Effect of Cold Working on Crack Growth from Holes in Fiber Metal Laminates

Cold expansion has proven to be an effective technique for extending the fatigue life of monolithic materials. Although fiber metal laminate materials show improved fatigue performance compared to their monolithic counterparts, the nucleation and growth of small cracks tends to occur early on in the fatigue life of the material, making acceptance of fiber metal laminates in aerospace more difficult. This work examined whether cold expansion could delay this process in fiber metal laminates and increase the fatigue life of these materials at the same time. The results showed that significant differences existed in the residual strain field between the mandrel exit and entry faces resulting in slower fatigue crack growth on the exit face, which also had higher residual strains in the region surrounding the cold expanded hole. Overall, cold expansion is effective at increasing the fatigue life of fiber metal laminate materials, but is less effective at delaying nucleation and growth of small cracks on the entry face.