Abstract: Under different stress states, the physical mechanisms of the process of fractures can be different.Traction facture occurs when brittle materials are subject to traction, andshearing fracture occurs when they are subject to pressure, which are twokinds of fracture patterns with different physical mechanisms. The Mohr'sstrength theory uses only one fracture condition, which is questionable. This problem is studied in combining withfracture damage tests of grey cast iron HT200 materials. The results indicate that Mohr's theory cannot rationally predict the strength of material fractures in the state ofcomplicated stresses.