Climbing Drum Peel Research Articles

Experimental study of partially-cured Z-pins reinforced foam core composites: K-Cor sandwich structures

This paper presents an experimental study of a novel K-Cor sandwich structure reinforced with partially-cured Z-pins. The influence of pultrusion processing parameters on Z-pins characteristics was studied and the effect of Z-pins on mechanical properties was disclosed. Differential scanning calorimetry (DSC) and optical microscopy (OM) methods were employed to determine the curing degree of as-prepared Z-pins and observe the implanted Z-pins in the K-Cor structure. These partially-cured Z-pins were treated with a stronger bonding link between face sheets and the foam core by means of a hot-press process, thereby decreasing burrs and cracking defects when the Z-pins were implanted into the Rohacell foam core. The results of the out-of-plane tensile tests and the climbing drum peel (CDP) tests showed that K-Cor structures exhibited superior mechanical performance as compared to X-Cor and blank foam core. The observed results of failure modes revealed that an effective bonding link between the foam core and face sheets that was provided from partially-cured Z-pins contributed to the enhanced mechanical performances of K-Cor sandwich structures.

Investigation of bond strength and energy absorption capabilities in recyclable sandwich panels

The paper investigates the energy absorption characteristics of honey comb sandwich panels manufactured from recyclable materials. In this study, honeycomb cores were manufactured from sawdust-polypropylene composites. The composites were thermoformed between matched-dies into half hexagonal profiles which were ultrasonically bonded at their nodes and facesheets to form recyclable sandwich panels. As the bonding efficiency at the nodes and the core-to-facing interface will be governed by several process parameters, an L8 partial factorial and full factorial design of experiments (DoE) based on the Taguchi method was used. Single lap shear test was used to determine the shear strength of the bond at the nodes, and climbing drum peel test was used to determine the skin-to-core bonding strength in the sandwich panels. The energy absorption characteristic of the honeycombs was determined experimentally by subjecting them to quasi-static compressive loads. The effect of the thickness-to-length ratio (t/l) of the cell walls on the energy absorption was evaluated using finite element (FE) modelling, and energy absorption maps were constructed for various t/l ratios. Taguchi analysis suggested setting all the parameters at their highest levels to obtain maximum lap shear strength at the nodes and indicated that the hold time has negligible influence, following which a L8 full factorial DoE based on orthogonal arrays was used to maximise the peel load, neglecting the hold time parameter. The results indicated a minor interaction between the welding amplitude and welding time, indicating the increase in welding amplitude should be complimented with the welding time. However, this interaction was marginal compared to the main factor effects. All the panels at different levels failed at the core-to-face interface but the highest peel load values were exhibited by specimens that were welded with amplitude of 22.5μm, a welding distance of 50% of the facing thickness from the welding interface and a welding time of 1s. Though the panels exhibited approximately 30% increase in shear strength values compared to the unreinforced cores, their specific shear strength was similar to those of the unreinforced ones (8×103Nm/kg). However, these were achieved at much lower cell densities or t/l values (0.07). The energy absorption diagrams revealed an increasing trend in energy absorption per unit volume with increasing t/l ratio of the honeycomb.

Bondline strength evaluation of cocure/precured honeycomb sandwich structures under aircraft hygro and repair environments

Effects of moisture accumulated into the adhesively bonded composite structures on their bondline mechanical strengths are investigated through a series of comparative experiments. Those composite structures include a honeycomb sandwich structures fabricated by the cocure and the precure processes. Mass of moisture accumulated into the closed cells of the honeycomb sandwich panel specimens has been calculated. A pressure due to the vapor expansion in each cell of the sandwich panel has also been obtained for the minimum repair pressure to be applied to the laminate patched area by vapor pressure–temperature relations from the thermodynamic steam table, the ideal gas state equation and two vapor pressure equations. The bondline strengths of the laminated skins on the flat surface of honeycomb have been compared by the flatwise tension test and climbing drum peel test for dry, wet and repair-after-wet specimens, respectively.

Using the Climbing Drum Peel (CDP) Test to Obtain a GIC Value for Core/Face Sheet Bonds

A method of measuring the mode I (peeling) fracture toughness of core/face sheet bonds in sandwich structures is desired, particularly with the widespread use of models that need this data as input. This study examined if a mode I critical strain energy release rate, GIC, can be obtained from the climbing drum peel (CDP) test. The CDP test is relatively simple to perform and does not rely on measuring small crack lengths such as required by the more commonly used double cantilever beam (DCB) test. Simple energy methods were used to calculate GIC from CDP test data on composite face sheets bonded to a honeycomb core. Face sheet thicknesses from 2 to 5 plies (0.51—1.27 mm) were tested to examine the upper and lower bounds on face sheet thickness requirements. Suggestions on conducting the test and on modifying the CDP apparatus to test composite face sheets (as opposed to metallic) are also presented. Results from the study suggest that the CDP test, with certain provisions, can be used to find the GIC value of a core/face sheet bond.

Using heterogeneous silane patterns to maintain adhesion and decrease water penetration into epoxy/aluminum interfaces

Silanes are used for surface modification to improve dispersion, bind biomaterials, improve adhesion, etc. Their use in improving adhesion in composite materials is widespread, and for these systems there is a growing need for both increased adhesion performance and resistance to water penetration across polymer/oxide interfaces. The present work reports adhesion results obtained using patterned, binary combinations of adhesion-promoting and non-adhesion-promoting silanes patterned onto an oxide surface. The effects of pattern shape, texture (feature size) and the fractional coverage of the adhesion promoter are explored for the bonding of epoxy matrices to aluminum oxide surfaces using combinations of γ-glycidoxypropyltrimethoxysilane (GPS), an adhesion promoter, and either octadecyltrichlorosilane (ODTS) as a hydrophobic non-adhesion promoter, or vinyltrimethoxysilane (VMS) as a water scavenger. In addition to dry tests, samples are submerged in boiling water for 48 h, and in 50°C water for 4 h to determine if water penetrated into the interface, thereby reducing the adhesion. Climbing drum peel tests reveal that heterogeneity of silane primer substrates can influence the adhesion and increase the durability to water penetration. Adhesion enhancement is attributed to the blocking of lateral diffusion of water by barriers due to the presence of the hydrophobic silane. Results show that due to exposure to water at 50°C for 4 h, the adhesion is reduced by 46% in the homogeneous samples but only by 20% in the heterogeneous samples. In contrast, the use of VMS was ineffective in preventing the loss of adhesion due to water penetration.

Enhancement of adhesion to heterogeneously patterned substrates

Organofunctional silanes have long been used as promoters of adhesion between polymers and mineral oxide surfaces. The present work reports adhesion results obtained using binary combinations of adhesion-promoting and non-adhesion-promoting silanes patterned onto an oxide adherend surface. The effects of pattern shape, texture (feature size) and the fractional coverage of the adhesion promoter are explored for the bonding of poly(vinyl butyral) (PVB) to aluminum (oxide) surfaces using combinations of γ-aminopropyl triethoxysilane (APS), an adhesion promoter and octadecyl trichlorosilane (ODTS), a non-adhesion promoter for this system. Climbing drum peel tests reveal that adhesion depends on feature size, shape and area ratio of the silanes, in many cases resulting in a reduction of adhesion compared with that for a pure APS film, but in other cases producing enhancements of as much as 80%. Among the conditions examined thus far, the greatest adhesion was achieved using square islands, 12 mm × 12 mm in size, surrounded by 1.5 mm wide borders of non-adhesive. Adhesion enhancement is attributed to arrest and confinement of crack propagation. As the crack propagates through a heterogeneous surface, it will blunt at the end of an adhesive patch, and must re-nucleate at the start of the next adhesive domain.

Comparison of Peel Tests for Metal–Polymer Laminates for Aerospace Applications

ABSTRACT Standard peel tests for aerospace laminates based on metal–polymer systems, namely floating-roller and climbing-drum peel methods, have been accommodated in a unified theory of peeling. This theory also accommodates more basic peel tests such as T-peel and fixed-arm peel and also newer methods such as mandrel peel. These five methods have been applied to two aerospace laminate systems to critically examine their use in the determination of adhesive strength. The theory has been used to unify the outputs from the tests in terms of adhesive fracture toughness. In this way, the comparative merits of the methods can be commented on. The validity of the standard methods has been put in doubt because of the absence of a correction for plastic bending energy and also because of the poor conformance of the peel arm to the roller system used in these methods. The unified theory and some measurements of peel-arm curvature help but not completely overcome some of these difficulties. A further complication that arises in peel is a change in the plane of fracture. This reflects a transition from cohesive fracture in the adhesive to an adhesive fracture at the interfaces among adhesive, primer, and substrate. It is likely that such plane-of-fracture phenomena are intrinsic to evaluation of the laminate and that contemplation of cohesive fracture toughness for the adhesive cannot accommodate such events.

Enhanced peel resistance of fiber reinforced phenolic foams

Abstract Short-fiber reinforced phenolic foams were synthesized and characterized by climbing drum peel tests and tensile tests. Significant improvements in mechanical properties were realized, including a multiple-fold increase in peel strength. Because peel strength is closely linked to the fracture toughness of foams, particular attention was focused on the mechanism responsible for the enhanced peel resistance. Scanning electron microscope observations revealed that phenolic foams reinforced with aramid fibers exhibited a unique ‘micro-peel’ process. This process was caused by a moderately weak interface between the flexible aramid fibers and the surrounding phenolic matrix, resulting in higher toughness relative to similar foams reinforced with stiffer glass fibers. In addition, a design-of-experiments analysis supported the expectation that fiber loading and length were primary factors contributing to the improved peel strength.

Bondline Strength Evaluation of Honeycomb Sandwich Panel For Cure Process and Moisture Absorption

In this paper, through a series of comparative experiments, effects of two different cure processing methods, cocure and precure, on the mechanical properties of honeycomb core materials for aircraft applications are considered. Mass of moisture accumulated into the closed cells of the sandwich panel specimen from the measured mass of moisture diffused to the full saturation state into the elements(skin, adhesive layer, Nomex honeycomb), consisting the honeycomb sandwich specimen has been calculated. Water reservoir of 70 was used to have specimens absorb moisture to see the influence of moisture absorbed into sandwich panel on its mechanical properties. For the repair condition holding for 2 hours at 177(350℉) temperature, a pressure due to the vapor expansion in each cell of the sandwich panel, which may result in the local separation of the interface between laminated skin and the surface of the honeycomb, has been estimated by vapor pressure-temperature relation from the thermodynamic steam table and compared to the pressure from the ideal gas state equation. The bonding strengths of the laminated skins on the flat surface of the Nomex honeycomb have been compared by the flatwise tension test and climbing drum peel test performed at room temperature for dry, wet and wet-repair specimens, respectively.

The mechanical performance of 3D woven sandwich composites

Composite sandwich structures were manufactured from a 3D woven fabric consisting of two face fabrics interconnected by pile yarns (Distance Fabric). Specimens were produced from Distance Fabric (DF) consolidated with vinyl ester resin with and without a polyurethane foam core and compared to specimens produced from a precast polyurethane foam core with composite skins added separately. Flatwise compression, edgewise compression, climbing drum peel and flexure tests were conducted and all demonstrated a dramatic improvement in properties from the combination of DF and foam core. These improvements are postulated to arise from the mutual reinforcement of the pile yarns and foam core.

Adhesive Properties of Cured Phenylethynyl-Terminated Imide Oligomers

Abstract As part of a program to develop structural adhesives for high performance aerospace applications, phenylethynyl-terminated imide oligomers are under evaluation. Imide oligomers with different molecular weights and compositions endcapped with either 4-(3-aminophenoxy)-4′-phenylethynylbenzophenone (3-APEB) or 4-phenylethynylphthalic anhydride (PEPA) have been prepared and characterized. These oligomers exhibit excellent processability. After heating to 350°C for 1 hour, the terminal phenylethynyl groups have reacted to provide chain extension, branching and crosslinking. The cured polymers exhibit excellent solvent resistance and high mechanical properties as neat resins and in various adhesive forms (tensile shear, sandwich flatwise tension and climbing drum peel specimens). The chemistry and properties of these phenylethynyl-terminated imide oligomers are discussed.

Influence of Chemical Pretreatments on Surface Morphology and Bondability of Aluminium

Abstract Wettability combined with a strong oxide layer are needed for adhesive bonding and these properties can only be examined by making joints with an adhesive and testing them. The best test for this purpose is the climbing drum peel test. Surfaces must be pickled and anodized. The acids used for pickling influence the anodized structure and chromic-sulphuric acid mixture followed by anodizing in chromic acid gives optimum results. Metal pickled in sulphuric acid shows lower peel strength on bonding. Electron-micrographs show a clear relation between surface morphology and bondability and a fine etchpit structure within coarser etchpits gives the most desirable structure. Low strengths are associated with either weak oxide or a weakened aluminium surface.

Polymer Viscoelasticity: Relation to Peel Strength in Structural Adhesives

Abstract The two tests of most importance in evaluating structural adhesives for metals are (1) lap shear strength and (2) peel strength. Epoxies perform well in the first due to high tensile and shear strength. They are poor in the second unless modified to reduce brittleness. We have developed a urethane modified epoxy for this purpose. By taking climbing drum peel data in which both the temperature and the peel rate are varied, the time-temperature superposition principle can be tested. This principle is most generally applicable to thermoplastic materials between Tθ and Tθ + 100 °C (Tθ = glass transition temperature), and serves as a measure of viscoelastic response in the polymer. First, good agreement was found for a thermoplastic adhesive (PE-AA film). This was done to verify that climbing drum peel data can be used in this manner. Next, data were taken for our urethane modified epoxy. Results showed adherence to the superposition principle only above the heat distortion temperature of the cured po...