Environmental strength evaluation of welded steel joint in sea water

Corrosion fatigue crack growth behaviours were experimentally evaluated for the parent metal, as-welded and PWHT specimens of SM53B steel. Multi-pass welding was done by a submerged arc welder. Metallographic observations along the weld fusion boundary were made to investigate the variation of microstructures through the thickness direction. PWHT was carried out at 650°C with holding time of 1/4hr and 40hr. The corrosion fatigue test was conducted in 3.5% NaCl solution with the frequency of 3Hz. In all cases, crack growth in corrosive environment was faster than that of in air. Besides, at the low †K region, crack growth was greatly influenced by corrosive environment and the heat treatment condition.     

A probabilistic analysis on variability of fatigue crack growth using the markov chain

Understanding the stochastic properties of variability in fatigue crack growth is important to maintaining the reliability and safety of structures. In this study, a stochastic model is proposed to describe crack growth behavior considering the variability of fatigue crack growth rates due to the heterogeneity of material. Fatigue life distribution is then predicted based on this model To construct this model, fatigue tests are conducted on a high strength aluminum alloy 7075 T6 under constant stress intensity factor range control. The variability of fatigue crack growth rates is expressed by random variablesZ and Γ based on the variability of material constantsC andm of the Paris-Erdogan equation. The distribution of fatigue life under constant stress intensity factor ranges is evaluated by the stochastic Markov chain model based on the Paris-Erdogan equation. The merit of the proposed model is that only a small number of tests are required to determine this function, and fatigue life required to reach certain crack length at a given stress intensity factor range can be easily predicted. Department of Mechanical Design and Production Eng.     

Numerical analysis for prediction of fatigue crack opening level

Finite element analysis (FEA) is the most popular numerical method to simulate plasticity-induced fatigue crack closure and can predict fatigue crack closure behavior. Finite element analysis under plane stress state using 4-node isoparametric elements is performed to investigate the detailed closure behavior of fatigue cracks and the numerical results are compared with experimental results. The mesh of constant size elements on the crack surface can not correctly predict the opening level for fatigue crack as shown in the previous works. The crack opening behavior for the size mesh with a linear change shows almost flat stress level after a crack tip has passed by the monotonic plastic zone. The prediction of crack opening level presents a good agreement with published experimental data regardless of stress ratios, which are using the mesh of the elements that are in proportion to the reversed plastic zone size considering the opening stress intensity factors. Numerical interpolation results of finite element analysis can precisely predict the crack opening level. This method shows a good agreement with the experimental data regardless of the stress ratios and kinds of materials.     

Analysis of short and long crack behavior and single overload effect by crack opening stress

The study analyzed the behaviors of short and long crack as well as the effect of single tensile overload on the crack behaviors by using fatigue crack opening behavior. Crack opening stress is measured by an elastic compliance method which may precisely and continuously provide many data using strain gages during experiment. The unusual growth behaviors of short crack and crack after the single tensile overload applied, was explained by the variations of crack opening stress. In addition, fatigue crack growth rate was expressed as a linear form for short crack as for long crack by using effective stress intensity factor range as fracture mechanical parameter, which is based on crack closure concept. And investigation is performed with respect to the relation between plastic zone size formed at the crack tip and crack retardation, crack length and the number of cycles promoted or retarded, and the overload effect on the fatigue life.     

Effect of sigma phase on near-threshold fatigue crack growth behavior and ultrasonic evaluation in AISI 316L stainless steel

In this work, we examined the influence of microstructural changes, such as an intermetallic sigma (??) phase, on the fatigue behavior of high-temperature aged AISI 316L stainless steel. Nondestructive ultrasonic test and fatigue crack growth tests were performed to determine the threshold stress intensity factor of these artificially aged specimens. Ultrasonic test results characterizing the microstructural changes were compared with those of the fatigue tests to propose an empirical formula capable of predicting the threshold stress intensity factor by a nondestructive method. We observed a strong correlation between the increase in the volume fraction of the ?? phase and the decrease of ??K_th. Ultrasonic velocity increased in response to the coarsening behavior of the ?? phase in the vicinity of the grain boundaries.     

Failure analysis of turbine blade in atomic power plant

The turbine blade in an atomic power plant may be fractured by fatigue, stress corrosion cracking and bad fitting. Especially, fatigue fracture is caused by low stress amplitude below the yielding stress. SEM fractography does not have striation, but AFM fractography does on the fatigue fractured surface of 12% Cr steel used for the turbine blade. Surface roughness R _q measured by AFM is linearly related to the stress intensity factor range, ΔK, and is increased linearly according to the load range ΔP. Therefore, in this study, the loading condition applied to a turbine blade is predicted by the relation between the intersection of the ΔK-R _q relation and load range ΔP.     

Effects of temperature and oxidation on threshold stress intensity factor of 12% Cr steel for steam turbine rotor component

Threshold stress intensity factor (ΔK _th ), one of the important parameters in plant design, is the value below which fatigue crack growth becomes unrecognizably slow. Although studies on the each effect of various factors such as temperature, frequency, stress ratio, oxidation and grain size on the ΔK _th have been reported widely, only few research works were reported on the complex effect of the plural factors acting simultaneously. In this study, the analysis of the complex effect of temperature and oxidation on ΔK _th of 12Cr steel, the turbine rotor material of the USC (ultra super critical) power plant, has been carried out. Two types of fatigue tests were performed in the air and in the nitrogen gas conditions. Experimental results showed that ΔK _th value increases with temperature increase and formation of oxidation scale. However it was observed that the magnitude of each effect is reduced when two factors are introduced simultaneously.     

Optimisation Method for Determination of Crack Tip Position Based on Gauss-Newton Iterative Technique

In the digital image correlation research of fatigue crack growth rate,the accuracy of the crack tip position determines the accuracy of the calculation of the stress intensity factor,thereby affecting the life prediction.This paper proposes a Gauss-Newton iteration method for solving the crack tip position.The conventional linear fitting method provides an iterative initial solution for this method,and the preconditioned conjugate gradient method is used to solve the ill-conditioned matrix.A noise-added artificial displacement field is used to verify the feasibility of the method,which shows that all parameters can be solved with satisfactory results.The actual stress intensity factor solution case shows that the stress intensity factor value obtained by the method in this paper is very close to the finite element result,and the relative error between the two is only-0.621％;The Williams coefficient obtained by this method can also better define the contour of the plastic zone at the crack tip,and the maximum relative error with the test plastic zone area is-11.29％.The relative error between the contour of the plastic zone defined by the conventional method and the area of the experimental plastic zone reached a maximum of 26.05％.The crack tip coordinates,stress intensity factors,and plastic zone contour changes in the loading and unloading phases are explored.The results show that the crack tip change during the loading process is faster than the change during the unloading process;the stress intensity factor during the unloading process under the same load condition is larger than that during the loading process;under the same load,the theoretical plastic zone during the unloading process is higher than that during the loading process.     

Statistical aspects of fatigue crack growth life of base metal,weld metal and heat affected zone in FSWed 7075-T651 aluminum alloy

The statistical aspects of fatigue crack growth life of base metal (BM), weld metal (WM) and heat affected zone (HAZ) in friction stir welded (FSWed) 7075-T651 aluminum alloy has been studied by Weibull statistical analysis. The fatigue crack growth tests were performed at room temperature on ASTM standard CT specimens under three different constant stress intensity factor range controls. The main objective of this paper is to investigate the effects of statistical aspects of fatigue crack growth life on stress intensity factor ranges and material properties, namely BM, WM and HAZ specimens. In this work, the Weibull distribution was employed to estimate the statistical aspects of fatigue crack growth life. The shape parameter of Weibull distribution for fatigue crack growth life was significantly affected by material properties and the stress intensity factor range. The scale parameter of WM specimen exhibited the lowest value at all stress intensity factor ranges.     

Fatigue crack growth behavior in ultrafine grained low carbon steel

Ultrafine grained (UFG) low carbon (0.15 wt.% C) steel produced by equal channel angular pressing (ECAP) was tested for investigating the effect of load ratio on the fatigue crack growth rate. Fatigue crack growth resistance and threshold of UFG steel were lower than that of as-received coarse grained steel. It was attributed to the less tortuous crack path. The UFG steel exhibited slightly higher crack growth rates and a lower ΔK_th with an increase of R ratio. The R ratio effect on crack growth rates and ΔK_th was basically indistinguishable at lower load ratio (R>0.3), compared to other alloys, which indicates that contribution of the crack closure vanishes. The crack growth rate curve for UFG steel exhibited a longer linear extension to the lower growth rate regime than that for the coarse grained as-received steel.     

The role of casting porosity in fatigue properties of Al−Si 319 lost foam cast alloy

The tensile, fracture toughness and fatigue properties of Al−Si 319 lost-foam-cast alloy were determined at room temperature. The fatigue properties of this alloy were also determined at 150°C. Fatigue cracks were always initiated at the largest casting pore. Initial pore sizes were measured using a scanning electron microscope. Surface replication showed that majority of the fatigue life was spent in fatigue crack propagation and permitted the estimation of the constants in the Paris power law and the threshold stress intensity factor (ΔK _th ). The role of internal casting porosity was quantified using a linear elastic fracture mechanics (LEFM) model for fatigue crack growth. The predicted lives agreed with the measured values within a factor of two.     

Fatigue crack propagation behavior in STS304 under mixed-mode loading

The use of fracture mechanics has traditionally concentrated on crack growth under an opening mechanism. However, many service failures occur from cracks subjected to mixed-mode loading. Hence, it is necessary to evaluate the fatigue behavior under mixed-mode loading. Under mixed-mode loading, not only the fatigue crack propagation rate is of importance, but also the crack propagation direction. In modified range 0.3≤a/W≤0.5, the stress intensity factors (SIFs) of mode I and mode II for the compact tension shear (CTS) specimen were calculated by using elastic finite element analysis. The propagation behavior of the fatigue cracks of cold rolled stainless steels (STS304) under mixed-mode conditions was evaluated by using K_I and K_II(SIFs of mode I and mode II). The maximum tangential stress (MTS) criterion and stress intensity factor were applied to predict the crack propagation direction and the propagation behavior of fatigue cracks.     

A comparative evaluation of K<Subscript>op</Subscript> determination and ΔK<Subscript>eff</Subscript> estimation methods

Methods for determination of the crack opening stress intensity factor (K_op) and for estimation of the effective stress intensity factor range (ΔK_eff) are evaluated for crack growth test data of aluminum alloys. Three methods of determining K_op, visual measurement, ASTM offset compliance method, and the neural network method proposed by Kang and Song, and three methods of estimating ΔK_eff, conventional, the 2/P10 and 2/PI methods proposed by Donald and Paris, are compared in a quantitative manner by using evaluation criteria. For all K_op determination methods discussed, the 2/PI method of estimating ΔK_eff provides good results. The neural network method of determining K_op provides good correlation of crack growth data. It is recommended to use 2/PI estimation with the neural K_op determination method. The ASTM offset method used in conjunction with 2/PI estimation shows a possibility of successful application. It is desired to improve the ASTM method.     

基于新裂纹扩展驱动力参量的焊接接头疲劳裂纹扩展的研究

传统上疲劳裂纹扩展速率以一个参量——应力强度因子幅(PARIS模型)或有效应力强度因子幅(ELBER模型)来表达。PARIS模型不能统计应力比效应和变幅加载历史。ELBER裂纹闭合模型虽被广泛应用,但确定其开闭口载荷的测量方法很多,且测量结果均存在主观性。最近研究表明,疲劳裂纹扩展不仅依赖于应力强度因子幅,还与最大应力强度因子有关。并且KUJAWSKI提出了两参量模型,该模型避开了有争议的裂纹闭合效应。基于一个载荷循环中柔度变化与裂纹尖端开闭口与弹塑性行为的关系,提出一个新的具有物理意义的两参量驱动力模型。针对Q345钢焊接接头各区域进行两种应力比R=0.1和0.5的疲劳裂纹扩展试验。使用该模型针对Q345钢焊接接头各区域的疲劳裂纹扩展数据进行验证。结果表明,提出的新模型在预测应力比对裂纹扩展速率的影响时比上述三个模型更有效。     

The effect of bridging on fatigue crack growth behavior in Aramid Patched Aluminum Alloy(APAL)

A new hybrid composite (APAL: Aramid Patched Aluminum Alloy), consisting of a 2024-T3 aluminum alloy plate sandwiched between two aramid/epoxy laminate (HK 285/RS 1222), was developed. Fatigue crack growth behavior was examined at stress ratios of R=0.2, 0.5 using the aluminum alloy and two kinds of the APAL with different fiber orientation (0°/90° and 45° for crack direction). The APAL showed superior fatigue crack growth resistance, which may be attributed to the crack bridging effect imposed by the intact fibers in the crack wake. The magnitude of crack bridging was estimated quantitatively and determined by a new technique on basis of compliances of the 2024-T3 aluminum alloy and the APAL specimens. The crack growth rates of the APAL specimens were reduced significantly as comparison to the monolithic aluminum alloy and were not adequately correlated with the conventional stress intensity factor range(ΔK). It was found that the crack growth rate was successfully correlated with the effective stress intensity factor range (ΔK _eff =K _br -K _ct ) allowing for the crack closure and the crack bridging. The relation between da/dN and theΔK _eff was plotted within a narrow scatter band regardless of kind of stress ratio (R=0.2, 0.5) and material (2024-T3 aluminum alloy, APAL 0°/90° and APAL±45°). The result equation was as follow:da/dN=6.45×10⁻⁷(ΔK _eff )^2.4.     

Environmental strength evaluation of welded steel joint in seawater

A study on corrosion fatigue was experimentally conducted for the as-welded and PWHT specimens of the steels, HT80 and SM53B in 3.5% NaCl solution. Submerged arc welding was done. PWHT was carried out at comparatively high temperature of 650° C. Besides, in order to simulate the residual stress in weld HAZ, the stress of 98MPa was applied during PWHT. Corrosion fatigue crack growth was dependent upon the materials and PWHT conditions. In the case of HT80, crack growth in corrosion environment was faster than that in air. However, the crack growth of the main crack for SM53B in 3.5% NaCl solution was decreased in comparison with that in air, unlike HT80. The sensitivity to corrosion environment was reduced due to PWHT. The applied stress in HAZ during PWHT acted to enhance the crack growth compared with that of the PWHT specimen without stress.     

硫化氢环境中低周疲劳裂纹扩展速率的研究

针对高强钢在硫化氢环境中腐蚀疲劳数据极为缺乏的现状,研究高压气瓶材料4130X在硫化氢腐蚀介质中的疲劳裂纹扩展速率.结合气瓶实际运行的环境和应力状态,应用改进型WOL(wedge-opening-loading)试样,在自行改造的专用低周腐蚀疲劳试验机上,完成0.006 7 Hz超低频率下饱和H_2S溶液、中等浓度H2S溶液和空气三种环境下的腐蚀疲劳试验,并用Paris公式进行两段式拟合,得出da/dN-ΔK的数学表达式.将试样微观断口的变化与宏观应力强度因子K的变化进行对比研究,给出不同环境中三个阶段K值的定量结果.结果表明:相同条件下,H_2S环境中的疲劳裂纹扩展速率比空气环境中大20倍以上;但当H_2S浓度达到一定范围后,对da/dN影响并不按比例增长,浓度相差11倍时,da/dN相差2.4倍,H_2S腐蚀介质的存在加速了疲劳破坏.     

Crack propagation behaviour in fretting fatigue

Fretting fatigue and normal, or unfretting, fatigue tests of a stainless steel SUS304L and an aluminium alloy A2024-T3 were carried out to investigate the effects of the contact pressure and the stress ratio on the crack propagation behaviour. The crack propagation behaviour was represented by the crack propagation rate da/dNversus the crack length a or the stress intensity factors ΔK_eff and K_max In fretting fatigue, crack propagation was divided into two stages, namely S_I and S_II. The value of da/dN in the S_I stage was very high, even under a stress intensity factor less than the threshold for normal fatigue, and decreased gradually with crack growth because of crack closure and the decreasing fretting effect. The decrease in da/dN was marked in the case of high contact pressure and low stress ratio such as when R = −0.33, where R denotes the minimum stress divided by the maximum stress. During fretting fatigue crack closure occurred at an oblique short crack in the early stages of crack propagation in both the SUS304L steel and the A2024-T3 alloy; it also occurred at the oblique cracked surface of the shear lips formed in the A2024-T3 alloy during crack growth. However, in the S_II stage, which followed the S_I stage, da/dN increased with crack growth as for normal fatigue.     

Growth behavior of short surface fatigue cracks in 2 1/4 Cr-1 Mo steel

Fatigue tests by axial loading (R-0.05) were carried out to investigate short fatigue crack growth behavior in 2 1/4 Cr-1 Mo steel at room temperature using smooth and a small notched flat specimen. All the data of the fatigue crack growth rate in the present tests were analyzed as a function of the stress intensity factor equation in conjunction with crack closure behavior. Analysis was performed accounting for the relation of surface effective stress range,Ua and depth effective stress range,Ub. In the case of isotropic crack growth properties,Ub=(ΔK_ta/ΔK_tb) ·Ua. By use ofUb obtained from the analysis, crack growth rates to surface direction coincide with those of depth direction.     

Fretting fatigue strength estimation considering the fretting wear process

In fretting fatigue process the wear of contact surfaces near contact edges occur in accordance with the reciprocal micro-slippages on these contact surfaces. These fretting wear change the contact pressure near the contact edges. To estimate the fretting fatigue strength and life it is indispensable to analyze the accurate contact pressure distributions near the contact edges in each fretting fatigue process.So, in this paper we present the estimation methods of fretting wear process and fretting fatigue life using this wear process. Firstly the fretting-wear process was estimated using contact pressure and relative slippage as follows:

W=K×P×S,