Mode I propagation is an in-plane incremental extension of a joint in pure opening mode (Figure 1). It occurs when a discontinuity, a flaw, or an already existing joint is oriented perpendicularly to the remote least principal stress (symmetric loading) such that the tensile stress at the tip is large enough to drive the propagation in a straight path. For more details, see 'Propagation of Joints.'

Figure 1. Pure mode I fracture propagation. From Pollard and Aydin (1988).