Effect of Lack of Penetration on the fatigue strength of high strength steel butt weld

Fatigue tests are performed to evaluate the fatigue strength of high strength steel containing partial penetration butt weld and full penetration butt weld. The influence of the unwelded ligament (Lack of Penetration) in the partial penetration welds on the fatigue life is analyzed for various LOP sizes. For full penetration welds, the fatigue crack initiated at the weld toe and propagated to the HAZ. For partial penetration welds, however, the fatigue cracks initiated at the LOP section and propagated to the weld metal (or weld toe) for the considered LOP sizes (from 2mm to 4mm) reducing the fatigue strength. Consequently, the increament of the LOP size yield in the fatigue life degradation by some extent.     

超高强度钢十亿周疲劳研究

利用20kHz压电超声疲劳试验技术，对六种抗拉强度高于1500MPa的低合金钢进行测试。结果表明，这些材料并不存在传统规范中所谓的“疲劳极限”，它们在超过10^7，甚至10^9应力循环后仍然继续发生破坏，而且其S－N曲线表现出“阶梯”型特征。研究还发现，这些材料都存在一个分界应力幅值区间（或应力循环数区间，大约在10^6到10^7的平台内），裂纹萌生由试件表面向其内部转换。实验证明了十亿周级超高周疲劳裂纹由试件内部萌生的机理。     

压弯组合应力下高强钢焊接板表面裂纹疲劳寿命计算

潜艇上浮下潜运动使得其耐压壳承受交变的外压载荷。潜艇耐压壳的疲劳热点为锥一柱过渡结构连接焊缝焊趾处，该处所受的应力特征为压弯组合应力。压弯组合应力下表面裂纹应力强度因子及其疲劳寿命的计算尚无报道。弄清楚压弯组合应力作用下带表面裂纹焊接板试件的疲劳特性对潜艇等结构的疲劳研究是必要的。文中提出用成一定角度的对接焊板试件和轴向加载获得压弯组合应力来模拟潜艇耐压壳锥一柱焊接结合区的应力特征的实验方法。用980高强钢作试件，研究焊趾处预制表面裂缝，并在压弯组合应力为特征的疲劳载荷作用下表面裂纹的疲劳行为。给出高强钢焊趾表面裂纹在压弯组合应力下应力强度因子及其疲劳寿命计算式。该结果可供海洋平台、压力容器及管道的某些受力特征为压弯组合应力的重要结构的疲劳设计时参考。     

某大桥扣索钢铰线断丝失效分析

对大桥施工中发生的扣索钢绞线断丝进行了成分、显微组织、断口及裂纹分析，找出了断裂原因，采取了措施。分析结果表明，扣索钢绞线断丝的断裂性质为疲劳断裂，其裂纹源的产生是由于扣索在索鞍轮处弯曲过大，内侧受到较大交变接触应力所致。风振及反复张拉调整，使裂纹不断扩展。     

Mechanism of reduction of fretting fatigue limit caused by hydrogen gas in SUS304 austenitic stainless steel

The fretting fatigue strength of pre-strained SUS304 is reduced in hydrogen gas. The mechanism of the reduction is discussed. In hydrogen gas, local adhesion between contacting surfaces occurred and many small cracks were formed at the adhered spots. The major crack propagated from one of the small cracks. The roles of the adhesion in relation to the initiation and propagation of the small cracks were examined by a two-step environment test. When adhesion was prevented by an oxidized film, no failure of the specimen occurred. It can be presumed that the stress conditions were more severe in hydrogen gas than that in air due to local adhesion.     

近疲劳强度载荷下WC/Co涂层对300M钢疲劳性能的影响

研究了在近疲劳强度载荷下超音速火焰喷涂WC/Co涂层和电镀铬层对300 M低合金钢疲劳性能的影响.结果表明：WC/Co涂层、电镀铬层试样和300 M钢基体的疲劳强度分别为753,600和720 MPa.在近疲劳强度载荷下,WC/Co涂层试样的疲劳寿命分别为基体和电镀铬层试样的1.5～2和5～10倍.三者的主裂纹源均为基体内部的Al2O3和SiO2夹杂物.电镀铬层中的次裂纹穿过界面并向基体中扩展,在基体表面形成新的次裂纹源,加快了基体中主裂纹源的形成和主裂纹的扩展;而WC/Co涂层中的次裂纹经过界面时向界面方向偏转,形成无支撑涂层,并最终使涂层剥落.     

Analysis of Fretting Fatigue of Cavitation Shotless Peened Ti–6Al–4V

Fretting fatigue behavior of cavitation shotless peened titanium alloy, Ti–6Al–4V coupons was investigated using finite element method and a critical plane-based multi-axial fatigue parameter. Cavitation shotless peening (CSP)-induced compressive residual stress, which was larger at the contact surface than its counterpart from the shot peening (SP). However, compressive residual stress decreased more sharply with distance from the contact surface in CSP than in SP. Analysis using a critical plane-based multi-axial fatigue parameter demonstrated that the crack initiation would occur inside the cavitation shotless peened specimen which matched with the experimental observations. On the other hand, crack initiation would occur on the contact surface in the shot peened specimen which again was in agreement with experiments. The analysis also showed that the crack propagation part of the total fretting fatigue life was longer in the shot peened specimen than in the cavitation shotless peened specimen while the crack initiation part was almost equal from both peening methods. Therefore, CSP could not improve the fretting fatigue life/strength as much as the SP did but it improved relative to the un-peened specimen.     

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.     

点焊搭接接头中的应力分布及接头疲劳行为研究

基于实际点焊接头硬度分布的分析,建立了包括熔核,热影响区和母材的单点点焊搭接接头计算模型。用三维弹塑性有限元分析方法,全面分析了点焊搭接头的应力分布情况。分析结果表明：接头搭接区内表面上焊点边缘处的应力集中是接头高周疲劳裂纹起裂和扩展的决定因素。焊点边缘附近区域中的存在的高应力,导致高周疲劳裂纹由强度较低的母材起裂并扩展。     

力学不均匀性对含横向裂纹焊接接头J积分的影响

采用 MARC有限元程序提供的 Delorenzi虚拟裂纹扩展方法计算了平面应力条件下不同强度匹配的焊缝纵向受载的含横向裂纹的焊接接头在不同载荷的广义 J积分。计算结果表明 :力学不均匀性对含横向裂纹焊接接头的 J积分有重要影响 ,低匹配焊接接头中积分路径穿越焊缝与母材界面的 J积分值高于积分路径在焊缝内的 J积分值 ,高匹配焊接接头中积分路径穿越焊缝与母材界面的 J积分值低于积分路径在焊缝内的 J积分值 ;载荷越大 ,焊缝与母材的强度差别越大 ;积分路径中包含的焊缝与母材界面长度越大 ,J积分的差值越大。力学不均匀性对焊缝纵向受载的含横向裂纹的焊接接头 J积分的影响是由焊缝与母材的强度差别及积分路径中包含的焊缝与母材界面长度的不同造成的     

Comparison of thermal fatigue behaviour of ASTM A 213 grade T-92 base and weld tubes

Super-heater tubes are subjected to alternate heating and cooling in power plants causing them to crack and eventually fail. This phenomenon is referred to as “thermal fatigue.” In this paper, a laboratory simulation for reproducing the thermal fatigue phenomenon is developed to determine the number of cycles necessary before failure occurs in super-heater tubes. The temperature and strain distributions along the specimen were computed theoretically using ANSYS software for the applied temperature condition. The thermal fatigue test was conducted for both base and shielded metal arc (SMA) welded tubes separately and both passed in the non-destructive tests. These tubes were subjected to thermal cycles from 800°C (accelerated temperature) to room temperature. Oxy-acetylene heating setup was utilized as a heating source, and a water bath was utilized for quenching purposes. The tests were carried out until open cracks were identified. Surface cracks were identified in the base and weld tubes after 90 and 60 cycles respectively. This study reveals that heating and cooling cause thermal fatigue, initiate cracks in the tubes.     

钝缺口试样疲劳短裂纹形成与扩展

对两种晶粒尺寸的中碳钢钝缺口试样进行了对称拉疲劳试验，应用复型方法观察了疲劳理解纹沿缺口根部的早期形成与扩展过程；测量了表面裂纹形状随裂纹长度的变化规律。结果表明，试样的破坏是由形成于缺口根部表面的短裂纹扩展、裂纹间连接形成穿透裂纹，穿透裂纹扩则引起的。     

Three-dimensional weld toe magnification factors for various welded joints

To effectively calculate the stress intensity factors in welded components, a weld toe magnification factor is introduced here that will allow for the influence of geometrical effects. This factor was derived from the data obtained in a parametric study performed by FEM. Several sets of weld toe magnification factor solutions have been presented, but these are applicable only to non-load-carrying cruciform or T-butt joints, due possibly to the requirement of very complicated calculations. In the majority of cases for various welded joints such as cruciform, cover plate and longitudinal stiffener joints, the currently used weld toe magnification factors do not adequately describe the behavior of weld toe cracks. In the present study, the weld toe magnification factor solutions with exponents and fractions that have polynomial functions in terms of a/c and a/t ?? i.e., crack depths normalized by corresponding crack lengths and specimen thickness ?? were developed through a parametric study using 3-dimensional finite elements for the above mentioned three types of welded joints. These weld toe magnification factor solutions showed a tendency to increase at a/t < 0.2. Meanwhile, for 0.2 < a/t < 0.7, the effect of the attachment on the weld toe magnification factor decreased asymptotically. When the a/t ratio exceeded this range, the weld toe magnification factor became almost one of unity. The fatigue crack propagation life was evaluated by using the proposed weld toe magnification factor and by considering the propagation mechanisms of multiple-surface cracks, and it showed good agreement-to within a deviation factor of 2-between the experimental and calculated results for the fatigue crack propagation life for all welded joint.     

Progression of fretting fatigue damage in Ti–6Al–4V

An investigation was conducted to explore the nature of fretting fatigue damage in the stages prior to crack formation. In the unique experimental apparatus employed in this study, where total slip never occurs, several locations on each test specimen exist where cracks can develop due to local contact conditions. Under the test conditions used, not all of the sites had cracks upon test completion. This study evaluated the condition of non-cracked sites on several fretted specimens in an effort to identify differences between these and sites where small cracks were observed.A single test condition of 620 MPa average applied static clamping stress and 250 MPa applied axial fatigue stress for R=0.5 was selected, which corresponds to a fretting fatigue life of 10⁷ cycles based on prior work. For specimens tested to 10⁶ cycles, or 10% of life, several destructive and non-destructive characterization methods were chosen: scanning electron microscopy (SEM), residual stress measurement and transmission electron microscopy (TEM). Each site at which crack nucleation could be expected was inspected in the SEM and was then characterized using surface X-ray diffraction to quantify the residual stresses field near that location. Then TEM foils were cut from one area on a specimen with tiny cracks and dislocation densities were observed. A novel technique was used which permitted TEM samples to be obtained from regions in close proximity on the original specimen.Comparisons were made between as-received (AR) and stress-relief annealed (SRA) specimens, on which the stress-relief was applied prior to fretting fatigue testing. SEM inspection was useful for qualitative analysis of wear debris and identification of cracks as small as 20 μm, but was unable to provide quantitative data on the level of fretting fatigue damage beyond crack size. Although differences were noted in the residual stresses for the SRA versus the AR specimens, no residual stress peaks were noted in the edge of contact regions where cracks would eventually develop. TEM observations in the vicinity of the crack nucleation region showed that the dislocation structure decayed rapidly into the specimen thickness. The cause of the dislocations was attributed to plastic deformation caused by the clamping stresses.     

Welding residual stress in roots between deck plate and U-rib in orthotropic steel decks

Fatigue cracks have recently been observed at weld roots between the deck plate and U-rib in orthotropic steel decks. Some cracks initiated from the weld roots and then propagated through the deck plate. The welding residual stress at the root appears to have a considerable influence on crack initiation and propagation. However, the stress has not yet been efficiently clarified in previous studies. In this study, the residual stress of model specimens with or without weld penetration and root gap were measured using the cutting method and magnetostriction method. In addition, thermo-elastic-plastic finite element analyses were carried out on parametric models of actual welded joints.     

全层状TiAl基合金拉伸试验断裂过程及机理

通过对全层组织TiAl基合金不同缺口试样进行原位拉伸试验,结合数据分析以及断口形貌的观察,研究TiAl基合金拉伸的断裂过程和断裂机理.研究发现在原位拉伸试验中,对于直缺口试样,裂纹起裂于缺口根部,其断裂过程主要是主裂纹(沿层裂纹)首先起裂、扩展、连接,然后形成解理断裂起裂源.这一起裂源形成的同时,在刚好能满足Griffiths脆性解理断裂所需要的临界裂纹尺寸时,引起整个试件发生脆性解理断裂.裂纹首先起裂于层间,层间是薄弱环节,断裂方式是穿层断裂和沿层断裂的混合体,以穿层断裂为主.对于大圆弧和平板试样,这种材料的断裂过程是:首先在较高外加载荷下,在标距范围内比较薄弱的区域沿层开裂,随着外加载荷的进一步增加,这些微裂纹扩展,当微裂纹引起的应力集中和外加载荷共同作用足以引起微裂纹进一步贯穿的瞬间,即能量积累到一定程度时,试样整体发生解理断裂.很多试样的断裂是直接起裂的过程,一旦产生裂纹试样立即解理断裂,即这种材料的强度较高,材料很难产生裂纹,也不易损伤.     

Characterization of fretting fatigue crack initiation processes in CR Ti–6Al–4V

A study was conducted to quantify fretting fatigue damage and to evaluate the residual fatigue strength of specimens subjected to a range of fretting fatigue test conditions. Flat Ti–6Al–4V specimens were tested against flat Ti–6Al–4V fretting pads with blending radii at the edges of contact. Fretting fatigue damage for two combinations of static average clamping stress and applied axial stress was investigated for two percentages of total life. Accumulated damage was characterized using full field surface roughness evaluation and scanning electron microscopy (SEM). The effect of fretting fatigue on uniaxial fatigue strength was quantified by interrupting fretting fatigue tests, and conducting uniaxial residual fatigue strength tests at R=0.5 at 300 Hz. Results from the residual fatigue strength tests were correlated with characterization results.While surface roughness measurements, evaluated in terms of asperity height and asperity spacing, reflected changes in the specimen surfaces as a result of fretting fatigue cycling, those changes did not correspond to decreases in residual fatigue strength. Neither means of evaluating surface roughness was able to identify cracks observed during SEM characterization. Residual fatigue strength decreased only in the presence of fretting fatigue cracks with surface lengths of 150 μm or greater, regardless of contact condition or number of applied fretting fatigue cycles. No cracks were observed on specimens tested at the lower stress condition. Threshold stress intensity factors were calculated for cracks identified during SEM characterization. The resulting values were consistent with the threshold identified for naturally initiated cracks that were stress relieved to remove load history effects.     

球墨铸铁疲劳裂纹扩展的微观机制

研究了在恒幅拉伸循环载荷作用下球铁的疲劳裂纹扩展的微观机制，分析了石墨形态对铸铁疲劳裂纹萌生及扩展行为的影响。结果表明：铸铁的石墨形态对疲劳裂纹的萌生及扩展有很大影响。灰铁由于在共晶团内石墨片相互连接，疲劳裂纹总是沿着与外应力相垂直的石墨片／基体边界扩展。球铁的疲劳裂纹扩展受基体组织及石墨形态所左右，但在石墨球／基体界面产生的滑移带、塑性区及裂纹尖端钝化区决定着疲劳裂纹萌生及扩展的整个过程。球铁的疲劳裂纹扩展规律是：裂纹连续扩展—急速扩展—裂纹尖端钝化—重新诱发裂纹—连续扩展。     

Initiation and propagation of surface cracks in rolling fatigue of high hardness steel

Surface cracks initiating flaking failure observed by Sugino in rolling fatigue were further studied.Surface crack initiation was influenced by either running or material conditions, affecting the residual tensile stress just below the rolling contact surface. The fatigue crack is thought to initiate very near the surface by the stress cycle of small tensile residual stress and large compressive contact stress. The amount and direction of the tensile residual stress due to rolling contact varies with the contact geometry, which probably determines the direction of the surface crack and the appearance of flaking. The transition from the surface crack to flaking failure or complete section fracture is material and running condition dependent and is explained by a fracture mechanics concept.     

Effects of Mn content and texture on fatigue properties of as-cast and extruded AZ61 magnesium alloys

Fatigue behavior of as-cast and extruded AZ61 magnesium alloys in ambient air (20 °C–55%RH) was investigated. It was found that size and distribution of cast defect influenced tensile and fatigue performance of the as-cast alloy. Fatigue limit of the as-cast alloy was significantly low compared to the extruded alloy. The casting defects served as stress concentration sites for fatigue crack nucleation. Fatigue tests were also carried out on a high Mn content alloy. All of the specimens failed from an inclusion near the specimen surface. Fatigue limit of Mg alloy with high Mn content was lower compared to that of the low Mn content alloy. Further, investigation on the effect of texture on fatigue and fatigue crack growth behavior of the extruded AZ61 magnesium alloy plate was carried out. The results showed that fatigue strength in the longitudinal direction to the extruded direction was higher compared to those in the transverse and 45° directions. Significant effect of specimen orientation on fatigue crack growth behavior for both short and long cracks was found near the threshold region. However, regardless of specimen orientation, the da/dN–ΔK_eff curves for all three kinds of specimens were in a narrow band. It is suggested that the difference in the fatigue life among the specimen orientations will be mainly due to the difference in the crack closure behavior. A transition of fracture mechanism was found for a long crack. Slip fracture mechanism was dominant above the transition point, whereas below the transition point, slip fracture mechanism was associated with cleavage fracture.