Complex sentence: These cracks propagate slowly at first during stage I crack growth along crystallographic planes with high shear stresses until they reach a critical size, then quickly during stage II crack growth in a direction perpendicular to the applied force.
Split into simpler sentences: These cracks propagate slowly at first during stage I crack growth along crystallographic planes, where shear stresses are highest. Once the cracks reach a critical size they propagate quickly during stage II crack growth in a direction perpendicular to the applied force.