Effect of strain rate on serrated load of indentation in Al-Mg alloy

Abstract

Indentation tests are used to determine the local mechanical properties of materials. Previously, the indentation strain rate was correlated with the strain rate in uniaxial tests based on the hardness, which was the obtained load divided by the cross-sectional area. However, the hardness can be influenced by pile-up of material after indentation. The purpose of this study was to relate the indentation strain rate with the uniaxial strain rate through serration behavior. The material used in this study was 5082 aluminum alloy, whose main alloying elements are aluminum and magnesium, and which is known to exhibit serration at certain temperatures and strain rates. Quasi-static uniaxial tensile tests were performed at strain rates from 10 -4 to 10 -1 s -1 at room temperature. Micro-indentation using a Berkovich indenter was performed at constant loading rates from 0.7 to 350 mN/s. The loading curvature, which was defined as the load divided by the square of the displacement, was used instead of the hardness to avoid the pile-up effect. As a result, the serrated loading curvature in the indentation tests was obtained as the decreasing loading rate. The effective strain rate, which was defined as the derivative of the load with respect to time divided by two times the applied load, decreased with increasing displacement. The serrated loading curvature changed its behavior as the effective strain rate decreased. It behaved similarly to the serration observed in uniaxial tensile tests. It was found that the indentation strain rate is correlated with the strain rate in uniaxial tensile tests through the serration behavior.

Effects of Testing Temperature on the Serration Behavior of an Al–Zn–Mg–Cu Alloy with Natural and Artificial Aging in Sharp Indentation

“…Therefore, there is a need to adopt such tests as an indentation test that can non-destructively evaluate the strength of a small area with accuracy comparable to that of uniaxial tests. In this study, we extend our previous work [8,18] to clarify the effects of testing temperature on the serration behavior during indentation tests. The microstructural changes in the Al-Zn-Mg-Cu alloy (7075 aluminum alloy) due to natural and artificial aging were employed [20].…”

“…where ε u is the strain rate and ε u is the strain during the uniaxial test. However, present authors [8] have, through experiment, proved that serration phenomena can be evaluated through indentation tests. Through continuous measurement of the load and the corresponding displacement, while loading and unloading in an indentation test, some mechanical properties that cannot be obtained through hardness tests could possibly be evaluated [9].…”

“…The effective strain rates for the indentations were calculated using Equation ( 6). Herein, β = 0.1 was used, based on previous studies [8,18]; the effective strain rate-displacement relationship is shown in Figure 6. The effective strain rate under displacement rate control, given by Equation ( 6), decreased as the displacement increased.…”

“…The classifications of serrations are briefly summarized below [7]. Figure 1 shows an example of variations in the serration behavior with a change in strain rate [8]. It shows an A-type stress fluctuation that repeatedly rises and falls (the wavy stress fluctuation at a relatively high strain rate, ε A ) and B-type saw-tooth-like stress fluctuations generated at intermediate strain rate (ε B ，ε A+B ).…”

“…Previous studies [8,18] indicated that the serration phenomenon in indentation could possibly be evaluated using the concept of effective strain rate. Indentation is performed using a sharp indenter, hence, a complicated deformation field is generated in the test material, whose deformation mechanism has only recently been clarified [19].…”