Importance of Displacement Measurement Location of Polymeric Dogbone Tensile Test Specimens

Rock supporting has a vital importance in longevity and sustainability of all underground openings. There are many types and forms of supporting systems available in the mining/tunneling industry. In recent decade, new polymer-based surface support material, thin spray-on liner (TSL), has started to take place of conventional surface support materials. Although TSL holds numerous merits and application purposes, the knowledge on mechanical properties of this material is in limited levels yet. In laboratory environment, researchers have generally studied the tensile failure mechanisms of TSLs. However, these experimental studies have observed to be carried out with misleading set-ups. Although the relevant test standards (ASTM -638) adopted from polymeric material testing to tensile testing of TSLs emphasize the exact displacement measurement location and methodology, researchers have neglected these procedures. In this study, importance of displacement measurement location of dogbone tensile test specimen on mechanical behavior determination of TSLs are studied. Axial extensometer and linear variable displacement transducer (LVDT) are used in the tensile testing of a polymeric TSL material (Meyco TSL-865, by BASF). Under the scope, 25 valid tensile tests are conducted and axial displacement is measured by two different ways simultaneously. Firstly, an LVDT transducer is mounted to the grips of the testing machine, as performed malpractice in the literature and secondly, an axial extensometer is attached to the gauge length of the dogbone specimen, as suggested by the standards. Resultant test findings reveal that the measured values show notable difference for tensile modulus and elongation at break for the TSL tested. LVDT measurements overestimates elongation at break by 3 times compared to axial extensometer. Consequently, the causes of measurement variations by two different methods are interpreted based on a fracture mechanics approach of the polymeric TSL material during tensile testing