Thick Adhesive Joints Research Articles

Healable adhesive paste development for thick adhesive joints

A new adhesive formulation is developed, enabling assisted self-healing for wind turbine rotor blade (WTRB) applications. The properties of this epoxy based formulation are compared to those of a state-of-the-art toughened epoxy adhesive (Sikapower®-830). An optimal content of 20 wt% poly(ε-caprolactone) (PCL) microparticles and 4 wt% fumed silica (NS) in a baseline epoxy fulfilled most requirements, such as sufficient rheological yield stress, adequate mechanical properties, glass transition temperature (Tg) higher than the operation temperature range and good adhesion, although the elastic modulus remained about 85 % of the desired value. In addition, 60 % healing efficiency was achieved in fractured samples after moderate heating. This approach provides new perspectives to control damage in thick bonded joints.

Combined computational-experimental investigation of residual stresses and pre-cracking in mode I behaviour of thick adhesively bonded GFRP composite joints

This paper presents a novel Finite Element (FE) simulation approach to examine the mode I fracture of thick adhesive joints used particularly in the trailing edge of the wind turbine blades. The approach involved FE models of the DCB specimens focusing on aspects overlooked in the existing literature. There has been limited investigation on residual stresses caused by thermal mismatch between composites and adhesives. Similarly, the impact of generating notches/pre-cracks in the adhesive layer during the preparation of Double Cantilever Beam (DCB) specimens on residual stresses has received minimal attention. Additionally, the Cohesive Zone Model, commonly used for simulating elastoplastic adhesives, may be inadequate due to its inability to account for the plastic deformation of the adhesive. In the present work, the pre-cracks were virtually generated in DCB FE models so that their effect on the stresses within the joint could be examined, making it a novel contribution to the field. The components were assigned with appropriate thermal expansion coefficients, and a simulation of the cool-down process was conducted to determine the thermal residual stresses. Furthermore, the Drucker-Prager plasticity criteria were used to capture the elastoplastic behaviour of adhesives in the FE simulations. Concurrently, the T-stresses were assessed through numerical investigations. For validation, experiments were conducted on DCB specimens made of two cross-ply composite laminates bonded with a ∼ 10 mm thick layer of an epoxy-based adhesive. A good agreement between computational and experimental results was observed, confirming the effectiveness and reliability of the proposed approach.

An experimental and analytical study of mode I fracture and crack kinking in thick adhesive joints

This study investigates the fracture behavior of thick adhesive joints manufactured with composite adherends and bonded with an epoxy-based structural adhesive common to the wind turbine industry. For that purpose, double cantilever beam specimens with an adhesive thickness of approximately 10 mm and different pre-crack lengths are manufactured and tested under mode I loading. Analytical approaches are compared to assess the energy release rate, including the simple beam theory, modified beam theory, compliance calibration method, and beam on an elastic and elastic-plastic foundation. In order to evaluate the applicability of the analytical approaches, an in-situ measurement method based on Digital Image Correlation is also employed to determine the energy release rate of the thick adhesive joints. The crack propagation angle is determined theoretically using the second-order crack kinking theory. A good correlation is observed between the theoretical predictions and experimental results. Furthermore, it is demonstrated that due to the T-stress, the crack tends to deviate from the middle of the joint and propagate towards the interface. By comparing different data reduction methods to evaluate the energy release rate of thick adhesive joints, recommendations for their fracture analysis are made, pinpointing the beam on an elastic and elastic-plastic foundation as the most suitable model.

New indicator for damage localization in a thick adhesive joint of a composite material used in a wind turbine blade

In this paper, a new indicator to localize fatigue damage in a fibre glass composite structure, i.e. spar cap to shear web thick adhesive joint of a wind turbine blade, is presented. This indicator is based on the effect of damping on the phase of the mode shapes of the structure. When fatigue damage occurs, damping increases in the defective area and this leads to an increase in the local energy dissipation. This non-uniformity in the energy dissipation throughout the structure causes the structure to vibrate with mode shapes whose structural elements no longer have the same phase creating complex mode shapes. A visco-elastic finite element (FE) vibration model is developed for a thick adhesive joint of a wind turbine blade. The mass, stiffness, and damping matrix extracted from the FE model are used to determine the complex mode shapes. The results show that the damaged area is located where the spatial derivative of the phase of the components of the mode shapes is minimum. Changes in the phase of mode shapes of the structural elements are strongly dependent on the location of damage. In the locations where the strain modal energy is greater, the change in the phase is also higher.

Fatigue damage and life evaluation of thick bi‐material double strap joints for use in marine applications

AbstractThe present‐day interest in the use of composite‐steel joints in primary marine structures requires an in‐depth knowledge of the fatigue performance of thick adhesive joints. This paper reports on experimental tests performed on compliant (i.e., thick and ductile) adhesive joints based on a two‐component methyl‐methacrylate adhesive in unaged and aged conditions. Bi‐material double strap joint with thick adhesive bondlines is subjected to quasi‐static and fatigue loading, during which the specimens are monitored by digital mage correlation and infrared thermography techniques to evaluate the types and extent of damage that occur in the joint during fatigue loading. The S‐N curve of the aged joint is evaluated along with its fatigue limit. It is found that the unaged specimens fail due to cohesive damage, hackles, and disbond at the adhesive‐adherend interface and the aged specimens fail due to delamination within the composite. The specimens that survived fatigue loading showed similar residual strength as specimens that were not fatigue loaded.

A generalized model for predicting stress distributions in thick adhesive joints using a higher-order displacement approach

As typical elements in lightweight applications, adhesive bonds bring many practical advantages over other joining technologies, such as their high strength-to-weight ratio, their damage-free joining process, and an over a wide area distributed load introduction into the adhesive layer. When modeling bonded joints for stress analysis or failure prediction, the adhesive layer is usually simplified to an interface, as it is very thin compared to the parts being joined. However, for thicker adhesive layers, these models no longer succeed in accurately predicting the stresses that occur, as many simplifications no longer apply. Therefore, we present a model that considers the adhesive layer as a continuum rather than an interface. Unlike previous works, we apply general energy principles enabling the determination of stresses as well as displacements throughout the adhesive layer with improved accuracy. A new, extended displacement approach with terms of higher order enables the capture of complex deformation effects that occur in thick adhesive layers. The new model is valid for various joint shapes, load cases, and material configurations and enables a quick, analytical prediction of stress and deformation distributions in the adhesive layer. For validation, we compare the results with those of previous models and finite element calculations.

The effect of the fatigue damage accumulation process on the damping and stiffness properties of adhesively bonded composite structures

Wind turbine blade spar cap to shear web adhesively bonded connections can suffer from damage at the bond-line which can propagate through the structure compromising blade integrity. This study investigates changes in the stiffness and damping of a thick adhesive joint test specimen during a fatigue test. The stiffness is calculated using an extensometer and damping is determined using vibration and thermography. Fatigue tests showed three distinct phases of damage. Firstly, transverse cracks and delamination initiate and grow with little change in stiffness observed while damping increases by 18.6% and 17.8% inferred from the vibration and thermographic methods, respectively. In the second phase, the number of transverse cracks increases reaching saturation and the stiffness reduces by 4.7%. The increase in the loss factor is 65.7% and 95.6% from the vibration and thermographic methods, respectively. In the final phase, the crack density remains constant whilst de-bonding of the joint begins and grows until failure. The loss factor increases by 111.4% and 116.9% from the vibration and thermographic methods, respectively though the stiffness shows a cumulative reduction of only 8.6%. The results show the potential for monitoring changes in damping to infer incipient damage in an adhesively bonded composite joint structure.

Review of fatigue of bulk structural adhesives and thick adhesive joints

ABSTRACT Fatigue of structural adhesives has been investigated through joints and a little number of works investigate bulk adhesive behaviour itself. Aerospace and automotive engineering focuses more on joint configuration studies, which are correlated with practical applications. Previous works showed that for thin adhesive joints, material properties measured by bulk adhesive testing and joint testing are similar despite the triaxial stress states developing in the adhesive bondlines. However, with the introduction of structural adhesives in construction industry, thicker bondlines have emerged where the bulk adhesive material dominates the joint behaviour. This review work summarises works on the fatigue of bulk structural adhesives used mainly in the construction industry investigating structural adhesives fatigue behaviour either through experiment on joints or on bulk adhesive specimens. The work focuses on thick adhesive bondlines in joints, and discusses the controversy that is over whether adhesive properties from joints or from bulk material should be used.

Characterisation of fatigue damage in a thick adhesive joint based on changes in material damping

The adhesively-bonded connections in a wind turbine blade, e.g. the spar cap to shear web joins, are key elements for the structural integrity of the blade. These joins can suffer from damage at the bond-line which can propagate through the structure and compromise the operation of the blade. In this paper, we determine the damping properties of a test specimen representative of that joining a spar cap and a shear web during a period of progressive damage. In addition to the experimental damping measurement, an analytical dynamic model based on the visco-elastic properties of the material is developed to relate the damping to the loss factor. The experimental results show that when a crack is initiated in the test specimen, the damping increases by around 5-7%. This value increases with the propagation of transverse cracks in the adhesive and reaches a value of 35% when the adhesive layer experiences crack saturation and the damping reaches 45 % before failure occurs. Although a significant change in the damping is observed, there is no significant change in the natural frequency (<1%) and by association little change in the stiffness of the test specimen.

Strength and Interface Adhesion Properties of in-Plane Shear Loaded Thick Adhesive Joint

This paper addresses the quality of the interface bonded joints in layers of timber elements. The shear performance was studied to assess the suitability of adhesive to bond timber with different thickness of glueline and timber densities. Since there is absolute test method in establishing the shear strength of the surface bonds between layers timber elements, two test methods were used namely Thick Adherend Shear Test [TAST] and Lap Shear Test. The adhesived used is Sikadur-30 and timber used are Sesendok, Bintangor and Kempas with average densities 400-600kg/m3, 600-800kg/m3 and 800-1000kg/m3 respectively. Obtained results suggest that the interface stress distribution are related to the thicknesses of adhesive and densities of timber.

Failure behavior of inclined thick adhesive joints with manufacturing defect

In this paper, numerical model of thick adhesive inclined joints has been prepared and validated against experimental test to study the effect of manufacturing defect on the joint strength. The inclined joint was made up of two laminate webs attached to a wedge by a layer of adhesive. Tensile tests were conducted on many thick adhesive joint specimens with two different geometries. One half of the symmetric test specimen was then modeled using finite element analysis in which cohesive zone modeling (CZM) was used to simulate the initiation and propagation of joint fracture. The progressive fracture through the adhesive layer and along adhesive-laminate interface was carefully examined. Based on inspection of the experimental test specimen, potential manufacturing defect types and locations were incorporated in the finite element model. The reduction in strength due to these manufacturing defects was used to predict the most critical flaw type in thick inclined joints. The differences between the “flawless” numerical model and the experimental test specimen was explained when these manufacturing defects were incorporated. The results were found to be consistent with the experimental tests.

Effects of aramid-fibre toughening on interfacial fracture toughness of epoxy adhesive joint between carbon-fibre face sheet and aluminium substrate

Brittle epoxy adhesive joints, between the carbon-fibre/epoxy face sheets and aluminium substrate, were toughened using randomly-distributed short aramid fibres. In this study, effects of the epoxy adhesive thickness on interfacial fracture toughness of the adhesive joints, with and without short aramid-fibre toughening, were investigated. Short aramid fibres of 6 or 14mm in length with an area density of 12g/m2 were inserted between the carbon-fibre face sheet and aluminium substrate during the laminating process. Two and six layers of aluminium foils were inserted at the interface to form the controlled thin and thick adhesive joints, which are around 20 and 70μm in thickness. The two “composite adhesive joints”, with different volume densities of short aramid fibres, reversed the adhesive-thickness influence on the interfacial toughness in comparison to that of the plain epoxy adhesive joints. However, both “composite adhesive joints” with low and high aramid-fibre densities resulted in significant improvement in the interfacial toughness. Analytical models, together with optical and scanning electron microscopy observations, were used to explain the experimental findings, and relevant toughening mechanisms.

Performance improvement by glass fiber of adhesively bonded metal joints at the cryogenic temperature

The performance of adhesively bonded joints composed of aluminum sheet and stainless steel foils was investigated at the cryogenic temperature for the secondary barrier LNG (Liquefied Natural Gas) containment system. Since the huge adhesive bonding area such as LNG carriers or floaters cannot be surface-treated economically by chemical treatment, new flame surface treatment method for the metal foils was evaluated and compared with other conventional surface treatments such as mechanical or chemical surface treatments at the cryogenic temperature. The surface wettability by contact angle measurement of the metal foil was investigated with respect to surface treatments and environmental aging conditions. The effects of adhesive thickness on the adhesive bond strength and fracture toughness have been also investigated at the cryogenic temperature.The experimental results showed that the bonding performances depended strongly on the surface treatment method, adhesive thickness and fiber reinforcement for the adhesive, such. It was also found that the thick adhesive joint had a reduced bonding strength at the cryogenic temperature, but the glass fiber reinforcement improved much the bonding strength and fracture toughness for thicker adhesive at the cryogenic temperature.