Coining Residual Stresses in Aluminium Ship Structures: Experiments and FE- simulations
The build-up and redistribution of residual stresses obtained by stress coining of aluminium plates containing a hole have been studied experimentally and numerically. The foil strain gauge technique has been employed to measure strains at the bore of a hole during and after the coining process. The finite element method has been applied for the computations of displacements, strains and stresses. Both axi-symmetric and fully three-dimensional models were used in the finite element analyses.
The coining process was found to create compressive residual hoop stresses at the bore of the hole with a magnitude primarily depending on the diameter of the mandrel used for the coining. Parameters of secondary importance were found to be the coining depth and the friction coefficient. Good agreement was obtained between the experimentally measured strains and the numerically calculated strains. A slight redistribution of the residual hoop stresses, leading to lower compressive values, at the bore of the hole was observed when the aluminium plates were subjected to a simulated alternating service load.
The possible beneficial effect of coining on the fatigue life has been quantified by calculating the propagation of a quarter elliptical corner crack and a half elliptical central crack at the bore of a stress coined hole. Results for the crack growth obtained from the empirically based and frequently used Paris law were compared with results from a more complex relation which also models the observed crack growth near the threshold value. Pertinent stress intensity factors were derived by the use of the weight function technique. A significant reduction of the crack propagation rate was found for quarter elliptical corner cracks growing from a coined hole.
The application of the coining technique is demonstrated on a typical ship structure. The finite element method was used to calculate the residual stresses at the corner of a longitudinal stiffener cut-out in a web-frame after fillet welding of the web-frame to the ship hull plating and after the coining process. It was found that the tensile residual welding stresses at the cut-out were reduced to almost zero by the coining process. Engineering estimations show that growth of edge cracks at the corner of the cut out will be reduced.