飞机谱载荷下裂纹扩展的三维约束效应

 研究裂纹端部三维应力约束、塑性约束和位移约束等对谱载疲劳裂纹扩展的影响。用适于三维应力状态的修正条带屈服模型 ( Modified Strip Yield Model)计算与裂纹扩展有关的三维约束因子。利用所得约束因子的理论解改进 NASA多年来发展的 FASTRAN-II寿命预测软件 ,使其避免了依赖经验确定的约束因子进行寿命预测的局限 ,仅利用一组常幅疲劳裂纹扩展数据和材料的常规机械性能便可预测飞机谱载下的裂纹扩展寿命。对多种谱型、应力水平、过载比和材料的组合情况进行了分析 ,预测寿命与试验结果吻合很好 ,证明本文方法和改进软件可以用于实际结构的寿命预测。

EFFECTS OF THREE-DIMENSIONAL CONSTRAINT ON CRACK GROWTH UNDER AIRCRAFT SPECTRUM LOADING

The objective of this paper is to study the effects of three dimensional constraints on fatigue crack growth under aircraft spectrum loading. A modified strip yielding model is proposed for stress states other than plane stress condition and used to evaluate the three dimensional constraint factors associated with crack growth. Then the constraint factors, describing the transition from plane stress to plane strain dominated crack growth, are used to correlate crack growth data under constant amplitude loading and to calculate crack growth life under standard and typical aircraft spectrum loading and block spectrum loading. The NASA's life prediction code FASTRAN II is modified by incorporating the constraint factors. The modified code is then applied to calculate crack growth in several finite thick sheets of aluminum alloy materials and high strength steel under TWIST spectrum loading, synthesized mission spectra of fighter and aerotransport and a block spectrum. The modified code can make prediction only based on one set of crack growth data under constant amplitude loading. In general, the predictions are within 16% of the test results under TWIST and within 33% under all of the spectra. The results demonstrated that the present method and the modified code can be used in engineering practice with higher accuracy.