Effects of precipitates on fatigue crack growth rate of AA 7055 aluminum alloy

The effects of precipitates on the fatigue crack growth rate of AA 7055 Al alloy subjected to different ageing treatments were investigated using transmission electron microscope and fatigue crack growth testing.The results show that the T77 treated samples exhibit the lowest crack growth rate,while the crack growth rate of over-aged samples is the highest.In terms of the model based on the reversibility of dislocation motion within the plastic zone close to the crack tip,the improved crack growth resistance is attributed to many precipitates that are coherent with Al matrix in the under-aged and T77 treated samples.When the precipitate is coherent with the Al matrix,the larger the precipitate is,the slower the fatigue crack grows.The effects of grain boundary precipitates and precipitate free zone on the fatigue crack growth resistance are less significant than those of precipitates within grains of the alloy.     

Stress corrosion crack growth rate in dissimilar welds

Summary

Dissimilar welds, used to join different sections in light water reactors, are potentially susceptible to stress corrosion cracking (SCC) in aqueous media, characteristic of nuclear plants. However, the study of these welds has been limited to evaluating weld material susceptibility in these media. Scarce data are available on crack growth rates due fundamentally to inadequate testing techniques. In order to address this lack of information, the crack growth rate at the interface of ferritic SA533B‐1 alloy and 1–82 alloy in a dissimilar weld (SA533B‐1/I‐82/316L) was studied. Experiments were conducted in water at 288 °C, 8 ppm of O₂ and 1 μS/cm conductivity.     

钢轨超声检测典型损伤B显图谱特征分析

以U75V重轨钢为研究对象,采用万能试验机、应变仪、扫描电镜等测定和分析其在线轧态和在线热处理态条件下的微观组织、断口形貌及三点弯曲断裂韧性,揭示其微观组织对断裂韧性的影响规律,为研究珠光体重轨钢热处理工艺提供参考。结果表明：U75V重轨钢热处理态和轧态的表面应变变化规律相似,但热处理态U75V重轨钢从弹性变形到塑性变形比轧态滞后;热处理态和轧态U75V重轨钢的断裂韧性K_IC值分别为45.122、42.048 MPa·m^1/2,热处理态的断裂韧性比轧态高;轧态U75V重轨钢断口为解理断裂,而热处理U75V重轨钢为准解理断裂;轧态和热处理态U75V重轨钢的珠光体片层间距分别为272.2、148.4 nm,热处理态U75V重轨钢的珠光体片层细密,存在显著的珠光体团簇,不利于裂纹的扩展,使得热处理态U75V重轨钢断裂韧性高于轧态。

     

Effect of electrolyte composition on crack growth rate in pipeline steel

Electrolyte composition can strongly affect the rate of crack growth in pipe steel X70. A weakly acidic citrate buffer (pH 5.5) and a mixture of NS4 solution with a borate buffer (pH 7) were used as background solutions under static and cyclic strain, respectively. Various compounds that affect steel dissolution and hydrogen absorption rates were added to the solutions. Crack growth is accelerated in the presence of metal dissolution activators but hindered upon addition of a corrosion inhibitor. Hydrogen absorption promoters do not accelerate the crack growth at the corrosion potential.     

Mixed-type crack growth rate in a 30KhGSA steel-VRP-10 solder bimetal

Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 9, pp. 49–50, September, 1990.     

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。     

Study of fatigue crack growth rate for austenitic Fe-Al-Mn alloys

A study was made of the crack growth rate (da/dN) versus stress-intensity variation ΔK behavior of Fe-Al-Mn alloys with different percentages of carbon, aluminum, and manganese at ambient temperature. The experimental results are described with respect to a Paris equation,da/dN = C(†K)ⁿ, where the exponent n, index for crack growth resistance of materials, was strongly influenced by alloy composition. It was found that higher manganese content provided better crack growth resistance, and that carbon and aluminum had an opposite effect. Scanning electron microscopy, x-ray diffraction, and mechanical properties evaluation were performed and correlated to the change of n values.     

时效时间对7075铝合金疲劳裂纹扩展速率的影响

采用透射电镜、显微硬度仪、拉伸试验和疲劳试验等研究了 7075铝合金在欠时效(120℃×8 h)、峰时效(120℃×24 h)与过时效(120℃×48 h)状态下的微观组织、硬度、拉伸性能方面与疲劳裂纹扩展速率方面的差异.结果表明:3种时效状态的合金在高低应力强度因子范围时,疲劳裂纹扩展速率呈现不同的规律.在低应力强度...     

超载对X52管线钢疲劳裂纹扩展速率的影响

用紧凑拉伸试样研究了不同应力比和超载比对X52管线钢疲劳裂纹扩展速率的影响,并对疲劳断口形貌进行SEM分析.结果表明,拉伸超载有阻滞X52钢疲劳裂纹扩展的作用,使疲劳裂纹扩展由超载扩展,减速扩展和恢复扩展三个阶段组成.在恒定的应力比下,超载比越大,裂纹扩展的阻滞效应也越明显.超载前后形貌差别越大;在恒定超载比下,低应力比的裂纹扩展阻滞现象更明显.     

Effect of corrosion medium composition on rate of crack growth in X70 pipeline steel

The growth rates of corrosion cracks in X70 pipeline steel are determined in weakly acidic (pH 5.5) buffer solution with additives of a number of compounds that are a part of the soil electrolyte or are steel corrosion inhibitors. It is shown that the effect of components of electrolyte on the rate of crack growth agrees fairly well with their stimulating or inhibiting effect on the anodic dissolution of iron. The rates of hydrogen permeation into steel are measured and the effect of hydrogen absorbed by the metal on the anodic process in the studied corrosion media is considered.     

淬火速率对砂铸Al-7Si-0.4Mg合金拉伸性能及疲劳裂纹扩展行为的影响

研究了淬火速率对峰值时效状态下的砂铸Al-7Si-0.4Mg合金拉伸性能及疲劳裂纹扩展行为的影响.结果表明,随着淬火速率的提高,合金的抗拉强度、屈服强度以及α(Al)相的显微硬度均有提高,抗拉强度和屈服强度与淬火速率的对数值近似成线性关系.淬水速率对合金的延伸率影响较小,但对合金的室温疲劳裂纹扩展速率有明显影响,较高的淬火速率可以明显提高合金的疲劳裂纹扩展阻力.     

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.     

Derivation of closure-free crack growth rate under biaxial fatigue loading conditions

The present paper develops a new methodology to derive the closure-free (effective) crack growth rate, da/dN, from fatigue life data under biaxial loading conditions using the fracture mechanics approach. The derived da/dN curve showed a good agreement with the measured crack growth rate data for 1045 steel. The method lowers largely the cost and time required to perform the conventional closure-free crack growth tests.     

Development of fast algorithms for estimating stress corrosion crack growth rate

A simplified method is proposed for modeling the chemistry and potential distribution in a stress corrosion crack in sensitized stainless steel in boiling water reactor (BWR) coolant environments. The model is based on an assumption that only those species that are present in the largest concentrations in the crevice determine the potential distribution down the crack. The advantage of this method is that it permits simplification of the mathematics and allows predictions to be made of the potential and concentration distributions without knowing various parameters, such as the equilibrium constants for homogeneous chemical reactions in the cavity and diffusion coefficients of species that are present at relatively low concentrations near the crack tip. In some important cases, analytical expressions can be obtained for the pH, potential near the crack tip, and crack propagation rate. The conditions for which the potential on the crack flanks and that in the vicinity of the crack tip coincide with the free corrosion potential in the local environment, and hence, for which there exists a balance between rates of the local anodic and cathodic partial processes, are determined. The impact of the potential drop in the external environment on the potential and concentration distributions down the crack and on the crack propagation rate is also investigated. Excellent agreement is obtained between calculated and measured crack growth rates.     

Testing study of subcritical crack growth rate and fracture toughness in different rocks

1 Introduction The time effect of rock engineering stability is attached importance to with enlarging of the rock engineering scale. The instability instances in-situ and rock laboratory examinations show that the rock instability is owing to crack growt…     

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。     

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.     

Explanation of an apparent abnormality in fatigue crack growth rate curves in titanium alloys

A surprising phenomenon is investigated where titanium alloys exhibit no threshold fatigue crack growth value if K_max in the K_max-constant testing procedure exceeds a certain value. The crack growth rate increases with decreasing ΔK up to final fracture. The phenomenon was found repeatedly for Ti–6Al–2Sn–4Zr–6Mo above K_max=21 MPa√m (equal to 72% of K_IC), and its causes were investigated. The same crack growth rates as in the K_max-constant test were reproduced by two independent experimental procedures, the so-called “jump” test and sustained K cracking experiments along with a calculation. It is demonstrated that the observed phenomenon is not a special crack growth feature or a new phenomenon, but simply caused by time-dependent crack growth, which is known to exist in titanium alloys or steels. Fractographic work revealed that intergranular crack growth along α and transformed β grain boundaries increases with decreasing ΔK and increasing K_max value, accompanied by creep deformation in the transformed β grains. The conditions for time-dependent cracking are believed to be a sufficiently high stress and strain field in the crack tip region, along with hydrogen-assisted cracking.