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.     

Effect of vibration fatigue on modal properties of single lap adhesive joints

As most existing studies focus on developing models and theories describing the static strength of adhesive joints as a function of the fatigue loading, there is a lack of understanding on how the fatigue of the adhesive joint affects dynamic modal properties of the bonded structure. In applications such as automobile components, modal properties are critical in determining their dynamic performances. To investigate the relationship between modal properties of single lap joints (SLJs) and the cyclic-vibration-peel loading, this study first carries out vibration fatigue tests and subsequent modal response measurements using steel–aluminum SLJ specimens. It is experimentally demonstrated that modal frequencies of the SLJ structure tend to decrease with increasing vibration fatigue cycles. Furthermore, it is also shown that this trend is related to the fatigue characteristics of the adhesive layer. The fatigue degradation effects of Young's modulus and contact area between the adhesive and the adherends on modal frequencies are then investigated using a finite element model. Simulation results reveal that dramatic reductions in modulus and contact area values are required to result in the modal frequency shifting observed in experiments, which may not be always realistic. Although the findings in this study are informative, more research effort is needed to further identify the critical reason(s) for the experimental trend of decreasing modal frequencies with increasing vibration fatigue cycles.     

Static and fatigue behavior of adhesive joints in SMC-SMC composites

This paper discusses the static and fatigue behavior of adhesively bonded single lap joints in SMC-SMC composites. Effects of lap length and adhesive thickness on the static and fatigue strength of SMC-SMC adhesive joints are studied. Effects of SMC surface preparation and test speed on the joint performance are evaluated. Finally, the effect of water exposure on the joint durability is also investigated. Results show that the static behavior of adhesive joints in SMC-SMC composites is significantly influenced by the lap length and adhesive thickness. With an increase in lap length from 12.7 mm to 38.1 mm, the joint failure load increases by 37%. The joint failure load also increases with the adhesive thickness, but it reaches a maximum at an adhesive thickness of 0.33 mm and then decreases. However, lap length and adhesive thickness have negligible effect on the ratio of fatigue strength to static strength. The fatigue strength at 10⁶ cycles is approximately 50% to 54% of the static strength for various adhesive thicknesses and lap lengths investigated in this study. Adhesive failure, fiber tear or combination of these two failure modes are observed during both static and fatigue tests.     

Fatigue and quasi-static analysis of a new type of surface preparation used for the CFRP repair of steel offshore structures

ABSTRACT

In recent years, there has been an increasing interest in repairing offshore steel structures by using adhesively joined carbon fiber reinforced polymers (CFRP). For such procedure, surface preparation plays a vital role to maintain the integrity of the joint and to ensure proper load transfer. The primary surface preparation used by the oil and gas industry is the grit blasting due to its known quality. However, the logistic required is a major drawback limiting the use of adhesively joined repairs. Other surface preparation procedures available are unable to promote proper treatment. In this paper, an alternative surface preparation methodology employing a portable machine that uses rotation and impact to treat the steel surface was evaluated by quasi-static and fatigue tests of CFRP/steel adhesively bonded using the double-lap joints. The joints were prepared using non-corroded and severely corroded steel surfaces treated by grit blasting or rotating impact machine. The corroded plate was used to evaluate the efficiency of the rotating impact machine in removing deeply penetrating oxides. Test results showed that the performance of the machine was comparable to grit blasting even for the severely corroded surface with deep pitting. Corrosion in the metallic substrate impaired the quasi-static and fatigue properties.     

Fatigue Behaviour of Adhesive Bonded Joints

This paper presents experimental results of the fatigue behaviour of adhesive bonded plastic-to-plastic joints and metal-to-plastic joints under both dynamic and static loading. The fatigue life of the joints was found to be independent of the test frequencies and humidity for the range of values tested, but dependent on the mean stress level and test temperature with greater reduction in fatigue life observed in metal-to-plastic joints at higher temperature. Empirical equations from which the fatigue life of joints could be predicted were obtained by regression analysis.     

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.     

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.     

Fatigue life uncertainty of adhesively bonded composite scarf joints – an airworthiness perspective

Adhesively bonded repairs provide a highly structurally efficient and cost-effective means of restoring residual strength to aircraft components. However, gaining airworthiness approval for bonded repairs to primary structures is a significant problem. This is largely because of the failure of current non-destructive inspection techniques to detect weak or non-durable adhesively bonded joints. Due to the presence of undetectable defects and anomalies, recent airworthiness policy ignores the contribution of adhesively bonded joints to the fatigue durability of repaired load-carrying aircraft structures. The key requirement for airworthiness is to demonstrate an acceptable low probability of repair patch disbonding during the remaining life of the structure. In order to satisfy this requirement, it is necessary to identify and control all manufacturing defects and anomalies that influence the durability of the bonded joint. In this study, a methodology has been developed to control manufacturing defects including porosity, unbonded area, and adhesive thickness and flatness variation of bond area. To evaluate the effectiveness of the developed methodology, fatigue tests were conducted, and corresponding uncertainty was analysed. It was found that these defects and anomalies have a significant influence on the fatigue life and fatigue life uncertainty of bonded joints, with minimal effect on their static strength.     

A review of finite element analysis of adhesively bonded joints

The need to design lightweight structures and the increased use of lightweight materials in industrial fields, have led to wide use of adhesive bonding. Recent work relating to finite element analysis of adhesively bonded joints is reviewed in this paper, in terms of static loading analysis, environmental behaviors, fatigue loading analysis and dynamic characteristics of the adhesively bonded joints. It is concluded that the finite element analysis of adhesively bonded joints will help future applications of adhesive bonding by allowing system parameters to be selected to give as large a process window as possible for successful joint manufacture. This will allow many different designs to be simulated in order to perform a selection of different designs before testing, which would currently take too long to perform or be prohibitively expensive in practice.     

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.     

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.     

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.     

Controlled reduced-strength epoxy-aluminium joints validated by ultrasonic and mechanical measurements

In this study a series of joint systems, consisting of aluminium substrates bonded using an epoxy adhesive, were produced. Several levels of adhesion were achieved by altering the substrate surface treatment and the curing cycle of the adhesive. The goal of this study was to produce reduced-strength epoxy-aluminium joints that could be used as reference samples for ultrasonic non-destructive testing (NDT) studies. There is clearly a continuing challenge to improve the quality of the adhesively-bonded joint inspection to ensure the durability of the bonds, to monitor repairs, and to evaluate the strength of the bonds. However, developing and qualifying innovative or advanced non-destructive testing requires an essential preliminary step: a method for repeatedly producing reduced-strength bonded test specimens must be developed. In this study, in addition to a rigorous protocol to produce bonded joints, complementary ultrasonic CSCAN were realised to validate the homogeneity of the joints and to ensure that samples met all requirements so as to be considered as reference samples. Mechanical tests were performed to evaluate the mechanical strength of each joint and Acoustic Emission (AE) was used during the tests in order to confirm the expected fracture mechanisms.     

Effect of Substrate Material on Fatigue Crack Propagation Rate of Adhesively Bonded DCB Joints

The effect of substrate material on the fatigue crack propagation rate was investigated using adhesively bonded DCB specimens with CFRP and aluminum substrates. The experimental results show that the increase in thickness of the adherend lowers the fatigue threshold, ΔG _th, and raises the crack growth parameter, n, irrespective of the substrate material, and that the crack growth parameter, n, for the aluminum joints is less than that for the CFRP joints. To elucidate the fatigue crack propagation behavior, fracture surface observation and finite element analysis have been conducted. Besides, Gurson's model is applied to the adhesive layer. SEM images show that numerous voids are formed in the fracture surface for the joints with aluminum substrate, but the growth of voids is suppressed for the joints with CFRP substrate. FEM results also show that the void area fraction for the joint with aluminum substrate is greater than that with CFRP substrate. Thus, the above experimental and numerical trends of voids correspond to the trends of the fatigue crack propagation behavior.     

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.     

Experimental Analysis of Static and Fatigue Strength Properties in Press-Fitted and Adhesively Bonded Steel–Aluminium Components

Press-fitted and adhesively bonded joints (Hybrid Joints) are increasingly used as an alternative way to traditional structural joining techniques. The main achievable benefits can be summarized in the possibility of maximizing the load transfer (torque or axial) and reducing both the weight and the stress field of the components, by taking advantage of the adhesive strength. Hybrid joints studies can be found in literature mainly on steel–steel components (Steel Hybrid Joints). The aim of this paper is to provide some relevant information on the static and fatigue strength properties in the case of steel–aluminium components (Mixed Hybrid Joints), from the experimental tests performed on a high strength, single-component adhesive which cures anaerobically. The use of the adhesive increases the press-fitted joint performances, with respect to its release force: the adhesive static shear strength is about 9 MPa, whereas the adhesive endurance limit is about 6 MPa, in presence of a stress ratio R = 0.1.     

Strength of Adhesively-Bonded Butt Joints of Tubes Subjected to Combined High-Rate Loads

The dynamic strength of adhesively-bonded joints was investigated experimentally. The strength of the bonded joints under combined high rate loading was measured using the clamped Hopkinson bar method. Tubular butt joints bonded by epoxy resin were used for the experiment. Combined stress waves of tension and torsion were applied to the specimens. The strength of the adhesively-bonded joint was determined by measuring the stress waves propagated in the load output tube of the specimen. It was found that the dynamic strength of the adhesive joints was greater than the static strength under tensile and shear load.     

Fatigue Growth of Cohesive Defects in T-Peel Joints

This paper uses 2D and 3D finite element models to predict the stresses within bonded and weld-bonded T-peel joints. Epoxy adhesive is modelled as a homogeneous layer providing a perfect bond between aluminium adherends. Knowledge of the critical tensile stresses enables the likely region of fatigue crack initiation to be predicted. The long term reliability and durability of a joint depend directly on its fatigue strength. This research elucidates the region of cohesive crack initiation, the subsequent direction of crack propagation and the relative duration of the different stages of fatigue crack growth. The various stages of embedded, surface and through-width fatigue growth of cohesive defects within a T-peel joint are compared. This establishes fatigue life from crack initiation to final joint fracture for typical bonded and weld-bonded T-peel joints.