(6f) Methods to Evaluate Residual Stress in FDM Printed Parts
Additive manufacturing has gained a lot of attention lately because of its ability to develop prototypes directly from a CAD model. However, due to the layer-by-layer sequential solidification of the printed material and large temperature differences that result during the printing process, residual stresses are prone to develop in AM parts, leading to geometrical inaccuracies like volume shrinkage, warpage and distortion of the printed part, adversely affecting the final product quality. Measuring and controlling such residual stresses can therefore help us better choose the printing parameters and hence greatly enhance the dimensional accuracy of the printed parts. In this study, Fused deposition Modeling (FDM) is employed to study the effect of printing speed, printing temperature and nozzle diameter on the residual stress generated in commonly used FDM filaments like ABS and PC printed parts. Volume shrinkage of ABS printed samples is quantified suing Dynamic Mechanical Analysis (DMA). The destructive hole-drilling technique coupled with a 2D subset-based digital image correlation (DIC) software is used to measure the residual strains. Non-destructive optical residual stress measuring techniques like RAMAN spectroscopy and birefringence are also used for characterization. Finally, an experimentally validated finite element analysis (FEA) model is developed to simulate the FDM printing process and the hole-drilling method. A non-linear temperature-dependent viscoelasticity model is also developed to predict the behavior change of the filament as it solidifies. The simulations and models developed can be directly applied to predict the residual stresses that are generated during the FDM printing process and hence fine tune the printing parameters leading to an improved final product quality.