Over the last fifty years, the
plastics industry has developed greatly, outstripping the steel industry also in technical applications. This has led to
new synthetic substances progressively replacing traditional materials and to a formal rethinking of structures, ergonomic shapes and production processes.
What has made use of these materials become so widespread is essentially the fact that they are
cheap, light, easy to work and it is possible to
design the desired mechanical properties. Increasingly
accurate and in-depth mechanical characterization is therefore necessary and it is in this context that the need arises to know and
study thevalue of residual stresses induced by machining processes in these materials.
Also polymer melt flow, pressure distribution, non-uniform temperature field, and density distribution all cause
residual stresses in polymer injection moldings and these stresses affect the
mechanical properties of plastic parts, can alter the final shape and significantly
reduce the life expectancy of the product, in addition to increasing the likelihood of
dimensional instability and
environment stress cracking. Although residual stresses are commonly found in plastics their magnitude can be difficult to predict, as it depends upon a
wide range of variables including the mold design, material and processing parameters. Consequently, it is important to have a
reliable technique to evaluate the stresses existing in plastic components.
The
hole-drilling strain gauge method allows
residual stress to be measured in a wide range of plastic moldings. It has the advantage that the
measurements can be made over a
smaller area. A special
strain gauge rosette is bonded to the surface of the specimen and a
hole is drilled precisely through the center of the rosette. The strains measured at the surface correspond to the
stresses relaxed during the drilling process. Using the measured strains and appropriate models (i.e. ASTM E837) it is possible to calculate the
stresses along the two principal axes and their direction.