Effects of peel angle on peel force of adhesive tape from soft adherend

In the case of the peeling of adhesive tapes from soft adherends, the contributions of the compressive force at the adhered portion as well as the larger deformation of adherend have essential roles in determining the peeling properties. In this paper, the peel force of an adhesive tape from a soft adherend has been measured to understand the peeling mechanism, which is greatly affected by the peel angle. A commercially available pressure-sensitive adhesive was used as the tape, and a cross-linked polydimethylsiloxane (PDMS) was used as the soft adherend. The purpose of this study is to clarify the effects of the peel angle on the peel behavior of this system at room temperature under different material specifications and different experimental conditions. The factors that affect the peel force of the PDMS adherend included the degree of cross-linking in PDMS, the thickness of PDMS, peel angle, and peel velocity. Two characteristic peel patterns were observed, which depended on the material specifications and different experimental conditions. The peel mechanism was discussed in terms of the deformation of the adherend.     

Analysis of stresses in adhesive joints applicable to IC chips using symbolic manipulation and the numerical method

In this study, a concentrated force is applied to both adherends bonded by an adhesive under pin–pin boundary conditions. First a mathematical model is derived with governing equations and boundary conditions. These complicated, and analytically problematic, coupled equations are solved numerically using symbolic manipulation and singular value decomposition (SVD). Also discussed are the effects of major factors, including the relative thickness of adherends, joint length and the action point of the concentrated force on the peel and shear stresses in the adhesive layer. This study identifies the conditions under which the upper adherend without breakage can be fully separated from the lower adherend. Particularly, it is found that the thickness of the lower adherend should be greater than ten times that of the adhesive layer but less than one-third that of the upper adherend, the adhesive layer should be relatively thin (h _a ≤ 0.01 mm), and the adhesive joint should be relatively short (thickness to length ratio γ ₁ ≥ 0.08).     

Peel mechanics for an elastic-plastic adherend

The force required to propagate a 180° bend in an elastic-plastic strip has been calculated from elementary bending theory. Measured forces for Mylar strips of various thicknesses, bent to various degrees, were in good agreement with these calculated values. The corresponding additional stripping force in a peeling experiment will depend upon the thickness of the elastic-plastic adherend, becoming zero both for infinitesimally thin adherends and for those exceeding a critical thickness t_c and passing through a maximum value at intermediate thicknesses. Published data are in good agreement with these conclusions. For a strongly adhering strip, higher peel strengths are found for a peel angle of 180°, compared to 90°, and the effect is greater than can be accounted for solely by plastic yielding of the adherend. It is attributed in part to greater energy dissipation within the adhesive layer.     

Analytical Peel Load Prediction as a Function of Adhesive Stress Concentration

Peel data for two epoxy adhesives and a recent model of the adhesive stresses in the peel geometry are used to investigate the effectiveness of two constitutive models and several adhesive failure criteria. The failure criteria are based on either the critical strain energy-release rate or the critical von Mises strain at the peel root, both taken as functions of the “loading zone length” (LZL), defined as a measure of the degree of stress concentration at the root of the peeling adherend. The peel model uses LZL as an independent parameter that captures the effects of the peel angle, adherend thickness, and the mechanical properties of the adhesive and adherend. Both the energy- and strain-based failure criteria can be used to predict the steady-state peel load with an average absolute error of less than 10% over the range of conditions that were examined.     

Strength of adhesive joints with adherend yielding: II. Peel experiments and failure criteria

Peel tests were conducted with an epoxy adhesive on nine rigid-flexible peel configurations: combinations of 1, 2, and 3 mm aluminum adherend thickness and 30°, 60°, and 90° peel angle. The peel model described in an accompanying paper was used to calculate the stress and strain distributions in the adhesive, the strain energy release rates, and the root curvature of the adherend corresponding to steady-state peel failure. Two failure criteria were examined: the critical von Mises strain and the critical fracture energy, G c . The first criterion was found to be essentially independent of the peel angle but dependent on the thickness of the peel adherend. It produced predictions of the peel force that had an average error of 11%. The fracture energy criterion showed that G c depended on the average phase angle of the loading. This criterion was preferred, having an average prediction error of 6% over the nine experimental cases, and requiring fewer free parameters.     

STRENGTH OF ADHESIVE JOINTS WITH ADHEREND YIELDING: II. PEEL EXPERIMENTS AND FAILURE CRITERIA

Peel tests were conducted with an epoxy adhesive on nine rigid-flexible peel configurations: combinations of 1, 2, and 3 mm aluminum adherend thickness and 30°, 60°, and 90° peel angle. The peel model described in an accompanying paper was used to calculate the stress and strain distributions in the adhesive, the strain energy release rates, and the root curvature of the adherend corresponding to steady-state peel failure. Two failure criteria were examined: the critical von Mises strain and the critical fracture energy, G c . The first criterion was found to be essentially independent of the peel angle but dependent on the thickness of the peel adherend. It produced predictions of the peel force that had an average error of 11%. The fracture energy criterion showed that G c depended on the average phase angle of the loading. This criterion was preferred, having an average prediction error of 6% over the nine experimental cases, and requiring fewer free parameters.     

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.     

Stress Distribution in Single Lap Joints with a Cracked Composite Adherend—Part II: Laminated Adherends

The effect of a crack in the overlap region of an adhesive single lap joint is studied on the shear stress distribution in adhesive layer. Each adherend is considered to be a laminated composite material with unidirectional fibers aligned in the direction of the applied load. Crack location is selected to be in the top adherend laminate, in the form of cut fibers and matrix bays. The crack can occur in any layer. The shear-lag model is used to derive the equilibrium equations which are then solved by means of eigenvector expansion. The effects of adhesive thickness, crack size, and location in the adherend, total number of layers in each adherends, volume fraction of fibers, and type of fibers are investigated on the shear distribution in the adhesive as well as load distribution in the intact fiber at the crack tip located in the top adherend. The effect of dissimilar laminated adherends is also investigated on the adhesive shear stress distribution. According to the results, in the presence of a crack, the peak shear stress in the adhesive layer and load concentration in the fibers are very susceptible to the adhesive thickness and number of layers in laminated adherends.     

Effect of Adherend Thickness and Mixed Mode Loading on Debond Growth in Adhesively Bonded Composite Joints

Symmetric and unsymmetric double cantilever beam (DCB) specimens were tested and analyzed to assess the effect of (1) adherend thickness and (2) a predominantly mode I mixed mode loading on cyclic debond growth and static fracture toughness. The specimens were made of unidirectional composite (T300/5208) adherends bonded together with EC3445 structural adhesive. The thickness was 8, 16 or 24 plies. The experimental results indicated that the static fracture toughness increases and the cyclic debond growth rate decreases with increasing adherend thickness. This behavior was related to the length of the plastic zone ahead of the debond tip. For the symmetric DCB specimens, it was further found that displacement control tests resulted in higher debond growth rates than did load control tests. While the symmetric DCB tests always resulted in cohesive failures in the bondline, the unsymmetric DCB tests resulted in the debond growing into the thinner adherend and the damage progressing as delamination in that adherend. This behavior resulted in much lower fracture toughness and damage growth rates than found in the symmetric DCB tests.     

Shear and peel stress analysis of an adhesively bonded scarf joint

The shear and peel stress distributions in a scarf joint made of two isotropic adherends with blunt adherend tips are analysed using a linear elastic analysis. The limits of the analysis with respect to adherend tip thickness have been investigated. A finite difference method is used to solve the differential equations for the shear and peel stress distributions over the joint. The boundary conditions used limit the analysis to the two adherends having the same thicknesses, lengths, and material properties. The adherends are modelled as plates with extensional and bending stiffnesses bonded together with an elastic interlayer. The stresses across the adhesive layer are assumed to be constant. The current analysis applied to cases known from the literature shows good agreement with the shear stresses but the peel stresses are overestimated.     

Theory and Analysis of Peel Adhesion: Adhesive Thickness Effects

The existing assumptions concerning boundary stress concentrations in peel adhesion are extended to treat the effects of adhesive thickness. In adhesive bonds involving the all-angle peeling of a flexible elastic adherend from a rigid substrate the varying of adhesive thickness is shown theoretically to predict a proportional increase of peel force (P) with adhesive interlayer thickness (a) when the product (βCa) of the cleavage stress concentration β, cavitation scale factor C, and adhesive thickness a is less than unity. When the product (βCa) becomes greater than unity the new theory predicts that cleavage stresses concentrate within a fractional layer of the total adhesive thickness f(a) and the peel force P tends to achieve a constant value P_max. This new theory is verified by experimental studies and the experimental analysis suggests new optimizations in the design and measurement of the peel adhesive bond.     

Plasticity in peeling

Contrary to classical theory, a high proportion of bond failures by peeling involve progressive plastic adherend flexural yield. Such yield occurs with adherend thicknesses below a critical value, T_c, which is shown calculable by combining elastic peel mechanics with plastic bending criteria. The geometry of such “peel with yield,” and thence the moment-controlled peel forces, can be accounted for only if the adhesive is also recognized as behaving essentially plastically. Subsequent plastic adherend unbending is important with highly extensible adhesives. The geometry of “legging” peel in such cases is best described by fully plastic mechanics. These are derived and shown to account for literature data on dependencies of peel force upon peel rate and adhesive thickness. “Stick-slip” peel phenomena are indicated to be controlled by recurring interacting plastic–elastic transitions, in both adhesive and adherend: adhesive strain rate is critical in such phenomena. Four regimes of peel behavior can therefore apply as adherend thickness (T) increases, with peel forces proportional respectively to T⁰, T^2/3, T^3/2 (above T_c) and finally controlled by moment limitations due to joint configurational constraints (“cleavage”).     

The behaviour of single lap joints under bending loading

Four-point bend tests were performed on single lap joints with hard steel adherends and a structural epoxy adhesive. The effect of the overlap, the adherend thickness and the adhesive thickness was studied. It was found that the length of the overlap has no significant effect on the strength of the joints. This is because the load transfer is occurring in a very localised area around the edges of the overlap, being the failure governed by peel mechanisms. The thickness of the adherends strongly affects the strength of the joints. The thicker the adherend, the stronger is the joint. The experimental results are compared with a finite element model and reinforce the fact that the failure takes place due to local strains at the ends of the overlap in tension. An analytical model is also given to predict in a simple but effective way the joint strength and its dependence on the adherend thickness.     

Failure load prediction of structural adhesive joints : Part 2: Experimental study

The objective of this study was to determine how the fracture of adhesive joints depends on elastic beam parameters describing the adherends and the applied loads. The basic specimen geometry was the cracked lap shear joint constructed of aluminium alloy with various adherend and bondline thicknesses. Loads were applied in different combinations of bending, tension and shear to generate a failure envelope for each adhesive and specimen geometry. It was found that crack propagation for precracked specimens occured at a critical strain energy release rate but was also a function of the G_I/G_II ratio and the bondline thickness. The experiments also showed that the loads required to propagate a crack in a precracked specimen were always lower than the loads required to break the fillet. Hence, by treating uncracked joints as being cracked, where the fictitious crack tip is assumed to coincide with the location of the fillet, a conservative estimate of the failure load is obtained.     

Shear testing of thick adhesive layers using the ENF-specimen

An existing experimental method to determine cohesive laws for adhesive layers loaded in shear is further developed. The method is based on differentiation of the energy release rate (ERR) with respect to the adhesive shear deformation at the crack tip. The test geometry used is an ENF-specimen for which the adherends are assumed to deform linearly elastic. The original method is expanded to account for situations where the thickness of the adhesive layer is not negligible as compared to the adherend thickness. To this end, a novel mathematical expression for the energy release rate (ERR) is derived. No assumptions on the form of the cohesive law are made; it is implicitly included in the derivation. The expression for the ERR contains the applied load and the shear deformation of the adhesive layer at the initial position of the crack tip, in addition to geometrical properties and the elastic modulus of the adherend material. Numerical simulations are performed to verify the accuracy of the mathematical expression for the ERR. Preliminary results from experiments performed on an epoxy adhesive are presented. The cohesive law of the adhesive layer is extracted by using a blunted crack tip. Verifying simulations confirm that the local pre-fracture behavior is accurately captured.     

Experimental study of the Mode I adhesive fracture energy in DCB specimens bonded with a polyurethane adhesive

The Mode I fracture energy of a polyurethane adhesive with low Young’s modulus was investigated. Metal adherends in standardized double cantilever beam (DCB) tests are typically too stiff for soft adhesives, making it difficult to measure the fracture energy accurately. However, soft adhesives, such as a single-component polyurethane adhesive tested in this paper, are in high demand in the automobile industry. Thus, accurate measurement techniques must be established. Flexible substrates composed of spring steel were used for the DCB tests to accommodate the deformation of the adhesive layer. First, the applicability of the flexible substrates was discussed using specimens bonded with an epoxy adhesive. For soft adhesives, however, the deformation of the adhesive layer must be considered in the calculation methods of the fracture energy. Although the deformation effect on the DCB tests has been discussed with Winkler’s elastic foundation, the crack length must be measured along with the load and displacement. To overcome the difficulty of measuring the crack length, a calculation method based on Winkler’s elastic foundation was introduced applying the compliance-based beam method (CBBM). Finally, the fracture energy of the polyurethane adhesive was discussed by comparing the calculation methods with and without measuring the crack length.     

Influence of metallic substrate surface engineering on peel resistance of adhesively bonded polymer film

The peel resistance of adhesively bonded polymer films to a stainless steel sheet substrate (SSSS) with different engineered surface characteristics was examined in two different loading directions and for two different peel speeds. The SSSS was laminated with two thin polymeric adherends using two different pressure-sensitive adhesives. The SSSS surface was altered by grinding and knurling techniques before lamination and the effects of surface alterations on peel resistance were compared with peel resistance of the adherend from as-received SSSS with a bright annealed surface condition. For ground surface, an increase in adherend peel resistance was observed and the increase was attributed to increase in contact area between the adhesive and SSSS surface. For knurled surfaces which involved deeper and less frequent grooves, however, a decrease in peel resistance was observed. This was attributed to a more complex stress state at the peel front in the SSSS groove region during peeling. An increase in peel speed enhanced the peel resistance from both ground and knurled surfaces.     

Relationship between the fracture energy and the mechanical behaviour of pressure-sensitive adhesives

Mechanical behaviours of two pressure-sensitive adhesives (PSAs) families, composed of elastomer copolymers or polyacrylate/acrylic copolymers, are characterised by peel tests. Fracture energy varies linearly according to the applied contact force between two levels, which depends on tackiness and cohesion of the PSA. Local fracture energies are measured by an original peeling system and they are related with the adhesive deformation. Mechanical behaviours of PSAs depend on their composition but majority of fracture energy is dissipated on the first millimetre near the bending zone where fibrils elongation is maximum. Observations of interfaces between PSAs and glass substrate underline that fracture energy varies linearly according to the contact area.     

Quasi-Static Adhesive Fracture

The concept of quasi-static crack propagation is used in the present paper to study quantitatively the effects of environmental fluids on fracture in adhesive joints. The mechanisms and mechanics of environmental adhesive fracture under rising loads are discussed. Two types of cracking behaviour were observed. (1) When the dissolution or the “surface energy reduction” mechanism prevailed, the fracture toughness of the adhesive joint in the environment was reduced. (2) However, when environment-enhanced crazes were formed in the adherend at the crack tip region, the local fracture toughness of the adhesive joint would be increased. But cracking was usually unstable so that crack velocities were not readily measurable.

Except in so far as the adhesive surfaces may have considerable effects, the fracture toughness of an adhesive joint is independent of the specimen geometries used in the present work. Also, the variation of fracture toughness with crack velocity for an Aluminum/ Araldite joint in a carbon tetrachloride solution is reported.