Damage Assessment of Glass-Fibre-Reinforced Plastic Structures under Quasi-Static Indentation with Acoustic Emission

Abstract

The use of fibre-reinforced plastics (FRPs) in various industrial applications continues to increase thanks to their good strength-to-weight ratio and impact resistance, as well as the high strength that provides engineers with advanced options for the design of modern structures subjected to a variety of out-of-plane impacts. An assessment of the damage morphology under such conditions using non-destructive techniques could provide useful data for material design and optimisation. This study investigated the damage mechanism and energy-absorption characteristics of E-glass laminates and sandwich structures with GFRP face sheets with PVC cores under quasi-static indentation with conical, square, and hemispherical indenters. An acoustic emission (AE) technique, coupled with a k-means++ pattern-recognition algorithm, was employed to identify the dominant microscopic and macroscopic damage mechanisms. Additionally, a post-mortem damage assessment was performed with X-ray micro computed tomography and scanning electron microscopy to validate the identified clusters. It was found that the specific energy absorption after impact with the square and hemispherical indenters of the GFRP sandwich and the plain laminate differed significantly, by 19.29% and 43.33%, respectively, while a minimal difference of 3.5% was recorded for the conical indenter. Additionally, the results obtained with the clustering technique applied to the acoustic emission signals detected the main damaged modes, such as matrix cracking, fibre/matrix debonding, delamination, the debonding of face sheets/core, and core failure. The results therefore could provide a methodology for the optimisation and prediction of damage for the health monitoring of composites.

Effects of Seawater on Mechanical Performance of Composite Sandwich Structures: A Machine Learning Framework

“…The conical indenter geometry led to an early onset of damage, while the seawater induced plasticisation and swelling of the epoxy matrix, affecting the adhesive bonding between the constituents of the FRPSS. At the microscale, this deterioration was characterized by a weaker fibre/matrix interface, as well as reduced intra- and interlaminar bonding, and debonding between the face sheet and the core at the macroscale [ 30 ]. In the case of indentation with larger contact areas, such as with hemispherical and flat indenters, the GSV samples were the most affected by the seawater exposure, experiencing decreases in the failure load of 38.8% and 46.1%, respectively.…”

“…Similarly, maintaining a constant pH level was crucial to prevent chemical reactions with the samples. Moisture absorption in composite structures occurs through three mechanisms: (i) moisture ingress into manufacturing-induced defects [ 25 ]; (ii) capillary/wicking along the fibre/matrix interface; (iii) combination of water molecules and hydrophilic resin groups [ 30 ]. Gravimetric measurements were conducted to determine the moisture gain in the exposed samples [ 15 ].…”

“…During the test, the indenter was aligned with the centre of the specimen with an offset of no more than 0.01 mm and was then applied until complete perforation of the sample. Further details regarding the QSI methodology employed can be found in our previous study [ 30 ].…”

“…The challenge of k -means++ is usually the need to specify the number of clusters a priori [ 23 , 35 , 36 ]. It is noteworthy that the optimum clusters for the k -means++ were identified in the previous work [ 30 ] using two cluster-validity indices, namely the normalized Calinski–Harabasz index and the Davies–Bouldin index.…”

“…This is based on the direct correlation between the damage to the material, its susceptibility to moisture ingress, and the variation in the AE features during loading. Although four damage modes were identified in a previous study and were shown to be influenced by specific indenter geometries [ 30 ], the novelty of this work is its analysis of the effects of seawater on the structural health of FRPSS.…”