Defect detection in additively manufactured natural fiber composites using active thermography

Active thermography is a fundamental technique for the non-destructive evaluation (NDT) of 3D printed biocomposites, as it allows the identification of internal defects invisible to the human eye. This method is based on analysing the thermal response of the material to an external energy source, such as infrared radiation, to detect anomalies beneath the surface. Its application is particularly relevant in Natural Fibre Composites (NFC), where internal voids and anisotropies generated during the printing process can compromise mechanical performance. One of the key advantages of this technique is its ability to perform accurate quality checks without damaging parts with complex geometries. In this research, parts with missing layer defects were deliberately manufactured by 3D printing using a Polylactic Acid-based NFC with recycled pine fibres. For the first time, a specific set-up was established to apply active thermography in lock-in mode, using a high-resolution camera and a portable heat source with modulated pulses. After acquiring thermal sequences, Principal Component Thermography processing was applied to identify internal defects. The results confirmed the feasibility of this technique as an NDT method also for materials with natural fibres, despite their particular thermal properties.