Fatigue crack growth in a Ni–Sn multilayered composite

Fatigue crack growth behavior in a Ni–Sn metallic multilayer composite is investigated using the three-point bend fatigue test because of very limited literature on the fatigue behavior of thin layered composites. The multilayer composites are sandwiched between two thick layers of the monolithic nickel by electroplating in order to fabricate bar-shaped samples suitable for fatigue tests. The bar-shaped sample is notched on one side in the nickel phase, and the primary fatigue crack is propagated towards the multilayers in the normal direction. It is observed for the first time that the primary fatigue crack propagation is effectively arrested when the crack reached the first Sn layer of the Ni–Sn composite. Consequently, the composite exhibited a remarkable fatigue damage tolerance. This unusual phenomenon is caused by the extensive intergranular cracking within the Sn layers after fatigue while the Ni layers remained intact. Consequently, the primary crack is branched by these intergranular cracks in Sn layers which reduced the stress intensity factor at its tip. Thus, the Sn layers acted as a barrier to the primary fatigue crack growth. It is also shown that the intergranular fatigue cracking is an intrinsic behavior of the pure Sn phase.