Enhancement of the durability of structural steels and their welded joints by plastic deformation

Conclusions The fatigue and corrosion-fatigue resistance of low-alloy steels increases as degree of plastic prestraining increases to 29%. The resistance of the 08kp steel prestrained and hardened by surface plastic deformation and its welded joints to fatigue and corrosion-fatigue fracture is higher than for the 08GSYuT alloyed steel. The procedures of pneumatic shot blasting and shot blasting prove to be efficient methods for increasing the cyclic and low-cycle durability of welded joints both in air and corrosive media. The corrosive medium considerably decreases the low-cycle durability of welded joints of sheet steels. The effect of the medium is more pronounced for alloyed steels and becomes weaker as the amplitude of strains increases. Translated from Fizyko-Khimichna Mekhanika Materially, Vol. 36, No. 3, pp. 121–122. May-June, 2000.     

Possibility of Inducing Compressive Residual Stresses in Welded Joints of SS400 Steels

Since the welded constructions produce easily stress corrosion cracking (SCC) or fatigue disruption in corrosive medium or under ripple load, two methods inducing compressive stress on structural surfaces by anti-welding-heating treatment (AWHT) and explosion treatment (ET) are presented. The results show that they are good ways to resisting SCC on the welded SS400 steel or other components.     

Einfluß korrosiver Umgebung auf die Lebensdauer zyklisch beanspruchter Bauteile

Influence of a corrosive environment on the fatigue life of cyclic loaded structures Fatigue life of structural components in practice is effected by corrosive environments, too. The failure mainly is caused by the interaction of cyclic straining and corrosive attack. Extensive research and test programmes during the last two decades had been carried out to investigate the parameters influencing corrosion fatigue. Results of the most effective parameters on corrosion fatigue are given in the present paper. The amount of degrading effect on fatigue properties depends on the susceptibility of the material to corrosive environment, the structural shape and the environment itself. The various parameters determing fatigue behaviour are superimposed in a synergistic manner, so their effects cannot simply be added. That may be the reason for very different conclusions on the effect of corrosion on fatigue life from ?disastrous”? to ?neglegible”?. Though there is still a number of unclear points, the good agreement of results of welded specimens (V-shaped specimens) and welded tubular joints indicates a positive outlook for the application of small specimen corrosion fatigue data for the design of structural components.     

Utilization of white,etch-resistant surface films for improving the resistance of steel to corrosion fatigue

Summary The presence of continuous white surface layers on quench-hardened steel test pieces increases their endurance limit both in air and, particularly (by a factor of 10), in a corrosive medium. This high resistance to corrosion fatigue affords new possibilities of using ordinary, medium-carbon or low-alloyed, quench-hardened steels with white surface films in many applications in which machine parts are subjected to the simultaneous action of cyclic stresses and corrosive media.     

Corrosion-fatigue resistance of steel in cyclic bending and torsion

Summary As a result of the action of corrosive media the ratio of the fatigue limit in torsion to that in bending (in the long endurance range) is increased to k=1, as compared with k=0.5–0.6 recorded in air. In other words, the torsional fatigue strength of steel in corrosive media is reduced much less than the fatigue strength in bending.In the low endurance range, a liquid corrosive medium may increase the fatigue strength as a result of its cooling action. Tests in corrosive and neutral media supported the view that the fatigue strength at high overloads depends to a large extent on the thermal effect associated with the internal friction of the specimen material.     

Fatigue strength of specimens with shrunk-on bushings

It was shown that the effect of shrinking-on operations on the fatigue strength of steel is observed in both neutral and active media (argon and air, respectively), the effect being more pronounced in air. Steel specimens with shrunk-on bushings were fatigue tested in rotating bending in air and in a corrosive medium. It was established that the shrinking-on operation produces more than a twofold reduction in the endurance of specimens fatigued in air and a considerably smaller reduction in the endurance of specimens tested in a corrosive medium. The bushing material (steel, brass) has practically no effect on the endurance of steel specimens in air or in a corrosive medium.     

Fatigue reliability study on T‐welded component considering load shedding

In this paper, a coupled reliability method for structural fatigue evaluation considering load shedding is first proposed based on probabilistic fracture mechanics in which the uncertainties of the structural parameters are taken into account. Then, the method is applied to predict the fatigue reliability of the T‐welded structure to the case of considering load shedding or not. The compared results show that by considering the load shedding, the structural fatigue reliability might be improved with less conservativeness. The influence rules of the load‐shedding coefficient on the fatigue failure probability of the T‐welded component are investigated, and some interesting results are obtained. That is, the influences of load‐shedding coefficient on the fatigue failure probability can be divided into three regions, namely the high, medium and low fatigue failure areas. The last area is the most intriguing when we try to design a T‐welded structure. The thickness of T‐welded structure along the crack propagation direction is found to be one of the important design variables for the design of fatigue reliability, in which the low‐fatigue failure zone is used as one of the reliability constraints. The basic design frame of T‐welded structure is established to constrain the fatigue failure probability within the low‐fatigue failure area.     

Untersuchungen zur Schwingungsrißkorrosion von unlegierten Stählen in wäßrigen Medien

Although a lot of work is done up to now in investigating corrosion fatigue many questions about the corrosion fatigue process of steel are still open. This applies for instance to the quantitative proportion at the complete damaging process which the corrosion strain component will assume and it applies moreover to special details of the crack initiation stage. The damaging process of corrosion fatigue of steel in the active state is generally supposed to proceed in the following three stages: “formation of pits”, “crack initiation” and “crack propagation”. The objective of the present study was at first to get by statistical means a quantitative assumption about the influence of the corrosive medium and secondly to get information about the contribution of pitting to the crack initiation process. Tension-compression experiments were conducted with specimen from steels Ck 15 and Ck 35 in the normalized state both without a corrosive influence (inert spindle oil) and with corrosive influence (desalted water, 3% NaCl-solution). It was found that the damaging process in its principal course is not affected by the kind of the corrosive medium but is hardly influenced by the extent of the load-amplitude at a given load-frequency.     

Influence of surface defects and corrosive medium on the fatigue resistance of ST17G1S steel

We study the influence of surface defects and corrosive medium on the characteristics of fatigue resistance of specimens of ST17G1S steel extensively used for the production of pipes in the oil-and-gas industry. The accumulated results enable us to estimate the degree of decrease in the limited fatigue strength for specimens of ST17G1S steel in the presence of surface defects and under the action of corrosive medium. __________ Translated from Problemy Prochnosti, No. 4, pp. 99–107, July–August, 2007.     

Multiaxial fatigue life estimation in welded joints using the critical plane approach

Many engineering structures experience multiaxial fatigue states of stress–strain in the vicinity of welded joints. Fatigue assessment of welded joints under proportional (in-phase) cyclic loading can be performed by using conventional hypotheses (e.g., see the von Mises criterion or the Tresca criterion) on the basis of local approaches. On the contrary, the fatigue life predictions of welded joints under non-proportional (out-of-phase) cyclic loading are generally poor if these conventional hypotheses are used. In the present paper, the critical plane-based multiaxial fatigue criterion proposed by Carpinteri and Spagnoli for smooth and notched structural components is extended to the fatigue assessment of welded joints under in- and out-of-phase loadings. The applicability of this criterion, expressed in terms of nominal stresses, to the fatigue life prediction of welded specimens is investigated by using experimental data available in the literature.     

Small fatigue crack initiation and growth behavior of high-strength low alloy steel in various environments

Fatigue tests under rotating bending and reversed torsion were carried out in air, distilled water and 3% saltwater, using smooth specimens of high-strength low alloy steel (Cr-Mo steel). The initiation and growth behavior of small fatigue cracks in each environment were evaluated based on detailed observations, and the effects of corrosive environment were also discussed. The fatigue strength decreased with increasing aggressiveness of test environment. The decreases in corrosive environment were due to earlier fatigue crack initiation. From the observed locations at which small fatigue cracks began, it was considered that the crack initiation was primarily governed by hydrogen embrittlement in distilled water and also affected by corrosive dissolution in 3% saltwater. The validity of the application of linear fracture mechanics for small fatigue cracks was established. The growth rates of small fatigue cracks were higher than for large through cracks, and not accelerated by the corrosive environment. Moreover, fatigue life in the corrosive environment was estimated by using the crack growth characteristics in air.     

Effect of stress-raiser sharpness on corrosion-fatigue of steel specimens

The effect of notch sharpness and corrosive media on the fatigue of 5-mm-diameter steel specimens was studied. It was established that the influence of stress raisers in a corrosive medium is weaker than in atmospheric air, though this difference may disappear in the case of specimens with very sharp notches.     

S‐N curve for riveted details in corrosive environment and its application to a bridge

A formula for stress‐life curve is proposed to predict the fatigue life of riveted bridges located in corrosive environments. The corrosive environment‐dependent parameters of the S‐N curve are determined based on the corrosion fatigue testing results of different types of steel specimens in air, fresh water, and seawater. Eurocode detail category 71 and UK WI‐rivet detail category represent the fatigue strength of riveted members. The proposed S‐N curve formula is compared with full‐scale fatigue test results of riveted joints, plate girders, and truss girders, which were tested in a corrosive environment. Thus, the validity of the formula is confirmed. The formula does not require any material parameter other than the code‐given fatigue curve of riveted details. The fatigue life of a riveted railway bridge is estimated by using the proposed formula, and the results are compared with conventional approaches. The applicability and significance of the proposed curve are confirmed.     

超声冲击法提高焊接接头疲劳特性研究进展

焊接结构的失效以疲劳断裂为主,且焊接结构强度主要是由焊接接头的疲劳强度决定的。因此,改善焊接接头疲劳性能将显著提高焊接结构的整体性能。超声冲击处理是一种有效改善焊接接头疲劳性能的表面强化技术。研究表明,该技术通过改善焊接接头几何外形,细化表层晶粒及引入有益残余压应力可大幅度提高焊接接头的疲劳强度和疲劳寿命。综述了超声冲击处理对焊接接头疲劳性能影响的研究现状,分析了影响焊接接头疲劳性能的因素,总结了超声冲击改善焊接接头疲劳性能的结果,对目前研究过程中存在的问题进行探讨,最后展望了超声冲击表面纳米化技术的应用前景。     

腐蚀环境下2024-T3铝合金疲劳裂纹扩展和剩余强度实验研究

对2024-T3铝合金在5种典型实验室环境和3种组合环境下的疲劳裂纹扩展和剩余强度进行了实验研究.通过实验获得的裂纹扩展数据,对Paris公式进行条件拟合,得到各种环境下的裂纹扩展常数,并作了对比分析.结果表明,腐蚀环境的参与使2024-T3铝合金的疲劳裂纹扩展速率明显加快,不同腐蚀环境对疲劳裂纹扩展速率的影响程度不同,其影响的严重程度由重到轻依次为:油箱结构区、厨房与厕所、油箱积存水、盐水、潮湿空气、高空环境、干燥空气.实验数据还进一步表明,腐蚀介质对临界裂纹长度的影响很小,说明环境对剩余强度能力无直接影响.     

Fatigue behavior of friction plug welds in 2195 Al–Li alloy

The focus of this paper will be on the fatigue behavior of friction stir welded 2195 Al–Li plates that contain friction plug welds. Tensile tests were performed for specimens containing base metal, friction stir welded 2195-T8, and friction stir welded 2195-T8 containing a friction plug weld consisting of a 2195-T8 plug. The ultimate strength was determined for base metal, friction stir welded material, and friction plug welded material. Fatigue properties were determined for both the friction stir weld and friction plug welded specimens in the medium to high cycle regimes. Comparison of the results show that the friction plug weld reduced both the UTS and fatigue life as compared to specimens containing only friction stir weld. The reduction in fatigue life is most likely due to the complication of weld geometry, interacting heat affected zones, and strength mismatch between base metal, friction stir weld, and plug material.     

A study of fatigue crack growth from artificial corrosion pits at welded joints under complex stress fields

The paper studies the effects of artificial corrosion pits and complex stress fields on the fatigue crack growth of full penetration load‐carrying fillet cruciform welded joints with 45° inclined angle. Parameters of fatigue crack growth rate of welded joints are obtained from S‐N curves under different levels of corrosion. A numerical method is used to simulate fatigue crack growth using different mixed mode fatigue crack growth criteria. Using polynomial regression, the crack shape correction factor of welded joints is fitted as a function of crack depth ratios. Because the maximum circumferential stress criterion is simple and easy to use in practice, fatigue crack growth rate is modified using this criterion. The relationship of effective stress intensity factor, crack growth angle and crack depth is studied under different corrosion levels. The simulated crack growth path obtained from the numerical method is compared with the actual crack growth path observed by fatigue tests. The results show that fatigue cracks do not initiate at the edge or bottom of pits but at the weld toes where the maximum stress occurs. The artificial corrosion pits have little effect on the effective stress intensity factor ranges and crack growth angle. The fatigue crack growth rates of welded joints with pits 1 and 2 are 1.15 times and 1.40 times larger than that of the welded joint with no pit, respectively. The simulated crack growth path agrees well with the actual one. The fatigue life prediction accuracy using the modified formulation is improved by about 18%. The crack shape correction factor obtained using the maximum circumferential stress criterion is recommended being used to calculate fatigue life.     

Combined effect of strength & sheet thickness on fatigue behaviour of resistance spot welded joint

Fatigue performance of spot welded lap shear joint is primarily dependent on weld nugget size, sheet thickness and corresponding joint stiffness. Two automotive steel sheets having higher strength lower thickness and lower strength higher thickness are resistance spot welded with established optimum welding condition. The tensile‐shear strength and fatigue strength of lap shear joint of the two automotive steel sheets are determined and compared. Experimental fatigue life of spot welded lap shear joint of each steel are compared with predicted fatigue lives using different stress intensity factor solutions for kinked crack and spot weld available in literature. Micrographs of fatigue fractured surfaces are examined to understand fracture micro‐mechanisms.     

Multi‐scale experimental study on fatigue damage behaviour and its effect on structural nonlinear response

Experimental analyses on the structural response caused by local fatigue damage accumulation in welded details are accomplished to perform failure process and nonlinear effect analysis at different structural levels. The experiment is carried out by using welded compact tension (CT) specimens and a scaled truss specimen, and all of them have a notch at the weld toe to facilitate damage initiation. Cyclic loads are applied to those specimens to generate accumulative fatigue damage, respectively. The process of fatigue accumulation including initiation and propagation of fatigue cracks in the welded detail and resultant structural responses of CT specimens and the truss are measured with integration of multiple testing techniques. Multi‐scale experimental results show that microscopic‐/mesoscopic‐concentrated strain and extension of plastic zone in the vicinity of notch tip are both affected significantly by the fatigue damage accumulation and present appreciable nonlinear behaviour; however, the macroscopic response such as the frequency and stiffness parameters of the welded truss specimen are less sensitive to the low‐level fatigue damage. It is concluded that the fatigue failure of the welded truss is a multi‐scale progressive process due to fatigue damage trans‐scale evolving, in which the local meso‐damage firstly affects local strain of plastic zone in the vicinity of the notch tip, and then fatigue damage evolving from meso‐ to macro‐scale affects nonlinear responses of the damaged components; lastly, the fatigue failure could be expected as the results of the propagation of macroscopic fatigue cracks.