Residual Stress, Shakedown and Failure in Carburized Hoisting Load Chain

Electric chain hoists lift their loads by passing carburized chain over a rotating load sprocket. A link passing over the load sprocket experiences relative motion with respect to adjacent links, giving rise to link/link interface frictional tractions that do not occur in those links not passing over the load sprocket. This study, largely conducted with finite element analysis, has focused on the contact stresses in both instances. Yield was assumed to obey the von Mises criterion, an associated flow rule employed and material hardening was taken to be isotropic. Factors complicating the analysis are multiple element groups representing the hardened outer case and the tough core, nonlinear material behavior and the contact itself. Validation of the finite element model was achieved with a convergence study and by comparing classical elasticity solutions to results obtained from analysis of linearly elastic materials in normal contact. Results of the frictionless problem with multiple nonlinear element groups show the peak von Mises stress to occur along the load line beneath the surface in the carburized case. However, yield was only observed in the core. By comparison, when friction was introduced into the problem the peak von Mises stress was found at the surface in front of the load line within the contact patch. Yield occurred in both the case and the core. However, merely exceeding the yield strength after one load application does not mean the design is poor or the chain is unusable. That issue can be resolved by examining the stress states after several load/unload cycles to explore shakedown. It was found that in both frictionless and tractive contact a state of elastic shakedown was achieved after one load cycle. In both instances this was attributed to the development of a system of protective residual stresses which arose in the vicinity of the case/core interface. Superposition integrals which allow calculation of all contact stress components in response to both normal and tangential tractions, given those tractions and the contact patch dimensions, have been developed from the work of Boussinesq and Cerruti. Agreement of the integrals with Sackfield's solution was quite good. A correction was also made in the analytical expression for txy in response to a concentrated normal force.