Mechanical Properties Of IR Glasses And Fibers

Fast and slow fracture of chalcogenide and fluoride glasses in air are compared and contrasted to silicate glasses as represented by silica glass. The fracture toughness, elastic modulus and hardness of silicates are, in general, superior to both chalcogenide and fluoride glasses. Thus, improvements are needed in these properties to successfully use these materials in a structural environment. The present and past studies have shown that changes in composition will not be sufficient to improve the desired mechanical property. Thus, some external techniques such as compressive surface stress, coatings, or additions of second phase will have to be employed to improve the strength and hardness of these materials. Slow crack growth due to stress corrosion processes were measured in two ways: direct measurement of crack extension using the double cantilever beam (DCB) technique and the strength-stressing rate (S-SR) technique. The former technique uses large cracks travelling at a constant stress intensity and the latter technique, used for fibers, uses small cracks propagated to failure during loading at a constant stressing rate. Both of these techniques result in a value of the stress corrosion susceptibility constant, n. In general, the higher the "n" the more resistant to stress corrosion. Values of n obtained from bulk glass (DCB) and fibers (S-SR) for chalcogenide glasses were found to be in the same range ( e.g. for As2S3 the values were between 17 & 20) but for fluoride glasses, the values were considerably different. Also, the n values were more dependent on composition than they were for toughness, elastic modulus or hardness. However, for both types of glasses, more data are needed in different environments to fully asses the structural reliability.