This document describes non-destructive testing using the transmission or reflection mode of terahertz time-domain spectroscopy (THz-TDS), and specifies the related detection equipment, calibration, and data analysis procedures to achieve the measurement of thickness of plastics and fiber-reinforced composite materials, as well as the assessment of defects such as delamination, pores, holes, inclusions, microcracks, etc., and the location of defects. This document is applicable to thermoplastic materials (such as polyethylene (PE), polypropylene (PP), etc.), thermosetting plastics (such as epoxy resin, unsaturated polyester, etc.), fibre-reinforced plastic (FRP) composites, including but not limited to glass fibre (GFRP), aramid fibre-reinforced plastics, and carbon fibre reinforced plastics (CFRP) under certain conditions (such as specific layup direction and limited thickness). NOTE 1 For CFRP, its detectability needs to be specifically evaluated based on factors such as fibre volume fraction, orientation, and terahertz wave frequency. The procedures are primarily intended for inspection of continuous unidirectional or multidirectional composites; however, this does not exclude their use on other formats of structural composites including woven and stitched fabrics and pultrusion. This document is applicable to products with flat or curved surfaces. For curved surface products, their geometries must be capable of ensuring effective coupling of terahertz beams with the detection surface. This document does not apply to the testing of defects in: - metal matrix composites; - composites containing a high proportion of conductive fibres; - specimens with rough surfaces; - specimens with high moisture content.