The Collapsing Mechanism of Aluminum Foams

The response of metallic foams to a high strain or high stress rate loading has received increased attention in recent years due to their potential to absorb large amounts of energy during plastic deformation and crushing (see, e.g., Thornton and Magee [1]). Research of the mitigation of blast effects indicates that the high-energy absorption characteristic of metallic foams makes them very useful as protective layers of critical structural elements. Consequently, understanding the material dynamic properties of metallic foams will enable engineers to better utilize their energy absorption characteristics. Aluminum foam (Al-foam) is a lightweight material with excellent plastic energy absorbing characteristics [2]. The implementation of bare Al-foam as a protective layer is not practical. The material needs to be a part of a multilayer structure (see, e.g., Seitzberger et al. [3]. The foam layer can be exploited as a protective layer in military vehicles where both lightweight and good energy absorption are needed. The ability of aluminum foams to reduce the explosion-generated blast-induced damage from concrete slabs has been demonstrated in the course of high-explosive (HE) field experiments (see, e.g., Hanssen et al. [4, 5] and Sadot et al. [6]). Several studies have been conducted in order to investigate the constitutive model parameters of Al-foams during the past three decades. The effect of the strain rate was one of the important issues needed to be resolved [7]. A constitutive numerical model was validated in the work of Hanssen et al. [4, 5]. The validation procedure was based on three levels: (a) material calibration; (b) non-uniformed compression test at the material level; and (c) numerical validation at the structural interaction level. Several material models from the LS-DYNA library were calibrated. However, discrepancies between the models were found even for relatively simple load configurations. The most important conclusion noted by the authors was the need for further development of more robust fracture models for the Al-foam. This conclusion is crucial especially since there are increasing numbers of Al-foam manufacturers. Various experimental facilities were used to dynamically load the Al-foam at large ranges of strain and stress rates. In the work of Dannemann and Lankford Jr. [8], closed-cell Al-foams were assessed under static and dynamic loads in the strain rate range of 400–2500 s⁻¹. This range was achieved by using split Hopkinson bar apparatuses. It was found that the strain rate effect is significant in high density Al-foams. Deshpande and Fleck [9] suggested that the initial elastic modulus was lower than that of fully dense alloys. Deformation in the cell walls led to stress concentration around the deformation zones, which resulted in a decrease of the modulus. Some inconclusive results regarding the dependence of the stress–strain curve on the strain rate were presented. Deshpande and Fleck [9] and Paul and Ramamurty [10] did not notice any strain rate dependency, in contrast to the findings of Dannemann and Lankford [8] and Paul and Ramamurty [10]. In a later work by Wang et al. [11], experiments were done using an Instron compression machine at strain rates ranging from 10⁻³ s⁻¹ up to 450 s⁻¹. Strain–stress curves constructed and distinct strain rate dependency was noted. In the work of Bastawros et al. [12] efforts were made to understand the morphology of the Al-foam during its collapse. Explanation was given to the cell deformation. However, some observations have been ignored and not fully explained even though some key elements of the deformation were identified. The dynamic behavior of Hydro/Cymat Al-foam material was investigated by Tan et al. [13] under different load conditions. The plastic collapse, the plateau range, and the strain at which the deformation occurred were found. It was demonstrated that the dynamic response depends on the direction of the load with respect to the plate manufacturing orientation. Some load enhancement was observed and was explained by micro-inertial effects. Postimpact observation of partly crushed specimens revealed that the deformation is through crush bends. Feng et al. [14] conducted experiments to investigate the rate dependence of Al-foams having different relative densities. They found that the effect of strain rate increases while increasing the Al-foam density as was found by others. For Al-foams with a relative density of 15 % there was little effect of the strain rate while for heaver foams significant strain rate effect was noted.