Selection of Densification Strain to Predict Dynamic Crushing Stress at High Impact Velocity of ALPORAS Aluminium Foam

Abstract

Cellular material such as aluminium foam has been considered as a potential material for energy absorption upon impact and blast loadings. One of the most important properties that contribute to this feature is the densification strain. At high impact velocity, prediction of the densification strain from quasi-static engineering stress-strain curve has been found inadequate. Furthermore, theoretical prediction using the equation proposed by Reid et al. always over-predicts the dynamic crushing stress. Formation of the shock wave at high impact velocity is believed to further increase the densification level of the foam. However, this effect is disregarded when determining the densification strain quasi-statically. The present study aims to address this issue by determining the densification strain experimentally from impact tests. Forty cylindrical aluminium foams with three different lengths were used as projectiles and were fired towards a rigid load cell by using a gas gun. The peak forces generated from the impact were recorded and analysed. The experimental densification strains were determined physically by measuring the deformation of the foam projectiles after the tests. It is concluded that, at high impact velocity, the densification strain varies with the initial impact velocity. Therefore an appropriate value of densification strain needs to be used in the equation of dynamic crushing stress for a better approximation.

Finite Element Analysis on a Newly-Modified Method for the Taylor Impact Test to Measure the Stress-Strain Curve by the Only Single Test Using Pure Aluminum

“…Here, the parameter C indicates the exponent of strain rate sensitivity. This model is quite similar to the Johnson-Cook model expressed in Equation (6). The difference is the second bracket on the right-hand side to show the rate sensitivity.…”

“…As a result of the test, maximum strain rates as high as 10 3 -10 5 s −1 can be easily obtained. The test is applicable to a wide variety of materials such as metals [2], polymers [3,4], foams [5,6] etc. The test was originally used for measuring the impact yield strength, and the fairly reliable value of the strength can be obtained.…”

“…In addition, these formulae only considering the deformation of the specimen, the external force is not measured and questions about the accuracy of the measurement results arise. From this point of view, Lopatnikov et al [5] and Yahaya et al [6] attempted to use measuring devices such as the Hopkinson pressure bar and a load cell made of quartz, with high accuracy and band width, by replacing the rigid wall. However, this was not applied to measuring the stress-strain curve by just one single number of the test, although the time history of the external force could be obtained.…”