Fatigue and hardness effects of a thin buffer layer on the heat affected zone of a weld repaired Bisplate80

Heat-affected zones (HAZs) in extensively weld-repaired Bisplate80, or the parent metal (PM), have been studied and examined using hardness measurements in conjunction with fatigue crack growth measurements and detailed scanning electron microscopy (SEM) observations. Three different groups of specimens have been prepared, i.e. as-received PM, weld-repaired PM with a thin 4 mm BL between PM and weld metal (WM), and weld-repaired PM without BL. The extended compact tension (E-CT) specimens for fatigue measurements are prepared according to the ASTM specifications, which are also used for hardness measurements. Hardness and fatigue crack growth variations across the boundaries between WM, BL and PM have been measured together with detailed SEM observations. It has been found that the welding process without BL reduced both hardness and fatigue resistance, especially around the weld interface and within the HAZ. Incorporation of a thin soft BL between WM and PM has increased both hardness and fatigue resistance around the weld interface and within the HAZ.     

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.     

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.     

Fatigue and corrosion fatigue behavior of an AA6063-T6 aluminum alloy coated with a WC-10Co-4Cr alloy deposited by HVOF thermal spraying

The present investigation has been conducted in order to study the fatigue and corrosion fatigue behavior of an AA6063-T6 aluminum alloy substrate coated with a WC-10Co-4Cr deposited by HVOF thermal spraying. It has been determined that the deposition of such a coating on the aluminum substrate gives rise to significant gains in fatigue life in comparison with the uncoated substrate, when testing is carried out both in air and in a 3 wt.% NaCl solution. It has been shown that during testing in air, the fatigue gain ranges between ~ 540 and 4300%, depending on the maximum alternating stress applied to the material. Larger fatigue gains are associated with low alternating stresses. Also, when fatigue testing is conducted in the NaCl solution, the gain in fatigue resistance varies between ~ 620 and 1460%. Fatigue cracks have been observed to initiate at the coating surface and then grow towards the substrate after propagating through the entire coating thickness. Crack growth along the coating has been observed to occur mainly along the regions formed by the agglomeration of W and W-Co-Cr-rich particles, flanking the tougher Co-Cr-rich areas. Although in the present work residual stresses were not measured, it is believed that the gain in fatigue life of the coating-substrate system is due to the presence of compressive residual stresses within the coating which hinder fatigue crack propagation. The deposition of the coating does not give rise to significant changes in the static mechanical properties and hardness of the aluminum alloy substrate. It has been observed that the WC-10Co-4Cr coating displays a significant indentation size effect and has a mean hardness of ~ 9.4 GPa.     

Fatigue Crack Growth of Peened Friction Stir-Welded 7075 Aluminum Alloy under Different Load Ratios

The effect of peening on the fatigue crack growth performance of friction stir-welded 7075 aluminum alloy was investigated. The fatigue crack growth rates were assessed for laser- and shot-peening conditions at stress ratios (R) of 0.1 and 0.7. The surface and through thickness residual stress distributions were characterized for the different regions in the weld. The results indicate a significant reduction in fatigue crack growth rates using laser peening compared to shot peening and the as-welded condition. The effect of the compressive stresses obtained through laser peening was deemed responsible for increasing the resistance to fatigue crack growth of the welds.     

Fatigue crack growth behavior of laser-annealed IN 718 alloy in hydrogen

The effect of hydrogen embrittlement on the fatigue crack growth of IN 718 plate and laser-annealed specimens in hydrogen containing environment were investigated. Although the differences in tensile strength and impact toughness between solution-annealed (S) and aged (A) IN 718 specimens were significant, the experimental results indicated that both specimens within the low ΔK regime exhibited a similar fatigue behavior. As the ΔK increased above 30 MPa , the solution-annealed specimen revealed a higher fatigue crack growth rate (FCGR) than the aged one. In general, the IN 718 alloy had a low sensitivity to hydrogen-enhanced fatigue crack growth, independent of hydrogen sources. Residual compressive stresses ahead of the crack tip were responsible for the improved resistance to fatigue crack growth in a laser-annealed specimen. For alloys with similar strength, IN 718 alloy trapped a huge amount of hydrogen in the matrix showing a less susceptibility to hydrogen-enhanced fatigue crack growth in comparison with the maraging steel. Additionally, fatigue-fractured appearance near crack initiation sites reveals quasi-cleavage fracture in embrittled specimens.     

Effect of buffer layer and notch location on fatigue behavior in welded high-strength low-alloy

A soft buffer layer (BL) between the weld metal (WM) and the parent metal (PM) was introduced in this study. To study the effect of a BL on mechanical properties, two types of joints made from welded high-strength low-alloy (HSLA) specimens with or without a BL were prepared. To investigate the influence of the notch location, the welded HSLA specimens machined with a U-notch in the WM or the PM. These conditions were examined to determine the effects of the BL and the notch location on the fatigue characterizations and the microscopic features of the fatigue-fractured surface. Extended-compact tension (E-CT) specimens were used for all tests. Paris fatigue curves and hardness distributions of the welded HSLA specimens with or without a BL were measured, and relevant scanning electron microscope (SEM) images were taken. The results showed that the presence of a BL significantly improved the fatigue crack growth behavior when a U-notch was in the WM. The notch location greatly influenced the fatigue behavior and the surface features of the fatigue-fractured specimens.     

The evolution of crack-tip stresses during a fatigue overload event

The mechanisms responsible for the transient retardation or acceleration of fatigue crack growth subsequent to overloading are a matter of intense debate. Plasticity-induced closure and residual stresses have often been invoked to explain these phenomena, but closure mechanisms are disputed, especially under conditions approximating to generalised plane strain. In this paper we exploit synchrotron radiation to report very high spatial resolution two-dimensional elastic strain and stress maps at maximum and minimum loading measured under plane strain during a normal fatigue cycle, as well as during and after a 100% overload event, in ultra-fine grained AA5091 aluminium alloy. These observations provide direct evidence of the material stress state in the vicinity of the crack-tip in thick samples. Significant compressive residual stresses were found both in front of and behind the crack-tip immediately following the overload event. The effective stress intensity at the crack-tip was determined directly from the local stress field measured deep within the bulk (plane strain) by comparison with linear elastic fracture mechanical theory. This agrees well with that nominally applied at maximum load and 100% overload. After overload, however, the stress fields were not well described by classical K fields due to closure-related residual stresses. Little evidence of overload closure was observed sometime after the overload event, in our case possibly because the overload plastic zone was very small.     

The effect of oxide coatings on fatigue properties of 7475-T6 aluminium alloy

The effect of hard anodic oxide and plasma electrolytic oxide coatings on the fatigue strength of 7475-T6 aluminium alloy has been investigated. The coated aluminium alloy was tested using constant load uniaxial tensile fatigue machine. Hard anodising led to an appreciable reduction in the fatigue strength of 7475-T6 alloy of about 75% for a 60 μm thick coating. Further, plasma electrolytic oxidation resulted in reduction of the fatigue strength of about 58% for a 65 μm thick oxide coating. The decrease in fatigue strength of the hard anodic oxide coatings was associated with the stress concentration at the microcracks in the coating. The better fatigue performance of the PEO coatings was attributed to the development of the compressive residual internal stress within the coatings. The reduction in the fatigue strength of the PEO coatings as compared to the uncoated material was associated with the development of the tensile residual internal stress within the substrate. This may cause an early crack initiation in the substrate adjacent to the coating.     

The role of residual stress in neutral pH stress corrosion cracking of pipeline steels – Part II: Crack dormancy

This investigation provides a quantitative analysis of the effect of Type I residual stresses on the occurrence of pitting and stress corrosion cracking (SCC) formation in pipeline steel exposed to neutral pH aqueous environments. It has been shown that SCC generated in neutral pH environments can be readily blunted due to plastic deformation (room temperature creep) and/or extensive anodic dissolution. As a result, a high positive tensile residual stress gradient is necessary for continued growth of SCC in pipeline steels exposed to this neutral pH environment. The tensile residual stress represents a large mechanical driving force for crack nucleation and short crack growth. Active cracks may become dormant as the near-surface residual stress gradient changes, due to self-equilibration, from highly tensile to a lower tensile state or to a compressive state. The change in residual stress level can occur within 1 mm of the surface, resulting in a large proportion of dormant SCC.     

水射流喷丸对渗碳钢表层组织与裂纹扩展性能的影响

对低碳钢渗碳淬火试样进行表面水喷丸处理,研究水喷丸对渗层表面组织结构及其疲劳裂纹扩展速率的影响。结果表明:添加细玻璃丸的水喷丸处理能显著细化表层亚结构,提高小角度晶界密度,并为渗层带来有效的残余压应力。在300 MPa水压及600 mm/min扫描速率工艺下,可实现100 μm左右深度,最高达1244 MPa的残余压应力,其表面粗糙度R_a=0.195 μm。疲劳裂纹速率试验结果显示,经过水喷丸处理后裂纹扩展速率da/dN有增加的趋势,这主要是因为表面压应力形成,增加了心部拉应力,且表层的变形强化降低了裂纹尖端塑性区。     

Fatigue properties of rolled AZ31B magnesium alloy plate

Fatigue strength, crack initiation and propagation behavior of rolled AZ31B magnesium alloy plate were investigated. Axial tension-compression fatigue tests were carried out with cylindrical smooth specimens. Two types of specimens were machined with the loading axis parallel (L-specimen) and perpendicular (T-specimen) to rolling direction. Monotonic compressive 0.2% proof stress, tensile strength and tensile elongation were similar for both specimens. On the other hand, monotonic tensile 0.2% proof stress of the L-specimen was slightly higher than that of the T-specimen. Moreover, monotonic compressive 0.2% proof stresses were lower than tensile ones for both specimens. The fatigue strengths of 107 cycles of the L- and T-specimens were 95 and 85 MPa, respectively. Compared with the monotonic compressive 0.2% proof stresses, the fatigue strengths were higher for both specimens. In other words, the fatigue crack did not initiate and propagate even though deformation twins were formed in compressive stress under the cyclic tension-compression loading. The fatigue crack initiated at early stage of the fatigue life in low cycle regime regardless of specimen direction. The crack growth rate of the L-specimen was slightly lower than of the T-specimen. Consequently, the fatigue lives of the L-specimen were longer than those of the T-specimen in low cycle regime.     

GH4169疲劳试样加工残余应力的表征及其对低周疲劳寿命的影响

为了探究GH4169合金低周疲劳试样加工过程中表面残余应力的变化及残余应力对低周疲劳性能的影响，采用X射线衍射法对“车 磨 抛”不同加工工艺的试样表面残余应力进行表征及对成品试样进行残余应力层深度测试，通过中子衍射法对试样内部进行残余应力测试。采用电液伺服万能试验机进行了低周疲劳试验。结果表明：GH4169合金低周疲劳试样加工表面残余应力随着切削量的增加由表面残余压应力变为残余拉应力。根据残余应力层深度测试，机加工对试样表面造成残余应力层深度小于0.01 mm，内部残余拉应力减小；根据疲劳试验结果，GH4169合金疲劳寿命与表面轴向残余压应力呈正相关关系。     

Laser shock processing induced residual compression: Impact on predicted crack growth threshold performance

Design credit is not currently taken for laser shock processing (LSP) induced compressive residual stresses in damage tolerant design. The inclusion of these and other compressive stresses in design practice has the potential to dramatically increase predicted fatigue crack growth threshold performance and damage tolerant design life. In the current effort, Ti-6Al-4V coupons will be subjected to shot peening, glass bead peening, and high intensity laser shock processing. The in-depth residual stresses due to processing will be analyzed and then input into a linear elastic fracture mechanics analysis code to predict fatigue crack growth threshold performance. This analysis establishes both the utility and feasibility of incorporating LSP-induced compressive residual stresses into damage tolerant design practice.     

激光冲击强化对2524铝合金疲劳寿命的影响

目的探索激光冲击工艺参数对2524铝合金疲劳寿命的影响。方法开展不同激光能量、不同冲击次数下的激光冲击强化实验,测试其残余应力和表面硬度,并进行裂纹扩展实验和显微组织观察。结果激光冲击强化能显著提高材料的表面硬度,且材料的硬度值随着冲击能量和冲击次数的增加而递增,但硬度增长率随冲击次数增多而降低。激光冲击强化在试样表层形成较大的残余压应力,使用6.25 J的激光能量冲击1次,最大残余压应力可达-222MPa,并且残余压应力随着激光能量和冲击次数增加而增加,但冲击强化次数存在阈值。相较于未冲击试样,激光冲击1次的试样的疲劳寿命提升32%,冲击2次的疲劳寿命提升41%。对试样断口进行微观形貌观察,在裂纹长度为28 mm处,未冲击试样、激光冲击1次和冲击2次试样的疲劳条带间距分别为1.06、0.628、0.488μm,裂纹扩展速率分别为1.06×10^-3、6.28×10^-4、4.88×10^-4 mm/N。结论激光冲击强化能显著提高2524铝合金的表面硬度,并在表面产生较大的残余压应力。激光冲击强化能够有效迟滞2524-T3铝合金的疲劳裂纹扩展速率,进而有效延长疲劳寿命。     

Influence of shallow and deep cryogenic treatment on the residual state of stress of 4140 steel

The present research work studies the effect of cryogenic treatment on the residual stress state in 4140 steel. Two kinds of cryogenic treatment, namely shallow (SCT, −80 °C × 5 h) and deep cryogenic treatment (DCT, −196 °C × 24 h) were carried out between quenching and tempering in conventional heat treatment process. The results evidenced an increase in the compressive residual stress in steel are subjected to cryogenic treatment before tempering. X-ray diffractometry revealed that residual stresses are relieved during tempering, according to the redistribution of carbon in martensite and the precipitation of transition carbides. While conventional heat treatment (CHT) and shallow cryogenic treatment (SCT) promote a tensile state of residual stress, DCT shows a compressive residual stress.     

Measurements of fibre bridging during fatigue crack growth in Ti/SiC fibre metal matrix composites

High spatial resolution synchrotron X-ray strain mapping has been used to map the elastic matrix and fibre strains in the vicinity of a fatigue crack in a Ti–6Al–4V/SCS6 SiC fibre composite. A 0.61 mm fatigue crack was initiated and grown in three-point-bending. By using an in-situ loading stage it was possible to map the crack opening (longitudinal) strain distribution at K_appl=K_max and K_appl=0. In the far field region, significant thermally induced stresses were evident, being compressive in the fibres and tensile in the matrix. Around the notch and in the wake of the crack tip essentially no residual strain and only small interfacial shear stresses were found in the unloaded case, indicative of a debonded/damaged interface. At K_max the maximum tensile stress in the matrix is in the vicinity of the crack tip, whereas for the SiC fibres the maximum stress is in the bridging zone in the wake of the crack. The perturbed zone extends about ±1.5 mm either side of the crack. It was at the boundary of this zone that the maximum interfacial shear stresses (∼80 MPa) were measured in the loaded stage. A small area of tensile strain in front of the crack tip in the unloaded condition suggests frictional resistance from the bridging fibres acts to keep the crack slightly open. A simple three-dimensional finite element model has been developed to help interpret the results. The crack is introduced statically by node release and the Coulomb friction law governs the interface strength. The results of the model are compared to the synchrotron strain measurements. This comparison confirms the degradation of the interface strength in the wake of the crack.     

GH4169疲劳试样加工残余应力的表征及其对低周疲劳寿命的影响

为了探究GH4169合金低周疲劳试样加工过程中表面残余应力的变化及残余应力对低周疲劳性能的影响,采用X射线衍射法对“车 磨 抛”不同加工工艺的试样表面残余应力进行表征及对成品试样进行残余应力层深度测试,通过中子衍射法对试样内部进行残余应力测试。采用电液伺服万能试验机进行了低周疲劳试验。结果表明：GH4169合金低周疲劳试样加工表面残余应力随着切削量的增加由表面残余压应力变为残余拉应力。根据残余应力层深度测试,机加工对试样表面造成残余应力层深度小于0.01 mm,内部残余拉应力减小;根据疲劳试验结果,GH4169合金疲劳寿命与表面轴向残余压应力呈正相关关系。     

Fatigue crack/residual stress field interactions and their implications for damage-tolerant design

Residual stress fields are now widely accepted to have significant influence on fatigue crack growth. Tensile stresses have detrimental effects on fatigue lives, whereas compressive residual stresses can be beneficial. Control of fatigue lives via residual stress is now established in many industrial applications, using techniques such as shot peening or cold expansion. However, knowledge of the processes that occur when a fatigue crack grows through a pre-existing stress field is far from complete. Although the residual stress field will clearly have an effect on crack growth, the crack will equally have an effect on the residual stress field. The determination of this effect is not trivial, and direct measurement may be the designer’s best safeguard. This article outlines the complementary effects that a growing fatigue crack and a residual stress field have on each other. Two types of residual stress field are considered: mechanically induced and thermally induced. The results are discussed in terms of the implications that residual stress interactions have for damage-tolerant-based design.