Cryogenic small-flaw strength and creep deformation of epoxy resins

Resin cracking, a cause of coil quench in superconducting magnets, occurs when a resin contains small flaws and sustains high thermal stress caused by coil restraint during cooling. Seven epoxy resins were chosen to evaluate thermomechanical properties, small flaw strength, and creep deformation at low temperatures. Although the plain specimen strengths consistently increased as the temperature decreased, the fracture toughness values resulting from large cracks reached maximum values at about 80 K, then decreased to the lower values at 4 K. The loss factor during cyclic loading behaved similarly because of low-temperature relaxation of the resin, having a maximum value at about 150 K. The strengths corresponding to small flaws leveled off to those of plain specimens; i.e., they deviated from linear fracture mechanics. Thermal stress in the epoxy resin caused by coil restraint was experimentally measured by simulating the coil-molding process. These stresses were close to the calculated ones obtained by using the elastic moduli and the coefficients of linear thermal expansion of the resin. However, they were slightly lower because of stress relaxation at about the glass transition temperature. By using the thermal stress values and the small flaw strengths at 4 K, we could calculate the allowable flaw sizes and the critical ratios of resin volume to conductor volume that produce coil quench. These values varied greatly among the resins. Even at as low as 77 K, creep deformation was significant, which may possibly cause a cryogenic delayed fracture of the resin and result in an unexpected coil quench during a steady-state persistent current operation. The ratios of the creep strain to the initial strain decreased uniformly for all resins as the difference between glass transition temperature and the test temperature increased. (author)