Jinyu Xu

A Modified Overstress Model to Simulate Dynamic Split Tensile Tests and Its Experimental Validation

The tensile strength of rock and rock-like materials is universally acknowledged to be much lower than their compressive strength and shearing strength (Meyers and Chawla 1999). Furthermore, the stability and reliability of rock structures are correlated remarkably well with the dynamic tensile strength of rock materials. Therefore, dynamic split tensile tests under high strain rate loads have been an active area of investigation in the field of rock mechanics. Carneiro (1943) first used Brazilian disc specimens to determine the tensile strength of brittle materials. Wang et al. (2004) improved these experiments by using flattened Brazilian discs (FBDs) instead to avoid stress concentration at the loading end. Wang’s method ensures that the initial crack only occurs in the central part of a specimen, which is vital for the validation of a split test. Recently, the split Hopkinson pressure bar (SHPB) has come into use as a general apparatus to determine the dynamic tensile strength of rock-like materials (Saksala et al. 2013; Xu et al. 2014). However, among the failure processes, crack propagation is difficult to observe, which makes it very difficult to judge whether or not the initial crack starts from the center. Finite element methods (FEMs) are widely used to simulate failure in brittle materials (e.g., Hang et al. 2015; Saksala et al. 2015). Compared with laboratory experiments, it is much easier and cost-effective to observe crack propagation in a simulation as long as an appropriate material model is built for the specimens. In this article, we build 3D models of FBD specimens in an SHPB apparatus and use these models to conduct numerical simulations of rock dynamic split tests. A modified overstress constitutive model is applied to describe the rock-like material. The failure patterns resulting from dynamic split tensile tests, as well as crack initialization and propagation, are studied with these simulations. The simulation results are verified by laboratory experiments using the SHPB and a high-speed camera.

Ultrasonic method to evaluate the residual properties of thermally damaged sandstone based on time–frequency analysis

Evaluation of the residual properties of thermally damaged rocks is of vital importance for rock engineering. For this study, uniaxial compression experiments and ultrasonic tests were conducted on sandstone specimens which experienced temperature treatments of different levels, including 25, 100, 200, 400, 600, 800 and 1000°C. Time–frequency analysis methods were applied to evaluate the deformation and strength properties of sandstone after being exposed to high temperature, confirming the effectiveness of the ultrasonic evaluation method. Linear correlations between the peak stress, deformation modulus and the longitudinal wave velocity confirm the effectiveness of ultrasonic time-domain properties in estimating the deformation behaviour of the thermally damaged sandstone. Synchronisation in the change of the peak stress and the kurtosis of frequency spectrum as temperature rises, defined in this paper to describe the spectrum distribution, as well as the centroid frequency, demonstrates the feasibility of ultrasonic frequency-domain properties in estimating the residual strength of the thermally damaged sandstone. The results have certain guiding significance for rock engineering in a high-temperature environment.

Research on Fracture Toughness of Flattened Brazilian Disc Specimen after High Temperature

Abstract Fracture toughness is an important parameter to study fracture characteristic of rock under external loads. Based on splitting tensile test on flattened Brazilian disc specimen of rock after high temperature, load–displacement curves of rock sample in the fracture process are obtained and three mechanical parameters of rock including fracture toughness, splitting tensile strength and elastic modulus are calculated according to the experiment. Then, the change rules of fracture toughness, splitting tensile strength and elastic modulus with temperature are discussed, and the relations between splitting tensile strength, elastic modulus and fracture toughness are established. Experimental results show that there exist two inflection points in load–displacement curves. The fracture toughness, splitting tensile strength and elastic modulus reduce gradually with increasing temperature, among which, the mechanical parameters of granite decrease approximately linearly while that of marble decrease approximately as exponential function. There is a close connection between splitting tensile strength, elastic modulus and fracture toughness, nearly a good linear relationship. Since the test method of splitting tensile strength is relatively simple and that of fracture toughness is complex, the fracture toughness can be roughly estimated by splitting tensile strength.