There is some debate in our department about whether or not traditional strain transformation equations applied to Tri-rosette strain gauges can be used when the gauges are applied to a material such as composites with a directionally dependent stiffness. These strain transformation will be used to derive the principal strains, maximum shear strain, and also to reorientation the normal strain grid between the loading axis and the surface fiber axis.

The department does acknowledge that going from principal strains to principal stress is more complex and requires something like classical laminate theory to switch between the two. This question focuses solely on the strain transformation aspect.

Are there any strain experts that can explain if strain transformations can be applied to composites using traditional strain transformation equations, or do we need more complex equations which involve classical laminate theory? If traditional strain transformations are valid, please can you explain or point to any resources.

Thanks,

Dan