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Fatigue growth of short cracks in Ti-17: Experiments and simulations

机译：Ti-17中短裂纹的疲劳增长：实验和模拟

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The fatigue behaviour of through thickness short cracks was investigated in Ti-17. Experiments were performed on a symmetric four-point bend set-up. An initial through thickness crack was produced by cyclic compressive load on a sharp notch. The notch and part of the crack were removed leaving an approximately 50 mu m short crack. The short crack was subjected to fatigue loading in tension. The experiments were conducted in load control with constant force amplitude and mean values. Fatigue growth of the short cracks was monitored with direct current potential drop measurements. Fatigue growth continued at constant R-ratio into the long crack regime. It was found that linear elastic fracture mechanics (LEFM) was applicable if closure-free long crack growth data from constant K_(Imax) test were used. Then, the standard Paris' relation provided an upper bound for the growth rates of both short and long crack. The short crack experiments were numerically reproduced in two ways by finite element computations. The first analysis type comprised all three phases of the experimental procedure: precracking, notch removal and fatigue growth. The second analysis type only reproduced the growth of short cracks during fatigue loading in tension. In both cases the material model was elastic-plastic with combined isotropic and kinematic hardening. The agreement between crack tip opening displacement range, cyclic J-integral and cyclic plastic zone at the crack tip with triangle open K_I verified that LEFM could be extended to the present short cracks in Ti-17. Also, the crack size limits described in the literature for LEFM with regards to plastic zone size hold for the present short cracks and cyclic softening material.
机译：在Ti-17中研究了贯穿厚度短裂纹的疲劳行为。实验是在对称的四点弯曲装置上进行的。最初的贯穿厚度裂纹是由尖锐缺口上的循环压缩载荷产生的。除去缺口和部分裂缝，留下约50μm的短裂缝。短裂纹在张力下承受疲劳载荷。实验在负载控制下以恒定的力幅值和平均值进行。用直流电势降测量监测短裂纹的疲劳增长。疲劳增长以恒定的R比率持续进入长裂纹状态。发现如果使用来自常数K_（Imax）测试的无闭合长裂纹扩展数据，则线性弹性断裂力学（LEFM）是适用的。然后，标准的巴黎关系为短裂纹和长裂纹的增长率提供了上限。短裂纹实验通过有限元计算以两种方式数值再现。第一种分析类型包括实验过程的所有三个阶段：预裂，去除缺口和疲劳增长。第二种分析类型仅重现了疲劳疲劳载荷下短裂纹的增长。在这两种情况下，材料模型都是弹塑性的，同时具有各向同性和运动学硬化相结合。裂纹尖端开口位移范围，裂纹尖端处的循环J积分和环形塑性区之间的一致性（三角形开口K_I）证明LEFM可以扩展到Ti-17中的当前短裂纹。同样，对于目前的短裂纹和循环软化材料，关于塑料区域尺寸的关于LEFM的文献中描述的裂纹尺寸极限仍然成立。

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