The role of adhesion in contact fatigue

By incorporating the effects of interfacial adhesion in the mechanics of ro unded contact between two bodies, a new approach is proposed for the quanti tative analysis of a wide variety of contact fatigue situations involving c yclic normal, tangential or torsional loading. In this method, conditions o f "strong" and "weak" adhesion are identified by relating contact mechanics and fracture mechanics theories. Invoking the notion that for strong and w eak adhesive contact, a square-root stress singularity exists at the rounde d contact edge or at the stick-slip boundary, respectively, mode I, II or I II stress intensity factors are obtained for normal, sliding and torsional contact loading, accordingly. A comparison of the cyclic variations in loca l stress intensity factors with the threshold stress intensity factor range for the onset of fatigue crack growth then provides critical conditions fo r crack initiation in contact fatigue. It is shown that the location of cra ck initiation within the contact area and the initial direction of crack gr owth from the contact surface into the substrate can be quantitatively dete rmined by this approach. This method obviates the need for the assumption o f an artifical length scale, i.e. the initial crack size, in the use of kno wn fracture mechanics concepts for the analyses of complex contact fatigue situations involving rounded contact edges. Predictions of the present appr oach are compared with a wide variety of experimental observations. (C) 199 9 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights rese rved.