Bounce Off Of Specimen And The Effect On The K Factor In Dynamic Fracture Test

The stress condition of Charpy impact specimen is three-point bending in static fracture test due to the low loading rates.While the specimens are tested in the dynamic impact tests,they will bounce off supports.But,the rule on the loss of contact of specimen with support is still unclear and the opinions of different researches are different about this phenomenon.These problems are discussed systematically in this paper.The Charpy specimen was impacted by the apparatus of Hopkinson pressure bar.And the phenomenon of the loss of contact between specimen and supports existed indeed.The phenomenon relates to the system stiffness essentially,which depend only upon the dimensions of specimens and span with a certain condition.The dynamic fracture tests about specimens with different dimensions were organized.The results show that the period of the loss of contact between the specimen and support is affect by the specimen dimension and span.The critical value of S/L varies with the wide W,and the critical condition of loss of contact on the dimensions was established.Thus scientific basis for determination of dynamic fracture toughness was provided.If the cracks initiate before supports beginning to be stressed,the so-called three-point bending test is in fact the one-point bending experiment.It results that expression of stress intensity factor based on the static fracture mechanical theory is not applicable,and the dynamic fracture toughness must be solved according to one-point bending condition.The Ansys software was applied to found a 3-demension model,including striker,loading bar,specimen and supports.The model simulated the dynamic three-point bending test,computed the J-integral around crack tip,and verified the path independence.