Improved test method for very low fatigue‐crack‐growth‐rate data
Improved test method for very low fatigue‐crack‐growth‐rate data
复制标题
极低疲劳裂纹扩展率数据的改进测试方法
DOI：
--
复制
发表时间：
2008
期刊：
影响因子：
--
通讯作者：
M. R. Hill
中科院分区：
文献类型：
--
作者：
J. Newman;J. Ruschau;M. R. Hill
目前，在北美，使用ASTM标准E647对阈值裂纹扩展进行了实验定义，在许多情况下，由于负载减少（LR）测试方法，该标准显示出异常。试验方法已被证明可以诱导远程闭合，从而过早地减缓裂纹扩展并产生异常高的阈值。本文采用LR试验方法和一种改进的试验方法，测定了Ti-6Al-4V （STOA）钛合金在阈值和近阈值状态下的疲劳裂纹扩展速率特性。改进后的方法使用由Pippan、Topper等人开发的“压缩-压缩”预裂，在接近阈值的恒定载荷下提供疲劳裂纹增长速率数据，而不受载荷历史影响。在三种不同宽度（25,51和76 mm）的致密C(T)试样上进行了大范围的应力比（R = 0.1-0.7）试验。在51毫米C(T)样品上使用了分切方法，以确认材料不包含来自成形和/或加工的显著残余应力。裂纹张开位移计用于监测裂纹的扩展。来自ASTM LR方法的数据给出了接近阈值的值，该值被发现依赖于试样宽度。然而，压缩预裂恒幅加载（CPCA）方法的数据给出了与试件宽度无关的近阈值数据。对LR和CPCA数据进行了裂纹闭合分析，以关联不同应力比下的数据。CPCA数据与有效应力强度因子范围与速率关系具有良好的相关性，而LR数据与应力比和试件宽度均表现出显著的阈值扇形。
Currently, in North America, the threshold crack-growth regime is experimentally defined by using ASTM Standard E647, which has been shown in many cases to exhibit anomalies due to the load-reduction (LR) test method. The test method has been shown to induce remote closure, which prematurely slows down crack growth and produces an abnormally high threshold. In this paper, the fatigue-crack growth rate properties in the threshold and near-threshold regimes for a titanium alloy, Ti-6Al-4V (STOA), are determined by using the LR test method and an improved test method. The improved method uses ‘compression–compression’ precracking, as developed by Pippan, Topper and others, to provide fatigue-crack-growth rate data under constant-amplitude loading in the near-threshold regime, without load-history effects. Tests were conducted over a wide range in stress ratios (R = 0.1–0.7) on compact C(T) specimens for three different widths (25, 51 and 76 mm). The slitting method was used on 51 mm C(T) specimens to confirm that the material did not contain significant levels of residual stresses from forming and/or machining. A crack-mouth-opening-displacement gage was used to monitor crack growth. Data from the ASTM LR method gave near-threshold values that were found to be dependent upon the specimen width. However, data from the compression precracking constant amplitude (CPCA) loading method gave near-threshold data independent of specimen width. A crack-closure analysis was performed for both the LR and CPCA data, to correlate data at the various stress ratios. The CPCA data correlated well with the effective stress-intensity factor range against rate relation, whereas the LR data exhibited significant threshold fanning with both stress ratio and specimen width.