Determination of mode I fracture toughness of rocks with field fitting and J-integral methods

Static fracture testing was performed on ISRM-suggested semi-circular bend (SCB) specimens with a constant CMOD increment rate of 0.0002 mm·s−1, and the digital image correlation (DIC) technique, as a practical and effective non-contact tool, was adopted for full-field deformation measurement and fracture process zone (FPZ) size determination. Two displacement-based methods, i.e. field fitting method and J-integral method, were employed to measure the fracture toughness of three types of rocks, i.e. red sandstone, marble, and granite. The effectiveness and robustness of two displacement-based methods were verified with synthetic images containing a mode I crack. Moreover, the sensitivity of the affecting factors in displacement-based methods, such as crack tip location, area for field fitting, the number of solution terms, and the integral paths were studied for more reliable fracture toughness of SCB specimens. Both displacement-based methods give the effective fracture toughness inherently considering the existence of the FPZ above the notch tip, without knowledge of the geometry of the specimen and applied loads. Experimental results indicated that the crack tip location estimated by the field fitting method is close to the merged point of the opening displacement contours, i.e. the tip of the FPZ. Both displacement-based methods generate comparable fracture toughness of three rocks, and an increase of 20%∼35% in fracture toughness is observed compared to the standard load-based method. Moreover, the effectiveness and applicability of the modified load-based method that takes the FPZ length into account are also verified with the two displacement-based methods.