疲劳裂纹在奥氏体/铁素体异种钢焊接接头中的扩展行为

采用三点弯曲试样研究了疲劳裂纹在奥氏体/铁素体异种钢焊接接头中的扩展行为与显微组织的关系,测得疲劳裂纹在Cr25Ni13/13CrMo44异种钢焊接接头中的扩展速率da/dN,并且讨论了疲劳裂纹扩展与显微组织之间的关系。实验结果表明,疲劳裂纹在异种钢焊接接头熔合区中扩展的路径,是接头中韧性最低的热影响区过热区,裂纹在铁素体材料侧,跟随熔合线并平行于熔合线5～25μm扩展,而马氏体层对疲劳裂纹有较大的抗力,疲劳裂纹的扩展路径主要受组织韧性的控制。疲劳裂纹在Cr25Ni13/13CrMo44异种钢接头的扩展速率为：da/dN=7.07×10-13(△K)3.863。     

Effects of welding residual stresses on fatigue crack growth behaviour in butt welds of a pipeline steel

In the present test the fatigue crack growth rate in the parent plate, weld and cross-bond regions was measured and the results were correlated with the stress intensity range ΔK and the effective stress intensity range ΔK_eff. It is indicated that the welding residual stresses strongly affect the crack growth rate. For the weld metal and cross-bond compact tension specimens in which crack growth is along the weld line the fatigue crack growth rate increases as the crack grows. However, for the T compact tension specimen in which crack growth is perpendicular to the weld line at a constant value of applied ΔK the crack growth rate initially decreases as the crack grows. Particularly, at a low constant value of applied ΔK the crack growth rate obviously decreases and the crack fails to grow after short crack growth. When the crack grows to intersect the welded zone, the fatigue crack growth rate gradually increases as the crack grows further. It is clear that the effect of welding residual stresses on the crack growth rate is related to the position of the crack and its orientation with respect to the weld line. Finally, the models of welding residual stress redistribution in the compact tension specimens with the growing crack and its influence on the fatigue crack closure are discussed. It appears that for a butt-welded joint one of the crack closure mechanisms may be considered by the bend or rotation deformation of crack faces due to the welding residual stress redistribution as the fatigue crack grows in the welded joint.     

Microstructure and fatigue properties of Mg-to-steel dissimilar resistance spot welds

The structural application of lightweight magnesium alloys in the automotive industry inevitably involves dissimilar welding with steels and the related durability issues. This study was aimed at evaluating the microstructural change and fatigue resistance of Mg/steel resistance spot welds, in comparison with Mg/Mg welds. The microstructure of Mg/Mg spot welds can be divided into: base metal, heat affected zone and fusion zone (nugget). However, the microstructure of Mg/steel dissimilar spot welds had three different regions along the joined interface: weld brazing, solid-state joining and soldering. The horizontal and vertical Mg hardness profiles of Mg/steel and Mg/Mg welds were similar. Both Mg/steel and Mg/Mg welds were observed to have an equivalent fatigue resistance due to similar crack propagation characteristics and failure mode. Both Mg/steel and Mg/Mg welds failed through thickness in the magnesium sheet under stress-controlled cyclic loading, but fatigue crack initiation of the two types of welds was different. The crack initiation of Mg/Mg welds was occurred due to a combined effect of stress concentration, grain growth in the heat affected zone (HAZ), and the presence of Al-rich phases at HAZ grain boundaries, while the penetration of small amounts of Zn coating into the Mg base metal stemming from the liquid metal induced embrittlement led to crack initiation in the Mg/steel welds.     

Effects of microstructure and residual stress on fatigue crack growth of stainless steel narrow gap welds

The effects of weld microstructure and residual stress distribution on the fatigue crack growth rate of stainless steel narrow gap welds were investigated. Stainless steel pipes were joined by the automated narrow gap welding process typical to nuclear piping systems. The weld fusion zone showed cellular–dendritic structures with ferrite islands in an austenitic matrix. Residual stress analysis showed large tensile stress in the inner-weld region and compressive stress in the middle of the weld. Tensile properties and the fatigue crack growth rate were measured along and across the weld thickness direction. Tensile tests showed higher strength in the weld fusion zone and the heat affected zone compared to the base metal. Within the weld fusion zone, strength was greater in the inner weld than outer weld region. Fatigue crack growth rates were several times greater in the inner weld than the outer weld region. The spatial variation of the mechanical properties is discussed in view of weld microstructure, especially dendrite orientation, and in view of the residual stress variation within the weld fusion zone. It is thought that the higher crack growth rate in the inner-weld region could be related to the large tensile residual stress despite the tortuous fatigue crack growth path.     

Fatigue crack propagation behavior in friction stir welding of AA6063-T5: Roles of residual stress and microstructure

Behavior of fatigue crack which was propagated at some representative areas in the friction stir welded (FSWed) joint of aluminum alloy 6063-T5 was studied. By extracting the T–L orientation specimens so that the loading axis on the fatigue test and the crack propagation direction were transverse and longitudinal to the welding direction, respectively, the crack propagation tests were carried out for both the as-welded and post-weld heat treated (PWHTed) FSWs at room temperature and 200 °C. The experiments showed that the fatigue crack propagation (FCP) rates were sensitive to the propagating location, the test temperature, and the PWHT condition as well. It was also found that the different FCP rates were driven by the microstructural influences in and around the welded zone. While the residual stress was remarkable in the shoulder limit areas, it had a minor effect on the FCP behavior.     

Fatigue crack growth behavior of the simulated HAZ of 800 MPa grade high-performance steel

The present study focuses on the fatigue properties in the weld heat-affected zone (HAZ) of 800 MPa grade high-performance steel, which is commonly used in bridges and buildings. Single- and multi-pass HAZs were simulated by the Gleeble system. Fatigue properties were estimated using a crack propagation test under a 0.3 stress ratio and 0.1 load frequencies. The microstructures and fracture surfaces were analyzed by optical microscopy, scanning electron microscopy, and transmission electron microscopy. The results of the crack propagation test showed that the fatigue crack growth rate of coarse-grained HAZ (CGHAZ) was faster than fine-grained HAZ (FGHAZ), although both regions have identical fully martensite microstructures, because FGHAZ has smaller prior austenite grain and martensite packet sizes, which can act as effective barriers to crack propagation. The fatigue crack growth rate of intercritically reheated CGHAZ (ICCGHAZ) was the fastest among local zones in the HAZ, due to rapid crack initiation and propagation via the massive martensite-austenite (M-A) constituent.     

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.     

Structure‐property investigations on a laser beam welded dissimilar joint of aluminium AA6056 and titanium Ti6Al4V for aeronautical applications. Part II: Resistance to fatigue crack propagation and fracture

Investigations were continued on the dissimilar laser beam welds of AA6056 and Ti6Al4V, fabricated by inserting Ti‐sheet into the profiled Al‐sheet and melting AA6056 alone. By using microstructure, hardness and strength as the criteria, sites exhibiting non‐uniform microstructure and localized plastic deformation due to strength mismatch were investigated in two orientations: ? crack parallel to the weld and ? crack perpendicular to the weld for fatigue crack propagation and fracture toughness at room temperature. Effect of temper of AA6056 on these properties was studied for two conditions; welding in T4 followed by post weld heat treatment T6, and welding in T6 and naturally aged for a defined period. The orientation “crack parallel to the weld” was investigated in 3 locations on the side of AA6056: the interface and the two changeovers on the Al‐side. Firstly, between the fusion zone and the heat affected zone (3 mm from the interface) and secondly, between (primary) heat affected zone and towards the base material (7 mm from the interface). Although brittle intermetallic TiAl₃ had been formed at the interface, uncontrolled separation or debonding at the interface was not observed. Insofar the bond quality of the weld was good. However, the ranking of interface was the lowest since fatigue crack propagation was relatively faster than that in the fusion zone and heat affected zone, and fracture toughness was low. Therefore, unstable fatigue crack propagation is observed when the crack propagates perpendicular to the weld from AA6056 towards Ti6Al4V. The results have shown that the dissimilar joints exhibit improved performance when laser beam welded in the T6 condition.     

Effect of microstructure of simulated heat‐affected zone on low‐ to high‐cycle fatigue properties of low‐carbon steels

To clarify the effect of microstructural changes on the fatigue property of the weld heat‐affected zone (HAZ), low‐ to high‐cycle fatigue tests were conducted on 16 types of simulated HAZ specimens that had been prepared using thermal processes. The results showed the fatigue S‐N curves of the HAZ to be widely scattered as a function of strength level. These fatigue data were divided into two groups: coarse grain (CG) and fine grain (FG) HAZ, when strain amplitude was used to represent S‐N curves. The fatigue data for the CGHAZ group showed a relatively short fatigue life. Based on surface observations, the initiated fatigue crack size of CGHAZ was larger than that of FGHAZ as a function of microstructural unit size. Hence, fatigue crack growth life, which is almost the same as total fatigue life of CGHAZ, decreased.     

Metallurgical investigations and corrosion behavior of failed weld joint in AISI 1518 low carbon steel pipeline

In this paper, failure analysis was carried out based on the available documents, metallographic studies and corrosion behavior of the welded joint pipe sample made of AISI 1518 low carbon steel. Nondestructive evaluations including penetration test (PT) and radiographic test (RT) were performed on the as-received pipeline and results indicated the presence of micro- and macro-cracks. Optical microscopic images and scanning electron microscopy (SEM) micrographs revealed various microstructures in the base metal (BM), heat affected zone (HAZ) and weld metal (WM). The microstructural variations may result in galvanic feature and lead to failure and rupture of the weld joint during the service. Microhardness measurements showed that hardness value was about 260 HV in the WM, while it declined in the HAZ and BM. Qualitative chemical analyses such as X-ray diffraction pattern (XRD) and SEM equipped with energy dispersive spectroscopy (EDS) confirmed the presence of corrosive media during weld joint rupture. Additionally, SEM and optical investigations indicated that micro-cracks were formed in HAZ due to residual stress as a consequence of improper welding condition. Surface fracture studies showed that the crack initiation, crack growth and finally crack propagation took place in the WM/HAZ interface. Electrochemical studies were conducted on the BM, HAZ and WM to investigate corrosion behavior of the failed joint sample. Finally, a proper corrosion mechanism is proposed based on the failure analyses and electrochemical studies.     

Fatigue crack propagation in API 5L X‐70 pipeline steel longitudinal welded joints under constant and variable amplitudes

Fatigue crack propagation (FCP) under constant and variable amplitude loading in base metal (BM), weld metal (WM) and heat affected zone (HAZ) of longitudinal welded joints of an API X‐70 pipeline steel was investigated. Constant amplitude loading tests were performed at R = 0.1 and 0.5, whereas for variable amplitude testing single peak tensile overloads (OLs) alternating between 75 and 100% of maximum load were applied at 2.5 mm intervals in crack growth. Results of SE(B) specimens tested under constant and variable amplitude loading revealed that BM, WM and HAZ regions subjected to R = 0.5 and low ΔK‐values presented the highest crack growth rates. At higher ΔK values FCP rates in all the studied regions were similar and the R effect on FCP rate was no more observed. Crack growth retardation due to OLs was observed at the three studied regions, showing a decrease on the FCP delay with a decreasing on ΔK.     

基于断裂力学的高强度钢材梁柱节点受力性能分析

为了研究国产Q460C高强度结构钢材梁柱节点的断裂行为,该文基于断裂力学理论,计算了Q460C高强度钢材焊缝及热影响区材料的断裂韧性,并且采用三维有限元断裂模型,以I型裂纹尖端应力强度因子K_I和J积分为定量的评价指标,分析了焊缝及热影响区不同长度的裂纹对梁柱节点断裂韧性的需求。弹性分析表明,K_I沿梁翼缘宽度方向呈W形分布,最大值出现在翼缘中心,且与名义弯曲应力呈线性关系,而焊根裂纹的断裂韧性需求比热影响区裂纹更高。弹塑性分析表明,J_I最大值出现在翼缘的边缘,热影响区裂纹的断裂韧性需求比焊根裂纹更高。研究结果表明,Q460C高强度钢材梁柱节点的断裂由焊根裂纹控制,断裂承载力与梁全截面塑性承载力相近,临界转角小于0.02rad,因此建议通过改善焊接工艺或局部构造来保证节点拥有足够的转动能力。     

电解充氢对堆焊熔合区疲劳裂纹扩展行为的影响

研究了阴极充氢前后堆焊熔合区疲劳裂纹扩展行为,发现堆焊熔合区对疲劳裂纹的扩展有阻碍作用,充氢对裂纹扩展速率无明显影响,但使熔合区出现大量二次裂纹,随着充氢时间的延长,二次裂纹将更加严重。     

An experimental investigation of local fracture resistance and crack growth paths in a dissimilar metal welded joint

An experimental investigation on the local fracture resistance and crack growth behavior in a Alloy52M dissimilar metal welded joint (DMWJ) between A508 ferritic steel and 316L stainless steel has been carried out by using the single-edge notched bend (SENB) specimens. The local J-resistance curves and crack growth paths of 13 cracks located at various positions in the DMWJ were determined, and the effects of the local strength mismatch on local fracture resistance, crack growth paths and integrity assessment for the DMWJ were analyzed. The results show that the cracks always deviate to the materials with lower strength, and the crack path deviations are mainly controlled by the strength mismatch, rather than toughness mismatch. The J-resistance curve with larger crack path deviation only reflect the apparent fracture resistance along the crack growth region, rather than the intrinsic fracture resistance of the material at the initial crack-tip region. Without considering the local fracture resistance properties of heat affected zone (HAZ), interface and near interface zone, the use of the J-resistance curves of base metals or weld metals following present codes will unavoidably produce non-conservative (unsafe) or excessive conservative assessment results. In most cases, the assessment results will be potentially unsafe. Therefore, it is recommended to obtain and use local mechanical and fracture resistance properties of all regions of the DMWJ if the complex local mismatch situation is a concern. And new integrity assessment methods based on local damage and fracture models also need to be developed for the DMWJs.     

Fatigue behavior of friction stir welded Al–Mg–Sc alloy

The effect of Friction Stir Welding on the fatigue behavior of Al–Mg–Sc alloy has been studied. To reveal the influence of the welding parameters, different travel speeds of the welding tool have been used to provide weld seams with varying microstructural features. Crack initiation as well as crack propagation behavior under fatigue loading has been investigated with respect to the local microstructure at the crack initiation sites and along the crack path. Fatigue cracks were mostly initiated around the stir zone and the adjacent thermo-mechanical affected zone independent from hardness distributions in the weld seams. In some specimens, defect-like feature was observed at the crack origins, which shortened the fatigue lives. It has been found that while the effect of the tool travel speed on the fatigue lifetime seems to be little, the varying and complex local microstructure in the weld seam basically affects both the crack initiation sites and the crack propagation paths.     

Role of plastic work in fatigue crack propagation in metals

The plastic work required for a unit area of fatigue crack propagation U was measured by cementing tiny foil strain gages ahead of propagating fatigue cracks and recording the stress-strain curves as the crack approached. Measurements of U and plastic zone size in aluminum alloys 2024-T4, 2219-T861, 2219 overaged, and A1-6.3 wt% Cu-T4, and a binary Ni-base alloy with 7.2 wt% A1 are herein reported. The results are discussed along with previously reported measurements of U in three steels and 7050 aluminum alloy. When U is compared to the fatigue crack propagation rate at constant ΔK along with strength and modulus, the conclusion is drawn that U is one of the parameters which determines the rate of fatigue crack propagation. The relation of U to microstructure is also discussed. “Homogeneous” plastic deformation in the plastic zone ahead of the crack seems desirable.     

The effect of frequency and environment on the fatigue-crack growth behaviour of ASTM A533 Grade B Class 1 weldment material

The effect of frequency and distilled water environment on the fatigue-crack growth characteristics of ASTM A533 Grade B Class 1 weldment material was studied with major emphasis placed on the crack growth along the weld centreline as well as along the heat affected zone (HAZ). A single deterministic fatigue-crack growth model based on the Four Parameter Weibull Survivorship Function is used to describe the dependence of crack growth per cycle, da/dN, on the alternating stress intensity, ΔK, over the entire range of ΔK. The probability distribution function of da/dN is determined to be log-normal by analysing the residuals of the linear regression model that is derived from the Weibull curve-fitting model. No significant effect of frequency is observed in the weld and HAZ materials in laboratory air. A decrease in frequency from 10 Hz to 0.5 Hz in distilled water tends to increase the crack growth rates over the baseline data. Distilled water environment tends to produce serious data scatter in both weld and HAZ materials. As a result, replication of fatigue tests is recommended in order to increase the amount of data which is essential for a better estimate of the median curve. The observed scatter could have some effect in fatigue-crack growth damage tolerance estimates.     

Fatigue crack growth property of laser beam welded 6156 aluminium alloy

Fatigue crack growth behaviours in different welding zones of laser beam welded specimens were investigated using central crack tension specimens for 6156 aluminium alloy under constant amplitude loading at nominal applied stress ratio R = 0.5, 0.06, ?1. The experimental results showed that base metal (BM) exhibited superior fatigue crack resistance compared to weld metal (WM) and heat‐affected zone (HAZ). Crack growth resistance of WM was the lowest. The exponent m values for BM and HAZ at different stress ratios are close and around 2.6, while m for WM at different stress ratio is around 4.7. The discrepancy between crack growth rates for WM and BM is more evident with increasing stress ratio, while it is a little change for HAZ and BM. Change of the microstructure in WM deteriorates the resistance of fatigue crack growth compared to BM. It was mainly due to grain boundary liquation and dissolving of second‐phase particles in the weld region. It was also found that the variety of fatigue crack resistance for different welding zones is in conformity with the change of hardness. BM with the highest hardness exhibited the maximum resistance for fatigue crack, and WM with the lowest hardness exhibited the minimum fatigue crack resistance.     

Fatigue crack propagation into the residual stress field along and perpendicular to laser beam butt‐weld in aluminium alloy AA6056

Laser beam butt welds in Al‐alloys are very narrow and are accompanied by steep residual stress gradients. In such a case, how the initial crack orientation and the distance of the notch tip relative to the weld affect fatigue crack propagation has not been investigated. Therefore, this investigation was undertaken with two different crack orientations: along the mid‐weld and perpendicular to the weld. Fatigue crack propagation ‘along the mid‐weld’ was found to be faster in middle crack tension specimens than in compact tension specimens. For the crack orientation ‘perpendicular to the weld’, the relative distance between the notch tip and the weld was varied using compact tension specimens to generate either tensile or compressive residual stresses near the notch tip. When tensile residual stresses were generated near the notch tip, fatigue crack propagation was found to be faster than that in the base material, irrespective of the difference in the initial residual stress level and whether the crack propagated along the mid‐weld or perpendicular to the weld. In contrast, when compressive weld residual stresses were generated near the notch tip, fatigue crack arrest, slow crack propagation, multiple crack branching and out of plane deviation occurred. The results are discussed by considering the superposition principle and possible practical implications are mentioned.     

Fatigue crack propagation behavior of friction stir welded 5083-H32 Al alloy

Fatigue crack propagation (FCP) behavior of friction stir welded (FSWed) 5083-H32 Al alloy was examined with the fatigue crack growing either along the dynamically recrystallized zone (DXZ) at variable ΔK or perpendicular to the DXZ at a constant ΔK value of 10, 13, 15, and 17 MPa√m, respectively. The FCP behavior of FSWed 5083-H32 specimen is substantially influenced by the presence of FSW zone, the trend of which is discussed based on residual stress measurement and fractographic observation.