表面裂纹在不同控制模式下的扩展特性

在应变、载荷和位移三种控制模式下,采用Gross板状试样,研究了10CrNi5Mo高强钢在悬臂弯曲加载方式下低周疲劳表面裂纹扩展特性,研究结果表明:表面裂纹扩展速率d(2c)/dN在位移和载荷控制模式下差别不大,且都高于应变控制模式;三种控制模式都可以用裂纹前缘名义总应变范围ΔεT作为统一参量来描述表面裂纹疲劳扩展速率,且表达式与Paris公式类似,均满足幂函数规律;试验中通过不断调整载荷或位移的大小来保证应变恒定,会增加试验的操作难度和工作强度。     

残余应变对表面裂纹扩展速率的影响

在悬臂弯曲加载方式下,采用逐级递增的多级名义应变控制表面裂纹前缘应变的试验方法,进行了在改换名义应变级别控制时残余应变调零和不调零时10CrNiMo结构钢的低周疲劳表面裂纹扩展速率试验,对试验结果进行了比较和分析,结果表明:残余应变会减缓表面裂纹的扩展速率,残余应变越大,这种减缓效果越显著;在过渡为下一级名义应变时残余应变调零将会获得较快的表面裂纹扩展速率。     

基于遗传规划算法的不同应力比下不同厚度7050铝合金疲劳裂纹扩展寿命预测

针对不同厚度7050铝合金试样进行了不同应力比条件下的一系列疲劳裂纹扩展试验,并运用遗传规划算法对疲劳裂纹扩展寿命进行预测。遗传规划算法是模拟自然界中生物的进化策略,通过交换、突变等遗传操作,搜索目标的最优解。建立7050铝合金疲劳裂纹扩展速率的遗传规划模型,并利用试验数据对模型进行测试,后与其他典型疲劳裂纹扩展模型进行比较。研究结果表明:GP模型预测的7050铝合金疲劳裂纹扩展寿命结果与试验值基本吻合,相对误差小于1.5%,且GP模型预测结果的准确性高于Paris模型和Walker模型。     

温度和应力比对航空铝合金疲劳裂纹扩展规律的影响及其机理

针对7475-T761航空铝合金中心开裂(MT)试样进行了不同温度、不同应力比条件下的一系列疲劳裂纹扩展试验,得到了相应试验条件下的疲劳裂纹扩展数据与规律,讨论了应力比、环境温度对疲劳裂纹扩展行为的影响,并利用扫描电镜(SEM)观测分析了疲劳断口。结果表明:7475-T761铝合金疲劳裂纹扩展速率随应力比、温度的增加而增加;消除裂纹闭合效应影响后,相同温度不同应力比下的da/dN-ΔKeff可由同一拟合公式描述;高温时弹性模量和材料抗拉强度的下降以及裂纹表面氧化导致裂纹扩展速率较快;对比不同条件下稳定扩展区疲劳条带宽度验证了试验分析结论。     

循环载荷压半周对疲劳裂纹扩展速率的影响

本文使用ＬＹ１１ＣＺ铝合金及１８ＭｎＨＰ钢两种板材的中心裂纹拉伸试件，在不同应力水平及应力比下，就应力比，特别是循环载荷压缩部分对疲劳裂纹扩展速率的影响进行了试验研究和分析，指出，现有裂纹闭合理论不足以解释载荷压半周的作用，并就两种材料给出了估算裂纹扩展速率的经验公式。在此基础上还指出，用线弹性断裂力学应力强度因子Ｋ作为裂纹扩展的控制参量，在理论上，有待进一步研究。     

倾斜应力作用下垂直于功能梯度材料夹层的币型裂纹扩展问题

分析了功能梯度材料中币型裂纹扩展问题。该裂纹体受有与裂纹面成任意角度的张应力或压应力,裂纹垂直于无限域中功能梯度材料夹层。假定非均匀介质的功能梯度材料夹层与两个半无限域完全结合,其弹性模量沿厚度方向变化。利用已发表的裂纹应力强度因子数据和线弹性断裂力学的叠加原理,将应力强度因子耦合于最小应变能密度因子断裂判据,讨论了裂纹扩展的临界荷载;并讨论了荷载方向和材料性质对临界荷载的影响。     

粉末冶金铝合金的疲劳裂纹扩展行为

疲劳性能是粉末冶金铝合金一项重要的使用性能,研究疲劳裂纹扩展行为是研究疲劳性能的一种重要的方法。总结了影响粉末冶金铝合金疲劳裂纹扩展速率的各种外部因素和内部因素,外部因素主要包括应力比、温度、制备方法等,内部因素有晶粒尺寸、夹杂物、铝基复合材料中的增强相颗粒等,并详细阐述了这些因素的影响机制。     

不同腐蚀环境对7475-T7351铝合金疲劳性能及裂纹扩展速率的影响

采用轴向加载疲劳和疲劳裂纹扩展速率性能测试方法,研究了不同腐蚀环境对7475-T7351铝合金厚板疲劳及裂纹扩展性能的影响.结果表明:腐蚀环境对7475铝合金的疲劳性能有较大影响,油箱积水和3.5%(质量分数,下同)NaCl溶液中光滑试样的疲劳强度较室温下降约68%,油箱积水和3.5%NaCl溶液环境对材料疲劳强度的影响程度基本相同;不同环境腐蚀(空气和3.5%NaCl)和不同温度(室温和125℃)对材料的低周疲劳性能影响不大;腐蚀环境对裂纹扩展有较明显的加速作用,油箱积水和3.5%NaCl溶液环境对裂纹扩展的加速规律基本一致.     

应力比对航空高强LD2CZ铝合金腐蚀疲劳裂纹扩展的影响

利用扫描电镜联合液压伺服试验机,并借助于Walker公式研究了应力比对预腐蚀不同时间航空高强LD2CZ铝合金疲劳裂纹扩展的影响,在应力比分别为0．05,0．5,0．7的条件下对预腐蚀0,15,30d的LD2CZ铝合金单边缺口板状试样进行了疲劳加载试验,得到了其疲劳裂纹扩展速率曲线,并拟合出了Walker公式中的材料常数。结果表明：裂纹扩展速率会随着应力比的增加以及腐蚀损伤的加深而增大,拟舍得到的Walker公式可用来定量化地表征应力比和腐蚀损伤对疲劳裂纹扩展速率的影响。     

CRACK INITIATION AND SMALL FATIGUE CRACK GROWTH BEHAVIOUR OF SQUEEZE-CAST Al-Si ALUMINIUM ALLOYS

Abstract— Fatigue strength, crack initiation and small crack growth behaviour in two kinds of squeeze-cast aluminium alloys, AC8A-T6 and AC4C-T6 were investigated using smooth specimens subjected to rotatary-bending fatigue at room temperature. Fatigue resistance of these alloys was almost the same as that of the wrought aluminium alloys because of their fine microstructure and of the decrease in defect size due to squeeze-casting. Fatigue crack initiation sites were at the eutectic silicon particles on the surface of specimens or at internal microporosity in the specimens. Crack initiation life, defined as a crack length of 50 μm on the specimen surface, was successfully estimated from an evaluation of initiation sites using fracture mechanics and the statistics of extrema. Small fatigue crack growth in the two kinds of alloys obeys the relation proposed by Nisitani et al. , namely that d(2c)/d N = C (σ_a/σ_B)ⁿ· (2 c ), where C is a constant and σ_B is the ultimate tensile strength. It is pointed out that an improvement in fatigue strength of cast aluminium alloys can be expected by refining the eutectic silicon rather than by an increase in static strength.     

A537钢疲劳裂纹扩展单次拉伸超载效应及裂尖形变,裂纹闭合行为

研究了恒定ΔＫ条件下，单次拉伸超载对Ａ５３７钢疲劳裂纹扩展速率的影响，并利用激光散斑技术原位研究超载前后的裂尖应变场，裂纹闭合效应。结果表明：超载后裂纹闭合效应呈增强趋势，裂尖应变呈下降趋势。伸超载有阻滞裂纹扩展的作用。     

轴承钢疲劳裂纹扩展速率的研究

研究了轴承钢碳化物及晶粒细化对轴承钢疲劳寿命的影响。结果表明;细化轴承钢中的碳化物可以使其疲劳裂纹扩展速率下降,而同时细化轴承钢中的碳化物和晶粒,会使其疲劳裂纹扩展速率下降更明显。     

玻璃纤维-Al 混杂复合层板的疲劳裂缝扩展行为

本文研究了一种新型结构复合材料玻璃纤维-Al 混杂复合层板(Glass Aluminum Laminates,GLALL)在不同外载荷作用下的疲劳裂缝扩展行为,以及疲劳破坏规律。试验发现,由于未断高强度纤维对裂缝的桥接作用降低了裂缝尖端的有效应力场强度因子,GLALL 的疲劳裂缝扩展速率远低于单一Al 合金。伴随 Al 合金层内疲劳裂缝的扩展,GLALL 疲劳裂缝附近区域会产生脱层破坏,脱层区宽度随外加载荷增大而增大。脱层区宽度越小,纤维对疲劳裂缝的桥接作用越强。     

CORRELATION OF FATIGUE CRACK GROWTH RATE AT DIFFERENT STRESS RATIOS FOR QUENCHED AND TEMPERED STEELS AND OTHER ALLOYS

Abstract— Measurements of the effect of stress ratio on the constant amplitude fatigue crack growth rates in four quenched and tempered steels in the Paris regime are reported. This data and published data for other alloys (including lower strength steels and non-ferrous alloys) are evaluated, and a correlation function suitable for practical fatigue life calculations is derived. In addition to stress intensity factor range and stress ratio, other significant parameters are the yield stress of the material and its thickness. For the four steels on which new measurements were made, the degree of dependence of the crack growth rate on stress ratio may be related to sensitivity to environmental conditions.     

残余应力与胶粘剂对ARALL层板疲劳裂纹扩展特性的影响

研究了粘剂性质、含量及残余应力状态对纤维－铝合金胶接层板（ＡＲＡＬＬ）疲劳裂纹扩展特性的影响，分析了裂纹扩展过程中的分层状态的变化，结果表明，ＡＲＡＬＬ层板内富胶层的剪切形变和伴随裂纹扩展的分层区越大，即这两方面耗散能量越多，则疲劳裂纺扩展速率越低；胶粘剂含量的影响不明显，给层板施加预应力极大降低了层板的疲劳裂纹扩展速率，其本质在于裂尖在同样的疲劳载荷下实际所受到的有效应力降低。     

THE RANDOMNESS OF FATIGUE CRACK GROWTH UNDER CONSTANT-AMPLITUDE LOADS

An analysis of fatigue crack growth from an statistical point of view has been carried out. Eighteen pre-cracked specimens obtained from the same sheet of aluminium alloy were subjected to identical load and environmental conditions. Tests were conducted under constant-amplitude loads in order to isolate the effect of material randomness. The experimental results exhibited two different types of dispersion: one showed as a change in the mean growth rate between tests and the other as nonuniform growth in the same specimen. At a subsequent stage, the statistical distribution of the experimental results was studied and a theoretical model was developed to account for the growth pattern observed. The proposed model uses a growth law comprising random parameters to account for the low-frequency component (slow changes). The comparison of the performance of two different laws, viz. the Paris-Erdogan law and the cubic law, is presented. Both were tested on the assumption of randomness in two of their fitting parameters. In addition to the above-mentioned law, the model uses a stochastic log-normal process to model the high-frequency component (rapid changes). The parameters for this process were determined by time series analysis of fatigue crack growth rate data.     

ACCELERATION OF FATIGUE CRACK GROWTH UNDER INTERMITTENT OVERSTRESSING WITH DIFFERENT MEAN STRESS LEVELS

Fatigue crack growth behaviour under intermittent overstressing was investigated in dry air with a low carbon steel under tension-compression and tension-tension loading with different mean stress levels. A very small number of cycles of overstress applied intermittently during a very large number of cycles of understress below threshold caused, in all cases, a significant acceleration in crack growth rate as compared to the case of steady cyclic stress. The acceleration for the case with tensile mean stresses was a little smaller than in the case without mean stress and it was related to a little higher crack closure level in the former case. Stress history in the precracking process had some effect on crack closure and, consequently, on the acceleration. A fairly good correlation was obtained between the acceleration and the effective stress intensity range. The acceleration under multi-level intermittent overstresses with different mean stresses could be predicted by linear summation from two-level intermittent overstress test data. No difference was recognized in the fracture surface morphology between the cases with different mean stress level including the cases of multi-level stresses, indicating that the micromechanism of crack growth acceleration would be the same in all cases.