Hygrothermal effects on mode II interlaminar fracture toughness of co‐bonded and secondary bonded composites joints

Abstract

Adhesive joints are being more extensively applied in the aeronautical industry, allowing for better integration between the structural parts and overall lower weight if compared with joints made with fasteners and rivets. However, a further evaluation of these new technologies is needed, once their critical fracture toughness under different environmental conditions is still unknown and this value is essential for design and certification of aircraft manufactured with these materials. Thus, this work focuses on the mode II fracture toughness characterization of carbon fiber composite laminates joined by co‐bonded (CB) and secondarily bonded (SB) techniques, using EA 9695 epoxy adhesive aged at different environmental conditions (room temperature ambient—RTA and elevated temperature wet—ETW). Dynamic mechanical analysis (DMA) was used to understand the effect of moisture absorption on the glass transition of materials and on the decreasing of Mode II fracture toughness after aging. The DMA results showed a reduction of 11% in Tg values for the GIIc values of ETW samples in comparison with specimens tested at RTA condition. Reductions about 92 and 94% in Mode II fracture toughnesses were obtained for CB and SB aged specimens, respectively when compared with the toughness values obtained for specimens tested at RTA. Further inspection of the fracture surfaces using scanning electron microscope proved that light fiber‐tear fracture occurred at both RTA and ETW conditions for CB joints, while fracture was mainly light‐fiber‐tear at RTA condition, becoming mostly cohesive after aging for SB joints. POLYM. COMPOS., 40:3220–3232, 2019. © 2018 Society of Plastics Engineers

Experimental characterization of Mode II fatigue delamination growth onset in composite Joints

“…This prepreg also contain thermoplastic polyamide particles on its system to reduce crack propagation and improve the toughness, as reported by. 13 All specimens had a [0°] 26 lay up with a Teflon film insert in the mid-plane of one end, acting as a crack starter with 40 mm extension from the edge of the laminate. Three types of joining techniques were considered: cocuring (CC), co-bonding (CB), and secondary bonding (SB).…”

“…The flexural modulus (E) was measured on eight samples for each type of joint technology, and the average values together with the standard deviation and COVs are presented in Table 1. [13][14][15] The flexural modulus was necessary to calculate the critical displacement associated with static delamination growth used in the analytical model for Mode II delamination. Although all specimens were made of the same material, they were obtained through different composite joining processes (CC, CB, and SB).…”

“…The first studies focused on bonded joints are dated back to the 70s 11 and this technology has been gaining increasing attention in recent decades. 12,13 In addition, bonded composite parts exhibit improved damage tolerant behavior, weight reduction, better stress distribution, flexibility in design, longer fatigue life, low manufacturing cost, and support vibrations. 14 Renton and Vinson 11 studied adhesive bonded joint aimed to evaluate the influence of the stress distribution and identify the design joint parameters.…”

“…Today, the studies significantly advanced when compared to the work developed by Renton and Vinson, 11 since several researchers are evaluating parameters that can interfere in the joint process, like adhesive, temperature, surface treatment, thickness, interface, and humidity. 13 Nowadays, co-curing (CC), co-bonding (CB), and secondary bonding (SB) are the most common bonding techniques employed in composite assemblies. [12][13][14] Cocuring corresponds to the process of joining and curing simultaneously two parts of the prepreg without additional adhesive interface.…”

“…13 Nowadays, co-curing (CC), co-bonding (CB), and secondary bonding (SB) are the most common bonding techniques employed in composite assemblies. [12][13][14] Cocuring corresponds to the process of joining and curing simultaneously two parts of the prepreg without additional adhesive interface. Co-bonding is the union of a cured structural part with another uncured part through an adhesive, which also needs to be cured.…”