Photoelastic Study of Crack Propagation In Composite Models

A two-dimensional dynamic photoelastic investigation was con ducted of crack growth in glass-plastic composite models. The models used were idealized fiber-reinforced composites and consisted of al ternating layers of glass and polyester resin. A crack configuration was introduced in the central glass reinforcing bar to insure crack initiation at that point. A falling weight system producing dynamic tension in the direction of the reinforcing bars was used. This dynamic loading was coordinated with the triggering of a high-speed Cranz-Schardin multiple-spark camera. Transient isochromatic fringe patterns during crack propagation were recorded with this camera operating at a rate of 200,000 frames per second. Some characteristic phenomena were observed. Precursor traces appeared before the crack. These traces did not indicate any evidence of cracking in tests where the crack was arrested and subsequent crack extension followed these traces faithfully. The arrested crack in the matrix phase had a peculiar horseshoe shape. Its propagation velocity was much lower than ex pected. Upon reaching the interface with the glass element the crack showed appreciable hackle dividing into many branches.