The Hertz contact theory allows the onset of yielding to be predicted for those contacts in which the effect of adhesion can be neglected. However in microscale contacts, such as those which occur in MEMS, yielding will occur for lower loads than predicted by the Hertz theory. For such cases, the JKR, DMT, and Greenwood-Johnson theories extend the Hertz theory to include the effect of adhesion. The present study provides yield conditions for the JKR, DMT, and Greenwood-Johnson theories of adhesion. Attention is first focused on the initiation of yield along the axis of symmetry of the contact. The results show that the critical loads for the three adhesion theories are close together, but differ significantly from that predicted by Hertz. In fact it is possible for yielding to occur due to adhesion alone, without an external applied load. A curve-fit formula is given to express the yield load as a function of an adhesion parameter for different Poisson's ratios. Results are also obtained for the onset of plastic deformation away from the axis of symmetry using the Greenwood-Johnson theory of adhesion.
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