首页|Cryogenic small-flaw strength and creep deformation of epoxy resins
Cryogenic small-flaw strength and creep deformation of epoxy resins
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NSTL
Resin cracking, a cause of coil quenching insuperconducting magnets, occurs when a resin contains smallflaws and sustains high thermal stress. Seven epoxy resins werechosen in order to evaluate thermo-mechanical properties, small-flaw strength, and creep deformation at low temperatures.Although the plain specimen strengths consistently increase as thetemperature decreases, the fracture toughness resulting from largecracks reaches a maximum at around 80 K and then decreases at 4K. The loss factor during cyclic loading behaves similarly, becauseof low-temperature relaxation of the resin, and has a maximumvalue at around 150 K. The strengths resulting from decreasingflaw size level off to those of plain specimens; that is, they deviatefrom linear fracture mechanics. Thermal stress (caused by coilrestraint) in the epoxy resin is experimentally measured bysimulating the coil molding process. These thermal stresses areclose to the calculated ones obtained by using the elastic moduliand the coefficients of thermal expansion of the resin. However,they are a little lower because of stress relaxation at around theglass transition temperature. Thermal stress and small-flawstrength at 4 K are used to calculate the critical volume-ratio ofresin to conductor that produces coil quenching as well as theallowable flaw sizes. The calculated ratios and sizes of the resinsvary considerably. Even at 77 K, creep deformation in the resins issignificant and may cause a cryogenic delayed fracture of the resinand result in unexpected coil quenching during steady-statepersistent-current operation. And the ratios of the creep strain tothe initial strain (in all resins tested) decrease uniformly as thedifference between glass transition temperature and test temperature increases.
NSTL
