Composite Adhesive Joints Research Articles

Bio-inspired patterned adhesive single-lap joints for CFRP and titanium

In this study, a bio-inspired patterned adhesive is proposed for a single-lap joint between carbon fiber reinforced polymer and titanium. Finite element analysis is performed with cohesive zone modeling to observe the damage propagation and adhesive strength of gecko-inspired hexagonal and lamellae patterns that are designed with high toughness and low toughness adhesives. Results show that adhesive fracture path and damage initiation location can be controlled by means of adhesive patterns, thereby enhancing composite adhesive joint strength. Large transitional stress peaks can be created by the hybrid adhesive patterns, resulting in increased energy absorption during failure. This work aims to employ bio-inspired concepts to path novel designs for patterned adhesive architectures to increase composite joint strength performances.

Study on delamination damage evolution of composite L-shaped adhesive joint based on cohesive behavior

The adhesive joint of composite materials is one of the typical structures in aircraft structures, and the delamination damage is one of the most important damage modes in composite adhesive joints. In this paper, static and fatigue tests were carried out on L-shaped adhesive joints to analyze the damage evolution and failure modes of delamination damage under static and fatigue loadings. Based on the cohesive constitutive models, the static and high-cycle fatigue delamination constitutive models were developed. The static and fatigue numerical models of composite L-shaped adhesive joints were established by using finite element software. The stress distribution, deformation modes and delamination propagation laws were systematically studied. The simulation results are in good agreement with the experimental results. The delamination damage initiation, evolution and failure mechanism of L-shaped adhesive joints under static and fatigue loads were revealed by combining the experimental and the numerical results, which will provide theoretical and engineering guidance for strength and fatigue analysis of composite adhesive structures.

Effects of anodizing surface treatment on the mechanical strength of aluminum alloy 5083 to fibre reinforced composites adhesive joints

In this study, the anodizing process based on the use of tartaric sulfuric acid solution (TSA) was carried out on metal substrate to evaluate for the first time its effect on the adhesion strength and corrosion resistance of aluminium alloy (i.e., AA5083) to fibre (i.e., basalt or glass) reinforced composite adhesive joints for nautical applications. Furthermore, some TSA anodized samples were soaked in a NaOH solution to investigate the influence of this post-immersion step on the joint performances. With the aim to improve the fibre-matrix adhesion in the composite substrate thus further increasing the overall mechanical response of the joint, glass and basalt fibres were treated with a silane coupling agent solution. The corrosion behaviour of the aluminium alloy was studied by electrochemical techniques. Samples morphology was analysed by scanning electron microscopy, while the interaction between aluminium alloy substrate and epoxy resin was studied through contact angle analysis and resin uptake tests. The effectiveness of the silane treatment was examined by means of Fourier transform infrared spectroscopy and quasi-static tensile tests carried out on dry glass and basalt fabrics. The mechanical response of the resulting joints was evaluated by means of quasi-static tensile tests in accordance to ASTM D3528 standard.

Application of the Pulse Infrared Thermography Method for Nondestructive Evaluation of Composite Aircraft Adhesive Joints.

In this article, we examine the possibility of using active infrared thermography as a nontraditional, nondestructive evaluation method (NDE) for the testing of adhesive joints. Attention was focused on the load-bearing wing structure and related structural joints, specifically the adhesive joints of the wing spar caps and the skins on the wing demonstrator of a small sport aircraft made mainly of a carbon composite. The Pulse Thermography (PT) method, using flash lamps for optical excitation, was tested. The Modified Differential Absolute Contrast (MDAC) method was used to process the measured data to reduce the effect of the heat source’s inhomogeneity and surface emissivity. This method demonstrated a very high ability to detect defects in the adhesive joints. The achieved results are easy to interpret and use for both qualitative and quantitative evaluation of the adhesive joints of thin composite parts.

Adhesively bonded joints of jute, glass and hybrid jute/glass fibre-reinforced polymer composites for automotive industry

Natural fibre-reinforced composites have attracted a great deal of attention by the automotive industry mainly due to their sustainable characteristics and low cost. The use of sustainable composites is expected to continuously increase in this area as the cost and weight of vehicles could be partially reduced by replacing glass fibre composites and aluminium with natural fibre composites. Adhesive bonding is the preferred joining method for composites and is increasingly used in the automotive industry. However, the literature on natural fibre reinforced polymer composite adhesive joints is scarce and needs further investigation. The main objective of this study was to investigate experimentally adhesively bonded joints made of natural, synthetic and interlaminar hybrid fibre-reinforced polymer composites. The effect of the number of the interlaminar synthetic layers required in order to match the bonded joint efficiency of a fully synthetic GFRP bonded joint was studied. It was found that the failure load of the hybrid jute/glass adherend joints increased by increasing the number of external synthetic layers (i.e. the failure load of hybrid 3-layer joint increased by 28.6% compared to hybrid 2-layer joint) and reached the pure synthetic adherends joints efficiency due to the optimum compromise between the adherend material property (i.e. stiffness and strength) and a diminished bondline peel stress state.

The influence of aging in salt chamber on strength of aluminum – CFRP single lap joints

Aluminum - composite adhesive joints are used in many technical fields, e.g. in the automotive, aviation, and construction industries. They are exposed to mechanical as well as environmental loads. The paper presents laboratory results for samples aged in the Weiss Technik SC 450 salt chamber. Aging of samples was done for four periods: 7, 14, 21, and 28 days. In addition, reference samples that were not exposed to salt spray were also tested. The tests were carried out on a total of 25 samples. Two-component “Distal Classic” epoxy adhesive was used to make the adhesive joint.The article analyzes both the strength of the joints and the energy needed for damage. As a result of research, it was found that after the first week of the samples being in the chamber, the force loss is almost 50%. The paper also presents macroscopic images of the surface after damaging the joints, which shows that the adhesive layer does not provide much protection against corrosion in the joint lap.

Composite stepped-lap adhesive joint analysis by cohesive zone modelling

The use of adhesive bonds has increased significantly in recent years and is now a predominant bonding technique in the aeronautical and automotive industries. Cohesive Zone Models (CZM), used together with the Finite Element Method (FEM), are a viable tool to predict the strength (Pm) of adhesive joints. The CZM combines strength criteria for damage initiation and fracture mechanics concepts for crack propagation. The main objective of this study is to numerically and experimentally evaluate the mechanical performance of adhesively-bonded stepped-lap joints with carbon-fiber reinforced plastic (CFRP) substrates under tensile loading. The influence of different overlap lengths (LO) and a ductile adhesive was also investigated. Experimental data was gathered and compared with numerical CZM results obtained using Abaqus®. A study of peel (σy) and shear (τxy) stresses in the adhesive layer was performed. A significant geometry effect was found on the maximum load (Pm) of the stepped-lap joints, with benefit for large LO; however, delamination should be taken into account mostly for the longer LO. The CZM technique was revealed to be a suitable tool in determining the behavior of composite stepped-lap joints.

The effect of metallic fiber geometry and multi-walled carbon nanotubes on the mechanical behavior of aluminum fiber-reinforced composite adhesive joints

The effect of the geometry of metallic fibers on the mechanical response of aluminum fiber-reinforced composite adhesive joints was studied experimentally. Moreover, a combination of multi-walled carbon nanotubes and aluminum fibers was used for reinforcing the composite adhesive joints. Different geometries for the aluminum fibers including straight, twisted and spring-shaped with different pitch lengths were considered. The results indicated that the composite adhesive joints reinforced with a combination of multi-walled carbon nanotubes and aluminum fibers experienced the highest improvement of 171% in the strength compared to the unreinforced specimen. It was found out that the aluminum fiber geometry can significantly influence the adhesive joint behavior. Incorporating the twisted aluminum fibers into the adhesive layer increased, while the spring-shaped fibers decreased the strength of the composite adhesive joints compared to the straight aluminum fibers. The fracture surfaces of the composite adhesive joints were analyzed using scanning electron microscope.

Glass fiber-reinforced polymer nanocomposite adhesive joints reinforced with aligned carbon nanofillers

In this paper, the effect of aligning multi-walled carbon nanotubes (MWCNTs) and graphene oxide nanoplatelets (GONPs) on the fracture behavior of glass-epoxy nanocomposite adhesive joints was studied experimentally. The nanofillers were aligned using an AC electric field along the adhesive thickness in double cantilever beam (DCB) specimens. A unidirectional carbon fiber layer was embedded as the second ply of each laminated substrate as an electrode inside the glass-epoxy composite laminates. Reinforcing the composite adhesive joints with randomly dispersed MWCNTs and GONPs resulted in the maximum improvements of 82% and 155% in the initiation fracture energy and the improvements of 19% and 69% in the maximum load, respectively. Furthermore, aligning MWCNTs and GONPs resulted in considerably higher improvements of 179% and 349% in the initiation fracture energy and 66% and 127% improvements in the maximum load, respectively and yet changed the failure pattern. The toughening mechanisms due to the addition of the nanofillers were investigated using scanning electron microscope (SEM).

Advanced ultrasonic NDT for weak bond detection in composite-adhesive bonded structures

Weak bond detection in composite-adhesive joints is a highly challenging task in the non-destructive testing (NDT) community. This paper aims to evaluate the bonding quality in composite-adhesive bonded structures with high-frequency high-resolution acoustic microscopy. Carbon fibre reinforced epoxy-epoxy bonded single-lap joints containing three different bonding quality -debonding, weak bond with less and more contamination, in addition to perfect bond were investigated. Shape-based feature extraction algorithm in the frequency domain was developed in order to detect weak bonds. The results show that high frequency focused transducers used in acoustic microscopy is a great choice to visualize interface quality in bonded structures. Developed post-processing algorithm performs well on the detection of weak bond, independent from the perfect bond inspection results.

Effects of temperature and moisture on the fracture behaviour of composite adhesive joints

Mode I interlaminar fracture toughness of CFRP joints co-cured and co-bonded was experimentally characterised under several environmental conditions. Two test campaigns were carried out: one with as-received specimens tested at −54 °C and 25 °C, and another with hygrothermally aged samples tested at 25 °C and 80 °C. Dynamic mechanical analysis and scanning electron microscopy were used to explain distinct results. For co-cure, the propagation onset for as-received samples started at the interlayer region between the two adherends before migrating deeper into the adherend, while for as-received co-bonded samples, propagation onset was mainly cohesive. After aging, both bonding techniques failed directly in the adherend, in a fibre-tear fashion. Thus, the aging process presented a higher influence on failure mechanisms than the testing temperature. Regarding performance, mode I fracture toughness did not present a great variation for co-cured samples, since failure locus was always in the adherend. However, the initiation mode I fracture toughness of co-bonding decreased between as-received and aged samples. This difference is attributed to the presence of the adhesive in co-bonded systems, which showed to be more affected by environmental conditions than systems formed by fibre and matrix only, such as co-cured ones.

Strength prediction and stress analysis of adhesively bonded composite joints using meshless methods

The applications of adhesive joints have grown in recent years in various industries, ranging from automotive to maritime. Compared to traditional joining techniques, such as bolts, welding or riveting, adhesive bonding has some advantages, namely, they are low weight, have high strength and have a more uniform stress distribution. However, they also have some disadvantages, many times they are impossible to disassemble, they have low peel strength and some adhesive require high curing temperatures. A particular advantage of using adhesives in composite joints is that they do not require holes in the composite to join the joint, which are associated with delamination failure. The present work aims at using radial point interpolation meshless methods to study the stress distributions and predict the strength of composite adhesive single lap joints (SLJ). To this effect, a brittle adhesive was tested in composite joints with varying overlap lengths (LO). The stress distribution obtained using meshless methods was compared to one obtained using the well-established Finite Element Method (FEM), resulting in a similar behaviour. The strength predictions, using the critical longitudinal strain (CLS) criterion, were similar to the experimental joint strength.

Achieving robust damage mode identification of adhesive composite joints for wind turbine blade using acoustic emission and machine learning

Interest in damage mode classification of composite structures by Acoustic Emission (AE) inspection technique and clustering analysis by machine learning has been increasingly growing. Furthermore, hyperparameters in clustering analysis promote the need for more robust clustering algorithms. This paper presents a clustering method by fast search and find of density peaks (CFSFDP), where robust identification for different damage modes can be achieved by means of similarities of AE signals. Based on the clustering analysis, matrix cracking and shear failure of the adhesive layer are demonstrated to be fundamental and characteristic damage modes, respectively. Meanwhile, similarities of AE signals for various damage modes in the subspace of AE features are explored in detail. The interface debonding (fiber/matrix interface debonding and adhesive failure) and the cracking of polymer (matrix cracking and cohesive failure) behave similarly in the subspace of selected AE features. As a different damage mode, fiber breakage is shown to be more similar to delamination, in contrast with other damage modes in the subspace. Moreover, effects of the selection of cluster number, the metric of spatial similarity and the value of cutoff index on the clustering results are shown to be negligible.

Effects of loading rates on mechanical property and failure behavior of single-lap adhesive joints with carbon fiber reinforced plastics and aluminum alloys

In this paper, the effects of loading rate on mechanical property and failure mode of carbon fiber reinforced plastic (CFRP) composite and aluminum alloy (Al) single-lap adhesive joints were investigated. Four loading rates (2 mm/min, 4 m/s, 8 m/s, and 12 m/s) were chosen to carry out the shear tests. Digital image correlation (DIC) technique was used to analyze the strain evolution of the joints. Results showed that the shear strength increased with the increase of loading rate. When the loading rate increased from 2 mm/min to 12 m/s, the average shear strength of the joint increased from 19.3 to 29.2 MPa. The load-displacement curves under dynamic loading had two peaks, which is mainly caused by local failure and progressive failure of the joints. There were stress concentrations at both ends of the bonding area, which caused cohesive failure and fiber-tear failure. At both ends of the bonding area, the failure mode was mainly cohesive failure of the adhesive, whereas the failure mode in the middle of the bonding area was mainly resin matrix failure of the CFRP. Wherein the peak loads had a certain correspondence with failure modes and failure regions.

Acoustic emission response and progressive failure behaviors of composite adhesively bonded joints loaded by Mode I and II

The analysis of progressive failure process is one of the critical issues in the research of composite adhesive joints. In view of this point, acoustic emission (AE) is applied to real-time monitoring of the dynamic damage process of composite adhesive joints in some loading modes such as Mode I (double cantilever beam (DCB)) and Mode II (end notch flexures (ENF)). Furthermore, the high speed camera and scanning electron microscopy are carried to analyses the damage mechanisms of composite bonded joints. Results show that there are significant differences in the load- deflection curves of the specimens under the loads of Mode I and II. The main failure mode of the two types of loading modes is adhesion failure, and the accumulation of damage observed at the edge of the adhesively bonded layer cause defeat of the composite bonded joints. In addition, AE parameters including amplitude, hits, energy, and duration are connected with the fail mechanism. Furthermore, the dynamic characteristics of the AE signal, especially amplitude spectrum distribution, can provide evidences for studying progressive failure behaviors of composite adhesive joints.

Experimental analysis of adhesively bonded joints in synthetic- and natural fibre-reinforced polymer composites

The application of adhesively bonded joints in automotive industry has increased significantly in recent years mainly because of the potential for lighter weight vehicles, fuel savings and reduced emissions. The use of composites in making automotive body components to achieve a reduced vehicle mass has also continuously increased. Natural fibre composites have recently attracted a great deal of attention by the automotive industry due to their many attractive benefits (e.g. high strength-to-weight ratio, sustainable characteristics and low cost). However, the literature on natural fibre-reinforced polymer composite adhesive joints is scarce and needs further investigation. The main objective of this study was to evaluate and compare the mechanical performance of adhesively bonded joints made of synthetic- and natural fibre-reinforced polymer composites. Similar and dissimilar single lap joints bonded with a modern tough structural adhesive used in the automotive industry, as well as the epoxy resin AR260 (the same resin used in composite fabrication) were tested. It was found that the average failure loads varied significantly with adhesive material strength and adherend stiffness. Furthermore, it was also observed that failure mode has a significant effect in failure load. The jute-based natural fibre composites joints, both hybrid and purely natural, were superior in strength compared to the sisal-based natural composites joints.

Impact and bending analysis of composite scarf adhesive joints modified with MWCNTs at room and hot temperatures

Scarf adhesive joints (SAJs) have attracted an increasing attention in joining/repairing of carbon-fiber reinforced epoxy (CFRE) composite structures due to their zero eccentricity, which provides better aerodynamic surfaces. This study evaluated for the first time, the performance of SAJs in CFRE composites under thermo-mechanical impact loads. The adhesive was modified with optimum percentage of multi-walled carbon nanotubes (CNTs). The impact tests were performed at room temperature of +25°C, +50°C and +75°C. The residual flexural properties of the unfailed impacted joints were measured using three-point bending test. Results from impact tests at temperature of 25°C, 50°C and 75°C showed improvement in the impact bending stiffness of the modified SAJs with CNTs by 8.3%, 7.4% and 11.8% and maximum contact force by 15.6%, 21.3% and 18.9%, respectively. The energy at failure of the CNT-SAJs was increased by 15.2% and 16.4% respectively at temperatures of 25°C and 50°C. At test temperature of 75°C the SAJs have hysteresis load-displacement behavior and energy-time curve with rebound energy of 35% and absorbed (damage) energy of 65%. The residual flexural strength of the modified and unmodified SAJs is 98.2% and 86.1% respectively, while their residual moduli have remarkable decrease to 71.7% and 81.3%.

Overview of Gc Tests Used to Evaluate Composite–Composite Adhesive Joints

The quality of composite–composite bonded joints is a very important issue for industries, especially for the aerospace sector. A deficient joint may stop the manufacturing process and its repairing may include very high costs. Nowadays, the quality of the joint is obtained by means of interlaminar fracture toughness tests, estimating the [Formula: see text] value (Energy Release Rate used to produce crack propagation) over coupons with (theoretically) the same conditions as those included in an actual part. Usually, [Formula: see text] is obtained by means of a Double Cantilever Beam (DCB) test. Although DCB is the reference test, it may present some drawbacks when a non-symmetrical coupon is used and when the adherents stiffness is low. An alternative to DCB is the Climbing Drum Peel (CDP) test which is able to address some of the DCB drawbacks. Nevertheless, both tests need the use of a universal testing machine, then they cannot be done in situ. In the present investigation, DCB and CDP main characteristics are analyzed and a new device, called Horizontal Drum Peel (HDP), which includes the advantages of previous tests and the possibility to perform the test over an actual part is presented.

Experimental analysis of composite scarf adhesive joints modified with multiwalled carbon nanotubes under bending and thermomechanical impact loads

Scarf adhesive joints have attracted an increasing attention in joining/repairing of carbon fiber reinforced epoxy composite structures due to their zero eccentricity, which provides lower stress distribution across the adhesive layer and better aerodynamic surfaces compared to other bonded joints. The main objective of this study is to evaluate the performance of the scarf adhesive joints in carbon fiber reinforced epoxy composites under thermomechanical impact loads, which is very important for the aerospace and automotive industries. The adhesive was modified with optimum percentage of multiwalled carbon nanotubes. The impact tests were performed at 25 ℃, 50 ℃, and 75 ℃. The residual flexural properties of the unfailed impacted joints were measured using three-point bending test. Results from impact tests at 25 ℃, 50 ℃, and 75 ℃ showed improvement in the impact bending stiffness of the modified scarf adhesive joints by 8.3%, 7.4%, and 11.8% and maximum contact force by 15.6%, 21.3%, and 18.9%, respectively. The energy at failure of the modified scarf adhesive joints with multiwalled carbon nanotubes was improved by 15.2% and 16.4% respectively at 25 ℃ and 50 ℃. At test temperature of 75 ℃, the scarf adhesive joints have hysteresis load–displacement behavior and energy–time curve with rebound energy of 35% and absorbed (damage) energy of 65%. The residual flexural strength of the modified and unmodified scarf adhesive joints is 98.2% and 86.1% respectively, while their residual moduli have remarkable decrease to 71.7% and 81.3%.

A cohesive model with a multi-stage softening behavior to predict fracture in nano composite joints

The scope of this research is presenting a new traction-separation law for composite adhesive joints in the presence of nanoparticles in mode I and II fracture. The experimental work involved evaluation of nanoparticles effects on bridging laws, cohesive mechanism, and traction-separation laws in mode I and II fracture. Nanocomposite samples have been undergone double cantilever beam, and end notched flexure tests. The experimental bridging laws were converted into traction-separation models applicable to user-defined interface/cohesive finite elements, and a new modified model was presented to simulate and predict fracture in mode I and mode II. The obtained traction-separation laws for various nanoparticles contents can be utilized in the finite element methods for numerical simulation. A damage evolution formulation was presented, and contributing parameters were modified. Numerical modeling in Abaqus software was implemented to compare experimental results of single lap joint strength and predicted mechanism within the new proposed model.