Mechanics of fatigue crack growth in a bonding interface

Experimental investigations have shown that cracks in a bonding interface such as adhesives or brazings under cyclic loading behave fundamentally different from cracks in homogeneous metals. In particular the slope of da/dN-vs.-IK-curves is much steeper and the threshold value higher than in homogeneous metals. In the present paper, the general case of a fatigue crack located in a thin layer of elasticaplastic material embedded in an elastic surrounding is considered analytically. Since the layer is thin compared to the size of the plastic zone, linear-elastic fracture mechanics is not applicable. Therefore Parisa law is formulated in terms of CTOD. It was found that a non-linear shielding mechanism reduces CTOD of a crack in the layer compared to a crack in bulk material under the same stress intensity. The shielding decreases with increasing stress intensity factor. This effect leads to the steep da/dN curves. Based on the hypothesis of a linear damage accumulation in the active plastic zone, an analytical formula is derived to estimate the fatigue crack growth rate as well as the threshold values of cracks in thin interlayers. The predictions agree well with the experimental data.