Abstract:Frozen soil is a kind of multi-phrase compound material. There are many defects and micro-cracks in frozen soil. Supposing the defects as the initial crack,using the theory and testing method of fracture mechanics,the nonlinear fracture process and character of frozen soil are investigated. The result shows that the nonlinear fracture process of frozen soil is composed of elastic period,micro-crack damage zone(MDZ) forming period and softening period. MDZ forming period is the main feature of nonlinear failure of frozen soil. The micro-crack is regarded as the virtual crack,considering the cement ice in frozen sold,the cementation force crack model of nonlinear fracture failure is presented. The material in MDZ is partially damaged but still able to carry cementation stress ,which is nonlinearly distributed over the length of MDZ. Then the method to calculate the length of MDZ is proposed. This model can offer some references for theory analysis and numerical calculation. Finally,the test method of nonlinear fracture toughness index involved in cementation force crack model and the corresponding results are given.