Griffith's theory of failure is based on the assumption that the low order of tensile strength in common materials is due to the presence of small cracks or flaws. Actual stresses may occur around these flaws, which are of the order of magnitude of molecular cohesion values, while the average tensile strength may be quite low. Mohr's theory predicts that failure of materials is due to failure in shear, whereas Griffith's theory postulates that it is due to failure at crack tips.