Stress distribution and fatigue crack propagation analyses in welded joints

An approach is presented, based on the weight function method to calculate the stress intensity factors of semielliptical surface cracks originating from the notch root of welded joints. The stress distribution along the potential crack plane required in the weight function method is constructed on the basis of the notch stress intensity factor approach in the highly stressed zone and of the equivalent linearized stress distribution and is compared with those determined by the finite element method and existing predictions. The stress intensity factors determined by the proposed approach are compared with available solutions. These comparisons show that the results determined by the proposed approach generally agree well with the existing solutions. For the cases where the agreement is poor, the reasons are identified. One important feature of the proposed approach is that the stress singularity at sharp notch tip can be considered, which cannot be appropriately simulated by the finite element method. Finally, to demonstrate the applicability of the proposed approach, the fatigue life and the fatigue crack shape evolution of welded joints are predicted and they are compared with experimental results.     

Experimental and analytical investigation of fatigue crack propagation of T‐welded joints considering the effect of boundary condition

T‐welded joints are commonly employed in ship and ocean structures. The fatigue failure of structure components subjected to cyclic loading always occurs in T‐welded joints because of the metallurgical differences, tensile residual stress fields and stress concentrations. The former researches about T‐welded joints fatigue have in common that the boundary condition needs to be taken into account as an influencing parameter to predict the crack propagation during cyclic loading. In this paper, the crack growth behaviour in T‐welded joint processed with Q345D steel (Pingxiang Iron & Steel Co., Ltd, Hukou, Jiangxi Province in China) under the fatigue loading was analysed via analytical model and verified via experiment. The results show that the influence of boundary condition should be considered in T‐welded joint structure during crack propagation in weld toe area. The correction factor concerning the effect of boundary condition and modified Paris' equation was proposed according to the experimental results and verified by the following repeated experiments.     

Research on fatigue crack propagation behaviour of 4003 ferritic stainless steel based on infrared thermography

Fatigue crack propagation (FCP) behaviour of 4003 ferritic stainless steel was investigated using infrared thermography. Four stages of superficial temperature evolution were observed during the FCP tests: an initial temperature decrease stage, a temperature equilibrium stage, a slow temperature increase stage and an abrupt temperature increase stage; a thermal model is developed to explain the observed temperature evolution. The experimental results indicate that: when the range of stress intensity factor (ΔK) is at a low level where the crack is located in slow propagation region, thermoelastic effect will be in dominant status; when the ΔK is at a high level where the crack is located in stable propagation region, the temperature rise can be used to describe FCP rate. The fatigue fracture surfaces were examined using scanning electron microscope (SEM) in order to understand the effect of the fatigue mechanisms on temperature variation.     

Consideration of the residual stress distributions in fatigue crack growth calculations for assessing welded steel joints

To better understand the crack closure and propagation, an analytical model is established. The residual stress effect on fatigue crack growth equations has been considered using the residual stress intensity factor (SIF) (K_res). The joint geometries, residual stress distributions (σ_res) and residual stress ratio (R_res) were considered also. K_res are calculated using the analytical weight function (WF) method and different residual stress distributions. It is to be emphasized that the current approach is little investigated. This is because the WF has already been developed to calculate SIF for an existing crack. The current approach calculates K_res for the crack that initiates and propagates until failure. Different stress distributions have been used, and R_res is defined. The validity of using the WF has been shown. SIF due to applied load (K_app) and applied stress ratio (R_app) have been considered. Fatigue crack growth rate was investigated in accordance with the current approach. The results have been verified and benchmarked.     

Influence of effective stress intensity factor range on mixed mode fatigue crack propagation

ABSTRACT The behaviour of fatigue crack propagation of rectangular spheroidal graphite cast iron plates, each consisting of an inclined semi‐elliptical crack, subjected to axial loading was investigated both experimentally and theoretically. The inclined angle of the crack with respect to the axis of loading varied between 0° and 90°. In the present investigation, the growth of the fatigue crack was monitored using the AC potential drop technique, and a series of modification factors, which allow accurate sizing of such defects, is recommended. The rate of fatigue crack propagation db/dN is postulated to be a function of the effective strain energy density factor range, ΔS_eff. Subsequently, this concept is applied to predict crack growth due to fatigue loads. The mixed mode crack growth criterion is discussed by comparing the experimental results with those obtained using the maximum stress and minimum strain energy density criteria. The threshold condition for nongrowth of the initial crack is established based on the experimental data.     

Fatigue crack propagation in tensile residual stress fields of welded joints under fully compressive cycling

Fatigue crack propagation rates in centre-crack-typed transverse butt-welded joints were measured at a constant stress intensity factor range obtained by decreasing the applied and mean loads on test specimens. The propagation rate was maintained constant except at extremely compressed stress ratios. Fatigue crack propagation properties under compressive loading were found to be similar to those under tensile loading. Only under highly compressive cycling did crack propagation rates decrease.     

Fatigue crack propagation in welded joints under variable-amplitude loading in synthetic sea water

Fatigue crack propagation rates and the fatigue threshold in transverse butt-welded joints of centre crack type were measured in synthetic sea water under variable-amplitude loading. The fatigue threshold was about a half of that in air, and the propagation rates near the fatigue threshold were high compared with those in air. The propagation rate for each cycle under variable-amplitude loading could be estimated from the constant-amplitude data, which were obtained in the crack-closure-free condition.     

Evaluation of fatigue crack propagation by mode I and mixed mode in 1050 aluminium

The mechanism of mixed‐mode fatigue crack propagation was investigated in pure aluminum. Push‐pull fatigue tests were performed using two types of specimens. One was a round bar specimen having a blind hole, one was a plate specimen having a slit. The slit direction cut in the specimen was perpendicular or inclined 45 degrees relative to the centre of the specimen axis. In both cases, cracks propagated by mode I or by the mixed mode combining mode I and shear mode, depending on the testing conditions. In these cases the crack propagation rate was evaluated with a modified effective stress intensity factor range. Crack propagation retardation was observed in some specimens. However, it was found that the crack propagation rate could also be evaluated by the effective stress intensity factor range independent of the crack propagation mode.     

A comparison of different methods to determine the threshold of fatigue crack propagation

Three different methods for determining the threshold value for fatigue crack growth — the load-shedding technique, the stepwise increase of load amplitude on specimens precracked in cyclic compression, and decrease of stress intensity range at a constant maximum stress intensity — were applied to a high-strength aluminium alloy. The load-shedding technique tended to lead to higher values of the threshold, especially at low R-ratios. The threshold determined with decreasing stress intensity range at a constant maximum stress intensity was larger than the effective threshold determined with stepwise increasing of load amplitude on specimens precracked in cyclic compression.     

Effect of microstructure on crack propagation in high-temperature fatigue of directionally solidified Ni-based superalloy

In order to clarify the crack propagation properties of an anisotropic material (Ni‐based directionally solidified superalloy), longitudinally loaded specimens (L‐specimens) and transversely loaded specimens (T‐specimens) with a crack are subjected to high temperature fatigue. The crack propagation rate is reasonably well correlated with the effective stress intensity factor range regardless of the propagation direction (specimens L and T), the stress range and the stress ratio. However, the crack propagation rate shows a notable fluctuation particularly in the T‐specimens. It is at most about five times faster than the average. The fracture surface features can be classified into four types with three transgranular and one intergranular types. In the former, though the crack is along the {100} or {110} planes on a macroscopic scale, it threads through the {111} or {100} planes on a microscopic scale. Crack propagation is notably accelerated in the intergranular region, while deceleration is caused by crack branching.     

Using fatigue crack propagation mechanism maps to predict changes in propagation mechanisms

The conditions determining the fatigue fracture mechanism in quenched and tempered steel are discussed with reference to fatigue crack propagation mechanism (FCPM) maps. Criteria for the change from one fatigue mechanism to another are presented.     

Overload effect on the fatigue crack propagation in large‐scale tubular joints

This paper examines the overloading effect on the fatigue crack propagations monitored in a large‐scale tubular X‐joint specimen under two separate cyclic tests. The first cyclic test applies a constant‐amplitude brace in‐plane bending to the joint, with a single cycle of 150% overload before the crack depth reaches the mid‐thickness of the chord. The second fatigue test applies two batches of cyclic loads, with the amplitude of the second batch at 66% of the former. The X‐joint specimen experiences a 150% overload cycle during the first batch of loading, followed by the second batch after it has recovered from the overload effect. The experimental results reveal that deep surface cracks experience more significant overload retardation than does a shallow fatigue crack. The Paris law estimation indicates that the single overload cycle applied in the first specimen leads to a 7% increase in the fatigue life of the X‐joint.     

Determination of the length dependence of the threshold for fatigue crack propagation

A rising load amplitude crack growth test on specimens pre-cracked in cyclic compression is presented as a procedure to determine the length dependence of the threshold of fatigue crack propagation described by the R(resistance)-curve for the threshold of stress intensity factor range. The experimental results show that the residual stress field in front of the pre-crack can significantly influence the R-curve.In order to measure the material specific R-curve which is not affected by the pre-cracking condition it is important to use the smallest possible load amplitude. To achieve this goal, a very small notch root radius is essential. It is shown that at notches machined by razor blade polishing technique the load amplitude for pre-cracking can be reduced to values where the load history does not influence the R-curve for the threshold of stress intensity range.     

A cumulative damage theory for fatigue crack initiation and propagation

A method for evaluating the cumulative damage resulting from the application of cyclic stress (or strain) sequences of varying amplitude is presented. Both the crack initiation and propagation stages of the fatigue failure process are included. The development is based on the concept of plastic strain energy dissipation as a function of cyclic life. The damage accumulated at any stage is evaluated from a knowledge of the fatigue limit in the initiation phase and an ‘apparent’ limit obtained through fracture mechanics for the propagation phase. The proposed damage theory is compared with two-level strain cycle test data of thin-walled specimens, and is found to be in fairly good agreement.     

Correlation of fatigue crack propagation in polyethylene pipe specimens of different geometries

Correlation in mechanisms and kinetics of step-wise fatigue crack propagation in polyethylene pipe specimens of different geometries is studied experimentally. It is shown that crack propagation in a non-standard specimen cut from a real pipe and conserving the pipe geometry can be effectively simulated using a standard compact tension specimen. Good correlation in both kinetics of step-wise crack propagation and fractography between the specimens is achieved if experimental conditions are chosen to assure equal values of (a) stress intensity factor and (b) stress intensity factor gradient at the initial notch tips. These results extend previous technique of fatigue accelerating slow crack growth used to predict lifetime of polyethylene pipes. This revised version was published online in August 2006 with corrections to the Cover Date.     

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.     

Modelling fatigue crack propagation in CT specimens

Although there are a great number of numerical studies focused on the numerical simulation of crack shape evolution, a deeper understanding is required concerning the numerical parameters and the mathematical modelling. Therefore, the objectives of the paper are the study of the influence of numerical parameters, particularly the radial size of crack front elements and the magnitude of individual crack extensions, the mathematical modelling of crack propagation regimes, and the linking of crack shape changes with K distribution. A relatively simple through-crack geometry, the CT specimen, was studied and the numerical model was validated with experimental results with a good agreement. The K distribution along crack front was found to be the driving force for shape variations. Shape variations were found to be one order of magnitude lower than K variations.     

U75V重轨钢弯曲疲劳裂纹扩展行为EI北大核心

为明确珠光体钢轨的疲劳裂纹扩展行为,测定U75V重轨钢轧态和热处理态两种条件下的三点弯曲疲劳裂纹扩展速率,采用光学显微镜、扫描电镜、EBSD对钢轨的微观组织、片层、断口形貌及裂纹扩展轨迹进行观察。结果表明:轧态和热处理态钢轨的疲劳辉纹平均间距分别为253,215 nm,轧态钢轨的疲劳断口呈现解理台阶与河流花样形貌,且河流花样趋于合并,而热处理态钢轨的疲劳断口呈现大量的解理台阶及较多的微裂纹和撕裂棱,河流花样以支流为主;热处理态钢轨的疲劳裂纹扩展速率远低于轧态,到达裂纹失稳阶段也较滞后;轧态和热处理态钢轨的疲劳裂纹扩展都是以穿晶断裂为主的穿晶断裂和沿晶断裂混合扩展方式进行,轧态和热处理态钢轨的珠光体片层间距分别为272,148 nm,其中热处理态钢轨的珠光体片层细密且方向多样,存在显著的珠光体团簇,裂纹扩展轨迹中出现较多的分支裂纹和裂纹桥接现象,对扩展起到阻碍作用,是热处理态钢轨抗疲劳裂纹扩展能力优于轧态的重要原因。     

Prediction of fatigue crack growth path in mechanical joints using a weight function approach

Cracks often initiate from the mechanical joints which are widely used in structural components. It has been reported that cracks in mechanical joints are under mixed‐mode condition and there is a critical angle at which mode I stress intensity factor becomes maximum. The crack propagates in an arbitrary direction and the prediction of fatigue crack growth path is needed to provide against crack propagation and examine safety. In this study, mixed‐mode fatigue crack growth tests are performed for horizontal and critical inclined cracks in mechanical joints. Fatigue crack growth paths are predicted using a weight function approach and maximum tangential stress criterion.     

Path prediction of I + III mixed mode fatigue crack 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 propagation rate expression and the path prediction. It is found that during the mode transformation process, the crack propagation rate is still controlled by the mode I stress intensity factor; and Paris equation also holds for the relationship between and ΔK_I . Crack propagation path can be predicted only when both the crack mode transformation rate and propagation rate are available.