Eco‐Friendly Laminates: From the Indentation to Non‐Destructive Evaluation by Optical and Infrared Monitoring Techniques

Abstract

In this work, the combined effect of indentation damage and of manufacturing defects of a hybrid laminate including jute hessian cloth (plain weave) and hemp fibres in an epoxy matrix has been investigated. With this aim, various non-destructive evaluation (NDE) techniques have been employed, such as near-infrared (NIR) reflectography, infrared thermography (IRT), holographic interferometry (HI) and digital speckle photography (DSP). In particular, two different methods of heating were applied during IRT data collection: pulse thermography and square pulse thermography (SPT). The first one using a mid-wave infrared (IR) camera, while the second one using a longwave IR camera. In the same way, two different cameras working into the near-and short-wave IR spectra were used, to compare different results from $ 0.74 to 14 lm. Data were processed applying principal component thermography (PCT), correlation and the robust secondorder blind identification (SOBI-RO) algorithms. The latter is used for the first time to our knowledge in this work. The defects found were enhanced by image subtraction between the reflectogram and the transmittogram, distance transform and image fusion. In particular, data fusion from IRT and DPS images allowed clearly defining the extension of the indentation damage.

Basalt fibre laminates non-destructively inspected after low-velocity impacts

“…3, the validity of the subtraction between images (i.e., the reflectogram and the transmittogram) loses its importance, as a proper image to be used as an input for the DT was seriously conditioned by the events happened after the impacts. Instead, in the case of the laminate impacted at 7.5 J, the validity of the method was preserved; indeed, the weaker zones arisen after the impacts encircled the damage by following the classical configuration already described in [18] . The speckle pattern shearing interferometry technique was able to retrieve the mechanical imprint due to the tip of the indenter, and the subsequent satellite defect [19] also detected by the method of subtraction of images.…”

Development of Infrared Techniques for Practical Defect Identification in Bonded Joints

“…PT and square pulsed thermography (SPT), where a longer heating pulse is used, has been applied to natural fibre laminates with static impact damage [36]. It was found that PT was able to identify different types of defects whereas SPT was reported to give information about the shape of the impact while undergoing the same post processing.…”

Detection of incipient defects in book bindings. A functional use of the infrared vision combined with optical and nuclear magnetic resonance (NMR) spectroscopy techniques

“…van Asperen de Boer [31] as a technique used to look through the paint layers (reflectogram) or to inspect the internal fibres distribution (transmittogram), it became more relevant in the composite materials field in 2013, when a digital subtraction performed in Matlab ® environment between the reflectogram and the transmittogram recorded at the same wavelength with the result obtained subsequently analyzed via distance transform (DT), has been applied for the first time [32].…”