Damage tolerance based shape design of a stringer cutout using evolutionary structural optimisation

This paper uses a modified evolutionary structural optimisation (ESO) algorithm for optimal design of a stringer cutout used in many pressured fuselages of transport aircrafts. Both stress and fracture based ESO algorithms are studied. Initially, a stress optimised shape of the stringer cutout is determined. It is found that the ESO method significantly reduces the peak stress on the boundary. A residual strength based optimisation is then performed. The fracture based evolutionary structural optimisation algorithm uses residual strength as the design objective. The formulation used here allows for numerous cracks to be located along the entire structural boundary. The fracture ESO algorithm considerably improves the residual strength by decreasing the maximum stress intensity factor. It also reduces the variability in the stress intensity factors for the optimum shape and produces a near uniform level of fracture criticality around the boundary. It is also shown that the shapes optimised for stress and fracture strength may differ, and a fracture strength based optimisation may produce a lighter design. This highlights the need to explicitly include fracture parameters in the design objective function.