Mode-I adhesive fracture energy measurement with modified single-lap joint test geometry

The adhesive fracture energy or fracture toughness of adhesively-bonded joints comprising carbon-fiber-reinforced polymer composite substrates and three different types of adhesives was detemined using a modified single-lap joint (MSLJ). This joint was made by implanting end pre-cracks in the adhesive layer at the center of the bondline of a conventional single-lap joint (SLJ). This modification ensured that the crack propagated from a sharp starter crack from both ends of the overlap during testing, reducing the effect of spew fillets on the measured adhesive fracture toughness scatter band. The MSLJ specimens were tested to failure and the adhesive fracture energy was calculated using the Kinloch–Osiyemi model. The values of the adhesive fracture energy obtained from the MSLJ tests were compared with those from SLJ and the double-cantilever beam (DCB) test geometries. The fracture energy values obtained from the MSLJ specimens were significantly lower than those from SLJ specimens and agreed well with those from DCB specimens. The three differenent types of aerospace grade film adhesives tested were Redux 322, Redux 335K and Redux 319A.     

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.     

Impact strength of joints bonded with high-strength pressure-sensitive adhesive

The impact strength of joints bonded with a double-coated high-strength pressure-sensitive adhesive (PSA) was experimentally investigated. PSA has recently been used to join parts of mobile devices such as smart-phones, which are often subjected to drop impacts. Consequently, the impact strength of PSA bonded joints has become important.Two types of specimens, butt joint specimens and double cantilever beam (DCB) specimens bonded with adhesives were utilized for the experiments. Quasi-static tests and impact tests of the specimens were carried out using a mechanical testing machine and an impact testing machine. The PSA layers in the specimens were observed using a high-speed digital camera. The deformation and strain distribution in the adherends of the DCB specimens were also measured using a novel high-speed digital camera with photoelastic imaging capability.Though the strength of the butt joints increased as the loading rate increased, the critical fracture energy of the DCB specimens decreased at high loading rates. This may be attributed to the transition to the brittle nature of the PSA in the loading range in which no cavitation occurred. To verify the critical fracture energy obtained with the DCB tests, finite element analyses (FEA) based on the cohesive zone model (CZM) were carried out, and the load–displacement curves of the DCB tests were simulated. The predicted results showed good agreements with the experimental results.     

Moisture and temperature degradation of double cantilever beam adhesive joints

In this work, the double cantilever beam (DCB) test is analysed in order to evaluate the combined effect of temperature and moisture on the mode I fracture toughness of adhesives used in the automotive industry. Very few studies focus on the combined effect of temperature and moisture on the mechanical behaviour of adhesive joints. To the authors’ knowledge, the simultaneous effect of these conditions on the fracture toughness of adhesive joints has never been determined. Specimens using two different adhesives for the automotive industry were subjected to two different ageing environments (immersion in distilled water and under 75% of relative humidity). Once they were fully degraded, they were tested at three different temperatures (?40, 23 and 80 °C), which covers the range of temperature an adhesive for the automotive industry is required to withstand. The aim is to improve the long term mechanical behaviour prediction of adhesive joints. The DCB substrates were made of a high strength aluminium alloy to avoid plastic deformation during test. The substrates received a phosphoric acid anodisation to improve their long term adhesion to the adhesive. Results show that even though a phosphoric acid anodization was applied to the adherends, when the aged specimens were tested at room temperature and at 80 °C, they suffered interfacial rupture. At ?40 °C, however, cohesive rupture was observed and the fracture toughness of the aged specimens was higher.     

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.     

Effect of interlayer deformation on blister test

The effect of interlayer deformation on blister test for measuring adhesive strength was analyzed by modeling the interlayer as a Winkler foundation. Critical load for the initiation of debonding along the interface between the interlayer and an elastic thin film was obtained as a function of the adhesive strength, interlayer deformation, elastic modulus of Winkler foundation, and blister size. The critical pressure increases with increasing the elastic modulus of Winkler foundation. The propagation of debonding was discussed, and the arrest of debonding was observed for the condition of constant deflection. The results provide a rational for characterizing the effect of interlayer deformation on the measurement of adhesive strength from a blister test.     

The relative merits of the Boeing wedge test and the double cantilever beam test for assessing the durability of adhesively bonded joints,with particular reference to the use of fracture mechanics

Water is one of the most aggressive environments in which adhesives can be exposed. Once water has entered the joint there are several ways in which it may cause weakening. The adhesive can be plasticised, which is sometimes a reversible condition, or it can crack, craze or hydrolyse which are irreversible conditions. Water can also attack the adhesive–adherend interface or cause the adhesive to swell, which creates stresses in the joint. It is therefore necessary to have reliable tests available which are inexpensive and easily understood so that a variety of substrates, surface treatments, and adhesives can be assessed under an assortment of environmental conditions. We have compared the Boeing wedge test (BWT), the forced wedge test (FWT), and the double cantilever beam (DCB) test and shown that the FWT is not to be recommended while the BWT and the DCB test expose the joints to quite different environments. However, the BWT and the DCB test can be usefully extended by calculating the fracture toughness from the raw results. While it is possible to refine these tests to reduce variability, the differences between good and bad joints are so great that such finesse is unnecessary.     

Comparison of Cyclic and Monotonic Loading of a Double Cantilever Beam Adhesion Test

Double Cantilever Beam (DCB) specimens were made from aluminium plates bonded with Hysol®EA9321 epoxy adhesive. These were tested both under monotonic and cyclic loading conditions. Experimental testing data were obtained from classic force and displacement measurements, as well as from backface strain recordings. Apart from the usual crack propagation monitoring, evolution of the process zone at the crack tip was studied during the experiment. Results of fatigue were compared with those of standard loading. Despite the viscoelastic nature of the adhesive, an elastic treatment of the results proved most satisfactory. In addition, good agreement was found between experimental and theoretical results obtained with a Timoshenko beam on Winkler elastic foundation model.     

Mode II fracture energy in the adhesive bonding of dissimilar substrates: carbon fibre composite to aluminium joints

The end-notched flexure (ENF) test calculates the value of mode II fracture energy in adhesive bonding between the substrates of same nature. Traditional methods of calculating fracture energy in the ENF test are not suitable in cases where the thickness of the adhesive is non-negligible compared with adherent thicknesses. To address this issue, a specific methodology for calculating mode II fracture energy has been proposed in this paper. To illustrate the applicability of the proposed method, the fracture energy was calculated by the ENF test for adhesive bonds between aluminium and a composite material, which considered two different types of adhesive (epoxy and polyurethane) and various surface treatments. The proposed calculation model provides higher values of fracture energy than those obtained from the simplified models that consider the adhesive thickness to be zero, supporting the conclusion that the calculation of mode II fracture energy for adhesives with non-negligible thickness relative to their adherents should be based on mathematical models, such as the method proposed in this paper, that incorporate the influence of this thickness.     

The Effect of Adhesive Thickness on the Mechanical Behavior of a Structural Polyurethane Adhesive

One parameter that influences the adhesively bonded joints performance is the adhesive layer thickness. Hence, its effect has to be investigated experimentally and should be taken into consideration in the design of adhesive joints. Most of the results from literature are for typical structural epoxy adhesives which are generally formulated to perform in thin sections. However, polyurethane adhesives are designed to perform in thicker sections and might have a different behavior as a function of adhesive thickness. In this study, the effect of adhesive thickness on the mechanical behavior of a structural polyurethane adhesive was investigated. The mode I fracture toughness of the adhesive was measured using double-cantilever beam (DCB) tests with various thicknesses of the adhesive layer ranging from 0.2 to 2 mm. In addition, single lap joints (SLJs) were fabricated and tested to assess the influence of adhesive thickness on the lap-shear strength of the adhesive. An increasing fracture toughness with increasing adhesive thickness was found. The lap-shear strength decreases as the adhesive layer gets thicker, but in contrast to joints with brittle adhesives the decrease trend was less pronounced.     

Effect of Adhesive Compliance in the Assessment of Soft Adhesives with the Wedge Test

Wedge tests are usually analysed assuming that the free, unbonded members may be treated as encastré cantilever beams. However, if the adhesive layer is sufficiently flexible (e. g., due to low elastic modulus), then significant strain in the bonded region may occur and lead to modification of the behaviour outside this region. Using in conjunction a sensitive strain gauge method on asymmetric wedge tests and a mathematical analysis developed from the work of Winkler, we conclude that the standard, simple beam theory approach significantly overestimates crack length for a supple adhesive layer. The present contribution mainly considers strain effects in the intact, bonded zone, rather than fracture per se. However, it is concluded that, if in fracture tests, the incorrect values of crack length obtained from the encastré beam assumption are employed to calculate fracture energy using the simpler model, there will be some self-compensation and little error in estimates of the latter will result (at least in the cases presently studied).     

The fracture threshold for an adhesive interlayer

Complementing an earlier paper which utilized an energy balance criterion for a continuum mechanics analysis of adhesive failure in a pressurized blister at the interface of an elastic material and a rigid substrate, the analysis is extended to include an additional elastic interlayer between them. An infinite lateral-length elastic plate strip bonded through a Winkler elastic foundation to a rigid substrate is assumed, in which the plate is separated from the adhesive layer by internal pressure. It is found that the important design parameters are the tensile modulus-to-thickness ratio of the adhesive layer and the adhesive fracture energy of separation of the respective materials. The results provide a basis for investigating changes in the chemical microstructure of the adhesive.     

Crack Path Selection in Adhesively-Bonded Joints: The Role of Material Properties

This paper investigates the role of material properties on crack path selection in adhesively bonded joints. First, a parametric study of directionally unstable crack propagation in adhesively-bonded double cantilever beam specimens (DCB) is presented. The results indicate that the characteristic length of directionally unstable cracks varies with the Dundurs' parameters characterizing the material mismatch. Second, the effect of interface properties on crack path selection is investigated. DCB specimens with substrates treated using various surface preparation methods are tested under mixed mode fracture loading to determine the effect of interface properties on the locus of failure. As indicated by the post-failure analyses, debonding tends to be more interfacial as the mode II fracture component in the loading increases. On the other hand, failures in specimens prepared with more advanced surface preparation techniques appear more cohesive for given loading conditions. Using a high-speed camera to monitor the fracture sequence, DCB specimens are tested quasi-statically and the XPS analyses conducted on the failure surfaces indicate that the effect of crack propagation rate on the locus of failure is less significant when more advanced surface preparation techniques are used. The effect of asymmetric interface property on the behavior of directionally unstable crack propagation in adhesive bonds is also investigated. Geometrically-symmetric DCB specimens with asymmetric surface pretreatments are prepared and tested under low-speed impact. As indicated by Auger depth profile results, the centerline of the crack trajectory shifts slightly toward the interface with poor adhesion due to the asymmetric interface properties. Third, through varying the rubber content in the adhesive, DCB specimens with various fracture toughnesses are prepared and tested. An examination of the failure surfaces reveals that directionally unstable crack propagation is more unlikely to occur as the toughness of the adhesive increases, which is consistent with the analytical predictions that were discussed using an energy balance model.     

Crack Path Selection in Adhesively-Bonded Joints: The Role of Material Properties

This paper investigates the role of material properties on crack path selection in adhesively bonded joints. First, a parametric study of directionally unstable crack propagation in adhesively-bonded double cantilever beam specimens (DCB) is presented. The results indicate that the characteristic length of directionally unstable cracks varies with the Dundurs' parameters characterizing the material mismatch. Second, the effect of interface properties on crack path selection is investigated. DCB specimens with substrates treated using various surface preparation methods are tested under mixed mode fracture loading to determine the effect of interface properties on the locus of failure. As indicated by the post-failure analyses, debonding tends to be more interfacial as the mode II fracture component in the loading increases. On the other hand, failures in specimens prepared with more advanced surface preparation techniques appear more cohesive for given loading conditions. Using a high-speed camera to monitor the fracture sequence, DCB specimens are tested quasi-statically and the XPS analyses conducted on the failure surfaces indicate that the effect of crack propagation rate on the locus of failure is less significant when more advanced surface preparation techniques are used. The effect of asymmetric interface property on the behavior of directionally unstable crack propagation in adhesive bonds is also investigated. Geometrically-symmetric DCB specimens with asymmetric surface pretreatments are prepared and tested under low-speed impact. As indicated by Auger depth profile results, the centerline of the crack trajectory shifts slightly toward the interface with poor adhesion due to the asymmetric interface properties. Third, through varying the rubber content in the adhesive, DCB specimens with various fracture toughnesses are prepared and tested. An examination of the failure surfaces reveals that directionally unstable crack propagation is more unlikely to occur as the toughness of the adhesive increases, which is consistent with the analytical predictions that were discussed using an energy balance model.     

The Adhesive Fracture Energy of Bonded Thermoplastic Fibre-Composites

The present paper first discusses the problems that occur when thermoplastic-based fibre-composite materials are bonded using structural engineering adhesives, such as epoxy and acrylic adhesives. A double-cantilever beam joint has been employed and it is shown that the value of the adhesive fracture energy, G_c, is very low when a simple abrasion/solvent wipe pretreatment is used for the thermoplastic fibre-composites. This arises from crack growth occurring along the adhesive/composite interface, which is relatively weak when such a pretreatment is employed. Secondly, it is demonstrated how very effective a corona surface pretreatment may be for these materials. Indeed, when such a pretreatment is used, interfacial crack growth is no longer observed but the crack now propagates either cohesively in the adhesive or through the composite substrate; both failure modes lead to relatively high values of G_c, with the former resulting in the highest values of G_c being recorded. Finally, from measuring the fracture properties of the composites and combining these data with a detailed analysis of the stresses in the DCB joint, calculated using a finite element analysis, the reasons for these different loci of failure may be readily understood and predicted.     

Numerical assessment of the Double-Cantilever Beam and Tapered Double-Cantilever Beam tests for the GIC determination of adhesive layers

ABSTRACT

Fracture mechanics-based techniques have become very popular in the failure prediction of adhesive joints. The most commonly used is cohesive zone modeling (CZM). For both conventional fracture mechanics and CZM, the most important parameters are the tensile and shear critical strain energy release rates (G_IC and G_IIC, respectively). The most common tests to estimate G_IC are the Double-Cantilever Beam (DCB) and the Tapered Double-Cantilever Beam (TDCB) tests. The main objective of this work is to compare the DCB and TDCB tests to obtain the G_IC of adhesive joints. Three adhesives with varying ductilities were used to verify their influence on the precision of the typical methods of data reduction. For both tests, methods that do not need the measurement of crack length (a) were tested. A CZM analysis was considered to reproduce the experimental load–displacement (P-δ) curves and obtain the tensile CZM laws of each tested adhesive, to test the suitability of the data reduction methods, and to study the effect of the CZM parameters on the outcome of the simulations. The CZM models accurately reproduced the experimental tests and confirmed that the data reduction methods for the TDCB test tend to underestimate G_IC for ductile adhesives.     

Comparative evaluation of the Double-Cantilever Beam and Tapered Double-Cantilever Beam tests for estimation of the tensile fracture toughness of adhesive joints

The continuous development observed in bonded joints, along with the improvements of the adhesives’ properties, are resulting in an increase of the bonded joint applications, as well as the variety of applications. Regarding the strength prediction of adhesive joints, two highly relevant methods are Fracture Mechanics and Cohesive Zone Models (CZM). By Fracture Mechanics, this is usually carried out by an energetic analysis. CZM enable the simulation of damage initiation and propagation. The tensile critical strain energy release rate (G_Ic) of adhesives is one of the most important parameters for predicting the joint strength. Two of the most commonly used tests are the Double-Cantilever Beam (DCB) and the Tapered Double-Cantilever Beam (TDCB). This work aims to assess the capability of the DCB and TDCB test to estimate the value of G_Ic of adhesive joints. Three types of adhesives with different levels of ductility are used, to study the accuracy of the typical data reduction methods under conditions that are not always consistent with Linear Elastic Fracture Mechanics (LEFM) principles. For both test protocols, methods that do not require measurement of the crack length (a) during the test are evaluated. In the DCB test, these are the Compliance Calibration Method (CCM), Corrected Beam Theory (CBT) and Compliance-Based Beam Method (CBBM). The methods used in the TDCB test are the Simple Beam Theory (SBT), CCM and CBT. With few exceptions, the results were consistent between the different methods considered for each test. The discrepancy of results is higher when comparing the two types of tests, except for the brittle adhesive. It was concluded that the data reduction methods for the TDCB test are too conservative to measure G_Ic of ductile adhesives.     

Fracture energy in structural adhesive joints of composite-aluminum under adverse environments conditions

This work analyzes the degradation of composite-aluminum adhesive joints when they are exposed to the weathering and environmental pollution in Madrid for a long period of time. Two adhesives (epoxy and polyurethane) and several surface treatments for adherends have been considered. End-notched flexure bending tests have been performed to evaluate the loss of mechanical properties (failure stress and fracture energy) of adhesive joints that were exposed to the weathering and environmental pollution. Tests results have shown that the environmental degradation of the adhesive leads to a loss of mechanical properties in the adhesive joints. Considering the relative percentage, the reduction of failure stress in the polyurethane is higher than in the epoxy (31.9% for the polyurethane and 21.1% for the epoxy). Similarly and considering relative percentage, fracture energy reduction is 42.0% for polyurethane and 41.5% for epoxy. Likewise, tests have shown that the loss of mechanical properties does not decrease linearly with the time when the samples have been exposed to the weathering. This reduction occurs during the first few weeks. In summary, tests results have allowed to conclude that adhesive joints with epoxy resist the environmental pollution better than the adhesive joints with polyurethane.     

Improved Analytical Models for Mixed-Mode Bending Tests of Adhesively Bonded Joints

In this paper, improved analytical models have been developed for more accurate determination of mode I and mode II compliances and strain energy release rates of adhesively bonded Double Cantilever Beam (DCB) type specimens. In these models, the effects of adhesive layer, elastic foundation and shearing deformation ahead of the crack tip due to the Saint-Venant end effect were considered. The improved analytical models were verified using finite element analysis (FEA), and excellent agreements were found for a variety of crack lengths. The comparisons of the measured and predicted testing machine loading point compliances with crack length at three different mixed-mode loading conditions for the adhesive layer thickness of 0.254 mm were also performed for further validation. In addition, contributions to the total mode I strain energy release rate from each effect are summarized.     

Accurate and continuous adhesive fracture energy determination using an instrumented wedge test

The wedge test and the related double cantilever beam test are practical methods of assessing structural adhesive fracture energy. In the former, and to a lesser extent the latter, a recognised problem is the difficulty of following the length of the growing crack, required to calculate fracture energy with any accuracy. We present a novel method of measurement of crack length that has the advantages of being accurate and allowing continuous assessment of crack-length evolution during the failure process. It is based on the attachment of a series of strain gauges to the outer surface of one of the beams constituting the adhesive assembly. Surface strain measurements are interpreted directly using simple beam theory. The method has been validated both with adhesive assemblies under failure conditions and by tests undertaken on “artificial” joints, where “bonding” is effected by clamping adherends together.