Bonded Joint Structures Research Articles

Step and scarf joint elastic and failure behavior comparison under tensile and flexural loading

Step and scarf joints are the two most commonly used joint types in composite laminate bonding, offering distinct characteristics and behavior in response to specific loading conditions. In order to gain a comprehensive understanding of the mechanical behavior step and scarf joints, a comparative study of adhesive bonded step and scarf joint carbon fiber reinforced composite laminate under tensile and flexural loading condition was performed. The test specimens were fabricated with carbon fiber reinforced composite laminate. The step and scarf joints were precisely manufactured using a precise milling machine, and the joints were bonded with an epoxy adhesive with controlled bondline thickness. To assess the behavior of the bonded joint structure under various loading scenarios, both tensile and four-point bending flexural tests were performed. Strain measurement during these tests was carried out using digital image correlation (DIC) technology. The step joint showed multiple sequential bondline crack propagations while the scarf joint experienced a single catastrophic failure. The test result showed that the step joint retained 10% and the scarf joint retained 32% of tensile strength compared to the no-joint test specimen. Also, the step joint retained 13% and the scarf joint retained 44% of flexural strength compared to the no-joint test specimen. As a result, the scarf joint had significantly higher ultimate tensile strength, tensile modulus, and tensile strain at failure. The scarf joint also showed higher ultimate flexural strength and flexural modulus compared to the step joint.

Super-Resolution Thermal Imaging Using Uncooled Infrared Sensors for Non-Destructive Testing of Adhesively Bonded Joints

To compensate for the inability of inexpensive uncooled infrared (IR) sensors to perform high-speed thermography detection with low noise and high sensitivity in the field of non-destructive testing (NDT), a post-processing method for uncooled IR sensors based on compressive sensing is proposed in this paper to perform super-resolution NDT thermography. Super-resolution IR phase images can be generated by first using a sparse representation of the low-resolution (LR) IR images and a sparse dictionary generated from randomly sampled high-resolution (HR) raw training image patches to generate an HR IR image sequence, and, subsequently, implementing IR thermography NDT. To verify the reliability of the proposed method, super-resolution IR images were combined with pulsed phase thermography (PPT) to verify bonded joint structures in carbon fibre-reinforced polymer (CFRP). Compared to other methods, the proposed method reduces the mean square error (MSE) by 11.13% and yields a better visual effect on the PPT phase image reconstructed using three-fold super-resolution. The proposed method not only produces sharp defect edges, but also retains the original texture, which is expected to enable wider application of uncooled IR sensors in NDT applications in the future.

Numerical investigation and structural optimization of bonded joints with fiber reinforce plastic sucker rods in offshore reservoirs

ABSTRACT Sucker rod constructed of FRP (fiber-reinforced plastic) is currently used in offshore oil and gas industries due to its high tensile strength, lightweight, and resistance to corrosion. Due to the fact that the surface of FRP sucker rod cannot be threaded, it is often attached to steel joints via adhesive technology, and the bonded joint of FRP sucker rod is particularly prone to breaking and coming off. The current research is primarily concerned with the mechanical properties of FRP sucker rods, with little emphasis on the adhesive performance of bonded connections. This research developed a numerical model with a novel bonded joint structure based on the cohesion failure criteria. An indoor test was used to determine and confirm the expected bonded joint failure load. Additionally, an orthogonal test was conducted to maximize the adhesion performance. The findings indicated that a) theaccuracy of numerical model was within 2% and b) when the groove angle was 30°, the length ratio was 2/23, each groove was 14 mm in length based on a triangular groove, and the anticipated failure stress of the bonded joint was around 443 kN, an increase of 10%.

A Review on Adhesively Bonded Aluminium Joints in the Automotive Industry

The introduction of adhesive bonding in the automotive industry is one of the key enabling technologies for the production of aluminium closures and all-aluminium car body structures. One of the main concerns limiting the use of adhesive joints is the durability of these system when exposed to service conditions. The present article primarily focuses on the different research works carried out for studying the effect of water, corrosive ions and external stresses on the performances of adhesively bonded joint structures. Water or moisture can affect the system by both modifying the adhesive properties or, more importantly, by causing failure at the substrate/adhesive interface. Ionic species can lead to the initiation and propagation of filiform corrosion and applied stresses can accelerate the detrimental effect of water or corrosion. Moreover, in this review the steps which the metal undergoes before being joined are described. It is shown how the metal preparation has an important role in the durability of the system, as it modifies the chemistry of the substrate’s top layer. In fact, from the adhesion theories discussed, it is seen how physical and chemical bonding, and in particular acid-base interactions, are fundamental in assuring a good substrate/adhesive adhesion.

Multiscale modeling and prediction of bonded joint failure by using an adhesive process zone model

In this work, a three-dimensional multiscale adhesive process zone model (APZM) is used to simulate dynamic failure process of bonded joint structures. APZM combines the ideas of the cohesive zone model (CZM), the Cauchy–Born rule, and the so-called virtual internal bond (VIB) theory to predict failures of bonded joints that involves adhesive failure, cohesive failure, and mixture of both. The proposed APZM has some unique advantages to describe mechanical behaviors of bonded joints under general loading conditions. First, in contrast to conventional FEM model, APZM model can simulate dynamic fracture process of structure without remeshing. Second, APZM replaces the ad-hoc traction–separation laws of interface used in CZM with full three-dimensional interphase model, which is based on the VIB theory. Consequently, APZM can naturally capture both mixed failure modes as well as mixture failure mode under complex loading conditions. Third, an empirical elastic energy density function for macromolecular polymers was constructed, and its material parameters are obtained by calibrating the model with material properties measured from experiments such as under pure tension and shear tests. In order to validate APZM model and to predict stress field in adhesive layer accurately, a series of numerical simulations of bonded joints under shear, tension and mixed loading conditions have been conducted. The numerical results indicate that APZM can accurately predict the failure modes as well as the dynamics fracture process of bonded joints.

Optimization of smart adhesively bonded single-strap composite joint

In order to reduce the stress concentration and improve the failure strength of adhesively bonded joint structures, a smart adhesively bonded joint structure has been developed by integrating piezoelectric layers into the adherends. For better design guide, a combined theoretical model and multi-objective optimization approach is employed to optimize the smart joint in this study. Firstly, the first-order shear deformation theory is utilized to model and analyze the detailed peel and shear stresses distribution in the adhesive layer. Further, considering the characteristics of the stress distribution, the interface debonding failure criterion is used to set up the objective; optimization variables are considered as the stacking sequence, geometries and applied electric fields of the integrated piezoelectric layer. Thus, with the aid of the Mathematica software, the optimal smart adhesively bonded joint system can be determined. Finally, some detailed examples are analytically solved to show the considerable difference between the preoptimized and optimized smart joint systems, which validate that the developed theoretical model and multi-objective optimization approach can be used to enhance adhesively bonded joint failure strength.

Detection of Moisture in Adhesive Bonds Using Electrochemical Impedance and Dielectric Spectroscopies

The ingress of moisture into an adhesive bonded structure can lead to a variety of effects. Moisture has the ability to plasticize the adhesive but may also segregate into microvoids. The water in the voids will have less of an effect as it does not plasticise the matrix and is unable to influence the interface between the resin and the substrate. Only if the T g of the matrix drops below ambient will it allow expansion of the voids and then there will be consequential lowering of the fatigue characteristics of the resin. Water also has the ability to interact with metallic surfaces leading to changes in the nature of the interaction between adhesive and adherend. Water has a large dipole signature and is easily studied in terms of its effect on the electrical properties of bonded joint structures. In this paper, applications of electrochemical impedance and dielectric spectroscopies are described. Both techniques measure the electrical response of the joint, however the nature of the measurement performed allows slightly different, but complementary, approaches to be adopted. Application of the techniques to similar joint structures illustrates the similarities and differences of the two approaches. The article demonstrates the value of using electrical methods to probe the changes that occur in joints on ageing and emphasizes the value of these methods in developing and understanding of the phenomena that are responsible for loss of bond integrity.

Residual stresses and practical adhesion: effect of organo-metallic complex formation and crystallization

Epoxy-amine liquid pre-polymers are often applied onto metallic substrates and cured to obtain painted materials or bonded joint structures. The overall performance of such systems depends on the interphase created between the epoxy-amine polymer and the metallic substrate. When epoxy-amine liquid mixtures are applied onto a metallic oxide layer, concomitant amine chemisorption and oxide dissolution occur leading to organo-metallic complex formation. Depending on the amine nature, as soon as the organo-metallic complex concentration is higher than the solubility product (e.g., isophoronediamine (IPDA)), these organo-metallic complexes crystallize as sharp needles. At the same time, the uncrystallized organo-metallic complexes react with the epoxy monomer to form, after curing cycle, a new network. Moreover, the crystal size increases with the solid/liquid contact time leading to an increase of intrinsic residual stresses and Young's modulus. When aliphatic diethylenetriamine (DETA) was used, no crystallization occurred, but the interphase formation was observed. The aim of this study was to understand and to establish the role of crystallization of organo-metallic complexes formed within the interphase on the practical adhesion performance. As the crystallization of the organo-metallic complex depends on the nature of the amine, two amine hardeners were used (IPDA inducing the formation of crystals and DETA without formation of crystals). For DGEBA-IPDA systems, the ultimate load decreases while residual stresses increase when the liquid/solid contact time increases. When no crystal formation was observed (e.g., DGEBA-DETA system), residual stresses, coating Young's modulus and ultimate load values all remained nearly constant irrespective of the liquid/solid contact time.

Loading nature of the interfacial cracks in a joint component under fusion-relevant thermal loads

One of the standard design concepts for divertor components in a fusion reactor is the bonded joint structure. Understanding the loading nature of interfacial cracks are significant for the assessment of structural integrity of divertor joint components. In this paper, the thermomechanical loading nature of interfacial cracks is discussed. A bi-material joint element consisting of the CFC/TZM system is considered. A typical fusion operation condition is simulated assuming a pulsed high heat flux loading. Stress singularities near the interfacial crack tips are characterized quantitatively in terms of the fracture mechanical parameters. The evolution of the stress intensity factors and the energy release rate during the given transient thermal load are determined. The difference in loading characteristics between the edge crack and the center crack is discussed. High heat flux cycling tests are performed on brazed CFC/TZM divertor elements in an electron beam test facility. The microstructures of the damaged interface agree with the predicted fracture modes. The loading nature and possible failure mechanisms are discussed for a fusion-relevant thermal loading.

Investigation of moisture ingress into adhesive bonded structures using high frequency dielectric analysis

The effects on the dielectric characteristics of aluminium-adhesive bonded joint structures with different surface treatments on exposure to moisture at elevated temperatures are reported. Analysis of the frequency domain data as a function of the exposure time allows three major ageing processes to be identified; plasticization of the adhesive, changes in the state of interfacial oxide and generation of disbonded areas. Time domain data allows identification of regions where changes in the characteristics of the bond have occurred and complements the data obtained from frequency domain data. Mechanical tests on joints aged for 733 days indicate a large decrease in the shear strength has occurred consistent with the detection of marked changes in the dielectric traces. Anodization of the substrate increases the durability of joints in comparison with etched samples. This study illustrates the potential of this method for in-situ characterization of changes occurring both within the adhesive and the adhesive substrate interface.