Characterization of Fatigue with the Field Theory of Deformation and Fracture

We conducted physical experiments with metal specimens undergoing fatigue loading. Using the optical interferometric technique Electronic Speckle-Pattern Interferometry (ESPI) we analyzed the temporal behavior of the displacement pattern formed while the specimen was experiencing cyclic loads. In traditional Fatigue Analysis of aircraft wings, cracks are formed due to oscillatory loads, these cracks propagate through a structure, and a failure occurs when stresses on these cracks are above a material's ultimate strength. ESPI data from our physical experiments indicates that shear instability is related to dislocation dynamics and it can be observed by unstable temporal behavior of the displacement field, leading us to a more formal physical description of fatigue. From the viewpoint of wave dynamics described by the Field theory of Deformation and Fracture we can describe the transition from deformation to fracture in solids. Deriving field equations that govern the displacement field of solids under deformation, we conducted numerical simulations based on our physical experiments. Our hypothesis is that the Field Theory of Deformation and Fracture characterizes fatigue on a fundamental theoretical level.