Mathew Joosten’s PhD thesis presents an investigation of triggering mechanisms designed to ensure energy absorbing composite structures exhibit a progressive crushing mode of failure. The crushing of composite structures provides significant dissipation of energy in a crash if a brittle buckling mode of failure is prevented from occurring. Triggers are needed to ensure that the failure initiation involves splitting and folding of the laminate over lengths comparable to the thickness of the laminate and that the failure develops into a stable crushing mode.

The research reported in the thesis focused on practical triggers that can be incorporated into the subfloor structure of helicopter airframes. The work was part of the CRC-ACS helicopter research program (2007 – 2010). The major research partner was the German Aerospace Center (DLR). A small-scale specimen that has become known as the DLR crush element was tested with a variety of triggers including chamfering the end of the laminate, offset ply-drop triggers and the inclusion of a steeple at the end of the section. The test program included quasi-static testing carried out in laboratories at the University of New South Wales and at the Defence Science and Technology Organisation in Australia, and dynamic testing at the DLR laboratories in Stuttgart as shown in Figure 1.

Figure 1:  Schematic representations and quasi-static failure modes of three configurations of DLR crush elements: (left) chamfered tip (middle) steeple trigger (right) offset ply-drop trigger

A further significant contribution is the design of triggers for sandwich panels. These panels are widely used in helicopter airframe structures and contribute to the energy absorbed in a crash. To the author’s knowledge this is the first work on triggering of sandwich panels constrained within a composite pi-joint. The pi-joint constraint ensured the sandwich web exhibited progressive crushing of the core material and surface plies as the panel was progressively loaded. An annotated load vs. displacement curve with the corresponding images illustrates the progressive failure of the sandwich/pi-joint configuration is shown in Figure 2. This configuration functioned as an effective energy absorber under both dynamic and quasi-static loading conditions and represents the practical implementation of a light weight sandwich energy absorber.

Figure 2:  Progressive failure of a triggered sandwich web contained within a representative composite pi-joint

The research included the application of a stacked-shell finite element modelling methodology for predicting the initial triggering and progressive crushing of laminates. A comparison of the predicted and experimental failure modes for a hat-shaped crush element are shown in Figure 3. This analysis methodology was validated against testing of specimens and the large test article. The study included chamfer, steeple and ply-drop triggers and makes recommendations on the most appropriate form of trigger for inclusion in structures that are expected to carry static loads but trigger when dynamically loaded.

Figure 3:  Failure modes of a eight ply carbon-epoxy hat-shaped crush element (left) experimental failure mode (right) numerical prediction

Finally a large scale test article was manufactured by CRC-ACS. The thesis details finite element prediction of the quasi-static and dynamic tests (performed at the DLR in Stuttgart) and compares the numerical predictions against experimental test data. The numerical models were able to predict the energy absorption and progressive failure mode of the test article for both quasi-static and dynamic loading conditions. A comparison between the experimental and numerically predicted failure is shown in Figure 4.

Figure 4:  Comparison of the failure mode of the 'large test article' impacted at 8.0 m/s (left) experimental (right) numerical prediction

The numerical modelling work contributed to a paper that was presented at the American Helicopter Society 67th Annual Forum. This paper¹ was awarded the best paper in the crash safety session

¹ Kindervater C, Mikulik Z, Mulcahy L, Joosten MW, Thomson RS, Johnson A, Validation of Crashworthiness Simulation and Design Methods by Testing of a Scaled Composite Helicopter Frame Section, American Helicopter Society (AHS) 67th Annual Forum & Technology Display, May 3-5, 2011, Virginia Beach, Virginia, USA