On the Behavior of Mechanical Stress Fields at Indentation of Materials with Residual Stresses

It is an obvious fact that residual stresses can have a detrimental effect on the mechanical integrity of structures. Measuring such stresses can often be a tedious task and for that reason sharp indentation testing has been proposed as an alternative for this purpose. Correlation between global indentation properties and residual stresses has been studied quite frequently, and a solid foundation has been laid down concerning this issue. Empirical, or semi-empirical, relations have been proposed yielding results of quite good accuracy. Further progress and mechanical understanding regarding this matter will require a more in-depth understanding of the field variables at this particular indentation problem and this is the subject of the present study. In doing so, finite element simulations are performed of sharp indentation of materials with and without residual stresses. Classical Mises plasticity and conical indentation are considered. The main conclusion from this study is that the development of stresses in materials with high or medium-sized compressive residual stresses differs substantially from a situation with tensile residual stresses, both as regards the level of elastic deformation in the contact region and the sensitivity of such stresses. Any attempt to include such stress states in a general correlation effort of indentation quantities is therefore highly unlikely to be successful.

Correlation of Global Quantities at Material Characterization of Pressure-Sensitive Materials Using Sharp Indentation Testing

Correlation of sharp indentation problems is examined theoretically and numerically. The analysis focuses on elastic-plastic pressure-sensitive materials and especially the case when the local plastic zone is so large that elastic effects on the mean contact pressure will be small or negligible as is the case for engineering metals and alloys. The results from the theoretical analysis indicate that the effect from pressure-sensitivity and plastic strain-hardening are separable at correlation of hardness values. This is confirmed using finite element methods and closed-form formulas are presented representing a pressure-sensitive counterpart to the Tabor formula at von Mises plasticity. The situation for the relative contact area is more complicated as also discussed.

Geometrically Necessary Dislocations on Plastic Deformation of Polycrystalline TRIP Steel

In this study, the main deformation behavior in terms of geometrically necessary dislocations (GND) was investigated on a transformation induced plasticity (TRIP) stainless steel by using sharp indentation at nanometric length scale. Results evidence that austenitic grains display an isotropic behavior on terms of GND, the main deformation mechanism being the Frank–Read source activated at local level.