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Disbond monitoring of adhesive joints reinforced with carbon nanofibres
| Content Provider | Semantic Scholar |
|---|---|
| Author | Ladani, Raj B. Wu, Shuying Mouritz, Adrian P. Kinloch, Anthony J. Ghorbani, Kamran Wang, Chun Hai |
| Copyright Year | 2015 |
| Abstract | This paper focuses on the ability of carbon nanofibre (CNF) networks for in situ monitoring of fatigue induced disbond damage in carbon fibre adhesive bonded joints. The mode I fatigue delamination behaviour of composite joints bonded with an unmodified epoxy adhesive and 0.7 wt% CNF modified epoxy adhesive are evaluated. The inclusion of CNFs in the epoxy adhesive increases its conductivity by five orders of magnitude while simultaneously retarding the delamination growth rate. The mode I critical strain energy of the CNF modified adhesive increases by about five folds from 88 J/m to 450 J/m under cyclic fatigue loading. The improved electrical conductivity is utilized to evaluate the ability of the CNF network to monitor and detect the fatigue induced disbond damage by in situ measuring the resistance changes using a four probe setup. The changes in total resistance was a function of the bulk electrical resistivity of the adhesive and the bond dimensions, which were related to the disbond length to model and determine the size of the disbond. Good agreement were found between the optical disbond observations and the calculated disbond length using the in situ resistance measurements, therefore proving the ability of CNFs to not only detect delamination as small as 1 mm in composite bonded joints but also retard its growth rate. 1 INTRODUCTION One of the key challenges for increasing the uptake of fibre composites is the need for joining during initial modular construction. Adhesive bonded joining provides many advantages such as high strength to weight ratio, low stress concentration, fewer processing requirements and good Raj B. Ladani, Shuying Wu, Adrian P. Mouritz, Anthony J. Kinloch, Kamran Ghorbani and Chun H. Wang environmental resistance[1]. However, a major drawback with adhesive joints is their inability to be disassembled for periodic inspection. Therefore, bolted joints are currently preferred in the industry for joining composite parts, in spite of their low bearing strength and the resultant higher stress concentration regions emanating from the discontinuity of the reinforcing fibres[2]. Thermosetting epoxy polymers are widely used for bonding purposes due to their superior mechanical property and good resistance to environmental degradation. However, due to their inherently brittle structure, they offer poor resistance to delamination or disbond damage[3]. An undetected barely visible damage in epoxy bonded joints can rapidly grow to failure under cyclic loads due to the low fatigue strain energy threshold of epoxy polymers[4,5]. Since regular visual aircraft inspection carried out during service are limited to the detection of flaws that lie near the surface, an underlying disbond could remain undetected between service intervals and therefor has mandated overdesigning of the composite structures[6]. Moreover, current damage inspection techniques are limited to ultrasonic, radiography and acoustic emission for adhesively bonded composite joints. Potential drop and eddy current technique cannot be used due to the absence of through-thickness conductivity in bonded composite joints as a result of the dielectric property of the epoxy matrix and adhesives[7]. With regular visual inspections limited to the detection of flaws that lie near the surface, there is a strong need for additional repeatable and reliable non-destructive inspection techniques that could be applied on field and in situ to monitor the integrity of bonded composite joints. In addition, a lightweight bonded joint design in only possible if the damage growth behaviour is sufficiently slow to enable the planning of an inspection schedule that enable reliable detection of damage before it becomes critical[8]. Recent studies have shown that conductive canbon nanofillers such as carbon nanotubes (CNTs)[9,10] or carbon nanofibres (CNFs)[11,12] can form conductive networks in polymeric materials at extremely low weight fractions while simultaneously improving the fracture toughness. These conductive fillers have been incorporated into fibre composites for damage detection[13–16]. More recently, Lim et al.[17] reported the use of CNT networks to monitor the initiation of damage in epoxy bonded lap joints under quasi-static and dynamic tensile loading. Similarly, Mactabi et al.[18] studied the electromechanical response of CNT networks incorporated in bonded lap joints under tensile fatigue loading. In adhesives containing conductive carbon nanofilles, any damage to the nanofiller network due to crack growth would result in a permanent electrical response which could be used to determine the size of the disbond or delamination damage. The potential application of this technique to monitor and detect mode I fatigue crack growth in epoxy bonded composite joints has never been investigated. The present study focuses on utilizing CNFs in epoxy adhesive bonded composite joints to in situ monitor disbond damage while simultaneously improving the resistance to crack growth under mode I fatigue. The mode I fatigue crack growth resistance of the CNF modified epoxy adhesive bonded composite joints is compared to the neat epoxy adhesive. The electrical response of the nanocomposite adhesive is measured in-situ during mode I fatigue test using DC potential drop technique. The in-situ resistance measurements are used to determine the size of the propagating crack. 2 MATERIALS AND EXPERIMENTAL The epoxy adhesive used for bonding composites was blend of five parts of bisphenol A ‘105’ and one part of the hardener ‘206’ (from West System). Commercially-available vapour-grown carbon nanofibres, Pyrograf® III PR-24-HHT and supplied by Applied Sciences Inc., USA, were employed as the nanofiller. Carbon fibre composite substrates were manufactured using 12 plies of unidirectional T700 carbon fibre/epoxy prepreg (VTM 264 supplied by Applied Composites Group). The substrates, with dimensions of 300 mm x 250 mm x 2.5 mm, were cured and consolidated in an autoclave at 120 C for 1 hr., in accordance with the manufacturer’s recommended cure process. The substrate surfaces were abraded using 320 grit aluminium oxide abrasive paper, cleaned under running tap water for about 2 minutes, degreased with acetone, and finally cleaned with distilled water to remove any surface impurities. A three-roll mill (Dermamill 100) was used to disperse the CNFs in the liquid epoxy resin. Firstly, 0.7 wt% of the CNFs were hand mixed with dispersion-aiding additives based upon solvent-free acrylate copolymers, namely Disperbyk-191 and -192 (supplied by BYK ®). The 20 International Conference on Composite Materials Copenhagen, 19-24 July 2015 dispersive surfactants that were added to the CNFs were equal to the weight of the CNFs, resulting in a mixture of CNFs:191:192 weight ratio of 1:1:1. The CNF-surfactant mixture was then added to the epoxy resin, with no curing agent yet added, and hand mixed for 5 minutes. This mixture was then passed four times through the three-roll mill at 150 rpm and the gap size was gradually reduced with each subsequent pass to achieve a homogeneous dispersion of CNFs. The surface treated carbon fibre composite substrates (150 mm x 250 mm) were placed between glass fibre frames which were used as a dam to prevent the epoxy mixture from flowing out of the joint. Spacers, 2 mm in thickness made of glass slides, were placed at both ends of the joint to control the thickness of the epoxy layer between the substrates. Teflon-coated tape about 30 mm long and 11 μm thick was placed at an approximately equal distance between the two substrates, at one end of the joint, to act as a crack starter. The amine-based curing agent was added to the dispersed CNF/epoxy resin mixture and hand-mixed for approximately 5 minutes. This CNF-modified epoxy resin mixture was then poured between the substrates. The epoxy adhesive was then cured at room temperature (i.e. 250 C) for 48 h in total to form the epoxy nanocomposite. The joints were allowed to cure under ambient conditions prior to cutting into 20 mm wide double-cantilever beam (DCB) adhesivelybonded specimens. Composite bonded joints with two different adhesives are investigated in this study namely, unmodified epoxy adhesive and 0.7 wt% CNF modified adhesive. For the 0.7 wt% CNF sample, the electrical contact were established by sanding the outer surface of the adherends to slightly expose the carbon fibres and strain gauge wires were bonded with conductive silver paint. The contact points were covered with an insulating tape. Non-conductive grips were used to isolate the DCB sample from the load frame. The mode I fatigue delamination test were conducted under displacement control at 5 Hz with a stress intensity ratio of 0.5 by adapting the ASTM D6115-97 standard on a 3 kN electromagnetic pulse driven Instron E3000 fatigue test rig. In situ resistance measurements were acquired by a four probe resistance measurement technique using Delloger80 dataker during mode I fatigue delamination tests. The crack growth increments were recorded using a travelling microscope. The fatigue test setup is shown in Fig. 1. Fig. 1: Experimental setup for in situ resistance monitoring during mode I fatigue test 3 RESULTS AND DISCUSSION 3.1 Crack growth resistance of the CNF modified adhesive Fig. 2 shows the delamination growth rate curves for the unmodified and CNF modified adhesive bonded composite joints recorded during mode I fatigue tests performed under displacement control and the stress intensity ratio of 0.5. The addition of 0.7 wt% CNF in the adhesive increased the critical strain energy by about five folds from 88 J/m to 450 J/m. Similarly, the threshold strain energy increased from 19.5 J/m to 55 J/m. This shows that the unmodified adhesive is a low toughness epoxy. However, the addition of just 0.7 wt% CNF increases its fracture toughness by about five folds which results in improved delamination resistance. Electrical conta |
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| Language | English |
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| Content Type | Text |
| Resource Type | Article |
