Composites fail via a wider variety of mechanisms than do simple
materials. Several theories to explain failure have been presented,
although some of their criteria are complicated and
difficult to apply. The definite number of the strength properties
of the materials cannot be defined because it depends on the failure
criteria adopted. However, for or thotropic materials such as
wood, the number of strength properties is limited to a few easily
measurable properties for which there are standard testing methods
for their measurement.