Peel Plies Research Articles

Evaluation of Peel Ply Effectiveness in Controlling Mould Release Agent Contamination of Epoxy/Carbon Fibre Composite Laminate Surfaces

The peel-ply effectiveness in controlling the mould release contaminations of resin transfer molded (RTM) RTM6/G0926 epoxy-carbon fibre composite laminate surfaces have been examined. X-ray photoelectron spectroscopy (XPS) was employed to evaluate the extent of transfer of mould release agents and other surface contaminants of RTM composite laminates at semi-cured and fully cured states. Fully cured and 55% semi-cure degree of RTM composite laminates were prepared using polyester and diatex peel plies. The effectiveness of polyester and diatex peel plies were evaluated by comparing the amount of Frekote mould release contaminants present in the laminate surfaces processed with and without peel plies. The XPS results shows that the composite laminates processed without peel plies at different cured states exhibit uniform contamination with Freakote calcium metal ions. At the comparisons of polyester and diatex peel plies, the diatex peel ply performs better in controlling the surface contaminations.

Effect of Peel Ply on Resin Flow during Vacuum Infusion.

Although various simulations have been conducted for the vacuum infusion process, most of the studies have considered only fabrics and flow medium and ignored the effect of peel ply. However, peel ply can affect resin flow because it is placed between the fabrics and flow medium. To verify this, permeability of two types of peel plies was measured, and it was found that the permeability between the peel plies differed significantly. Moreover, the permeability of the peel plies was lower than that of the carbon fabric; thus, peel ply can cause a bottleneck in the flow in the out-of-plane direction. Some 3D flow simulations were conducted in cases of no peel ply and for two types of the peel plies to confirm the effect of peel ply, and experiments were also conducted for two types of the peel plies. It was observed that filling time and flow pattern were highly dependent on the peel plies. The smaller permeability of peel ply has, the greater effect of peel ply is. These results indicate that the permeability of peel ply is one of the dominant factors and should be considered in process design in vacuum infusion. Additionally, by adding one layer of peel ply and applying permeability, the accuracy of flow simulations can be improved for filling time and pattern.

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

AbstractComposite 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.

Experimental investigation of modified co-curing process for carbon fiber/epoxy-laminates

In this study, thermoset-based carbon fiber-reinforced polymer structures manufactured by the so-called modified co-curing process are analyzed and compared to well-established co-curing and co-bonding. The modified co-curing process allows manufacturing geometrically complex parts without traditional core technologies by producing laminates from a un-cured half and a pre-cured half in contrast to using two un-cured halves (co-curing) or a fully cured half plus an un-cured half (co-bonding). The interlaminar fracture toughness under Mode I loading, [Formula: see text], was determined in double cantilever beam (DCB) tests. [Formula: see text] displays a correlation of the degree of cure and the joint properties, with the co-curing laminates having 11% and 33% higher fracture toughness than the modified co-curing configurations. However, modified co-curing in all cases results is superior or equal to co-bonding. To assess the influence of surface properties for the bonding quality, different peel plies were compared with respect to the resulting joint properties. The results with up to 50% loss in [Formula: see text] values indicate the high importance of appropriate surface preparation. Subsequent tests also show that the negative influence of the peel ply on the joint properties can be reversed by abrasive surface treatment. It was found that at higher degrees of partial curing before co-curing, crack growth increasingly occurs in the interface of the bonded laminates. Therefore, the properties of the surface before joining were analyzed and modified to assess its relevance for the bonding properties and the potential for improvement.

Infrared spectroscopy of peel ply: simple and efficient analysis of specific components

Peel plies as indispensable auxiliary materials have gained broad attention in composite material forming process. However, the detection method of specific components of peel plies has rarely been reported. In this paper, shredding-infrared spectroscopy technology was put forward to investigate the specific components of peel plies. There were two ways to test the specific components of peel plies using infrared spectroscopy: direct dissolution and pretreatment (shredding) before dissolution. Compared with the sample prepared by direct dissolution, the sample fabricated via pretreatment displayed significant merits, such as saving time cost and excellent detection accuracy. In addition, the testing mechanism was illustrated.

Surface Topography Influences on the Fatigue Behavior of Composite Joints

The surface condition of carbon fibre reinforced plastic (CFRP) substrates is decisive to obtain high bond strength and lifetime of adhesively bonded parts. Those surfaces were adjusted in terms of their microscopic topography by means of peel plies and release foils. The subsequent surface treatment via atmospheric pressure plasma jet or vacuum blasting allowed the modification of the microscopic roughness as well as the surface chemistry. Those configuration were assessed using surface analytic methods as well as quasi-static and cyclic fracture tests on single lap shear specimens. The microscopic surface roughness, if at all, only showed a small influence on the bond strength. Despite release agent residues, fracture was found within the fiber-matrix interface, which caused difficulties in evaluating the effect of surface pretreatments on the adhesion strength. Fatigue tests revealed a lifetime reduction of uneven microscopic rough surfaces, which was assigned to stress concentrations at the tip of asperities. The crack propagation was accelerated in case of release agent residues. If surfaces were free of contaminations, no differences between microscopically smooth and slightly structured surfaces were found. Overall, fatigue testing on single lap shear specimens showed an increased sensitivity with regard to the assessment of surface morphology.

Investigation of fluorine-based release agents for structural adhesive bonding of carbon fibre reinforced plastics

Peel plies can be used during the manufacture of fibre-reinforced plastics for two reasons: to protect the surface during transport and storing the parts as well as during subsequent work steps, such as adhesive bonding, the removal of the peel ply can result in bondable surface with required surface characteristics. However, the use of peel plies is not straightforward. It can be difficult to remove peel plies and a surface produced by a peel ply is altered in terms of roughness and elemental composition. In the present work, the influence of fluorine-based release agents on adhesive joining of carbon fibre reinforced composites is investigated. Within the scope of the screening, 14 fluorine-based release agents—ETFE release films, PTFE coated glass fabrics as well as fabrics made of PTFE fibres—were investigated. Preliminary studies (Meer, in: Deutscher Luft- und Raumfahrtkongress 2014, Augsburg, 2015) have shown that ETFE films have advantages in terms of adhesion. The study covers a number of aspects: the determination of the tear strength of the release agent by peel test; the determination of the element composition (XPS) and surface characteristics (SEM) before and after atmospheric pressure plasma pre-treatment, characterization the topology and the characterization of the adhesive strength by centrifugal adhesion test.

Debonding characteristics of adhesively bonded carbon-epoxy composite laminates

Abstract Adhesive bonding through co-curing of composite parts becomes more practical in higher end applications such as aerospace industries. These bonding technologies avoid mechanical fasteners, stress concentrating cut-outs and offers flexibility in designing complex geometries to simpler units. The strength of the adhesive joining is characterized using fracture toughness values obtained from the failure analyses. In this work, the debonding characteristics of FM 300 epoxy adhesive between 5HS carbon fiber reinforcements and RTM6 resin RTM moulded composite laminates were determined experimentally by mode I, double cantilever beam test (DCB). The mode of crack propagation was observed and the fracture toughness of the adhesive bonding between the laminates was evaluated using load-displacement curves of DCB tests. The influences of laminate surface contaminations on the bonding characteristics were also examined. From the results, it was seen tortuous crack propagation and a non-linear fracture toughness values was obtained to the contaminated laminate joints moulded without peel plies. Moreover, the adhesive bond peeled out easily with clean and flat fracture surfaces at the adhesive-laminate interfaces. The results also showed that the use of peel plies during laminate moulding controlled the surface contaminations evidencing uniform crack propagation and increased fracture toughness values at the adhesive joints.

Mechanical and Thermal Properties of Carbonized PAN Nanofibers Cohesively Attached to Surface of Carbon Fiber Reinforced Composites

SummaryUnidirecitonal pre‐preg carbon fibers of ten peel plies were laid up at 0‐, 45‐, −45‐, and 45‐degree stacking sequences on a flat and smooth aluminum (Al) plate, and then carbonized electrospun polyacrylonitrile (PAN) nanofibers were placed on top of the last ply prior to vacuum curing in a vacuum oven. The PAN electrospun fibers were oxidized at 280°C in an ambient condition for 1 hr and then carbonized at 850°C for 1 hr in an argon (Ar) gas atmosphere. The resultant composite panels were cut into small pieces and subjected to a number of different characterization techniques. Thermal mechanical analysis (TMA) measurements clearly showed that significant reinforcement was achieved for the pre‐preg/carbonized PAN fiber composites because of the enhanced interfacial bonding between the PAN nanofibers and the matrix. Dynamic mechanical analysis (DMA) tests exhibited a shift of the glass transition temperature of the carbonized PAN nanofiber/composite, which may be helpful for high‐temperature applications of the present composites. A Raman spectroscopy peak around 897 cm−1 indicated formation of the γ‐phase of the carbonized PAN fibers. The highest stretching peak of the CH2 group was recognized within the range of 2,500–2,800 cm−1 for the carbonized fibers. The vibration peak of the CN group also appeared at 1,452 cm−1 spectrum. TMA determined the coefficient of thermal expansion (CTE), indicating an improvement in stability of the composite material, which can be useful for structural health monitoring (SHM) as well as lightning strikes and electromagnetic interference shielding applications of new carbon fiber composites.

Interface modification of glass fibre–polyester composite–composite joints using peel plies

This work includes an investigation of six different surface treatments for joining composites by overlamination. The durability of the pre-treated glass fibre-unsaturated polyester composite joints is investigated accounting for a 200-day water immersion at 64±1°C. Degradation due to accelerated aging, by water and elevated temperature, is analysed using Fourier transform infrared spectroscopy (FTIR), water absorption measurement, tensile testing and a systematic sequence of four different fracture test methods. Based on the fracture tests, the mechanical abrading treatment outperformed all other treatments yet suffered an adverse effect due to the combined moisture and elevated temperature. We observed irreversible chemical degradation in the bulk composite in terms of permanent 2.3% weight loss. The softening and plasticization of the polyester matrix seemed to have played a fundamental role in the rupture of adhesion at the peel ply and tear ply pre-treated overlamination interfaces. However, the adhesion loss was partly reversible and it was concluded to signify the domination of mechanical interlocking at peel ply and tear ply modified interfaces.

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.

An Engineer Asks: Is it Really More Important that Paint Stays Stuck on the Outside of an Aircraft than that Glue Stays Stuck on the Inside?

The purpose of this article is to draw attention to two problems encountered with modern aircraft: the difficulties in making adhesive and paint adhere to composite substrates and the lack of any after-the-fact inspection that can prove that there will not be any interfacial failures at some time during the service life. It is also observed that the response to paint peeling off is more rapid and thorough than to a discovery of separations between internal components that were once believed to have been bonded together. Because there is so much similarity between the processes of making paint and adhesive adhere, it is suggested that some of the efforts to improve adhesion of the paint might also help improve the processes for making adhesives stick. The article focuses on a series of anecdotes about problems and their resolutions, with the hope that the solutions might help others solve or avoid future such problems. It is pointed out that the cost of improving the adhesion of both paint and adhesive has always been insignificant in comparison with the sometimes enormous costs incurred as a result of fleet-wide occurrences of what were perceived to be bond “failures” but which should more properly be characterized as initially undetected nonbonds. A critical issue is the acknowledged absence of any nondestructive inspection capable of distinguishing between bonds that will “fail” in service and those that will not. Experience has shown that none of the apparent interfacial failures to date have occurred on grit-blasted surfaces. Equally, it must be conceded that not all of the bonded composite structures made using peel-ply surfaces can be expected to fail, even though those associated with released peel plies or prebond moisture probably will, because these conditions have been associated with so many of the past failures. The distinction between interfacial failures and impact damage to properly bonded structures is that the former can extend throughout the entire structure, whereas the broken fibers and interlaminar matrix failures associated with the latter will not extend far beyond the impact area. This is one reason why it is so important to use only surface preparations that ensure the absence of interfacial failures. It is also noted that there is no counterpart, for the bonding of composite structures, of the peel-type test that was so instrumental in solving the equivalent bonding problem that was widespread in bonded metal structures some 30 years ago. It is recommended that there should be, because the use of only shear-load tests has been found to be insufficient to ensure bond durability for both metallic and composite structures.

Interlaminar Tests for Marine Applications. Evaluation of the Influence of Peel Plies and Fabrication Delays

This paper presents results from a study of the influence of surface preparation on the mechanical performance of overlaminated polyester composites. Panels of 16 woven glass plies have been prepared in two halves by hand lay-up. After the first 8 plies were laminated the surface was either protected by a peel ply or left in air. The overlamination of the second half of the composite thickness was completed after different periods and interlaminar shear, flexure and mode I fracture specimens were tested. The results enabled the influence of the delay and the surface condition to be related to mechanical performance. Surfaces protected with peel plies show very low GIc propagation energy release rates. Results are compared to those from continuous lamination of the 16 ply composite. Interlaminar fracture tests are shown to be much more sensitive to overlamination conditions than the traditional short beam shear test. Results are interpreted in terms of fracture surface features.

Adhesive Bonding of Composite Structures—Progress to Date and Some Remaining Challenges

The purpose of this paper is to describe and explain some highlights in the history of adhesive bonding of fibrous composite structures. Successes are described, along with opportunities to do better in future. The importance of making simple details is stressed, in the context of making high-strength composite parts free from wrinkles in the fibers. A case is made that secondary bonding is frequently less expensive than co-curing of complex “single-step” parts, despite the consistent projections that the opposite should be true. Some explanations are presented to explain how this misconception arises, based mainly on the issue that the estimates used for comparison are traditionally based on the costs of making only the first articles, whereas what should have been compared was the cost of making possibly very differently designed lower-cost parts and structures for series production. Some common misunderstandings about variable-thickness adhesives layers are put to rest by explaining how intense load transfer occurs only in small areas, where the adhesive layer is thinnest, with the rest of the bond area inevitably lightly stressed. The problem created by variable thickness bonds is not that of weak bonds. The thicker, and often porous, bond lines usually shed most of the load they were intended to transmit to nearby thin and stiffer areas. Indeed, the loss of load caused by softening can exceed the loss of strength caused by the porosity. No, the problem caused by variable thickness adhesive layers is that it increases the local stresses in the skin whenever the glue layer is thinnest. Traditional problems with release-agent-coated peel plies that prevent adhesion of the glue to the substrate are recounted. This issue is now fairly well known. What is far less understood is the adverse influence of pre-bond moisture that is unable to escape during the bond cycle. This prevents adhesion perhaps even more effectively than a layer of silicone. The importance of bonding only dry parts is explained. This is far easier to do at the time of original manufacture and less easy for in-service repairs and repainting. The paper concludes with a discussion of the important factors to be considered during design and analysis of bonded composite joints.

Effect of absorbed water in CFRP composites on adhesive bonding

This paper reports the results of wedge testing adhesive-bonded joints made from dry, and water-immersed and dried, carbon fibre-reinforced plastic (CFRP) laminates based on Ciba Fibredux 914. Five groups of specimens were tested. Those bonded with Hysol EA 9394 adhesive were tested on dry, and immersed and dried, CFRP adherends using mechanical abrasion as the surface preparation. Hysol EA 9390 repair resin was tested on abraded dry samples and on dry, and immersed and dried, peel ply surfaces. Only the initially dry specimens that were also dried before bonding and given the mechanical abrasion treatment gave cohesive failure (now considered to be matrix failure) after 1388 days' immersion. All the peel ply surfaces — whether initially dry, or immersed and then dried — gave interfacial failure. Unfortunately, in the case of the immersed and dried specimens, the two lowest temperature drying cycles were both given to the specimens which were prepared by mechanical abrasion and the two highest temperature drying cycles were given to the peel ply samples. All the peel ply samples gave interfacial failure but the immersed and dried peel ply surfaces gave the same result, in terms of fracture energy and durability, as the initially dry ones, indicating that the higher temperature drying cycles were adequate. One strange and unexpected result was that the fracture energy obtained with Hysol EA 9394 on the immersed and dried surfaces was higher than on the dry surfaces by a large amount. This suggests that, for this particular adhesive, a small amount of moisture actually improves the cure and the fracture toughness, even though a cohesive failure was not obtained. In view of the extensive use of peel plies in industry it would seem that further research is required to achieve more durable bonds. From a repair point of view, where mechanical abrasion is the most common method of surface preparation, it was encouraging to find that in some cases repair bonds could actually be more durable than those made by the manufacturer if peel plies are used.

The effect of fabrication-induced surface roughness on the compressive and flexural properties of carbon fibre-reinforced plastics

Carbon fibre composite panels have been fabricated using a range of different peel-ply materials. The peel plies based on fabrics imprinted a surface texture on moulded panels, the roughness of which was related to the coarseness of the peel-ply materials. Compressive and flexural tests were performed on the materials. Compressive strength was unaffected by the surface roughness, but the smoothest panels, made with a peel ply comprising a smooth film, had higher flexural strength and stiffness than the materials with a pronounced surface texture. Suggestions for these findings are proposed.