Influence of the constraint effect on the fatigue crack growth rate in S355 J2 steel using digital image correlation

Displacement fields around the fatigue crack tip for a constant value of stress intensity factor (SIF) range were measured using digital image correlation (DIC) technique on the S355 J2 steel grade. The data obtained were resolved into the T‐stress evaluation and quantification of its influence on the fatigue crack growth rate. The higher order terms of the Williams expansion (WE) were calculated as well. The displacements of a set of points outside of the plastic zone were selected for application of the over‐deterministic method (ODM) to obtain several initial WE terms. The computed values of T‐stress show good agreement with finite element analysis and literature. It was shown by collected experimental displacement, that the level of constraint influences the fatigue crack propagation rate.     

Extended digital image correlation with crack shape optimization

The methodology of eXtended finite element method is applied to the measurement of displacements through digital image correlation. An algorithm, initially based on a finite element decomposition of displacement fields, is extended to benefit from discontinuity and singular enrichments over a suited subset of elements. This allows one to measure irregular displacements encountered, say, in cracked solids, as demonstrated both in artificial examples and experimental case studies. Moreover, an optimization strategy for the support of the discontinuity enables one to adjust the crack path configuration to reduce the residual mismatch, and hence to be tailored automatically to a wavy or irregular crack path. Copyright © 2007 John Wiley & Sons, Ltd.     

Probabilistic simulation of fatigue crack growth by damage accumulation

A fatigue crack growth simulation model based on probabilistic damage accumulation is presented. The modified weakest-link hypothesis determines the reinitiation of voids ahead of the crack, which in turn determines the crack advance. From a linearly cumulated-damage standpoint, this is shown to be approximated by a statistical Miner's law. The historical damage on the material elements at the crack-tip vicinity, neglected in previous analysis, is quantified, and shown to be significant. The model includes a prediction of the fatigue threshold stress-intensity factor range, and a comparison with published crack growth rate data for three different structural metals.     

AFM image reconstruction for deformation measurements by digital image correlation

The scanner drift of the atomic force microscope (AFM) is a great disadvantage to the application of digital image correlation to micro/nano-scale deformation measurements. This paper has addressed the image distortion induced by the scanner drifts and developed a method to reconstruct AFM images for the successful use of AFM image correlation. It?presents such a method, that is to generate a corrected image from two correlated AFM images scanned at the angle of 0° and 90° respectively. The proposed method has been validated by the zero-deformation test. A buckling test of a thin plate under AFM has also been demonstrated. The in-plane displacement field at the centre point of the buckling plate has been successfully characterized by the application of the image correlation technique on reconstructed AFM images.     

Investigation of sub-critical fatigue crack growth in FRP/concrete cohesive interface using digital image analysis

The cohesive stress transfer during the sub-critical crack growth associated with the debonding of FRP from concrete under fatigue loading is experimentally investigated using the direct shear test set-up. The study focused on high-amplitude/low-cycle fatigue. The fatigue sub-critical crack growth occurs at a load that is smaller than the static bond capacity of the interface, obtained from monotonic quasi-static loading, and is also associated with a smaller value of the interfacial fracture energy. The strain distribution during debonding is obtained using digital image correlation. The results indicate that the strain distribution along the FRP during fatigue is similar to the strain distribution during debonding under monotonic quasi-static loading. The cohesive crack model and the shape of the strain distribution adopted for quasi-static monotonic loading is indirectly proven to be adequate to describe the stress transfer during fatigue loading. The length of the stress transfer zone during fatigue is observed to be smaller than the cohesive zone of the interfacial crack under quasi-static monotonic loading. The strain distribution across the width of the FRP sheet is not altered during and by fatigue loading. A new formulation to predict the debonding crack growth during fatigue is proposed.     

Investigation of fatigue crack closure using multiscale image correlation experiments

Two full-field macroscale methods are introduced for estimating fatigue crack opening levels based on digital image correlation (DIC) displacement measurements near the crack tip. Crack opening levels from these two full-field methods are compared to results from a third (microscale) method that directly measures opening of the crack flanks immediately behind the crack tip using two-point DIC displacement gages. Of the two full-field methods, the first one measures effective stress intensity factors through the displacement field (over a wide region behind and ahead of the crack tip). This method reveals crack opening levels comparable to the limiting values (crack opening levels far from the crack tip) from the third method (microscale). The second full-field method involves a compliance offset measurement based on displacements obtained near the crack tip. This method delivers results comparable to crack tip opening levels from the microscale two-point method. The results of these experiments point to a normalized crack tip opening level of 0.35 for R ∼ 0 loading in grade 2 titanium. This opening level was found at low and intermediate ΔK levels. It is shown that the second full-field macroscale method indicates crack opening levels comparable to surface crack tip opening levels (corresponding to unzipping of the entire crack). This indicates that effective stress intensity factors determined from full-field displacements could be used to predict crack opening levels.     

Fatigue crack growth in Haynes 230 single crystals: an analysis with digital image correlation

Fatigue crack growth was investigated in Haynes 230, a nickel‐based superalloy. Anisotropic stress intensity factors were calculated with a least squares algorithm using the displacements obtained from digital image correlation. Crack opening/sliding levels were measured by analysing the relative displacement of crack flanks. Reversed crack tip plastic zones were calculated adopting an anisotropic yield criterion. The strains measured in the reversed plastic zone by digital image correlation showed a dependence on crystallographic orientation. Finally, a finite element model was adopted to examine plasticity around the crack tip. Results were compared with the experimentally observed strains.     

High temperature fatigue crack growth in Inconel 718

Abstract

The nickel base superalloys are extensively used in high temperature applications, so it is important to know their behaviour under conditions of high-temperature fatigue. This paper studies the influence of ΔK, loading frequency, stress ratio and temperature on the high temperature fatigue crack growth rate of nickel base superalloys. This study is based on fatigue tests carried out in corner crack specimens of Inconel 718 at 600°C and at room temperature. Three stress ratios (R = 0.05, 0.5 and 0.8) and loading frequencies ranging from 0.0017 to 15 Hz were considered in the tests. For frequencies below 0.25 Hz, the load wave shape was trapezoidal with different dwell times at maximum load. At relatively high frequencies the propagation is cycle dependent, while for lower frequencies it is time dependent. At intermediate frequencies a mixed crack growth occurs. The transition frequencies from cycle dependent to mixed regime and from mixed to time dependent regime were obtained for each R. The increase of R increases the transition frequencies, i.e., extends the time dependent crack growth to higher frequencies. The increase of R also produces an increase of cyclic crack growth rate for all regimes of crack growth. In the time dependent regime, a higher variation is observed, that can be explained by an acceleration of oxidation damage promoted by the increase of maximum stress. An approach for modelling the high-temperature fatigue crack growth in nickel base superalloys is presented. A good agreement was observed between time dependent fatigue results and mathematical models based on static load results.     

High resolution digital image correlation using proper generalized decomposition: PGD‐DIC

A new method is proposed to measure the finite element (FE) displacement field from a deformed image in comparison with a reference one. In opposition to standard FE approaches, the unknown displacement is sought as a sum of products of separated dimension functions. With the problems in each dimension being uncoupled, the method involves only one‐dimensional meshes and one‐dimensional problems. An algorithm that builds successive best rank‐one approximations is proposed and integrated into the nonlinear iterations of the correlation problem. Although the method can be applied to spaces of any dimension, this paper focuses on two‐dimensional images. Many synthetic examples are provided to evaluate the performance of the method. In addition, it is shown that, even with this separated representation, the introduction of a regularization operator is convenient. The latter makes it possible to perform a pixel‐wise measure with huge computational savings. Copyright © 2012 John Wiley & Sons, Ltd.     

疲劳裂纹扩展中裂纹尖端应变变化的详细考察

传统的ASTM2%偏差法和卸载弹性法是以柔度轨迹的线性区段确定开闭口载荷(Pop和Pcl),往往因人工误差导致结果相差很大.本文注重裂纹尖端塑性变形带给柔度变化的物理意义,描述了一个载荷循环下柔度变化与裂纹开闭口以及弹塑性行为的关系.采用自行开发的高精度局部柔度测量法,针对结构钢进行了疲劳试验,记录了裂纹尖端应变与载荷关系的系列迟滞回线,并以微分法定量求出迟滞回线上的特征载荷.根据试验考察结果,分析了文献中几种疲劳裂纹扩展参量的关系.结果表明,ΔKRPG比ΔKcl和ΔKop更适合作为裂纹扩展驱动力参量.     

Early fatigue crack growth as the damage accumulation process

A probabilistic approach to modeling of the initial stage of fatigue crack growth is suggested based on the concepts of continuum damage mechanics. The material is presented as a set of microstructural elements with randomly distributed properties. Both the grains and intergranular boundaries are considered as the elements of microstructure. The parameters of resistance of each element to damage accumulation are considered as random variables. These parameters are distributed among the elements independently that allows to model the damage process in polycrystalline materials. The damage measure depends on the characteristic normal and tangential stresses in order to take into account the tensile and shear fracture modes for each element of microstructure. It is assumed that a nucleus of a crack is initially present near the body surface as a single completely ruptured element. The final damage of an element is considered as the crack advancement. The crack is modelled as a sequence of ruptured grains for the transgranular fracture, and as a sequence of couples of neighboring ruptured grains when the intergranular rupture is considered. Numerical simulation is performed to illustrate feasibility of the proposed model. In particular, non-planar crack propagation, blunting, kinking and branching of cracks at the early stage is demonstrated. The non-monotonous pattern of the short crack growth process is observed. Statistical scattering of the current crack size and the crack growth rate as functions of the cycle number and the crack depth is studied.     

Low-cycle fatigue crack growth prediction by strain intensity factor

The validity of the strain intensity factor for representing driving force of fatigue crack growth under the large scale yielding condition was shown in this study. First, a crack growth test technique using plate specimens was developed in order to apply a fully-reversed cyclic load and to measure the global strain for calculating the strain intensity factor. Fatigue crack growth tests using Type 316 stainless steel revealed that the growth rates correlated well with the strain and effective strain intensity factor ranges for various stress or strain ranges, specimen geometries and loading modes (stress- and strain-controlled). Based on the test results, validity and physical meaning of the strain intensity factor were discussed. It was concluded that the strain intensity factor was an effective parameter for predicting crack growth under the small and large scale yielding conditions.     

High temperature fatigue crack growth in superalloy blade materials

Abstract

The main factors influencing high temperature fatigue crack growth in superalloy blade materials are discussed. The significance of temperature, frequency, environment, and minimum to maximum load ratio R are considered in turn. Transgranular cycle dependent and intergranular time dependent controlled cracking processes are identified. Conditions favouring each mechanism are clarified and it is shown that cumulative damage concepts can be applied to predict interaction effects. It is found that cycle controlled processes are most likely to dominate at high frequency and low values of R. Creep and oxidation mechanisms, which are favoured by low frequencies, high temperatures, and high values of R, have been identified as contributing to the time-dependent component of cracking. It is shown that creep and oxidation can enhance crack growth/cycle significantly and reduced blade lifetimes.

MST/517     

A unified approach to damage accumulation and fatigue crack growth

An approach is proposed to the theory of fatigue cracks propagation based on the following postulate: a growing crack at least once in a cycle becomes a nonequilibrium one (in the Griffith's sense) under the condition that the resistance to crack growth is calculated with an account of damage accumulated at the crack tip during the loading history. The theory is used for nonuniaxial stress states including jumplike growth, stops, kinking and branching phenomena. The general structure of differential equations is discussed for the averaged crack growth rate under nonuniaxial loading.     

Cyclic analysis of a propagating crack and its correlation with fatigue crack growth

Stress and strain field of a propagating fatigue crack and the resulting crack opening and closing behavior were analysed. It was found that a propagating fatigue crack was closed at tensile external loads due to the cyclically induced residual stresses. Strain range value $\text{Δ?}{}_{\mathrm{y}}$ in the vicinity of the crack tip was found to be closely related with the effective stress intensity factor range $\text{ΔK}{}_{\mathrm{eff}}$ which was determined on the basts of the analytical crack opening and closing behavior at its tip. Application of this analysis to the non-propagating fatigue crack problem and the fatigue crack propagation problems under variable stress amplitude conditions revealed that both $\text{Δ?}{}_{\mathrm{y}}$ and $\text{ΔK}{}_{\mathrm{eff}}$ were essential parameters governing fatigue crack growth rate.     

Full‐field characterisation of crack tip deformation and fatigue crack growth using digital image correlation—a review

In recent years, digital image correlation has been widely used in the analysis of crack problems. This review will examine digital image correlation, as a full‐field measurement technique, in the studies of crack tip mechanical behaviour under cyclic loading conditions. In particular, topics including determination of fracture mechanics parameters and evaluation of crack closure will be discussed. Micromechanical aspects of crack growth under cyclic loading will also be explored in crack driving and attenuation effects.     

A relative crack opening time correlation for corrosion fatigue crack growth in offshore structures

A considerable amount of research has been carried out on the prediction of mean stress effects on fatigue crack growth in structures. Newer types of structure are now being developed for use in highly dynamic, harsh marine environments, particularly for renewable energy applications. Therefore, the extent to which mean stresses can enhance corrosion‐assisted fatigue damage in these structures needs to be better understood. A new theoretical model that accounts for mean stress effects on corrosion fatigue crack growth is proposed. The model is developed based on the relative crack opening period per fatigue cycle and by considering only the damaging portion of the stress cycle. The baseline data for the modelling exercise are the data obtained at a stress ratio of 0.1 in air and seawater tests conducted on compact tension specimens. The model is validated by comparison with experimental data and with other fatigue crack propagation models. The proposed model correlates fairly well with experimental data and the other models examined.