Peel Experiments Research Articles

Investigation of resistance spot weld failure in tailor hot stamped assemblies

A novel experimental method, dubbed the “Mode I Caiman test”, is introduced to characterize spot weld failure and propagation within spot weld groups. The test configuration comprises a partially welded pair of channel sections subjected to Mode I tensile loading through an opening mode loading applied at the un-welded end of the channels. Experiments were performed considering hot stamped USIBOR® 1500-AS in a fully hardened condition (fully quenched) or in a tailored condition in which the strength of the welded flanged regions was altered using in-die heating (IDH). Both static and dynamic loading conditions are considered. The static loading utilized a conventional tensile frame while the dynamic experiments were performed on an instrumented crash sled at velocities of 7.5 m/s.A high speed thermal imaging camera was utilized to detect the temperature rise associated with plastic work during individual weld failure. This proved to be a rather novel and effective technique to detect weld failure and enabled tracking of weld failure propagation through the weld group for the different parent metal (flange) conditions.The experiments served to demonstrate the complex interaction between weld nugget, heat affected zone (HAZ) and parent metal strengths. For high parent metal strengths (fully quenched, martensitic condition), the welds exhibited interfacial and weld pull-out failure modes with rapid unzipping of the weld group resulting in relatively low energy absorption. In contrast, the lower strength tailored flanges exhibited a more stable (slower) failure propagation through the weld group and considerably higher energy absorption which is largely attributed to greater dissipation through deformation of the flange itself. Numerical simulations of the experiments were performed using spot weld models calibrated from single weld lap shear, cross tension, and coach peel experiments. The models captured the peak loads rather well, but predictions of rate of failure propagation within the weld line exceeded that seen in the experiments, pointing to the need for improved post-failure models for spot weld simulation.In automotive structures in which numerous welds are present, the amount of energy absorbed in weld failure can significantly affect the impact response of the structure. The Mode I Caiman test presents a new opportunity to calibrate the energy release rate of spot weld damage models to improve their accuracy when modelling spot welded connections involving multiple spot welds.

Peel behavior of polyurethane/Monel 400 interface under load in saline water

ABSTRACT This study investigates the coupling effects of saline water and cable weight on the adhesive energy of polyurethane/Monel 400 interface using a dead weight peel test. Cables play a crucial role in naval structures serving as a central nervous system. The cables and their connectors are generally molded, encapsulated, or potted with polyurethanes to ensure that they are insulated and waterproof. During service, exposure of some of these cables and their connectors to the marine environment results in the premature failure of the polyurethane/metal interfaces of connectors. Three sets of experiments were carried out to better understand this phenomenon: an initial 90° fixed arm peel test to determine the strength of the interface, a hanging weight peel test in saline water at an elevated temperature to determine the effects of weight variation and saline water degradation, and a dead weight peel test at varying saline water temperatures to determine the acceleration factor. According to the findings, no peel was recorded in the peel experiments conducted in air indicating that the strength of the interface is higher than that of the peel arm. In the dead weight peel tests, increasing the masses of the hanging weights increased the peel velocity. Additionally, for low peel velocities, a linear relationship was observed between adhesive energy and peel velocity. Furthermore, a more cohesive primer failure was observed at the end of the peel process, indicating that peel velocity increased with longer exposure to saline water.

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.

Development of miniaturized peel experiments to quantify interfacial adhesion between 2D monolayers and viscoelastic polymers

ABSTRACT In this work, a miniaturized peel-testing setup capable of performing peel tests for viscoelastic polymer strips attached to rigid substrate at different peel rates (20 nm/s–20 µm/s) and at constant peel angles (45°–135°) was developed. A mathematical framework accounting for competition between the delamination from rigid substrate as well as bending and extension of the peel arm during the peel test was employed for modeling the viscoelastic response of the soft polymeric strips attached to a rigid substrate. The efficacy of the novel miniaturized peel-testing setup was evaluated for linear elastic and viscoelastic peel arms attached to rigid substrates. The adhesion measurements for the linear elastic peel arms were found to be insensitive to peel angles and the values were in great agreement with standardized macroscale adhesion measurements for the same interface. The peel tests revealed that the interfacial adhesion measurements for the viscoelastic PDMS peel arms were highly sensitive to the nominal peel rates. An elaborate specimen fabrication process was developed to evaluate the interfacial adhesion between large area 2D materials and soft polymer substrates. A series of peel-testing experiments were performed to ensure that the developed fabrication process does not adverse effects on the interfacial adhesion measurements. The peel-testing methodology was then employed to quantify the interfacial adhesion between graphene monolayers and PDMS substrates. Due to the weak van der Waals interactions associated with the graphene/PDMS interface, the interfacial adhesion measurements for this interface was significantly lower than the adhesion values for a PDMS/PDMS interface.

Highly Efficient Rubber-to-Stainless Steel Bonding by Nanometer-Thin Cross-linked Polymer Brushes.

Stainless steel (SS) surfaces were grafted with poly(glycidyl methacrylate) (PGMA) brushes that were post-modified using allylamine, diallylamine, and propylamine as reagents. Likewise, poly[2-(diethylamino)ethyl methacrylate] brushes were synthesized. All samples were compression molded with uncured ethylene-propylene-diene M-class rubber and dicumyl peroxide and vulcanized for 12 min at 170 °C. The efficiency of the novel bonding solution was evaluated through peel experiments. Two parameters, the fracture toughness () and the cohesive-to-adhesive fracture ratio (Ar), were calculated to evaluate the strength and the performance of the coupling, respectively. For the nanometer-thin PGMA films modified with allylamine, in particular, full cohesive fracture was obtained. The obtained values of (15.4 ± 1.1 N mm–1) and Ar (1.00 ± 0.01) matched those obtained for a micrometer-thick commercial bonding agent. Cross-linking of polymer brushes by intermolecular reactions by the primary amines proved to have a significant impact on the type of fracture (cohesive/adhesive) and the performance of the adhesives.

The Mechanical Theory behind the Peel Test

The peel test is a very simple and fast method to determine the adhesion of interconnector ribbons to solar cell metallizations. It is part of the solar cell standard DIN EN 50461 and is, due to its ease of use, widely accepted to qualify cell metallizations and the soldering process. In the standard a force of 1N per mm of joint width is specified but other relevant quantities are missing, for example the peeling angle. We show that this lack of specification enables the manipulation of peel testing results. We therefore apply the mechanical theory of Kinloch [1] where measured peel forces are translated into adhesive fracture energies GA. The fracture energy is a geometry-independent parameter that describes the energy to break the interfacial bondings at the peel front. It incorporates the dimensions of the ribbon and its stress-strain-curve. We perform 86 peel experiments at 90°, 135° and 180° of ribbons on continuous front side busbars of cells from one stringing batch. While the median forces for 90°(3.07N), 135°(2.35N) and 180°(3.39N) differ by up to 30.4% we find the median adhesive fracture energies to deviate by only 17.4%. Using the same adhesive fracture energy (260J/m2) for a 45° peel test we expect peel forces of 7.45N which is factor 2.4 (2.2) higher than the 90° (180°) peel forces.

The influence of bond line thickness and peel arm thickness on adhesive fracture toughness of rubber toughened epoxy–aluminium alloy laminates

The adhesive fracture toughness ( G A) of rubber toughened epoxy–aluminium alloy laminates is studied using two types of peel test, namely fixed arm peel and roller-assisted mandrel peel. In the fixed arm tests, the peel angle is varied in the range 45–135° and in the mandrel peel experiments, the mandrel roller size is selected in order to ensure conformance of the peel arm to the roller during fracture. The principal aims of the study are two-fold: (i) To investigate the influence of adhesive bond line thickness on G A. (ii) To investigate the influence of peel arm thickness on G A. The peel crack growth experiments were complemented with measurements of cohesive fracture toughness using tapered double cantilever beam geometry with a range of values for bond line thickness. Throughout the work, it is important to identify the mode of peel fracture and in particular to establish whether the peel cracks are cohesive (propagating in the adhesive) or interfacial (propagating at the interface between adhesive and substrate). To this effect, confocal laser scanning microscopy was used in order to study the peel arms after testing and to measure the thickness of adhesive coating on the peel arms. The experimental work was conducted on a wide range of laminates where two adhesives with a common substrate were employed.

The ATP Binding Cassette Transporter AtMRP5 Modulates Anion and Calcium Channel Activities in Arabidopsis Guard Cells

Stomatal guard cells control CO(2) uptake and water loss between plants and the atmosphere. Stomatal closure in response to the drought stress hormone, abscisic acid (ABA), results from anion and K(+) release from guard cells. Previous studies have shown that cytosolic Ca(2+) elevation and ABA activate S-type anion channels in the plasma membrane of guard cells, leading to stomatal closure. However, membrane-bound regulators of abscisic acid signaling and guard cell anion channels remain unknown. Here we show that the ATP binding cassette (ABC) protein AtMRP5 is localized to the plasma membrane. Mutation in the AtMRP5 ABC protein impairs abscisic acid and cytosolic Ca(2+) activation of slow (S-type) anion channels in the plasma membrane of guard cells. Interestingly, atmrp5 insertion mutant guard cells also show impairment in abscisic acid activation of Ca(2+)-permeable channel currents in the plasma membrane of guard cells. These data provide evidence that the AtMRP5 ABC transporter is a central regulator of guard cell ion channel during abscisic acid and Ca(2+) signal transduction in guard cells.

Effect of Surface Segmental Mobility on Adhesion of Acrylic Soft Adhesives

The dependence of adhesion on the surface segmental dynamics of polymer substrates in thin-film poly(methyl acrylate) (PMA)− and poly(methyl methacrylate) (PMMA)−adhesive tape systems was probed via tape peel tests. From 90° angle tape peel experiments at 28 °C of very thin films of PMA adsorbed on glass slides, it was found that the peel velocities varied linearly with applied force and the force−velocity curves were dependent on film thickness. Extrapolation of the peel−velocity curves to zero velocity yielded an estimate of the fracture energies, which varied from about 50 to 20 J/m2 for thicknesses from 50 to 2000 nm. The fracture energies were also found to depend exponentially on film thickness with a decay length of 110 nm. In contrast to the behavior of PMA, PMMA films showed larger fracture energies but no thickness dependence. The differences in the behavior of the two types of films are believed to be due to differences in segmental mobilities in the films which give rise to differences in the motions of the “anchoring sites” across the interface. For PMMA, the glassy polymer was rigid at the interface, regardless of the thickness, and hence, no thickness dependence was observed. For PMA, a rubbery polymer at room temperature, the segments at the adhesive interface were influenced by the restrictions posed by interactions of the polymer with the glass surface. A model based on differing thicknesses of the polymer interphases is proposed to account for the behavior observed.

Fingerprinting the processing behavior of polyethylenes from transient extensional flow and peel experiments in the melt state

The flow curves of linear (linear-low and high density) and branched polyethylenes are known to differ significantly. At increasing shear rates, the linear polymers exhibit a surface melt fracture or sharkskin region that is followed by an unstable oscillating or stick-slip flow regime when a constant piston speed capillary rheometer is used. At even higher shear rates, gross melt fracture appears. Unlike their linear counterparts, branched polyethylenes rarely exhibit sharkskin melt fracture and although gross melt fracture appears at high shear rates there is no discontinuity in their flow curve. The various flow regimes of these two types of polyethylenes are examined by performing experiments in the melt state using a unique extensional rheometer (the SER by Xpansion Instruments) that is capable of performing accurate extensional flow and peel experiments at very high rates not previously realized. The peel strength curves of these linear and branched polyethylenes exhibit all of the distinct flow regimes exhibited in their respective flow curves, thereby providing a fingerprint of their melt flow behavior. Moreover, these extensional flow and peel results in the melt state provide insight into the origins and mechanisms by which these melt flow phenomena may occur with regard to rapid tensile stress growth, melt rupture, and adhesive failure at the polymer wall interface.

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.

Adhesion of latex films. Part IV. Dominating interfacial effect of the surfactant

Our aim was to study the effect of various surfactants on the adhesion properties of latex films. For this purpose, a model monodisperse, surfactant-free, core-shell latex was synthesized. The core was a random copolymer of styrene and butyl acrylate. The shell consisted of a styrene/butyl acrylate/methacrylic acid terpolymer with 10 wt% of methacrylic acid. Sodium dodecyl sulfate (SDS) and various ethoxylated nonionic surfactants were post-added to the latex at increasing concentrations. The adhesion of the corresponding films was measured by peeling at 180°. The shapes of the curves relating the peel energy to the surfactant concentration were similar, irrespective of the nature of the surfactant, namely a decrease to a constant plateau level. This means that in these systems, the influence of a weak boundary layer at the interface dominates all other possible effects of the surfactant. This was confirmed by surface analysis and peel experiments with surfactant-free films contacted with spin-coated SDS layers of increasing thickness. The reasons why the deleterious interfacial effect of the surfactant overwhelms all others are discussed.

Involvement of Cyclic AMP in ABA- and Ca2--Mediated Signal Transduction of Stomatal Regulation in Vicia faba

The focus of this study is to investigate possible involvement of cyclic AMP in regulation of Vicia stomatal movements. The presence of 0.1 mM 8-Br-cAMP, a membrane-permeable analogue of cAMP, alone in the incubation medium did not affect stomatal opening in the light in leaf epidermal peel experiments. However, addition of 0.1 mM 8-Br-cAMP completely reversed exogenous ABA- and Ca2+-induced inhibition of stomatal opening. Consistent with these results, patch-clamping experiments showed that intracellular addition of 0.5 mM or 1 mM cAMP significantly reversed the inhibition of whole-cell inward K+ currents by internally supplied 13 μM Ca2+ or 10 μM ABA in stomatal guard cell protoplasts, respectively. Furthermore, intracellular addition of either 10 μM prostaglandin E1 (PGE1, an adenylate cyclase activator) or 1 mM 3-isobutyl-1-methylxanthine (IBMX, a phosphodiesterase inhibitor) mimicked the effect of exogenous cAMP on the removal of ABA- or Ca2+ inhibition of inward K+-current. These results suggest that a cAMP signaling pathway is involved in signal transduction in stomatal regulation by interacting with ABA and Ca2+ signaling cascades. A hypothetical mechanism by which cAMP may regulate K+ in stomatal guard cells is also discussed.

Stomatal responses ofVicia fabaL. to indole acetic acid and abscisic acid

Evidence is presented that stomata in isolated epidermal peels of Vicia faba L. open in darkness in response to the external presence of indole acetic acid (IAA) in the incubation medium. The effect of IAA is found to be overcome completely in the presence of either TRIS or MES buffers. In the absence of buffer, V. faba stomata are shown to be influenced by IAA in a concentration-dependent trend which reached a maximum at an [IAA] of 10 −3 mol m −3 . Further investigations reveal that stomata in this species can be shown to respond to the presence of IAA and another phytohormone, abscisic acid (ABA). IAA and ABA are demonstrated to be antagonistic in their effects provided the incubation conditions are suitable. The data are discussed in relation to stomatal responses of other species in different treatment conditions. Recommendations are made with respect to standardization of incubation media during epidermal peel experiments

Stomatal responses to light in the facultative Crassulacean acid metabolism species, Portulacaria afra

Guard cell responses to light are mediated by guard cell chlorophyll and by a specific blue light photoreceptor. Gas exchange and epidermal peel techniques were employed to investigate these responses in the facultative Crassulacean acid metabolism (CAM) species, Portulacaria afra (L.) Jacq. In P. afra individuals performing C3 metabolism, red light stimulated an increase in leaf conductance in intact leaves and stomatal opening in isolated epidermal peels, indicating the presence in guard cells of the chlorophyll‐mediated response to light. Under a background of continuous red illumination, conductance exhibited transient increases following pulses of blue but not red light, indicating that the specific stomatal response to blue light was also operative. In contrast, in CAM individuals, conductance in gas exchange experiments and stomatal opening in epidermal peel experiments were not stimulated by red light. In CAM plants, conductance did not increase following blue light pulses administered over a range of temperatures, vapor pressure differences (VPD), ambient CO2 concentrations and background red light intensities. These results indicate that P. afra does possess typical guard cell responses to light when performing C3 metabolism. The metabolic pathways mediating these responses are either lost or inhibited when CAM is induced.

Comparison of Peel and Lap Shear Bond Strengths for Elastic Joints With and Without Residual Stresses

Abstract Debonding energies have been calculated from peel and lap shear experiments on rubber strips bonded together with a pressure-sensitive acrylic adhesive layer. In some cases, one strip was held stretched during bonding, to create joints with built-in stresses. Good agreement was obtained in all cases, provided that elastic strain energy was taken into account, the work of debonding being about 180 J/m2. For thick rubber strips, about 3–4 mm or greater, the strain induced by peel or shear forces was rather small and the assumption of linear elastic behavior was found to be satisfactory. Good agreement was then obtained with the relations derived by Kendall.1,2

Adhesion Promotion by Silanes: A Study of their Interfacial Chemistry in a Model Polystyrene Coating by XPS and SIMS

Abstract Adhesion of a polystyrene coating to solvent cleaned steel is increased two-fold by addition of 0.5% wt/wt of aminosilane (A1120). A study has been carried out on the coating-substrate interfacial chemistry to gain an understanding of the mechanism of adhesion promotion. It is shown that in peel experiments the coating fails adhesively between the polystyrene and an adsorbed layer of aminosilane on the steel surface. The improvement in adhesion results from displacement by the silane of the 1.4 nm thick layer of residual carbonaceous contamination on the steel surface. It is proposed that this leads to a stronger substrate-coating interaction either through improved intermolecular contact between the segregated silane and the polymer or through secondary bonding between the amine groups of the silane and the polarisable aromatic rings of the polystyrene.

Morphology and rheology of peel of model adhesives

AbstractThe morphology of surfaces produced during peel experiments was studied to increase the understanding of peel phenomena. Thin films of polystyrenes of narrow molecular‐weight distribution and a polydisperse polyester on flexible supports were peeled from rigid substrates at temperatures above their glass transitions with equipment that also permitted quenching and preservation of the peeled surfaces which then could be studied by scanning electron microscopy. The results show many types of cavitation including foams and the coalescence of cavities to form fibers. Cavitation occurred throughout and beyond the region of transition from cohesive to interfacial failure where peel force decreased abruptly; however, cavity growth was abruptly restricted as the failure mode changed. The master curves and shift factors obtained in this work provided strong evidence that the quenching used to preserve the peeled surfaces did not introduce large perturbations in the results. The reason was that failure occurred at or close to the heated substrate.