Peel Ply Research Articles

Ultrasonic Nondestructive Evaluation of Composite Bond Strength: Quantification through Bond Quality Index (BQI)

This article presents a concept, materials, and methods to devise a Bond Quality Index (BQI) for assessing composite bond quality, approximately correlating to the respective bond strength. Interface bonding is the common mechanism to join two composite structural components. Ensuring the health and quality of the bond line between two load-bearing composite structures is crucial. The article presents the classification and data-driven distinction between two types of bond lines between similar structural components. The interface bonds in composite plates were prepared using polyester peel ply and TX-1040 nylon peel ply. For all the plates, ultrasonic inspection through scanning acoustic microscopy (SAM) (>10 MHz) was performed before and after localized failure of the plate by impinging energy. Energy was impinged 0–10 J/cm2 of in the 16-ply plates, and 0–25 J/cm2 were impinged in 40-ply plates. Followed by bond failure and SAM, a new parameter called the Bond Quality Index (BQI) was formulated using ultrasonic scan data and energy data. The BQI was found to be 0.55 and 0.45, respectively, in plates with polyester peel ply and TX-1040 nylon peel ply bonds. Further, in 40-ply plates with polyester peel ply resulted in a BQI equivalent to 3.49 compared to 0.75 in plates with a TX-1040 nylon peel ply bond. Currently, the BQI is not normalized; however, this study could be used for AI-driven normalized BQIs for all types of bonds in the future.

Tailoring adherend surfaces for enhanced bonding in CF/PEKK composites: Comparative analysis of atmospheric plasma activation and conventional treatments

Here, we propose the utilization of atmospheric plasma activation (APA), which outperforms peel-ply (PP) treatment and mechanical abrasion (MA) in achieving high-performance adhesively bonded carbon fiber/polyetherketoneketone (CF/PEKK) composites. This study covers several key aspects, including the chemical and morphological characterization of treated surfaces and mechanical performance assessments of single lap-joints (SLJs) under tensile and flexural loading conditions. In addition, in-situ acoustic emission (AE) monitoring is employed during tensile tests to determine dominant damage types and failure modes in the SLJs. Surface analysis shows that MA increases roughness, PP treatment decreases wettability, while APA enhances wettability by modifying the surface chemistry. Tensile and flexural tests reveal that APA-treated joints surpassed non-treated (NT) ones, with up to 5- and 7-times higher load-carrying performance, respectively, while fracture analysis suggests a shift from adhesive to cohesive failure. AE results show that increased AE events related to cohesive failure align with improved interface interactions.

Effect of atmospheric plasma treatment on Mode-I and Mode-II fracture toughness properties of adhesively bonded carbon fiber/PEKK composite joints

This study aims to assess the influence of peel-ply (PP), mechanical abrasion (MA), and atmospheric plasma activation (APA) treatments on Mode-I and Mode-II fracture toughness of carbon fiber/ poly-ether-ketone-ketone (CF/PEKK) composite joints. A comprehensive examination of the topography and wettability of the adherend surfaces is conducted using various methods. The CF/PEKK adherends are produced through an automated fiber placement (AFP) process, and the CF/PEKK bonded joints are prepared using two different structural adhesive films, one of which has a lower strength, while the other has a higher strength. To evaluate their fracture toughness properties, double cantilever beam (DCB) and end-notched flexure (ENF) tests are carried out in accordance with ASTM standards. Acoustic emission sensors are used to monitor the test specimens during DCB tests, allowing for an in-depth evaluation of the failure modes and damage propagations in the joints. The results show that the GIC and GIIC values of the APA-treated CF/PEKK bonded joints are remarkably higher than those of the untreated ones, with a range of improvement of 34.0–84.8 times and 7.5–17.4 times, respectively. Adhesive failure is the dominant failure mode on the surfaces of non-treated (NT) and PP samples, while cohesive failure is more prominent in those treated with MA and APA. The failure modes of the treated samples varied depending on the adhesive used, with APA-treated samples always exhibiting a cohesive failure. It is observed that the AE counts increase more slowly in APA-treated samples compared to MA-treated joints as delamination progresses more slowly with cohesive failure dominant, which leads to a lower release of AE energy.

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.

Inner defect detection of glass fiber reinforced polymer sandwich panel using pulsed terahertz imaging based on smoothing and derivative

We conducted an experimental investigation to localize and detect internal defects in a sandwich panel made of glass-fiber-reinforced polymer (GFRP) using terahertz (THz) time-domain spectroscopy and signal processing techniques, which included smoothing and derivative analysis to enhance the defect detection performance. A simulated defect was created using a peel ply and scanned using a pulsed THz scanner with a reflection geometry specifically designed for this purpose. The reflected THz wave was analyzed to improve defect detection performance through time-domain signal processing using smoothing and derivative analysis. To quantify the performance of the proposed signal processing method, we introduced the Defect to Noise Ratio and evaluated its effectiveness accordingly. As a result, noise was removed and defect detection performance was enhanced, enabling the visualization of foreign insert defects in the GFRP sandwich panel.

Increasing strength and fracture toughness of carbon fibre-reinforced plastic adhesively bonded joints by combining peel-ply and oxygen plasma treatments

Multifunctional surface treatment combined with mechanical interlocking and chemical bonding is one of the directions to further expand the field of surface treatment of composite materials. In this study, plasma modification was conducted on the surface of carbon fibre reinforced plastic (CFRP) pretreated by peel-ply, and then comparative analyses were conducted. The adherend surface and the bonding interface were analysed from microstructure, element distribution and chemical bonding to study the strengthening mechanism. Experimental results showed that the bonding strength and fracture toughness of the joints significantly improved after oxygen plasma treatment, which removed and transformed the silicon-containing impurity and reorganised the surface groups. Surface properties of the treated specimens were also improved, as indicated by their higher surface energy values and mechanical interlocking between substrate and adhesive. Compared to plasma treatment taken individually, the CFRP bonded joints had better performance after peel ply plus plasma treatments, and there was less strength loss and higher toughness when the surface damage resulted from the long-duration plasma treatment. These findings may provide insights into solving the challenge of simultaneously increasing the strength and toughness of CFRP bonded joints.

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.

Variation of Mesh Peel Ply and Gelcoat on Mechanical Properties of Carbon Twill-Vinyl Ester Composites Using Vacuum Assisted Resin Infusion Manufacturing Method

This study discussed the modifications made to the Vacuum Assisted Resin Infusion (VARI) method to make composites. Composites made using the standard VARI method sometimes have values that have not reached the desired standard. In addition, there was a modification to add Gelcoat and mesh peel ply from previous studies that used e-glass fiber and epoxy resin, which positively affected the mechanical properties of the composite. In this study, carbon twill material was used, which is relatively better in mechanical properties than e-glass fiber and vinyl ester resin with different viscosity from epoxy. The finite element simulation was also performed to compare the load versus displacement graph with the testing. The tensile test results of the VARI with double mesh peel ply increased by 1.61%, while VARI with the addition of gelcoat increased 4.34% from the composite with the standard method of 527.62 MPa. The tensile test results of the VARI with double mesh peel ply decreased 17.20%, while the VARI with the addition of gelcoat decreased 30.24% from the composite with the standard method of 29.97 MPa.

Vol4 num2-16 CFRP advanced surface characterization prior to adhesive bonding. Peel ply treatment vs UV laser

Surface treatment is one of the most important issues for obtaining high quality adhesive bonding. The main objective of this treatment is the removal of surface contaminants, the modification of surface roughness and the increase of surface free energy, without damaging the surface of the composite material. Nowadays, the treatments which are approved by the aeronautical sector for the surface preparation of CFRP prior to adhesive bonding are manual and some of them depend on the skills of the operator. Because of this, different automatable alternatives are being studied in order to substitute these traditional methods. One of these alternatives is the use of UV laser radiation. In this work the advanced surface characterization of CFRP surfaces treated with peel ply and processed with UV laser have been performed. Advanced techniques have been used in order to evaluate the changes that occur in the surface and which can be related to adhesiveness. Thus, techniques such as scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), surface free energy determination and 3D optical confocal profilometry have been used to evaluate chemical and physical properties of treated surfaces. Obtained results show that UV laser radiation leads to surface changes which would improve the adhesiveness of the surfaces, although these changes suggest that the mechanism by which adhesion happens is different from the one which is related to peel ply. &nbsp

Surface Treatment of Composites with Bismaleimide Resin-Based Wet Peel Ply for Enhanced Adhesive Bonding Performance.

Surface treatment is typically required to improve the bonding performance of carbon-fiber-reinforced composites. Herein, a wet peel ply was prepared using bismaleimide (BMI) resins as a matrix resin. The temperature–heating rate extrapolation method and rheological method were employed to study the reaction characteristics and viscosity-temperature characteristics of the matrix in the BMI wet peel ply. The curing temperatures of the BMI wet peel ply and the BMI prepreg were the same (200 °C), making this wet peel ply suitable for co-curing with the BMI prepreg. After treatment with the wet peel ply, the bonding strength of the BMI composite joint showed a mean shear strength of 35.5 MPa, which was 1.72% higher than that of the sanded composite and 17.5% higher than that of the composite treated with the dry peel ply. In addition, the BMI composite treated with the BMI wet peel ply exhibited good bonding stability with a coefficient of variation of 3.9. After damp-heat aging for 1440 h, the retention rate of shear strength at room-temperature was 82.3%. The relatively loosely woven carrier in the BMI wet peel ply increased the surface roughness of the composite, thus improving the bonding strength.

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.

Effects of Bleeder Schedule to Fiber Mass Fraction and Composites Surface Topography on Wet Lay-up Manufacturing Process

The thermal protector materials of the rocket’s motor are made by a wet lay-up manufacturing process. Since the fiber mass fraction of the product is low, several experiments were conducted to solve this problem including changing the type of the epoxy resin and selecting the most suitable bleeder schedule. Bleeder cloth application results in increasing the fiber mass fraction. The fiber mass fraction of thermal protector material manufactured by hand lay-up can reach a maximum of 56,78%, whereas vacuum bagging can reach a maximum of 66,43%. Peel ply and breather fabric combination are the best bleeder schedule for the hand lay-up method meanwhile perforated release film and breather fabric are the best bleeders for the vacuum bagging method. Composite surface topography obtained from peel ply is visible on the surface. The imprints of the nylon peel ply weave are visible through SEM analysis. Meanwhile, the surface topography obtained from the perforated release film is not visible. The vacuum bagging method helped reduce the number of voids and ductile polymer fractures from the composite surface. This paper recommends peel ply usage in the thermal protector manufacturing process to replace the sanding or filling method that the author use nowadays.

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.

Effect of surface pre-treatment on the behaviour of adhesively-bonded CFRP T-joints

This work aims to better comprehend the mechanical behaviour, including failure sequence and failure modes, of differently surface-treated CFRP T-joints when subjected to out-of-plane loadings.Specimens consisted in two CFRP adherends – skin and stiffener – adhesively bonded with a structural film adhesive. The adherends' untreated and treated (by peel ply or abrasion) surfaces were firstly characterized using scanning electron microscopy (SEM), profilometry and wettability measurements. Stiffener pull-off tests (SPOT) were then experimentally conducted under quasi-static loading. The fractured surfaces were analysed by fractography to identify the failure mechanisms.The peel ply treatment has led to the worst results in terms of maximum load, while the abrasion-treated specimens performed the best of the three series. Surface characterization and fractography analysis were properly correlated with the SPOT results. Fracture damage typically initiates as mode I within the tip of the stiffener's flange, but as the fracture extends along the stiffener's foot, it does it under predominant mode II.

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.

Comparison of Surface Treatments for Secondarily Bonded Joints of Marine Grade Composites

Abstract Secondarily bonded joints will be required for large, >50 m, modularly constructed composite hull vessels, as opposed to the primary single infusion currently used in smaller hulls. The objective of this study is to characterize the experimental response of and provide documentation for the use of E-glass fiber and epoxy resin composite secondarily bonded joints fabricated using vacuum-assisted resin transfer molding and surface treatment methods common in modular marine construction. Primary single infusion benchmark joints are compared to secondarily bonded joints using four surface treatments: polyamide release-treated peel ply fabric, polyester peel ply fabric, isopropyl alcohol cleaning agent, and acetone cleaning agent. The experimental methods used to compare the surface treatment methods consist of two measuring apparent strength, single lap shear, and cleavage peel experimentation, and one measuring Mode I interlaminar fracture toughness, using the double cantilever beam experimental method. The investigation results concerning the surface treatments indicate the use of polyester peel ply over polyamide peel ply and isopropyl alcohol over acetone as a surface cleaning agent. The investigation concerning the experimentation methods indicates the acceptable use of a single-sided lap shear to determine statistically significant variations between surface treatments. Single primary infusion joints showed less variation and higher characteristic values than secondarily bonded joints.

Studies on the influence of surface treatment type, in the effectiveness of structural adhesive bonding, for carbon fiber reinforced composites

Primary intent of the present paper is to correlate the effectiveness of surface preparation type (pre-bonding), such as solvent cleaning, sanding (mechanical abrade), chemical etching (alkaline and acid), and peeling of sacrificial surface layer (peel ply) of carbon fiber reinforced composite (CFRC) test specimens, with the corresponding effect to the final strength of adhesive bonding. The analysis was performed by means of several spectroscopic and microscopic techniques, such as XPS, SEM, AFM, and a contact angle measurement technique which provides a direct, fast and easy measurement of the surface energy of the fiber composites. The adhesively bonded carbon fiber composites based on the different surface treatments were mechanically tested by employing the ASTM D 5528-0 (reapproved 2007) standard. The double cantilever beam (DCB) specimen geometry was used to evaluate the value known as opening Mode I inter-laminar fracture toughness (GIC) in adhesive bonds of carbon fiber composite materials. The results were varying between 1 and 1.8 kJ/m2 for GIC analysis and around 50 mm for delamination lengths. The mechanics of the linear elastic fracture is considered a tool for the failure of the laminate in composite materials with cracks or cuts in the plane, due to the presence of damaged zones in the tip of the crack; which is therefore applied to the study of delamination or fracture toughness.

Effect of peer ply and cooling rate on the tensile properties of Al/Gf/PP laminate prepared by hot pressing

The glass fiber-reinforced polypropylene/AA2024 hybrid composite (short for Al/Gf/PP laminate) was prepared by hot pressing with anodized 2024-T3 aluminum alloy as the skin and thermoplastic glass fiber-reinforced polypropylene core as a substitute of thermosetting glass fiber-reinforced polymer (based on epoxy) in order to increase the productivity and reduce the production costs during manufacturing procedure. The effect of peel ply (release cloth) and cooling rate on the laminate was investigated elaborately in this work. The orientation of glass fiber is mainly influenced by the peel ply structure, resulting in the resin’s overflow during hot press. Moreover, different cooling rates due to various cooling methods significantly affected both the degree of crystallization of the matrix and deformation of the metal skin during the curing process. Finally, a new peel ply structure was adopted to prevent the fiber from spilling during hot press process, while bubbles were allowed to flow out normally along with partial resin. Finally, a novel cooling method to reduce the degree of polypropylene crystallization and control metal deformation was proposed.

A new method for ultrasonic detection of peel ply at the bondline of out-of-autoclave composite assemblies

Out-of-autoclave materials have long been an established material system for secondary structural applications; however, recent advancements in material properties allow for more advanced structural applications. Even though certain out-of-autoclave properties have achieved parity with autoclaved cured materials, out-of-autoclave materials are cured at reduced temperatures and pressures resulting in less compaction and homogeneity. The consequence is extraneous ultrasonic signals, due to internal reflections and refractions that cause attenuation, potentially masking defects leading to unidentifiable indications. Advanced algorithms were developed to improve the signal to noise ratio between constituents of similar acoustic impedance in bonded out-of-autoclave carbon fiber reinforced polymer assemblies. Conventional ultrasonic nondestructive testing techniques and analysis software cannot consistently achieve signal to noise ratios that meet quantifiable rejection thresholds of accurately sized peel ply inserts at the bonded interface of composite assemblies. Ultrasonic pulse echo with full waveform capture was used to inspect a reference standard with peel ply inserts placed between the adhesive and three-dimensional-woven fabric preform. The ultrasonic signal was produced by a 64 element array transducer with a central frequency of 2.8 MHz. Waveform post-acquisition analysis with post processing software was used to analyze and enhance the signal response between the peel ply and the bondline resulting in the final algorithm. To verify the results, the signal to noise ratio of each insert was calculated for both the raw and processed data. As the measure of detectability, the method relies on principles of statistical measurement to provide an industry standard signal to noise response of 3:1.

Effect of infrared laser surface treatment on the microstructure and properties of adhesively CFRP bonded joints

Adhesive bonding is considered to offer advantages over the connections of the aircraft composite structures. The infrared laser surface processing of various laser process parameters and peel ply treatment are performed in this study. To investigate the microstructure and properties of adhesively bonded composite single-lap joints obtained by infrared laser surface treatment and peel ply processing, the examinations of microstructures, surface free energy and shear strength of the joints are conducted. The results indicate that the adhesively bonded joints acquired by infrared laser treatment have lower average shear strength of 20.158 MPa, comparing with the joints of 32.574 MPa in average shear strength treated by peel ply processing. In addition, the surface roughness test is conducted and the surface roughness of 4.926 μm of the specimen after peel ply treatment is higher than that of the specimen after infrared laser treatment.