Determination Of Double-K Fracture Parameters Using Semi-Circular Bend Test Specimens

According to double-K fracture model, the stress intensity factor created at the initial crack tip by the initial cracking load Pini, is defined as the cracking initiation toughness Klcini,and the critical stress intensity factor created at the critical crack tip by the maximum load Pmax, is defined as the unstable fracture toughness Klcun. It is well known that when structures subjected to loading, prior to fracture undergoes three stages of crack propagation viz:crack initiation, stable crack propagation and unstable fracture. The double-K fracture model can describe these three important stages of crack propagation in concrete. In the present research, an attempt is made to evaluate the double-K fracture parameters using semi-circular bend (SCB) test specimens. Fracture tests were carried out on SCB specimens with three different diameters (200, 400 and 500 mm) and four different concrete strength series (C2, C3, C4 and C5). The effect of specimen size and concrete strength on double-k fracture parameters was analyzed. The results indicated that both the double-K fracture parameters Klcini and Klcun for SCB specimens are size dependent. Also it is noticed that concrete strength had affected the values of double-K fracture parameters. All the fracture parameters, such as critical crack length ac, critical crack mouth opening displacement CMODc and unstable fracture toughness Klcun increases as the specimen size increases; while, initiation cracking toughness Klcini decreases with the increase of specimen size.