Crack propagation on sheet materials is a useful phenomenon to characterize material toughness. In particular, it has been known that crack-propagation speed jumps abruptly as a function of applied strain. We have proposed a model which reproduces the velocity jump. This model uncovers the physical origin of the velocity jump as a glass transition that occurs in the vicinity of a crack tip. This theory suggests that the velocity jump can be universally observed for a certain class of viscoelastic materials. In fact, we recently succeed in detecting the velocity jump in semicrystalline porous polypropylene sheets. This work clarifies the importance of the dynamic test, in which sheets is extended with a constant speed during crack propagation. We further explore the potential of crack-propagation tests in industry on the basis of our recent findings.