Abstract
This paper reports an original inverse perforation tests on foam core sandwich panels under impact loading. The key point is the use of an instrumented Hopkinson pressure bar as a perforator and at the same time a measuring device. It aims at a high quality piercing force record during the whole perforation process, which is a weak point of common free-flying projectile-target testing schemes.
This new testing arrangement allows for the measurement of piercing force–displacement curves under quasi-static and impact loadings of sandwich samples, which is made of 40
mm AlSi
7Mg
0.5 Cymat foam cores and 0.8
mm thick 2024 T3 aluminium sheets as top and bottom skins (incident and distal side facesheets). Compared with quasi-static top skin peak loads (the maximal load before the perforation of top skins) obtained under same geometric and clamping conditions, a significant enhancement under impact loading (25%) of the top skin peak load is found. However, the used foam core and skin sheet are known and have been confirmed to be hardly rate sensitive by separate tests on foam cores as well as on the skin sheets.
A possible explanation of these puzzling results is following: the foam core under the perforator was locally more compressed under impact loading because of the inertia effect. As the used foam cores has a quite important strain hardening behaviour, the strength of foam cores before the failure of the top skin is higher than that under quasi-static loading, which leads to an increase of the top skin peak load under impact loading. Tests on a uniformly pre-compressed sandwich sample exhibit indeed higher top skin peak loads, which supports also this aforementioned concept.