Crack initiation and Extension Characteristics of Ceramic/Metal Joint under Cyclic Loading.

Abstract

Crack initiation and extension characteristics of tension side, view side and compression side of ceramic/metal joints were investigated under cyclic bending loading. Location of cracks and propagations were investigated by SEM and X-ray compositional analysis methods. The stress and strain distribution under cyclic bending loading were investigated by FEM analysis. The results obtained are as follows. (1) Crack initiation occurred at ceramic/interlayer interface at the specimen corner under cyclic bending loading. The unstable fracture occurred from the interface crack. At the 84∼90% of mean strength, crack initiated at ceramic/interlayer interface at the specimen corner. (2) After a main crack occurred many microcracks appeared in order near the main crack region with the increase of maximum stress. Finally they united to the main crack under cyclic loading. The length of crack was increased with increasing maximum stress. The curve slopes of crack lengths versus stresses also increased with increasing maximum stress. Under constant cyclic bending loading, the cracks intermittently grew. (3) At compression side, when the absolute value of maximum stresses were higher than those of tension side, cracks started on the ceramic/interlayer interface and grew up to about 40μm, and finally halted. (4) The crack initiations occurred at the ceramic/brazing filler interface. The location of crack initiation under cyclic bending loading was same as monotonic strength. (5) The crack length increment was decelerated with increasing energy release rate, then increased. The difference of the relationship between crack length increment and energy release rate was less than that of crack length increment and maximum stress. (6) From the FEM analysis, the plastic deformation progressed in interlayer when specimen was loaded and the plastic strain of interlayer left when unloaded. In the interlayer of interface, as compressional plastic strain was constrained by materials around it, tensile residual stresses occured at the compression side at end of unloading. That is why crack initiation occurred in the compression side.