Improvements in the Hole-Drilling Test Method for Determining Residual Stresses in Polymeric Materials

Abstract

Abstract This article describes some improvements of the hole-drilling test method to increase the accuracy in determining residual stresses in polymeric materials. Unlike traditional applications of the hole-drilling technique to metals, when applied to polymers, it is essential to take into account important aspects such as the influence of ambient temperature, local heating, and thermal and mechanical effects generated by drilling operations. Unfilled polymers have a great coefficient of linear thermal expansion range, but the strain gauges for residual stresses available on the market are self-compensated only for some metallic materials. Furthermore, their low thermal diffusivity and the strong temperature dependence of their mechanical properties, notably, elastic modulus, creep, and relaxation, could determine local heating problems. The article investigates the hole-drilling technology on these materials and presents a testing procedure suitable for minimizing sources of error. The tests are carried out in a temperature-controlled test environment, and drilling is performed using the automatic drilling technique with a remote control, rather than a manual, drilling technique. Selections of the drilling rotational speed, feed rate, and type of drilling cutter are examined in detail. In order to validate the measurement approach, the experimental results obtained with a bending test bench on a polyethylene terephthalate sample are shown.