Hey guys I have a few questions to ask: 1.) Why are materials whose yield stresses are highly strain-rate dependent more susceptible to brittle fractures than those materials whose yield stresses do not exhibit marked strain-rate dependence? 2.) How would you define 'toughness' in the case of elastic deformation; plastic deformation; and fast fracture? 3.) Under what conditions can materials that generally exhibit a yeilding stress and final fracture stress can still fail below the yeild stress. I appreciate the help in this matter since I am not too familiar with material science engineering.