Elsevier

Composite Structures

Volume 118, December 2014, Pages 423-431
Composite Structures

Static and dynamic evaluation of a multifunctional crashworthy pi-joint/sandwich energy absorbing structure

https://doi.org/10.1016/j.compstruct.2014.07.023Get rights and content

Abstract

Experimental evaluation of a multifunctional crashworthy pi-joint/sandwich structure is described herein. The energy absorbing sandwich webs were bonded into a representative pi-joint, thereby, producing a load carrying joint that also functions as a constraint during crushing. Ply-drop triggers were incorporated into both face sheets of the sandwich web to initiate a controlled failure leading to a progressive crushing failure mode. The energy absorbing structure was evaluated under quasi-static and dynamic loading rates. Stable progressive crushing was observed for all specimens, demonstrating the robustness of the developed configuration. The steady state crushing load recorded during dynamic testing was 11.5% lower than the same configuration evaluated quasi-statically. The reduction in steady state crushing load was attributed to a change in the failure mode (brittle fracture) compared to the lamina bending (splaying) failure mode observed during quasi-static testing. The consistency of the results demonstrate that an energy absorbing pi-joint/sandwich structure, such as the configuration described in this paper, has the potential to be integrated into the crashworthy subfloor of future helicopters and aircraft.

Introduction

Modern aerospace structures are increasingly fabricated from high performance composite materials. Historically aerospace structures have been assembled using mechanical fasteners. The use of load carrying bonded joints offers enormous potential for cost and weight savings in aircraft structures, provided the challenges associated with meeting structural performance requirements, manufacturing robustness and non-destructive inspection can be met. Recently, a European framework project MOJO ‘Modular Joints for Aircraft Composite Structures’ developed a number of bonded joint configurations including ’Pi’ ‘T’ and ‘H’ profiles. An idealised representation of a pi-joint is shown in Fig. 1 [1].

As part of a collaborative project between NASA, the US Air Force Research Laboratory, Lockheed Martin and Northrop Grumman, a three-dimensional woven preform (pi-joint) was developed as part of the F-35 Joint Strike Fighter Program. The joint, shown in Fig. 2 [2], consists of a composite sandwich panel integrated into a woven joint which was consolidated in a single cure cycle. Integrating these types of structural joints into the subfloor of future aerospace platforms presents the designer with unique challenges, particularly those associated with meeting the certification requirements for crashworthiness. The progressive failure and energy absorption mechanisms of these types of structures must be understood before the cost and weight savings of this configuration can be fully realised.

A significant amount of research investigating energy absorbing composite structures has been performed over the last 25 years with the majority of this work focussing on monolithic structures. Only a limited number of researchers have investigated the crushing response of sandwich structures [3], [4], [5], [6], [7], [8], [9]. These investigations have either analysed unconstrained specimens [3], [4], [7], [8] or used a constraint [5], [6] which would be impractical to integrate into a crashworthy aerospace subfloor structure.

The failure of unconstrained sandwich specimens was investigated by Mamalis et al. [3] The unconstrained specimens failed in one of three failure modes; Mode I, unstable sandwich column buckling with core shear failure, Mode II, unstable sandwich disintegration with buckling of face sheets, or Mode III, progressive end-crushing of the sandwich panels. The specimens which failed in a progressive crushing mode of failure absorbed three to four times the amount of energy as the specimens exhibiting a Mode I or Mode II failure. To fully utilise the energy absorbing potential of a composite/sandwich design, the structure must be designed to fail in a progressive crushing mode, thereby, maximising the energy absorption for a given configuration. This can be achieved by including a triggering mechanism to induce a progressive failure mode and prevent catastrophic structural failure.

Lindström and Hallström [4] conducted a series of experiments with unconstrained sandwich specimens. Several triggering mechanisms were analysed including, chamfered face sheets and a series of groove triggers. The inclusion of a triggering mechanism reduced the peak (or triggering) load compared to the pristine panels without a geometric trigger. The specific energy absorption for the triggered specimens was slightly larger than for the un-triggered specimens. It should be noted that these specimens exhibited a progressive crushing failure mode. The triggering mechanisms were located at the free edge of the panels and are not suitable for integrating into a structural pi-joint.

Stapleton and Adams [5] investigated the crushing response of un-triggered sandwich panels with externally applied ‘plug type’ constraint fixtures. The constraint fixtures drove the face sheets inwards (or outwards) to drive a specific failure mechanism, thereby allowing the specimens to exhibit a progressive crushing mode. It was concluded that sandwich specimens can absorb large amounts of energy through careful design of the specimen and application of an external constraint fixture. The external plug type trigger is not suitable for aerospace applications due to the large weight penalty which would be introduced. Additionally, this configuration does not form a load carrying joint.

The motivation for this work was to evaluate an energy absorbing sandwich structure that also functions as a load carrying structural joint. Previous work by the authors [9], [10], [11] focussed on the development and evaluation of a representative composite pi-joint that could provide lateral constraint to a sandwich web to ensure that the structure exhibited a stable progressive crushing failure mode. For the previous experiments the interface between the sandwich web and the constraint fixture was un-bonded and was therefore not representative of a load carrying joint. The work described herein extends the previous work by evaluating a multifunctional crashworthy structure that can react in-service loads and also function as an energy absorbing structure once a predefined triggering load has been exceeded. The crushing response of these multifunctional structures was evaluated under quasi-static and dynamic loading rates.

Section snippets

Specimen description

The multifunctional crashworthy pi-joint/sandwich energy absorbing structure consists of two main components, namely, the sandwich web and the pi-joint constraint. The main functions of the pi-joint constraint are to provide a constraint mechanism to the sandwich web and to allow the transfer of loads into the sandwich web. Load is transferred from the pi-joint into the sandwich web through an adhesive interface. The functions of the sandwich web are to react in-service loads and absorb energy

Test parameters

The specimens were designed to be self-supporting and were tested without any external fixture. The representative pi-joint has been shown to be capable of providing an adequate amount of lateral constraint to the sandwich web during progressive crushing [10]. The quasi-static test specimens had pi-joints bonded to the top and bottom of the sandwich webs. A more conservative approach was adopted for the dynamic test specimens as the structural configuration had never been evaluated dynamically.

Quasi-static testing

Quasi-static testing was conducted to establish the baseline performance of the specimens, to confirm that a progressive crushing mode could be established and that the failure progression was similar to previous tests with an un-bonded interface between the pi-joint and the sandwich web [9], [10]. The quasi-static load vs. displacement curves of four bonded pi-joint crush specimens are shown in Fig. 5. It should be noted that the S-BP-3-3-BTR-A specimen was only crushed down by 26.0 mm. This

Triggering and crushing mechanisms of composite sandwich structures contained within a pi-joint

Analysis of the quasi-static and dynamic test results indicated that the failure of an axially compressed bonded pi-joint/sandwich web with an embedded offset ply drop triggering mechanism can be divided into five phases. These are represented by the characters shown on the top of the representative force vs. displacement curve of Fig. 12, and are described in the following sections.

Trigger configuration

Previous work by the authors investigated different trigger configurations in un-bonded pi-joint configurations [9], [10]. A comparison of the load vs. displacement response of two sandwich webs with different trigger configurations is shown in Fig. 15. It is evident that the trigger configuration defines the load vs. displacement response in the bottoming out phase. The sandwich web in the PL-2-1 specimen was not bonded into the pi-joint and the triggering mechanism was located 3.0 mm from the

Conclusions

The quasi-static and dynamic response of a multifunctional crashworthy pi-joint/sandwich web has been demonstrated. The webs exhibited consistent failure behaviour under dynamic and quasi-static loading. Under dynamic loading the steady state crushing loads and energy absorption were reduced by 11.5% and 3%, respectively. The reduction in steady state crushing load, observed during dynamic testing, is attributed to a change in failure mode with increased loading rate. With further refinement

Acknowledgements

This work is part of the Research Program of the Cooperative Research Centre for Advanced Composite Structures Ltd (CRC-ACS), established and supported under the Australian Government’s Cooperative Research Centres Program, and their financial support is kindly acknowledged. The authors acknowledge the contribution provided by the German Aerospace Center (DLR), in particular A.F. Johnson and C.M. Kindervater, in supporting this collaborative research program. H. Abu El-Hija provided invaluable

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