Effect of Applied and Residual Stresses on the Analysis of Mechanical Properties by Means of Instrumented Indentation Techniques

Abstract

Various methods have been proposed in recent years for the determination of mechanical properties of a material by using instrumented indentation testing. These load and depth sensing indentation techniques imply the measurement of a characteristic load-indentation depth curve by the aid of which numerous materials properties can be extracted. On the other hand in many publications the effect of applied or residual stresses on the results of hardness readings is investigated. Methods are proposed to estimate applied or residual stresses by means of instrumented indentation testing. Based on this obvious inconsistency between these procedures on the use of information of instrumented hardness testing the influence of residual stresses as well as applied stresses on continuous microhardness readings is systematically investigated for steel samples. Experimental investigations were supplemented by finite element simulations of ball indentation tests on equi-biaxially prestressed materials states. These simulations show that the registered force-indentation depth curves as well as the geometry of the indentations are affected by loading and residual stresses in a characteristic way. For hardness values changes of up to 35% are determined with reference to the unstressed initial state.

In situ study of structural integrity of low transformation temperature (LTT)-welds

“…On the one (1) the high cooling rates observed close to the FZ, (2) an increase in ␣ -phase fraction due to stress induced martensite formation (see Section 3.2) and (3) as a result of martensite formation increased compressive lattice strain (see Section 3.4) counteracting the indentation deformation upon hardness testing (such effects were reported before (e.g. [25]) may cause the increased hardness values towards the FZ in this region). The OHAZ displays the typical softening zone indicating that the local welding temperature exceeded the tempering temperature with the consequence of the observed significant drop in hardness through the entire plate thickness.…”

Instrumented Indentation Testing on Uniaxially Prestressed Steel Samples

“…Furthermore the indentation modulus E IT , which is an estimate for Youngs modulus, is defined in ISO EN 14577 according to The effect of loading and residual stresses on characteristic values determined by depth sensing indentation were examined in detail in [7]. In already published papers [8,9] the effect of loading stresses on the Martens hardness of steel samples subjected to uniaxial testing were presented. The results of the experimental studies were supported by finite element simulations of ball indentation tests on steel samples [9,10].…”

“…In already published papers [8,9] the effect of loading stresses on the Martens hardness of steel samples subjected to uniaxial testing were presented. The results of the experimental studies were supported by finite element simulations of ball indentation tests on steel samples [9,10].…”