Research On Testing And Analysis Method Of Resistance Characteristic Of Steel With Crack Under Different Loading Rates

The use of metal materials meets the needs of structural diversity and lightweight in engineering.However,it was found in practice that there were many unexpected accidents in metal structures.And the reason why the accidents took place was the reduction of the strength of structures caused by cracks.Fracture mechanics is a subject that studies crack problem specially,which analyzes stress field,strain field at crack tip and crack propagation law.It provides guidance for the evaluation of structures with cracks.Based on the existing specifications,experiments of resistance characteristic（resistance curve）under different loading rates were carried out in this paper.At the same time,numerical simulation was used to analyse the related problems.The main works of this paper were as follows:（1）The resistance curves and fracture toughness of specimens made by 11Mn Ni Mo steel were measured by the quasi-static compact tensile test.It was found that the initial crack length had a great influence on the calculation results,which the traditional J integral theory could not explain this phenomenon.Therefore,the two-parameter theory was considered to explain the differences in resistance characteristic.（2）Charpy pendulum impact tests on standard specimens and prefabricated crack specimens made by 11Mn Ni Mo and 15Mn Ti were carried out.The resistance curves and fracture toughness were calculated by Schindler method.The fracture toughness calculated from standard specimens was larger than that from prefabricated crack specimens.Therefore,they were not suitable for engineering evaluation.In addition,the method of calculating fracture toughness of prefabricated crack specimens by confirming fracture initiation point was discussed.When the average of the yield load and the maximum load on the experimental curve was used as the load of initial extension of crack to calculate the fracture toughness,the result will be too conservative.（3）The method of combining experiment and numerical simulation to calculate the fracture toughness was discussed.Based on the results of the three-point bending test about11Mn Ni Mo and 15Mn Ti steels on high-speed material testing machine,the Gurson-Tvergaard-Needleman（GTN）damage model was used for finite element simulation.The resistance curves and fracture toughness of these two materials were calculated,and the feasibility of this method of combining experiment and numerical simulation was verified.In addition,the damage parameters of the two materials identified can also be used for subsequent study.（4）The compact samples with different sizes and crack lengths of 11Mn Ni Mo steel were tested under quasi-static conditions.The resistance curves and fracture toughness were calculated.The stress fields were solved by finite element software.Further,the constraint parameters A₂were solved.The constraint parameter A₂was considered to modify the general power function expression of the resistance curve.The correctness and effectiveness of the theory was proved by the height fitting of the resistance curves calculated according to the modified resistance curve expression and the experimental results.