At the American Helicopter Society 67^th Annual Forum held at Virginia Beach in May 2011, the paper entitled “Validation of Crashworthiness Simulation and Design Methods by Testing of a Scaled Composite Helicopter Frame Section” was award Best Paper of the Crash Safety sessions. This paper gave an overview of the three year CRC-ACS project, Design Capability for Crashworthy Helicopter Structures. This collaborative project between CRC-ACS and the DLR Institute of Structures and Design, involved CRC-ACS participants Australian Aerospace, DSTO, Pacific ESI and UNSW.

Modern helicopters make extensive use of composite materials to reduce structural weight. These include in-crash energy absorption devices integrated into the subfloor. Despite significant advances in computer-based design tools, the design of a composite energy absorbing structure is normally performed by semi-empirical methods supported by extensive testing. The paper described the development of improved methods for the design of composite energy absorbing structures. A building block approach as shown in Figure 1 was used in which, as specimens of increasing complexity were tested, simulation methods were developed and validated. These tests ranged from material characterisation through to large scale crash testing.

Figure 1: The building block approach adopted in the development of the crashworthy design methods

Detail modelling methods were developed in PAM-CRASH, using the global ply continuum damage model and a stacked-shell technique. This couples relative computational efficiency with the ability to predict the complex failure modes that characterise composite crushing. Material parameters for the modelling were derived from a range of coupon and fracture tests. The crushing modelling technique was developed using the results of crush elements of various geometries, lay-ups and thicknesses, as shown in Figure 2.

Figure 2: Comparison of experimental and predicted crushing response of a trapezoid crush element 

Final validation of the simulation methods was achieved through the design and test of a representative helicopter frame section. The test article included a structural composite frame capable of sustaining the loads expected during crash. Below this, a trapezoidal energy absorbing element was located in the subfloor region, designed to crush at a specific load thereby limiting the acceleration of the occupants to acceptable bounds. Three test articles were manufactured by CRC-ACS and one tested statically and two dynamically at DLR as shown in Figure 3. The predicted energy absorption, crushing force and strain levels agreed closely with the experimental data as shown in Figure 4.

 Figure 3: Test setup in the DLR drop tower

 Figure 4: Comparison of predicted and experimental failure characteristics for a dynamically tested specimen (skin hidden in the numerical model for clarity)

The results showed that the methods can be used to design complex composite energy absorbing structures, and that they have the potential to enable improved designs, while significantly reducing physical testing requirements. The design methods can be an enabling technology for improved designs, while significantly reducing the demands for expensive physical testing. The implementation of these design methods is essential to meet the requirements for improved crashworthiness in the next generation of helicopter platforms, both military and civilian.

Reference:

Christof Kindervater, Rodney Thomson, Alastair Johnson, Matthew David, Mathew Joosten, Zoltan Mikulik, Lex Mulcahy, Sebastian Veldman, Andrew Gunnion, Adrian Jackson, Stuart Dutton, Validation of Crashworthiness Simulation and Design Methods by Testing of a Scaled Composite Helicopter Frame Section, AHS International 67^th Annual Forum and Technology Display, May 3-5, 2011, Virginia Beach, Virginia, USA.