Fracture and fatigue behavior of carbon/epoxy laminates modified by nanofibers

Abstract

This research focused on fracture and fatigue response of carbon/epoxy laminates modified with electrospun nylon 66 nanofibers. For this purpose, AS4/8552 composite laminates interleaved by nanofibrous mat were considered under mode-I quasi-static and fatigue loading conditions. According to test outcomes, the fracture toughness increased about 133% under quasi-static tests. On the other hand, the plotted fatigue curves showed that the crack growth rate significantly decreased in modified samples and the threshold energy release rate enhanced about 128%. Finally, scanning electron microscopy (SEM) was also conducted to investigate the damage mechanisms.

Introduction

High-strength carbon/epoxy laminates are being applied in various industrial applications, such as automotive and aerospace structures. One of the most widespread matrices used in these type of materials is thermoset polymers like epoxy. Notwithstanding many advantages, such as low density and perfect mechanical performance, laminated composites can be delaminated easily under mechanical loadings which is because of the brittle nature of thermosets [1], [2]. They can be failed by interlaminar cracks, which lead to the initiation of delamination and its propagation, resulting in final fracture.

Although the application of thermoplastics in the form of very thin film can be regarded as one of the most common toughening methods [3], [4], the associated penalty of sacrificing the elastic modulus of the final product has limited their use in many industries [5]. Consequently, thermoplastic polymers in the form of nanofibers have gained rapid acceptance since 2001 which is introduced for the first time to increase fracture toughness of thermosets-based laminates [6]. High surface-to-volume ratio and high porosity of nanofibrous mat increased their capability in comparison with the film layers, and on the other hand remove their mentioned problem [7], [8]. Various types of polymers, such as polysulfone [9], [10], phenoxy [11], polyvinylidene fluoride [12], [13], polyethersulfone (PES) [14], [15], polyvinyl butyral [16], [17], aramid [18], polycaprolactone [19], [20], [21], carbon [22], [23], [24], [25], [26] and nylon [15], [27] have been applied as the toughener between composite layers, but the most attractive one is the latest, i.e. nylon [28].

Different types of nylon, i.e. 6, 66, and 69, have been used for toughening composite laminates [2], [29], [30]. Most researchers have focused on the nylon 66 and its effect during mode-I and mode-II fracture tests [29], [31], [32]. Brugo and Palazzetti [33] considered the influence nanomat thickness and also the laminate type, unidirectional or plain-woven, on fracture toughness. The outcomes showed that increasing the thickness led to enhancement of fracture toughness and, on the other hand, samples made of woven type were more affected by nanofibers. Gholizadeh et al. [32] investigated the effect of nylon 66 nanofibers on different damage modes using the acoustic emission method. The results showed that matrix cracking, debonding, and fiber breakage reduced about 82%, 53%, 64%, respectively, by interleaving nanofibers. Some researchers also conducted low-velocity impact tests on the reference and nanomodified laminates [2], [34], [35]. Akangah et al. [2] interleaved AS4/3501-6 composite laminates by nylon 66 and impacted under 0.46–1.8 J. According to the outcomes: threshold impact force increased about 60% and the rate of impact damage growth rate decreased to one-half with impact height. In another study, Saghafi et al. [35] conducted tests on the curved glass/epoxy laminates. Their results showed adding nanofibers did not change impact parameters like maximum load and displacement, but could reduce the delaminated area significantly (about 60%).

Although many papers have been published regarding fracture and impact behavior of nanomodified laminates, the study about fatigue response of these laminates is very limited [36], [37], [38]. For instance, Polat et al. [36] examined the influence of graphene nanoplates/nylon 66 nanofiber mats on the fatigue life of composite to aluminum single-lap joint. The tests were done with various maximum stress levels as 20%, 30%, 40%, 50% and 60% of breaking shear tensile strength, and the results showed that the nanomaterial could increase the life cycle incredibly. In this study, pure nylon 66 nanofibers are used as the toughener of carbon/epoxy to consider the toughening mechanism under mode-I fracture and fatigue loadings. The fatigue tests were conducted under constant displacement ratio and various ratios of fracture toughness, (G_Iimax/G_IC). On the other hand, the aim is to assess the constant crack growth rate section of the Paris law diagrams for the reference and nanomodified specimens and also determining physical strain energy release rate (G*) for both samples. Finally, scanning electron microscope (SEM) was applied for investigating the fractured surface of laminates.