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.     

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.     

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.     

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％。     

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。     

STATISTICAL INVESTIGATION OF THE EFFECT OF LABORATORY AIR ON THE FATIGUE BEHAVIOUR OF A CARBON STEEL

In order to clarify the effect of the atmospheric conditions on fatigue damage, rotary bending fatigue tests were carried out on smooth specimens of a normalized 0.37% carbon steel in controlled laboratory air. The air conditions used in the tests were moist air at 20 °C, moist air at 35 °C and dry air at 35 °C. The influence of atmosphere on crack initiation and propagation behaviour was investigated in detail based on successive observations of the surface. Experimental results showed that the fatigue life was superior at 20 °C compared to 35 °C by a factor of 2, while the effect of moisture was small compared to that of temperature. The statistical investigation of crack initiation and propagation behaviour indicated that the temperature strongly affects the crack initiation process; conversely, moisture plays an important role in the propagation process of cracks smaller than 0.3  mm. Moreover, the distribution characteristics of crack initiation life, crack propagation life, fatigue life and crack growth rate were analysed by assuming either a Weibull distribution or a log-normal distribution.     

不锈钢表面粗糙度对超高分子量聚乙烯摩擦磨损性能的影响

以超高分子量聚乙烯软骨材料为销样,316不锈钢硬骨材料为盘样,在自制的销－盘式磨损试验机上考察了不锈钢盘样表面粗糙度对超高分子量聚乙烯摩擦磨损性能的影响,并利用光学显微镜观察了摩擦副表面的形貌,结果表明,在干摩擦条件下,表面粗糙度对超高分子量聚乙烯的摩擦磨损有较大影响,存在着适合的表面粗糙度范围,使超高分子量聚乙烯摩擦系数,磨损率最小。     

EFFECT OF BIAXIAL MEAN STRESS ON CYCLIC STRESS-STRAIN RESPONSE AND BEHAVIOUR OF SHORT FATIGUE CRACKS IN A HIGH STRENGTH SPRING STEEL

Abstract— Biaxial fatigue tests were conducted on a high strength spring steel using hour-glass shaped smooth specimens. Four types of loading system were employed, i.e. (a) fully reversed cyclic torsion, (b) uniaxial push—pull, (c) fully reversed torsion with a superimposed axial static tension or compression stress, and (d) uniaxial push—pull with a superimposed static torque, to evaluate the effects of mean stress on the cyclic stress—strain response and short fatigue crack growth behaviour. Experimental results indicate that a biaxial mean stress has no apparent influence on the stress—strain response in torsion, however a superimposed tensile mean stress was detrimental to torsional fatigue strength. Similarly a superimposed static shear stress reduced the push—pull fatigue lifetime. A compressive mean stress was seen to be beneficial to torsion fatigue life. The role of mean stress on fatigue lifetime, under mixed mode loading, was investigated through experimental observations and theoretical analyses of short crack initiation and propagation. Using a plastic replication technique the effects of biaxial mean stress on both Stage I (mode II) and Stage II (mode I) short cracks were evaluated and analysed in detail. A two stage biaxial short fatigue crack growth model incorporating the influence of mean stress was subsequently developed and applied to correlate data of crack growth rate and fatigue life.