Research On Feasibility Of Evaluation Of Fracture Toughness In The Ductile-to-brittle Transition Region Of Q345R By Small Punch Tests And Beremin Model

Small punch testing technology is used for quantifying the mechanical properties of bulk materials with greatly reduced volume. However, as the volume decrease, the ductile to brittle transition region of the specimen will move to lower temperature. That means a small punch specimen and a bulk specimen may fracture under different regime. For this case, it’s difficult to obtain fracture toughness of bulk materials from the small punch tests. In this paper, a new approach, which is based on Beremin local approach, to evaluate the fracture toughenss of bulk materials in the ductile to brittle transition region by small punch tests has been proposed. To make the approach theoretically practicable, there precondictions should be met:(I) Beremin local approach could be used to predict the cumulative failure probability of small punch specimens;(II) Weibull parameters (m,(σu) were independent of specimen size. That means the Weibull paremeters obtained by small punch tests were equal to the parameters obtained by bulk materials;(III) The relationship between Weibull shape parameter, au, and temperature could be described by a function which could be determined by small punch tests carried out at low temperature. Hence, the applicability of Beremin local approach to the small punch tests and the relationship between Weibull parameters and specimen size and betweenσu and temperature have been investigated to verify the theoretically practicability of the new approach proposed in this paper using small punch tests. The accuracy of evalutation results and the factors which affected the accuracy also have been investigated. The main contents and conclusions of the thesis are as follows:(1) Applicibility of Beremin local approach to the small punch tests has been investigated using0.5mmLNSP specimens (linear notched small punch specimens),1mmLNSP specimens,0.5mmCHSP speicmens (central holed small punch specimens) and0.8mmCHSP specimens. The researchs shows that Beremin local approach could be used to predict the cumulative failure probability of small punch speicmens. However, the accuracy of prediction might be affected by thickness, structure and the method used to manufacture the notch. Fortunately, these influence could be avoided by choosing the specimen of which the thickness and the radius of notch are small and the notch was manufactured without residual strain. For this type of specimen, the precondition (Ⅰ) is met.(2) The relationship between Weibull parameters and specimen size and betweenσu and temperature have been investigated using0.5mmCHSP specimens and0,4T SE(B) specimens. The research shows:(i) For the original Beremin model, the Weibull parameters are dependent on the specimen size;(ii) For the model modified by strain proposed by Gao, Weibull parameters are independent of the specimen size and σu depends on temperature following a power function. For Gao’s model, the precondition (Ⅱ) and (Ⅲ) are met.(3) Fracture toughness of1T SE(B) has been evaluated by using0.5mmLNSP specimens and0.5mmCHSP specimens following the procedure of new approach proposed in this paper. The research shows the approach is practicable but the accuracy of evalution results deviated from the real value. Then several suggestions for improving results and future studies were proposed.