In this paper we present experimental data on dynamic crack propagation in square PMMA plates of two types - 3.5 and 20 mm thick. Samples were loaded dynamically (mode I loading type) and crack tip position was registered using high speed camera. Explosion of a copper wire due to high electrical current was used to load faces of the initially prepared cracks. In order to investigate stress intensity factor (K _I ) history, method of caustics was applied. Thick samples demonstrated considerably higher values of final crack travel distance and higher crack velocity values. Additionally, dependence of stress intensity factor on crack velocity was observed.