Behaviour Of Adhesive Joints Research Articles

Predicting lifetime of adhesive bonds for naval steel by time-temperature superposition

There is a lack of knowledge about the long-term behaviour of adhesive joints in the marine environment, and for hence, its reliability. The life cycle expected for a ship is twenty-five years and conduct duration tests would last decades. The importance and originality of this study are that it provides a methodology for predicting the durability of adhesive bonds. For this purpose, three adhesives for bonding naval steel were tested at several temperatures for determination of their shear strength on standard single-lap-joint specimens. Subsequently, using the time temperature superposition principle, these results were combined into one master curve for each adhesive that can be considered as a prediction method of the durability in a long-term period. The usual parametric models of Arrhenius and Williams-Landel-Ferry were used to obtain the master curve for each adhesive by manual shifting. The results were compared with those obtained applying an open-source software developed by the authors in R language, which specifically implements a non-parametric methodology based on the shift of the first derivative curves, following parameters of Explainable Artificial Intelligence. It was found a good correlation between both methodologies, supporting the durability lifetime obtained and therefore the applicability on ships of this adhesive technology.

Experimental behaviour and fatigue life prediction of bonded joints subjected to variable amplitude fatigue

The variable amplitude fatigue behaviour of adhesive joints has not been extensively considered in literature and considerable knowledge gaps regarding its underlying mechanisms still exist, hindering accurate life prediction. This study investigates the effect of loading sequence and number of load transitions on single lap joints (SLJs) bonded with a toughened epoxy adhesive. Life prediction is also conducted using established cumulative rules: Palmgren–Miner (PM), Broutman and Sahu (BS) and, Hashin and Rotem (HR). To overcome their limitations, a modified Cortan–Dolan approach is proposed. For this, SLJs were subjected to single transition (low-to-high and high-to-low load transitions at different stages of the fatigue life and with different magnitudes) and multi-block loading spectra with different numbers of transitions.Results showed an extension of the fatigue life for high-to-low transitions. This was attributed to a combination of load interaction effects, as evidenced by an elastoplastic numerical simulation, and non-linear damage accumulation. Conversely, low-to-high transitions exhibited a damaging effect. For single transition spectra, BS and HR models were seen to improve prediction over PM by indirectly considering these effects. The proposed approach further improved this prediction. Multi-block results showed no significant influence of the number of load transitions and were satisfactorily predicted using PM.

Viscoelastic-adhesive modeling of hybrid adhesively bonded double-lap joints: analytical and numerical studies

ABSTRACT There are a limited number of studies examining the viscoelastic and viscoplastic behavior of adhesive joints. If there is more than one adhesive in the overlap region, the problem becomes more complex and viscoelastic modeling of the overlap region becomes difficult. There is no study in the literature that covers such a bonding (overlap) scheme and examines the viscoelastic behavior of the double lap joint (DLJ). In this study, the effect of viscoelasticity of polymeric adhesives on shear stress and strain distributions in the hybrid-adhesive layer of a DLJ is studied. Maxwell-Wiechert (MW) viscoelastic material model is used to define the viscoelastic behavior of the adhesives. Analytical and finite element (FE) results are compared for both mono- and hybrid-adhesive joints under the step load. Good matches are observed between the analytical and numerical results. Four different hybrid-adhesive joints with different combinations of the adherend thicknesses are investigated by considering only the analytical method. In order to investigate the effect of the loading type on the viscoelastic behavior, step and ramp loads were applied to the joint separately. It is observed that applying the ramp load to the hybrid-adhesive joints affects the shear stresses occurred at the ends of the overlap more than the mono-adhesive joints.

Moisture sorption behavior of an epoxy-based structural adhesive and its impact on the mode I interlaminar fracture toughness of bonded joints in the oil and gas industry

Structural adhesives are widely used across industry sectors and, specifically in oil and gas, they appear as an alternative for structural joining instead of welding, to mitigate the risks arising from sparks in an environment with highly flammable products. In this sense, this article aims to evaluate the mechanical behavior of adhesive joints using the Double Cantilever Beam (DCB - to determine the GIC, mode I interlaminar fracture toughness) test before and after exposing these joints to severe environmental conditions, such as high humidity and high temperature. Also, three different surface treatments (manual sanding with abrasive sponge, solvent cleaning, and peel-ply application) of the adherend (carbon fiber/epoxy composite) are examined, as well as two different thicknesses of the adhesive layer (0.5 and 1.0 mm) to verify possible influences on the GIC behavior before and after conditioning. The results show that reducing manual surface treatment processes is a better option, to reduce the factors that cause greater dispersion in GIC values. In particular, cleaning the adherend with just isopropyl alcohol (solvent) before applying the adhesive yielded better GIC results among the tested techniques. Regarding thickness, the findings suggest that a greater thickness of the adhesive layer can lead to an increase in the plastic regions, enhancing the dissipation energy and improving the mode I interlaminar fracture toughness. Hygrothermal conditioning revealed the adverse effects of humidity on the adhesive, resulting in a 20–30 % reduction in GIC. Differential Scanning Calorimetry (DSC) analyses demonstrate that conditioning caused a notable reduction in the glass transition temperature (Tg) of the adhesive.

Effects of cyclic ageing frequencies on the ageing and mechanical behaviour of adhesive materials: Experimental analysis and numerical study

In marine structures adhesive joint structures are often exposed to cyclic conditioning where the ambient humidity changes cyclically during their service. Though some comprehensive studies on the aging of adhesives exist, these researches mainly focus on monotonic aging conditions where the adhesive joints are exposed to a wet condition continuously for a long time. However, the few investigations performed on the cyclic moisture absorption of adhesive materials show that the parameters obtained in monotonic aging conditions are not suitable for estimation of the aging behaviour of adhesive exposed to alternating humidity. An important question that can arise is whether or not this frequency affects the mechanical behaviour of adhesive joints under cyclic aging condition. In this investigation bulk dogbone and square samples were manufactured, subjected to cyclic aging conditions with four different aging frequencies and tested. The results show that the moisture diffusion constant of adhesives exposed to higher aging frequencies increase more than those exposed to lower aging frequency conditions. In addition, the moisture content only affects the degradation of strength and stiffness of the tested adhesives in different aging frequencies.

Monitoring Crack Tip Position in Adhesively Bonded Joints Under Mixed I+II Mode Loading

The use of adhesive joints has gained favour in recent years, due to their capacity for weight reduction, improved stress distribution, and enhanced fatigue resistance. However, the widespread use of adhesive joints in structural applications is hindered, because reliable and mature inspection and defect detection techniques are not yet available. Adhesive joints in structural applications are subject to fatigue loading, during which debonds can initiate and propagate, ultimately leading to potential failure. In the majority of cases, joints operate under mixed-mode loading conditions. In this work, steel Cracked Lap Shear specimens were tested to investigate the crack growth behaviour of adhesive joints under mixed I+II mode loading. To address the challenge of monitoring these joints, a range of experimental techniques was employed. These techniques included Visual testing, Digital Image Correlation, Optical Backscatter Reflectometry, and localization by Acoustic Emission. The different methods were evaluated and compared to determine their effectiveness in locating the crack tip within the joints during fatigue propagation.

Fatigue behavior of adhesive joints under modes I and II fracture in carbon-epoxy composites, influence of exposure time in a saline environment

This work investigates the fatigue crack growth behavior of adhesive joints under pure modes I and II within epoxy matrix composites reinforced with unidirectional carbon fibers. Experimental tests are made using Double Cantilever Beam (DCB) and End-Notched Flexure (ENF) setups for modes I and II respectively, considering exposure periods of one week and twelve weeks in a salt spray chamber. Control specimens are also studied for comparison.Static tests were conducted to securely establish the levels of Energy Release Rate (ERR) that were subsequently used to obtain the fatigue initiation curves (G-N) and fatigue crack growth curves (G-da/dN). A probabilistic model based on a Weibull distribution is applied to analyze fatigue initiation data.The fatigue limit in mode I, for all aging periods, is around 25 % of the static strength, while in mode II, it is around 20 %. These results are very close at all aging levels (0, 1, and 12 weeks). From this, it is inferred that aging in a saline environment of the studied joints does not have a significant impact on the fatigue limit.In the crack growth zone, for mode I, the velocity is higher in the specimens aged in both periods than in the unaged specimens. The same cannot be said for mode II, where a clear trend cannot be appreciated.

Numerical and experimental study of the residual strength of CFRP laminate single-lap joint after transverse impact

The adhesive joint is one of the efficient joining method in the composite structures, and it is important to comprehend the behavior of adhesive joints during impact in order to design stronger and safer structures. This paper investigates the residual strength of CFRP single-lap joints (SLJs) after transverse impact using both finite element analysis (FEA) and experimental methods. The impact damage FEA model and static tensile strength FEA model are established using the Hashin failure criterion and cohesive zone model. The impact damage results of the SLJs are transferred to static tensile strength FEA model using the restart analysis method. The CFRP single-lap joint models with different impact energies and lap lengths are analyzed. According to the research requirements, a total of 24 SLJ specimens with [45/0/-45/90]s lamination are manufactured, and the impact test and tensile strength test are conducted sequentially. For the SLJs with a 20 mm overlap length but different impact energies (1 J, 2 J, and 4 J), the maximum and minimum residual strength errors are 12.6 % and 6.2 % respectively. The results show that the tensile strength of the joint decreases significantly after impact. Specifically, the residual strength of the SLJ after a 4 J impact is 2902 N, whereas the strength of the non-impact SLJ is 4088 N. When considering the 20 mm, 30 mm, and 40 mm overlap length SLJs with a 4 J impact energy, the errors in the residual strength coefficients between FEA and experiment are all within 3 %. Additionally, the residual strength coefficient of a 40 mm overlap length increases by approximately 10 % compared to a 20 mm overlap length. These results demonstrate that the proposed FEA model can accurately simulate the impact and residual strength of SLJs with reasonable accuracy.

Mode II Delamination under Static and Fatigue Loading of Adhesive Joints in Composite Materials Exposed to Saline Environment.

This study investigates the fatigue delamination behavior of adhesive joints in epoxy carbon composite materials under Mode II fracture loading. The joints were characterized using the End-Notched Flexure (ENF) test, comprising adhesive joints formed by bonding two unidirectional carbon fiber epoxy matrix laminates with epoxy adhesive. These joints were subjected to different exposure periods (1, 2, 4, and 12 weeks) in a saline environment. Prior to dynamic fatigue testing, critical Mode II energy release rate values were determined through quasi-static tests, serving as a reference for subsequent fatigue characterization. This study aimed to comprehend how exposure duration to a saline environment affected the initial stage of fatigue delamination growth and employed a probabilistic model based on the Weibull distribution to analyze the experimental data. The results, gathered over a two-year experimental program, revealed varying behaviors in adhesive joint resistance to delamination based on exposure duration. A noteworthy reduction in fatigue strength capacity was observed, with fracture energies for infinite fatigue life reaching approximately 20% of their static loading capacity. This study sheds light on the deterioration of adhesive joints when exposed to a saline environment.

Analysis of peel and shear strains in cracked lap shear specimens subjected to fatigue loading using digital image correlation

Adhesive bonding presents many advantages, such as efficient manufacturing and improved structural performance [1]. However, in structures subjected to fatigue, cracks might initiate and propagate in joints, leading to in-service failure [2]. Most adhesively bonded joints are subjected to combination of peel and shear loads, so mixed I+II mode loading conditions are present [3]. In this work, Cracked Lap Shear specimens, which feature mixed I+II mode loading conditions, were tested under fatigue loading. During tests, crack growth was monitored using Visual Testing and Digital Image Correlation. With Digital Image Correlation, opening and sliding displacements in the bondline were extracted from the substrates’ displacement fields and compared against a Finite Element Model, revealing a highly strained process zone ahead of the crack tip. Results highlight the usefulness of DIC in capturing the deformation behaviour of adhesive joints under mixed mode loading conditions.

Strength prediction of a single lap joint under impact using meshless methods

The use of adhesive joints in structures subjected to dynamic loads, such as wind turbines and cars, makes it important to study them under those conditions. Numerical models are an integral part of that. Commonly the Finite Element Method (FEM) is used, but meshless methods can be an interesting alternative. These models do not require elements, and as such they can model complex geometries more easily. The current work aims at performing a first study on adhesive joints under impact using a meshless method, the Radial Point Interpolation Method (RPIM). Since the strength prediction of adhesive joints is also an important field and because the commonly used Cohesive Zone Models (CZM) have some limitations, like the use of special cohesive elements, this work also aims to expand the use of the ISSF criterion to impact conditions. The results show that the RPIM can be used in this type of problem without numerical difficulties, and the ISSF gives acceptable strength predictions, with errors between 30.5% and 13.5%.Adhesive bonding is a joining technique that offers some advantages, when compared to other common joining techniques, like bolting or riveting. One of those advantages is that adhesive joints are generally lighter than the alternatives, which is very important in the search for more efficient modes of transportation since lighter vehicles consume less energy. Given the interest in the use of adhesive joints, it is important to study their behaviour under different conditions. Currently, the static behaviour of adhesive joints is very well documented, with many research works dedicated to it. However, the number of publications on their dynamic behaviour is still scarce, with only a few works dedicated to fatigue, impact and free vibrations. Additionally, the use of meshless methods to study adhesive joints is also currently mostly limited to static analysis, and even in that case it is still very incipient. Therefore, this work aims at extending the use of meshless methods to the dynamic analysis of adhesive joints, to help in the advancement of both fields.

A family of models of hard and soft interfaces with damage

In many industrial applications in aeronautical, civil, energetic, automotive and biomedical engineering, adhesion techniques have recently emerged as assembly procedures alternative to conventional joining. Adhesive bonding is advantageous for many aspects, such as mitigation of stress concentration, better resistance to corrosion and water sealing, and the possibility of assembly dissimilar materials. In the literature, interface models are a classical modeling approach to the description of the mechanical behavior of adhesive joints. In this paper, an original generalized interface model is presented for both soft and hard adhesives, characterized by lower and higher stiffness than the adherents, respectively. The proposed model is able to capture a damaging behavior of the adhesive material, the elastic properties of which are obtained by homogenizing a microcracked media. The cracks density in the adhesive is the damage parameter, and a kinetic law based on the generalized standard material model describes its evolution. The role and relevance of damage velocity is illustrated through some simple examples and with a comparison with experimental data taken from literature. The numerical results show how the mechanical properties of the adhesive and the joint degrade as damage evolves, indicating a transition from quasi-brittle to brittle behavior as the damage velocity decreases.

Investigating Failure Behavior of Adhesive Joints with Dissimilar Materials of Steel and Additively Manufactured PLA

Investigating Failure Behavior of Adhesive Joints with Dissimilar Materials of Steel and Additively Manufactured PLA

From High Strain Rates to Elevated Temperatures: Investigating Mixed-Mode Fracture Behaviour in a Polyurethane Adhesive.

The investigation of the behaviour of adhesive joints under high strain rates is an active area of research, primarily due to the widespread use of adhesives in various industries, including automotive manufacturing. Understanding how adhesives perform when subjected to high strain rates is crucial for designing vehicle structures. Additionally, it is particularly important to comprehend the behaviour of adhesive joints when exposed to elevated temperatures. Therefore, this study aims to analyse the impact of strain rate and temperature on the mixed-mode fracture characteristics of a polyurethane adhesive. To achieve this, mixed-mode bending tests were conducted on test specimens. These specimens were subjected to three different strain rates (0.2 mm/min, 200 mm/min, and 6000 mm/min) and tested at temperatures ranging from -30 °C to 60 °C. The crack size was measured using a compliance-based method during the tests. For temperatures above Tg, the maximum load supported by the specimen increased with an increasing loading rate. GI increased by a factor of 35 for an intermediate strain rate and 38 for a high strain rate from low temperature (-30 °C) to room temperature (23 °C). GII also increased for the same conditions by a factor of 25 and 95 times, respectively.

Experimental study on the influence of environmental conditions on the fatigue behaviour of adhesive joints

The fatigue behaviour of adhesive joints under the influence of environmental conditions has been the subject of several studies. However, a significant knowledge gap still exists, particularly regarding the understanding of failure mechanisms associated with these phenomena. This research presents a novel investigation into failure modes under varying environmental conditions of adhesively bonded single lap joints (SLJ), particularly focusing on the transition between static and fatigue loading. Utilising numerical simulation and experimental observations, the study offers valuable insights into the failure mode transition and its dependence on the environment.Results shown that, both temperature and humidity reduced the fatigue strength of the joints, mostly for lower fatigue lives, up to 10 times. Fatigue changed the failure mode from cohesive to mixed cohesive–adhesive. Thus, static tests proved inadequate in predicting fatigue fracture path. Adhesive failure area increased with decreasing load level (up to 60%). These phenomena were ascribed to the numerically determined SLJ’s stress concentration. The findings contribute to improving the design of adhesive joints for enhanced fatigue resistance in different environments.

On enhancement of fracture resistance of adhesive joints by surface micropatterning using a femtosecond laser

This study focuses on the influence of surface micropatterns, including uniform and nonuniform grooves fabricated by selective removal of a designed volume from aluminum alloy substrates using a femtosecond laser, on the mode I fracture behavior of adhesively bonded interfaces. The morphology, wettability, chemistry and microstructure of the patterned surfaces have been analyzed. The mode I fracture behavior of adhesive joints was characterized by measuring the fracture resistance using a J-integral approach, and the fracture process in the joint was investigated numerically using a continuum damage model. The results show that the laser patterning has modified the surface roughness, wettability and surface chemistry such that the fracture resistance could be greatly increased. It also reveals the significance of patterning uniformity across the surfaces and the existence of a limiting effective patterning ratio (the ratio of the patterned area to the flat bonding area) on enhancing the fracture resistance. Local plastic deformation that occurred in the adhesive at the patterned structures due to stress concentration was found to be one toughening mechanism although it tended to induce crack growth close to one substrate-adhesive interface.

Influence of Temperature, Humidity and Load Coupling on Mechanical Properties of Adhesive Joints and Establishment of Creep Model

To study the creep and property degradation behavior of adhesive joints under the coupling action of temperature, humidity and load, polyurethane shear joints were prepared and tested. Different static loads were applied to joints at high temperature (80 °C) and high temperature and humidity (80 °C/95% RH) to test and analyze the creep deformation, and a suitable creep model was established. At the same time, the performance degradation test of the joints under the effect of multifactor coupling was carried out to obtain the variation law of the failure load, and the failure mechanism was discussed based on the failure section. The research shows that the creep strain of the joint at high temperature and humidity was significantly larger than that at high temperature, and the failure fracture time was shorter, in which water molecules played a role of softening and hydrolysis. The viscoelastic multi-integral creep model was used to analyze and predict the creep behavior of the joints. It was found that the creep model could better describe the creep behavior of the joints under uniaxial constant loading. Under the coupling effect of temperature, humidity and load, the failure load decreased with time, and with the increase in static load, the decline range and rate of failure load increased. It was found that the mechanical properties in the high temperature and humidity environment decreased significantly more than those in the high temperature environment. When a static load was applied during creep, cracks easily occurred inside the adhesive layer, and water molecules easily diffused inside the cracks, which increased the decay rate of the mechanical properties. This study provides good theoretical significance and engineering value for the application of polyurethane adhesion structures in rail vehicles.

Determination of Mode I cohesive law of structural adhesives using the direct method

ABSTRACT The Mode I fracture behaviour of adhesive joints, bonded with two different epoxies, was evaluated applying the direct method to obtain the tensile cohesive law, together with Digital Image Correlation (DIC) analysis. The Double Cantilever Beam (DCB) test was performed, and the fracture toughness was attained by applying the J-integral. The DIC measurements were analysed resorting to a Python script, that was used as a post-processing tool, able to minimize the data noise stemming from the experimental results. This tool proved to be of utmost importance to guarantee acceptable results. Finally, the direct method could predict the main features of the cohesive law. Also, the law was used to simulate the fracture behaviour in a different test specimen, to evaluate the influence of the specimen geometry on the results. The obtained data indicates that the direct method is dependent on the joint geometry and the constraining condition of the adhesive.

Novel mode III DCB test setups and related evaluation methods to investigate the fracture behaviour of adhesive joints

In this work, two test setups in proximity to the familiar Double Cantilever Beam and Tapered Double Cantilever Beam tests are proposed which allow for determining the critical mode III energy release rate (ERR) of adhesive joints. The experiments are performed on an elastic–plastic adhesive (SikaPower® 498) and evaluated using data reduction schemes based on nonlinear elastic fracture mechanics (J-integral) and linear elastic fracture mechanics (Irwin-Kies equation). Next to the data reduction schemes based on linear elastic fracture mechanics in accordance with ISO 25217, crack length independent and corrected beam theory approaches are presented which incorporate measurements of the rotational angles of the load introduction points. Crack length and process zone length are measured during the experiments using backface strain measurements. Furthermore, a multi-specimen compliance calibration is employed to allow simplified testing in future studies. The obtained critical ERRs and cohesive laws are compared and discussed thoroughly to gain insight into the advantages and disadvantages of the different data reduction schemes. The results implicate that the J-integral evaluation method is the most reliable for evaluation of the mode III fracture energy, as no assumptions need to be made about the behaviour of the adherends when determining the mode III ERR.

Insights into the micromechanical response of adhesive joint with stochastic surface micro-roughness

Micro-roughness at adhesion surface yields significant influences on the structural behaviour of adhesive joints. Investigations into the micromechanical mechanism are extremely limited. This works developed a novel particle-based model of joints with stochastic microstructural features of roughness, which can capture refined multi-scale responses as first of this kind. Aluminium adherends with mechanical surface treatments were firstly scanned using 3D laser scanning microscope. The statistical features and reconstruction method of micro-roughness profiles were determined. Single lap shear tests on joints made of epoxy adhesive (Loctite EA 9497) and treated aluminium adherends were performed to provide testing data and observations on failure modes. The refined numerical models were subsequently developed to examine the influences of the actual micro-roughness on the micromechanical behaviors and failure mechanism. The mechanical interlocking, mitigation on crack propagation due to the irregular roughness were investigated. It is followed by introducing the reconstructed roughness of various magnitudes and further numerically examining the micromechanical responses by key stochastic parameters such as root mean square roughness and correlation length. The results indicate that the mechanical interlocking contribute more to enhancing the joint strength than the increase of adhesion area by micro-roughness. A rougher surface in either horizontal or vertical directions does not exhibit a consistent improvement of joint strength, which also depends on the threshold of roughness and the surface skewness triggering the transition of failure modes.