Deformation in the clamp can be compensated for, giving an improved strain mea surement, as discussed by Setterholm  and Setterholm and Gertjejansen . Therefore, the measured stress-strain curve can be demonstrated to be a valid mate rial characteristic. On the other hand, compression failure is essentially the transition form in-plane deformation to out-of-plane deformation. This deformation is restricted by the plates. For this reason, the compression strength measured by this apparatus may be artificially high.