Steady-state Peeling Research Articles

Effect of Temperature/Humidity Treatment Conditions on Interfacial Adhesion Energy between Inkjet-Printed Ag and Polyimide

The effect of temperature/humidity treatment conditions on the interfacial adhesion energy of inkjet-printed Ag/polyimide systems was investigated for metallization of inkjet printing techniques in flexible printed circuit board applications. The interfacial adhesion energy was determined from 180° peel tests by calculating the plastic deformation energy of peeled metal and polyimide films from the energy balance relationship during the steady-state peeling process. After 76 h of temperature/humidity treatment, there is an increase in interfacial adhesion energy. This is a chemical bonding effect because there exists good quantitative correlations between the interfacial bonding energy and the Ag* peak area fraction from X-ray photoemission spectroscopy (XPS) data of peeled metal surfaces. After 196 and 388 h of temperature/humidity treatment, there is a decrease in interfacial adhesion energy, which seems to degrade polyimide by hydrolysis reaction with water molecules.

Peeling Single-Stranded DNA from Graphite Surface to Determine Oligonucleotide Binding Energy by Force Spectroscopy

We measured the force required to peel single-stranded DNA molecules from single-crystal graphite using chemical force microscopy. Force traces during retraction of a tip chemically modified with oligonucleotides displayed characteristic plateaus with abrupt force jumps, which we interpreted as a steady state peeling process punctuated by complete detachment of one or more molecules. We were able to differentiate between bases in pyrimidine homopolymers; peeling forces were 85.3 - 4.7 pN for polythymine and 60.8 +/- 5.5 pN for polycytosine, substantially independent of salt concentration and the rate of detachment. We developed a model for peeling a freely jointed chain from the graphite surface and estimated the average binding energy per monomer to be 11.5 +/- 0.6 k(B)T and 8.3 +/- 0.7 k(B)T in the cases of thymine and cytosine nucleotides, respectively. The equilibrium free-energy profile simulated using molecular dynamics had a potential well of 18.9 k(B)T for thymidine, showing that nonelectrostatic interactions dominate the binding. The discrepancy between the experiment and theory indicates that not all bases are adsorbed on the surface or that there is a population of conformations in which they adsorb. Force spectroscopy using oligonucleotides covalently linked to AFM tips provides a flexible and unambiguous means to quantify the strength of interactions between DNA and a number of substrates, potentially including nanomaterials such as carbon nanotubes.

Effect of Ar+ Radiofrequency Plasma Treatment Conditions on the Interfacial Adhesion Energy Between Atomic-Layer-Deposited Al2O3 and Cu Thin Films in Embedded Capacitors

The effects of Ar+ radiofrequency (RF) plasma pretreatment conditions on the interfacial adhesion energy of a Cu/Cr/Al2O3 system were investigated for thin-film capacitors in embedded printed circuit board applications. The interfacial adhesion energy was evaluated from 90 deg peel tests by calculating the plastic deformation energy of peeled metal films from the energy balance relationship during the steady-state peeling process. The interfacial adhesion energy was fivefold higher after RF plasma pretreatment of the surface of 50-nm-thick Al2O3 prepared by atomic layer deposition. Atomic force microscopy, Auger electron spectroscopy, and x-ray photoemission spectroscopy results clearly reveal that this increase can be attributed to both mechanical interlocking and chemical bonding effects.

Determination of interfacial mechanical parameters for an Al/Epoxy/Al 2o 3 system by using peel test simulations

Peel test measurements and simulations of the interfacial mechanical parameters for the Al/Epoxy/Al2O3 system are performed in the present investigation. A series of Al film thicknesses between 20 and 250 microns and three peel angles of 90, 135 and 180 degrees are considered. Two types of epoxy adhesives are adopted to obtain both strong and weak interface adhesions. A finite element model with cohesive zone elements is used to identify the interfacial parameters and simulate the peel test process. By simulating and recording normal stress near the crack tip, the separation strength is obtained. Furthermore, the cohesive energy is identified by comparing the simulated steady-state peel force and the experimental result. It is found from the research that both the cohesive energy and the separation strength can be taken as the intrinsic interfacial parameters which are dependent on the thickness of the adhesive layer and independent of the film thickness and peel angle.

Peeling experiments of ductile thin films along ceramic substrates – Critical assessment of analytical models

Two types of peeling experiments are performed in the present research. One is for the Al film/Al 2O 3 substrate system with an adhesive layer between the film and the substrate. The other one is for the Cu film/Al 2O 3 substrate system without adhesive layer between the film and the substrate, and the Cu films are electroplated onto the Al 2O 3 substrates. For the case with adhesive layer, two kinds of adhesives are selected, which are all the mixtures of epoxy and polyimide with mass ratios 1:1.5 and 1:1, respectively. The relationships between energy release rate, the film thickness and the adhesive layer thickness are measured during the steady-state peeling process. The effects of the adhesive layer on the energy release rate are analyzed. Using the experimental results, several analytical criteria for the steady-state peeling based on the bending model and on the two-dimensional finite element analysis model are critically assessed. Through assessment of analytical models, we find that the cohesive zone criterion based on the beam bend model is suitable for a weak interface strength case and it describes a macroscale fracture process zone case, while the two-dimensional finite element model is effective to both the strong interface and weak interface, and it describes a small-scale fracture process zone case.

A numerical analysis of the elastic-plastic peel test

The adhesive fracture energy, G c, is determined from two types of elastic-plastic peel tests (i.e. the single-arm 90° and T-peel methods) and a linear-elastic fracture-mechanics (LEFM) test method (i.e. the tapered double-cantilever beam, TDCB method). A rubber-toughened epoxy adhesive, with both aluminium-alloy and steel substrates, has been used in the present work to manufacture the bonded joints. The peel tests are then modelled using numerical methods. The overall approach to modelling the elastic-plastic peel tests is to employ a finite-element analysis (FEA) approach and to model the crack advance through the adhesive layer via a node-release technique, based upon attaining a critical plastic strain in the element immediately ahead of the crack tip. It is shown that this ‘critical plastic strain fracture model (CPSFM)’ results in predicted values of the steady-state peel loads which are in excellent agreement with the experimentally-measured values. Also, the resulting values of G c, as determined using the FEA CPSFM approach, have been found to be in excellent agreement with values from previously-reported analytical and direct-measurement methods. Further, it has been found that the calculated values of G c are independent of whether a standard LEFM test or an elastic-plastic peel test method is employed. Therefore, it has been demonstrated that the value of the adhesive fracture energy, G c, is independent of the geometric parameters studied and the value of G c is indeed a characteristic of the joint, in this case for cohesive fracture through the adhesive layer. Finally, it is noted that the FEA CPSFM approach promises considerable potential for the analysis of peel tests which involve very extensive plastic deformation of the peeling arm and for analysing, and predicting, the performance of more complex adhesively-bonded geometries which involve extensive plastic deformation of the substrates.

Analytical Peel Load Prediction as a Function of Adhesive Stress Concentration

ABSTRACT 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.

A calibrated finite element model of adhesive peeling

A finite element model of the flexible-rigid peel test was compared with experimental steady-state peel forces for nine configurations (three adherend thicknesses and three angles) in order to investigate the von Mises critical strain as a failure criterion. It was found that the critical von Mises strain was approximately constant for a given adherend thickness at the three peel angles, but that it increased significantly as the adherend thickness increased. Modification of the functional dependence of adhesive yield stress on hydrostatic stress did not correct for this thickness dependence. The steady-state peel geometry and loads were determined by executing the model incrementally through small load steps beginning with the initial undeformed shape of the adherend. This simulation of the transient stage of the peel test illustrated the large differences between the peel force and geometry at the onset of adhesive failure and at steady state.

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.

Relationship Between the External Force and the Specific Work of Fracture R for Steady-State Tearing and Peeling of Elastoplastic Materials

A simple relationship is obtained between the external force F and the fracture toughness R for thin sheets in steady state elastoplastic combined tearing and peeling along self-similar paths. The relationship depends only on the material properties (E, σy, and α for an elastoplastic material with linear hardening) and strip cross section (B and H). An earlier analysis (which incorporates transient tearing and peeling) requires lengthy computations over the whole length of the strip. The present analysis avoids that complication. Experiments in steady-state agree with the theory.

Analysis of the Symmetrical 90°-peel Test with Extensive Plastic Deformation

A numerical 2-D study of the symmetrical 90°-peel test (a similar geometry to the T-peel test) in which extensive plastic deformation occurs in the adherends is presented in this paper. A traction-separation relation is used to simulate failure of the interface, and the conditions for both crack initiation and steady-state crack growth are investigated. The numerical predictions for the steady-state peel force are compared with those based on elementary beam theory. It is shown that two competing effects dominate the mechanics of the peel test to such an extent that the results of beam-bending analyses cannot be used to predict the peel force. At one extreme range of parameters, delamination is driven by shear rather than by bending, resulting in a lower peel force than would be predicted by beam-bending analyses. At the other extreme, where delamination is bending-dominated, the constraint induced by the interfacial tractions cause an increase in the peel force. The numerical results are compared with the results of experiments in which adhesively-bonded specimens are tested in the symmetrical 90°-peel configuration. Excellent agreement between the numerical and experimental results validates the numerical approach.

Adhesion induced by mobile cross-bridges: Steady state peeling of conjugated cell pairs

Recent experiments on cell-cell adhesion indicate that (i) bonds between two cells migrate towards the region of conjugation during peeling, and (ii) cell membrane tension increases with the rate of peeling. Analytical methods that would allow the derivation of intrinsic parameters of adhesion from such experimental data have yet to be explored. In this study we introduce a specific adhesion model to investigate the complex interaction between rate of peeling, lateral diffusivity and the rate of detachment of bonds. Our analysis show that adhesive energy density increases with the rate of peeling and with lateral diffusivity of the bonds but decreases with increasing rate of detachment. The rate dependency of the adhesive energy is due to the modulation of the number density distribution of bonds by the speed of peeling.

Parameters of failure of adhesive joints in symmetric peeling

1. A scheme of symmetric peeling of a coating from a substrate was proposed and examined in systems with a relatively weak adhesive reaction; it permits characterizing the failure of AJ with two values: the peeling force and angle, determined in simultaneous separation of two symmetric strips of coating from a flat substrate with force normal to the plane of the adhesive contact. 2. Important properties of the proposed scheme were established: preservation of the invariability of the direction of the effect of the total peeling force with respect to the plane of the adhesive contact during the entire process of failure of the AJ, despite the change in the slope of the peeled coating strips and movement of the boundaries of the peeling front, and the mutual compensation of the shear components of the peeling forces, which load the adhesive joint through the peeled coating strips, resulting in improvement of the metrological indexes. 3. It was shown that the process of failure of AJ in symmetric peeling of a coating from a substrate is characterized by the presence of a steady-state period with constant values of the peeling force and angle, which are parameters bearing information on the cohesive properties of the coating and its adhesion to the substrate for a given pair of coating and substrate materials. It was hypothesized that the value of the steady-state peeling angle is correlated with the ratio of the indexes of these properties.

Elastoplastic analysis of the peel test

A theoretical analysis of the peeling of a thin elastoplastic film bonded on an elastic substrate is presented in this paper. The moment-curvature relation for pure bending of an elastoplastic beam under conditions of plane strain is derived and slender beam theory is used to analyze the deformation of the adherend. Large deformation finite element analysis is used to study in detail the stress and deformation fields in the area near the tip of the interfacial crack. An analysis of steady slate peeling and a method for the calculation of the work expenditure during steady state peeling are presented. An energy balance is used to relate an experimentally measured peel force to the specific fracture energy.

Mechanics of Film Adhesion: Elastic and Elastic-Plastic Behavior

A peel test is a useful method for comparing the behavior of various adherends and adhesives. An exact analysis of the mechanics of the peel test would be of great help in the interpretation of test results in terms of the bulk properties of the materials, and of the failure mechanism of the bond. The existing theories of peeling apply to elastic peel films, very thin elastic or viscoelastic adhesive, and a rigid substrate. In many applications the film is metallic, stressed beyond its elastic range; the elasticity of the substrate is often similar to that of the adhesive; and the adhesive may be quite thick compared to the film, or may be wholly absent as in electroplated components. In this paper, the effects of non-elastic behavior of the film are analyzed. Results from the use of computer programs that incorporate an analytical model of steady state peeling are presented and compared with experimental data.

Peel adhesion. I. Some phenomenological aspects of the test

Cellophane sheets were coated with acrylic latex polymers, pressed together in pairs at elevated temperature, and peeled apart at constant relative humidity and temperature. The effect of the rate of peeling and the adhesive layer thickness upon the peeling force was investigated. When a laminate is peeled, the steady-state peeling force is generally not constant, but the frequency distribution of the instantaneously measured steady state force values is Gaussian. The mean value of the force is well defined and is equal to the median force. This type of variability is probably due to sample nonuniformity. The second type of variability in the steady-state peeling force is inherent in the binder/substrate system. Here failure in the glue line is not initiated continuously but periodically, and the failure propagates faster than the rate of peeling. Consequently the steady-state peeling force passes through well-defined maxima and minima. It was also observed that the distance between two adjacent maxima or minima was constant and insensitive to the testing rate. When the force is randomly oscillating, the mean force generally either increases with the rate of testing or is insensitive to it. It always increases with the thickness of the adhesive layer. A special case was extensively investigated where at low rates there was cohesive failure and the peeling force increased with the testing rate, while at high rates the force was rate independent and produced adhesive failure. The cohesive failure force could be represented by a single master curve when the force was plotted against Rtan, where R is the peeling rate, ta is the thickness of the adhesive layer, and the exponent n is a constant characteristic of the system. When the binder/substrate system gives a steady-state force with well-defined periodicity, the maxima and minima in the force generally decrease with increasing testing rate. The periodicity of the steady-state force can be eliminated by having a rough instead of smooth binder/substrate interface.