From equation 15, the 'interaction' terms S_{16} and S_{26} are both non-zero and this indicates that, under off-axis loading, normal stresses produce shear strains (as well as normal strains) and shear stresses produce normal strains (as well as shear strains). This tensile-shear interaction is also present in laminates, but does not occur if the loading system is applied along the principal axes of a single isolated lamina, in which case S_{16} = S_{26} = 0 as in equation 13.

\[{\eta _{xyx}} = {E_x}{\overline S _{16}}\;\; and\;\;{\eta _{xyy}} = {E_y}{\overline S _{26}}\]

The extent of this tensile-shear interaction is quantified by the parameters η_{xyx} and η_{xyy} (Click to open pop-up)

### Balanced laminates

Tensile-shear interactions are undesirable as they lead to distortions and local microstructural damage and failure. A laminate whose interaction ratios are zero is said to be **'balanced' **. Use the model below to investigate the variation of η_{xyx} with loading angle.

*Note: This animation requires Macromedia Flash Player 8 and later, which
can be downloaded here.*