Underload‐induced crack growth behaviour of minor cycles of pipeline steel in near‐neutral pH environment

Despite decades of research, the mechanisms of near‐neutral pH stress corrosion cracking of pipeline steel are still not fully understood. This investigation was aimed to understand the effect of minor cycles with very high R ratios (minimum stress/maximum stress) on crack growth in air and in near‐neutral pH environments. It has been demonstrated that the minor cycle, even with an R ratio as high as 0.9, could significantly contribute to the crack growth of pipeline steels in the presence of a large underload cycle with a low R ratio of 0.5 in near‐neutral pH environment. Comparing with constant amplitude tests, an increase of crack growth rate by a factor of 3 and 5 was observed when some non‐propagating minor cycles were combined with an underload cycle for tests in air and in near‐neutral pH solution, respectively. Based on the test results, the crack growth mechanisms during minor cycle loading in near‐neutral pH environments were discussed and practical strategies aimed to minimise crack growth during pipeline operation were also proposed.     

Stress Corrosion Crack Growth in Pipe Grade Steels in Near Neutral pH Environment

This paper is concerned with an accelerated testing and modeling of stress corrosion cracking (SCC) phenomena in pipe grade steels in near neutral pH environment. In modeling of SCC, the authors adopt the crack layer theory that provides formalism to account for contributions to crack growth rate such processes as electro-chemical corrosion, hydrogen embrittlement and mechanical loading. Special attention is paid to the hydrogen diffusion, a precursor to hydrogen embrittlement. The energy-momentum tensor (Eshelby's tensor) is employed to evaluate the thermodynamic forces responsible for SC crack growth. Griffith' crack equilibrium condition is used to derive a quasi-equilibrial SC crack growth equation. A parametric study and comparison with the experimental results of corrosion fatigue tests for various maximal stress, stress ratio and electric potential are performed to examine the validity of the proposed model.     

Thermodynamically modeling the interactions of hydrogen,stress and anodic dissolution at crack-tip during near-neutral pH SCC in pipelines

A thermodynamic model was developed to clarify the interactions of hydrogen, stress and anodic dissolution at crack-tip during near-neutral pH stress corrosion cracking in pipelines by comprehensively considering the electrochemical reactions occurring in the pipeline steel in deoxygenated, near-neutral pH solution. By analyzing the change of the free-energy of steel due to the presence of hydrogen and stress, it is demonstrated that a synergism of hydrogen and stress promotes the cracking of steel. The enhanced hydrogen concentration in the stressed steel significantly accelerates the crack growth. An exact expression of the hydrogen concentration factor, i.e., the dependence of anodic dissolution rate of steel at crack-tip on the hydrogen concentration, is essential at the quantitative prediction of the crack growth rate.     

Corrosion Fracture of Structural Metallic Materials: Effect of Electrochemical Conditions in Crack

Abstract: The work is a compressed review based on the summarised results and the original approach for study of corrosion crack growth, taking into account local electrochemical conditions in the crack tip, which was developed at the Karpenko Physico‐Mechanical Institute of NASU. The model scheme of the pre‐fracture zone in the corrosion crack tip, which can be defined by the local values of pH of solution, electrode potential of metal E and stress intensity factor K_I is proposed. For its realisation, the special method and testing equipment for corrosion crack growth study and local electrochemical measurements in the crack were developed. The variation of the electrochemical conditions in corrosion cracks was studied, and it has been found that some stabilised levels of the pH and E values can be achieved in the tip of a non‐propagating and a propagating crack under static and cyclic loading during of exposure time. On this ground, the method for forecasting of the threshold stress intensity factor K_ISCC under stress corrosion cracking was proposed using these characteristic values of pH and E. This method was also adopted for the determination of the threshold stress intensity factor K_th under corrosion fatigue. The special method for determining corrosion fatigue crack growth rate diagrams based on consideration of extreme electrochemical conditions in the crack tip was developed. It has been proven that such diagrams reflect the extreme influence of the environmental factor on corrosion fracture of material, and they may be recommended as the base for the remaining lifetime calculation of the structural elements exploited under environmental conditions.     

Crack growth prediction for underground high pressure gas lines exposed to concentrated carbonate–bicarbonate solution with high pH

A crack growth model for high pH stress corrosion cracking of pipeline steels is presented based on the assumption that the cracking is dominated by the repeated rupture of passive film at the crack tip. The model is validated by the experimental data available. It provides a reasonably good prediction to effects of various factors relating to materials, environment and loading conditions. Although the fatigue damage produced in operation of gas lines is negligible, the contribution due to cyclic load-promoted crack tip dissolution needs to be considered in remaining life prediction. Finally, the procedure for field application is discussed.     

加载历史和裂纹闭合对疲劳裂纹扩展行为影响的数值模拟

采用不同应力比条件下的16MnR钢紧凑拉伸试样,设计了三种有限元分析模型,即不考虑加载历史效应的静态裂纹扩展模型,同时考虑加载历史和裂纹闭合的动态裂纹扩展模型以及仅考虑加载历史的伪动态裂纹扩展模型,对疲劳裂纹闭合过程、裂纹尖端的应力-应变迟滞环、疲劳损伤和裂纹扩展速率进行了数值模拟与分析,进而着重探讨了加载历史和裂纹闭合影响疲劳裂纹扩展行为的交互作用机制。结果表明:对于同类分析模型,应力比越大越不容易产生裂纹闭合;而在应力比相同的情况下,加载历史引起的残余压应力对裂纹闭合有明显的促进作用。裂纹闭合效应阻碍了平均应力的松弛,减小了裂纹尖端附近的应力-应变场强度、疲劳损伤和裂纹扩展速率,而加载历史引起的残余压应力则加快了平均应力的松弛和抑制了棘轮效应。与实验结果比较发现,只有同时考虑了裂纹闭合效应和加载历史影响的动态裂纹扩展模型,才能对疲劳裂纹扩展行为进行准确、定量的模拟。     

STRESS STATE-RELATED FATIGUE CRACK GROWTH UNDER SPECTRUM LOADING

Abstract— The fatigue crack growth behaviour in aluminium alloy sheets of 2024-T3 and 7475-T761, subjected to standardized spectra (TWIST and FALSTAFF), was investigated using centre-cracked specimens. A strip crack closure model was used to interpret experimental data, and to make predictions for the crack growth.
The strip model is based on the Dugdale concept, but modified to keep plastically-stretched materials on the crack surface so that the crack opening load can be determined, and the fatigue crack growth can be analysed according to Elber's crack growth assumption. Differing from other models of the same kind, a variable constraint factor was introduced to account for the gradual transition of stress state at the crack tip resulting from the crack growth. It has been shown that the transition of stress state at the crack tip causes the unusual behaviour of the fatigue crack growth in sheets. Both experiments and predictions show that a crack may grow faster at a low load than at a higher one in a certain applied load range due to the crack tip stress state transition. The crack tip stress state also contributes to the thickness effect observed for the crack growth in sheets. In agreement with experimental results, it has been shown that a plane stress state will prevail at the crack tip in a thin sheet compared to that in a thick sheet. The plane stress state results in a higher crack opening level which leads to a longer fatigue life for thin sheets.     

不同应力比下Cu/WCp复合材料疲劳裂纹扩展行为及影响机制分析

通过粉末冶金工艺制备了一种高压电触头用Cu/WCp颗粒增强复合材料。研究了不同应力比下Cu/WCp颗粒增强复合材料的疲劳裂纹扩展行为,并结合裂纹闭合模型和两参数驱动力模型分析了应力比对Cu/WCp颗粒增强复合材料疲劳裂纹扩展速率的影响机制。研究结果表明:随着应力比R的增大裂纹扩展速率增大,尤其在近门槛值附近裂纹扩展速率差别最明显。裂纹闭合模型和两参数驱动力模型均可以较好地将不同应力比R下(da/d N-ΔK)关系曲线关联起来,且两参数驱动力模型的相关性更好。这说明导致不同应力比R下Cu/WCp颗粒增强复合材料疲劳裂纹扩展速率差异的原因主要是Kmax引起裂纹尖端单调损伤,其次是裂纹闭合效应。根据SEM断口分析发现高应力比的断面较低应力比的粗糙,低应力比时断口以基体撕裂为主而高应力比时以颗粒基体脱粘为主。     

A multiparameter approach to the prediction of fatigue crack growth in metallic materials

A multiparameter approach is proposed for the characterization of fatigue crack growth in metallic materials. The model assesses the combined effects of identifiable multiple variables that can contribute to fatigue crack growth. Mathematical expressions are presented for the determination of fatigue crack growth rates, d a /d N , as functions of multiple variables, including stress intensity factor range, Δ K , stress ratio, R , crack closure stress intensity factor, K _cl , the maximum stress intensity factor K _max , nominal specimen thickness, t , frequency, Ω , and temperature, T . A generalized empirical methodology is proposed for the estimation of fatigue crack growth rates as a function of these variables. The validity of the methodology is then verified by making appropriate comparisons between predicted and measured fatigue crack growth data obtained from experiments on Ti–6Al–4V. The effects of stress ratio and specimen thickness on fatigue crack growth rates are then rationalized by crack closure considerations. The multiparameter model is also shown to provide a good fit to experimental data obtained for HY-80 steel, Inconel 718 polycrystal and Inconel 718 single crystal. Finally, the implications of the results are discussed for the prediction of fatigue crack growth and fatigue life.     

Load sequence effects in fatigue crack growth of thick-walled welded C–Mn steel members

Many welded steel structures in marine, offshore, and infrastructural industries are subjected to variable amplitude (VA) fatigue loads. It is well known that the level and sequence of the load cycles can cause crack growth retardation or acceleration and thus influence the fatigue life. An important sequence effect is generated by a large stress cycle followed by smaller stress cycles. Whereas the effect of single large stress cycles in a further constant amplitude (CA) load on central through cracks in thin-walled aluminium sheet is well established, studies into the effects of practical VA loads on cracks in thick-walled welded steel structures are less common. This paper presents the results of CA tests with large stress peaks and VA tests on 70 mm C–Mn steel butt welded 4-point bending specimens with crack growth in thickness direction. It is demonstrated that loading by a sequence of accelerating and subsequent decelerating stress cycles cause significant retardation of the crack growth and that the same stress cycles but placed in random sequence hardly result in retarded crack growth. The obtained crack growth versus number of cycles for as-welded and stress relieved specimens have been simulated using two relatively simple crack rate retardation models, being the well-known Willenborg model and the Space-state model developed by Ray and Patankar. The latter model is also used to simulate crack growth of semi-elliptical surface cracks in welded steel structures tested by others. The Space-state model is able to predict experimental results with reasonable to good accuracy. A proposal is put forward for future improvement of the model.     

PROPAGATION AND CLOSURE OF SHORT FATIGUE CRACKS AT NOTCHES UNDER COMPRESSIVE MEAN STRESSES

Abstract— Single-edge-notched specimens of a low-carbon steel were fatigued under cyclic in-plane bending with compressive mean stresses. The development of crack closure with crack growth was studied both experimentally and theoretically. The relation between the crack opening stress and the crack length was a function of the minimum (compressive) applied stress, irrespective of the maximum stress. The effective stress intensity range was a unique parameter in correlating the crack growth rate, even if the crack was embedded in the compressive plastic zone. Under a constant minimum stress, the length of nonpropagating cracks became longer with increasing maximum applied stress. A theoretical model was proposed for predicting the crack opening stress on the basis of the compressive stress distribution at the minimum applied stress. The predicted value agreed fairly well with the experimental result. The model gave upper bounds of the crack growth rate and the length of nonpropagating fatigue cracks within the plastic zone.     

Fatigue crack growth in residual stress fields

A fatigue crack growth (FCG) model for specimens with well-characterized residual stress fields has been studied using experimental analysis and finite element (FE) modeling. The residual stress field was obtained using four point bending tests performed on 7050-T7451 aluminum alloy rectangular specimens and consecutively modeled using the FE method. The experimentally obtained residual stress fields were characterized using a digital image correlation technique and a slitting method, and a good agreement between the experimental residual stress fields and the stress field in the FE model was obtained. The FE FCG models were developed using a linear elastic model, a linear elastic model with crack closure and an elastic–plastic model with crack closure. The crack growth in the FE FCG model was predicted using Paris–Erdogan data obtained from the residual stress free samples, using the Harter T-method for interpolating between different baseline crack growth curves, and using the effective stress intensity factor range and stress ratio. The elastic–plastic model with crack closure effects provides results close to the experimental data for the FCG with positive applied stress ratios reproducing the FCG deceleration in the compressive zone of the residual stress field. However, in the case of a negative stress ratio all models with crack closure effects strongly underestimate the FCG rates, in which case a linear elastic model provides the best fit with the experimental data. The results demonstrate that the negative part of the stress cycle with a fully closed crack contributes to the driving force for the FCG and thus should be accounted for in the fatigue life estimates.     

Dynamic ductile crack growth and transition to cleavage – a cell model approach

The fracture behavior of ferritic steel in the transition regime is controlled by the competition between ductile tearing and cleavage. Many test specimens that failed by catastrophic cleavage showed significant amounts of ductile tearing prior to cleavage fracture. The transition from ductile tearing to cleavage has been attributed to the increase in constraint and sampling volume associated with ductile crack growth. This work examines the role of dynamic ductile crack growth on the fracture mode transition by way of a cell model of the material. The cell model incorporates the effects of stress triaxiality and strain rate on material failure characteristics of hole growth and coalescence. Loading rate and microstructure effects on the stress fields that evolve with rapid (ductile) crack growth are systematically studied. The stress fields are employed to compute the Weibull stress which provides probability estimates for the susceptibility to cleavage fracture. A center-cracked panel subjected to remote tension is the model problem under study. The computational model uses an elastic-viscoplastic constitutive relation which incorporates enhanced strain rate hardening at high strain rates. Adiabatic heating due to plastic dissipation and the resulting thermal softening are also accounted for. Under dynamically high loading rate, our model shows the crack speed achieves its peak value soon after crack initiation and quickly falls off to slower speeds with further crack growth. Remarkably, the Weibull stress follows a similar pattern which suggests that the transition to the cleavage fracture is most likely to occur, if at all, at the peak speed of ductile crack growth. Key words: Dynamic fracture, ductile tearing, crack growth, transition regime, cleavage fracture, cell model, finite element.     

Crack closure in fibre metal laminates

GLARE is a fibre metal laminate (FML) built up of alternating layers of S2-glass/FM94 prepreg and aluminium 2024-T3. The excellent fatigue behaviour of GLARE can be described with a recently published analytical prediction model. This model is based on linear elastic fracture mechanics and the assumption that a similar stress state in the aluminium layers of GLARE and monolithic aluminium result in the same crack growth behaviour. It therefore describes the crack growth with an effective stress intensity factor (SIF) range at the crack tip in the aluminium layers, including the effect of internal residual stress as result of curing and the stiffness differences between the individual layers. In that model, an empirical relation is used to calculate the effective SIF range, which had been determined without sufficiently investigating the effect of crack closure. This paper presents the research performed on crack closure in GLARE. It is assumed that crack closure in FMLs is determined by the actual stress cycles in the metal layers and that it can be described with the available relations for monolithic aluminium published in the literature. Fatigue crack growth experiments have been performed on GLARE specimens in which crack growth rates and crack opening stresses have been recorded. The prediction model incorporating the crack closure relation for aluminium 2024-T3 obtained from the literature has been validated with the test results. It is concluded that crack growth in GLARE can be correlated with the effective SIF range at the crack tip in the aluminium layers, if it is determined with the crack closure relation for aluminium 2024-T3 based on actual stresses in the aluminium layers.     

Mechanisms and modeling of low cycle fatigue crack propagation in a pressure vessel steel Q345

The fatigue process near crack is governed by highly concentrated strain and stress in the crack tip region. Based on the theory of elastic–plastic fracture mechanics, we explore the cyclic J-integral as breakthrough point, an analytical model is presented in this paper to determine the CTOD for cracked component subjected to cyclic axial in-plane loading. A simple fracture mechanism based model for fatigue crack growth assumes a linear correlation between the cyclic crack tip opening displacement (ΔCTOD) and the crack growth rate (da/dN). In order to validate the model and to calibrate the model parameters, the low cycle fatigue crack propagation experiment was carried out for CT specimen made of Q345 steel. The effects of stress ratio and crack closure on fatigue crack growth were investigated by elastic–plastic finite element stress–strain analysis of a cracked component. A good comparison has been found between predictions and experimental results, which shows that the crack opening displacement is able to characterize the crack tip state at large scale yielding constant amplitude fatigue crack growth.     

A probabilistic multiparameter framework for the modeling of fatigue crack growth in concrete

This paper presents a probabilistic multiparameter framework for the modeling of fatigue crack growth in three grades of concrete. The framework relies on the use of ranked fatigue crack growth rate data (with specified occurrence probability levels) in the formulation of multiparameter fatigue crack growth expressions. These relate ranked fatigue crack growth rates to crack driving force parameters such as the stress intensity factor range, maximum stress intensity factor, stress ratio and occurrence probability level. A probabilistic framework is then presented for the estimation of material reliability or failure probability due to fatigue crack growth. The probabilistic model is then validated for the available data.     

A MODEL FOR SMALL FATIGUE CRACK GROWTH

Abstract— Based on the assumption that normalized Kitagawa-Takahashi diagrams for different materials are the same, a unified model for microstructurally small fatigue crack and physically small fatigue crack growth rates was developed to describe their behaviour under different fatigue stress ranges. The stress-sensitive blocking effect of microstructural barriers to small fatigue crack growth is satisfactorily simulated by the model. Incorporated with the materials fatigue limit and microstructural barrier spacing, this model can be easily used in the prediction of small fatigue crack lifetime. Small fatigue crack growth rates of previous experimental studies in 7075-T6 Aluminium alloy and HT60 steel under different stress ranges are in an envelope between two boundary prediction curves corresponding to the largest and smallest stress ranges applied in the experiments. Problems concerning model accuracy and model application are also discussed in the present paper.     

2D modelling of inclined intergranular stress corrosion crack paths

Inclined high pH stress corrosion cracking (SCC) is a type of intergranular environmental cracking in gas pipelines, which differs from typical SCC by propagating at an angle from the wall direction. Investigations of Australian and Canadian inclined SCC colonies have not provided a clear indicator of a cause for the abnormal crack growth direction. This paper addresses the possibility of crack tip strain enhanced electrochemistry causing the inclination. Potentiodynamic tests were conducted to quantify the influence of strain on the electrochemistry, and strain was found to increase current density up to 300% in the SCC region. A model was developed that incorporates crack tip strain driven SCC growth, which showed good agreement with field grown cracks, and the aspect ratio of the grains was shown to have an effect on the inclination angle. The results indicate that crack tip strain enhanced electrochemistry is a plausible cause for inclined SCC.     

基于灰色理论的高强铝合金应力腐蚀开裂预测模型的建立与应用

采用3.5%NaC1溶液中预制裂纹的方法测试了2124高强铝合金的应力腐蚀裂纹扩展长度随时间的变化,获得了裂纹扩展速率随腐蚀时间的变换规律及应力腐蚀断裂韧性界限值,并对断口进行分析.根据裂纹扩展的基本规律,运用灰色理论GM(1,1)模型,依据2124铝合金应力腐蚀开裂裂纹扩展长度的原始数据进行了灰色预测,并对预测结果进...