Experimental investigation of crack growth direction in multiple cracks

In this paper, the interaction between multiple cracks in crack growth direction is studied in an aluminium alloy under static and fatigue loading. Self similar as well as non‐self‐similar crack growth has been observed which depends on the relative crack positions defined by crack offset distance and crack tip distance. On the basis of experimental observations, the criterion for crack coalescence and crack growth direction are expressed in terms of the crack positions defined by crack offset and crack tip distances. The criterion presented in this study can be used to determine the limiting value of crack tip and crack offset distance and to determine the mode in which cracks coalesce during their growth process. Experimental results and crack interaction criterion presented under various crack positions and size conditions could be used to derive a new evaluation method of crack growth in multiple crack geometry.     

Experimental study of I + III mixed mode fatigue crack transformation propagation

In this paper, compact tension specimens with tilted cracks under monotonic fatigue loading were tested to investigate I + III mixed mode fatigue crack propagation in the material of No. 45 steel with the emphasis on the mode transformation process. It is found that with the crack growth, I + III mixed mode changes to Mode I. Crack mode transformation is governed by the Mode III component and the transformation rate is a function of the relative magnitude of the Mode III stress intensity factor. However, even in the process of the crack mode transformation the fatigue crack propagation is controlled by the Mode I deformation.     

Characterization of Loading Rate Effects on the Interactions between Crack Growth and Inclusions in Cementitious Material

The microcapsule-enabled cementitious material is an appealing building material and it has been attracting increasing research interest. By considering microcapsules as dissimilar inclusions in the material, this paper employs the discrete element method (DEM) to study the effects of loading rates on the fracturing behavior of cementitious specimens containing the inclusion and the crack. The numerical model was first developed and validated based on experimental results. It is then used to systematically study the initiation, the propagation and the coalescence of cracks in inclusion-enabled cementitious materials. The study reveals that the crack propagation speed, the first crack initiation stress, the coalescence stress, the compressive strength and the ultimate strain increase with the loading rate. The initiation position, the propagation direction, the cracking length and the type of the initiated cracks are influenced by the loading rates. Two new crack coalescence patterns are observed. It is easier to cause the coalescence between the circular void and a propagating crack at a slow loading rate than at a fast loading rate.     

Numerical investigation of dynamic crack branching under biaxial loading

Dynamic crack growth and branching of a running crack under various biaxial loading conditions in homogeneous and heterogeneous brittle or quasi-brittle materials is investigated numerically using RFPA2D (two-dimensional rock failure process analysis)-Dynamic program which is fully parallelized with OpenMP directives on Windows. Six 2D models were set up to examine the effect of biaxial dynamic loading and heterogeneity on crack growth. The numerical simulation vividly depicts the whole evolution of crack and captured the crack path and the angles between branches. The path of crack propagation for homogenous materials is straight trajectory while for heterogeneous materials is curved. Increasing the ratio of the loading stress in x-direction to the stress in y-direction, the macroscopic angles between branches become larger. Some parasitic small cracks are also observed in simulation. For heterogeneous brittle and quasi-brittle materials coalescence of the microcracks is the mechanism of dynamic crack growth and branching. The crack tip propagation velocity is determined by material properties and independent of loading conditions.     

INTERACTIVE EFFECT OF TWO COPLANAR CRACKS ON PLASTIC YIELDING AND COALESCENCE

A theoretical study of the conditions for coalescence of two fatigue cracks in a multi-crack system is presented. As both cracks grow, the near-side plastic zone sizes of the interacting cracks increase more rapidly than the off-side zone sizes which results in higher overall crack propagation rates for an interacting crack growth system than the simple addition of individual crack growth rates. The use of a single crack growth model to predict failure in the presence of two or more cracks may therefore underestimate the interaction between cracks and lead to un-conservative predictions. Some of the theoretical predictions were confirmed by published results of experimental measurements of crack interaction distances in multi-crack systems.     

Small crack growth rates from simple sequences containing underloads in AA7050-T7451

Fatigue crack growth in materials that display confined slip show crack path changes that are dependant on the loading history. In these materials certain variable amplitude loading patterns can produce strong slip bands ahead of the crack tip. One of these patterns of loadings involving bands of high R cycles followed by one or two underloads also produce distinct features or progression marks on the fracture surface that have been used to delimit small blocks of constant amplitude cycles. The same loading pattern also produces strong slip bands ahead of the fatigue crack both in the plane of the crack and out of plane. These slip bands affect the direction and possibly the rate of propagation of the fatigue crack. Thus these loading patterns make an ideal marker to look at small crack growth rates in the presence of slip bands.This paper reports on the crack growth rates for a series of fatigue cracks grown in AA7050-T7451 coupons, from near initiation to near failure. The aim of this work was to generate constant amplitude crack growth data for use in predictions that is more useful for predicting crack growth lives than that obtained from long crack constant amplitude tests. Three simple sequences which applied small bands of constant amplitude loading were used in the fatigue tests preceded by a loading sequence to produce a progression mark to delimit the bands. The fatigue cracks in the coupon initiated from etch pits on the surface of the coupons. The width of the bands of constant amplitude growth in these sequences were measured under a microscope. The growth in these sequences was found to be faster than for long cracks under constant amplitude loading.     

Modeling of structural failure of Zircaloy claddings induced by multiple hydride cracks

Zirconium alloys have been serving as primary structural materials for nuclear fuel claddings. Structural failure analysis under extreme conditions is critical to the assessment of the performance and safety of nuclear fuel claddings. This work focuses on simulating structural failure of Zircaloy tubes with multiple hydride defects through modeling explicit crack propagation in ductile media. First, we developed an integrated cladding failure model by taking into account both crack initiation induced by hydride/matrix interface separation and ligament tearing-off between activated hydride cracks. Second, to accommodate the initiation, propagation, and coalescence of multiple cracks in finite plastic media we incorporated this structural failure model into a coupled continuous/discontinuous Galerkin (DG) based finite element code, a traditionally preferred implicit numerical framework. Third, to improve the adaptive placement of DG interface elements for crack propagation and to identify potential coalescence of cracks due to the interaction between adjacent hydride cracks, we defined a special failure index for the assessment of potential failure zones using both true plastic strain developed and predicted failure strain based on the Johnson–Cook material failure criterion. Finally, by calibrating the proposed material failure model using a cluster of Zircaloy material experimental tests, we successfully simulated a complete failure process of a fuel cladding tube with multiple hydride cracks.     

拉—拉疲劳下15MnMoVN多裂纹扩展研究

为分析单裂纹或多裂纹在裂纹面承受疲劳拉伸载荷作用下尖端应力强度因子变化规律和裂纹形貌变化以及疲劳寿命情况,以含不同初始长深比的半椭圆单裂纹或双裂纹的薄片试样为研究对象,对试样在应力比R=0.1的疲劳拉伸载荷下单裂纹或双裂纹情况进行了仿真分析。建立含裂纹试样的有限元模型,仿真分析了裂纹在扩展过程中尖端应力强度因子的分布情况,并将单裂纹扩展结果与双裂纹相互作用影响下的结果进行了对比研究;进行含裂纹试样的疲劳实验,分析了含单裂纹或双裂纹的试样的断裂面的形成原因,并验证仿真结果正确性。结果表明,裂纹面之间的相互作用会逐渐影响裂纹的扩展方向、扩展速率以及在扩展过程中尖端应力强度因子的变化趋势;而且初始形貌为半椭圆形的双裂纹在相互作用影响下会逐渐过渡到半圆形。     

Fatigue crack growth behavior of fine-grained steel S460N under proportional and non-proportional loading

In the present investigations a series of fatigue crack growth experiments has been conducted using thin-walled, hollow cylinders with a notch. The cylinders are made of fine-grained steel S460N. Cyclic tension-compression, torsion and proportional as well as non-proportional combinations of both loadings have been applied. For these experiments the crack-initiation locations and the crack growth lives as well as the crack growth curves and the crack paths have been identified. Depending on the loading type initiation of two to four cracks at different positions of the notch has been observed. The location of crack-initiation correlates well with the location of the maximum notch root stress amplitude calculated assuming elastic material behavior. The crack propagation lives vary depending on the type of loading. They are slightly longer under non-proportional loading than under proportional loading.     

Fatigue crack propagation assessment based on residual stresses obtained through cut-compliance technique

Crack growth rate versus crack length curves of heavily overloaded parent material specimens and fatigue crack propagation curves of friction‐stir‐welded aluminium samples are presented. It is shown that in both cases the residual stresses have a strong effect on the crack propagation behaviour under constant and variable amplitude loading. As a simplified engineering approach, it is assumed in this paper, that in both cases residual stresses are the main and only factor influencing crack growth. Therefore fatigue crack propagation predictions are performed by adding the residual stresses to the applied loading and by neglecting the possible effects of overloading and friction stir welding on the parent material properties. For a quantitative assessment of the residual stress effects, the stress intensity factor due to residual stresses K_res is determined directly with the so‐called cut‐compliance method (incremental slitting). These measurements are particularly suited as input parameters for the software packages AFGROW and NASGRO 3.0, which are widely used for fatigue crack growth predictions under constant and variable amplitude loading. The prediction made in terms of crack propagation rates versus crack length and crack length versus cycles generally shows a good agreement with the measured values.     

A new method for modelling the coalescence and growth of two coplanar short cracks of varying lengths in AA7050-T7451 aluminium alloy

The growth, interaction and coalescence of two coplanar short cracks of varying lengths in AA7050-T7451 aluminium alloy were studied under low amplitude cyclic loading. Fractographic studies showed that the way in which the fracture surfaces developed was dependent on the relative crack sizes, however interactions between the cracks did not significantly affect the crack growth rates before the tips of the cracks touch to form a single crack. Subsequently, the longitudinal growth rates of the crack were retarded for a period which appeared necessary for the newly coalesced crack to form a single semi-elliptical shape before resuming growth rates of a single crack. A new mathematical model was developed to account for this behaviour.     

Crack initiation and crack propagation under thermal cyclic loading

Theoretical and experimental investigations of crack initiation and crack propagation under thermal cyclic loading are presented. For the experimental investigation a special thermal fatigue test rig has been constructed in which a small circular cylindrical specimen is heated up to a homogeneous temperature and cyclically cooled down under well defined thermal and mechanical boundary conditions by a jet of cold water. At the end of the cooling phase the specimen is reheated to the initial temperature and the following cycle begins. The experiments are performed with uncracked and mechanically precracked specimens of the German austenitic stainless steel X6CrNi 1811.

In the crack initiation part of the investigation the number of load cycles to initiate cracks under thermal cyclic load is compared to the number of load cycles to initiate cracks under uniaxial mechanical fatigue loading at the same strain range as in the cyclic thermal experiment. The development of initiated cracks under thermal cyclic load is compared with the development of cracks under uniaxial mechanical cyclic load.

In the crack propagation part of the investigation crack growth rates of semi-elliptical surface cracks under thermal cyclic loading are determined and compared to suitable mechanical fatigue tests made on compact-tension and four-point bending specimens with semi-elliptical surface cracks. The effect of environment, frequency, load shape and temperature on the crack growth rate is determined for the material in mechanical fatigue tests.

The theoretical investigations are based on the temperature distribution in the specimen, which is calculated using finite element programs and compared to experimental results. From the temperature distribution, elastic and elastic-plastic stress distributions are determined taking into account the temperature dependence of the material properties. The prediction of crack propagation relies on linear-elastic fracture mechanics. Stress intensity factors are calculated with the weight function method and crack propagation is determined using the Paris relation.

To demonstrate the quality of the crack growth analysis the experimental results are compared to the prediction of crack propagation under thermal cyclic load.     

An adaptive multiscale method for crack propagation and crack coalescence

This work presents a new multiscale technique for the efficient simulation of crack propagation and crack coalescence of macrocracks and microcracks. The fully adaptive multiscale method is able to capture localization effect mesh independently. By modeling macrocracks and microcracks, the extended finite element method offers an accurate solution and captures cracks and their propagation without changing the mesh topology. Propagating and coaliting cracks of different length scales can therefore be easily modeled and updated during the computation process. Hence, the presented method is an efficient and accurate option for modeling cracks of different length scales. This is demonstrated in several numerical examples showing the interaction of microcracks and macrocracks. Copyright © 2012 John Wiley & Sons, Ltd.     

Fatigue growth analysis of interacting and coalescing surface defects

Fatigue crack-growth is simulated for multiple surface cracks in a plate subjected to different combinations of tension and bending loads. The numerical technique employed is based on the step-by-step integration of a Paris' type of fatigue crack-growth law at a set of points of crack front, which enables the crack shape change during propagation to be traced. The stress intensity factors along the crack front are estimated by the three-dimensional finite element method. The technique has also accommodated an automatic procedure for the regeneration of finite element models as the crack grows, so that the prediction of complex crack shape change undergoing particularly during the crack coalescence is made possible and easy. The fatigue growth behaviours at three typical growth stages, i.e. pre-coalescence, coalescence and post-coalescence, are analysed for two multiple crack configurations. The predicted results are also compared with those obtained by the ASME XI code and a ‘no interaction and immediate transition (NIIT)’ simplified method. It is shown that each individual crack before they touch propagates almost independently and the interaction between them is generally limited despite being dependent on actual crack configurations. The analyses of fatigue lives show that both the ASME XI code and the NIIT method give more conservative results than the present simulation technique; however, the NIIT method is relatively less conservative than the ASME XI code which is excessively conservative for the crack configuration examined. This revised version was published online in July 2006 with corrections to the Cover Date.     

Growth and coalescence of two collinear indentation cracks in glass

Subcritical growth and coalescence of two collinear cracks of different lengths were investigated using small Knoop indentation cracks in glass. Indentation cracks subjected to bending in water showed anomalous crack growth in terms of the stress intensity factor, K_I. The crack growth velocity, dc/dt, was initially high, decreased and thereafter increased with increasing K_I. The effective stress intensity factor, K_I,eff, was calculated by adding a term describing the state of residual stress to explain this anomalous growth. Before crack coalescence, a large crack showed a crack velocity higher than expected from the coalescent crack. The coalescent crack velocity increased with K_I,eff and the slope of dc/dt−K_I,eff curves differed from that for a single crack, depending on the crack length.     

爆炸载荷下空孔缺陷与爆生裂纹扩展行为研究

空孔在岩石巷道直眼掏槽爆破中具有重要作用,为研究空孔及其缺陷在爆炸荷载作用下的扩展行为和作用机理,以PMMA代替岩石材料,利用预制裂纹代替空孔缺陷,借助动态焦散线系统和理论分析为手段,研究不同间距下空孔、空孔处预制裂纹、爆生裂纹动态扩展规律及机理,分析不同"径距比"与掏槽效果的关系。研究结果表明:在装药量一定的情况下,随着炮孔与空孔距离的变化,爆生裂纹扩展距离呈现递增而预制裂纹扩展距离呈现递减的趋势,但都存在极值;当炮孔与空孔距离较小时,爆生裂纹和预制裂纹扩展及相互作用最复杂,爆生裂纹扩展经历由压缩应力波为主,表现为直线的前期扩展;由空孔处发射应力波和压缩应力波共同作用下,爆生裂纹偏离炮孔与空孔连心线的中期扩展,以及由空孔应力集中区作用使爆生裂纹向空孔方向偏移的后期阶段;预制裂纹扩展经历由空孔处应力集中作用下,表现为直线的前期扩展,以及由爆生裂纹处反射拉伸波作用使其向爆生裂纹发展的后期阶段;当炮孔与空孔距离较大时,反射应力波及应力集中效应对爆生裂纹和预制裂纹扩展在减弱,爆生裂纹与预制裂纹扩展行为仅有前期直线扩展阶段。"径距比"的大小对爆破效果影响较大,直眼掏槽爆破应以最优"径距比"作为掏槽爆破参数设计的依据。     

Growth behaviour of small surface fatigue cracks in AISI 304 stainless steel

A series of axial tensile fatigue tests (R = 0.1) was carried out to investigate the initiation and the growth behaviours of very small surface fatigue cracks under two different surface conditions (viz. smooth and pitted surfaces) of AISI 304 stainless steel at room temperature. This paper deals with both of the two approaches regarding the analysis of fatigue: the approach based on the concept of fracture mechanics and low cycle fatigue. In particular, both the initiation and growth of cracks and the coalescence of small cracks by fatigue in the specimen have been investigated by the methods of surface replicas and photomicrographs. Quantitative information such as the initiation period, growth and coalescence behaviours of small cracks, and crack growth properties were systematically obtained. The results show that the accurate determination of these parameters is critical for the application of fracture mechanics to fatigue life assessment.     

Effect of Cyclic Loading on Cracking Behaviour of X-70 Pipeline Steel in Near-Neutral pH Solutions

The cracking behaviour of X-70 pipeline steel in near-neutral pH solutions was studied under different modes of cyclic loading. The crack propagation process of X-70 pipeline steel under low frequency cyclic loading condition was controlled mainly by stress corrosion cracking (SCC) mechanism. Under mixed-mode cyclic loading, both higher tensile stress and shear stress made cracks easier to propagate. Applied cathodic potentials and high content of carbon dioxide in solutions also promoted the propagation of cracks. The propagation directions of cracks were different under different cyclic loading conditions. Under mode I (pure tensile stress) cyclic loading condition, cracks were straight and perpendicular to the tensile stress axis, while under mixed-mode I/Ill (tensile/shear stress) cyclic loading, cracks were sinuous and did not propagate in the direction perpendicular to the main tensile stress axis. Under the mixed-mode cyclic loading, cracks were much easier to propagate, suggesting that shear stress intensified the role of tensile stress. In addition, shear stress promoted the interaction between cracks, resulting in easier coalescence of cracks.     

INITIAL FRACTURE MECHANISMS IN NICKEL ALLOYED PM STEEL

Abstract— Initial fatigue crack propagation mechanisms at near threshold conditions were studied for four nickel-alloyed, powder-metallurgy (PM) steels. Fatigue fracture surfaces were obtained by testing smooth rectangular specimens at 30 Hz and under constant amplitude and zero mean stress conditions. Materials based on Distaloy AE were used in two densities, namely 7.15 and 7.45 g/cm³.
All the fracture surfaces were composed of three morphological regions (i) a macrocrack initiation region Rl where cracks propagated preferentially through particles (ii) a macrocrack growth region R2 and (iii) an unstable crack growth region R3 where cracks propagated preferentially between particles. Initial fatigue crack growth, in region R1, was controlled by the propagation of short cracks whose dimensions were comparable to the material microstructure. The subsequent fatigue crack growth in regions R2 and R3 was controlled by ductile rupture between microvoids. Transparticle fracture in region R1 was independent of pore distribution, while interparticle fracture in regions R2 and R3 was dependent on pore distribution.