Terahertz time-domain spectroscopy of weld line defects formed during an injection moulding process

The terahertz time-domain spectroscopy (THz-TDS) imaging technique was used to detect the weld line defect of an injection-moulded short glass fiber-reinforced composite. To determine the effect of the weld lines on tensile strength, tensile tests were conducted with the specimens fabricated with NORYL (amorphous blends of polyphenylene ether resin and polystyrene) having a short glass fiber content ranging from 0% to 30%. The time delay phenomenon of the THz waves was observed with respect to the weld line and short glass fiber contents. The interaction between the THz waves and structural characteristics of the weld line, such as density and fiber orientation, were studied and different time delay phenomena of THz signal were observed owing to the different refractive index of the weld line defect. The refractive index of THz wave was also obtained by an analytical calculation and compared with the experimental results in order to determine the correlation between degradations of tensile strength and changes in the reflected THz wave caused by the weld line. Finally, the weld line defect of the specimens was successfully detected and identified using a THz-TDS imaging technique, showing that this technique is a nondestructive inspection method that can be utilized to evaluate the reliability of injection moulded short glass fiber-reinforced composite structures.