A promising method for the detection of subsurface defects in impacted composite materials made by natural fibers

In this paper, infrared vision and optical nondestructive testing (NDT) techniques were applied for the evaluation of emerging defects after impacts in composite materials made by natural fibers. The samples were manufactured using wool and jute fibers. Pulsed thermography (PT) and square pulsed thermography (SPT) were used as stimulated heating procedures, while two algorithms, named principal component thermography (PCT) and pulse phase thermography (PPT), were applied in order to enhance the acquired thermographic data. For comparison purposes, near-infrared reflectography (NIRR) and transmittography (NIRT), as well as laser speckle imaging techniques, were also included. Additionally, ultraviolet (UV) imaging technique was used on the samples both as complementary approach and as reference for the future inspections. Two particular zones highlighted by the latter technique were characterized using an XRF spectrophotometer which revealed a reverse concentration of the Al, Si and Cl elements. This work can be considered as an innovative approach in the characterization of eco-friendly laminates after an impact loading.