Transient growth in 3D of a semi-infinite, plane brittle crack in an isotropic, elastic
solid is considered. Growth is mixed-mode, caused by in-plane and normal point
forces on each face of an existing semi-infinite crack. An analytic solution is obtained
for the case of dynamic similarity, i.e., crack edge speed is subcritical and may vary
continuously with direction, but is time-invariant. The dynamic energy release rate
criterion, with kinetic energy included, is imposed. A nonlinear differential
equation for crack edge speed results, and allows the description of crack
contour, i.e., the curve formed by the crack edge in the crack plane. Study
indicates that forces of a type that increase rapidly from zero can create a
fracture initiation phase in which crack growth rate indeed does not vary with
time.
