Effect of Adhesive Ductility on Cyclic Debond Mechanism in Composite-to-Composite Bonded Joints

An investigation of an adhesively bonded composite joint with a brittle adhesive was conducted to characterize both the static and fatigue debond growth mechanism under mode I and mixed mode I-II loadings. The bonded system consisted of graphite/epoxy adherends bonded with FM-400 adhesive. Two specimen types were tested: (1) a double-cantilever-beam specimen for mode I loading and (2) a cracked-lap-shear specimen for mixed mode I-II loading. In all specimens tested, failure occurred in the form of debond growth either in a cohesive or adhesive manner. The total strain-energy-release rate is not the criterion for cohesive debond growth under static and fatigue loading in the birttle adhesive as observed in previous studies with the ductile adhesives. Furthermore, the relative fatigue resistance and threshold value of cyclic debond growth in terms of its static fracture strength is higher in the brittle adhesive than its counterpart in the ductile adhesive.     

Effect of Adherend Thickness and Mixed Mode Loading on Debond Growth in Adhesively Bonded Composite Joints

Symmetric and unsymmetric double cantilever beam (DCB) specimens were tested and analyzed to assess the effect of (1) adherend thickness and (2) a predominantly mode I mixed mode loading on cyclic debond growth and static fracture toughness. The specimens were made of unidirectional composite (T300/5208) adherends bonded together with EC3445 structural adhesive. The thickness was 8, 16 or 24 plies. The experimental results indicated that the static fracture toughness increases and the cyclic debond growth rate decreases with increasing adherend thickness. This behavior was related to the length of the plastic zone ahead of the debond tip. For the symmetric DCB specimens, it was further found that displacement control tests resulted in higher debond growth rates than did load control tests. While the symmetric DCB tests always resulted in cohesive failures in the bondline, the unsymmetric DCB tests resulted in the debond growing into the thinner adherend and the damage progressing as delamination in that adherend. This behavior resulted in much lower fracture toughness and damage growth rates than found in the symmetric DCB tests.     

Bonded joints with through-thickness adhesive stresses – reinforcing the F/A-18 Y470.5 bulkhead

A bonded composite reinforcement is being developed for the Royal Australian Air Force F/A-18 Y470.5 bulkhead crotch region. Due to the curvature of the surface in this area any bonded reinforcement was predicted to have high through-thickness stresses present in the adhesive under load. A representative specimen, the “curved beam specimen” (CBS) was designed using finite element analysis to identify the stress state in the joint and determine the through-thickness stresses in the adhesive. The specimen was then mechanically tested to failure under static loads and also under spectrum fatigue loading to determine the viability of the repair. The data from the above program was used to determine an estimate for the through-thickness stress design allowable for the structural film adhesive Cytec FM73. It is suggested that the CBS will prove to be a suitable specimen for providing generic data on the through-thickness static and fatigue strength of adhesives.     

Simulation of Mixed-Mode I/II Fatigue Crack Propagation in Adhesive Joints with a Modified Cohesive Zone Model

A model to predict fatigue crack growth in bonded joints under mixed mode I/II conditions is developed in this work. The model is implemented in the finite element software ABAQUS using the related USDFLD subroutine. The present model is based on the cohesive zone (CZ) concept, where damage develops according to the value of the opening/sliding at the bondline under static loading, and according to a cyclic damage accumulation law under fatigue loading. The damage accumulation law is obtained by distributing the cyclic crack area increment over the process zone ahead of the crack tip, where the cyclic crack area increment is calculated according to a Paris-like law that relates the crack growth rate to the applied loading. In this way, the experimental crack growth rate is related directly to damage evolution in the cohesive zone, i.e., no additional parameters have to be tuned besides the quasi-static cohesive zone parameters.     

Time Dependent Crack Growth and Loading Rate Effects on Interfacial and Cohesive Fracture of Adhesive Joints

Double cantilever beam fracture specimens were used to investigate rate dependent failures of model epoxy/steel adhesively bonded systems. Quasi-static tests exhibited time dependent crack growth and the maximum fracture energies consistently decreased with debond length for constant crosshead rate loading. It was also possible to cause debonding to switch between interfacial and cohesive failure modes by simply altering the loading rate. These rate dependent observations were characterized using the concepts of fracture mechanics. The time rate of change of the strain energy release rate, dG/dt, is introduced to model and predict failure properties of different adhesive systems over a range of testing rates. An emphasis is placed on the interfacial failure process and how rate dependent interfacial properties can lead to cohesive failures in the same adhesive system. Specific applications of the resulting model are presented and found to be in good agreement when compared with the experimental data. Finally, a failure envelope is identified which may be useful in predicting whether failures will be interfacial or cohesive depending on the rate of testing for the model adhesive systems.     

Similarity Concepts in the Fatigue Fracture of Adhesively Bonded Joints

Cyclic debond data obtained from fatigue testing of four different specimen geometries having the same adhesive is considered. Fatigue properties of the adhesive are characterized in terms of linear elastic fracture mechanics concepts whereby debond growth rates are correlated to appropriate mixed mode fracture parameters. Stress analyses of the four specimens under maximum load indicate that in most cases inclusion of geometric nonlinearities is required for the determination of the fracture parameters. For three of the specimens considered, the debond growth laws based on total energy release rate as correlating mixed-mode fracture parameter were found to be similar. A number of potential reasons for the lack of similarity in debond growth laws in all four specimens are explored.     

A Fracture Mechanics Approach to Characterizing Cyclic Debonding of Varied Thickness Adhesive Joints to Electroprimed Steel Surfaces

Applications of adhesive bonding for automotive structures have been increasing in recent years due to improvements in the types of adhesives available and in improved knowledge of bonding procedures. Consequently, there exists a demand for design techniques to assess the influence of bondline thickness on adhesive joint strength. One design approach currently being used is based on limiting shear stresses in the adhesive while designing to eliminate peel stresses. Another design approach is based on fracture mechanics and accounts for shear and peel stresses and both static and fatigue modes of failure. The present study applies fracture mechanics to investigate the mixed-mode response of cracked-lap-shear (CLS) joints bonded with unprimed and electroprimed steel surfaces. Three bondline thicknesses equal to 0.254, 0.813, and 1.27 mm were evaluated for unprimed and primed bondlines. For the experimental portion of the study, debond growth rates (da/dN) were measured using a remote imaging system over a range of applied cyclic loads. Corresponding changes in the strain release rates (ΔG) were calculated, through finite element analyses, as a function of debond length and applied load level. The computations for ΔG applied a finite element formulation to determine both the peel component, ΔG_i, and the shear component, ΔG_ii. When computed ΔG values were plotted against the measured debond growth rates, da/dN, the results showed a power law relationship which characterizes the debond behavior of a given material system and bondline thickness.     

Analysis and design of adhesively bonded joints for fatigue and fracture loading: a fracture-mechanics approach

An experimental–computational fracture-mechanics approach for the analysis and design of structural adhesive joints under static loading is demonstrated by predicting the ultimate fracture load of cracked lap shear and single lap shear aluminum and steel joints bonded using a highly toughened epoxy adhesive. The predictions are then compared with measured values. The effects of spew fillet, adhesive thickness, and surface roughness on the quasi-static strength of the joints are also discussed. This fracture-mechanics approach is extended to characterize the fatigue threshold and crack growth behavior of a toughened epoxy adhesive system for design purposes. The effects of the mode ratio of loading, adhesive thickness, substrate modulus, spew fillet, and surface roughness on the fatigue threshold and crack growth rates are considered. A finite element model is developed to both explain the experimental results and to predict how a change in an adhesive system affects the fatigue performance of the bonded joint.     

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.     

Fatigue analysis of composite bonded repairs

Abstract

The present work intends to describe all procedures developed in order to predict the fatigue/fracture behaviour of single-strap repairs of carbon-epoxy composites. The main goal is to validate a mixed-mode I + II cohesive zone model for high-cycle fatigue based on the modified Paris law. A preliminary static fracture characterisation in mode I, mode II and mixed-mode I + II is necessary in order to achieve the static energetic criterion describing fracture of the bonded joint. Subsequently, the same tests were carried out under high-cycle fatigue loading in order to determine the evolution of the modified Paris law parameters as function of mode ratio. These fatigue/fracture characterisation tests were also used to validate the cohesive mixed-mode I + II zone model appropriate for high-cycle fatigue. The model was then used to predict fatigue life of the single-strap repairs and revealed good performance when compared with experimental results. Finally, the model was utilised to assess the influence of specimen geometry on the fatigue life of these structural repairs. It was concluded that such type of models can be considered appealing tools concerning the optimisation of repaired structures fatigue life.     

Fatigue enhancement of concrete beam with ECC layer

The pseudo strain-hardening behavior of Engineered Cementitious Composites (ECC) is a desirable characteristic for it to replace concrete to suppress brittle failure. This widespread use of ECC in the industry is, however, limited by its high cost. To achieve higher performance/cost, ECC can be strategically applied in parts of a structure that is under relatively high stress and strain. In this paper, layered ECC-concrete beams subjected to static and fatigue flexural loads were investigated by experiments. The static test results showed that the application of a layer of ECC on the tensile side of a flexural beam increased its flexural strength and the degree of improvement increased with the thickness of ECC applied. In addition, the layer of ECC enhanced the ductility of the beam and the failure mode changed from brittle to ductile. Under four-point cyclic loading, the ECC layer significantly improved the fatigue life of the beam. Moreover, in comparison to plain concrete beams, layered ECC beams could sustain fatigue loading at a larger deflection without failure. The great improvement in fatigue performance was attributed to the effectiveness of ECC in controlling the growth of small cracks. The experimental findings reflect the feasibility of using ECC strategically in critical locations for the control of fatigue crack growth.     

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.     

Influence of mechanical surface treatment on fatigue life of bonded joints

Adhesively bonded joints can support a longer fatigue life if compared to conventional joining techniques, provided that a set of requirements is fulfilled. One of the most important requirements is the mechanical preparation of the bonded joint surface, which improves the joint interface adhesion. The aim of this work is to investigate the influence of surface roughness of mild steel substrates on fatigue behavior in adhesive bonded plates. To accomplish this objective, three different surface treatments were used on A36 steel substrate specimens, namely sand blasting, grit blasting, and bristle blasting. Bonded plate specimens, using end-notched flexure format, with a thin adhesive epoxy layer were manufactured and tested, under mode II loading condition, in both static and dynamic tests. The results confirm the importance of surface treatment of the substrate on the fatigue life, confirming that adhesively bonded joints have significant performance differences when subjected to static and dynamic loadings.     

An investigation of fatigue failure prediction of adhesively bonded metal/metal joints

A fracture mechanics-based model for fatigue failure prediction of adhesive joints has been applied in this work. The model is based on the integration of the kinetic law of evolution of defects originated at stress concentrations within the joint. Final failure can be either brittle (fracture toughness-driven) or ductile (tensile/shear strength-driven) depending on the adhesive. The model has been validated against experiments conducted on single-lap shear joints bonded with a structural adhesive. Three different kinds of adhesives, namely a modified methacrylate, a one-part epoxy and a two-part epoxy supplied by Henkel, have been considered and three different overlap lengths have been tested. Fracture toughness and fatigue crack growth properties of the adhesives have been determined with mode I tests. The number of cycles to failure has been successfully predicted in several cases. It is interesting to notice that in the case of joints loaded at the same average shear stress, the shorter the joint, the longer the duration. This fact is also captured by the model.     

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.     

Evaluation of different modelling conditions in the cohesive zone analysis of single-lap bonded joints

Cohesive Zone Models (CZM) are widely used for the strength prediction of adhesive joints. Different simulation conditions, such as damage initiation and growth criteria, are available for use in CZM analyses to provide the mixed-mode behaviour. Thus, it is highly relevant to understand in detail their influence on the simulations’ outcome. This work studies the influence of different conditions used in CZM simulations to model a thin adhesive layer in single-lap joints (SLJ) under a tensile loading, for an estimation of their influence on the strength prediction under diverse geometrical and material conditions. Validation with experimental data is considered. Adhesives ranging from brittle to highly ductile and overlap lengths (L_O) between 12.5 and 50 mm were considered. Different studies were considered: Variation of the elastic stiffness of the cohesive laws, different mesh refinements, study of the element type, and evaluation of several damage initiation and growth criteria. The analysis carried out in this work confirmed the known suitability of CZM for static strength prediction of bonded joints and pointed out the best set of numerical conditions for this purpose. Inaccurate results can be obtained if the choice of the modelling conditions is not the most suitable for the problem.     

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.     

Fatigue and Durability of Silane-Bonded Epoxy/Glass Interfaces

Fatigue (slow) crack growth in epoxy/glass interfaces bonded with the silane coupling agent 3-aminopropyltriethoxysilane was studied under static and cyclic loading at 23°C, 95% RH using the double cleavage drilled compression test. Crack growth rates under cyclic loading were significantly greater than under static loading, in contrast to crack growth rate results in monolithic glass. After aging up to 34 h at 94°C in distilled water, the silane-bonded epoxy/glass specimens exhibited somewhat greater resistance to fatigue crack growth than the unaged samples; however, after aging at 98°C in distilled water and at 70°C in an aqueous KOH solution at pH 10, crack growth became cohesive and exhibited fractal behavior. Mechanisms for fatigue crack growth at silane-bonded epoxy/glass interfaces are proposed.     

Fatigue and Durability of Silane-Bonded Epoxy/Glass Interfaces

Fatigue (slow) crack growth in epoxy/glass interfaces bonded with the silane coupling agent 3-aminopropyltriethoxysilane was studied under static and cyclic loading at 23°C, 95% RH using the double cleavage drilled compression test. Crack growth rates under cyclic loading were significantly greater than under static loading, in contrast to crack growth rate results in monolithic glass. After aging up to 34 h at 94°C in distilled water, the silane-bonded epoxy/glass specimens exhibited somewhat greater resistance to fatigue crack growth than the unaged samples; however, after aging at 98°C in distilled water and at 70°C in an aqueous KOH solution at pH 10, crack growth became cohesive and exhibited fractal behavior. Mechanisms for fatigue crack growth at silane-bonded epoxy/glass interfaces are proposed.     

Effect of moisture on pure mode I and II fracture behaviour of composite bonded joints

The effect of moisture on the fracture properties of composite bonded joints under pure mode I and pure mode II was analysed in this work. The double cantilever beam and end notched flexure tests were used for mode I and mode II fracture characterisation, respectively. Three different moisture conditions (55% and 75% of relative humidity (RH) and immersion in distilled water (IW)) were tested to assess its influence on the fracture behaviour under both pure loading modes. It was verified that fracture energy is drastically affected for the immersion in water in both loading modes. A cohesive zone model was also used to estimate the influence of RH on the cohesive parameters defining the law that mimics accurately the fracture process for each case. It was concluded that alterations on the cohesive laws reflect an increase of material brittle behaviour with the increase of the moisture uptake.