Peel Mechanics

Published treatments of peel mechanics are shown to yield inconsistent relations for the dependence of peel force upon the angle of peel. The paradox is resolved by limiting the stress analyses to small bending deformations of the detaching strip in the still-attached region. This condition holds when the moment arm of the applied peel force is much larger than the length of the high-stress region in the bond, which must therefore be considered a prerequisite for use of the published bond stress distributions.

Failure of some experimental results to conform to the theoretical dependence of peel force upon peel angle are ascribed to inelastic deformation or stretching of the detaching strip.     

THE DEVELOPMENT OF A MANDREL PEEL TEST FOR THE MEASUREMENT OF ADHESIVE FRACTURE TOUGHNESS OF EPOXY–METAL LAMINATES

A mandrel peel test is established for measuring the adhesive fracture toughness of a metal/rubber-toughened epoxy laminate system. By adopting an energy balance analysis it is possible to determine directly both adhesive fracture toughness and plastic work in bending the peel arm around the mandrel. The suitability of the procedure is examined for various types of metal peel arms, which are classified in terms of their ability to deform plastically during the test. The plastic work is also predicted theoretically, and comparisons are made between the measured and calculated values. The fracture energies determined from the mandrel tests are compared with those obtained from 90° fixed-arm peel tests. For the calculations of plastic work in bending in the fixed arm test, various options are used when modelling the tensile stress-strain behaviour of the peel arm material. In addition, the adhesive layer thickness is considered in terms of its influence on the calculation of adhesive fracture toughness.     

THE DEVELOPMENT OF A MANDREL PEEL TEST FOR THE MEASUREMENT OF ADHESIVE FRACTURE TOUGHNESS OF EPOXY-METAL LAMINATES

A mandrel peel test is established for measuring the adhesive fracture toughness of a metal/rubber-toughened epoxy laminate system. By adopting an energy balance analysis it is possible to determine directly both adhesive fracture toughness and plastic work in bending the peel arm around the mandrel. The suitability of the procedure is examined for various types of metal peel arms, which are classified in terms of their ability to deform plastically during the test. The plastic work is also predicted theoretically, and comparisons are made between the measured and calculated values. The fracture energies determined from the mandrel tests are compared with those obtained from 90° fixed-arm peel tests. For the calculations of plastic work in bending in the fixed arm test, various options are used when modelling the tensile stress-strain behaviour of the peel arm material. In addition, the adhesive layer thickness is considered in terms of its influence on the calculation of adhesive fracture toughness.     

Protocols for the Measurement of Adhesive Fracture Toughness by Peel Tests

This article reports on the work of the European Structural Integrity Society Technical Committee 4 (ESIS TC4) and its activities in the development of test protocols for peel fracture. Thirteen laboratories have been working on peel test methods in ESIS TC4 since 1997 and their activities are ongoing.

The aim of the work is to develop robust and credible test methods for the determination of adhesive fracture toughness by peel tests. Several geometric configurations have been used, namely, multi-angle fixed arm peel, T-peel, and roller assisted peel in the form of a mandrel test.

The starting point of their work is an established analysis of a peel method that is often developed from a global energy approach. The adopted analysis is combined with an experimental approach in order to resolve ambiguities in the determination of adhesive fracture toughness (G_A). The test methods involve the measurement of peel strength in order to calculate the total input energy for peel (G) and the calculation of the plastic bending energy (G_P) during peel. The latter is often obtained from a measurement of the tensile behaviour of the peel arm. Adhesive fracture toughness is then G - G_P.

Four ESIS TC4 projects are described. The first relates to fixed arm peel whilst the second and third involve both fixed arm and T-peel. The fourth project combines mandrel peel and fixed arm peel. Each project uses different types of polymeric adhesives in the form of quite different laminate systems. The selection of the laminate system enables all characteristics of laminate property to be embraced, for example, thin and thick adhesive layers, polymeric, and metallic peel arms and a range of flexibility in the laminates.

The development of the enabling science required to establish the test protocols is described and software for conducting all calculations is referenced.     

Protocols for the Measurement of Adhesive Fracture Toughness by Peel Tests

This article reports on the work of the European Structural Integrity Society Technical Committee 4 (ESIS TC4) and its activities in the development of test protocols for peel fracture. Thirteen laboratories have been working on peel test methods in ESIS TC4 since 1997 and their activities are ongoing.

The aim of the work is to develop robust and credible test methods for the determination of adhesive fracture toughness by peel tests. Several geometric configurations have been used, namely, multi-angle fixed arm peel, T-peel, and roller assisted peel in the form of a mandrel test.

The starting point of their work is an established analysis of a peel method that is often developed from a global energy approach. The adopted analysis is combined with an experimental approach in order to resolve ambiguities in the determination of adhesive fracture toughness (G _A ). The test methods involve the measurement of peel strength in order to calculate the total input energy for peel (G) and the calculation of the plastic bending energy (G _P ) during peel. The latter is often obtained from a measurement of the tensile behaviour of the peel arm. Adhesive fracture toughness is then G – G _P .

Four ESIS TC4 projects are described. The first relates to fixed arm peel whilst the second and third involve both fixed arm and T-peel. The fourth project combines mandrel peel and fixed arm peel. Each project uses different types of polymeric adhesives in the form of quite different laminate systems. The selection of the laminate system enables all characteristics of laminate property to be embraced, for example, thin and thick adhesive layers, polymeric, and metallic peel arms and a range of flexibility in the laminates.

The development of the enabling science required to establish the test protocols is described and software for conducting all calculations is referenced.     

Experimental determination of the peel adhesion strength for metallic foils

This work addresses the problem of experimental measurement of peel adhesion in cases where a non-recoverable (plastic) deformation energy of the peeled foil, plus frictional losses, constitutes a significant portion of the total peel energy. In standard tests, when the true adhesion strength is desired, the plastic energy has to be calculated and deducted from the total energy. Several studies have been dedicated to the modelling and calculation of the energy dissipated through plastic deformation so that the net adhesion energy could be deduced. These calculations are cumbersome and impractical for general use. A simple experimental technique for the determination of the net adhesion strength is proposed. Experimental results with ～ 0.1 mm thick foils of stainless steel, nickel, and titanium confirm the theoretical predictions regarding the energy balance during peeling. Using the proposed methodology, there is no need to calculate or otherwise determine the deformation energy losses of the peeled foil or the frictional dissipation. The method is not limited to a particular material and can be used successfully for strain hardening plastic as well as metallic foils. Peel tests on adhesively bonded specimens of stainless steel and nickel and of a thermal spray-coated Ti alloy foil were carried out to demonstrate the applicability of the proposed method.     

An experimental partitioning of the mechanical energy expended during peel testing

Peeling of polyimide coatings bonded to aluminum substrates was analyzed from a thermodynamic perspective with the intent of determining how the energy expended in separating the bonded materials is consumed. The mechanical work expended and the heat dissipated during peeling were simultaneously measured using deformation calorimetry. The surfaces exposed by peeling were analyzed by electron microscopy and electron spectroscopy. The thermodynamics of tensile drawing for polyimide were studied using deformation calorimetry and thermomechanical analysis. When polyimide coatings were peeled from aluminum substrates at a peel angle of 180°, almost all of the mechanical energy was consumed in propagating the bend through the coating being peeled. The fraction of peel energy dissipated as heat was 48 ± 1.3% and nearly all of the remainder was stored as latent internal energy in the peeled polyimide. When the bend is propagated through aluminum, which has a limited capacity to store latent internal energy, 97-100% of the mechanical energy is dissipated as heat.     

An Elasto-Plastic Investigation of the Peel Test

This work outlines an elasto-plastic investigation of two common peel tests which use high and low yield strength aluminium adherends. An elastic, large-displacement, finite element program has been extended to include elasto-plastic material behaviour. This has been used to analyse both peel tests. The adhesive stresses near the crack tip have been shown to be finite while the corresponding strains remain singular. A failure criterion based on a maximum adhesive strain has been used to predict the relative strengths of the peel test. The amount of energy dissipated in the plastic deformation of the peeling adherends has been assessed by a series of tests and has been shown to be a considerable amount of the total energy supplied to the peeling system. Further, although the two aluminium alloys considered have grossly different yield strengths the energies dissipated in plastic deformation are similar. Material data for the finite element analysis and the plastic work calculations have been obtained from uniaxial tensile tests of both the adherends and the adhesive and actual peel strengths have been measured in a series of peel tests.     

An Elasto-Plastic Investigation of the Peel Test

This work outlines an elasto-plastic investigation of two common peel tests which use high and low yield strength aluminium adherends. An elastic, large-displacement, finite element program has been extended to include elasto-plastic material behaviour. This has been used to analyse both peel tests. The adhesive stresses near the crack tip have been shown to be finite while the corresponding strains remain singular. A failure criterion based on a maximum adhesive strain has been used to predict the relative strengths of the peel test. The amount of energy dissipated in the plastic deformation of the peeling adherends has been assessed by a series of tests and has been shown to be a considerable amount of the total energy supplied to the peeling system. Further, although the two aluminium alloys considered have grossly different yield strengths the energies dissipated in plastic deformation are similar. Material data for the finite element analysis and the plastic work calculations have been obtained from uniaxial tensile tests of both the adherends and the adhesive and actual peel strengths have been measured in a series of peel tests.     

Energy and force distributions during mandrel peeling of a flexible tape with a pressure-sensitive adhesive

Peel tests are commonly used to investigate the strength of adhesive joints. In the mandrel peel test the curvature of the flexible tape within the zone of detachment is controlled by the radius of the mandrel and this, coupled with an energy balance approach, allows separate determinations of the energy terms associated with any plastic or inelastic deformation of the tape and the de-adhesion or peel energy. If, in addition, the force on the mandrel is monitored then the position of the force vector within the separation or de-adhesion zone can be established. This, in turn, permits a closer comparison with predictions from a peel model which allows non-linear behaviour of both the tape and the adhesive layer. The model has been validated by peel tests on cellulose, PVC, aluminium and Teflon tapes carried out at sufficiently low speeds for rate effects within the adhesive to be small. Both the measured values of the de-adhesion energy, which varied from 60 to 160 J/m², and the positions of the force vector within the de-adhesion zone correlated well with those predicted from load-extension tests on samples of the tapes and bulk samples of similar polymeric adhesives carried out at similar rates of deformation.     

On the free volume evolution in a deformed epoxy composite. A positron annihilation study

After fabrication of an epoxy system filled with aluminum powder, followed by inelastic deformations under compression of the specimens, Positron Annihilation Lifetime Spectroscopy (PALS) was used to follow the evolution of the free-volume holes in the matrix. In order to describe the micromechanical deformation mechanism that takes place in the matrix around the inclusions, the experimental free-volume holes data were analyzed in terms of a model specifically developed. This model involves a hydrostatic internal stress resulting from the fabrication process of the composites and the deviatoric part of the applied stress during inelastic deformation. The influence of both kinds of stresses on the modification of the free-volume sizes in the matrix is discussed.     

Study of a 'T'-type peel test on a metal/polymer/metal sheet sandwich

This paper describes the specific 'T'-type peel mode in the case of a metal/polymer/metal sheet sandwich and gives experimental results on the influence of plastic deformation in the metallic substrates on the peel energy. We propose an experimental method of carefully determining the peel energy of a metal-polymer interface in a sandwich structure. Based on the mechanical properties of the stainless steel substrates and the maximum curvature of the metallic sheet measured experimentally during the peel test, several expressions for the clastoplastic deformation energy of the metal substrates are given. It is noteworthy that the curvature of the metal substrate layers depends not only on the mechanical properties of the material, but also on the work necessary to overcome the interfacial or cohesive forces. It is shown that even for thin metallic substrates (0.1 mm thick stainless steel), the work absorbed by the deformation represents roughly 50% of the total measured energy. During peeling the same specimen at different rates, the propagation peel force is higher or lower than the initiation force depending on the previous curvature of the metal sheets.     

Deformation mechanisms at the interface between grafted polyethylene and ethylene/vinyl alcohol copolymer

The adhesion of grafted polyethylene (PEg) to an ethylene/vinyl alcohol copolymer (EVOH), has been studied using different peel configurations and angles. Overall peel energies have been obtained and found to depend on peel angle. Experimental and theoretical studies of local peel arm curvature and opening angles near the crack front led to good agreement, the latter being based on elastic foundation theory and global elasto-plastic analysis. Having established the validity of the analysis used, the contribution to the peel energy pertaining to bulk bending of the peel arm(s) was estimated, allowing the local adhesion energy to be isolated. This was found to be virtually independent of peel angle. Scanning electron microscopy examination revealed a plastic, fibrillar craze zone in the PEg corresponding to a Dugdale zone. Nevertheless, adhesion energy was higher than expected from the Dugdale model. Energy dissipation in the vicinity of the Dugdale zone associated with shear deformation, and thus without apparent cavitation, may contribute to fracture energy. A rough estimate of the energy expended during the observed change in orientation of fibrils in the relaxation zone after the crack tip shows this contribution to be significant.     

A Practical Criterion for Rheological Modeling of the Peeling of Pressure Sensitive Adhesives

The transient extensional viscosity of non-crosslinked pressure sensitive adhesives (PSA's) and physically-crosslinked PSA's is measured and compared with theoretical predictions based on the linear viscoelastic (LVE) properties of the PSA's and the use of linear and quasi-linear constitutive equations. Based on a previously-derived expression for the relative contributions of individual relaxation modes of a polymeric material to its transient extensional viscosity, a criterion for whether large extensional deformations can be modeled on the basis of the LVE spectrum is proposed and evaluated for each PSA. The relevance to adhesion is demonstrated in peel tests, where the deformation of adhesive is quantified in images of the peel front under the assumption of uniaxial elongation and used to obtain theoretical peel forces in excellent agreement with measurements. This demonstrates the applicability of the criterion to the peeling process.     

COHESIVE ZONE MODELS AND THE PLASTICALLY DEFORMING PEEL TEST

The peel test is a popular test method for measuring the peeling energy between flexible laminates. However, when plastic deformation occurs in the peel arm(s) the determination of the true adhesive fracture energy, G c , from the measured peel load is far from straightforward. Two different methods of approaching this problem have been reported in recently published papers, namely: (a) a simple linear-elastic stiffness approach, and (b) a critical, limiting maximum stress, σmax , approach. In the present article, these approaches will be explored and contrasted. Our aims include trying to identify the physical meaning, if any, of the parameter σmax and deciding which is the better approach for defining fracture when suitable definitive experiments are undertaken.

Cohesive zone models Fracture mechanics Laminates Peel tests Plastic deformation     

Cohesive zone models and the plastically deforming peel test

The peel test is a popular test method for measuring the peeling energy between flexible laminates. However, when plastic deformation occurs in the peel arm(s) the determination of the true adhesive fracture energy, G c , from the measured peel load is far from straightforward. Two different methods of approaching this problem have been reported in recently published papers, namely: (a) a simple linear-elastic stiffness approach, and (b) a critical, limiting maximum stress, σ max , approach. In the present article, these approaches will be explored and contrasted. Our aims include trying to identify the physical meaning, if any, of the parameter σ max and deciding which is the better approach for defining fracture when suitable definitive experiments are undertaken. Cohesive zone models Fracture mechanics Laminates Peel tests Plastic deformation     

A Practical Criterion for Rheological Modeling of the Peeling of Pressure Sensitive Adhesives

The transient extensional viscosity of non-crosslinked pressure sensitive adhesives (PSA's) and physically-crosslinked PSA's is measured and compared with theoretical predictions based on the linear viscoelastic (LVE) properties of the PSA's and the use of linear and quasi-linear constitutive equations. Based on a previously-derived expression for the relative contributions of individual relaxation modes of a polymeric material to its transient extensional viscosity, a criterion for whether large extensional deformations can be modeled on the basis of the LVE spectrum is proposed and evaluated for each PSA. The relevance to adhesion is demonstrated in peel tests, where the deformation of adhesive is quantified in images of the peel front under the assumption of uniaxial elongation and used to obtain theoretical peel forces in excellent agreement with measurements. This demonstrates the applicability of the criterion to the peeling process.     

塑料异型材口模构型要素对挤出形变的影响

塑料异型材的挤出胀大及其引起的不规则挤出形变,是异型材挤出制品尺寸控制的难点所在。针对此类问题,采用有限元方法研究了典型异型材口模构型要素如T型口模分支立臂位置、90°夹角L型口模过渡区域转角、L型口模两臂夹角等对挤出形变和模流平衡的影响。结果表明,随T型口模分支立臂由挤出中心位置向右偏移,挤出后的型材挤出中心位置发生偏移,立臂顶端向右上变形,从而使得挤出形变加剧,需要在模具设计时将立臂顶端向挤出中心方向进行补偿。在90°夹角L型口模内外转角处设置过渡圆角,有助于模具出口处的模流平衡。L型异型材两臂夹角在挤出后变大,且初始设计夹角为60°和90°时变化较大,需要根据相关实验找出其初始设计夹角与挤出后的实际夹角对应关系,以便在模具设计过程中进行参照。上述研究结果对逆向挤出问题的求解、异型材挤出模具的设计和口模构型的确定有着一定的参考意义。     

Effect of Deformation-induced Residual Stress on Peel Strength of Polymer Laminated Sheet Metal

The adhesion between adhesively bonded polymer film and a metallic sheet substrate in a polymer laminated sheet metal (PLSM) subjected to large deformation, such as in a forming process, is influenced by two deformation-induced factors. These are (i) evolution of surface roughness of metallic substrate with applied strain and (ii) development of residual stress in the polymer adherend (polymer film with a thin uniform adhesive layer on one side) arising from significant differences in the deformation behavior of metal and polymeric components. A new experimental methodology was devised in this study to decouple the effects of substrate surface roughness and residual stress on interfacial peel strength (IPS) of uniaxially deformed PLSMs. This methodology was based on 180° peel testing of PLSM specimens prepared under two different lamination conditions, one involving systematic pre-straining in uniaxial tension of the metallic substrate prior to laminations and the other involving post-lamination pre-straining of the PLSM. The role of pre-strain and peel test speed, for the above laminations conditions, were critically analyzed for their effect on IPS of two differently tailored PLSM systems. The IPS results were attributed to the effect of deformation-induced residual stress and metallic surface roughness. The analysis suggests that IPS is strongly dependent upon the residual stress induced by uniaxial deformation but only marginally on substrate surface roughness depending upon the constituents (film and adhesive) of the adherend. The magnitude of pre-strain was inversely and non-linearly related to IPS for both deformed PLSMs. Peel test speed, on the other hand, showed a more complex behavior in terms of IPS for the two PLSM systems.     

Determination of Quasiequilibrium Work of Adhesion of Elastic Coatings

Present methods for the determination of adhesion bonding of elastic polymeric materials entail certain experimental difficulties. In particular, the necessity of strict centering of the test specimen, and the difficulty associated with application of a homogeneously distributed stress over the whole cross-sectional area (homogeneous detachment or shear), or the excessive expenditure of work resulting from polymer deformation (peel).^1,2

We are suggesting a method to determine the quasi-equilibrium work of adhesion during the peeling process for elastic polymeric coatings, the value of which, as was demonstrated experimentally, does not depend on the coating thickness, deformation or rate of peeling.