A Test Method for Accelerated Humidity Conditioning and Estimation of Adhesive Bond Durability

The ability to determine the durability of adhesive bonds remains an elusive task, especially when the service environment involves exposure to diluents such as water. Moisture continues to be of major concern for many adhesive bond systems for a number of reasons including:

1) many adhesives are hydrophilic, picking up significant amounts of moisture over time;

2) most adhesives and some adherends allow moisture permeation, eventually reaching the adhesive/adherend interface;

3) the high surface energies of metallic and certain other substrates result in moisture migrating to the adherend surfaces and displacing the adhesive from the substrates, and possibly oxidizing the adherend, etc., and

4) absorbed moisture induces swelling stresses which can reduce the bond strength.

Recognition of this susceptibility to moisture has led to extensive studies aimed at evaluating the effects of moisture, developing an understanding of the responsible mechanisms, and predicting the performance of adhesive bonds subjected to humid environments. While some studies have focused on the effect of humidity on neat adhesive samples, most studies have recognized the significance of the adhesive/adherend interactions, and have evaluated strength of actual bonded joints. Unfortunately, the time required for typical bonded geometries to reach moisture equilibrium can be quite long. Single lap joints (SLJ) and double cantilever beam (DCB) specimens with a width of 25mm may take several years to equilibrate, depending on the temperature and adhesive. Such lengthy conditioning times hamper the development of improved adhesives, and may delay the acceptance of these adhesives because of the time required to certify them. Methods to accelerate the conditioning of test specimens would be of significant benefit to adhesive formulators and users.     

Assessment of test methods for a cryogenic liquid containment system

This paper presents a study demonstrating the selection and use of adhesive joint test methods for the design and validation of an adhesively bonded, foam-composite membrane, cryogenic insulation system for the marine transportation of liquefied natural gas (LNG). The study considered the performance of epoxy and polyurethane adhesives under ambient and sub-zero operating temperatures. Double-lap, sandwich panel and double cantilever beam (DCB) joint tests, essential in “calibrating” the interpretation of finite element analysis (FEA), were performed along with FEA in order to assess the stress states (in-plane, peel and shear stress) in the adhesive layer that, under defined loads and extensions, are comparable with the stress levels in the LNG container under service conditions.The study reinforces the view that the presence of barrier film substrates has a major effect on performance, and that the critical state of stress for the integrity of the flexible composite barrier film (FSB) to rigid composite barrier film (RSB) bond in the cryogenic containment system is the tensile peel stress at the ends of the joint. Sandwich panel tests conducted using the two adhesives indicate that failure tends to occur when the peel stress exceeds the tensile strength of the bulk adhesive with the polyurethane adhesive exhibiting more robust adhesion properties than the epoxy with consequences for future design of LNG containers.     

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.     

Properties of compression densified wood. Part I: bond performance

Bond performance of hygro-thermally compression densified wood was studied using hygro-thermally treated and control yellow-poplar wood (Liriodendron tulipifera). Opening mode double cantilever beam fracture testing and cyclic boiling were used to evaluate bond performance. Phenol–formaldehyde (PF) film and polymeric diphenylmethane diisocyanate (pMDI) adhesives were used to bond specimens for fracture testing. Fracture toughness of hygro-thermal samples bonded with PF film was significantly higher than control samples, while no difference was found for densified samples. Fracture toughness of densified samples bonded with pMDI was significantly higher than control samples, however no change was seen for hygro-thermal samples. Cyclic boiling reduced the fracture toughness of hygro-thermal fracture samples only, irrespective of adhesive type.     

Crack-growth behavior of epoxy adhesives modified with liquid rubber and cross-linked rubber particles under mode I loading

The crack-growth resistance (R-curve) of bulk single-edge notch bend (SENB) and adhesively bonded double cantilever beam (DCB) specimens was investigated under mode I loading conditions using two types of rubber-modified epoxy adhesive: one was a liquid rubber (CTBN)-modified adhesive and the other was a cross-linked rubber particle (DCS)-modified adhesive. As a result, for both the SENB and DCB specimens, the gradient of the R-curve for the DCS-modified adhesive was steeper than that for the CTBN-modified one, however, the difference in fracture toughness between DCS- and CTBN-modified adhesives is smaller for DCB than for SENB specimens. To elucidate such behavior, crack-growth simulation based on Gurson's model was conducted, where the DCS- and CTBN-modified adhesives were characterized by both the initial void fraction and nucleation. The difference in the behavior of R-curves was also observed in simulations. Moreover, it was found that the difference in fracture surface roughness observed by SEM for both adhesives correspond to the variation in R-curves.     

Cohesive and continuum mixed-mode damage models applied to the simulation of the mechanical behaviour of bonded joints

The objective of this work is to discuss the adequacy of cohesive and continuum damage models for the prediction of the mechanical behaviour of bonded joints. A cohesive mixed-mode damage model appropriate for ductile adhesives is presented. The double cantilever beam and the end-notched flexure tests are proposed in order to evaluate the cohesive properties of the adhesive as a thin layer under mode I and mode II, respectively. A new data reduction scheme based on the crack equivalent concept is also proposed to overcome crack-monitoring difficulties during propagation in these fracture characterization tests. An inverse method to determine the cohesive parameters of the trapezoidal softening law is discussed. A continuum mixed-mode damage model is developed in order to better simulate the cases where adhesive thickness plays an important role. The model is applied to evaluate the effect of adhesive thickness on fracture characterization of adhesive joints. Some important conclusions about the advantages and drawbacks of cohesive and continuum damage models are reported.     

The Use of Large Diameter Monofilament for Improving Peel Resistance of a Brittle High Temperature Adhesive

Peel force measurements as a function of adherend thickness are reported for adhesively bonded specimens based on a cyanate ester resin and aluminium adherends. It has been demonstrated that by incorporating large diameter (0.28mm) PTFE monofilament within the adhesive bond then the peel force and associated fracture energy can be increased significantly over that for specimens based on adhesive alone. Fracture energy measurements are derived for specimens with peeling adherend thickness of up to about 0.6 mm using the 90° peel test. Fracture energies are also derived for peeling of more practically-representative 1.6mm thickness adherends using a single cantilever beam experiment. In-situ photoelasticity and SEM microextensomctry experiments are reported which show the stress fields and displacements associated with the presence of the monofilament. It is believed that the reported increase in measured fracture energy is partly due to the crack pinning effect of the monofilament, and partly due to the monofilament creating a “load shadowed” region between adherend and monofilament which prevents the interfacial crack from propagating between adherend and adhesive.     

The Effects of Pre-Bond Moisture on the Fracture Behaviour of Adhesively-Bonded Composite Joints

The results of an investigation into the effects of pre-bond moisture absorbed by fibre-composite substrates prior to bonding with various structural epoxy adhesives are presented. Substrates were bonded in the as-received condition (where substrates had been exposed to atmospheric moisture for periods of greater than three months) and were also bonded in the fully-dried condition (after drying under vacuum at 105°C for 28 days). Additionally, substrates were conditioned by water submersion for various durations prior to bonding. Double cantilever beam tests were performed on the resulting joints to determine the adhesive fracture energy, G_IC. The effect of pre-bond moisture on the glass transition temperature of the adhesive was also determined. One adhesive was shown to exhibit an extreme sensitivity to pre-bond moisture. A severe reduction in fracture energy accompanied a change in the fracture morphology and T_g. Other adhesives were shown to be relatively insensitive to the levels of pre-bond moisture introduced.     

Role of bond thickness on adhesive failure

An analytical, numerical and experimental program is described which establishes the basic fracture mechanics properties of an adhesive joint. A finite element analysis of a homogeneous finite tapered double cantilever beam is first presented and the results compared with elasticity and strength of materials solutions. Using analytical results developed in another paper, a finite bond line thickness correction factor is introduced to determine the crack tip stress intensity factor as a function of crack length. An experimental program is described wherein the crack tip stress intensity factor for the cantilever beam adhesive joint is measured by the compliance method and the results compared with those obtained by analytical and numerical methods. Finally, the critical value of the adhesive crack tip stress intensity factor is determined using the analytical and experimental techniques presented.     

Fracture mechanics testing of the bond between composite overlays and a concrete substrate

This paper presents a methodology for assessing the bond strength of composite overlays to concrete utilizing a fracture toughness test. The principles and practices of existing ASTM standards for determining the fracture toughness of adhesive bonds between double cantilever beam (DCB) metallic and composite specimens (D 3433-93 and D 5528-94a) have been extended to cover the case of an elastic composite layer bonded to a rigid concrete/masonry substrate. In the theoretical section, the dominant loading conditions, relevant ASTM standards, and the development of energy release rate concepts for analyzing a disbonding composite layer modeled as an elastic cantilever beam are presented. The experimental section covers specimen fabrication and preparation, experimental setup, test procedures, post-test evaluation of the specimens, and data processing. The discussion of test results focuses on explaining the variability in measured strain energy release rate, and identifies trends between the measured strain energy release rate and the fraction of the fracture surface retaining cement paste after disbonding. It was found that good-quality composite-to-concrete bond is associated with high fracture toughness of the adhesive and location of the crack path in the concrete substrate. Strict enforcement of surface preparation and adhesive handling procedures was found to play an important role in promoting good bond strength and high fracture toughness. The fracture toughness test developed in this paper can be used for screening various composite-repair systems, to assess the effect of different environmental attacks, and as a quality control tool.     

Effect of bond thickness on creep lifetime of adhesive joints under mode I

An experimental investigation has been carried out on double cantilever beam specimens with different bond thicknesses to study the effect of bond thickness on lifetime of adhesive joints under mode I. This paper describes an approach to predict the rate of crack propagation. The approach is based on principles of linear elastic fracture mechanics and uses elevated temperature to accelerate the crack propagation under constant loads. The fracture energy of the joint is studied as a function of bond thickness. The results from short-term tests are analyzed and a simple model has been proposed to predict the variation of two kinetic parameters of the Paris law with bond thickness.     

Comparison of fracture behavior between acrylic and epoxy adhesives

An experimental study was conducted on the strength of adhesively bonded steel joints, prepared epoxy and acrylic adhesives. At first, to obtain strength characteristics of these adhesives under uniform stress distributions in the adhesive layer, tensile tests for butt, scarf and torsional test for butt joints with thin-wall tube were conducted. Based on the above strength data, the fracture envelope in the normal stress-shear stress plane for the acrylic adhesive was compared with that for the epoxy adhesive. Furthermore, for the epoxy and acrylic adhesives, the effect of stress triaxiality parameter on the failure stress was also investigated. From those comparison, it was found that the effect of stress tri-axiality in the adhesive layer on the joint strength with the epoxy adhesive differed from that with the acrylic adhesive. Fracture toughness tests were then conducted under mode l loading using double cantilever beam (DCB) specimens with the epoxy and acrylic adhesives. The results of the fracture toughness tests revealed continuous crack propagation for the acrylic adhesive, whereas stick-slip type propagation for the epoxy one. Finally, lap shear tests were conducted using lap joints bonded by the epoxy and acrylic adhesives with several lap lengths. The results of the lap shear tests indicated that the shear strength with the epoxy adhesive rapidly decreases with increasing lap length, whereas the shear strength with the acrylic adhesive decreases gently with increasing the lap length.     

The Effects of Pre-Bond Moisture on the Fracture Behaviour of Adhesively-Bonded Composite Joints

The results of an investigation into the effects of pre-bond moisture absorbed by fibre-composite substrates prior to bonding with various structural epoxy adhesives are presented. Substrates were bonded in the as-received condition (where substrates had been exposed to atmospheric moisture for periods of greater than three months) and were also bonded in the fully-dried condition (after drying under vacuum at 105°C for 28 days). Additionally, substrates were conditioned by water submersion for various durations prior to bonding. Double cantilever beam tests were performed on the resulting joints to determine the adhesive fracture energy, G _IC. The effect of pre-bond moisture on the glass transition temperature of the adhesive was also determined. One adhesive was shown to exhibit an extreme sensitivity to pre-bond moisture. A severe reduction in fracture energy accompanied a change in the fracture morphology and T_g. Other adhesives were shown to be relatively insensitive to the levels of pre-bond moisture introduced.     

Analysis of the cohesive zone and crack length correction in the adhesively bonded double cantilever beam specimen

The double cantilever beam specimen has been increasingly employed to enable the development of cohesive zone models for adhesive joints. Evaluation of the traction–separation law (TSL) requires elaborate experimental techniques and usually relies on data measured until the crack initiation point. Nonetheless, current standards stipulate fracture energy measurements under steady-state crack propagation. This paper investigated the influence of the cohesive zone on the commonly used corrected beam theory data reduction scheme. Analytical solutions for the elastic–perfectly plastic, bilinear, and trapezoidal laws were developed using a beam model. The role of the elastic traction decay zone was found to be significant for high strength moderately tough adhesives. Nevertheless, the results showed that the sensitivity of the crack length correction to the cohesive zone can be exploited to obtain approximate TSLs.     

Studies on Effective Bond Strength of Kendu Fruit Adhesive

Certain investigations on the effective bond strength of gum adhesive extracted from kendu (Diospyros cordifolia Roxb) fruit have been studied with respect to different interfaces such as general-purpose wood-wood, teakwood-teakwood, plywood-plywood, glass-glass, PMMA-wood, PVC-PVC. LDPE-LDPE, HDPE-HDPE, and PMMA-PMMA. The effects of different parameters such as curing temperature, curing time, thickness of specimens, and presolvent treatments on the bond strength of kendu fruit adhesive (KFA) are investigated. The bond strengths of KFA under different hazardous environments such as boiling water, saline water, and kerosene oil, acidic as well as alkaline solutions for the teakwood-teakwood interface have also been examined. A comparative account of studies of the effective bond strength of KFA with three different commercial synthetic adhesives such as Varnicol, Quickfx, and Araldite is reported. The KFA shows a superior bond strength for general-purpose wood-wood adhesion. Its bond strength is also comparable with synthetic adhesives for certain plastic-plastic as well as teakwood-teakwood adhesions. The retention of effective bond strength of KFA in various hazardous atmosphere remarkably encourages an industrial-grade adhesive formulation through proper R&D investigations. Adequate modifications and informations are required for an improved adhesive formulation from this natural adhesive to meet the industrial standard.     

Experimental investigation of an adhesive fracture energy measurement by preventing plastic deformation of substrates in a double cantilever beam test

A method to prevent substrate damage in a double cantilever beam (DCB) test was experimentally investigated by changing bond-line width. Highly toughened adhesives are developed due to an increase in the demand for structural adhesives. Furthermore, structural materials that are lightweight and excellent in mechanical properties but have limitations in thickness are developed. In contrast, plastic deformation of the substrates is expected when the DCB test is performed with conventional DCB specimens using the aforementioned adhesives and materials. The reduction in the maximum stress on the substrate surface by narrowing a bond-line width is a practical method to prevent plastic deformation when a substrate possesses limited thickness. In this study, the influence of the bond-line width on an adhesive fracture energy in mode I was experimentally investigated by manufacturing DCB specimens in which the bond-line width is narrower than the substrate width. Additionally, the maximum bond-line width to prevent plastic deformation of the substrates was theoretically derived. The evaluated criteria for examining the existence of the plastic deformation by changing the bond-line width exhibited good agreement with the experiment results.     

Influence of graphene nanoplatelets (GNPs) on mode I fracture toughness of an epoxy adhesive under thermal fatigue

Abstract

The contribution of graphene nanoplatelets (GNPs) for enhancing the fracture toughness of a commonly used room-cured epoxy, used to bond E-glass/epoxy composite adherends, is evaluated. A comprehensive experimental investigation is conducted to examine the performance and degradation of adhesively bonded joints subject to cyclic thermal loading using the standard double cantilever beam (DCB) specimens. Several groups of DCB specimens were fabricated using the adhesive reinforced with four different GNPs weight-percentages (i.e. 0.0, 0.25, 0.5 and 1%). The specimens are subsequently subjected to various numbers of thermal cycles (to a maximum of 1000 heating/cooling cycles), and then tested, and the resulting mode I fracture toughness values are evaluated and compared. The extent and modes of damage captured through microscopy and scanning electron microscopy images are presented and discussed. In addition, a computational framework, using the cohesive zone modeling technique, is developed for predicting the response of the adhesives and their damage evolution.     

Strengthening of the adhesive bond using a mixture of adhesives between dissimilar adherends in a single lap joint

Abstract

Adhesive bonding is the best alternative to riveting in aircraft structures but the strength of the adhesive bonded joint is low and is limited by strength of adhesive. Strengthening of adhesive bonding is an important requirement. In this work, an attempt has been made to strengthen the adhesive bonding by mixing different quantities of brittle adhesive in the ductile adhesive and vice-versa. Two different adhesives, one brittle (AV138) and another ductile (Araldite-2015) adhesive have been considered. Initially single lap joint has been constructed between the CFRP and aluminium with individual adhesives, then the mixture of adhesives have been used in the bonded region in varied proportions. The X-ray radiography and ultrasonic testing have been performed to check the quality of bonding. Uniaxial tensile tests have been conducted on the lap joints along with Digital Image Correlations (DIC) to obtain the individual and mixed adhesive bond strength. The failure patterns have been identified using optical and scanning electron microscope. These studies indicate that strengthening of the adhesive bonding achieved by mixing of two adhesives and highest bond strength obtained when the mixture of AV138 and Araldite-2015 adhesives are used in equal proportions.     

Effect of Pressure Cycling on Fracture Energy of Polyurethane/Aluminum Adhesive Bonds

An experimental study was conducted to investigate the effect of pressure-cycling on adhesive bond fracture energy of polyurethane/aluminum adhesive bond joints. Initially, two types of peel tests were conducted to characterize adhesive bond strength and challenges associated with pre-mature polyurethane cracking and failure during these tests are discussed. A modified double cantilever beam (MDCB) specimen configuration was specially designed and opening-mode loading conditions were employed to determine the interfacial adhesive bond energy (G_C). The test specimens were pressure-cycled in water-filled tanks for 1 to 4 weeks with an increment of 1 week. The G_C of pressure-cycled specimens was compared with both control and water-soaked samples (without pressure-cycling). The results indicated that pressure-cycling decreased G_C values to those of the control and water-soaked samples: hence, prolonged pressure-cycling could be problematic to polymer/metal adhesive bonds of hardware installed outboard of submarine pressure hulls.     

Dielectric spectroscopy of anaerobic adhesive cure

Dielectric spectroscopy (DS) is shown to be a most useful tool for the study of the surface initiated redox cure in model anaerobic acrylic adhesives. These models are deliberately designed to exhibit different levels of cure heterogeneity. The cure is initiated by the surfaces of two substrates on either side of a bondline. The heterogeneous cure situation is referred to as low cure through volume (CTV). Poor CTV is an undesirable feature that leads to mechanical weakness in an adhesive bond. Thus, experimental techniques based on DS are developed to investigate the cure characteristics and in particular the CTV phenomenon in the model anaerobic adhesives.