Full-scale analysis of deformation and stress distribution for constrained composite bearing elements under compressive yielding conditions

Abstract

Five hundred composite pads with nominal diameter 250 mm and thickness 40 mm are used as bearing elements in a redesigned ball-joint, maximum loaded at 150 MPa under normal working conditions. Above their yield strength, softening of the top surface is favourable for smooth sliding while the dimensional stability should be controlled by reinforcements. Pads are therefore incorporated into separate steel holes and the top surface is additionally stabilised by a carbon fibre/epoxy ring. A “local” study on a single bearing element by full-scale static deformation and creep tests with an evaluation of stresses and deformation by finite element simulations is presented in this paper. Yielding was observed above 50 MPa, with consequent increase in stiffness due to the constraining action of the steel sample holder. Long time creep at 60–150 MPa is stabilised after total constraint, with the composite pad under hydrostatic stress conditions. Finite element modelling is used for short-time and long-time deformation as a function of different pad geometries (thickness, diameter and fixation methods) and Young’s moduli. Mainly deformation of a polymer lip at the top surface and extrusion near circumferential polymer grooves, containing a rubber ring for axial fixation, is studied. The contact between a convex counterface and the polymer top surface is modelled, and full contact occurs when loaded above 50 MPa, showing a complex interaction between immediate elastic deformation, long-term creep and variable contact geometry as reflected in the stress–strain characteristics. Under high loads, there is a transition in bulk modulus implied by the test geometry. Finally, the strength of the composite pads and the steel constraining walls is verified.