An experimental study on the effect of the length-to-diameter ratio on the post-peak behaviour of rock

Whether the post-peak behaviour of rock is a characteristic is currently a matter of debate. One of the debatable issues is whether specimen length affects post-peak behaviour. To investigate the effect of specimen length on post-peak behaviour, a series of laboratory tests were conducted in parallel at three laboratories. Each laboratory tested at least two rock types: a soft and weak rock (Class I) and a hard and strong rock (Class II). The length-to-diameter (L/D) ratios of the specimens were 1, 2.5, and 4. The radial strain was measured using a Linear Variable Differential Transducer (LVDT) placed around the specimen at mid-height. The axial strain was measured over a fixed gauge length in one laboratory, whereas it was measured over the entire specimen length in the other two laboratories. The test results showed that when the L/D ratio increased from 2.5 to 4, the post-peak stress–strain curve of the specimens remained unchanged when the gauge length was fixed on the specimen; however, it became dependent on specimen length when the measurement was taken over the entire specimen length. A computational algorithm was developed to explore the relationship between the post-peak behaviour of rock and energy conversion. The results indicate that the post-peak stress–strain curve reflects the characteristic behaviour of rock when the specimen L/D ratio is equal to or greater than 2.5 and the strains are measured within the fracture zone. • The post-peak stress–strain curve is a characteristic behaviour when the axial strain is measured in the fracture zone. • The PPB is influenced by the L/D ratio when the axial strain is measured over the entire specimen length. • An algorithm was developed to calculate the PPB by accounting for energy conversion.