Dynamic uniaxial crushing of wood

Experimental results are provided from a series of tests on the uniaxial dynamic crushing of cylindrical specimens of five species of wood selected for the density range they cover and tested up to impact velocities of approximately 300 ms(-1). An account of the macro-deformation and micro-deformation modes resulting from quasi static and dynamic uniaxial compression is given. Measurements of the force pulses generated by the impact of the wood specimens on the end of a Hopkinson bar load cell show that significant enhancements of the initial crushing strengths of the specimens occur under dynamic loading conditions. The deformation mechanisms of wood are localised under quasi-static compression and under dynamic loading conditions they become even more localised and propagate through the material as crushing wave fronts which have some of the characteristics of shock waves. A simple shock model based upon a rate-independent, rigid, perfectly-plastic, locking (r-p-p-l) idealisation of the stress-strain curves for wood is proposed to provide a first order understanding of the dynamic response. This model is particularly successful in predicting the dynamic enhancement of the crushing strength of specimens loaded across the grain as confirmed by comparisons between the experimental data and theoretical results. It is less successful for those compressed along the grain. The source of the discrepancy is discussed and explanations are provided for the fairly constant crushing stress enhancement factor observed at low to moderate impact velocities, for the high impact velocity at which shock-type response is initiated and for the existence of clearly delineated crush fronts which characterise these specimens. (C) 1997 Elsevier Science Ltd.