Experimental evaluation of the crush energy absorption of triggered composite sandwich panels under quasi-static edgewise compressive loading

Abstract

An experimental evaluation of the energy absorption of constrained triggered sandwich structures is presented. Four configurations of embedded ply-drop triggering mechanisms were analysed and compared. A composite pi-joint was then developed to provide a constraint fixture that is representative of an in-service integrated energy absorbing structure. The specimens tested within the pi-joints obtained slightly lower specific energy absorption compared to the equivalent specimens tested in a rigid test fixture. The potential of an integrated energy absorbing triggered sandwich structure contained within a composite pi-joint was demonstrated. The interface between the sandwich panel and the pi-joint was not bonded, and further research will focus on the development of a bonded joint configuration suitable for structural applications.

Introduction

The primary structure of the next generation of transport aircraft will be principally fabricated from high performance composite materials. A composite design can tailor the strength and stiffness to the applied loading, which results in a structure with increased specific strength and stiffness compared to an equivalent metallic construction. Modern aerospace platforms include several structural configurations including monolithic structures and sandwich designs. A sandwich construction can produce a structure with increased stability, strength and stiffness with a minimal increase in structural weight. Aerospace sandwich structures are commonly fabricated by bonding a carbon fibre reinforced polymer (CFRP) skin onto a honeycomb or foam core.

With careful design a CFRP crashworthy structure can offer superior specific energy absorption [1] (SEA) compared with an equivalent metallic design. A significant issue associated with energy absorbing composite structures is the tendency to fail catastrophically in a global brittle buckling mode, thereby absorbing a negligible amount of energy. To address this issue, energy absorbing composite structures often include a triggering mechanism to initiate the failure at a specific location and ensure a sustained continuous crushing mode of failure is achieved.

The energy absorption of triggered composite structures has been investigated by a number of authors [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]. Triggering mechanisms which are commonly employed for monolithic plates include chamfers (single or double) [5], [11], [12] or a saw tooth arrangement [5], [11]. A wider variety of triggering mechanisms can be incorporated into closed or open section energy absorbing elements, which are often designed with either a chamfer [2], [4], [8], [13], [14], tulip (or steeple) trigger [2], [13], embedded ply-drop [10], [13], [14] or tapered geometry [7].

The failure and energy absorbing characteristics of sandwich structures have been investigated by Mamalis et al. [15], Velecela et al. [1] and Stapleton and Adams [9]. Mamalis et al. [15] investigated the crushing response of sandwich panels subjected to quasi-static edgewise compressive loading. The specimens investigated as part of their work did not have a triggering mechanism to promote a crushing mode of failure. Each of the sandwich configurations were tested without any lateral support. The unconstrained and un-triggered sandwich specimens exhibited three failure modes a global buckling mode, de-bonding of the sandwich faces (followed by buckling of the faces) and progressive crushing. The specimen which progressively crushed absorbed a significant amount of energy, the specimens which exhibited a buckling failure mode absorbed a minimal amount of energy.

The crushing failure of glass fibre reinforced composite sandwich structures were investigated by Velecela et al. [1]. As a part of their work three configurations of triggered sandwich structures were evaluated. Each of the triggers were machined off the top of the sandwich structure. However this type of triggering mechanism is generally unsuitable for aircraft structures. For maximum efficiency, an aerospace crashworthy structure should also carry the in-service flight loads. A steeple type of triggering mechanism cannot carry in-service flight loads and is unsuitable for aerospace applications because of the extra weight it adds to the aircraft. The test specimens were also supported in a test fixture, which prevented the skins from buckling locally and de-bonding from the core. Therefore, the tests results are not indicative of an unsupported energy absorbing element.

Stapleton and Adams [9] investigated the energy absorption of composite sandwich structures under edgewise compressive loading. Externally applied triggering mechanisms were employed to initiate a crushing failure mode. The energy absorption of the tested concepts showed a large degree of scatter and the crushing load fluctuated significantly over the duration of the crushing stroke without exhibiting a steady-state crushing region. Whilst the experimental investigations conducted by Stapleton and Adams [9] compared the suitability of plug and wedge initiators, these types of triggering mechanisms are difficult to integrate into a vehicle’s crashworthy structure without introducing a significant weight penalty. The externally applied triggering mechanisms do not form a load carrying joint and would be incapable of reacting any in-service flight loads. This makes the externally applied triggering mechanisms, investigated by Stapleton and Adams [9] less efficient than the integrated ply-drop triggering mechanisms reported in this paper.

The primary aims of the experimental test program reported in this paper were to evaluate several embedded ply-drop triggering mechanisms and to develop a structural support that could be integrated into the crash management system of an aerospace platform. Both triggered and un-triggered specimens were analysed and their influence on the energy absorption investigated. As an outcome of this work, an effective composite pi-joint was developed. This type of constraint could be integrated into the subfloor structure of rotorcraft, and is to this extent representative of an integrated energy absorbing structural element. However, the interface between the specimen and the composite pi-joint was not bonded as would be required in a fully representative joint. This experimental investigation explores the potential for this type of structural configuration while omitting the complexity of bonding.