The prediction of crack growth in bonded joints under cyclic-fatigue loading I. Experimental studies

The performance of adhesively-bonded joints under monotonic and cyclic-fatigue loading has been investigated using a fracture-mechanics approach. The joints consisted of an epoxy film adhesive which was employed to bond aluminium-alloy substrates. The effects of undertaking cyclic-fatigue tests in (a) a ‘dry’ environment of 55% relative humidity at 23°C, and (b) a ‘wet’ environment of immersion in distilled water at 28°C were investigated. In particular, the influence of employing different surface pretreatments for the aluminium-alloy substrates was examined. In addition, single-lap joints were tested under cyclic fatigue loading in the two test environments, and a back-face strain technique has been used which revealed that crack propagation, rather than crack initiation, occupied the dominant proportion of the fatigue lifetime of the single-lap joints. In Part II, the data obtained in the present Part I paper will be employed to predict theoretically the lifetime of the adhesively-bonded single-lap joint specimens.     

Debond and failure characteristics of a double-lap adhesively bonded joint

A novel double-lap joint design was used to bond steel adherends using a structural epoxy adhesive. Different levels of debond were built into the joint using a mold release agent during fabrication. The damping capacity measurements of the debonded specimens were obtained using a FFTbased impulse-frequency response vibration technique. The joint strengths were obtained by loading the specimens to failure in a servo-hydraulic MTS 850 test system. It was observed that the failure strength of the joint correlated well with the loss factor (a measure of damping). Empirical equations for predicting the strength of the joint in terms of the loss factor or resonant frequency are presented. A torsional vibration test rig was also used to evaluate the damping properties and to predict the mechanical properties of the bulk adhesive used in the fabrication of the adhesive joint. SEM fractographs of both the bulk adhesive specimen and the debonded joints are examined and the modes of failure presented.     

Ultrasonic evaluation of adhesive bond degradation by detection of the onset of nonlinear behavior

Quantitative nondestructive evaluation (QNDE) of the degradation of adhesive bonds remains one of the most challenging problems in QNDE. The objective of this research was to approach this problem by the detection of nonlinearity due to bond deterioration. The paper starts with experimental observations of the reflection of ultrasonic signals by adhesive bonds. The specimens for these tests had been subjected to cyclic loading which was expected to cause bond deterioration. Differences in the reflections could, however, be observed only if the adhesive bonds were subjected to static loads simultaneously with the ultrasonic testing. The higher the number of fatigue cycles, the lower the required load to display the differences in signals with unloaded cases. The second part of the paper presents a theoretical explanation of these ultrasonic measurements based on the postulate of nonlinear stress-strain behavior of the deteriorated bond. The ultrasonic tests provide the slope of the stress-strain curve and the results can therefore be used to determine the deviation of the stress-strain curve from linear behavior. For the higher numbers of fatigue cycles, this deviation, which is indicative of bond deterioration, starts at smaller load values.     

On the fatigue durability of clad 7075-T6 aluminium alloy bonded joints representative of aircraft repair

The fatigue durability of bonded joints representative of repairs to aircraft structure with and without the presence of a clad layer was investigated by testing aluminium alloy 7075-T6 double lap shear joint specimens. This was done by changing the bonding interface of the outer strap. The joint geometry, central adherend material, adhesive and surface preparation method were all kept the same. On two of the specimen types, the strap material was clad 7075-T6, with one type fabricated with the clad layer left on prior to surface preparation, and the other with the clad layer removed. On the last specimen type, the strap material was unclad 7075-T6. The test results showed that the fatigue durability was lowest when the clad layer was left in-situ, followed closely by those with the clad layer removed. The unclad specimens achieved a fatigue life one order of magnitude greater than those with the clad layer physically removed. Under constant amplitude loading, adhesive fatigue cracking was observed at the location of peak load transfer, which progressed to the interface. Analysis showed that the cracking caused a substrate stress concentration which may have caused the clad fracture. Further analysis, supported by test observations, showed that once a small notch had formed at the interface, damage progression through the outer strap was rapid.     

Fatigue Analysis of Adhesive Joints Under Vibration Loading

Adhesively bonded joints have been used extensively for many structural applications. However, one disadvantage usually limiting the service life of adhesive joints is the relatively low strength for peel loading, especially under dynamic cyclic loading such as impulsive or vibrational forces. Moreover, accurately predicting the fatigue life of bonded joints is still quite challenging. In this study, a combined experimental–numerical approach was developed to characterize the effect of the cyclic-vibration-peel (CVP) loading on adhesively bonded joints. A damage factor is introduced into the traction-separation response of the cohesive zone model (CZM) and a finite element damage model is developed to evaluate the degradation process in the adhesive layer. With this model, the adhesive layer stress states before and after being exposed to various CVP loading cycles are investigated, which reveals that the fatigue effect of the CVP loading starts first in the regions close to the edges of the adhesive layer. A good correlation is achieved when comparing the simulation results to the experimental data, which verifies the feasibility of using the proposed model to predict the fatigue life of adhesively bonded joints under the CVP type of loading.     

An innovative approach to fatigue disbond propagation in adhesive joints

An innovative approach to characterize the resistance of adhesively bonded joints to fatigue disbond propagation (FDP) is presented. A constitutive equation, known as the modified crack layer (MCL) model, is employed to extract parameters characteristic of the adhesive joint's resistance to FDP. These parameters are γ', the specific energy of damage, which reflects the fatigue disbond resistance of the adhesive joint and the dissipative characteristic of the joints, β'. Stress-controlled tension-tension fatigue experiments were conducted on lap joints fabricated from aircraft grade aluminum 2024-T3 and 3M structural adhesive. The disbond length was measured periodically along the edges of the bonded area at the four corners and the corresponding number of cycles was recorded. This is in order to calculate the disbond growth rate. The hysteresis loop was also recorded for each measurement from which both the energy release rate, J*, and the change in work, W_i, were determined. It was found that the proposed model describes the behavior of the adhesively bonded joints over the entire range of the energy release rate. Thus, the proposed model can provide a basis upon which the relationships between the microstructure and/or the processing conditions and the resistance of adhesively bonded joints to FDP can be constructed. Such relationships can guide the development of adhesively bonded joints with superior resistance to debonding and should aid in their lifetime assessment.     

Development of adhesive bonds with reduced fracture strength as NDE benchmarks

A method for producing adhesively bonded aluminum joints with a predictable loss of fracture strength was developed and evaluated. The method uses an open-faced specimen geometry and a humid high-temperature environment to promote adhesive degradation. The rate of degradation was greatly increased over previous accelerated degradation schemes through the use of MgSO₄ as a contaminant. The contaminant was applied as an aerosol in a purpose-built duct having a controlled airflow. Specimens were prepared and subjected to accelerated aging under a variety of conditions and then fractured using a DCB loading jig. It was found that the contaminant surface concentration was a strong determinant of the fracture strength after hot-wet aging. Exposure to the hot-wet environment was shown to have little effect beyond an initial threshold. Standard ultrasonic imaging techniques were incapable of differentiating between fresh and hot-wet aged specimens, in spite of significant differences in the fracture strength. This is consistent with the hypothesis that the approach produced specimens that simulated the effects of environmental attack, since standard ultrasonic methods, such as those used in the present study, cannot detect such losses of fracture strength in the absence of any delamination between adhesive and adherend. FESEM and EDX analysis of the fracture surfaces showed residual aluminum, suggesting an intra-oxide locus of failure consistent with other accelerated degradation methods. The technique can be used to generate adhesive joint specimens to aid the development of ultrasonic methods capable of detecting the loss of fracture strength associated with environmental degradation.     

Nondestructive evaluation methods for damage assessment in fiber‐metal laminates

The Structures, Materials and Propulsion Laboratory of the NRC Institute for Aerospace Research (IAR) is engaged in a collaborative project with Bombardier Aerospace. The main objective of the project is to evaluate the potential of applying fiber‐metal laminates (FML) to aircraft types manufactured by Bombardier. As a part of this project, nondestructive evaluation (NDE) procedures have been developed and used at IAR to determine the extent of damage caused by impact, corrosion and fatigue loads in a commercial FML material (GLARE). X‐rays using radioopaque fluids as well as conventional and air‐coupled ultrasonic and eddy current methods have been investigated. This report describes the NDE procedures employed at IAR to assess damage in FML and provides examples of the results obtained utilizing each of the inspection methods and the damage types investigated. Also, the ability of the investigated NDE methods to determine damage size and the accuracy of damage measurements is discussed.     

Fracture and fatigue behavior of scrim cloth adhesively bonded joints with and without rivet holes

_—Tensile and fatigue disbond propagation studies on scrim cloth structural adhesive lap joints without and with rivet holes were performed. The geometry of the rivet holes is similar to that in a fuselage part of an aircraft. The joints were cycled in tension-tension fatigue at a frequency of 3 Hz and a maximum load, below the linear limit of the joint, which was obtained from the tensile tests of similar joints. The disbond length at each corner of the joint was viewed using a travelling optical microscope attached to a video camera. It was found that the static-tensile behavior of both types of joints (without and with rivet holes) consists of three stages: a linear stage followed by a region of increased non-linearity and then a 'yield' region. It is within this yield region that the rivet holes affect the strength of the joint. Stress analysis of the disbond problem under static loading revealed a strong mixed mode between the opening and shear mode stress intensity factors for both types of joints. The fatigue disbond kinetics of adhesively bonded joints without and with rivet holes were found to display an S-shaped curve with three stages of the disbond propagation rate. Failure analysis of the fatigue failed joints (without and with rivet holes) revealed three distinct regions on each half of the failed joint: an interfacial region with bare metal, a cohesive region, and an interfacial region with the adhesive adhered to the substrate. Scanning electron microscopic analysis of the disbond surface showed that the cohesive region of the fatigue fractured joints is more tortuous compared with the statically failed joints.     

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.     

Experimental study of the influence of adhesive reinforcement in lap joints for composite structures subjected to mechanical loads

The paper deals with experimental investigations on reinforcing the adhesive in single lap joints subjected to mechanical loads such as tensile, bending, impact and fatigue. The adhesive used for bonding was an epoxy reinforced with unidirectional and chopped glass fibres as well as micro-glass powder. The adherends were glass reinforced composite laminates. The bonding surfaces were prepared before joining. In the case of unidirectional fibres in the adhesive region, the fibre orientations considered were 0°, 45° and 90°. The volume fraction of fibres in the adhesive layer in all the cases was 30%. The volume fractions of micro-glass powder were 20%, 30% and 40%. The tensile, bending, impact and fatigue tests on the prepared specimens were conducted according to ASTM standards. The results show that except the 90° unidirectional orientation, reinforcing the adhesive with glass fibres or powder increases the joint strength. The use of volume fraction of 30% of micro-glass powder gave the best performance in the above loading conditions. The fatigue life increased by 125%, the ultimate joint strength in tension increased by 72%, the bending ultimate joint strength increased by 112% and the impact joint strength increased by 63%. The microstructure of the debonded area was examined and three modes of failure could be observed namely cohesive failure, light fibre-tear failure and thin layer cohesive failure.     

Impact of multiwall carbon nanotubes on the fatigue strength of adhesive joints

This paper presents the results of research undertaken to determine the possibility of improving the fatigue properties of peel-loaded adhesive joints by dispersing multiwall carbon nanotubes (MWCNTs) into epoxy-based adhesives. The fatigue strength tests were carried out on an electromagnetic inductor with the resonance frequency of the adhesively bonded joint specimen. The tests were conducted for three types of epoxy adhesives whose properties were modified through the introduction of multiwalled carbon nanotubes, into their structure. Carbon nanotubes were synthesized by means of the Chemical Vapour Deposition (CVD) method with Fe-Co catalysts. A quantity of 1 wt.% of the dried material was dispersed into the epoxy adhesives. The results of the fatigue strength tests revealed a significant improvement of the fatigue lifetime of adhesive joints due to MWCNT introduction as filler for epoxy adhesives. In the case of the Epidian 57/PAC adhesive composition, a more than twofold increase in the fatigue lifetime was obtained (an increase of 106.8%). For the Bison Epoxy adhesive composition, the fatigue lifetime increased by 69.3%. The fatigue strength for the best result increased by about 13%.     

Fatigue crack initiation and damage characterization in Brazilian test specimens for adhesive joints

The present study is focused on the fatigue failure initiation at bimaterial corners by means of a configuration based on the Brazilian disc specimens. These specimens were previously used for the generalized fracture toughness determination and prediction of failure in adhesive joints, carried out under static compressive loading. Under static loading, local yielding effects might affect the asymptotic two-dimensional linear elastic stress representation under consideration. Fatigue loading avoids this fact due to the lower load levels used. The present tests were performed using load control; video microscopy and still cameras were used for monitoring initiation and crack growth. The fatigue tests were halted periodically and images of the corner were taken where fatigue damage was anticipated. Damage initiation and subsequent crack growth were observed in some specimens, especially in those which presented brittle failure under static and fatigue tests. These analyses allowed the characterization of damage initiation for a typical bimaterial corner that can be found in composite to aluminium adhesive lap joints.     

An investigation into the crack initiation and propagation behaviour of bonded single-lap joints using backface strain

In this paper, the backface strain (BFS) measurement technique is used to characterise fatigue damage in single-lap adhesive joints subjected to constant amplitude fatigue loading. Different regions in the BFS plots are correlated with damage in the joints through microscopic characterisation of damage and cracking in partially fatigued joints and comparison with 3D finite element analysis (FEA) of various crack growth scenarios. Crack initiation domination was found at lower fatigue loads whereas crack propagation dominated at higher fatigue loads. Using the BFS and fatigue life measurement results, a simple predictive model is proposed which divides the fatigue lifetime into different regions depending upon the fatigue load. The model can be used with experimental BFS measurements to determine the residual life of the joint in different regions of damage progression during the fatigue life.     

Effects of environment and surface coatings on the fatigue properties of polystyrene

The influence of surface condition, environmental media and surface coatings on the fatigue lifetime of polystyrene specimens has been explored. It is shown that surface flaws such as machining marks, are much more detrimental to fatigue lifetime than to static strength. The effect of alcohols on fatigue lifetime is primarily one of plasticization rather than of molecular size and mobility. Hence n-butanol is a more aggressive environment for polystyrene than is methanol. For a wide variety of organic media, a fairly good correlation was found between fatigue lifetime and solubility parameter. Highly polar media, like glycerol and water, are shown to be favourable media, rather than aggressive ones, in that they increase average fatigue lifetimes of polystyrene specimens by about one decade. It is suggested that any media that inhibit or delay crazing, either by increasing surface energy or by blunting flaws and reducing stress concentration, should also be beneficial to fatigue performance. A surface coating that performs this latter function is a 600 molecular weight polystyrene oligomer. It is shown that application of this compatible, viscous coating to polystyrene specimens increases the average fatigue life, for both polished and unpolished specimens by a decade or more.     

Determination of the impact tensile strength of structural adhesive butt joints with a modified split Hopkinson pressure bar

The impact tensile strength of structural adhesive butt joints was determined with a modified split Hopkinson pressure bar using hat-shaped specimens. A typical two-part structural epoxy adhesive (Scotch weld^® DP-460) and two different adherend materials (Al alloy 7075-T6 and commercially pure titanium) were used in the adhesion tests. The impact tensile strength of adhesive butt joints with similar adherends was evaluated from the peak value of the applied tensile stress history. The corresponding static tensile strengths were measured on an Instron testing machine using joint specimens of the same geometry as those used in the impact tests. An axisymmetric finite element analysis was performed to investigate the static elastic stress distributions in the adhesive layer of the joint specimens. The effects of loading rate, adherend material and adhesive thickness on the joint tensile strength were examined. The joint tensile strength was clearly observed to increase with the loading rate up to an order of 10⁶ MPa/s, and decrease gradually with the adhesive thickness up to nearly 180 μm, depending on the adherend materials used. The loading rate dependence of the tensile strength was herein discussed in terms of the dominant failure modes in the joint specimens after static and impact testing.     

Failure behavior of adhesively bonded joints with a rubber modified epoxy adhesive under tensile-shear combined cyclic loading

Rubber-modified epoxy adhesives are used widely as structural adhesive owing to their properties of high fracture toughness. In many cases, these adhesively bonded joints are exposed to cyclic loading. Generally, the rubber modification decreases the static and fatigue strength of bulk adhesive without flaw. Hence, it is necessary to investigate the effect of rubber-modification on the fatigue strength of adhesively bonded joints, where industrial adhesively bonded joints usually have combined stress condition of normal and shear stresses in the adhesive layer. Therefore, it is necessary to investigate the effect of rubber-modification on the fatigue strength under combined cyclic stress conditions. Adhesively bonded butt and scarf joints provide considerably uniform normal and shear stresses in the adhesive layer except in the vicinity of the free end, where normal to shear stress ratio of these joints can cover the stress combination ratio in the adhesive layers of most adhesively bonded joints in industrial applications.

In this study, to investigate the effect of rubber modification on fatigue strength with various combined stress conditions in the adhesive layers, fatigue tests were conducted for adhesively bonded butt and scarf joints bonded with rubber modified and unmodified epoxy adhesives, wherein damage evolution in the adhesive layer was evaluated by monitoring strain the adhesive layer and the stress triaxiality parameter was used for evaluating combined stress conditions in the adhesive layer. The main experimental results are as follows: S–N characteristics of these joints showed that the maximum principal stress at the endurance limit indicated nearly constant values independent of combined stress conditions, furthermore the maximum principal stress at the endurance limit for the unmodified adhesive were nearly equal to that for the rubber modified adhesive. From the damage evolution behavior, it was observed that the initiation of the damage evolution shifted to early stage of the fatigue life with decreasing stress triaxiality in the adhesive layer, and the rubber modification accelerated the damage evolution under low stress triaxiality conditions in the adhesive layer.     

Mode I characterization of toughened epoxy adhesive joints under shock-wave loading

With the advances in adhesive technology, the use of structural adhesive joints has extended to the broader engineering field as an alternative to traditional joining methods such as bolting, riveting, and welding. Therefore, characterization the adhesive joints under different loading and environmental conditions has been becoming more significant in designing adhesives joints for an engineering application. Since most of the polymer-based adhesives have non-linear mechanical behavior and loading rate sensitivity caused by their viscoelastic properties, testing adhesive joints under quasi-static loading cannot give adequate information to predict the response of adhesive joints to high loading conditions. It is therefore imperative to characterize the adhesive joints under high loading rates in order to integrate them into the applications that require high impact resistance. This study focused on the bending (mode I) characterization of adhesive joints under shock-wave loading generated by a large-scale shock tube. A specially designed adhesive joint that transfers the shock-wave loading to the bond in the mode-I form was designed, fabricated and tested. A series of shock-wave loading experiments were carried out with two different adhesive joints: aluminum-epoxy and steel-epoxy and their performances were compared. In addition to the experimental work, an FEM parametric study by an inverse problem-solving technique was used to estimate the mechanical properties of adhesive in both adhesive joints under different shock wave loading conditions. This technique also allowed to estimate the energy absorption capabilities of aluminum-epoxy and steel-epoxy joints.     

Effect of notch size on the fatigue damage behaviour of toughened epoxy adhesive specimens

ABSTRACT

In the present work the influence of notch size on the fatigue damage behaviour of toughened epoxy adhesive specimens is investigated. Notched and un-notched bulk adhesive specimens were fatigue tested at room temperature under tension-tension cyclic loading at a stress ratio of 0.1. The investigation was based on the analysis of fatigue life (SN) and stiffness degradation curves, which were correlated with notch size and applied stress. Finite element analysis (FEA) was carried out in order to evaluate the notch-dependent stress concentrations. Fatigue results evidenced a reduction of lifespan with increased applied stress amplitude and a possible relationship between the inverse slope of SN curves and notch size. Most notched samples exhibited lower fatigue strength in comparison to un-notched, except in the low cycle fatigue range where un-notched and notched samples had similar fatigue strength. Stiffness degradation showed a correlation with applied stress, i.e. an increase in applied stress was accompanied by faster and stronger degradation. For higher loads, un-notched and 0.2 mm notch samples presented greater stiffness degradation prior to failure than other notched samples.