Analysis of Crushing Response of Composite Crashworthy Structures

Abstract

The increase usage of polymer composite materials in the aerospace and automotive industry has generated considerable interest in using composite materials for crashworthy structures that absorb impact energy through a controlled failure in progressive crushing. By tailoring the fiber type, matrix type, fiber-matrix interface, fiber stacking sequence and fiber orientation, composite crashworthy structures have been shown to have excellent energy absorption performance characteristics. To understand the energy absorption and failure mechanisms of crashworthy structures, DLR has developed a chamfered tube segment specimen, which is easy to fabricate and gives reproducible axial crush failures under quasistatic and dynamic loading conditions. This may be used for screening of different energy absorbing composite materials and provides design data for crashworthy design studies, see [1].The paper describes a successful methodology for crush testing of chamfered tube segment specimens in both quasi-static and dynamic loading conditions with an analysis method to compare crushing characteristics and energy absorption performance. The specimen is a flanged circular segment which is self-stabilising without edge supports and when triggered fails by steady crushing without buckling, as seen in Fig. 1. The specimens are easily fabricated in open moulds and are clamped in the test at their base, which avoids special support fixtures used in similar tests with flat plate specimens. A test programme is described [2] which compares quasi-static crush behaviour with tests carried out at impact velocities of 2 -10 m/s in a high rate servo-hydraulic test machine. Tests on carbon fibre fabric/epoxy test specimens are presented in detail, with particular emphasis on the influence of loading rate on energy absorbed and crush failure mechanisms. The testing methodology included the use of a high speed camera to capture the crushing behaviour of the specimen during the crush tests. In addition to capturing the crushing behaviour of the specimen during the crush tests, Highresolution Computed Tomography (HRCT) scanning of the specimen was performed to enable a detailed analysis of crushed specimen using the nanotom® CT system by phoneix‫|‬xray. This permits detailed observation of the composites failure surface by X-ray without the need for sectioning and microscopic analysis. The quasi-static crush specimen exhibited the most favourable energy absorption performances in terms of Crush force

Crush responses of composite cylinder under quasi-static and dynamic loading

“…In contrast, many authors [7,8,18,24] tested only one specimen per test configuration. The observed damage modes in the present study were also compared against the literature.…”

“…Other more complex geometries have also been investigated, including C sections [17,18] and I sections [15]. Triggering has been shown to be an important aspect of energy absorbent structure design, with chamfering on the loading surface being most common [19].…”

Design and testing of crashworthy aerospace composite components

“…DLR has developed chamfered tube segment specimens, see [28], which are easier to fabricate than tubes, are self-stabilising without lateral supports and give reproducible axial crush failures under both quasi-static and dynamic loading conditions. Quasi-static tests on the chamfered tube segment specimens are discussed here, and dynamic crush tests are described further in [39]. The test specimens consist of a half-circle cylindrical segment with flanges at the longitudinal free edges to inhibit local buckling in axial compression.…”

Cohesive zone models and impact damage predictions for composite structures

“…David and Johnson considered thin-walled open sections consisting of a half circle and two flanges experimentally [353]. The numerical simulation of these test involved the finite element modeling of the composite profile using the stackedshell approach with cohesive interface elements [354].…”