Residual stress analysis in polymer materials using the hole drilling method - basic principles and applications

Residual stresses are mechanical stresses in materials free of external loads, moments or temperature gradients. These residual stresses form a self-equilibrated and inhomogeneous stress state, which is then superimposed upon external stresses applied to the material. A distinction is made between macroscopic residual stresses, which are comparable to loading stresses, and microscopic residual stresses, which take the inhomogeneous microstructure of materials into account. In polymer materials, residual stresses are generally produced during the processing phase due to changing pressure history and inhomogeneous cooling. Residual stresses are mostly unwanted in polymer materials, because they induce geometrical changes, such as warpage. These deformations can be predicted with finite element analysis. Residual stresses also play a role in the mechanical integrity of workpieces. In fact, they influence the stress field around a crack, and even small amounts of residual stresses can cause early defects, for example, in pressurized polymer pipelines. Moreover, tensile loads and tensile residual stresses are known to have a high tendency to crack when exposed to an aggressive chemical environment. Since this environmental stress cracking is one of the most frequently occurring failures in polymer materials, it is necessary to be able to measure residual stresses to better predict the lifetime of products. For example, in medical devices, the intensive use of chemicals can lead to cracks on the surface of components. Inducing compressive residual stresses near the surface of components could be an effective way to increase the resistance to environmental stress cracking, by limiting the influence of possible external tensile loads. Therefore, residual stresses can also have positive effects on the mechanical properties of polymer materials. Altogether, residual stress measurements are needed to validate results from finite element analysis and to predict the short-term and long-term mechanical behaviour of products.