Crack propagation in calcium fluoride single crystals

Structural design of optical components launched into space requires fracture mechanics properties. To perform component design of a calcium fluoride (CaF2) prism, the fracture toughness and slow crack growth (SCG) parameters were measured on the {100}, {110}, and {111} low index planes. The fracture toughness is lowest on the {111} plane at 0.35 ± 0.01 MPa√m with a very flat cleavage surface exhibited during both fracture toughness and strength testing. SCG was significant on the {111} plane with a power law exponent of n = 30 ± 8. For engineering purposes, SCG was insignificant on the {100} and {110} planes with n > 75. The facture surfaces have distinct patterns that are indicative of the cleavage plane. Biaxial testing with disks implies that design for general multiaxial states should be based on {111} strength and crack growth properties.