Shear/compressive strength of GFRP under mixed load condition at low temperature

Insulation of superconducting magnet systems requires excellent electrical insulating properties, compressive strength and flexibility so that it can bear the compressive stress of the electromagnetic force and the shear stress caused by the deformation of each conductor in these magnets. GFRP is suitable for these insulation systems and most superconducting magnet systems use it. Although the interlaminar shear strength of GFRP is about a tenth of its compressive strength, this strength increases under a combination of stresses. GFRP strengths under shear/compressive loading are specified for optimum designs. Therefore, we can apply GFRP against shear/compressive loading for which static and fatigue strengths are the dominant factors in magnet life assessment. The coefficient of friction of the surface affects the static and fatigue behavior at low temperature. Two types of tests were carried out to simulate the combined stresses, and shear/compressive static and fatigue tests were performed at 77 K on GFRP. Employing different angle test fixtures, GFRP specimens were loaded with various levels of shear and compressive stress. We evaluated the strength of insulators that sustain compressive and frictional shear stresses to take into account the stress redistributions for cases both with and without the occurrence of surface slips. A new criterion for the shear/compressive static and fatigue failure is proposed in this study.