The mechanical properties, such as failure strength, fracture angle, and ductility, are function of state of stress, temperature and strain rate (e.g. Paterson and Wong, 2005). In this book, only the effects of stress state are discussed. As mentioned in the preceding chapter, the stress state can be specified by the three principal stresses, σ1 (the maximum principal stress), σ2 (the intermediate principal stress) and σ3 (the minimum principal stress) (compressive stress condition is taken positive).