the dimensionless, scale-independent attributes, such as height-to-width ratio of asperities, fractal dimensions, dimensionless roughness parameters,etc., do not represent the aspects of crack geometry that are of primary importance in the crack closure phenomena. Furthermore, it is shown that the scaledependent characteristics, such as average asperity height, do represent the aspects of crack geometry that play an interactive role in the closure process. These observations have implications concerning the validity of geometry-dependent, closure-based models for fatigue crack growth.     

A general method for estimation of fracture surface roughness: Part II. Practical considerations

Surface roughness is an important attribute of fracture surfaces. An assumption-free method for estimation of surface roughness presented in Part I^[1] is analyzed further here. It is shown that three vertical sectioning plane orientations mutually at an angle of 120 deg contain sufficient information for a reliable estimation of surface roughness; in most of the cases, the sampling error due to measurements on a limited number (three) of vertical section orientations should be less than ±6 pct with a confidence limit of 95 pct. A simplified procedure is presented for calculation of a profile structure factor from the measurements of the profile frequency function. A practical example of application of the present analysis involving measurement of the fracture surface roughness of a metal-matrix composite is discussed.     

A general method for estimation of fracture surface roughness: Part I. Theoretical aspects

An assumption-free general method is developed for quantitative estimation of fracture surface roughness from the measurements performed on the fracture profiles generated by sectioning planes which are normal to the average topographic plane of the fracture surface. The input data are the profile roughness parameter,R _L , and angular orientation distribution of line elements on the fracture profile,f(α). It is shown that (fx1193-01) whereR _S is the fracture surface roughness parameter andψ is the profile structure factor, which is completely determined by the profile orientation distribution function,f(α).R _L · ψ the expected value of the productR _L · ψ on a set of sectioning planes normal to the average topographic plane of the fracture surface; measurement ofR _L andψ on few such sectioning planes can give a reliable estimate of the fracture surface roughness,R _S . The result is geometrically general, and it is applicable to fracture surfaces of any arbitrary complexity and anisotropy.     

A general method for estimation of fracture surface roughness: Part I. Theoretical aspects

An assumption-free general method is developed for quantitative estimation of fracture surface roughness from the measurements performed on the fracture profiles generated by sectioning planes which are normal to the average topographic plane of the fracture surface. The input data are the profile roughness parameter,R _L, and angular orientation distribution of line elements on the fracture profile,f(α). It is shown that

A general method for estimation of fracture surface roughness: Part II. Practical considerations

Surface roughness is an important attribute of fracture surfaces. An assumption-free method for estimation of surface roughness presented in Part I^[1] is analyzed further here. It is shown that three vertical sectioning plane orientations mutually at an angle of 120 deg contain sufficient information for a reliable estimation of surface roughness; in most of the cases, the sampling error due to measurements on a limited number (three) of vertical section orientations should be less than ±6 pct with a confidence limit of 95 pct. A simplified procedure is presented for calculation of a profile structure factor from the measurements of the profile frequency function. A practical example of application of the present analysis involving measurement of the fracture surface roughness of a metal-matrix composite is discussed.     

Some considerations on fatigue crack closure at near-threshold stress intensities due to fracture surface morphology

Summary It is noted that at near-threshold levels, in addition to the role of plasticity-and oxide-induced crackclosure, fracture surface roughness or morphology may promote significant closure effects in plane strain, as similarly noted by Minakawa and McEvily.This is considered to result from the fact that, where maximum plastic zones sizes are small compared to the grain size, fatigue crack growth proceeds by a single shear decohesion mechanism (Stage I) with associated Mode II+I displacements. The resulting serrated or faceted fracture surfaces (“microstructurally-sensitive growth”) coupled with Mode II crack tip displacements thus induce high closure loads (i.e., K _cl/K _max ~0.5) by wedging the crack open at discrete contact points. At higher growth rates where the plastic zone encompasses many grains, striation growthvia alternating or simultaneous shear mechanisms (Stage II) produces a more planar fracture surface, with pure Mode I displacements, and a corresponding reduction in closure loads. Such concepts of roughness-induced closure are shown to be consistent with observations of the role of coarse grain sizes in reducing near-threshold crack growth rates at low load ratios and of the absence of this effect at high load ratios. R. O. RITCHIE and S. SURESH, both formerly with Massachusetts Institute of Technology     

Effect of precipitant surface roughness on cementation kinetics

Empirical correlation equations for the cementation of cadmium on zinc, of copper on zinc and of copper on cadmium have been developed.For each of the three different systems when the cementation reaction is carried out under an inert atmosphere using a rotating disc geometry under laminar flow conditions, the correlation equation in the initial stage follows the Levich expression written in dimensionless form: Sh = 0.62 (Re)^0.5 (Sc)^0.333For the enhanced rate period, the data were correlated by Sh = A (Re)^0.74 (Sc)^0.29 The constant A depends on the actual deposit morphology and has the values of 0.37, 0.15 and 0.11 for the cadmium/zinc, copper/zinc and copper/cadmium precipitation systems when the deposition is from dilute aqueous solutions.