Deformation Relief of the Surface as a Characteristic of Fatigue Damage of Clad Aluminum Alloys. Part 2. Fatigue Degradation of a Cladding Layer

The cladding of aircraft aluminum alloys for their corrosion protection will deteriorate the fatigue strength. This phenomenon can be explained by more intensive plastic deformation and loosening (degradation) of the cladding layer as compared to the base material. A phenomenological model is proposed to describe the degradation that is based on the concept of the change in the cladding layer volume with its surface area due to the formation and evolution of the deformation relief. An increase in the volume may be associated with the accumulation of continuity defects (microcracks, pores) in the surface layer, which initiate the fatigue cracks in the base material. A relative increase in the material volume is characterized by the degradation measure, which is determined by the surface plastic strain and ratio fractal parameter equal to the deformation relief area – their perimeter. Within the proposed model and the change in the cladding layer degradation with the surface saturation a deformation relief clusters was investigated available experimental data. The degradation measure against saturation is described by power, functions and has two sections. The degradation intensity starts growing when the deformation relief clusters occupy approximately 25% of the surface area. The intensification of degradation becomes more pronounced if represented as its measure against the area–perimeter ratio of the deformation relief clusters. It gives grounds to advance the fractal parameter as a diagnostic characteristic of cladding layer damageability under cyclic loading at the stage preceding the fatigue crack nucleation.