Crush Stability and Crush Compression Ratio (CCR)
Whilst striving for materials and laminates with ever higher crush performance or crush stresses it is important to keep a close eye on some other aspects of the material performance. The success of a crush structure relies on the progressive crushing of the material at the crush front and the ability of the rest of the structure (backup structure) to withstand the crushing forces imparted without collapse.
Fundamentally the sustained crush stress of the material is lower than its static compressive strength, often by a factor of 6 or 7 for high performance materials. The Crush Compression Ratio (CCR) of a material is the static compressive strength divided by the sustained crush strength. If the CCR is too low it can be difficult or impossible to design a structure to progressively crush, as the high forces developed lead to failure in areas in the backup structure with any appreciable stress concentration. A CCR of 2 or 3 may only be suitable for ideal shapes with ideal loading scenarios eg a uniform box or channel section in axial crush. Real world scenarios such as imperfect loading directions or crush members with features such as bends, notches or holes usually require CCR’s upwards of 3 and preferably towards 6 or 7. The higher the CCR, the easier it is to design a robust crush structure.
Related to the CCR is the crush stability, which is calculated from the shape of the measured crush data. Ideally a material will exhibit a flat crush response like that shown in figure <Inset graphic>. Many acceptable crush materials have a more varied response such as shown in figure <Inset graphic>, while some composites just do not crush as indicated in figure <Inset graphic>. Engenuity have automated procedures for characterising the crush stability direct from tested data, and use this as one of the more important parameters in the material selection process.