EARLY DEVELOPMENT OF FATIGUE CRACKING AT MANUAL FILLET WELDS

Abstract— An experimental study within the Canadian Offshore Corrosion Fatigue Research Programme was performed on the early development of fatigue cracking along the wavy toe of manual fillet welds between structural steel plates. Stress relieved and as-welded cruciform joints were tested under R = −1 and R = 0 loading at different stress amplitudes. The depth and the opening level of cracks as small as 10–20 μm were monitored using miniature strain gauges installed along the toe apex, in combination with beach marking. Most of the "initiation life" (25% to 50% of total life), conventionally defined by a crack depth of 0.5 mm, is consumed in short crack propagation. Three types of short crack development for different combinations of local mean stress and stress range are identified and analyzed. Growth rates in as-welded specimens are faster than in stress relieved specimens, which results in shorter "initiation lives". This is associated with a higher effective stress range, particularly under R = - 1 loading where cracks are open over nearly the full stress range. The V-notch stress intensity factor is a promising parameter to rationalize the crack "initiation life". It takes into account the thickness effect experimentally observed. Under R = - 1 loading of as-welded joints, using R = 0 data and taking the whole stress range gives a reasonably conservative approximation of the crack "initiation life".     

EFFECT OF GRAIN SIZE ON COLLECTIVE DAMAGE OF SHORT CRACKS AND FATIGUE LIFE ESTIMATION FOR A STAINLESS STEEL

In the short crack regime of the fatigue process, grain boundaries in steels are barriers against crack growth. In this paper, we use: (1) a method involving crack density; and (2) a method of dimensional analysis, to evaluate the effects of grain size and grain-boundary resistance on short crack behaviour and fatigue life. The results show that the fatigue life increases with a decrease in grain size and an enlargement in the obstacle effect of a grain boundary. An experimental investigation is consequently performed and four groups of stainless steel specimens are used with different grain sizes. The experimental measurements show the dependence of fatigue properties on grain size, which are in good agreement with the theoretical results.     

FATIGUE DESIGN OF SPOT-WELDED AUSTENITIC AND DUPLEX STAINLESS SHEET STEELS

The fatigue strength of spot-welded stainless sheet steels has been investigated. The main part of the fatigue tests was performed on a cold rolled austenitic stainless sheet steel (AISI304) in air at ambient temperature. For comparison, a duplex stainless steel (SAF2304) of similar yield strength as AISI304 was also incorporated into the test programme. Since the fatigue strength of spot-welded joints depends on the mode of loading, both shear-loaded and peel-loaded joints were tested. The fatigue strength of the spot-welded stainless steels was found to decrease with decreasing sheet thickness. Furthermore, the fatigue strength for peel-loaded joints is lower than that of shear-loaded joint for sheets of equal thickness.
The local loading conditions at the weld edge have been analysed in terms of finite element calculations and fracture mechanics. A design parameter derived from a fracture mechanics analysis was defined for spot-welded stainless sheet steels. It was shown to predict the fatigue life of the present steels and joint configurations in a satisfactory way.     

SURFACE MICRO-CRACKS IN BLOCK AND RANDOM FATIGUE CYCLING OF 500 MPa STRENGTH STEEL AND WELD METAL

Abstract— Crack density, main crack length and the sum total length of micro-cracks, which initiated and extended on the surface of a 500 MPa strength steel and its weld metal, were investigated by a replica technique applied during fatigue tests. The base and weld metal specimens were subjected to constant amplitude, random and block strain cycling. The base metal specimens were further tested under 4 kinds of block cycling and 2 kinds of incremental strain cycling. All the cumulative cycling patterns in random and block modes followed the so-called p -distribution.
As a result of an analysis of micro-cracks, it was shown that the most useful parameter to estimate the accumulated fatigue damage was the sum total length of micro-cracks in a unit area, which increased exponentially with cycle ratio. Empirical formulae were obtained expressing the increasing tendency as a function of the cycle ratio for three groups of the equivalent strain amplitude. The formulae were applicable to all strain cycling patterns investigated in the present study.     

钢结构角焊缝抗剪疲劳性能的试验研究

国内外学者对钢结构的焊缝连接在名义正应力荷载条件下的疲劳性能进行了广泛的研究,但是所见文献中对焊缝在名义剪切应力下的疲劳性能研究较少。为此,以Q345B钢材制作了原状处理的侧面和正面角焊缝连接的2组板材试件,采用高频疲劳试验机进行常温剪切疲劳试验,并进行数据拟合。试验结果发现：该文试验中的2类焊接试件的疲劳试验结果具有非常大的离散性。由中国钢结构规范GB 50017-2017得到的200万次下的疲劳强度值比侧面角焊缝试验拟合曲线计算得到的值小,两者曲线吻合较好;正面角焊缝试验所得疲劳强度高于各国规范;正面角焊缝接头抗剪切疲劳强度高于侧面角焊缝接头;正面角焊缝疲劳破坏形式为贯通焊缝裂纹,侧面角焊缝为焊趾向热影响区延伸裂纹。研究结果表明：现有中国规范评价侧面和正面角焊缝剪切性能都偏于安全,但是使用规范推荐的曲线评估侧面角焊缝剪切疲劳性能较为合适。     

MICROSTRUCTURE AND FATIGUE PROPERTIES OF A WELDED DUPLEX STAINLESS STEEL

Abstract— In response to the increasing structural applications in duplex steels for welded structures, fatigue behaviour of a SAF 2304 grade duplex stainless steel was investigated, considering both the base metal and GTAW welded joints. Fatigue curves and fatigue limits under rotary bending fatigue were obtained. The study focused attention on the microstructural features of fatigue crack propagation of the two series of experiments, thereby permitting an evaluation of the tortuous crack path of welded joints and the mechanisms related to threshold microstructural barriers.     

SMALL FATIGUE CRACKS IN AN AUSTENITIC STAINLESS STEEL

The present study concerns nucleation and growth of small surface cracks during the low-cycle fatigue of a nitrogen-containing austenitic stainless steel. Metallographic replicas as well as longitudinal sectioning were used to record the developing crack pattern on the specimen surface. The influence of grain size and nitrogen content is considered. Small surface cracks are observed after about 10% of the fatigue life. The nucleation of cracks continues until about half of the lifetime, when the crack density saturates. This saturation phenomenon is related to the local unloading effect of growing cracks.
The mean crack length increases continuously as a power-law until specimen failure. However, small grains and a low nitrogen content amplify the effect of crack–grain boundary interactions resulting in an intermediate retardation in growth.
At high nitrogen contents, the crack growth characteristics are very much related to the slip bands formed. This results in a more simultaneous growth of cracks, a more jagged feature of the cracks introducing a higher roughness-induced crack closure effect, and, consequently, better fatigue properties.     

THE SIGNIFICANCE OF CRACK TIP DEFORMATION FOR SHORT AND LONG FATIGUE CRACKS

Abstract— The behaviour of physical short mode I cracks under constant amplitude cyclic loading was investigated both numerically and experimentally. A dynamic two-dimensional elastic-plastic finite element technique was utilised to simulate cyclic crack tip plastic deformation. Different idealisations were investigated. Both stationary and artificially advanced long and short cracks were analysed. A parameter which characterises the plastically deformed crack tip zone, the strain field generated within that zone and the opening and closure of the crack tip were considered. The growth of physically short mode I cracks under constant amplitude fully reversed fatigue loading was investigated experimentally using conventional cast steel EN-9 specimens. Based on a numerical analysis, a crack tip deformation parameter was devised to correlate fatigue crack propagation rates.     

HOT SPOT STRESS APPROACH TO FATIGUE STRENGTH ANALYSIS OF WELDED COMPONENTS: FATIGUE TEST DATA FOR STEEL PLATE THICKNESSES UP TO 10 MM

Abstract— The use of the hot spot stress approach to the fatigue analysis of welded components is briefly described. Results are presented of fatigue tests on arc welded steel joints (C-Mn and stainless), carried out at Lappeenranta University of Technology between 1980 and 1993, based on the hot spot approach. Based on experimentally-measured hot spot strains, the fatigue capacities of around 100 specimens of C-Mn steel joints, and 80 stainless steel joints, were found to be consistent. The fatigue class FAT 100, or in many cases FAT 112 or higher, can be used as the design hot spot fatigue strength for toe failure of welded joints of moderate thickness, i.e. up to 10 mm. A sharp transition at the fusion zone from the base metal to the weld was clearly shown to be detrimental, leading to a fatigue capacity below average. The log, value of the standard deviation of fatigue life, or the fatigue capacity (Δσ³ N ), was typically 0.13 within a series of C-Mn joints. Statistical analysis of all test data concerning weld toe failure gave a standard deviation of 0.24. By considering all the specimens in one series, a mean fatigue strength of FAT 148, and a characteristic fatigue strength of FAT 107, were obtained.     

钢结构角焊缝低温抗剪疲劳性能的试验研究

国内外学者对钢结构的焊缝连接在常温下的疲劳性能进行了广泛的研究,但是所见文献中对构造细节在低温下的疲劳性能研究较少。为此,以Q345B钢材制作了原状处理的侧面和正面角焊缝连接的两组板材试样,采用高频疲劳试验机在0℃、-20℃、-40℃下进行低温疲劳试验,并进行数据拟合。试验结果发现：对于侧面角焊缝试样而言,低温会提高构造细节的疲劳寿命,而低温对正面角焊缝试样的影响并不明显。低温下的正面角焊缝抗疲劳强度高于侧面角焊缝。正面角焊缝疲劳破坏形式为贯通焊缝裂纹,侧面角焊缝为焊趾向热影响区延伸裂纹。研究结果表明：低温对于不同的构造细节形式的节点疲劳寿命的影响没有统一的结论,有待更多试验进行研究并分析。     

NUCLEATION OF INTERGRANULAR CRACKS DURING HIGH TEMPERATURE LOW CYCLE FATIGUE OF SUS316 STAINLESS STEEL

Fundamental behaviour of intergranular cracking of SUS316 stainless steel which characterises the high temperature low cycle fatigue process has been studied with a special emphasis on the interaction between oxidation and a grain boundary sliding. Two types of specimens were prepared for fatigue experiment to extract a sole effect of surface oxidation on crack nucleation. One was heat-treated in air and the other was in vacuum so that the specimens had the same history of heat treatment except oxidation to the surface. Results of fatigue tests of these specimens well explain the relationship between oxidation and surface cracking as follows. The morphology of the oxidised surface of the specimen subjected to low-cycle fatigue at 700°C is quite different from that of the oxidised surface caused by simply holding at the same temperature in air for several hours with no applied stress. Localised oxidation along the grain boundary is a characteristic feature for the specimen fatigued at 700°C, while no localised oxidation was observed when the specimens were simply held at the same 700°C, i.e. with no fatigue loading. Accordingly, intergranular cracking in high temperature low cycle fatigue in air occurs when grain boundary sliding due to cyclic loading is accelerated by localised oxidation along the grain boundary.     

INFLUENCE OF SHOT-PEENING ON SHORT CRACK BEHAVIOUR IN A MEDIUM CARBON STEEL

The results of research into short fatigue crack behaviour in shot peened, medium carbon steel specimens under reversed torsion are presented. Characteristic features of short crack growth were established on the basis of optical and electron-optical observations of the replicas and micro-sections of the samples. Surface crack growth analysis, supported by plots of crack development, crack growth rate and crack length distributions against cycle ratio show that significantly slower crack growth and lower crack densities occur in shot peened specimens than in non-treated samples. That results can be linked to the effect of grain distortion, the packing of laminar grains, a high dislocation density in the plastically deformed surface layer and the introduction of compressive residual stresses. A fractographic analysis of the fracture surfaces has assisted an understanding of the mechanisms of fracture in shot peened specimens.     

循环荷载下奥氏体型和双相型不锈钢材料本构关系研究

为研究循环荷载下不锈钢材料的本构关系,对奥氏体型S30408不锈钢和双相型S220503不锈钢材料进行了单调拉伸和大应变超低周循环加载试验。采用三种常用的单调拉伸本构模型对所得应力-应变曲线进行拟合,得到相应单调荷载下材料本构参数;采用Ramberg-Osgood本构模型对循环骨架曲线进行拟合,得到材料循环强化参数;利用Chaboche塑性本构模型,标定了两种材料的循环本构参数。结果表明：在单调拉伸荷载下,G-R-O本构模型更适用于拟合不锈钢材料的单调拉伸本构;在循环荷载下,不锈钢材料滞回曲线饱满,且随着应变增大,两种材料在加载后期均表现出了明显的循环强化现象;Ramberg-Osgood本构模型对骨架曲线拟合较好,有限元计算结果和试验滞回曲线吻合度高;表明该文标定出的强化参数、循环本构参数可用于结构体系地震响应分析之中,为准确分析不锈钢结构在地震作用下的受力性能提供参考。     

盐对SUS321和SUS309不锈钢高温耐腐蚀性的影响

通过试验研究了盐对SUS321和SUS309不锈钢高温耐腐蚀性的影响.试验结果表明,盐的作用使SUS321和SUS309不锈钢的高温耐腐蚀性大大下降,使其可以承受的温度分别从原来的830℃和880℃都降低到了750℃以下.     

THE INFLUENCE OF AGEING AT INTERMEDIATE TEMPERATURES ON THE MECHANICAL BEHAVIOUR OF A DUPLEX STAINLESS STEEL: Part II. FATIGUE LIFE AND FATIGUE CRACK GROWTH

Abstract— The mechanical behaviour of AISI 329 steel has been investigated for ageing times up to 20,000 h at temperatures of 475, 425, 375, 325 and 275°C. The study has concentrated on the changes in the response to cyclic strains, in the low-and the high-cycle fatigue regimes, and in the resistance to fatigue crack propagation as a function of temperature and time of ageing.
It is shown that ageing increases the fatigue resistance in the high-cycle fatigue regime, but the opposite occurs in the low-cycle fatigue regime. Ageing increases the LEFM threshold stress intensity factor range for fatigue crack propagation which reaches high values in these alloys, and is influenced by the fatigue load ratio. Crack closure contributes to the LEFM threshold stress intensity factor range for crack propagation only in the annealed condition of the AISI 329 steel.     

不锈钢和碳钢在含酚酸性废水中耐蚀性

用恒电位极化曲线法和旋转挂片失重法研究了３１６Ｌ、３０４不锈钢和２０号碳钢在３０℃，ｐＨ＝２．１０的含酚酸性废水中的耐蚀性。结果表明３１６Ｌ和３０４不锈钢具有优良的耐蚀性，而２０号碳钢不耐蚀。含酚酸性废水中所含甲醛对３１６Ｌ和３０４不锈钢有缓蚀作用，与相同ｐＨ的盐酸溶液中腐蚀速度相比，其缓蚀率为７０％。     

铝热-离心法制备的不锈钢内衬复合钢管不锈钢层裂纹成因分析

研究了铝热-离心法制备的不锈钢内衬复合钢管中的残余热应力及不锈钢层的组织结构,分析了复合钢管不锈钢层中裂纹产生的原因。研究表明,不锈钢内衬层中的裂纹是钢中的夹杂及析出物使钢变脆以及冷却过程中热应力共同作用的结果。     

316L-16MnR复合板不锈钢侧晶间腐蚀原因分析

采用GB／T4334．5—2000标准方法对不锈钢-低合金钢复合板进行晶间腐蚀试验,对不锈钢侧产生裂纹的原因进行了分析。结果表明,由于不锈钢板与低合金钢板的结合面上存在着不可抗拒的碳迁移现象,致使不锈钢板侧过渡区内的奥氏体晶界上聚集着大量的网状碳化物,从而造成晶界上的缺陷,使其在经过硫酸．硫酸铜溶液的腐蚀后,进行弯曲时产生了大量的晶间腐蚀裂纹。