Adhesively bonding has been increasingly used for numerous industrial applications to meet the high demand for lightweight and safer structures. Debonding of adhesively bonded joints is a typical ...mixed mode failure process. It is highly depended on the interactional effects of material properties and geometric definitions of the constituents, which is very complicated. The existing studies in identifying fracture modes of joints based on either experiments or finite element analysis are often prohibitively time and computational expensive. This paper proposed an innovate method by combining Finite Element Analysis (FEA), Latin Hypercube Sampling (LHS) and Genetic Programming (GP) to understand the effect of the physical attributes on the fracture modes of adhesively single lap joints. A dataset of 150 adhesive joint samples has been generated using LHS, including different combinations of adherend and adhesive’s material properties and thicknesses. The mixed mode ratios of the 150 samples are calculated using Strain Energy Release Rate (SERR) outputs embedded in Linear Elastic Fracture Mechanics (LEFM), which has been validated by experimental tests. Finally, a GP model is developed and trained to provide an extracted explicit expression used for evaluating the early-state failure modes of the adhesively bonded joints against the design variables.
The tensile performance of adhesively bonded CFRP joints has been investigated experimentally. In this study, overlap length, adherend thickness, adherend width and scarf angle were chosen as design ...parameters. All load–displacement curves are linear, except that the thicker single-lap joints behave slightly nonlinearity due to the bending effect caused by eccentric loading. The lap shear strength is not directly proportional to overlap length, adherend thickness, adherend width and scarf angle for the brittle adhesive studied in the paper. The major failure mode includes adhesive shear failure and adherend delamination failure, sometimes accompanying with some fiber pull-out. Finally, the lap shear strength of three different lap types with similar bonding area (W=25mm, L=10mm, θ=5.71°) and adherend thickness (0.96mm) was analyzed. It is found that the double-lap joint has the highest ultimate failure load. However, when considering the lap region weight, the scarf-lap joint is the most efficient.
Over the past three decades, the strengthening and repair of existing civil engineering structures using FRP laminates has attracted a great deal of attention. With the advances in polymer science, ...adhesive bonding has become a common joining technology in these applications. Despite numerous studies that address the short-term behaviour of adhesively bonded FRP/steel joints, uncertainty with respect to long-term performance still remains. This knowledge gap is regarded as a critical barrier, hindering the widespread application of FRPs to strengthen and retrofit steel structures. This paper presents the state of the art in terms of the durability of FRP/steel joints used in civil engineering applications. Important influential factors relating to the durability of adhesively bonded joints are reviewed and different damage mechanisms are discussed. Moreover, related investigations of the combined environmental durability of these joints are critically reviewed and the findings are presented. The paper concludes with a discussion to motivate future research topics, while it is emphasised that the generalisation of the available results is questionable.
The structural integrity of several structures could be determined by their joints strength. Over the years, adhesively bonded joints have been often chosen to achieve a compromise between mass ...reduction and higher mechanical strength. Among others, the reduction in stress concentrations, the ability of producing smooth surfaces with no discontinuities and the reduced weight penalties are some of the factors that make this type of joints so attractive. Normally, to increase the bond strength, the materials to be bonded must be subjected to a kind of surface treatment. For metals, and more specifically, for aluminium alloys, phosphoric acid anodizing and chromic acid anodizing have been the most used treatments worldwide. However, recent investigations show that these kinds of anodizing are detrimental to health due to the release of carcinogenic substances. With this in mind, it is of the utmost importance to find alternative surface treatments that can ensure an effective bond. In this paper, a vast experimental study was performed based in the single lap joint ASTM D 1002 standard method, with the objective of determining the best alternative surface treatment (Sulfuric Acid Anodizing and Boric-Sulfuric Acid Anodizing), for aluminium-to-aluminium joints, using two types of adhesives, namely the AF 163 and the EA 9658 AERO. Results show that the optimum surface treatment is different for each type of adhesive and this fact has a huge influence on mechanical behavior of this type of aeronautical adhesive joints.
•The mechanical performance regarding adhesively bonded joints in aeronautical structures is addressed.•The effect of surface treatment on adhesively bonded joints and its related strength is characterized and evaluated.•The different type of failures regarding the different combinations of treatment vs adhesive are evaluated.
•Backface strain measured with optical fibres could locate the crack-tip in the adhesive under fatigue crack propagation.•The combination of optical fibres and digital image correlation could ...identify the length of the process zone under fatigue.•Results of the optical fibres were confirmed by comparison with DIC and visual evaluation.
The feasibility of applying Optical Backscatter Reflectometry as a backface strain monitoring method for bonded joints subjected to mode I fatigue crack propagation was studied. Visual evaluation and Digital Image Correlation methods were applied to track the crack-tip position and the onset of plasticisation in the adhesive, respectively. The methods were compared to understand the relationship between the changes in the optical fibre measurements and the crack propagation. The absolute maximum strain values gave important information about the crack propagation stage (stable or high velocity) within the adhesive and fast-rate crack propagation, which resulted in a sudden increase in strain values. Finally, the strain peak position was found to be a valuable indicator of crack tip position.
Due to their good mechanical performance and lightweight advantages, adhesively bonded CFRP-aluminum alloy joints have seen increasingly widespread use in the automotive industry. However, the ...mechanical properties of adhesive joints may degrade when exposed to a hygrothermal environment and to alternating loads. In this study, the influence of alternating load on the residual strength of hygrothermally aged joints was investigated. CFRP-aluminum alloy joints bonded with epoxy adhesive were aged in constant and cyclic hygrothermal (Con-H\Cyc-H) environments, after which the fatigue life and residual strength of the joints were measured. A differential scanning calorimetry (DSC) analysis of Araldite® 2015 and CFRP was performed to evaluate the effect of hygrothermal aging. The failure modes were also investigated to explain the failure mechanism. The results show that the adhesive, CFRP and joints all degenerate after hygrothermal aging. For the materials, the decrease in the glass transition temperature (Tg) is more significant under Con-H environmental conditions, while the static failure strength and fatigue performance of the joint after Cyc-H aging are the lowest. As the number of cycles of alternating load increases, the damage in the adhesive system accumulates continuously, resulting in the strength of the aged and unaged joints declining, first slowly and then quickly. The decrease in strength is more apparent at higher load. After 15 days of hygrothermal exposure, nearly complete cohesive failure is more likely to be observed relative to CFRP fiber tear for initial joints, and the interfacial failure tends to occur at the boundary of the CFRP-adhesive interface under the influence of alternating load conditions. Finally, environmental and fatigue degradation factors were defined based on the test data and incorporated into the numerical simulation to predict the residual strength of CFRP-Al alloy bonding joints exposed to alternating load.
•Effects of rubber modification on fracture toughness of epoxy were investigated.•Fracture toughness tests were conducted under mode I and II loadings.•The fracture toughness values were evaluated ...using two types of J-integral values.•The rubber modification effect depended on the type of J-integral value.
In the present study, the effects of rubber modification on the joint strength and fracture toughness of epoxy adhesives under mode I and II loading were investigated using Arcan-type adhesively bonded joints. The results of the fracture tests under mode I loading revealed the occurrence of stress whitening owing to triaxial tensile stresses near the crack tip. The joint strength and fracture toughness were calculated using the J-integral value, where the adhesive layer was considered as an elastic–plastic material. The crack-growth energy was also calculated using the J-integral value, where the adhesive layer was considered as an elastic material. The results revealed that rubber modification under mode I loading induced an increase in the joint strength, fracture toughness, and crack-growth energy. Rubber modification under mode II loading did not induce stress whitening near the crack tip owing to low triaxial tensile stresses. However, macroscopic void growth and significant plastic deformation were observed in the adhesive layer. Rubber modification induced a slight decrease in the joint strength; however, it induced a significant increase in the fracture toughness owing to the growth of the plastic zone in the adhesive layer. The calculated crack-growth energy decreased slightly, in accordance with the joint strength. It was concluded that rubber modification contributed to an increase in the crack-growth resistance under mode I loading and not under mode II loading. Furthermore, a correlation was observed between the joint strength and crack-growth energy.
Adhesive bonding is an effective technique for connecting fiber reinforced composites (FRPs) and plays a crucial role in determining load transfer capability and overall structural safety. However, ...long-term exposure to temperature and moisture can degrade adhesive joints, highlighting the significant importance of accurately predicting their durability when designing reliable adhesive structures. This study aims to address the industry's demand and academic interest by proposing a convenient and accurate prediction method to evaluate the durability of adhesive structures subject to complex stresses. Laboratory accelerated aging experiments were conducted on butt joints (experiencing pure normal stress) and shear joints (experiencing pure shear stress). Quasi-static tensile tests and adhesive FTIR tests were performed to establish a dataset of adhesive functional group absorbance and failure strength under single stress conditions. To compare and assess the predictive performance of different models for aging behaviors, Artificial Neural Network (ANN), Support Vector Regression (SVR) and Random Forest (RF) models were selected. The model hyperparameters were further optimized using the Genetic Algorithm (GA). The results demonstrated that the ANN model exhibited superior correlation and accuracy compared to the SVR, and RF. The characteristic importance analysis revealed that the hydrolysis and post-curing of adhesives were the primary factors affecting joint performance under hygrothermal environments. By incorporating environmental degradation factors and defining initial failure criteria, the CZM constitutive model was modified and a predictive method was established for assessing the hygrothermal aging behavior of CFRP/aluminum alloy bonding structures. This method effectively eliminates the effect the intricate aging process (e.g. aging time, aging conditions) and establishes a direct correlation between the molecular structure of adhesive and the macroscopic mechanical properties of the bonded joint, which serves as a valuable reference and guidance for the safety design of automotive composite material bonding structures throughout their entire lifespan.
Among the myriad joining techniques, the adhesive bonding technique is widely used to join complex large-scale composite structures because of its numerous advantages compared to traditional joining ...techniques. This article profusely analysed the various techniques for ameliorating the performance of composite joints, such as bonding methods (secondary bonding, co-bonding, co-curing, and multi-material bonding), surface modification techniques (plasma, laser surface treatment, surface grinding, etc.), additional reinforcement techniques (Z pin, wire mesh, nanofiller, etc), and different joint geometries (stepped joints, half-stepped joints, balanced joints, and scarf joints). Also, the effect of various adhesives and fabrication techniques on the static and dynamic performance of CFRP and GFRP-based joints was studied in detail. Moreover, this review addresses the finite element modelling and optimisation techniques on adhesively bonded joints. It has been observed that the bonding methods, surface modification to enhance the roughness of the adherend, addition of nanofillers, and variations in joint geometry greatly influence the shear strength, fracture toughness, fatigue, and vibration behaviour of FRP composite joints.
The structural applications of adhesively bonded joints on transportation industries have been increasing, and it is expected that this rising trend persists in the future. The appropriate design of ...these joints should address two main issues: fatigue behaviour and environmental effects. Environmental effects consist of the degradation of the bonded joints by means of harmful influence of temperature, moisture or both simultaneously. These effects can have an impact on the fatigue behaviour of bonded joints because they influence the quality of the bonding. The combination of environmental effects and fatigue lead to synergetic consequences resulting in premature and unpredictable rupture, which transforms these issues into relevant and actual research topics. The present paper describes the most recent works addressing the referred subjects. Experimental works and analytical/numerical approaches are also described aiming to give a picture of the real state‐of‐the‐art. Actual limitations and perspectives of future evolution are also discussed.
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