Fatigue crack growth of titanium alloy joints by electron beam welding

Electron beam welding（EBW） has been widely used in the manufacture of titanium alloy welded blisk for aircraft engines. Based on fatigue crack growth tests on titanium alloy electron beam welding（EBW） joints, mechanism of fracture was investigated under scanning electron microscope（SEM）. The results show that fatigue crack growth rate increases as the experimental load increases under the same stress ratio and stress intensity factor range. At the beginning of crack growth, the extension mechanism of fatigue crack is the typical mechanism of cleavage fracture. In the steady extention stage, crack extends along the weld seam firstly.Then, crack growth direction changes to extend along the base metal. The extension mechanism of fatigue crack in the weld seam is the main mechanism of cleavage fracture and the extension mechanism of fatigue crack in the base metal is the main extension mechanism of fatigue band. In the instantaneous fracture stage, the extension mechanism of fatigue crack is the typical dimple-type static fracture mechanism.Crack growth was simulated by conventional finite element method and extended finite element method.     

Fatigue crack growth behavior of heat affected zone in P92 steel weldment

In this study, the effect of microstructure on fatigue crack growth behavior of HAZ was investigated using newly developed P92 steel weldment. Fatigue crack growth rates of the fusion line and fusion line+2 mm in HAZ were found to be faster than that of welds or base metal, while fatigue crack growth rate of the fusion line+1 mm was the slowest. The fracture appearance of the fusion line and fusion line+2 mm revealed mainly transgranular, cleavage-like faceted fracture surfaces and the fusion line+1 mm showed quasi-cleavage fractures with scattered dimples, which increased resistance to fatigue crack growth.     

A7075搅拌摩擦焊疲劳裂纹扩展速率试验分析

对航空用铝合金A7075-T651的搅拌摩擦焊接头的母材、焊核区和热影响区的疲劳裂纹扩展速率（da／dN）进行了试验研究。疲劳裂纹扩展试验采用CT（紧凑拉伸）试样,在810 Material Test System试验机上进行。用递增多项式法求得疲劳裂纹扩展速率（da／dN）和（△K）,并分别将母材、焊核区和热影响区同一组试样的（da／dN）和（△K）数据点合在一起进行了整体回归分析。试验中得到了以Paris公式表达的铝合金A7075-T651母材、焊核区、热影响区da／dN与△K的关系式。结果表明,热影响区疲劳裂纹扩展速率高于母材和焊核区,是该合金搅拌摩擦焊接头最薄弱的区域。     

Fatigue crack growth behavior of laser-processed 304 stainless steel in air and gaseous hydrogen

Fatigue crack growth test was performed to evaluate fatigue behavior of 304 stainless steel specimens with or without laser processing (welding and surface treatment) in air and gaseous hydrogen. As the crack propagation normal to the laser welding or scan direction, the laser-processed specimens exhibited a higher resistance to crack growth in the low stress intensity factor range (ΔK) than the as-received steel plates regardless of testing environments. However, the marked retardation of crack growth behavior vanished for welded specimens subjected to a 850 °C/h stress relief treatment or with a shorter distance from notch tip to the weld centerline in the test.Fatigue-fractured appearance of the steel plate tested in air was composed of mainly transgranular fatigue fracture and some flat facets, along with a small amount of intergranular fracture. While quasi-cleavage fracture and few twin boundary separations were observed for the same specimen in hydrogen. On the other hand, the lower crack growth rate of laser-processed specimens in both air and hydrogen was accompanied with rubbed areas on the fracture surfaces. It was found that the extent of quasi-cleavage fracture was related to the formation of strain-induced martensite, which would contribute to an increased fatigue crack growth rate of all specimens in gaseous hydrogen.     

Effects of transformation temperature on variant pairing of bainitic ferrite in low carbon steel

The effects of transformation temperature on the variant pairing tendency of bainitic ferrite (BF) and the orientation relationship (OR) between BF and austenite matrix are investigated quantitatively in low-carbon, low-alloy steel by means of electron backscatter diffraction analysis. By numerical reconstruction of the austenite orientation it was found that BF holds a near Kurdjumov-Sachs OR with respect to the austenite matrix. The angular deviation of the close-packed planes (CPPs) becomes smaller with decreasing transformation temperature, while the small misorientation angle between the close-packed directions (CPDs) remains nearly the same. BF variants with small misorientations are formed side by side at high transformation temperatures by sharing the same Bain correspondence. In contrast, BF variants are formed adjacently at lower transformation temperatures by sharing the same parallel relations for CPP. In particular, pairing of twin-related variants is dominant in this case. Such a variant pairing tendency with respect to transformation temperature is discussed from the viewpoint of self-accommodation of transformation strain and inter-variant boundary energy.     

Determination of corrosion‐assisted stress crack growth rate in 304L stainless steel welds

The corrosion assisted stress crack growth rate of 304L stainless steel welds, was determined by the rising displacement test method. The experiments were carried out in a Boiling Water Reactor (BWR) environment, using a refreshed autoclave loop system at 288 °C and 8 MPa of pressure. Pre‐fatigued CT specimens were used and the crack length was monitored by the potential drop technique. After testing, fractographical analysis and measurement of the initial and final crack length were made. The displacement rates of 3.6 and 2.37 μm/h allowed the initiation and propagation of cracks by stress corrosion cracking, and the test results agree with the crack growth rate values determined with another method. The fracture surfaces are evidence of stress corrosion crack propagation with a transgranular appearance in some cases and transgranular with localized areas of intergranular propagation in others.     

AA6082铝合金搅拌摩擦焊接头疲劳扩展速率

研究了6082-T6铝合金搅拌摩擦焊接头不同位置的疲劳裂纹扩展速率,并分析了接头的组织及疲劳断口形貌。试验结果表明,疲劳裂纹扩展速率最快的区域为接头焊核细晶区;当裂纹在热影响区扩展时,在较小的应力强度因子范围(ΔK)条件下,裂纹的扩展速率低于其在母材中的扩展速率,伴随着ΔK的逐渐增加,裂纹的扩展速率明显加快并高于其在母材中的扩展速率。断口形貌表明,疲劳裂纹在焊核区扩展主要由脆性的准解理断裂形貌组成,扩展速率较快;而热影响区及母材区的断口形貌主要由光滑的疲劳条纹组成。     

16Cr3NiWMoVNbE齿轮钢的疲劳性能与裂纹萌生

采用真空感应熔炼+真空自耗重熔(VIM+VAR)工艺制备16Cr3NiWMoVNbE齿轮钢。测定了试验钢的疲劳极限和S-N曲线。通过观察断口,分析疲劳萌生类型和影响因素。结果表明:疲劳极限强度达到773 MPa,疲劳裂纹萌生于表面驻留滑移带、表面缺陷、近表面夹杂物和次表面夹杂物。表面驻留滑移带萌生疲劳裂纹占13%,表面缺陷萌生裂纹占33.3%,近表面夹杂物萌生占40%,次表面夹杂物萌生占13%。当疲劳裂纹萌生于内部夹杂物时,疲劳寿命随应力的增大而减小;在一定实际应力作用下,疲劳寿命随夹杂物尺寸的增大而减小。随着实际应力增加,疲劳裂纹萌生的夹杂物临界尺寸减小。     

Fatigue crack growth in the welding nugget of FSW joints of a 6060 aluminum alloy

Friction stir welded butt joints were performed on 8 mm thick sheets made of AA6060 T6 aluminum alloy by means of a CNC machine tool, at feed rates between 117 and 683 mm/min and tool rotational speed between 838 and 1262 rpm. Tensile tests, metallographic analyses and micro-Vickers tests were carried out to evaluate the mechanical properties of the joints as a function of the process parameters. The fatigue behavior was studied by means of crack growth tests performed according to ASTM E647 standard on CT specimens, with propagation in the middle of joint along the weld nugget. The results show the influence of welding process parameters on mechanical properties and fatigue behavior. Reduction of UTS of about 20–30% with respect to base material occurred with rupture in the softened zone of welding, usually HAZ. In this range, slight variations of joint efficiency were observed with f/S ratio, while the width of the softening area increases for decreasing values of this parameter. Fatigue crack growth was always slower than that in the base material at low ΔK below 12 MPa m^1/2. The effect of non-optimal welding parameters was evident at intermediate and high ΔK, due to defects, such as tunnels, that cause dramatic increase of propagation rate up to five times higher than the base material.     

Fatigue mechanics of WC–Co cemented carbides

The fracture and fatigue behavior of a fine-grained WC–10 wt% Co hardmetal is investigated. Mechanical characterization included flexural strength and fracture toughness as well as fatigue limit and fatigue crack growth (FCG) behavior under monotonic and cyclic loads, respectively. Considering that fatigue lifetime of cemented carbides is given by subcritical crack growth of preexisting defects, a linear elastic fracture mechanics (LEFM) approach is attempted to assess fatigue life–FCG relationships for these materials. Following the experimental finding of an extremely high dependence of FCG rates on the applied stress intensity for the hardmetal studied, the LEFM analysis is concentrated, from a practical design viewpoint, on addressing the fatigue limit–FCG threshold correlation under infinite fatigue life conditions. Thus, fatigue limit associated with natural flaws is estimated from FCG threshold experimentally determined for large cracks under the assumptions that (1) similitude on the FCG behavior of small and large cracks applies for cemented carbides, and (2) critical flaws are the same, in terms of nature, geometry and size, under monotonic and cyclic loading. The reliability of this fatigue mechanics approach is sustained through the excellent agreement observed between estimated and experimentally determined values for the fatigue limit under the different load ratios investigated.     

Fatigue crack propagation behavior in dual-phase steel

The fatigue crack propagation in dual-phase steel was studied with the objective of developing ferriticmartensitic microstructures via intercritical annealing and thermomechanical processing. It was found that the changes in fatigue crack propagation rates and in the threshold stress intensity range, ΔK _th, resulting from microstructural variations, were directly related to tensile strength in the same manner that was observed in other types of structural steels. It was also observed that the relationship between tensile strength and fatigue crack propagation in intercritically annealed and thermomechanically processed dual-phase steel was much the same as for conventional steels of similar strength level.     

Fatigue crack growth rate in underwater wet welds: out of water evaluation

The characteristics of fatigue crack propagation in welds produced in conventional conditions (welds made out of water) and in underwater repair welding conditions (underwater wet welding) were evaluated out of water. The fatigue crack growth rates da/dN showed considerable dependence on pore density and distribution, factors that vary significantly with the welding process and the environment. Variations in the fatigue crack propagation rate were correlated with analysis of the fracture surface in the zone of stable crack propagation. The results of this study show that the underwater wet welding procedure produces weld metal resistant to fatigue, which is suitable for use at low applied stresses in offshore structures.     

Stress-induced martensitic transformation in metastable austenitic stainless steels: Effect on fatigue crack growth rate

This paper addresses the influence of cyclic stress-induced martensitic transformation on fatigue crack growth rates in metastable austenitic stainless steels. At low applied stress and mean stress values in AISI type 301 stainless steel, fatigue crack growth rate is substantially retarded due to a cyclic stress-induced γ-α′ and γ-ε martensitic transformation occurring at the crack-tip plastic zone. It is suggested that the transformation products produce a compressive residual stress at the tip of the fatigue crack, which essentially lowers the effective stress intensity and hence retards the fatigue crack growth rate. At high applied stress or mean stress values, fatigue crack growth rates in AISI type 301 steels become almost equal to those of stable AISI type 302 alloy. As the amount of transformed products increases (with an increase in applied or mean stress), the strain-hardening effect brought about by the transformed martensite phase appears to accelerate fatigue crack growth, offsetting the contribution from the compressive residual stress produced by the positive volume change of γ → α′ or ε transformation.     

激光表面熔凝处理过共晶Al-Si合金的疲劳裂纹扩展行为

试验研究了Al-20%Si合金在激光表面熔凝处理后的显微组织特征及激光表面熔凝处理对合金疲劳裂纹扩展形为的影响。结果表明,激光表面熔凝处理使试验合金的显微组织得到了明显细化,熔凝层主要由细小的初生Si、-αAl及Al-Si共晶相组成,熔凝层的硬度显著提高;在应力比R为0.1和0.5的试验条件下,激光表面熔凝处理显著提高了试验合金的疲劳裂纹扩展抗力。     

Cold hole expansion effect on the fatigue crack growth in welds of a 6061-T6 aluminum alloy

Compact test specimens were extracted from a 6061-T6 aluminum alloy welded plate with a thickness of 9 mm to analyze the cold hole expansion effect on fatigue crack growth tests conducted in mode I cyclic loading. At R = 0.1, a sharp crack in base metal, weld metal and heat affected zone was propagated from 17 to 24 mm. The fatigue crack growth at 24 mm (α = a/W = 0.3) was delayed by drilling a hole at the crack tip and applying a cold hole expansion of 4.1%. The residual stress fields due to cold hole expansion were determined with the finite element method. The fatigue crack growth testing was continued up to a crack length of 35 mm (α ∼ 0.43) at the same R, and crack opening displacements of the post-expansion crack were also determined with the finite element method. The results were expressed in terms of crack length versus number of cycles, as well as, fatigue crack growth rate as a function of applied and effective stress intensity factor range. The cold hole expansion contributed to delay the fatigue crack growth in base metal, and to a lesser extent in the weld metal and heat affected zone. A crack closure effect was determined by means of load versus crack opening displacement curves of the post-expansion crack, which was, completely or partially closed, in welded zones with compressive residual stress fields. The fracture surfaces of each welded zone were analyzed to elucidate the crack nucleation zone and its relation with the residual stress field. In all cases the crack was initiated at the surface of the specimen where the residual stresses were positive.     

Effect of electron beam surface hardening on fatigue crack growth rate in AISI 4340 steel

This paper studies the effect of electron beam (EB) surface hardening on the fatigue crack growth rate in AISI 4340 steel. The heat treatment conditions were varied to consider the influence of microstructure and residual stress. The results show that increasing the EB heat input increases the compressive residual stress in the hardened layer. Thus EB surface-hardening treatment improves the fatigue crack growth resistance. This effect increases with increasing EB heat input but disappears as the ΔK value increases. The fracture mechanism of the hardened layer is intergranular fracture, while that of the base material is transgranular quasi-cleavage.     

不同热处理条件下贝氏体钢的微观组织和疲劳裂纹扩展

以贝氏体钢为研究对象,设计了4种热处理工艺,研究了不同热处理工艺下试验钢的显微组织及疲劳裂纹扩展速率。结果表明,热轧态试验钢的微观组织以粒状贝氏体为主,其上分布有少量的板条贝氏体、马氏体和粗大块状M/A岛,残留奥氏体的体积分数为16.2%,但稳定性较差,裂纹能够直接穿过粗大的块状M/A岛继续扩展,疲劳裂纹扩展速率最快。经900 ℃奥氏体化+空冷后,显微组织以板条贝氏体和马氏体为主,M/A岛仍为粗大的块状,残留奥氏体含量减少至12.3%,疲劳裂纹扩展速率略有降低。经900 ℃奥氏体化+380 ℃盐浴等温30 min +空冷后,显微组织以细密、有序的板条贝氏体为主,残留奥氏体含量减少至10.2%,以薄膜状伴生在板条贝氏体间,板条状贝氏体及板条间的残留奥氏体薄膜会使裂纹端钝化、分叉、偏折,阻碍裂纹扩展的能力增强;经350 ℃回火240 min后,显微组织以马氏体和板条贝氏体为主,残留奥氏体含量比热轧态略微降低,为14.9%;而经450 ℃回火240 min后,显微组织以板条状贝氏体为主,其上分布有少量的马氏体,残留奥氏体体积分数减少至8.6%,也以薄膜状伴生在贝氏体板条间,同时有大量的碳化物析出,裂纹扩展速率最慢。     

Fatigue properties of welded joints using steel with high resistance to fatigue crack growth

It has been generally recognized that the fatigue life of welded joints is little influenced by the strength of steels owing to the high-stress concentration and the tensile residual stress near the weld toe. In this paper, improvement of the fatigue life of welded joints using steel with high resistance to fatigue crack growth (ferrite/martensite (F/M) steel) is investigated. F/M steel has a microstructure with an elongated and banded martensite phase distributed in a ferrite matrix and a fatigue crack growth rate of about one-half to one-tenth in the thickness direction, compared with conventional steel. As a result, the fatigue life of an out-of-surface gusset-welded joint increases with the decrease of the fatigue crack growth rate. The fatigue life of welded joints using F/M steel with the highest resistance to fatigue crack growth increases to about twice that of joints using conventional steel. Whereas the fatigue crack growth rate decreases significantly, the fatigue life of welded joints increases only slightly. This can be attributed to the stress ratio independent of the fatigue crack growth rate. In other words, the fatigue crack growth rate of F/M steel increases with the increase of the stress ratio, approaching that of conventional steel. In the case of welded joints, even if a fatigue test is carried out at a low-stress ratio, the region near the weld toe is under a high-stress ratio due to tensile residual stress. Therefore, improvement of the fatigue life of welded joints becomes comparatively small so that the effect of fatigue crack retardation of F/M steel decreases.     

搅拌摩擦焊AA7075-T651铝合金接头的疲劳裂纹扩展(英文)

研究12 mm厚AA7075-T651铝合金板搅拌摩擦焊接头的疲劳裂纹扩展行为。从搅拌摩擦焊接头以及母材中截取试样,对试样进行疲劳裂纹扩展实验。对搅拌摩擦焊接头以及母材的横向拉伸性能进行评估。用光学显微镜和透射电镜分析焊接接头的显微组织。用扫描电镜观察试样的断裂表面。与母材相比,焊接接头的ΔKcr降低了10×10-3 MPa·m1/2。搅拌摩擦焊AA7075-T651接头的疲劳寿命明显低于母材的,其原因可归结于焊缝区的析出相在搅拌摩擦焊接过程中的溶解。