船体结构钢低周疲劳表面裂纹扩展规律

采用悬臂弯曲加载方式,以总应变范围（ΔεＴ）作为受检参数和控制参数,对船体结构钢低周疲劳表面裂纹扩展行为进行了研究,得到表面裂纹扩展速率ｄ（２ｃ）／ｄＮ与ΔεＴ的关系,提出了结构疲劳寿命指标的估算方法和表面裂纹低周疲劳扩展机制。     

缺口小裂纹的疲劳扩展速率

将复型技术应用于疲劳小裂纹扩展试验中的裂纹长度测量。在等载荷比R=0.1、不同平均载荷水平影响的疲劳条件下,板试样V型缺口小裂纹疲劳扩展速率做了试验测试;通过结果分析,提出了缺口小裂纹疲劳扩展速率表达式,并以ε_P为控制参数,求出45^*钢的计算式。     

环境对316L钢塑性疲劳短裂纹萌生与扩展行为的影响

研究在对称恒幅塑性应变（Ｒ＝－１，△εＰ＝２×１０－３控制下，真空条件及温度效应对３１６Ｌ钢的塑性疲劳短裂纹萌生和扩展行为的影响。通过观测表面主裂纹长度及裂纹密度随循环周次发展规律给出材料表面损伤的定量分析。结合对表面主裂纹长度与深度的分析建立起疲劳短裂纹前沿扩展速率与循环Ｊ积分及基于塑性应变控制下等效应力强度因子的关系。     

高强度热轧双相钢的低周疲劳性能

从热轧双相钢的实际应用需求出发,研究了高强度热轧双相钢DP600的低周疲劳性能。采用轴向应变控制方法对DP600钢进行了低周疲劳试验,并对试验数据进行拟合计算,得到DP600钢的循环应力-应变曲线、应变-寿命曲线和过渡疲劳寿命。通过扫描电镜观察疲劳断口,结果显示低周疲劳条件下,DP600钢断裂裂纹起源于试样表面,裂纹扩展前期呈现部分脆性断裂特征,后期则以明显的韧性断裂为主。     

复杂应力状态2.25Cr-1Mo钢的高温低周疲劳裂纹扩展规律

用带预裂纹的缺口试件研究2.25Cr-1Mo钢高温低周疲劳裂纹扩展规律,通过疲劳试验观察裂纹扩展寿命,应用ANSYS计算裂纹尖端应力应变分量和当量弹、塑性应变范围,利用当量J积分范围表征2.25Cr-1Mo钢在复杂应力状态下的低周疲劳裂纹扩展速率.结果表明:疲劳裂纹扩展速率da/dN与当量J积分范围△Jf的关系不受试件缺口型式和加载应变范围的影响,用当量J积分来评价2.25Cr-1Mo钢的高温低周疲劳裂纹扩展规律是有效的.     

枝晶间一次相对熔铸GH136合金低周疲劳性能的影响

本文对熔铸GH136合金的研究结果表明:存在于枝晶间的大的未溶一次相粒子能降低材料的低周疲劳性能。粒子的作用机构有两个,其一是激发主裂纹的早期萌生,其二是自身形成内疲劳源,通过内源的生长及与主裂纹的联结加速材料的破坏。
这两种作用机构均与裂纹扩展速率有关。在高裂纹扩展速率下(或高应力幅值),粒子的有害作用被减弱,反之,其有害作用加强。采用提高固溶温度的方法可以有效地去除这些一次相粒子,使材料在低交变应力范围的性能得以改善。     

银离子注入多晶α－Fe的低周疲劳研究

本文研究了多晶α－Ｆｅ经９０ｋｅＶ，６×１０（１６）ｉｏｎｓ／ｃｍ２银离子注入前后的低周疲劳性能，结果表明，离子注入改善了α－Ｆｅ的低周疲劳性能，疲劳韧性系数和疲劳韧性指数的绝对值分别从未注入时的０．０７６和０．３２增加到０．２１６和０．４２。作者认为，注入离子阻碍了体心立方基体螺位错的交滑移是疲劳韧性提高和低周疲劳性能改善的主要机制；另外，注入引起的缺陷强化和固溶强化对提高低周疲劳性能也有一定作用。     

裂纹扩展速率的分形模型

裂纹扩展速率的分形模型类维生，苏燕，陈丙森ＡＦｒａｃｔａｌＭｏｄｅｌｏｆＣｒａｃｋＰｒｏｐａｇａｔｉｏｎＳｐｅｅｄ￥ＬｅｉＷｅｉｓｈｅｎｇ；ＳｕＹａｎ；ＣｈｅｎＢｉｎｇｓｅｎ１前言材料形变与断裂的研究是分形（Ｆｒａｃｔａｌ）理论应用的主要领域之一。对...     

Frequency dependent low cycle fatigue crack propagation

The frequency modified Coffin-Manson low cycle fatigue expression phenomenologically describes the influence of the cycling frequency on the fatigue life. This expression relates only to the fatigue life and as such does not enable the separation of the frequency influence on crack nucleation from that on propagation. The approach taken here was to study directly the propagation phase of low cycle fatigue and to this end a frequency modified crack growth expression is presented. The experiments reported here were performed on A286 (an iron-base superalloy) cycled at 1100°F with plastic strain limits. The influence of the cycling frequency is described in terms of two frequency regimes. At the lowest frequencies (below 0.05 cpm) varying the frequency did not change the time to failure. The crack growth rate is thus more a result of stress rupture than fatigue. At higher frequencies both time and cycles determine the crack propagation behavior.     

Low cycle fatigue of Ti-Mn alloys: Microstructural aspects of fatigue crack growth

Fatigue crack path behavior of a series of Ti-Mn alloys heat treated to produce volume fractions of the alpha phase ranging from 0 to 97.5 pct was investigated. Both Widmanstätten and equiaxed morphologies of the α phase were used in this study. Interior and surface crack paths are discussed in terms of slip behavior and microstructural details.     

A transgranular fatigue crack growth model based on restricted slip reversibility

This article presents a transgranular fatigue crack growth model based on a restricted slip reversal process where the transgranular crack growth rate is related to the cyclic plastic strain range ahead of the crack tip. Upon applying deformation and fracture kinetics theories, a physically based constitutive law for fatigue crack growth rate is derived. In the absence of any environmental contributions to crack growth, the model takes the form of the Paris equation with a power law exponent of 3 at positiveR values. The model expresses the fatigue crack growth rate explicitly in terms of material properties, such as yield strength, work-hardening coefficient, microstructural quantities such as activation energy, activation volume, and work factor, as well as test constraints such asΔK andR. The absence of a fatigue threshold is predicted for test conditions where environment does not influence the crack growth process and the material microstructure remains stable. Formerly Graduate Student, Department of Mechanical Engineering, University of Ottawa.     

High cycle fatigue and fatigue crack growth of the oxide dispersion strengthened alloy MA 754

The high cycle fatigue (HCF) behavior of the oxide dispersion strengthened (ODS) MA 754 alloy has been determined as a function of specimen orientation. The fatigue life showed anisotropic behavior with the longest and shortest lives in the longitudinal and short transverse directions, respectively. Surface porosity, due to oxidation, was found to affect fatigue life in the long transverse orientation more than in the longitudinal orientation. The fatigue crack growth behavior in MA 754 exhibited a directional dependence. In general, the crack growth rates in the longitudinal direction were lower than those in the long transverse direction. The ΔK _th was ∼11 MN ·^-3/2 and 9 MN · m^-3/2 for the longitudinal and the long transverse orientation, respectively. This behavior was explained on the basis of the unusual grain structure and the texture exhibited by this alloy as well as different crack closure effects. It was found that a consideration based on the crack growth rates results, obtained from fracture mechanics specimens, could not explain the anisotropic behavior of the HCF properties of MA 754. However, the anisotropic HCF properties could be rationalized on the basis of the differences in the modes of crack initiation.     

Intrinsic fatigue crack growth

The influences of microstructure and deformation mode on inert environment intrinsic fatigue crack propagation were investigated for Al-Li-Cu-Mg alloys AA2090, AA8090, and X2095 compared to AA2024. The amount of coherent shearable δ (Al₃Li) precipitates and extent of localized planar slip deformation were reduced by composition (increased Cu/Li in X2095) and heat treatment (double aging of AA8090). Intrinsic growth rates, obtained at high constantK _max to minimize crack closure and in vacuum to eliminate any environmental effect, were alloy dependent;da/dN varied up to tenfold based on applied ΔK or ΔK/E. When compared based on a crack tip cyclic strain or opening displacement parameter (ΔK/(σ_ys E)^1/2), growth rates were equivalent for all alloys except X2095-T8 which exhibited unique fatigue crack growth resistance. Tortuous fatigue crack profiles and large fracture surface facets were observed for each Al-Li alloy independent of the precipitates present, particularly δ, and the localized slip deformation structure. Reduced fatigue crack propagation rates for X2095 in vacuum are not explained by either residual crack closure or slip reversibility arguments; the origin of apparent slip band facets in a homogeneous slip alloy is unclear. Better understanding of crack tip damage accumulation and fracture surface facet crystallography is required for Al-Li alloys with varying slip localization.     

Mechanisms of near-threshold fatigue crack growth in a low alloy steel

The near-threshold fatigue crack growth behavior of CrMoV steel was investigated in the temperature range of 24–427°C. At load ratios of 0.1 and 0.5, the value of threshold stress intensity range, ΔK_th, experienced a minimum as test temperature increased from 24 to 427°C. Increasing load ratio generally decreased the effect of temperature on ΔK_th. Oxide and roughness-induced crack closure offered an explanation for the influence of temperature on near-threshold crack propagation rate properties. Moreover, extensive oxide thickness and surface roughness measurements substantiated the explanation. At 24°C, oxide thickness increased with decreasing ΔK due to fretting oxidation. Furthermore, increasing load ratio decreased oxide thickness. In contrast, at higher temperatures of 149, 260 and 427°C, oxide thickness generally decreased with decreasing ΔK, due to thermal oxidation, irrespective of load ratio. While oxide thickness increased with increasing temperature, surface roughness decreased with increasing temperature. At 427°C, at low load ratios oxide-induced crack closure was responsible for the increased crack closure level with decreasing ΔK while at high load ratios, plasticity-induced crack closure was suggested to be the controlling mechanism in affecting the crack closure level. At a fixed ΔK value, the crack closure level increased with increasing load ratio.     

Elevated-temperature fatigue crack growth

The fatigue crack growth behavior of MAR-M200 single crystals was examined at 982 °C. Using tubular specimens, fatigue crack growth rates were determined as functions of crystallographic orientation and the stress state by varying the applied shear stress range-to-normal stress range ratio. Neither crystallographic orientation nor stress state was found to have a significant effect on crack growth rate when correlated with an effective ΔK which accounted for mixed-mode loading and elastic anisotropy. For both uniaxial and multiaxial fatigue, crack growth generally occurred normal to the principal stress direction and in a direction along which ΔK _II vanished. Consequently, the effective ΔK was reduced to ΔK_I and the rate of propagation was controlled by ΔK _I only. The through-thickness fatigue cracks were generally noncrystallographic with fracture surfaces exhibiting striations in the [010], [011], and [111] crystals, but striation-covered ridges in the [211] specimen. These fracture modes are contrasted to crystallographic cracking along slip bands observed at ambient temperature. The difference in cracking behavior at 25 and 982 °C is explained on the basis of the propensity for homogeneous, multiple slip at the crack tip at 982 °C. The overall fracture mechanism is discussed in conjunction with Koss and Chan’s coplanar slip model.