Due to the high specific strength and the very cost efficient manufacturing process by injection moulding, reinforced polymeric materials are more and more used in structural application in automotive and other industries. Since such parts are often developed for long service life under various environmental conditions, it is essential to get a good knowledge about the fatigue behaviour of reinforced plastics. To consider the lifetime of a part in an early state of a development process, it is indispensable to deal with all effects on the material from the manufacturing process to the real load history. A closed simulation chain is established to estimate the life time of a part based on local mechanical properties, depending on the fibre orientation from an injection moulding process. Other stress based influences like the mean stress or the stress peaks at notches are also covered. The behaviour of polymer based materials show a strong dependency on environmental conditions. Therefore it is also necessary to understand whether and how these conditions influence the mechanical behaviour. This work focus on how the absorption of water affects the dimensions and mechanical properties of a short glass fibre reinforced polyamide. Therefore quasi-static and cyclic tests were performed after aging the specimens in water. The test results show a significant decrease of the mechanical properties over the aging time. The absorption of water leads to an increase in volume and to remarkable residual stresses, caused by anisotropic swelling. A FE-simulation was set up, so that it was possible to cover the influence of residual stresses in a closed simulation chain as described before. The simulation results show a good correlation between the measured and the simulated increase in volume.