The Impact of Axial Load Distribution on Brazilian Tensile Testing on Rock

Tensile strength of rock and rock-like materials is a critical material property in rock engineering and the design of infrastructure that interacts with the geological environment. This study presents findings from an indirect Brazilian Tensile Strength (BTS) laboratory testing program in which platens were aligned to facilitate even loading across the disc specimen and then compared with measured results when platens are misaligned, but within acceptable thresholds defined by the International Society for Rock Mechanics and Rock Engineering (ISRM) Suggested Methods, causing uneven loading across the BTS disc specimen. New data processing techniques involving the use of foil strain gauges were used to measure the horizontal strain response under load and calculate strain thresholds of true tensile strength (TTS) and splitting tensile strength, also referred to as BTS. To test the variability in strain readings at mid-height on a disc specimen, a second BTS laboratory testing program consisting of specimens instrumented with a 10 mm foil strain gauge on one side of the specimen and a 20-mm foil strain gauge on the other was completed. Results from experiments with different strain gauge foil lengths show that for NQ and NQ3 size core, the measured strain response from 10- and 20-mm foil length gauges were similar and can both be effectively used for the measurement of different strain thresholds in BTS tests. However, the results from the platen alignment program demonstrate that uneven loading across the specimen results in significantly lower measured tensile strength values, even though both platen configurations are within the tolerance prescribed by the current ISRM Suggested Methods. We strongly recommend (i) a new maximum tolerance for platen alignment, specifically 0.06°/0.001 radians (26% of the current standards), and (ii) measurement of horizontal strain on both sides of a BTS specimen to validate the platen geometry and tensile strength results, be widely adopted to improve standard practice of BTS laboratory testing.