裂纹扩展中名义压缩载荷部分的作用

 本文意图:从总体上揭示名义压缩载荷在裂纹扩展各阶段中的作用。文中以现有资料为基础,针对典型载荷情况,对循环载荷的名义压缩部分在裂纹起始、短裂纹和长裂纹扩展阶段中的作用进行了分析。在两处关键的地方补充做了新的试验工作。发现名义压缩载荷部分在裂纹扩展各阶段、各种载荷谱作用下,对疲劳寿命起着非常不同的作用。指出有效尾迹塑性区长度ι_o这个参数起着重要的作用。

EFFECTS OF NOMINAL COMPRESSIVE LOAD ON VARIOUS STAGES OF CRACK GROWTH

Based on data available for typical load spectra, effects of nominal compressive load on crack initiation, short and long crack growth are analyzed. The results listed in table III show that the effects of compressive load on fatigue life of a structure are very complicated and depend on both the spectra（for instance, constant cycle loading, single overload loading or multiple overload loading）and the stages of crack growth. In some cases the compressive part in the spectra can be neglected with little error （for instance, in constant cycle loading, or in compressive part after multiple overload loading of a long crack）. But this conclusion is unsuitable for short crack, crack initiation or the compressiive part in long crack growth under single overload.Based on Elber's crack closure concept, the effective length of the wake plastic zone l0 is introduced in this paper. It, together with the other parameters, can explain the effects of nominal compressiive load on crack growth.Finally, the specially designed tests show that the compressive load under the overload is unable to lower the crack retardation any more when the length of the multiple overload wake plastic zone reaches l0≈（1-2）rp. The tests also show that da/dN of short crack is higher than that of long crack under the condition of constant cycle loading △K. This phenomenon becomes more evident when the nominal load contains compressive part.