Transverse anisotropy of compressive failure in European oak – a digital speckle photography study

The mechanical behaviour of European oak ( Quercus robur L.) was studied in radial and tangential compression. Young's modulus and the yield strength were approximately 1.7- and 1.6-fold higher, respectively, in the radial direction. Strain fields were determined by digital speckle photography (DSP). Strains and the effective Poisson ratio could be determined separately in earlywood and latewood during deformation and failure events. In radial compression, strain data showed that rays contributed significantly to the high modulus. In addition, multiseriate ray microbuckling was observed to control yield strength. The microbuckling was localised in the low-density earlywood. In tangential compression, yield strength was controlled by vessel collapse in the low-density regions of the latewood. The strain field data provide direct evidence that the rays are the main micro-structural factor controlling transverse anisotropy in European oak.