Measurement ofmode I fracture characteristics,such as critical opening displacement and fracture process zone(FPZ)length,is fundamental for the analysis and prediction of crack propagation.Currently,the newer direct measurement in laboratory mainly involves acoustic emission(AE),digital imaging correlation(DIC)and fiber bragg grating(FBG).However,only the FBG can supply information of crack displacementinside the rock of practical engineering.In view of the insufficient investigations,the application of FBG for characterizing fracture is worth verifying.In this paper,mode I fracture test was performed on Sichuan unconsolidated sandstone using three-point bending,and four bare fibers were fixed perpendicular to the fracture path to characterize the fracture process,which was simultaneously detected by AE and DIC.Experiment results showed,the property of horizontal displacement discontinuity in a small zone near crack was well fitted for FBG measurement,and the opening displacements obtained by grating region was well consistent with the displacement from DIC.At the onset of unstable propagation(peak load),the critical opening displacement δc was equal to 76 μm.The position of δc determined by FBG can be regarded as the boundary of FPZ and traction-free crack.This identification approach was well consistent with AE analysis,for example,70%-90%of AE energy was focused on the process-zone,and few AE energy was concentrated in traction-free zone.The length of fully developed FPZ was 20 mm(±2 mm).Finally,FBG was well verified for describing the mode I fracture characteristics(namely the critical opening displacement and process-zone length)inside the rock-like materials.