The Use of Peel Ply as a Method to Create Reproduceable But Contaminated Surfaces for Structural Adhesive Bonding of Carbon Fiber Reinforced Plastics

In the fabrication of fiber-reinforced plastics materials peel plies are commonly used as an additional layer on top of the laminates to sponge up the surplus resin and to create an activated surface for adhesive bonding or coating by peel ply removal. In theory, the peel ply removal results in a new and uncontaminated fracture surface that is activated by polymer chain scission. The peel ply method is often presented as being a good surface treatment for structural bonding.

In this study carbon fiber-reinforced plastics (Hexcel® 8552/ IM7) were produced by the use of five different peel plies and a release foil made of polytetrafluorethylene (PTFE). The peel plies themselves and the surfaces on the CFRP created by peeling were examined by scanning electron microscopy (SEM), x-ray photo electron spectroscopy (XPS), energy-dispersive x-ray spectroscopy (EDX), infrared (IR) spectroscopy, atomic force microscopy (AFM), and contact angle measurements to characterize the surfaces produced. Furthermore, the bond strength of lap shear and floating roller peel samples was determined with and without additional plasma treatment. For bonding, a room temperature-curing two-component-epoxy adhesive (Hysol® 9395) was used to prove the applicability of different peel plies for structural adhesive bonding under repair conditions.     

ENF test in the adhesive bonding of aluminium–composite joints and evaluation of its reliability with Weibull distribution

The composite materials of the polymeric matrix reinforced with carbon fibre have an extensive industrial application as they provide light and resistant structures. However, in many products (automobiles, aircraft, etc.), the composite materials must be joined to other components manufactured with aluminium alloys. The use of structural adhesive to bond these materials may be a good alternative if a specific design of the adhesive joint is carried out by maximising its performance and reducing its limitations. In the current work, the end-notched flexure (ENF) fracture test is used to assess the mechanical behaviour of the adhesive joint to facilitate the choice of the best adhesive and surface treatment of the adherends. However, in industrial applications of a great technical requirement (where the safety of staff or property may be at risk), the former experimental results are not enough and must be complemented with methods that can provide additional guarantee for a suitable reliability. For this purpose, a statistic analysis of the obtained experimental data has been carried out by means of the application of a Weibull distribution, in order to propose the adhesive and surface treatment that best combines the mechanical performance and high reliability. At the end, we can conclude that the epoxy adhesive with sand-blasting treatment for the aluminium and the peel ply for the carbon fibre has the greatest reliability (more than 90% for loads until 900?N).     

The Influence of Moisture Absorption-Drying of Composite Materials on the Bonding Performance of the Joints

Composite materials are susceptible to environmental threats correlated to moisture absorption, which affects  the bonding performance of the joints. Therefore, drying should be taken into account during actual manufacturing operations. In this work, the influence of moisture absorption-drying of composite materials on the bonding performance of the joints is investigated to provide a theoretical direction for the co-bonding process. Firstly, the surfaces of composite materials are treated with three methods: sanding, dry peel ply, and wet peel ply. Secondly, the composite materials are subjected to moisture absorption-drying treatment at 26 °C/65% relative humidity (RH) and 70 °C/85% RH. Finally, the above materials are bonded to evaluate the bonding performance. These results show that the bonding performance of the joints treated with two peel plies decreases significantly after moisture absorption. After moisture absorption-drying at 26 °C/65% RH, the bonding performance of the joints treated with dry peel ply cannot fully recover, while wet peel ply can be fully recoverable. However, the bonding performance of all joints cannot fully recover after absorbing moisture-drying at 70 °C/85% RH, whose recovery has relations with surface treatment.     

Hygrothermal ageing of adhesive joints with nanoreinforced adhesives and different surface treatments of carbon fibre/epoxy substrates

The effect of water absorption on the strength of single lap adhesive joints subjected to accelerated hygrothermal ageing (55 °C, 95% relative humidity, 800 h) was analysed. Two different variables were studied: the surface treatment of the carbon fibre/epoxy laminates (peel ply, grit blasting and atmospheric pressure plasma) and the addition of carbon nanofillers (0.5 wt% nanofibres and 0.25 wt% nanotubes) to the epoxy adhesive. The joint strength and the failure mode of the joints were investigated. Furthermore, the amount of water absorbed by the adhesive was determined.Adhesive joints with peel ply-treated laminates exhibit an increase in their strength, which is attributed to a relaxation of stresses in the adhesive/laminate interface; with grit blasting, this property remains almost constant. Plasma treatment provides the worst ageing behaviour because this treatment results in a surface with a higher surface free energy, which is more susceptible to environmental attack. The nanoreinforcement of the adhesive has a beneficial effect: it decreases the amount of absorbed water.     

Investigation of optimal surface treatments for carbon/epoxy composite adhesive joints

Although an adhesive joint can distribute the load over a larger area than a mechanical joint, requires no holes, adds very little weight to the structure and has superior fatigue resistance, but it not only requires a careful surface preparation of the adherends but also is affected by service environments. In this paper, suitable conditions for surface treatments such as plasma surface treatment, mechanical abrasion, and sandblast treatment were investigated to enhance the mechanical load capabilities of carbon/epoxy composite adhesive joints. A capacitively coupled radiofrequency plasma system was used for the plasma surface treatment of carbon/epoxy composites and suitable surface treatment conditions were experimentally investigated with respect to gas flow rate, chamber pressure, power intensity, and surface treatment time by measuring the surface free energies of treated specimens. The optimal mechanical abrasion conditions with sandpapers were investigated with respect to the mesh number of sandpaper, and optimal sandblast conditions were investigated with respect to sandblast pressure and particle size by observing geometric shape changes of adherends during sandblast process. Also the failure modes of composite adhesive joints were investigated with respect to surface treatment. From the peel tests on plasma treated composite adhesive joints, it was found that all composite adhesive joints failed cohesively in the adhesive layer when the surface free energy was higher than about 40 mJ/m², because of high adhesion strength between the plasma treated surface and the adhesive. From the peel tests on mechanically abraded composite adhesive joints, it was also found that the optimal surface roughness and adhesive thickness increased as the failure load increased.     

Mode-I adhesive fracture energy of carbon fibre composite joints with nanoreinforced epoxy adhesives

The effect of the addition of carbon nanoreinforcements to an epoxy adhesive on the strength and toughness of carbon fibre/epoxy composite joints was studied. The laminate surfaces, treated with peel ply, were characterised by profilometry, image analysis and wettability. The mechanical properties of the joints were determined by lap shear testing and double cantilever beam testing. The fracture mechanisms were studied by scanning electron microscopy.The addition of carbon nanofibres and carbon nanotubes caused an increase in the mode-I adhesive fracture energy, G_IC, of the joints while their lap shear strengths remained approximately constant. This improvement in the fracture behaviour was attributed to the occurrence of toughening mechanisms when carbon nanoreinforcements were added to the epoxy adhesive. Additionally, the use of carbon nanotubes improved the interfacial strength between the adhesive and the substrate, changing the crack growth behaviour and the macroscopic failure mode.     

Adhesion Improvement of Glass-Fibre-Reinforced Polyester Composites by Gliding Arc Discharge Treatment

A gliding arc is a plasma that can be operated at atmospheric pressure and applied for plasma surface treatment for adhesion improvement. In the present work, glass-fibre-reinforced polyester plates were treated using an atmospheric pressure gliding arc discharge with an air flow to improve adhesion with a vinylester adhesive. The treatment improved wettability and increased the polar component of the surface energy and the density of oxygen-containing polar functional groups at the surfaces. Double cantilever beam specimens were prepared for fracture mechanics characterisation (fracture resistance as a function of nominal mode mixity) of the laminate adhesive interface. It was found that gliding arc treatment significantly increases the interfacial fracture energy and fracture resistance in comparison with a standard peel ply treatment, although the mixed mode fracture energy of the gliding arc treated specimen was not as high as that of the laminate itself.     

Multiple Cracking of Unidirectional and Cross-PlyCeramic Matrix Composites

This paper examines the multiple cracking behavior of unidirectional and cross-ply ceramic matrix composites. For unidirectional composites, a model of concentric cylinders with finite crack spacing and debonding length is introduced. Stresses in the fiber and matrix are found and then applied to predict the composite moduli. Using an energy balance method, critical stresses for matrix cracking initiation are predicted. Effects of interfacial shear stress, debonding length and bonding energy on the critical stress are studied. All the three composite systems examined show that the critical stress for the completely debonded case is lower than that for the perfectly bonded case. For cross-ply composites, an extensive study has been made for the transverse cracking in 90° plies and the matrix cracking in 0° plies. One transverse cracking and four matrix cracking modes are studied, and closed-form solutions of the critical stresses are obtained. The results indicate that the case of combined matrix and transverse crackings with associated fiber/matrix interfacial sliding in the 0° plies gives the lowest critical stress for matrix cracking. The theoretical predictions are compared with experimental data of SiC/CAS cross-ply composites; both results demonstrated that an increase in the transverse ply thickness reduces the critical stress for matrix cracking in the longitudinal plies. The effects of fiber volume fraction and fiber modulus on the critical stress have been quantified. Thermal residual stresses are included in the analysis.     

Failure behaviour of the single lap joints of angle-plied composites under three point bending tests

Abstract

In this paper, the response of adhesively-bonded single lap joints (SLJs) with angle-plied composite adherends subjected to flexural loading was investigated. The experiments were carried out for the adherends, glass reinforced polymer matrix, with three kinds of stacking sequence. A three-dimensional finite element (FE) model was developed using ABAQUS/Explicit. The three dimensional Hashin failure criterion with an appropriate damage evolution law was used to characterize the damage inside a ply. Cohesive zone elements were used to model the damage in the adhesive layer (AF163-2K) and the interply failure, that is, the delamination. The developed numerical model was verified with the performed experiments. The SLJs of [±20]_5s and [±45]_5s failed due to failure in the adhesive layer and the delamination between the plies, whereas that of [±10]_5s failed mainly due to the former failure. The intralaminar damage was not noticed for any case. The influence of the fiber angle of plies in the adherends, adherend thickness, overlap length, and the thickness of adhesive layer on the damage in the adhesive layer and the delamination were investigated in terms of the competition between these two failures and activation of different failure modes in each thoroughly.     

The Fiber/Matrix Bond Strength of CFRP Deduced from the Strength Transverse to the Fibers

The strength transverse to the fibre (or the transverse fracture strain) of CFRP is directly related to the fibre/matrix bond strength. This enables the determination of a fibre matrix bond strength based on transverse fracture data. In crossply laminates, the transverse ply usually fails many times, so that principally these experiments deliver a treasure of data. A method, developed to make use of these data, produces a Weibull distribution for transverse fracture based on the data of, basically, a single specimen. A model developed to separate the influences of defects, matrix ductility and constraint finally leads to a procedure to determine the strain at interphase(or interface) failure. The usefulness of the model and the determined strain at interface failure is demonstrated with several examples, such as the influence of constraint, of test temperature and of the fibre surface treatment on transverse cracking and the fibre/matrix bond strength.     

Analytical solutions for adhesively bonded composite single-lap joints under mechanical loadings using full layerwise theory

In this paper, analytical solutions for adhesively bonded composite single-lap joints (SLJs) are presented within the framework of the full layerwise theory (FLWT). The adhesively bonded composite SLJ is divided into a large number of mathematical plies through the thickness and three regions along its length. The equilibrium equations of each region are obtained using the principle of minimum total potential energy. The three sets of fully-coupled governing equations then are simultaneously solved by introducing the state space variables. The effects of adhesive thickness and loading conditions including uniaxial tension and bending moment on the interfacial peel and shear stress as well as the von Mises stress distributions along the length and through the thickness of the adhesive layer are studied. The present results, which are verified via analytical, experimental, and numerical investigations available in the literature, can be introduced as scaling solutions to verify the authenticity of other methods.     

Analytical solutions for nonlinear analysis of composite single-lap adhesive joints

This paper presents analytical nonlinear solutions for composite single-lap adhesive joints. The ply layups of each composite adherend can be arbitrary, but in the overlap region the ply layups of the upper and lower adherends are assumed to be symmetrical about the adhesive layer. In the present formulation, equilibrium equations of the overlap are derived on the basis of geometrical nonlinear analysis. The governing equations are presented in terms of adherend displacements by taking into account large deflections of the overlap adherends and adhesive shear and peel stresses simultaneously. Closed-form nonlinear solutions for adherend displacements, an edge moment factor and adhesive stresses are formulated and then simplified for practical applications. To verify the present analytical solutions for nonlinear analysis of composite single-lap joints, the geometrically nonlinear 2D finite element analysis is conducted using commercial package MSC/NASTRAN. The numerical results of the edge moment factor, deflections and adhesive stresses predicted by the present solutions correlate well with those of the geometrically nonlinear finite element analysis. This indicates that the present analytical solutions capture key features of geometrical nonlinearity of composite single-lap adhesive joints.     

Numerical study of lap joints with composite adhesives and composite adherends subjected to in-plane and transverse loads

In this paper, single lap joints for joining fibre composites were modeled and a three-dimensional finite element method was used to study the joint strength under in-plane tensile and out-of-plane loadings. The behaviour of all the members was assumed to be linear elastic. The adherends were considered to be orthotropic materials while the adhesive could be neat resin or reinforced one. The largest values of shear and peel stresses occurred near the ends of the adhesive region, as expected. The values and the rate of variation in peel stress was more than that of shear stress. By changing the properties and behaviour of adhesive from neat epoxy (isotropic) to fibre composite adhesive (orthotropic) and with various fibre volume fractions of glass fibre, the ultimate bond strength increased as the fibre volume fraction increased, in both tensile and transverse loadings. Also, changing the orientation of fibres in the adhesive region with respect to the global axes influenced the bond strength.     

Surface preparation effect on performance of adhesively-bonded graphite/epoxy composites subjected to impact

The influence of pre-bond surface preparation on performance of adhesively-bonded composites subjected to impact was investigated in this study. Impact test was carried out on adhesively bonded graphite/epoxy composite specimens at different low impact energies ranging from 5 to 20 J using the drop-weight impact test. Post-impact ultrasonic evaluation was performed in order to determine the resulting internal damage due to impact on the adhesive bondline. The ultrasonic C-scan of the gated ultrasonic wave signal was acquired and the ensuing debond area in the adhesive bond was determined quantitatively for specimens made from substrates with different surface preparations such as paper peel ply, sandblasting, and sandpaper abrasion. In order to determine the flexural load bearing capacity and stiffness reduction after impact, a three-point bending test was conducted on unimpacted and impacted specimens. A comparative study was performed to evaluate the performance of adhesively-bonded composites with different surface preparations. The results revealed that paper-peel ply performed the best in terms of resistance to debond area formation in the adhesive layer, as well as in terms of retention of flexural load bearing capacity and stiffness after impact.     

Effect of thermal treatment on the tensile and in‐plane shear behavior of carbon fiber‐reinforced poly(phenylene sulfide) composite specimens

The effect of PPS matrix evolution occurring during thermal treatment of carbon fiber‐reinforced PPS plies prior to their consolidation to laminates on the mechanical behavior of the composite material has been investigated. The thermal treatments were performed at temperatures and times, which are relevant for processing PPS composites. All thermal treatments were carried out in an oven in air to facilitate the presence of oxygen, while the subsequent consolidation was performed in an autoclave. The tensile and in‐plane shear behavior of both, thermal‐treated and untreated materials, was investigated. Differential scanning calorimetry and microscopy analyses were made to evaluate the effect of the performed thermal treatments on degree of crystallinity and porosity of the laminates. The mechanical tests carried out have shown an appreciable degradation of the mechanical properties investigated. The observed degradation increases with increasing thermal treatment temperature and time when thermal treatments were carried out on each single composite ply prior to the consolidation. On the other hand, when, prior to the consolidation, the whole set of plies was subjected to thermal treatment, improved mechanical properties were observed. The results were discussed under the viewpoint of PPS matrix evolution during processing of the composite plies in the presence of oxygen. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

3D FE delamination induced damage analyses of adhesive bonded lap shear joints made with curved laminated FRP composite panels

This paper deals with the evaluation of inter-laminar stresses in the adhesive layer existing between the lap and the strap adherends of lap shear joints (LSJ) made with curved laminated fibre reinforced plastic (FRP) composite panels for varied embedded delaminations between the first and second plies of the strap adherend. Non-linear finite element analyses have been carried out using contact and multi point constraint (MPC) elements. The use of contact elements ensures avoidance of inter-penetration of delaminated surfaces. Sequential release of MPC elements facilitates computation of individual modes of Strain Energy Release Rates (SERR). The effects of varied delamination lengths on variations of peel and inter-laminar shear stresses and different modes of SERR are seen to be very significant. Their variations on both the delamination fronts, for each size of the delamination, are found to be much different from each other indicating different propagation rates at the two delamination fronts. The structural integrity of the LSJ in the presence of delaminations, thus, can be predicted with adaptive finite element (FE) simulations. It is further seen that the peak stress magnitudes and SERRs are higher in the LSJs made with curved FRP composite panels as compared to the flat laminates. This may be due to the stiffening effects induced by the curvature geometry of the curved composite panels.     

Surface preparation for adhesive bonding of polycyanurate‐based fiber‐reinforced composites using atmospheric plasma treatment

In this article, the effects of atmospheric plasma treatment on the microstructural, chemical, and mechanical behavior of epoxy‐bonded polycyanurate composites are investigated. Adhesive bond strength of plasma‐treated specimens exhibited strength increases of over 35% to that of peel‐ply and solvent‐wiped surface preparation techniques. The improvements were as much as 50% greater than those obtained using abrasive surface preparation techniques. X‐ray photoelectron spectroscopy analysis showed an increase in the surface concentration of oxygen as a function of plasma treatment passes. However, the levels were substantially lower than that of epoxy composites treated under identical conditions. In addition, the concentration of carboxyl groups (O CO), which have been associated with improved adhesive strength in epoxy‐based composites, was shown to saturate in cyanate ester composites after a much lower exposure period than what was observed when treating epoxies. The effect of plasma surface treatment on the surface morphology of the cyanate ester composite was also studied using scanning electron microscopy and atomic force microscopy. Atomic force microscopy analysis showed a progressive increase in surface roughness with treatment; however, this increase only translated into a marginal increase in surface area and is not believed to contribute significantly to adhesive strength. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Quantitative Detection of Peel Ply Derived Contaminants via Wettability Measurements

Abstract

The theoretical relationship between surface energies and adhesion are briefly reviewed. Surface energies obtained from contact angles measurements can be extremely useful but difficult to obtain in manufacturing and field environments. A novel technique for probing surface energies via liquid drops, created using ballistic deposition, was investigated. The use of this technique as a quality assurance tool for detecting peel ply derived siloxane contaminants is discussed. It is shown that a readily detectable threshold amount of contamination is required to affect fracture toughness, and that the amount is similar for several distinct adhesive systems.     

Reaction‐to‐fire properties of polymer matrix composites with integrated intumescent barriers

The integration of an intumescent barrier between the plies of prepreg based polymer matrix composite and sandwich panels is investigated in detail with regard to reaction‐to‐fire properties. Incident heat flux, panel thickness and insertion depth within the panel were varied systematically. Fire retarding effects are compared to the application of an intumescent and top coating on the surface. All tests were carried out with a commercial material: HexPly® 8552/IM7 by Hexcel. Design rules for an effective improvement of reaction‐to‐fire properties are derived. Two practical applications were identified not interfering with mechanical properties: A metal mesh as support for the intumescent material underneath a single top ply and the one‐sided integration in a sandwich with the possibility to expand into the honeycomb. Degradation mechanisms are characterized by cone calorimetry and temperature development throughout the specimens. Copyright © 2014 John Wiley & Sons, Ltd.     

A ply-level electrical resistance approach to monitor crack evolution in a laminate SiC/SiC composites

In the aim of providing a reliable technique to monitor the development of damage in 0°/90° melt-infiltrated SiC-fiber reinforced prepreg laminate ceramic-matrix composites, it was hypothesized that the electrical resistivities of different layers of this material were significantly different due to their free Si content and morphology. Three distinct layers: a 0° fiber ply, a 90° fiber ply and a matrix only ply, were distinguished in the composite architecture. Free silicon is the most conductive phase in this composite system; however, the Si content and morphology were different in each of the three types of plies. Unidirectional and [0°/90°]_2s specimens enabled quantification of ply-level resistivities. An electric circuit model was constructed; it consists of parallel resistors where each resistor represents a ply in the composite system. This ply-level electrical model was validated using composites of different vintages which contained different silicon contents. A room temperature stepped fatigue test was conducted and the ply level circuit model was used to discern crack morphology with the support of acoustic emission and digital image correlation.