Thermal Fatigue Cracking and Extension Behaviors of Squeeze Infiltrated Al18B4O33/Mg Functionally Graded Materials

This paper presents the experimental and analytical investigation on the thermal fatigue fracture mechanism of Al18B4O33/Mg systems in the temperature range of 150–350 � C. Three types of functionally graded Al18B4O33/Mg composites which consisted of 2, 3 and 4 layers and where volume fractions of Al18B4O33 were gradually changing from 0 to 35% were fabricated using graded preform and squeeze infiltration process. A simple quenching method of thermal shock test was used to simulate the operating state of the automotive composite cylinder liner system. On the other hand, thermal stress intensity factors in FGMs were obtained by the ANSYS finite element code. Through the study of the microstructure in thermal fatigue test, it was found that the thermally induced cracks appeared in two forms, vertical crack and interfacial crack. By the comparison between numerical and experimental results, the vertical cracks considered to be generated by the mode I stress intensity due to the coupled effect of thermal stress and temperature gradient in FGM systems. The depth of the vertical cracks may correspond to a location of mode I stress intensity being equal to the fracture strength of the FGMs. The interfacial cracks were fatigue cracks which were initiated and extended by thermal loads in mixed I and II mode. Both the mode I and II stress intensities were largely reduced with the number of FGM layer. As the stress intensity decreased, the thermal fatigue resistance of Al18B4O33/Mg system was clearly increased with the number of the layer.