PSI - Issue 33

R. Nobile et al. / Procedia Structural Integrity 33 (2021) 685–694 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

686

2

potential in various high-performance composite sandwich structures which are now widely used in a wide range of applications, i.e. for energy absorption [Cho et al. (2012), Kara et al. (2014)]. Metal foams, such as aluminum foam, present a significant volume of porosity (up to 95%) that provide a considerably lighter structure than that of dense metal. Aluminum foam has been widely used in automotive and aerospace industries as engine mounting brackets, floor panel and protective devices [Srinath et al. (2010), Saharudin et al. (2013)]. These components usually do not require high density material and are potentially exposed to high impact loads. Fiber-reinforced polymer (FRP) laminated composites are characterized by peculiar mechanical properties such as high resistance to damage induced by shock loads. For example, glass fiber GFRP employs glass fibers reinforced with a polymer matrix [Mohd et al. (2015)]. Although the mechanical strength of GFRP is relatively lower than that of carbon fiber reinforced polymer (CFRP), GFRP exhibits higher ductility and lower manufacturing costs than CFRP [Hashim et al. (2014)]. In addition, sandwich structures are increasingly used in applications requiring high stiffness and flexural strength combined with low weight. These components are used in various manufacturing industries to improve constituent materials properties, as shown in the research conducted by Ahmad et al . (2011) where the CFRP and aluminum foam core sandwich structure showed greater energy absorption capacity than their components tested separately. Therefore, sandwich structures with aluminum foam core and GFRP skin combine lightness and rigidity in addition to advantageous costs; furthermore, in case of bending, the stiffness and strength increase rapidly with the thickness of the structure [Villanueva and Cantwell, (2003)]. Sandwich structures are well-known for their high strength/weight ratio when compared to conventional materials, however these structures can have complicated breaking mechanisms [Hosur et al. (2007)]. Triantafillou and Gibson (1987) studied the various failure modes of a sandwich subjected to bending test, classifying them as plastic deformation of the skin, instability of the skin in compression (or wrinkles), breakage by shear, tension or compression of the foam, indentation of the foam by the upper roller, the core / skin interface is broken. However, in the literature several studies demonstrate that most sandwich panels tested up to failure tend to be damaged by the delamination mechanism, i.e. when the front plate detaches from the surface of the core [Frostig (1992), Wadee and Blackmore (2001)]. Several researches have been conducted on different damage mechanisms of sandwich structures and recent studies investigated the effects of the pre-existing damage on static and fatigue behavior of composite structures [Dattoma et al. (2018)]. In this research, static three-points bending tests are carried out on two series of GFRP- aluminum sandwich panels (AFS) to compare the flexural behavior in the absence or in the presence of simulated impact damages. The experimental data results and damage modes are analyzed in both impact and bending tests; furthermore, phased array ultrasonic and pulsed thermographic investigations are carried out for damage zones assessment, showing a size variability of impacted regions due to non-uniform morphology and foam density. Finally, IRT and UT data are analyzed in MATLAB environment to compare the size evaluation of two NDT techniques to provide a complete characterization and quantify the damage in terms of size and shape of the impacted area.

Nomenclature AFS

Aluminium Foam Sandwich GFRP Glass Fiber Reinforced Polymer NDT Non-destructive Testing

2. Materials and methods 2.1. Manufacturing of Al-GFRP sandwich panels

In this work, ALULIGHT closed cell aluminum (Al) foam (20 and 30mm thickness) and GFRP (Glass Fiber Reinforced Polymers) laminates on flat Al surfaces of the sandwich are used for the manufacturing of sandwich panels. ALULIGHT foams are produced by Alulight International GMBH with decomposition of particles method