Roller Peel Test Research Articles

A multiscale study of heat treatment effects on the interlayer mechanical properties of 3D printed continuous carbon fiber‐reinforced composites

AbstractThe main goal of this study was to investigate the effect of heat treatment on the microstructural and interlayer mechanical properties of 3D printed continuous carbon fiber‐reinforced composites (CCFRCs). The influence of raster orientations on the interlayer properties of composites was systematically studied by the floating roller peel test. The multiscale morphological investigation was applied to analyze the influence of heat treatment on the interlayer mechanical properties and change of pores microstructure for composites. The results showed that heat treatment improved the interlayer mechanical properties of CCFRCs. During heat treatment, the pores merged, migrated, and deformed as the softening and collapse of the matrix. The average peeling strength after heat treatment for specimens increased by 10.17%–51.30%. Besides, the increase in matrix fluidity and the change of pore shape in CCFRCs were more significant than that of matrix specimens after heat treatment, since more thermal channels in CCFRCs.Highlights The interlayer mechanical properties of 3D printed continuous carbon fiber‐reinforced composites (CCFRCs) were improved by heat treatment. The pores of composites merged, migrated, and deformed as the softening and collapse of the matrix. The addition of continuous carbon fiber (CCF) made the composites have more thermal channels than those with matrix specimens.

Experimental evaluation of interface adhesion of a flax fiber composite patch with epoxy and polyurethane adhesives for the reinforcement of steel structures

Using fiber-reinforced composite patches for repairing damaged structures made of metal or/and concrete is an interesting and widely available solution on the market using synthetic materials. These repairing patches are bonded on the structures’ surfaces to increase their strength against internal stresses, as well as protect them from external physico-chemical attacks, thereby limiting crack propagation. Natural fibers offer a potential alternative to replacing glass or carbon fibers commonly used for bonded repair patches. Similarly, bio-based polymers represent an important sustainable alternative for partially or entirely replacing the petroleum-based polymers. In this study, an epoxy matrix reinforced with flax fiber is proposed as the material for the patches, and bonded to a steel plate using four different types of adhesive materials, including a castor-oil derived polyurethane resin. Floating roller peel tests were performed to assess the adhesion and viability of these new patches. The resulting peeling loads and fracture surface analysis are presented. Polyurethane demonstrates promising performance for epoxy-to-steel joints, but major improvements of the bio-based polyurethane application process and curing conditions may be necessary for its successful industrial implementation.

Identifying interfacial failure mode in aerospace adhesive bonds by broadband dielectric spectroscopy

The most widely accepted test for bond durability analysis in aerospace metal-bonded structures is the bondline corrosion test introduced in the late 90s. Little progress has been made however on non-destructive testing methods that allow determining the bond quality after years of use. Here, a non-destructive method based on dielectric spectroscopy is introduced to evaluate the state of a metal-adhesive-metal bond exposed to salt fog spray up to 180 days. Several samples were evaluated with broadband dielectric spectroscopy (BDS), floating roller peel (FRP) tests and bondline corrosion (BLC) after exposure to salt fog spray test for different times. Relaxation processes and conductivity phenomena extracted from the BDS data (e.g. apparent conductivity relaxation time (τmax) using electric modulus) are found to correlate well with the bond strength measured in peel test and BLC progression. The BDS-based protocol was able to identify the local interfacial degradation stages in a non-destructive mode and with high resolution. The protocol has the potential to be further developed into a test method for durability on coupon level.

Semiconducting properties of surface-treated titanium and their effect on peel resistance: Experimental and modelling studies

Adhesively-bonded titanium components enable new design concepts in lightweight engineering but often lack long-term joint durability and endurance. Major challenges are related to the fact that the titanium/adhesive interface is susceptible to environmental ageing which leads to interfacial deadhesion. In the present work, the issue is addressed from the standpoint of semiconducting properties of surface TiO 2 films. Examination of surface treated, anodized cp-Ti has been conducted using electrochemical impedance spectroscopy (EIS). Semiconducting properties of titanium oxide films were derived from Mott-Schottky analysis whereas roller-peel tests were considered for joint strength assessment before and after accelerated ageing in climate chambers. The experimental study is complemented by a computational study on the adhesion force distribution during roller peel testing based on a cohesive zone FEM model. The studies show clear indication for a complex interaction between the adhesion contributing underlying physio-chemical mechanisms. Crucial insights into the electrochemistry at the respective interfaces and their relevance for joint design are provided.

Influence of laser surface structuring of steel sheets on the mechanical properties of electrodeposited ZnNi coatings

The influence of laser surface structuring on the formation and the internal strength of electrodeposited ZnNi coatings on steel sheets was investigated. Surface structuring by extended direct laser interference patterning (xDLIP) was used to modify the cohesive strength in the ZnNi layer. An SEM analysis of the crystal structures shows perpendicular nucleation during ZnNi layer formation and areas of crosslinked crystalline structures, which results in an interlocking of crystal structures within the layer. The interlocked crystalline structure leads to an increase of the cohesive strength in the ZnNi layer, which was observed in roller peel tests according to DIN 1464 for adhesively bonded joints.

Adherend effect on the peel strength of a brittle adhesive

Peel tests are widely used to characterize the peel strength of bonded joints and control adhesion quality. There are various configurations of peel test, such as the T-peel test, peel testing at 180°, the floating roller peel test, and the climbing drum peel test. These methods have been widely used mainly in the aeronautical industry, as a way of assessing the peel strength of metallic joints. However, with the growing use of composite materials in industry, it is necessary to characterize bonded joints with these materials when subjected to peeling loads. In this research, the adherend effect on the peel strength of a brittle adhesive is experimentally studied using the floating roller peel test with the aim of evaluating how adherend changes affect adhesion properties of brittle adhesives and also to assess the viability of using the floating roller peel test in composite-to-composite and composite-to-aluminum joints, as well as make a comparison with aluminum-aluminum joint performance. It is also intended to prove the applicability of this test for quality control of adhesion and determination of peel strength in joints with composite materials. The results show the Araldite® AV138 performance falls within the characteristic values of peel strength of other structural adhesives, particularly when composite adherends are concerned, and with reasonable repeatability considering it is a brittle adhesive.

Analysis of the resistance of adhesively bonded joints with rigid-flexible substrates

Hydrogen, as an energy source, appears as a valid alternative to traditional fossil resources and supports the required reduction of CO2 emissions. The most recent hyperbaric tanks built for hydrogen storage (type IV) are made of composite material. For the manufacturing of these tanks, an adhesive layer is being used to join the flexible internal polymeric core, the liner, with the external composite shell. This work aims to study the strength of the liner-composite joint using the Floating Roller Peel (FRP) test. To this purpose, finite element analyses of the peeling process are carried out using the cohesive model. In order to establish a framework for the investigation of the joint properties as a function of manufacturing parameters (e.g., adhesive type and surface preparation), this paper describes a potential procedure that can be used for the identification of joint properties.

Temperature-dependent peel performance of adhesively rebonded hybrid joints

Structural bond integrity of reassembled (adhesively bonded) hybrid joints is assessed through floating roller peel tests in remanufacturing. Improper peeling, inconsistencies in the surface preparation and residual adhesive significantly affect the floating roller peel behavior of adhesively bonded and rebonded joints. In this work, the peel behavior of hybrid joints consisting of glass fiber reinforced polymer (GFRP) composite, Ti-6Al-4V adherends (with different surface preparations) and structural epoxy adhesive as a function of temperature was measured. Further, the peel performance of rebonded joints made up of thermo-mechanically debonded composites without removing the residual epoxies (R1) and reconditioned with multiple surface preparation techniques (R2) were evaluated. In this paper, the effect of residual adhesive on the peel load, failure modes as a function of temperature and flexural deformation of the Ti-6Al-4V adherend were studied. Improper surface reconditioning of debonded composites resulted in poor bond quality and peel strength as compared to the pristine joints.

Investigation of the effect of green laser pre-treatment of aluminum alloys through a design-of-experiments approach

The present study aims to investigate a surface treatment process performed by a pulsed Ytterbium green fiber laser on aluminum 2024 coupons. An evaluation of the process parameters effect as well as a statistical optimization of the process is attempted with the aim to improve the mechanical performance of adhesively bonded joints. The process parameters under investigation are the scanning speed, the pulse frequency and the hatching distance whereas the targeted objectives to be optimized are the surface wettability as well as the quality of adhesion as characterized by the Sessile Drop Technique and the Floating Roller Peel Test respectively. A design of experiments approach is implemented to search as effectively as possible the process parameters space. A Box-Behnken three-level factorial design was used to define the minimum number of experimental trials and build an appropriate test matrix. Finally, Analysis of Variance was implemented to assess the influential parameters. The set of parameters that optimize the objective of high peel strength and surface (super)hydrophilicity were identified following a response surface methodology. In order to understand and evaluate the experimental results and the underlying mechanisms, scanning electron microscopy (SEM) and chemical analysis with X-ray photoelectron microscopy (XPS) of the laser-treated surfaces were performed. In conclusion, it has been demonstrated that the laser surface treatment of aluminum alloy is an extremely promising dry treatment method, capable of improving the performance of adhesive bonding.

Enhanced Comprehension of the Continuous Fusion Bonding Process of Multi-Material Structures

In comparison to monolithic composite structures, tailored multi-material structures offer high potential considering lightweight design approaches in combination with cost efficient manufacturing processes. Roll forming enables flexible large scale production of hybrid structures, due to the continuous manufacturing process as well as high degree of automation. The multi-material structures consist of steel sheets which are selectively reinforced by unidirectional carbon fibre reinforced thermoplastics (CFR-TP). In view of minimizing process steps and decreasing cycle times, both materials are joined by fusion bonding. Therefore, CFR-TP is heated above melting temperature of thermoplastic matrix and joined to the steel surface under defined pressure and time. However, joining of both materials within a continuous process is still challenging due to a lack in terms of process comprehension. Consequently, multi-material specimens were manufactured depending on various process parameters as temperature of either material or processing speed and tested mechanically by floating roller peel test for the evaluation of the adhesion between both materials. Furthermore, viscosity of matrix was determined and investigations of CFR-TP interface were performed by Fourier transform infrared spectroscopy. The results show the requirement of a defined CFR-TP temperature and the change in crystalline structure of the matrix in dependency of the processing.

Interlaminar adhesion assessment of carbon-epoxy laminates under salt water ageing using peel tests

The aim of this study is to assess the interlaminar adhesion of carbon-epoxy laminates under salt water condition. Carbon-epoxy laminate specimens were immersed in a salt water tank for 60 days. Some specimens were then dried at room temperature for 280 days, until recovering their initial weight. Specimens were tested using the composite peel test, an adaptation of the floating roller peel tests for composite materials. The results showed a degradation of peel strength in some areas due to the ageing process. The drying process did not affect the test results. A scanning electron microscopic analysis carried out on the fracture surface of the specimens revealed a typical mode I failure microstructure. A mixture of matrix failure and fibre/matrix interfacial failure was observed in non-aged specimens. Finally, a chemical characterization of the fracture surfaces with energy-dispersive spectroscopy confirmed the penetration of salt water in regions near the edge of the specimens. A degradation of the fibre/matrix interface adhesion was observed in affected areas. Floating roller peel tests proved to be a fast and effective method to access the interlaminar adhesion performance of composite laminates.

Interface adhesion assessment of composite-to-metal bonded joints under salt spray conditions using peel tests

An experimental study has been carried out to assess the adhesion quality of composite-to-metal bonded joints under salt spray ageing conditions. The tests were performed according to the ASTM standard of floating roller peel tests with a new specimen layup. The layup and geometry of the specimen was defined in order to have the Carbon Fiber Reinforced Polymer (CFRP) and carbon steel as the rigid and flexible substrate, respectively. Specimens were exposed to salt spray (or salt fog) for 30 and 90days. The results show that the adhesion performance (i.e. average peel load) of the joints progressively decreases with increasing the ageing time. The fracture surfaces of dry specimens (non-aged) exhibit a cohesive failure within the adhesive layer, which indicates a good adhesion between the CFRP-steel interfaces. Interface degradation is indicated by a drop in peel load and adhesive failure. The percentage of adhesive failure increases with ageing times. Fracture surfaces of the adhesive failure exhibit deposition of NaCl crystal at the interface. Peel test successfully assessed the interface adhesion in aged and non-aged conditions, and can be used as a fast, easy and reliable test to study the long term durability in case of composite-metal bonded joints.

Effect of Anodic Aluminum Oxide Chemistry on Adhesive Bonding of Epoxy

The transition to Cr(VI)-free production is a great challenge in the global aerospace industry that currently still relays on it for the preparation of aluminum for bonding. Proper surface pretreatment is a prerequisite for strong and durable adhesive joint. Despite decades of experience, the nature and contribution of the different adhesion forces between the aluminum and organic adhesive remain under discussion. Herein we studied the adhesion of epoxy resin as a function of the surface chemistry of barrier-type anodic oxides prepared in sulfuric acid (SAA), phosphoric acid (PAA), and mixtures of phosphoric–sulfuric acids (PSA) and chromic acid (CAA) at different anodizing temperatures. X-ray photoelectron spectroscopy (XPS) data measured on model specimens were curve-fitted to calculate the relative amounts of O2–, OH–, PO43–, and SO42– species at the surface. The amounts of these species were then related to the mechanical performance of the joint measured by the floating roller peel test. Results show...

Investigation of peel resistance during the fibre placement process

In this study, the influence of compaction load, layup speed and temperature on the adhesive properties of automated fibre placement grade towpreg has been investigated on the ply-tool interface where higher peel forces are required to permit the deposition of subsequent plies. The automated layup process was simulated on a CNC milling machine, using a roller assembly and the adhesion properties of the towpreg were determined using a floating roller peel test. The processing window for the towpreg was determined using a dynamic mechanical analyser and a two-level, full factorial design of experiments was developed for the three factors, to understand their effects on the peeling force, both individually and synergistically. The design of experiments analysis indicates a strong temperature effect, with the towpregs requiring a higher layup temperature to accommodate higher layup speeds. A strong load-temperature interaction was detected, with a negative temperature effect at lower loads and a strong positive temperature effect at higher loads. The predicted factor settings to achieve a peeling force of 246 N/m are, 1 kN compaction load, 65℃ layup temperature, and a layup speed of 120 mm/min. Experimental tests, carried out at the predicted factor settings, agree well with the analysis, yielding a peel force of 256 N/m with a standard deviation of 25 N/m.

Test method to assess interface adhesion in composite bonding

This paper introduces a new type of peel tests dedicated to composite bonding: Composite Peel Tests. This test is inspired on the standard floating roller peel test widely used for metal bonding. The aim of this study is to investigate the potential of the Composite Peel Test to assess interface adhesion in composite bonded structures. To this end, peel tests were performed with nine different types of adhesives and at two environmental temperatures, room temperature and +80°C. The results were compared with the standard floating roller peel tests with Aluminium adherends. The results show that when using the Composite Peel Test good interface adhesion results either in cohesive failure of the adhesive or intra-laminar failure of the composite, while bad adhesion results in adhesive failure. In most cases of good interface adhesion, increasing the temperature favors cohesive failure of the adhesive in detriment of intra-laminar failure of the composite. Peel strengths can be used as a quality indicator of interface adhesion only if using exactly the same type of flexible adherend (peeling-off member). Nevertheless, if cohesive failure is the dominant failure mode, the comparison between adhesives’ peel strength is consistent disregarding of the type of peel-off adherend. Composite Peel Tests are suitable to assess interface adhesion of composite bonded structures.

Adhesion Properties of Bonded Composite-to-Aluminium Joints Using Peel Tests

In this research, the adhesion properties of bonded composite-to-aluminium joints are evaluated using floating roller peel tests. Tests were performed using two different adhesives and different adherend lay-ups: composite-to-aluminium, composite-to-composite and aluminium-to-aluminium. The results show that floating roller peel tests, widely used in metal bonding, can also be used to assess adhesion properties of composite bonding and composite-to-aluminium bonding. However, attention should be paid on which results are important to take from the peel tests. In adhesion tests, the failure mode is more important than the failure load. The peel load can only be compared when using exactly the same type of flexible adherend. Even when the adhesion properties are good (cohesive failure), the peel load value can decrease up to a factor of 10 when peeling off a composite flexible adherend instead of an aluminium flexible adherend.

The use of power ultrasound energy to support interface formation for structural adhesive bonding

A reliable performance is what the engineers need to take advantage of for the weight-saving potential of adhesive bonded structures. The present paper presents a new approach to ensure improved and quality assured adhesive bonds by using power ultrasound in combination with the adhesive application to support interface formation. In contrast to various known publications in this paper ultrasound is not used at low power-level as for inspection by nondestructive testing (NDT) but to influence interface cleaning and to improve contamination tolerance of the adhesive bonding process. Power ultrasound with an intensity of 10–100 W/cm 2 is used to transfer energy for a short time into the liquid adhesive. The general idea and various advantages for adhesive bonding by supplying energy for the developing adhesive/substrate interface are presented and discussed. Power ultrasound can be used: to remove surface contaminations, to obtain contamination tolerance for the bonding process, to improve substrate wetting, to mix adhesives, to outgas adhesives and the substrate surface or to establish chemical reactions at the interface. Results from adhesive bonded lap shear and roller peel tests with contaminated samples, rheological measurements and pictures from high speed videos demonstrate the effectiveness of the approach.

Floating Roller Peel Test Indicates Excellent Current Condition

The ageing behaviour of adhesive bonds has been the subject of numerous studies, research programmes and component qualification processes, primarily because of the fundamental importance of these bonds for the safety and reliability of the components in question. The outcome of these time-consuming investigations is almost always a series of relatively inconclusive forecasts. Against this background, the results described in this article of studies of samples that have aged over 40 years of practical use are highly interesting.

Macro- and micro-surface engineering to improve hot roll bonding of aluminum plate and sheet

Hot roll bonding of aluminum plate and sheet currently is the primary manufacturing method of fuselage skin sheet for aircraft and brazing sheet for automotive applications. Tremendous challenges and opportunities exist to improve quality and productivity of these products. This work explored the effects of macro- and micro-surface engineering on hot roll bonding process. It was found that micro-surface engineering yielded better bonding quality than macro-surface engineering did. The critical surface roughness of the core was about 0.58 μm below which bonding quality in terms of area contact and bonding energy can be significantly improved. However, no marked improvement was observed when the surface roughness was reduced further to 0.03 μm. Also, the oxide layer on the surface of the core and local deformation on the surface of liner played a very important role. Ultrasonic test and roller peel test were employed to assess the bonding strength and quality. The quantitative ultrasonic test results are in good agreement with the roller peel test results.

Small scale aluminium/epoxy peel test specimens and measurement of adhesive fracture energy

A small 2 nun wide, 25 mm long peel specimen is described which has been used to obtain the adhesive fracture energy for specimens comprising a toughened epoxy adhesive with aluminium adherends. The specimen is conveniently made from the unpulled ends of existing floating roller peel test specimens used by the aerospace industry as part of the routine quality control procedures. Specimens were prepared with the thinner adherend polished down to give specimens with a range of different adherend thicknesses, with failure occurring at or near the interface between adhesive and the adherend oxide. Estimates of adhesive fracture energy for failure in this region were made by using recent analytical methods. Measurements are also reported on the force required to bend thin strips with the same dimensions as those used for the peeling experiments. These served as a simulation of that part of the peel load due to energy dissipation in the peeling adherend arm, and the results were compared with predictions. The influence of peel specimen width on measured peeling load is also reported.