Journal of Adhesion

Budzik, Michal K.; Wolfahrt, Markus; Reis, Paulo; Kozłowski, Marcin; Sena-Cruz, José; Papadakis, Loucas; Nasr Saleh, Mohamed; Machalicka, Klara V.; Teixeira de Freitas, Sofia; Vassilopoulos, Anastasios P.

doi: 10.1080/00218464.2021.1953479pmid: N/A

The development of new adhesives has allowed to expand the application of bonding into the most diverse industrial fields. This review article presents the commonly used experimental methods for the investigation of mechanical performance of adhesively bonded joints in the aerospace, wind energy, automotive and civil engineering sectors. In these sectors, due to their excellent intrinsic properties, composite materials are often used along with conventional materials such as steel, concrete and aluminium. In this context, and due to the limitations that the traditional joining techniques present, adhesive joints are an excellent alternative. However, standardized experimental procedures are not always applicable for testing representative adhesive joints in these industries. Lack of relevant regulations across the different fields is often overcome by the academia and companies’ own regulations and standards. Additional costs are thus mitigated to the industrial sectors in relation with the certification process which effectively can deprive even the biggest companies from promoting adhesive bonding. To ensure continuous growth of the adhesive bonding field the new international standards, focusing on actual adhesive joints’ performance rather than on specific application of adhesive joints are necessary.

Grothe, Richard; Striewe, Jan; Meinderink, Dennis; Tröster, Thomas; Grundmeier, Guido

doi: 10.1080/00218464.2021.1957676pmid: N/A

In this study, nanocrystalline ZnO (NC ZnO) films are considered as interface layers for polymer/metal bonds consisting of hot-dip galvanised steel (HDG) and epoxy resin. NC ZnO films with a thickness of less than one micrometre were deposited on HDG through hydrothermal deposition from aqueous Zn nitrate/hexamethylenetetramine electrolytes. Furthermore, 3-aminopropylphosphonic acid (APPA) and 3-aminopropyltriethoxysilane (γ-APS) were considered for molecular adhesion-promoting layers. 90°-peel tests of water-saturated epoxy films confirm the improvement of bond strength under humid environments. Electrochemical properties of the thin film coatings analysed by current density potential curves indicate effective inhibition of oxygen reduction and metal alloy dissolution. Scanning Kelvin probe (SKP) measurements confirm that the corrosive delamination of the polymer from the metal substrate is significantly slowed down by the considered surface modifications. The combination of NC ZnO films with APPA as an adhesion promoting molecule showed the best performances.

Chen, Hongli; Na, Jingxin; Mu, Wenlong; Tan, Wei; Luan, Jianze

doi: 10.1080/00218464.2021.1958681pmid: N/A

To investigate the influence of sealing treatment on the resistance of lap joints to hygrothermal environment, Al-CFRP single lap joints (SLJ) and Al-CFRP-Al double lap joints (DLJ) were selected as the research objects, and hygrothermal-cyclic aging test was carried out based on the vehicle service environment. The failure strength and failure mode of the joints before and after the sealing treatment and hygrothermal aging were analyzed. Fourier transform infrared (FTIR), thermogravimetric (TG), and scanning electron microscopy (SEM) analyses were used to explain the aging behavior of the adhesive and CFRP. The failure strength of the lap joints decreases with the aging time, but the rate of decline of the joints strength after the sealing treatment is considerably lower. The failure mode of the lap joints is cohesive failure before aging. At the beginning of the aging process, interface failure appears at the edge of the failure surface of the joints and gradually leads to the fiber tearing. Moreover, due to the diffusion mechanism of water molecules, the aging behavior at the edge and the center of the interface is different. When the joints is sealed, the aging behavior caused by aging time and aging path will be considerably weakened.

Garcia Momm, Guilherme; Fleming, David

doi: 10.1080/00218464.2021.1959323pmid: N/A

The extensive aeronautic application of structural bonding motivated the development of numerous analytical models over the past century. Most of these models consider the adhesive to be uniform and in pristine conditions. Real-life bonded joints often contain localized manufacturing defects – e.g., porosity, partial cure – and degradation under environmental conditions – e.g., humidity, temperature. These deviations can reduce joints’ residual strength, contributing to premature failure in-service. This work proposes and evaluates two efficient continuum-mechanics-based closed-form methods for the stress analysis of complex bonded joints made of composite adherends and ductile adhesives in non-pristine conditions and subjected to general loading conditions. These models were based on a model for linear elastic functionally graded adhesives, which was generalized to incorporate adhesive plasticity using a linear equivalent modulus method and a non-linear deformation theory. The results indicate conservative residual strengths and strain distributions, agreeing with an existing analytical model for degraded joints and a detailed finite element model. Both proposed models demonstrated suitability for damage tolerance analyses and forensic investigations of real-life joints and repairs. The computational efficiency and robustness of the linear model are particularly applicable for sensitivity analyses and automated processes.

Lai, Tianmao; Guo, Mingli; Chen, Yuguo

doi: 10.1080/00218464.2021.1969922pmid: N/A

The evolution of millimetric-range electrostatic force between a silicon cantilever and a negatively charged dielectric sample was studied by recording “suspended” force curves at a fixed distance on an atomic force microscope (AFM). Results show that the long-range force increases sharply at first with time, then increases moderately until reaching a global maximum, and at last decreases slightly and exponentially. The sharp increase was attributed to carrier drift at a high velocity. Moreover, a repelling electric field due to accumulated charges leads to the moderate increasing trend. In addition, the exponential decrease was attributed to charge decay on the sample surface. With the absence of the sample for a while (absent time) and presence again, the force increases sharply again with a lower value. The longer the absent time, the larger the decreasing rate. This was attributed to carrier diffusion due to the concentration difference. The proposed method can be used to measure millimetric-range electrostatic forces at a fixed distance, and investigate its evolution with time, and study the effect of some factors on the charge dissipation. Furthermore, the outcomes may suggest that charge carriers inside an AFM silicon cantilever play a part in the electrostatic force contribution.

Kaufmann, Marvin; Flaig, Florian; Müller, Michael; Stahl, Lukas; Finke, Jannis; Vallée, Till; Fricke, Holger

doi: 10.1080/00218464.2021.1971081pmid: N/A

Currently, adhesive bonding processes are developed and optimised in a time-consuming trial and error procedure, which rarely leads to an optimal solution due to the high complexity of the adhesive flow behaviour during application. The ideal adhesive layer has precise geometric specifications; entrapped air bubbles or overfilling of the bond should be avoided. Numerical methods, such as Computational Fluid Dynamics (CFD), are only capable of calculating squeeze-flow processes to a limited extent. Apart from high computing times, mesh and convergence problems often occur due to the small ratio of adhesive layer thickness to adhesive layer length and width. The Generalised Partially Filled Gap (GPFG) model, published in a companion paper uses fundamental characteristics of every bonding process to derive clever assumptions, and thus provide an efficient simulation tool for adhesive squeeze-flow. The GPFG model simplifies the squeeze-flow to a two-dimensional problem, as the flow in thickness direction can be neglected for most bonding processes – without significant loss of accuracy compared to analytical or CFD solutions. The experimental validation of the model is presented in this study. Both stresses and flow geometry were evaluated, and a very good agreement between experiments and model was proven.