On the use of electron paramagnetic resonance in fracture mechanics

Electron spin resonance (ESR) provides a sensitive tool by which microscopic bond rupture can be monitored simultaneous with measurements of macroscopic deformation and failure. The use of this tool to study fracture of semicrystalline polymers, rubber at very low temperatures, and rubber in the presence of ozone is discussed. In these cases, a large number of ruptured bonds precedes macroscopic failure. The micromacro behavior of these materials will be discussed in terms of some various stages of fracture and some recently proposed models of polymers. To reliably predict the strength of a polymer from the strength of the molecular chains, it is important to understand the distribution in effective length of these load‐carrying chains. ESR provides a means of obtaining insight into such information. Straining rubber before lowering its temperature to −100°C results in crystallization and subsequent fracture behavior very similar to that of polycrystalline polymers. Rate of molecular bond scission is successfully correlated by a Griffithtype energy balance to the strain energy release rate during ozone cracking of rubber.