Fracture Toughness Control in Functionally Gradient Material

东北大学与华盛顿大学在功能梯度材料断裂韧性评价方面的国际合作，在共同研究课题的基础上进行了3年。东北大学负责FGM试件的制作、通过燃烧器加热试验测试热冲击性能，以及通过新设计的重复维克斯压痕技术评估FGM的断裂韧性。另一方面，华盛顿大学参与了对由成分梯度层引起的抗断裂性的精确评估。一种同时使用光学测量和数值计算的非传统混合方法，对氧化锆/不锈钢功能梯度材料进行了燃烧器加热热冲击试验，结果表明，裂纹的萌生取决于热应力的产生和材料内部的缺陷分布。在燃烧器加热试验的冷却过程中，观察到陶瓷表面垂直裂纹的形成，发现梯度层有效地抑制了裂纹的扩展。反复维氏压痕试验为我们提供了方便和快速的评估功能梯度材料的断裂韧性。细观力学和断裂力学的考虑表明，反复维氏压痕法是适合的FGM的断裂韧性的评估，特别是对于薄FGM层。混合方法为梯度材料中裂纹的扩展提供了更为详细的信息，为阐明梯度材料中裂纹的扩展和终止机制提供了一种可行的方法，为梯度材料断裂力学研究提供了基础资料，对梯度材料的研究具有重要的意义。

The international collaborative work between Tohoku University and The University of Washington on the evaluation of fracture toughness of FGMs were performed for three years with sharing research subjects between both contributors. Tohoku University took charge of the fabrication of FGM test pieces, testing of thermal shock property by burner heating test, as well as the valuation of the fracture toughness of FGMs by newly devised repeated Vickers indentation technique. The University of Washington, on the other hand, took part in the precise assessment of the fracture resistance that will be induced by the composition-graded layr. An unconventioanl hybrid approach using simultaneously optical measurement and numerical aclculation.The burner heating thermal shock test on a FGM of zirconia/stainless steel showed that the initiation of cracks depends on the thermal stress generation and on defect distribution within the material. The formation of vertical cracks on the ceramic surface during cooling period of the burner heating test were observed, and the propagation of the cracks was found to be effectively suppressed by the graded layr. The repeated Vickers indentation test provides us easy and fast assessment of the fracture toughness of FGMs. Micromechanical and fracture mechanical considerations showed that the repeated Vickers indentation method is suitable for the assessment of the fracture toughness of FGMs particularly for the thin FGM layrs. The hybrid method provides us the more detailed information on the propagation of the cracks in the graded layr, and will be a viable approach for the clarification of the mechanisms of the crack propagation and its cessation.The present collaborative work has provied fundamanetal information on the fracture mechanics of FGMs, and will contribute much to the progress of the FGM researches.