LOW-CYCLE THERMAL FATIGUE TESTING OF BERYLLIUM

A novel technique has been used to test the relative low cycle thermal fatigue resistance of different grades of US and Russian beryllium, w hich is proposed as plasma facing armor for fusion reactor first wall, limiter and divertor components. The 30 kW electron beam test system at Sandia National Laboratories was used to sweep the beam spot along one direction at 1 Hz. This produces a localized temperature 'spike' o f 750 degrees C for each pass of the beam. Large thermal stresses in e xcess of the yield strength are generated, due to very high spot heat flux, 250 MW m(-2). Cyclic plastic strains on the order of 0.6% produc ed visible cracking on the heated surface in less than 3000 cycles. An in-vacuo fiber optic borescope was used to visually inspect the beryl lium surfaces for crack initiation. Grades of US beryllium tested incl uded: S-65C, S-65H, S-200F, S200F-H, SR-200, I-400, extruded high puri ty, HIP'd spherical powder, porous beryllium (94 and 98% dense), Be/30 % BeO, Be/60% BeO, and TiBe12. Russian grades included: TGP-56, TShGT, DShG-200, and TShG-56. Both the number of cycles to crack initiation and the depth of crack propagation, were measured. The most fatigue re sistant grades were S-65C, DShG-200, TShGT and TShG-56. Rolled sheet B e (SR-200) showed excellent crack propagation resistance in the plane of rolling, despite early formation of delamination cracks. Only one s ample showed no evidence of surface melting, Extruded (T). Metallograp hic and chemical analyses are provided. Good agreement was found betwe en the measured depth of cracks and a 2-D elastic-plastic finite eleme nt stress analysis. (C) 1997 Elsevier Science S.A.