Question

Why are materials whose yield stresses are ighly strain rate dependent more susceptible to brittle fracture than those materials whose yield stresses do not exhibit marked starin rate dependce

Answer 1

A higher strain rate would correspond to a shorter loading time, equivalent to lower temperatures causing the ductile-to-brittle transition of the specimen. This ductile-to-brittle transition may be due to the significant influence of strain rate effect. With strain rate and multiaxiality increasing, the local stress peaks become so high that
they can reach the microscopic cleavage fracture strength oj of the material. Brittle
fracture is expected, when the local value of the maximum principal stress exceeds aJ
over a characteristic distance X, which depends on the microstructure of the material.