Residual stress on polymers

Over the last fifty years the plastics industry has developed greatly, outstripping the steel industry also in technical applications. Accurate and in-depth mechanical characterization is therefore necessary, and it is in this context that the need arises to know and study the value of residual stresses induced by machining processes in these materials.

The hole-drilling strain gage method can be applied on several materials including unfilled polymers with the introduction of some variations in the measurement procedure. When applying this method to such materials, it is necessary to minimize the influence of thermal and mechanical effects due to both temperature variations and hole-drilling process.

Moreover, polymers are very sensitive to the influence of temperature when compared to metals, due to the higher thermal expansion coefficients. Due to the lack of a dedicated strain gage rosette available on the market, a dummy strain gauge can be installed on an unstressed specimen and maintained at equilibrium in the same environment with the component under test.

A special version of the MTS3000-Restan system can be used to determine residual stress on polymeric materials.

The MTS3000-Restan system for polymeric materials is provided with a controller that makes this test completely automatic, easy and fast: an automatic device allows to manage drilling process (feed rate and drilling speed) and strain gage acquisition. Regarding strain acquisition, the technique of dummy gage can be used for avoiding any apparent thermal effect on the acquired strain.

The drilling system is powered and automatically controlled by the electronic control system and drilling control software, thus making the drilling process fully automated.

The mechanical system is powered and managed by a controller and a dedicated software, thus making the drilling process fully automatic.

The whole measurement apparatus is remotely operated: this option is advisable as it minimizes the external influence of the operator during the measurement process.

residual stress polymers rosette hbm

Thanks to a dedicated drilling slide working at low speed, drilling process does not generate any local melting and burning around the drilled hole, thus avoiding undesirable effects on acquired strains and calculated residual stresses.

residual stress measurement chain

The entire measurement chain is validated using both standard bending and 4-point bending test rig configuration.

More information regarding theoretical topics and experimental validation of the hole-drilling method on polymeric materials can be found in a dedicated article titled “Improvements in the Hole-Drilling Test Method for Determining Residual Stresses in Polymeric Materials” published on ASTM Journal “Materials Performance and Characterization”.