Interlaminar Fracture Toughness Estimation of Aerospace Composites by Weighted Residual Approach

Abstract

Introduction. Preventing failure of composite material systems has been an important issue in engineering design. There are two types of physical failures that occur in laminated composite structures and interact in complex manner are intralaminar and interlaminar failures. Intralaminar failure is manifest in micromechanical components of the lamina such as fiber breakage, matrix cracking, and debonding of the fiber-matrix interface. Generally, aircraft structures made of fiber reinforces composite materials are designed such that the fibers carry the bulk of the applied load. Interlaminar failure such as delamination refers to debonding of adjacent lamina. The possibility that intralaminar and interlaminar failure occur in structural components is considered a design limit, and establishes restrictions on the usage of full potential of composites. Due to the lack of through-the-thickness reinforcement, structures made from laminated composite materials and adhesively bonded joints are highly susceptible to failure caused by interfacial crack initiation and growth. The delamination phenomenon in a laminated composite structure may reduce the structural stiffness and strength, redistribute the load in a way that the structural failure is delayed, or may lead to structural collapse. Therefore, delamination is not necessarily the ultimate structural failure, but rather it is the part of the failure process which may ultimately lead to loss of structural integrity.Most of the components on the aircraft are increasingly being replaced with composite materials. The main attraction is the effective reduction in mass with a comparative increase in stiffness, strength, fatigue and impact resistance, thermal conductivity and corrosion resistance. Through these replacements, the structural weight can be reduced, which will in turn lead to a more economical commercial aircraft [1]. The major structural applications for fiber-reinforced composites are in the field of military and commercial aircrafts, for which weight reduction is critical for higher speeds and increased payloads. Ever since the production application of boron fiber-reinforced epoxy skins for F-14 horizontal stabilizers, the use of fiber-reinforced polymers has experienced a steady growth in the aircraft industry. Carbon fiber-reinforced epoxy has become the primary material in many wings, fuselage, and empennage components. The structural integrity and durability of these early components have built up confidence in their performance and prompted developments of other structural aircraft components, resulting in an increasing amount of composites being used in military aircrafts. The F-22 fighter aircraft also contains nearly 25% by weight of carbon fiber-reinforced polymers. The outer skin of B-2 and other stealth aircrafts is almost all made of carbon fiber-reinforced polymers. The stealth characteristics of these aircrafts are due to the use of carbon fibers, special coatings, and other design features that reduce radar reflection and heat radiation [2].A...

Adhesive Through-Reinforcement Improves the Fracture Toughness of a Laminated Birch Wood Composite

“…Hence, there is significant interest in improving the fracture toughness of laminated composites, particularly those used for aircraft construction [1,3]. Modern high performance aircraft uses a range of components made from laminated carbon-fibre/epoxy composites [4], but in less demanding aerospace applications the laminated wood composite plywood is still used (Euro Plywood 2016) [5]. Plywood consists of thin layers of wood veneer that are glued together, with adjacent layers having their wood grain rotated up to 90 degrees to one another [6].…”

“…The susceptibility of laminated composites to delamination depends on a variety of intrinsic and extrinsic factors, but the lack of reinforcement of adhesive bond lines is a fundamental reason why cracks propagate between laminae [4]. Therefore, one obvious route to increasing the fracture toughness of laminated composites is to provide such reinforcement.…”