The effect of strain rate on the mechanical properties of AA5xxx series aluminum alloys containing solute Mg atoms (AA5005, AA5021, AA5082 and AA5182) and pure aluminum (A1070) was investigated within a wide strain rate range of 1.0 × 10−4 to 1.0 × 103 s−1 at room temperature. The A1070 exhibited a positive strain rate dependence of material strength at the investigated strain rates. However, the AA5xxx series aluminum alloys primarily exhibited the negative strain rate dependence of material strength and serration caused by the Portevin-Le Chatelier effect on the Mg content and strain rate. As a result of using the material constitutive equation for the negative strain rate dependence, it was found that the flow stress may change in the dynamic strain rate range. However, it was found that the strain rate dependence of material strength differed in the AA5082 and the AA5182 alloys. It would be caused by less solute Mg of the Al phase in the AA5182 alloy than in the AA5082 alloy, because more Mg2Si compounds precipitated on Mn bearing particles as precipitation sites in the AA5182 alloy.

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.

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The effect of strain rate on mechanical properties of Al-2.3wt.%Mg alloy (AA5021) and commercial pure aluminum (purity 99.7wt.%: A1070) was investigated at room temperature. The tensile tests were conducted at strain rates from 1.0×10−4 to 1.0×103 s−1. The universal testing machine was used for strain rate 1.0×10-4 to 1.0×10−1 s−1. For the strain rate 1.0×100 s-1, the servohydraulic testing machine, which was developed by our laboratory, was used. The impact strain rate 1.0×103 s−1 was obtained using the split Hopkinson pressure bar method. The pure aluminum showed positive strain rate dependence of material strength at the investigated strain rates. In contrast, the Al-2.3wt.%Mg alloy showed the negative strain rate dependence at strain rates from 1.0×10−4 to 1.0×100 s−1. However, Al-2.3wt.%Mg alloy showed the positive strain rate dependence at strain rates from 1.0×100 to 1.0×103 s−1. It was surmised that the effect of dislocation locking by the solute Mg atoms became negligible at strain rate of approximately 1.0×100 s−1. It was confirmed that material properties for the Al-Mg alloy at the strain rate of 1.0×100 s−1 were important, since the strain rate dependence changed negative to positive around this strain rate.

Abstract. Indentation is widely used to investigate the elastic and plastic properties of mechanical materials, which includes the strain rate sensitivity. The indentation exhibits an inhomogeneous strain distribution in contrast to compression and tensile tests with homogeneous deformation. Thus, the strain rate of the indentation may form the inhomogeneous distribution. Therefore, the effect of strain rate distribution of the indentation on pure aluminum with respect to the strain rate dependence of strength in order to clarify the effect of the strain rate on the indentation technique. First, the numerical simulation was established using the Cowper-Symonds equation as the dynamic constitutive equation. Secondary, the strain rate distribution was calculated from the equivalent plastic strain distribution. The strain rate distribution was quite different from the strain distribution, which showed that the strain rate at the crater rim was higher than that beneath the indenter. Finally, we try to perform the averaging of strain rate distribution in order to make an index of strain rate in the indentation. The average of strain rate distribution was calculated using the equivalent plastic strain above a boundary value that is the critical strain and the representative strain. There is correlation between the average strain rate and the loading curvature, which shows that the average strain rate can express as the representative of strain rate for the indentation technique.

Strain rate effect of strength is a crucial factor for material characterization. Attempts have been made to evaluate strain rate effect by indentation tests. An indentation causes a non-uniform stress and strain field inside a specimen. This must induce a non-uniform strain rate field. However, little has been reported about strain rate distribution beneath the indenter. So far, various indenter control methods have been used. In previous studies, no direct comparisons were available as to how strain rate distribution was affected by different control methods. In this study, we report on the strain rate effect of indentation with two indenter control methods: constant loading rate (CLR) and constant indentation strain rate (CISR). The finite element method was designed to reproduce deformation caused by a conical indenter of a half apex angle of 70.3°. Pure aluminum (99.999 mass% purity), which showed high strain rate dependence of strength, was chosen as a specimen. Material properties were obtained from low strain rate (10 −4 , 10 −2 /s) to high strain rate (10 2 /s) tests, and results were incorporated into a FEM analysis using the Cowper-Symonds equation. Four constant loading rates (from 0.7 to 350 mN/s) and constant indentation strain rates (from 0.006 to 6/s) were used, and both results were compared. Differences between both indenter control methods were displacement-dependent. Loading curvature, which has been defined as a material constant in the indentation, was calculated from load divided by square of displacement. Although loading curvatures were decreased with increasing displacement for CLR, they were constant for CISR. Results also showed that values of strain rate decreased as displacement increased for CLR, whereas they were the same for CISR. Similarities of both indenter control methods were found as follows. The highest strain rate regions were observed at the edge of the indenter. In addition, higher strain rate region was distributed hemispherically from the edge of the indenter.

Indentation is widely used to evaluate mechanical properties of structural materials. It has been known that a measured hardness value increases with increase of the indentation strain rate. By using this inconvenient phenomenon, a new method for determining of constants for Cowper-Symonds constitutive equation by using a sharp indentation is proposed in this research. The first report evaluates the influence of strain rate on strain field, and the later report will propose the determining method based on the principle which is shown in the first report. The first report shows that a finite element model with Cowper-Symonds constitutive equation can simulate the strain rate effect such as a load increase with strain rate increase and a penetration increase during a holding time, and then the authors investigate on several parameters which may affect indentation results for a material, which shows dependence of strain rate. The investigated parameters are a critical strain, an indenter velocity, an indenter angle and a roundness of indenter tip. The obtained results are the following; the maximum strain rate appears near indentation crater rim and the strain value at this area is close to the value of the critical strain which describes important strain region for indentation load. A sharper indenter angle produces higher strain rate and makes indentation load higher due to strain rate effect. The roundness of indenter tip decreases the strain rate under the indenter, but an error caused by the roundness of indenter tip can be easily corrected with the conventional technique.

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