Peel Load Research Articles

Curing of epoxy adhesives between thin metal foils by means of inductive heating

Metal foils are being widely used, from the chemical or electronics sector to household appliances. The joining of these foils by adhesive bonding is often the preferred method due to discolouring and warping under the thermal stresses of other joining methods, such as welding. However, long curing times are a disadvantage of adhesive bonding compared to welding. The use of electromagnetic induction is a promising solution for accelerated curing. This work investigates induction heating for accelerated curing of 1-C epoxy adhesives for bonding of thin nickel foils. Process parameters for rapid curing of the adhesives were determined based on reaction kinetics using differential scanning calorimetry measurements. According to those results peel test specimens were fabricated, and the peel resistance was evaluated using a 90° peel load.

Study on precise weld diameter validations by comparing destructive testing methods in resistance spot welding

The torsion test is rarely used for resistance spot-welded joints since they are not subjected to torsion in applications. Normal, shear, and/or peel loads are usually the main stresses. Extensive scientific investigations in the context of Kunsmann’s dissertation date back more than 50 years. These investigations are still the basis of ISO 17653 and the German guideline DVS 2916-1. Recent scientific investigations only use torsion tests, but do not describe the reason for its use. A decisive advantage of the torsion test over the other standardized destructive testing methods lies in the types of fracture modes that occur and the properties of the fracture surfaces. Torsional loading results in either interfacial or button-pulled fracture modes. No material residues occur on the fracture surfaces for ductile and advanced high-strength steels. Hence, the measurement of weld diameter is achievable with minimal constraints, resulting in reduced variability and facilitating objective assessments of spot welds. This article delineates these attributes through a comparative analysis of various destructive testing methods employing statistical approaches. Additionally, the article expounds on the design concept of the developed rig for conducting torsion tests on spot welds.

Evaluation of surface pretreatment methods on thin austenitic stainless steel foils for adhesive bonding applications

ABSTRACT Stainless steel is used across all industries, from the chemical or construction sector to household appliances. To produce load bearing joints, a surface treatment of the passive surface of austenitic stainless steel is generally necessary. However, the treatment of thin-walled foil materials inhibits challenges concerning the deformation under thermal loads. This work investigates the effectiveness and mechanisms of different surface treatments on 50 µm thin AISI 316 L steel foil. Basic cleaning, surface finishing with an aluminum oxide abrasive, flame-pyrolytic deposition of silicon dioxide and application of a primer, etching with hydrochloric acid, treatment by atmospheric plasma, ns-pulsed laser surface cleaning, and fs-laser structuring are investigated. The effects of these treatments are analyzed in surface energy investigations by contact angle measurement, roughness measurements, and SEM-EDX. The bonding strength is tested with epoxy adhesives and a methyl methacrylate adhesive under 90° peel load. The failure mechanisms are further investigated by FTIR spectroscopy and SEM-EDX. The results show that the used coating with primer produces the strongest bonds, followed by the fs-laser treatment. In particular, sub-microscopic surface structures induced by fs-laser show promising results. SEM images show a full wetting of the capillary structures, providing the possibility of mechanical interlocking on a sub-microscopic level.

Self-induced crystallization to form a shish-kebab structure on PLA-based Janus membrane to promote water transmission and interlayer binding force

Janus membranes are widely used in fields such as oil-water separation and wound dressings due to their asymmetric wetting properties. However, the weak interface binding force between the hydrophobic and hydrophilic layers can affect the liquid transport performance and membrane separation. In this study, we have reported a method using self-induced crystallization to grow shish-kebab (SK) structures in electrospinning polylactic acid-based fiber membranes. Peel testing revealed that the peel load after induced crystallization increased from 0 to 0.05 N to 0.05–0.15 N, and the time for droplets to penetrate from the hydrophobic side to the hydrophilic side decreased from 16S to 3S. This indicates that SK structures can enhance the adhesion between the hydrophilic and hydrophobic interface, as well as facilitating droplet transport. Through the oil-water separation experiment, it was discovered that the presence of SK structures can filter out more microemulsions. For different emulsions (kerosene, petroleum, n-hexane, petroleum ether, tea tree oil), the separation efficiency of Janus-SK membrane increased by 1.67 %, 2.5 %, 3.0 %, 9.7 %, 4.3 %, respectively. The development of Janus membranes has been provided with a new direction.

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.

Analysis of peel and shear strains in cracked lap shear specimens subjected to fatigue loading using digital image correlation

Adhesive bonding presents many advantages, such as efficient manufacturing and improved structural performance [1]. However, in structures subjected to fatigue, cracks might initiate and propagate in joints, leading to in-service failure [2]. Most adhesively bonded joints are subjected to combination of peel and shear loads, so mixed I+II mode loading conditions are present [3]. In this work, Cracked Lap Shear specimens, which feature mixed I+II mode loading conditions, were tested under fatigue loading. During tests, crack growth was monitored using Visual Testing and Digital Image Correlation. With Digital Image Correlation, opening and sliding displacements in the bondline were extracted from the substrates’ displacement fields and compared against a Finite Element Model, revealing a highly strained process zone ahead of the crack tip. Results highlight the usefulness of DIC in capturing the deformation behaviour of adhesive joints under mixed mode loading conditions.

Design Concepts for Peel-Dominant Adhesive Joints in Aeronautic Applications

The adhesive bonding technique is employed from the aeronautical/aerospace industry to current house products. To comply with the requirements of distinct applications, different joint configurations are available to the designer. While single-lap joints (SLJ) are the most common in application and research, double-lap joints, scarf joints and T-joints find specific applications. T-joints are seldom studied in the literature, but these are used, for instance, in aircraft to bond the stiffener beams to the skin, or in the cars between the B-pillar and the rocker. Due to the high stress concentrations, T-joints often fail under average stresses much lower than the adhesive strengths, giving rise to the necessity for proper design and strength improvement methodologies. This work initially aims to validate the cohesive zone modelling (CZM) technique with experiments, and then use it to numerically evaluate and optimize the performance of T-joints subjected to peel loads. CZM is nowadays regarded as the most powerful strength prediction tool for adhesive joints, and can be a valuable tool to improve T-joints. Different features are addressed for a complete analysis: adhesive type, geometrical parameters, dual-adhesive technique for strength improvement, and composite joints. The evaluated geometrical parameters are the base adherend thickness (a), T-part thickness (t), overlap or bonding length (l) and curvature radius (r). As a result of this work, the model was successfully validated, and clear design guidelines were provided to define the ideal geometric and material (adhesive) conditions for best performance.

P‐12.3: Analysis and Application of COF Peeling Failure in Display

In order to solve the problem of COF peeling on OLB (Outer Lead Bonding) side of high curvature display screen, this paper studied the peeling failure strength of COF Bonding so as to guide product design through the combination of experiment and simulation. Firstly, basic peeling test of the COF was carried out under the condition of 60 ℃/90%RH HTHHO, and the COF peeling load was obtained; Then, the linear stress of COF under the peeling load was 0.18 N/mm by finite element numerical simulation; Finally, the COF twist model of 32‐inch R1000 module was established, and the maximum COF twist value was 3mm under the maximum linear stress of 0.18 N/mm at COF Bonding; The module has passed the 60 ℃/90%RH HTHHO 500h test, which verified the accuracy of COF peeling failure judgment method. Considering the safety factor of 2, the linear stress intensity standard of this Bonding process was determined to be about 0.09 N/mm. Based on the above‐mentioned linear stress intensity standard, the 27‐inch curved surface module was optimized from double PCB to single PCB, which passed the verification of high temperature and high humidity condition and met the product quality requirements. The method proposed in this paper can guide module optimization design, reduce the risk of peeling failure at COF Bonding on the source side of curved screen, and improve product reliability.

Influence of Injection Molding Parameters on the Peel Strength between Plasma-Treated Fluoropolymer Films and Polycarbonate

Light guiding is used to direct light from an emitting source to a different location. It is frequently realized through a clad-core structure with a difference in the refractive index of the materials. This paper explores the possibility of combining a fluoropolymer (THV) film of low refractive index, serving as a cladding layer, with a polycarbonate (PC) core, via injection molding. Pristine THV lacks adherence to the PC. However, when treated with O2 plasma prior to overmolding, bonding can be established that was quantified in peel tests. The effect of this surface treatment was further investigated by adjusting the plasma treatment duration and time to overmolding. Furthermore, parameter studies comprising the four molding parameters, namely packing pressure, injection speed, melt temperature, and mold temperature, were performed. Numerical injection molding simulations assessed the prevailing temperatures at the PC-THV boundary. Consequently, the temperature-time integral could be calculated and linked with the measured peel strengths by fitting a proportionality constant. While the plasma treatment duration showed minor influence, the activation diminished with time, halving the measured peel loads within 24 h. The adhesion was experimentally found to increase with a lower packing pressure, faster injection speed, and higher melt and mold temperature. Those same molding relations influencing the peel loads were also found with the temperature-time integral when scaled by the proportionality constant in the simulations (R2=85%). Apparently, adhesion is added by molding settings which promote higher interface temperatures that prevail for longer. Hereby, the faster injection speed increases the melt temperature through shear heating. A higher packing pressure, in contrast, presumably increases the heat transfer at the PC-THV interface, accelerating the cooling. The measured peel loads were 0.3-1.6 N/mm for plasma-treated samples and nearly zero for pristine THV.

Experimental and numerical investigation of the influence of multiaxial loading conditions on the failure behavior of clinched joints

Destructive testing methods are used to determine the joint characteristics of mechanical joining technologies by testing the joints under different types of loads. In real load cases, however, combined loading conditions arise in complex structures that cannot be investigated experimentally at present. The objective of this study is to adapt a so-called L-specimen geometry for mechanical joints to introduce combined peel and shear tensile loading, taking into account plastic pre-deformation of the joined parts. A new specimen geometry is presented to investigate three different clinched joints using different materials, namely HCT590X as well as EN AW-6014 T4. The load capacity behavior, as well as the failure behavior, is analyzed based on failure modes for pure shear, pure peel, and combined loading. Furthermore, the L-specimen is investigated by numerical simulation and compared with the experimental results.

Effect of hygrothermal acid rain environment on the shear bonding performance of CFRP-concrete interface

In this study, forty-five carbon fiber-reinforced polymer (CFRP)-concrete single shear specimens were designed to investigate the effect of hygrothermal acid rain environment. At first, the hygrothermal acid rain environment was simulated through a mechanical high-temperature dry-wet cycle and by artificially preparing acid rain solution with a pH value of 1.5. Subsequently, the effects of concrete strength and corrosion times on the interface failure mode, bond shear capacity, ultimate displacement, load–displacement behavior, and strain distribution in bonding interval were studied through the single shear test of CFRP-concrete. Then, the interface constitutive relation model was built based on the influence coefficient of hygrothermal acid rain. Finally, a reference method was proposed to divide the corrosion degree in hygrothermal acid rain environment, and the interface failure mechanism was expounded using a scanning electron microscope (SEM) for detection. The results indicated that the interface bonding performance was enhanced with the increase of the concrete strength, and the interface peeling position tended to change to the adhesive layer. The interfacial bonding performance increased first and then decreased with the increase of the corrosion times. When the corrosion number reaches ten cycles, the interfacial bond performance gets the best, the interfacial peel load and ultimate displacement of the three types of strength concrete(C30\\C40\\C50) were examined as 3.04 %, 3.50 %, 5.78 %, and 0.50 %, 0.49 %, 0.95 % higher than those of the uncorroded specimens, respectively. The SO42- ions in the acid rain invaded the surface of the concrete to synthesize an expansive substance CaSO4·2H2O, thus temporarily enhancing the interfacial bonding performance at the early corrosion stage. The load–displacement curve comprised four stages, including rising, oscillating, strengthening, and falling. In this study, an interface constitutive model was proposed based on the influence coefficient of hygrothermal acid rain, which is well consistent with the existing experimental data and exhibits high accuracy and safety. The relevant research results can provide theoretical support and design guidance for CFRP reinforcement projects in high-humidity and high-heat acid rain areas.

Evaluation of Viscoelastic Adhesion Strength and Stability of Composite Waterproofing Sheet Using Non-Hardening Viscoelastic Synthetic Polymer-Based Rubber Gel

Recently, a non-hardening type synthetic polymerized rubber gel (hereinafter referred to as NHV-SPRG) composite waterproofing sheet has been used on construction site as a new waterproofing technology. In this study, a test method is proposed in which a composite waterproof sheet is attached to an area of 150 mm in length and 100 mm in width on the mortar-based substrate specimen, and subsequently peeled off at 180 ° vertically to measure the “peel load (N)” at 10 points of 10 mm intervals (P1~P10, from 30 mm point to 120 mm point). Value obtained by dividing the average value of the measured peeling load by a width of 100 mm was defined as “viscoelastic adhesion strength (N/mm)”. The viscoelastic adhesive strength evaluated for the NHV-SPRG composite waterproof sheet of 4 types (VG-1, VG-2, VG-3, VG-4) was an average of 1.99 N/mm. To examine the effectiveness of the viscoelastic adhesive strength, the adhesion stability was evaluated based on the peel load’s coefficient of variation, grade of the linear regression line of the peel load, and peel load percent relative range in peel load for each section of the composite waterproofing layer. As a result, the VG-2 type was measured to have the highest viscoelastic adhesive strength, but the VG-1 type was confirmed to have the highest adhesive stability. Considering these results, it is judged that even a product with high adhesion can have varying stability in terms of performance that can affect construction precision. The test method and performance standard value for the viscoelastic adhesion strength proposed as an evaluation index through this study are expected to be used as a quality control standard to secure the on-site adhesion stability of the NHV-SPRG composite waterproof sheet.

Peeling angle effect on soft adhesive: Mixed-mode CZM considering fibrillation

Soft adhesive has gained broad application in flexible electronics and biomedicine. Because the soft adhesive layer is frequently subjected to peeling load, it is critical to investigate the mixed-mode effect on the cohesive failure of the soft adhesive layer. In this paper, a mixed-mode CZM considering the fibrillation as well as the shear flow of the soft adhesive layer has been proposed, and shows a better agreement on the experimental data compared to the bilinear CZM. The complex evolution of the mode-mixity with the peeling angle, which is difficult to be experimentally investigated, can also be well characterized. The soft adhesive layer exhibits the mode-I dominating failure at the large peeling angle due to the significant fibrillation, indicating that its adhesive performance can be well correlated with that under the probe-tack test. The mixed-mode effect has a significant impact on the adhesive performance of the soft adhesive layer at the small peeling angles, and the mode-mixity varies dramatically with the peeling angle. The parametric study reveals that the failure mode of the soft adhesive layer can be changed with the material properties. When compared to the material property of the soft adhesive layer, decreasing the adherend stiffness results in a considerable change in the cohesive failure mode from mode-I failure to mixed-mode failure. The proposed CZM is a promising approach for describing the mixed-mode cohesive failure of the soft adhesive layer during the peeling test.

Mechanical and physical characterization of natural and synthetic consolidants

The mechanical behavior of adhesives is strongly influenced by a large number of variables, relating to a complex interaction of mechanical-physical-chemical factors, such as its loading direction (shear, peel), the temperature and the environmental relative humidity (RH). These variables can have a large influence on the durability of restored art objects where thermoplastic adhesives have been used as a consolidant. This study aims to characterise the mechanical and physical behavior of some adhesives commonly used polymers by conservators as consolidants to restore cultural objects such as canvas paintings or historic wooden furniture. Twelve commercially available natural and synthetic adhesive materials were tested. The influence of RH at room temperature on the mechanical and physical properties of the adhesives was investigated. Shear and peel experiments were performed on adhesively bonded wood and canvas coupon to establish mechanical characterisation. The physical properties of the adhesives were determined by performing moisture adsorption measurements and Differential Scanning Calorimetry (DSC). The results of this study demonstrate that synthetic adhesive products are able to resist higher shear and peel loads than natural types. Moreover, the influence of important changes in RH on the mechanical properties of the adhesives was demonstrated. Reflecting on the combined data derived from shear and peel tests with the adhesive's sensitivity to moisture will help conservators to select the most suitable adhesives for their applications to achieve optimal durability and the best mechanical performance in versatile environmental conditions.

Understanding creep behavior of carbon fiber/epoxy interface via molecular dynamics simulation

This work focuses on molecular creep behavior of carbon fiber/epoxy interface under sustained loads at different levels. Threshold stress and energy barrier for the onset of interfacial creep failure under peeling loads are found greater than those under shearing. Microstructural changes during creep show that the epoxy has a large and irreversible deformation under peeling loads, denoted as a yielding process in the peeling case. The yielding process in the peeling case dissipates the applied energy in the epoxy structure. Therefore, the interface shows a stronger resistance to the peeling and the shearing is more critical during composite failure.

Interfacial peeling loads in the TBC with an air hole: Analytical solutions and viscoplasticity modelling

This work evaluates the temperature field, stresses, and peeling loads in thermal barrier coatings (TBCs) with air holes subjected to cyclic thermo-mechanical loads during a complete flight. Analytical formulae for the steady-state temperature field and interfacial peeling loads are developed considering thermal gradients in the thickness and longitudinal directions. In addition, viscoplasticity finite element modelling is conducted for the mechanical behaviors of the TBC system. A unified Chaboche-Lemaitre constitutive model that combines a power flow rule with non-linear anisothermal evolution of isotropic and kinematic hardening is integrated into the numerical framework. The results show tensile peeling stress and shear stress are generated in the vicinity of the hole, which motivates the initiation and propagation of hole-edge cracks. The maximum peeling and shear stresses locate close to the thermally grown oxide/bond coat interface and they increase with increasing hole radius. The interfacial peeling moment and shear force could characterize edge cracking in TBCs. As the hole radius increases, the interfacial peeling moment and shear force increase quickly when the hole radius is less than 1 mm but remain stable when the hole radius is beyond 2 mm. This work not only helps to explain the failure mechanisms of hole-edge delamination and spallation in TBCs but also guides the design of thermal barrier coated hot-section components in gas turbines.

Mode I creep fracture of rubber-like adhesive joints at constant crack driving force

This work contributes to the description of time-dependent mechanical mode I creep fracture of viscoelastic adhesive joints under pure peel loading at finite deformations. The focus is on the dependency of crack resistance and resulting crack propagation rate on the forming of minor cracks beside the macroscopic major crack tip. In this work, Double Cantilever Beam tests were carried out at polyurethane-based flexible adhesive joints. The experiments were externally controlled on a constant J-integral, which was calculated in-situ from measured data to obtain a constant crack driving force and a statically loaded crack tip, respectively. The resulting creep crack propagation was investigated using two cameras targeting both sides of the specimen. With help of image processing and the beam compliance method, crack extension and crack propagation rate were determined. The respective results were used to draw comparisons between these two methods. The slight difference between the methods was attributed to the appearance of minor cracks, which seemed to reduce the stress intensity at the macroscopic crack tip, yielding a lower crack propagation rate.

Composite-to-metal joining using interference fit micropins

Interference fit micropinning is a novel joining method that involves inserting thin pins with a size interference into an array of ultra-thin holes to create high strength structural joints. An experimental and finite element (FE) modelling study demonstrates the high structural properties of composite-to-metal joints using interference fit micropins. The joint studied consisted of plates of carbon-epoxy laminate and aluminium alloy joined using an array of steel micropins (505 μm diameter) with a close interference (1%) to the holes. The interference fit micropins are effective at creating composite-to-metal joints with high specific ultimate strength and ductility. The capacity of micropins to provide composite-to-metal joints with high structural properties under lap shear and peel load conditions is demonstrated. The effect of the spacing and number of micropins on the joint properties is evaluated.

Numerical analysis of composite-composite bonded joints. Fracture toughness determination

Nowadays, the use of composite materials in primary structures has considerably increased, in particular in what refers to primary structures and bonded joints. Thus, numerical tools and experimental tests have to be implemented in order to understand failure modes and critical loads in this kind of structures. Usually, bonded joints quality is tested based on the interlaminar fracture test DCB (G1c). Nevertheless, these tests have some limitations. The aim of the present study is to create a feasible and accurate numerical model which allows the quality of a bonded joint as function of the peeling load to be determined. The idea is then an actual alternative to offer in-situ test by means of a transportable and easy to use device. In this study, a research analyzing two tests able to identify the quality of a bonded joints, as an alternative to the interlaminar fracture test, based on a classic peeling test on carbon laminates, is presented. The numerical simulation is based on a Finite Element Analysis using the commercial software Abaqus®. Onset and propagation of adhesive damage is taken into account by means of a cohesive zone model. A virtual test of the peeling test is done and its behavior is compared with the experimental results from an extensive laboratory test campaign. Numerical results will allow us to verify the feasibility to compute the value of G1c from experimental results.

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.