Novel in-situ residual strain measurements in additive manufacturing specimens by using the Optical Backscatter Reflectometry

Material extrusion (MEX) is a well established production method in additive manufacturing. However, internal residual strains are accumulated during the layer-by-layer fabrication process. They bring about shape distortions and a degradation of mechanical properties. In this paper, an in-situ distributed measurement of residual strains in MEX fabricated thermoplastic specimens is achieved for the first time. This innovative measuring system consists of an Optical Backscatter Reflectometry (OBR) interrogation unit connected to a distributed fiber optic strain sensor which is embedded during the MEX process. The characteristic residual strain distribution inside 3D printed components is revealed and numerically validated. The main mechanisms of residual strain creation and the sensing principles of in-situ OBR are described. A minimum measuring range of 4 mm and a spatial resolution of 0.15 mm were experimentally demonstrated. The potential of in-situ OBR technology for detecting invisible manufacturing defects was shown by a trial experiment.