微机辅助近门坎区疲劳裂纹扩展速率测试

Microcomputer Aided Fatigue Crack Propagation Rate Measurement in the Near-Threshold Region

本文介绍了用降 K 增 K 法测疲劳门坎值ΔK_(th)及近门坎区疲劳裂纹扩展速率的 Apple-Ⅱ微机辅助测试系统。提出了有效利用降 K 法时,如何给定变载系数 C与降载扩展间隔(Δa)_D 以及如何正确确定门坎值。最后给出了系统用于 Lc9铝合金近门坎区腐蚀疲劳裂纹扩展速率的试验结果。

In the early papers about microcomputer aided measurement system for fatigue crack propagation(FCP)threshold and FCP rate in the near-threshold region, only the selection technique of load-variation coefficient C_D,C_I for the cases of K-decreasing and K-increasing measurement has been researched[1,2],but the method of selection of load-variation growth interval(Δa)_D,(Δa)_I for the cases of K-decreasing and K-increasing measurement has not been studied.Al- though various microcomputer aided measurement systems for FCP threshold have been set up later[3-10],the testing parameters for the cases of K-decreasing and K-increasing measurement have still not been investigated.As the testing parameters used by previous authors are not the optimum ones,speed of testing, the amout of information obtainable and accuracy of measurement have been adversely affected.Compared with previous papers,the new contributions of the present study are as follows: (1)The efficient selection formulas of growth interval(Δa)_D and(Δa)_I for the cases of K-decreasing and K-increasing measurement have been derived (See formulas (6)～(19)in the full paper). (2)The optimum relationship among(Δa)_D、(Δa)_I、C_D and C_I has been given(See formula(12)of the full paper). (3)It has been proved that the FCP rate in the near-threshold region should be carefully taken in accordance with explanatory remarks in the full paper concerning figure 4,otherwise a false FCP rate on the unsafe side is used as was generally done in previous papers. On the basis of the author's contribution,An Apple-Ⅱ microcomputer aided measurement system for FCP rate has been set up.Compared with other systems. this system can increase the measurement accuracy,shorten testing time and obtain more information.Furthermore,the hardware of the above-mentioned system is inexpensive.