Experimental study on development characteristics and size effect of rock fracture process zone

•The development of FPZ of SCB specimens is studied with the help of DIC method.•The length of FPZ is about 2–2.5 times its width at the peak.•The crack opening mode is divided into three stages during the loading process.•FPZ begins to develop around the 65% pre-peak, and the shape of FPZ is strip.•The length and width of FPZ increase with the size of the SCB specimen. In order to investigate the development characteristics and size effect of rock fracture process zone (FPZ), the GCTS RTX-3000 rock mechanics testing machine was used to carry out on semi-circular bend (SCB) sandstone specimens with diameters of 50 mm, 100 mm, 150 mm and 200 mm, respectively. Combined with the digital image correlation (DIC) method, the variations of the displacement field at the crack tip of specimens with different sizes were analyzed. The results show that the crack opening mode of the SCB sandstone specimen has experienced three stages of slow, linear and accelerated growth. Its critical crack opening displacement is about 20 μm. When the load reaches around 65% pre-peak, the FPZ begins to develop. The range of the FPZ reaches the maximum at the peak. The closer to the peak, the faster the development of the FPZ, and the length of the FPZ that increases from 90% pre-peak to peak accounts for 50% of the total FPZ length. The FPZ length of specimens of different sizes at the peak is about 2–2.5 times of the width. As the size of the specimen increases, the length and width of the FPZ increase. The thickness of the specimen has a significant effect on the width of the FPZ.