3D characterization of internal fatigue cracks initiation and propagation in a Ti alloy during gigacycle fatigue

When submitted to a very large number of mechanical cycles (~ 1e9 cycles) at very low load levels, structural components generally fail because of the apparition of internal fatigue cracks. Experimental data on their growth rate is scarce but X-ray tomography appears to be the adequate technique to detect their initiation and to follow their propagation in metals. For that reason, a new in situ ultrasonic fatigue machine has been developed to detect and monitor the evolution of internal cracks in a cast-aluminum thanks to X-ray microtomography and infrared thermography. However, because of the material large grain size, the observed fatigue cracks possessed complex 3D shapes which were extremely challenging to model. Consequently, because this type of experiment remains the only one that can provide reliable experimental data on internal fatigue crack propagation, this proposal aims at using the same setup on Ti alloy specimens with a larger grain size to obtain smoother crack fronts.当结构部件在极低载荷水平下承受极大量机械循环（约10^9次循环）时，通常会因内部疲劳裂纹的萌生而失效。目前关于疲劳裂纹扩展速率的实验数据较为匮乏，但X射线断层扫描（X-ray tomography）似乎是检测金属内部疲劳裂纹萌生并追踪其扩展过程的合适技术。鉴于此，研究人员开发了一款新型原位超声疲劳试验机（in situ ultrasonic fatigue machine），可借助X射线显微断层扫描（X-ray microtomography）与红外热成像（infrared thermography）技术，检测并监测铸铝试样内部裂纹的演化过程。然而，由于该材料晶粒尺寸较大，观测到的疲劳裂纹呈现出复杂的三维形态，这给建模工作带来了极大挑战。因此，鉴于此类实验仍是获取内部疲劳裂纹扩展可靠实验数据的唯一途径，本项目拟采用相同的试验装置，对晶粒尺寸更大的钛合金（Ti alloy）试样开展试验，以获得更为平滑的裂纹前沿。