Evaluation of dynamic fracture toughness in ductile steel by means of a split Hopkinson pressure bar 3PB technique

The objective of this work is the tune of a methodology for the determination of the fracture toughness in ductile materials at high strain-rate by means of dynamic 3 Point Bending (3PB) tests. The specimens made in high strength steel have been pre-cracked in fatigue with a single edge notch. The dynamic loading conditions have been reached by means of a modified Split Hopkinson pressure Bar setup, in which two small diameter bars are used as transmitted bars instead of the standard single one. In order to obtain the maximum amount of information from each test, different measuring systems have been adopted, in addition to the strain measurements on the bars performed using semiconductor strain-gages. On the specimen surface, one or two crack-gages (one for each side for specimens) have been bonded for obtaining data about the crack propagation and the crack tip position vs. time curve. Moreover, a high speed camera has been used for obtaining the measurements of the Crack Opening Displacement (COD) and the opening velocity: digital images have been processed using tracking techniques following some points on the specimen notch and input bar tip. Starting from the experimental measurements in term of force, displacement and crack-gage data, a protocol for fracture toughness calculation at the initiation stage of the crack growth in dynamic condition has been defined and a hybrid experimental – numerical methodology has been used. One of the key variables for the determination of dynamic initiation fracture toughness is the crack time to initiation. Many efforts have been done in order to better determine a precise method to experimentally measure this entity: this datum has been obtained from the analysis of both the force and crack-gage vs. time signals and the COD measurement on high speed video. Good agreement has been found between the two methodologies. At this stage, a 3D model, which simulates the SE(B) specimens during the Hopkinson 3PB test, has been done in order to determine the toughness (J integral) as a function of time. Knowing the initiation time, the fracture toughness at crack initiation can directly be determined.