Showing papers by "Domenica Paoletti published in 2013"

Falling weight impacted glass and basalt fibre woven composites inspected using non-destructive techniques

Abstract: A limited number of comparative studies on falling weight impact properties of different composites exist, especially using non-destructive techniques (NDTs). In this work, two types of woven fabric composites, reinforced respectively with E-glass fibres and basalt fibres, were subjected to low velocity impact at different energies (7.5, 15 and 22.5 J). Comparative indications were offered by impact hysteresis cycles and the integration of data between different enhanced vision methods, namely interferometric and IR thermographic techniques. The integrated application of these techniques suggests that the increased directionality of impact damage observed in basalt fibre reinforced composites, though their impact performance appears to be slightly superior, may represent a limitation on the predictability of their behaviour.

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 long-wave 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 μm. Data were processed applying principal component thermography (PCT), correlation and the robust second-order 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.

Nondestructive testing of externally reinforced structures for seismic retrofitting using flax fiber reinforced polymer (FFRP) composites

Abstract: Natural fibers constitute an interesting alternative to synthetic fibers, e.g. glass and carbon, for the production of composites due to their environmental and economic advantages. The strength of natural fiber composites is on average lower compared to their synthetic counterparts. Nevertheless, natural fibers such as flax, among other bast fibers (jute, kenaf, ramie and hemp), are serious candidates for seismic retrofitting applications given that their mechanical properties are more suitable for dynamic loads. Strengthening of structures is performed by impregnating flax fiber reinforced polymers (FFRP) fabrics with epoxy resin and applying them to the component of interest, increasing in this way the load and deformation capacities of the building, while preserving its stiffness and dynamic properties. The reinforced areas are however prompt to debonding if the fabrics are not mounted properly. Nondestructive testing is therefore required to verify that the fabric is uniformly installed and that there are no air gaps or foreign materials that could instigate debonding. In this work, the use of active infrared thermography was investigated for the assessment of (1) a laboratory specimen reinforced with FFRP and containing several artificial defects; and (2) an actual FFRP retrofitted masonry wall in the Faculty of Engineering of the University of L’Aquila (Italy) that was seriously affected by the 2009 earthquake. Thermographic data was processed by advanced signal processing techniques, and post-processed by computing the watershed lines to locate suspected areas. Results coming from the academic specimen were compared to digital speckle photography and holographic interferometry images.