Adhesive Joints Research Articles

Enhancing mechanical properties of GFRP–aluminum joints through Z pinning: a low velocity shear impact study

Studying the mechanical responses of GFRP and aluminum samples under varying loads is crucial for developing methods to enhance their mechanical properties. This research investigates the impact of z-pinning on single-lap adhesive joints through low-velocity shear impact tests. The study involves testing two types of joints: single-lap adhesive joints and single-lap hybrid pin-adhesive joints with six reinforcing pins, subjected to drop weight tests at load levels of 22.5, 27.5, and 32.4 J. The findings demonstrate that z-pinning can increase the joints’ load-bearing capacity by up to 18.87% and energy absorption by 20.58%. This enhancement is attributed to the additional composite substrate layers in the hybrid joint, which help absorb impact loads. In contrast, in the single-lap adhesive joint, only the layer adjacent to the adhesive bears the impact load. Observations from the tests indicate that in the single-lap adhesive joint, the first composite layer next to the adhesive undergoes complete delamination, whereas in the hybrid joint, all laminates fracture at the steel pins. In summary, this study underscores the significant potential of z-pinning to improve single-lap adhesive joints, providing an innovative approach to enhance their load-bearing capacity and energy absorption.

Fatigue damage tolerance of CFRP/Al adhesive joints with thermal effects

In order to promote application of adhesive connection in primary load-bearing structures for engineering practice, tolerance of fatigue damage signifies a critical parameter in line with the guidance of FAA AC 20-107B. This study aims to investigate this issue from four aspects, namely, statistics of fatigue cracks, residual performance, critical point for fatigue damage tolerance, and degradation of adhesive property. To carry out a dedicated study, the carbon fiber reinforced polymer and aluminum (CFRP/Al) adhesive joint specimens with different degrees of fatigue damage were considered by monitoring real-time variation of stiffness in the course of pre-fatigue loading. The residual strengths at different temperatures and related degradation mechanisms were explored through the experimental tests and numerical analysis. The study suggests the 92% normalized stiffness to be a critical point, at which the structural safety factor is 2.5. It is found that a safety factor of 1.5 may not be sufficient for the structure under fatigue condition as the joint is fairly susceptible in the structure. This study unveiled that a reliable management for residual fatigue performance can be achieved. It is anticipated to provide an effective guide for damage-tolerant design, reliable safety monitoring, and punctual repair of damaged adhesive structure.

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.

Experimental and virtual testing of mode II and mixed mode crack propagation under dynamic loading

Under static loading, measuring experimentally the mode II and mixed mode fracture toughness of composite materials and adhesive joints is well standardised. However, under dynamic loading, no standard procedure has been defined yet. Therefore, this paper proposes an experimental methodology to measure the mode II and mixed mode interlaminar fracture toughness of composite materials and adhesive joints. The methodology is based on a modified split Hopkinson compression bar. Two different data reduction schemes are explored and compared, one based on measuring the crack length, and another based on measuring the force from the strains in the transmitted bar. The two data reduction methods provided considerably different results. By using the method based on measuring the force, the mode II and mixed mode fracture toughness for both interlaminar and adhesive joints decreased for higher strain rates, while the opposite was found with the other approach. The method based on the crack length measurement was deemed to be unreliable due to the difficulties in measuring it.

Impact of surface preparation method on the fracture toughness of adhesive joints subjected to diverse loading modes

While research has explored surface treatment effects on fracture energies in pure modes (I or II), there is a lack of research under mixed-mode loading. The aim of this paper is to investigate the influence of surface treatment approaches on the fracture behaviour and fracture energy of adhesive joints at different loading modes. To achieve this, double cantilever beam (DCB) adhesive joints were manufactured considering two different surface treatment methods namely grit blasting and mechanical bristle blasting. The joints were subjected to five different mode mixities. Fracture energy for each tested condition was calculated based on a compliance-based approach. The fracture energies in mode I reached an increase of 55 % when grit blasting surface treatment was applied, while for mode II the fracture energy reached a value three times higher for mechanical bristle blasting surface treatment, both energies from the mixed mode tests. The failure mechanisms were also evaluated for joints with different surface treatments as a function of loading mode. In all tested conditions, a cohesive failure mode was observed; however, the results underscore the substantial influence of the surface treatment method on the fracture energy of adhesive joints. This is attributed to the distinct failure mechanisms observed under varying surface treatment conditions.

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.

Features of the formation of cohesive and adhesive strengths by non-structured and structured polyvinyl acetate films during wood gluing

The formation of a higher number of hydrogen bonds in the structured polyvinyl acetate film compared to the unstructured one was established by means of infrared spectroscopy method. It was concluded that the structured polyvinyl acetate film has a higher cohesive strength than unstructured. It was revealed that the films subjected to moisture loads (with the amount of sorbed moisture lower than the swelling limit of the adhesive film) would show greater cohesive strength compared to the films which were not subjected to moisture loads due to the formation of a higher number of hydrogen bonds. Based on the obtained results, it was predicted that the system "structured film after moisture loads/wood substrate" would exhibit the highest adhesive strength, explained by an increase in the number of hydrogen bonds between the film components and the wood cellulose. It was theoretically predicted that in order to provide high cohesive and adhesive strengths of polyvinyl acetate adhesive joints of wood ensuring, moisture saturation should be lower than the saturation limit of the wood fiber and the adhesive film simultaneously.

Effects of Carbon Fiber Reinforced Plastic Laminate on the Strength of Adhesive Joints

Carbon fiber composites are being widely considered for many classes of heavily loaded components. A common feature of such components is the need to introduce local or global loads into the composite structure. The use of adhesive bonding rather than mechanical fasteners offers the potential for reduced weight and cost. The use of carbon fiber reinforced plastics (CFRP) for the repair of steel structure is also advantageous, for example to avoid welding or bolting which can weaken structure and add fire risk. This study is based on adhesive bonded double lap shear (DLS) joints where the inner adherend is steel and the outer adherend is cross plies unidirectional (UD) CFRP. Overlap lengths of 25-125 mm with CFRP thickness of 3 and 6 mm were considered and two different orientation angles [0o,90o] and [90o,0o] laminates have been considered. The overall objectives of the paper are to investigate the delamination of the CFRP laminates within the DLS joints under quasi-static loading and both experimental and numerical techniques were used. The numerical study was based on 2-D model using strength-limit method, taking into consideration the UD properties of the plies and the resin layers separating these. As a result, it was concluded that the data obtained from 2-D finite element analysis were coherent with experimental results and additional to that fiber orientation angles of the laminate markedly affected the failure load of joints, failure mode and stress distributions appeared in adhesive and composite

The Use of Bilinear Cohesive Zone Model to Study the Fracture of Mode I in Adhesive Joints

Adhesively bonded joints can be numericallysimulated using the Cohesive Zone Model (CZM) concepts.CZM are widely used for the strength prediction of adhesivejoints. The critical strain energy release rate and criticalinterface strength are the parameters which must be knownwhen cohesive elements in ABAQUS software are used. Theformulation of the cohesive finite elements is based on theCZM approach with the bilinear traction-separation law. Inthis work, the parameters of two industrial adhesivesHuntsman Araldite 2015 and resin LY3505/XB3405 forbonding of epoxy composites are identified. DoubleCantilever Beam (DCB) test data was used for theidentification. Finally, cohesive parameters are identifiedcomparing numerically simulated load-displacement curveswith experimental data retrieved from literature.Parametric study is performed to evaluate the variation ofinput parameters like initial stiffness, element size, peakstress and energy release rate ‘G’. From the numericalevaluation, it was noted that CZM simulation relies largelyon element size and peak cohesive strength.

An energy-based 2D model for predicting mechanical behavior of adhesively bonded CFRP laminates

This paper presents a two-dimensional energy-based model for the mechanical behavior of adhesively bonded carbon fiber-reinforced polymer (CFRP) joints subjected to tensile loading. The proposed energy model is better suited than available discrete models to capture failure modes in adhesively bonded CFRP joints of different configurations having complex geometries. The governing equations are first developed for a three-stepped lap joint configuration by employing the minimum potential energy theorem and then solved using the finite element method (FEM). The disbond behavior of the joint configuration is modeled using the bilinear cohesive law. Subsequently, the developed model is extended to capture the mechanical response of adhesive joints for simple lap and scarf joint configurations. The proposed energy model is verified against 3D-FEA and experimental studies. Thereafter, a comparative analysis is undertaken for the three configurations of adhesive joints studied here. This is followed by a parametric study for the three-stepped lap joint to understand the influence of geometric and material properties of the adhesive and adherends on the stiffness and load-carrying capacity of the adhesively bonded CFRP joint. Finally, a design criterion is proposed for the three-stepped lap joint configuration, aimed at improving its load-carrying capacity/strength by reducing the peak stress levels at joint corners/overlap edges.

Comparative fracture surface analysis of adhesively bonded dissimilar lap joints: Nanosilica effect

AbstractThis research presents a comparative analysis of the fracture surfaces of adhesively bonded dissimilar single lap joints (DSLJs) and investigates the effect of nanosilica (NS) additives on their fracture behavior and performance. The study focuses on DSLJs composed of aluminum (Al), stainless steel (SS), copper (Cu), and carbon fiber reinforced polymer (CFRP) adherends bonded with an epoxy adhesive. The fracture surface of DSLJs is further analyzed by introducing two parameters: fractional total fracture area (aT) and the surface sensitivity of each adherend (aSurface). These parameters serve as benchmarks to predict the final load capacity of the adhesive joints. It is found that a decrease in and an increase in the sensitivity of the adherend with a lower Young's modulus lead to higher load capacity. Tensile tests revealed that incorporating NS up to a critical weight percentage of 0.6 wt% improves the proposed surface parameters and significantly contributes to higher levels of load capacity and absorbing greater amounts of energy to failure. Furthermore, numerical simulations offer insights into stress distribution and the mechanism of plastic deformation propagation, providing additional support for the experimental findings.Highlights NS up to 0.6 wt% enhances the mechanical properties of DSLJs. NS–epoxy interactions characterized through Fourier transform infrared and transmission electron microscopy analysis. Fracture surface patterns help to predict the final load capacity of DSLJs. Plastic propagation in epoxy verifies fracture surface pattern in DSLJs.

Investigation of geometric and material effects on the fatigue performance of composite and steel adhesive joints

This study experimentally investigates the combined effect of several parameters on the fatigue behaviour of adhesive bonded single lap joints, including: substrate material (steel, glass fibre reinforced polymer (GFRP) and carbon fibre reinforced polymer (CFRP)); adhesive material (epoxy and methacrylate); as well as joint geometry (overlap length and adhesive thickness).Results showed that despite the lower bulk strength of the ductile methacrylate adhesive, in terms of the ratio of fatigue load to ultimate failure load, joints bonded by this adhesive had a higher fatigue performance when compared to joints bonded by the epoxy one, with differences in fatigue life of up to 20 times for the same load ratio. In a similar substrate configuration, flexible GFRP substrates reduced the fatigue life of the joints bonded by the methacrylate adhesive more than 10 times when compared to steel joints for the same load. However, in a dissimilar configuration, an increase in fatigue life was observed which, based on a finite element analysis, was attributed to the ductility of the adhesive and its effects on the stress field. In CFRP joints bonded by the epoxy adhesive, a deterioration in fatigue performance was observed due to the occurrence of interfacial failure.Regarding the geometrical factors, increasing overlap length led to a decrease of the fatigue life for the same nominal shear stress. Similar trend was obtained for adhesive thickness. A parametric analysis revealed that overlap length is the parameter that affects more the fatigue performance of the joints.

Combination of scanning electron microscopy and digital image correlation for micrometer-scale analysis of shear-loaded adhesive joints

ABSTRACT To understand adhesive bonds and their macroscopic behavior, a new experimental method is presented in this paper that allows to investigate micromechanical effects down to nanometer scale under load. The method combines ion milled cross sections, low-voltage scanning electron microscopy (SEM), digital image correlation, and a miniaturized testing machine in the SEM-chamber. Experimental results from shear-loaded adhesively bonded carbon fiber-reinforced polymer specimens illustrate the method’s benefits. Through a plasma etching process, the specimen cross section is textured, creating a stochastically distributed speckle pattern visible in SEM images. A miniaturized testing machine placed in the SEM is used to apply a load to the specimens while capturing images simultaneously. The resulting image series can be analyzed by image correlation algorithms. This method enables precise statements regarding strain distribution on the specimen at micro- and nanometer scales. While conventional coupon tests on bonded specimens can only depict the effects of a composite in a homogenized manner, the new method allows to gain additional insights into the underlying mechanisms.

Multi-Objective Optimization of Adhesive Joint Strength and Elastic Modulus of Adhesive Epoxy with Active Learning.

Studying multiple properties of a material concurrently is essential for obtaining a comprehensive understanding of its behavior and performance. However, this approach presents certain challenges. For instance, simultaneous examination of various properties often necessitates extensive experimental resources, thereby increasing the overall cost and time required for research. Furthermore, the pursuit of desirable properties for one application may conflict with those needed for another, leading to trade-off scenarios. In this study, we focused on investigating adhesive joint strength and elastic modulus, both crucial properties directly impacting adhesive behavior. To determine elastic modulus, we employed a non-destructive indentation method for converting hardness measurements. Additionally, we introduced a specimen apparatus preparation method to ensure the fabrication of smooth surfaces and homogeneous polymeric specimens, free from voids and bubbles. Our experiments utilized a commercially available bisphenol A-based epoxy resin in combination with a Poly(propylene glycol) curing agent. We generated an initial dataset comprising experimental results from 32 conditions, which served as input for training a machine learning model. Subsequently, we used this model to predict outcomes for a total of 256 conditions. To address the high deviation in prediction results, we implemented active learning approaches, achieving a 50% reduction in deviation while maintaining model accuracy. Through our analysis, we observed a trade-off boundary (Pareto frontier line) between adhesive joint strength and elastic modulus. Leveraging Bayesian optimization, we successfully identified experimental conditions that surpassed this boundary, yielding an adhesive joint strength of 25.2 MPa and an elastic modulus of 182.5 MPa.

Research on technological parameters affecting the quality of adhesive joints in casting model assemblies and assessment of their operational properties in liquid and humid environments

Modern casting model kits, often created using adhesive technologies, can experience deformations leading to warping and even destruction. This can occur due to differences in thermal expansion and moisture absorption rates between the material and the adhesive. The study evaluated the behavior of the adhesive joint in model kit elements under operational conditions in liquid and humid environments, assessing the hardness and strength of the joint. The analysis revealed a correlation between the density of the plastic and the quality of the adhesive joint.

Predicting the mixed-mode fracture propagation in single-leg bending using a radial point interpolation meshless method

Adhesive bonding is a widely used joining technique with increasing applications in the automotive and aircraft industries. However, the numerical simulation of adhesively bonded joints is challenging due to the intricate mechanical behavior of the bonding layer, especially when structures are subjected to mixed-mode loading. This work proposes a meshless fracture propagation algorithm to simulate mixed-mode fracture propagation in single-leg bending (SLB) adhesive joints. The proposed algorithm combines the radial point interpolation method (RPIM) with a mixed-mode strain-based criterion to predict crack growth. The RPIM permits a flexible discretization of the fracture region and eases the geometric remeshing of the integration cells to account for the crack tip propagation. Additionally, the RPIM provides accurate and smooth strain/stress fields, allowing implementation of the mixed-mode fracture criterion. The numerical model was validated against experimental data for SLB adhesive joints with a brittle adhesive. The results showed that the proposed algorithm can accurately predict the resistance curves and critical energy release rates of the adhesive joints.

Effect of hybridization and composite architecture symmetry in the bonded joint performance of jute/carbon fibre-reinforced composites

This study investigates the effect of hybridization and composite architecture symmetry in the bonded joint performance of jute/carbon fibre-reinforced composites. Jute and carbon fibre bidirectional fabrics were used as reinforcement fibres for interlaminar hybrid epoxy composite adherends. The effect of the number of carbon fibre layers (1 and 2) as well as layup symmetry on the mechanical performance of the bonded joints was studied by testing single lap joints (SLJs) bonded with two adhesive types (a structural ductile bi-component epoxy adhesive and a brittle one). It was found that the hybridization of jute fibre reinforced composites with carbon fibres significantly improved the bonded joint performance for all tested specimens, when compared to the neat jute fibre reinforced composites (maximum of approx. 69 %). However, when compared to the neat carbon fibre (CFRP) adherend joints, the hybrid jute/carbon adherend joint configurations studied here achieved up to approximately 94 % (ductile adhesive system) and 97 % (brittle adhesive system) of maximum joint load efficiency of the CFRP synthetic joint. The effect of composite architecture symmetry was more pronounced for the brittle adhesive joints, whereas the failure loads of the ductile adhesive joints exhibited a plateau-like trend (ductile adhesives accommodate better the bending moments during SLJ loading, while brittle adhesives are more sensitive to adherend stiffness). A jute/carbon fibre hybrid composite bonded structure presents a potentially cost-effective and efficient alternative for reducing the structural carbon footprint, without a significant compromise in mechanical properties.

Analysis of the influence of damp-heat aging on the shear failure of the adhesive joint of composite materials

Abstract In this study, a comprehensive temperature and humidity environment chamber was used to accelerate the artificial aging of carbon fiber bonded joint specimens in wet and hot environments, and shear tests were conducted on non-aging and wet and hot aging specimens, respectively. At the same time, acoustic emission technology was used to monitor the damage to the specimen, and the influence of wet and heat aging on the shear properties and damage forms of the bonded joint was explored. It is found that under shearing load, the unaged bonded joint specimens first show delamination degumming damage with an energy distribution of about 1000 (mV*mS) and then fiber fracture damage with an energy distribution of more than 4000 (mV*mS). According to the load strain curve, it can be seen that the shear strength of the moisture-heat aging specimen decreases by 30% compared with that of the non-aging specimen, from 33.6 kN to 22.3 kN. According to the load energy curve, it can be seen that the rubber layer of the specimen is damaged at the inflection point of the load curve, the rubber layer of the non-aging specimen is damaged at 15 kN, and the rubber layer of the moisture-heat aging specimen is damaged at 8 kN. The test results show that the wet heat aging reduces the interlayer properties of the specimens. The structural adhesive delamination is serious, which leads to the damage of the aged specimens in advance compared with the non-aged specimens. The overall deformation of the specimens increases.

New Hybrid Sublayers for Heat- and Water-Resistant Adhesive Joints of Cast Polyurethane with Metals

New Hybrid Sublayers for Heat- and Water-Resistant Adhesive Joints of Cast Polyurethane with Metals

Failure mechanisms in pre-tensioned bonded hybrid joints

ABSTRACT This study investigated the failure mode of pre-tensioned bonded hybrid joints in steel structures using Digital Image Correlation (DIC) alongside Finite Element Analysis (FEA) and compared the results with those of adhesive bonded joints of identical geometry. While all joint types initially exhibited ideal adhesive joint behaviour at low loading levels, with linear elastic response, differences emerged as loading increased. In particular, hybrid joints exhibited out-of-plane deformations near the ends of the splice plates, a feature not observed in bonded joints. Shear deformations consistently peaked at the ends of the overlap for all joint types and increased with increasing load levels. In contrast to bonded joints, hybrid joints demonstrated resilience under escalating loads, maintaining resistance due to the presence of bolts despite the increased deformations. This study contributes to the understanding of the observed failure mode: while hybrid joints initially resembled ideal bonded joints at lower loads, increasing loads caused critical cracks to initiate at the overlap ends in the adhesive layer, culminating in bonded joint failure. In conclusion, this study has elucidated the failure mode of hybrid joints, providing insights for modelling and dimensioning in steel structures.