An Experiment-Based Model to Determine Eigenstrain in Fibre Metal Laminates Induced by Laser Peen Forming

Fibre metal laminates are new sorts of composite materials with superior fatigue and impact properties. Forming parts from FMLs is very attractive to reduce the process cycle and labor cost. Laser peen forming (LPF) is a promising method to form FMLs with its design flexibility and adaptability to produce complex shapes. To predict the deformed shape of FMLs after laser peen forming, eigenstrain-based modelling is a helpful method with advantage in spending less computational time, but it is difficult to precisely determine eigenstrain distribution regarding material with layered structure. In the present research, an efficient and effective experiment-based method is proposed to determine eigenstrain in the metal layer of FMLs, which can avoid the complicated numerical simulation. The determined eigenstrain is assumed to be uniformly distributed in each metal layer while no plastic deformation is generated in composite layer due to its high strength yield. An analytical model is developed to relate the bending deformation to eigenstrain field. Firstly, the equivalent external bending moment applied on the samples is deduced from eigenstrain, and then the relation of deformation and bending moment is provided by a beam bending model. Chemical etching is utilized to remove metal layer by layer to calculate eigenstrain in each layer. With removal of each metal layer, the bending profiles will spring back or bend further due to change of applied moment and bending stiffness. The eigenstrain in each layer can be inversely determined by matching the residual bending profile after each etching with the developed bending model. One scanning strategy of LPF with reciprocating line and 50% overlapping rate is performed in experiments to apply laser shocks on the entire top surface of strip samples, which are prepared from glass laminate aluminum reinforced epoxy with a unidirectional orientation. Eigenstrain in the top and bottom metal layer is successively determined by the proposed analytical model and chemical etching. Finite element analysis is utilized to verify the determined eigenstrain by comparing the simulated shape with experiments and good agreement is obtained.