PSI - Issue 13

Guiyun Gao / Procedia Structural Integrity 13 (2018) 51–56

52

Guiyun Gao/ Structural Integrity Procedia 00 (2018) 000 – 000

2

1. Introduction

In engineering practice such as underground blasting and hydraulic fracturing stress measurement, rocks are under confining pressure and dynamic or impact load at the same time and behave differently as compared to being subjected solely to either static stress or dynamic loading(Saberhosseini et al., 2014; Zhu et al., 2015). However, researches on the crack propagation of rock under dynamic loading have rarely been reported.

Nomenclature K Ic

fracture toughness

K I K II

mode I stress intensity factor mode II stress intensity factor horizontal displacement

u x u y

vertical displacement

Confining stress influences the dynamic tensile strength and fracture toughness of rock materials. Zhou et al. (2014) and Wu et al. (2015; 2016) have experimentally studied the dynamic tensile failure of rock under static pre tension. They found that the dynamic tensile strength of rocks decreases with the increase of the pre-tension, while the increment of the tensile strength decreases with the hydrostatic confinement. Schmidt and Huddle (1977) measured the fracture toughness of the Indiana limestone as a function of hydrostatic pressure. They find that the fracture toughness increase from 0.93 MN m − 3/2 at atmospheric pressure to 4.2 MN m − 3/2 at a confining pressure of 62 MPa. Abou-Sayed (1978) used an internally notched thick walled cylindrical specimen to test the fracture toughness of Indiana Limestone. And the demonstration show that the fracture toughness is independent of overburden stress or pore pressure but increases substantially with confining stresses. Müller (1986) , Thallak et al. (1993) and Al-Shayea et al. (2000) also found similar phenomena using three point bending test under confining stress. However, few works are conducted on the dynamic fracture behaviors of rock plate under confining stress, and related experimental methods need to be developed. The objective of this paper is to characterize the dynamic fracture behavior of rock plate under uniaxial compression using the digital image correlation (DIC) method combined with ultra-high speed photography. Using a modified hydraulic pump, the uniaxial compression was exerted at the top and bottom ends of the rock plate. Dynamic crack propagation tests of plate specimen were conducted using split Hopkinson pressure bar (SHPB) and the fracture processes were captured by an ultra-high speed photography. The displacement and strain fields of the dynamic fracture process were calculated using DIC. By setting the virtual digital extensometer, the crack-tip position, crack propagation velocity and the dynamic fracture toughness were obtained. Digital image correlation (DIC) method is one of the non-contact optical measurement method, which has many advantages such as wide measurement range, no special requirement for specimen, etc. It could be used to measure the surface deformation of specimen with high precision combined with proper design of spackle patterns(Hall, 2012). The crack propagation information and fracture parameters could be obtained from this method combined with the theory of the fracture mechanics(Abshirini et al., 2016). This method will be extended to obtain the dynamic crack propagation velocity and the fracture toughness of confined rocks from the deformation fields based on DIC results. Consider a specimen that illuminate by a light source. The reference image and the deformed image are divided into several sub images. By maximizing the correlation coefficient, the location of a sub image (a square subset (2N+1)×(2N+1) centered at the considered point) in the deformed image is detected and the displacement components of this subset center can be determined using the correlation function ZNCC. The horizontal displacement u x and vertical displacement u y can be determined by optimizing the correlation function. The same tracking procedure is repeated on other points of interest, and the full-field displacement of the zone of interest (ZOI) can thus be obtained. 2. Measurement techniques and principles