Nucleation and propagation of fatigue cracks in Al-304 stainless steel laminate composite

Abstract

A study has been made of fatigue crack nucleation and propagation in Al-stainless steel (30 vol%) laminate composites. A Paris type power relationship between the crack growth rate, da/dN, and the alternating stress intensity, ΔK, was obtained over the crack growth rates ranging from 10⁻⁷ to 10⁻⁴ mm/cycle, with an exponentm of 2.7. The cracks nucleated first in Al strips and then in stainless steel strips accompanied by some interface decohesion. The fatigue crack propagated in two stages. In the first stage, where the Al-steel interface was largely intact, the crack propagated in a plane strain mode (flat fracture surface with striations, each striation consisting of a cluster of interstriations). In the second stage, where there occurred extensive Al-steel interface delamination and the concomitant loss of mutual constraint, the crack propagated in the plane stress mode (slant fracture with voids). The crack growth was faster in Al than that in steel since the apparent striation spacing was larger in the former than in the latter. No one to one correspondence existed between the apparent striation spacing and the macroscopic crack growth rate.

Thus, although, microscopically, the crack front was not planar; macroscopically, it could be regarded as planar, and a Paris type power relationship did characterize the macroscopic fatigue crack growth in this laminate system over the applied stress amplitude studied. Comparing the fatigue crack growth rates among Al-steel laminate, commercial or pure aluminium and 304 stainless steel, the Al-steel laminate has the lowest crack growth rate. This plus the weight and cost saving benefits make Al-steel laminate quite attractive.