Stress analysis of bias extension specimens

The Bias Extension test is commonly used for material characterisation in shear, providing essential input to composites forming simulations. Specimens of different sizes are used, with some preference for the minimum length-to-width ratio of 2. A static equilibrium analysis is presented to derive the forces and stress distribution in a test specimen of this minimum aspect ratio. The stress distribution is shown to depend on two variables, while usually only one (the pulling force) is measured.

The central region of relatively longer specimens is generally subject to non-homogeneous boundary conditions, causing non-uniform deformations which can hamper accurate material characterisation. In addition, the analysis demonstrates that the major part of the pulling force is carried by the outer fibres, further emphasizing the need for critical preparation and evaluation of bias extension testing if high accuracy characterisation is needed.