Stress trajectories for Mode I fracture

The theoretical stress trajectories (isostatics) for Mode I fracture are calculated. This is achieved by combining the general equations for isostatics and the expressions for the state of stress at a point under mode I loading. The trajectory is valid only in the elastic region at the tip of the crack and not in cases where plasticity exists. It would be relatively easy to check that the predicted stress trajectories correspond to the true trajectories by deriving them from isoclinics determined in a photoelastic model. However, this was not undertaken in the present study.