Abstract
The determination of residual stresses in engineering materials using sharp indentation testing is studied analytically and numerically. The numerical part of the investigation is based on the finite element method. In particular, the effect from elastic deformations on global indentation properties is discussed in detail. This effect is essential when residual stresses are to be determined based on the change of the contact area due to such stresses. However, standard relations for this purpose are founded on the fact that the material hardness is invariant as regards residual (applied) stresses. Presently, this assumption is scrutinized and it is shown that it is only valid at dominating plastic deformation around the contact region. The hardness dependence of residual stresses can, however, be correlated in the same way as in the case of stress-free materials, indicating that the wealth of characterization formulas pertinent to indentation hardness is available also for the purpose of residual field determination. Only cone indentation of elastic-perfectly plastic materials is considered, but the generality of the results is discussed in some detail.
Highlights
Residual stresses can be a very dangerous feature when it comes to reduction in load-carrying capacity and strength in general
The investigation is based on the finite element method and the material properties, and the residual fields are described by the Johnson (Ref 16, 17) parameter K, in Eq 1 and 6, and the stress ratio rres/ry
The present numerical results based on Eq 14 and 15 are introduced in Fig. 9 and 10, and again, these results indicate that in this case ln K = 3 constitutes an approximate border between level II and level III indentation and that the explicit hardness values fall right on the master curve defined by the stress-free hardness values in Fig. 7, see especially Fig. 10
Summary
Residual stresses can be a very dangerous feature when it comes to reduction in load-carrying capacity and strength in general. Further pertinent investigations include (Ref 3-12) (just to mention a few) introducing more theoretical approaches to the analysis of the mechanics of the problem In most of these studies, progress was made based on the fact that hardness was not affected by residual stresses and the deformation at the contact contour could be directly correlated with the magnitude of the residual (or applied stresses) present in the indented material. This is somewhat surprising as the few pertinent results that exist regarding this matter, cf (Ref 12, 14), indicate that when elastic effects are noticeable, invariance is lost This is not an issue for standard metallic materials, but could be so, for example, for polymers and ceramics where elastic deformations around the contact region are in the same order as the plastic ones. It is the intention of the present paper to investigate this matter in some detail
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