Detection and Evaluation of Pre-Preg Gaps and Overlaps in Glare Laminates

Abstract

Gaps and overlaps between pre-preg plies represent common flaws in composite materials that can be introduced easily in an automated fibre placement manufacturing process and are potentially detrimental for the mechanical performances of the final laminates. Whereas gaps and overlaps have been addressed for full composite material, the topic has not been extended to a hybrid composite material such as Glare, a member of the family of Fibre Metal Laminates (FMLs). In this paper/research, the manufacturing, the detection, and the optical evaluation of intraply gaps and overlaps in Glare laminates are investigated. As part of an initial assessment study on the effect of gaps and overlaps on Glare, only the most critical lay-up has been considered. The experimental investigation started with the manufacturing of specimens having gaps and overlaps with different widths, followed by a nondestructive ultrasonic-inspection. An optical evaluation of the gaps and overlaps was performed by means of microscope image analysis of the cross sections of the specimens. The results from the non-destructive evaluations show the effectiveness of the ultrasonic detection of gaps and overlaps both in position, shape, width, and severity. The optical inspections confirm the accuracy of the non-destructive evaluation also adding useful insights about the geometrical features due to the presence of gaps and overlaps in the final Glare laminates. All the results justify the need for a further investigation on the effect of gaps and overlaps on the mechanical properties.

Application of Dual Focused Ultrasonic Phased Array Transducer in Two Orthogonal Cross-Sections for Inspection of Multi-Layered Composite Components of the Aircraft Fuselage

“…The obtained results show the possibility to characterize the gap within thin laminates [13]. In addition, the investigation and evaluation of internal defects (overlaps and gaps) was performed by phased array testing, C-scan acquisition and attenuation analysis over gap regions [7,13]. The central frequency of single-element transducers in the through-transmission arrangement was 10 MHz.…”

“…of the necessary material properties for specific application conditions defined by fatigue, stress, etc. However, the application of non-destructive testing methods to test the structure of such materials during development, manufacturing and/or in-service inspection is complicated enough due to the presence of dissimilar materials (e.g., metal and fibre reinforced composite) layers possessing different material and acoustic properties.There are different existing types of GLARE construction [7]. The main constructional component is prepreg reinforced by unidirectional glass fibres and FM 94 adhesive.…”

“…The central frequency of single-element transducers in the through-transmission arrangement was 10 MHz. It was stated that this is the most sensitive frequency for assessment of GLARE quality [7,13,14]. The ultrasonic NDT procedure was developed in order to inspect the composite laminates in the presence of overlaps and gaps [7,13,14].…”

“…The application of phased array testing (frequency 5 MHz) and immersion through-transmission setup has been demonstrated for detection of delamination damages due to impacts [10]. However, the technique is not suitable for in-service applications and requires access to both sides of the specimen [7,8,10].The objective of this work is to apply the technique we proposed to enhance the 3D spatial resolution of ULPAT in the cases of detection and sizing demonstration of internal defects possessing spatially complex geometry, and during the inspection of defective multi-layered thin composite components (e.g., GLARE) of the aircraft fuselage.These particular tasks cover the achievement of higher accuracy of defect sizing inside the composite structure of joined dissimilar materials (e.g., lightweight metals and fibre reinforced plastics) and their possibility to be applied in the case of only single-side access to the surface of the sample.The solution is based on the application of a specially designed delay line with the fluid-based convex cylindrical lens [15], which was proposed for contact type inspection during quality control, NDT and material characterization of the multi-layered composites fabricated from joined dissimilar materials (e.g., metal and GFRP). The obtained results demonstrate that the spatial resolution of the 3D ultrasonic beam of ULPAT and the accuracy of defect sizing might be improved by applying the technique of using a spatially dual focused beam in two orthogonal cross-sections, both by lens and electronically.…”

“…There are different existing types of GLARE construction [7]. The main constructional component is prepreg reinforced by unidirectional glass fibres and FM 94 adhesive.…”