Finite Element Modeling of Part Distortion

Airframe designs for modern commercial and military aircraft contain hundreds of monolithic metal structural components machined from solid plate or forgings of aluminum and titanium. The start-to-finish weight ratio for these components is easily 20:1 and greater, meaning that more than 95% of the initial material is machined away. The resulting thin-walled structure often distorts due to the removal of bulk material stresses and the application of machining-induced stresses. To alleviate the distortion, current practices rely on taking small depths of cut and flipping the part several times. This results in long cycle times, high cost and under utilization of large machining equipment. A finite element model has been developed specifically to predict and control these distortions. The model takes into consideration the machining induced residual stresses as well as the bulk stresses in the material from the manufacturer. It is the combination of these two residual stress components coupled with the geometry of the part and type of material that allows us to accurately predict, manage and control the shape and magnitude of deformation in large thin walled structures. The model can be used to determine the ideal cutting conditions for the process as well as determine the best location within the stock plate to machine the part from. Multiple machining tests have been done in order to validate the accuracy of the model.