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.     

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.     

Characterization of fracture in adhesively bonded double-lap joints

A broad finite element study was carried out to understand the stress fields and stress intensity factors behavior of cracks in adhesively bonded double-lap joints, which are representative of loading in real aerospace structures. The interaction integral method and fundamental relationships in fracture mechanics were used to determine the mixed-mode stress intensity factors and associated strain energy release rates for various cases of interest. The numerical analyses of bonded joints were also studied for various kinds of adhesives and adherends materials, joint configurations, and thickness of adhesive and different crack lengths. The finite element results obtained show that the patch materials of low stiffness, low adhesive moduli and low tapering angles are desirable for a strong double-lap joint. In the double-lap joint, the shearing-mode stress intensity factor is always larger than that of the opening-mode and both shearing and opening mode stress intensity factors increase as the crack length increases, but their amplitudes are not sensitive to adhesive thickness. Results are discussed in terms of their relationship to adhesively bonded joints design and can be used in the development of approaches aimed at using adhesive bonding and extending the lives of adhesively bonded repairs for aerospace structures.     

Piezoelectric monitoring of the reliability of adhesive joints

Since the reliability of adhesively bonded joints for composite structures is dependent on many parameters such as the shape and dimensions of joints, type of applied load, and environment, so an accurate estimation of the fatigue life of adhesively bonded joints is seldom possible, which necessitates an in-situ reliability monitoring of the joints during the operation of structures. In this study, a self-sensor method for adhesively bonded joints was devised, in which the adhesive used works as a piezoelectric material to send changing signals depending on the integrity of the joint. In order to validate the method, the piezoelectric properties of the adhesive were measured during the fatigue test. Electrically conducting adherends were used as electrodes without embedded sensors, and the adhesively bonded joint was modeled as the equivalent parallel circuit composed of electric charge and capacitance. From the investigation, it was found that the electric charge increased gradually as cracks initiated and propagated in the adhesive layer, and had its maximum value when the adhesively bonded joint failed. So it is feasible to monitor the integrity of the joint during its lifetime. Finally, a relationship between the piezoelectric property of the adhesive and crack propagation was obtained from the experimental results.     

Analysis of aircraft adhesive joints under combined thermal and mechanical cyclic loadings

The effect of thermal aging on the static and fatigue behavior of adhesively bonded aircraft joints has been investigated. The aging cycle consisted of high and low temperatures at different levels of humidity in accordance with ASTM D1183-70 test procedure C for exterior land and air conditions. Single lap joints of aluminum 7075-T6 and 3M structural adhesive prepreg were used. The static loading behavior showed no effect of thermal cycling on the load-carrying capacity of the joints. However, the joints' static deformation increased with increasing number of aging cycles. The specific energy of damage concept was used to extract parameters characteristic of the joint resistance to fatigue loading. These parameters are the specific energy of damage, y', and the energy dissipation coefficient, β'. It was found that the specific energy of damage, γ', was reduced from 35.40 kJ/m³ for joints without thermal cycling to 28.9, 27.1, and 25.9 kJ/m³ for joints subjected to 2, 4, and 6 thermal aging cycles, respectively. However, the energy dissipation coefficient, β', increased from 23 x 10^-5 for joints without thermal aging to 26.5 x 10^-5, 27.6 x 10^-5, and 28.8 x 10^-5 for joints with 2, 4, and 6 aging cycles, respectively. The decrease in the value of y' and the increase in β' indicate a loss of resistance to fatigue crack propagation. The greatest loss of fatigue resistance was encountered after the first two thermal aging cycles. While the static results showed an enhancement in the joints' fracture toughness, based on the area under the load-displacement curve, the fatigue crack propagation data showed a considerable loss in resistance due to aging. Hence this paper emphasizes that it is important to evaluate the resistance of adhesive joints to combined thermal and mechanical cycling if the joints are expected to serve in such conditions.     

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.     

Fatigue life prediction of adhesively bonded single lap joints

Better fatigue performance of adhesively bonded joints makes them suitable for most structural applications. However, predicting the service life of bonded joints accurately remains a challenge. In this present study, nonlinear computational simulations have been performed on adhesively bonded single lap ASTM-D1002 shear joint considering both geometrical and material nonlinearities to predict the fatigue life by judiciously applying the modified Coffin-Manson equation for adhesive joints. Elasto-plastic material models have been employed for both the adhesive and the adherends. The predicted life has close agreement in the high cycle fatigue (HCF) regime with empirical observations reported in the literature.     

On the use of fracture mechanics in designing a single lap adhesive joint

The design of adhesively bonded joints is a quite difficult task, due to the stress singularity that arises at the edges of the adhesive adjacent to the loaded substrate. This stress singularity makes any design approach based on elastic stress analysis inconvenient. A more convenient design tool for an adhesive joint should be based on its mode of failure. Most of the adhesive joints fail at the adhesive/adherend interface or very close to it in the adhesive layer. Therefore, a fracture theory such as linear elastic fracture mechanics (LEFM) can be used to analyse the failure of an adhesive joint. In this paper, the design of a single lap joint using a fracture mechanics parameter, i.e. the strain energy release rate (SERR), is discussed. The SERR is extracted from a finite element model using Irwin's virtual crack closure integral. A design equation relating the lap length to the adherend thickness through some design parameters is derived.     

Crack growth in adhesively bonded joints subjected to variable frequency fatigue loading

The main aim of this article is to investigate the effect of frequency on fatigue crack propagation in adhesively bonded joints. Adhesively bonded double-cantilever beam (DCB) samples were tested in fatigue at various frequencies between 0.1 and 10 Hz. The adhesive used was a toughened epoxy, and the substrates used were a carbon fibre-reinforced polymer (CFRP) and mild steel. Results showed that the crack growth per cycle increases and the fatigue threshold decreases as the test frequency decreases. The locus of failure with the CFRP adherends was predominantly in the adhesive layer, whereas the locus of failure with the steel adherends was in the interfacial region between the steel and the adhesive. The crack growth was faster, for a given strain energy release rate, and the fatigue thresholds lower for the samples with steel adherends. Tests with variable frequency loading were also carried out, and a generalised method of predicting crack growth in samples subjected to a variable frequency loading was introduced. The predicted crack growth using this method agreed well with experimental results.     

CRACK GROWTH IN ADHESIVELY BONDED JOINTS SUBJECTED TO VARIABLE FREQUENCY FATIGUE LOADING

The main aim of this article is to investigate the effect of frequency on fatigue crack propagation in adhesively bonded joints. Adhesively bonded double-cantilever beam (DCB) samples were tested in fatigue at various frequencies between 0.1 and 10 Hz. The adhesive used was a toughened epoxy, and the substrates used were a carbon fibre-reinforced polymer (CFRP) and mild steel. Results showed that the crack growth per cycle increases and the fatigue threshold decreases as the test frequency decreases. The locus of failure with the CFRP adherends was predominantly in the adhesive layer, whereas the locus of failure with the steel adherends was in the interfacial region between the steel and the adhesive. The crack growth was faster, for a given strain energy release rate, and the fatigue thresholds lower for the samples with steel adherends. Tests with variable frequency loading were also carried out, and a generalised method of predicting crack growth in samples subjected to a variable frequency loading was introduced. The predicted crack growth using this method agreed well with experimental results.     

Diagnosis criterion for damage monitoring of adhesive joints by a piezoelectric method

Adhesive joints have been widely used for fastening thin adherends because they can distribute the load over a larger area than mechanical joints, require no holes, add very little weight to the structure and have superior fatigue resistance. Since the reliability of an adhesive joint is dependent on many parameters, such as the shape of joint, type of applied load and environment, an accurate prediction of the fatigue life of adhesive joints is seldom possible, which necessitates an in situ damage monitoring of the joints during their operation. Recently, a piezoelectric method using the piezoelectric characteristics of epoxy adhesives has been successfully developed for adhesive joints because it can continuously monitor the damage of adhesively bonded structures without producing any defects induced by inserting a sensor. Therefore, in this study, the damage of adhesive joints was monitored by the piezoelectric method during torsional fatigue tests in order to develop the diagnosis criterion for damage monitoring of adhesive joints by the piezoelectric method. The diagnosis criterion was developed by analyzing damage monitoring signals under various test conditions and adopting normalized parameters.     

Fatigue Crack Growth Rates in Adhesive Joints Tested at Different Frequencies

Mode I fatigue crack growth tests were conducted on joints bonded with a filled adhesive (A) at 20 Hz and 2 Hz and on joints bonded with a filled and toughened adhesive (B) at 20 Hz, 2 Hz, 0.2 Hz and 0.02 Hz. Strain energy release rate, G, and J-integral were evaluated based on elastic and elastoplastic finite element analyses (FEA) of the joints bonded with adhesive A and B, respectively. For the configurations considered, J was found to be path-independent and did not differ much from G. The fatigue crack growth rate (FCGR), da/dN, in the joints bonded with adhesive A was relatively independent of frequency while it increased with decreasing frequency at given δ for the joints bonded with adhesive B. The fatigue processes in both adhesives involved the cracking of the filler particles and subsequent linkage of the resultant microcracks. The process zone in adhesive B is larger than that in adhesive A and it increases with decreasing frequency. It is suggested that this variation in process zone size can account for the observed fatigue behaviour. The fatigue crack growth velocity, da/dt, was also calculated for the joints bonded with adhesive B and the variation of da/dt with test frequency at given δG is much smaller than the variation in da/dN, suggesting a creep effect in the fatigue crack growth.     

An evaluation of strength wearout models for the lifetime prediction of adhesive joints subjected to variable amplitude fatigue

Almost all structural applications of adhesive joints will experience cyclic loading and in most cases this is irregular in nature, a form of loading commonly known as variable amplitude fatigue (VAF). This paper is concerned with the VAF of adhesively bonded joints and has two main parts. In the first part, results from the experimental testing of adhesively bonded single lap joints subjected to constant and variable amplitude fatigue are presented. It is seen that strength wearout of bonded joints under fatigue is non-linear and that the addition of a small number of overloads to a fatigue spectrum can greatly reduce the fatigue life. The second part of the paper looks at methods of predicting VAF. It was found that methods of predicting VAF in bonded joints based on linear damage accumulation, such as the Palmgren–Miner rule, are not appropriate and tend to over-predict fatigue life. Improved predictions of fatigue life can be made by the application of non-linear strength wearout methods with cycle mix parameters to account for load interaction effects.     

Fatigue strength estimation of adhesively bonded tongue and groove joint of thick woven composite sandwich structures using genetic algorithm approach

In the present work, the fatigue behavior of tongue and groove joints bonded by a toughened epoxy adhesive was investigated. Axial cyclic tests were performed by different design configuration conditions and the effects of design parameters were evaluated. The bonding strength of adhesives under fatigue loading is influenced by many factors such as, the length of bondline, adhesive thickness, traverse pre-stress on near the free edges of bond line and material of the joining parts. Since all these factors affect the fatigue strength of the adhesively joined parts, the effects of these parameters need to be investigated. The present paper describes the use of the stochastic search process that is the basis of a Genetic Algorithm, in developing fatigue strength estimation of adhesively bonded thick woven E-glass/vinyl ester laminates. Non-linear estimation models were developed using genetic algorithm. Developed models are validated with experimental data. Genetic Algorithm Fatigue Strength Estimation Model for Tongue and Groove Joints was developed to estimate the fatigue strength of the adhesively bonded joint. The strongest adhesively bonded joints can be achieved by selecting optimum design parameters obtained from the models. The logarithmic number of cycles was increased 2.46 times by selecting aluminum EN AW 5083 insert instead of composite insert materials. The joint fatigue strength was significantly improved by selecting appropriate design parameter values.     

Fatigue Crack Growth Rates in Adhesive Joints Tested at Different Frequencies

Mode I fatigue crack growth tests were conducted on joints bonded with a filled adhesive (A) at 20 Hz and 2 Hz and on joints bonded with a filled and toughened adhesive (B) at 20 Hz, 2 Hz, 0.2 Hz and 0.02 Hz. Strain energy release rate, G, and J-integral were evaluated based on elastic and elastoplastic finite element analyses (FEA) of the joints bonded with adhesive A and B, respectively. For the configurations considered, J was found to be path-independent and did not differ much from G. The fatigue crack growth rate (FCGR), da/dN, in the joints bonded with adhesive A was relatively independent of frequency while it increased with decreasing frequency at given δ for the joints bonded with adhesive B. The fatigue processes in both adhesives involved the cracking of the filler particles and subsequent linkage of the resultant microcracks. The process zone in adhesive B is larger than that in adhesive A and it increases with decreasing frequency. It is suggested that this variation in process zone size can account for the observed fatigue behaviour. The fatigue crack growth velocity, da/dt, was also calculated for the joints bonded with adhesive B and the variation of da/dt with test frequency at given δG is much smaller than the variation in da/dN, suggesting a creep effect in the fatigue crack growth.     

The effects of surface roughness and bond thickness on the fatigue life of adhesively bonded tubular single lap joints

Since the surface roughness of adherends greatly affects the strength of adhesively bonded joints, the effect of surface roughness on the fatigue life of adhesively bonded tubular single lap joints was investigated analytically and experimentally by a fatigue torsion test. The stiffness of the interfacial layer between the adherends and the adhesive was modelled as a normal statistical distribution function of the surface roughness of the adherends. From the investigation, it was found that the optimum surface roughness of the adherends for the fatigue strength of tubular single lap joints was dependent on the bond thickness and applied load.     

Development of a Fatigue Failure Model for the Adhesively Bonded Tubular Single Lap Joint under Dynamic Torsional Loading

The adhesively bonded tubular single lap joint shows nonlinear torque transmission capability and deformation characteristics under static torsional loading because of nonlinear properties of the adhesive. However, the dynamic or fatigue torque transmission capability can be calculated with linear, analysis because the stress-strain relation under torsional fatigue loading is linear, due to the small dynamic transmission capability compared with the static torque transmission capability.

In this paper, a failure model for the adhesively bonded tubular single lap joint under torsional fatigue loading was developed with respect to the adhesive thickness, which is the critical factor for the static torque transmission capability. Also, a design method for the adhesively bonded tubular single lap joint under torsional fatigue loading was proposed.     

The Use of a Modified Mixed Mode Bending Test for Characterization of Mixed-mode Fracture Behavior of Adhesively Bonded Metal Joints

This paper summarizes recent mixed-mode I and II fracture experiments on adhesively bonded metal joints using a modified mixed-mode bending (MMB) test fixture and double cantilever beam (DCB)-type specimens. The MMB test had been previously developed and used for mixed-mode I and II delamination testing of composite laminates, but in the present research it is adapted and modified for fracture testing of adhesively bonded joints with metallic adherends. Strain energy release rates were evaluated by the use of improved analytical models. Mixed-mode fracture behavior of AA5754-0 aluminum alloy specimens bonded with a tough one-part epoxy adhesive (Dow Automotive Betamate 4601 ® ) was characterized.     

Evaluation of mode II fatigue disbonding using Central Cut Plies specimen and distributed strain sensing technology

The lack of a widely-accepted test standard for characterizing the mode II fatigue disbond growth behavior of adhesively bonded interfaces is a challenge to the research community in terms of producing consistent and repeatable results. Typically, researchers apply the End Notch Flexure specimen, which is already used for static delamination studies. However, the needs for static and fatigue disbond growth characterization are not the same, resulting in some undesirable effects in such specimen. This study looks at a particular mode II test configuration known as the Central Cut Plies (CCP) specimen. A critical evaluation of the suitability of this specimen, including the influence of geometry, disbond measurement approaches and the stability of the disbond growth is carried out through a combination of numerical and experimental investigations. A distributed strain sensing system based on Rayleigh Backscattering provided a surface strain profile from which disbond growth rate data was obtained. A finite element model was used to verify the experimental results and determine the disbond length from the strain profiles. Results of this evaluation have shown that the CCP specimen is a promising specimen configuration for characterizing fatigue disbond growth; however, it also presents several challenges that require consideration in its application.     

In situ cure monitoring of adhesively bonded joints by dielectrometry

Since the reliability of adhesively bonded joints is much dependent on the curing status of thermosetting adhesives, the in situ cure monitoring during the cure of adhesive joints could improve the quality of adhesively bonded joints as it enables one to control the cure parameters. In this work, a dielectric method which measures the dissipation factor of the adhesive during the cure of joints and converts it into the degree of cure of the adhesive was devised. Steel adherends were used for the adhesively bonded joints because the steel adherends worked as the electrodes for the measurement of dissipation factor without additional electrodes. The relation between the dissipation factor and the degree of cure of adhesive was investigated, which could eliminate the temperature effect on the dissipation factor that is largely affected by the degree of cure and temperature of adhesive. Comparing the results obtained by the method developed with those by DSC (differential scanning calorimetry), it was found that the dissipation factor showed a trend similar to the cure rate of the adhesive.