Adhesive bonding has emerged as an attractive solution for the joining of lightweight structures, yet accurate stress analysis remains computationally demanding when relying on Finite Elements (FE). This paper introduces a novel plate Macro-Element (ME) formulation that extends previous beam-type approaches to enable three-dimensional stress analysis of bonded joints. High-order polynomial expansions are employed to describe the displacement field of the adherends, while the adhesive is modeled as an elastic foundation. Governing equations are derived using a variational principle and integrated within a standard FE framework. Through the derivation of a special stiffness matrix, a ME can simulate an entire overlap with just one element. The proposed methodology is validated against FE results for a single-lap bonded joint with a thin adhesive layer. The influence of different higher-order displacement assumptions and constitutive models is investigated. The results show that their inclusion in the formulation improves the solution accuracy.

- To improve the mechanical strength and the stiffness of sealed and bolted sealants (BS) joints, the load transferred by the sealant layer shall be increased. In this way, the influence of Silicon Carbide (SiC)-reinforced sealant on static and fatigue strength of sealed and BS joints has been investigated for both uncoated and coated aluminum substrates. The configuration of double-lap aluminum joint with one fastener under net-fit installation is chosen in the present study. Reinforcing the sealant significantly improves the mechanical strength and stiffness of the sealant but reduces its toughness. The mechanical behavior of double-lap sealed joints under tensile relies on the failure mode (cohesive, adhesive or mixed) and the joint thickness. The analysis, based on the Hart-Smith and Volkersen models, is relevant for simulating the mechanical behavior of the sealant joint in three successive regimes: initial non-linear regime, transient linear regime, followed by a progressive degradation of the sealant related to combined effects of shear, peeling and geometric non-linearities. Concerning the mechanical behavior of double lap bolted-sealed joints under tensile, the sealant nature (reinforced or not), sealant layer thickness, and substrate coating do not affect the general sequence of steps-initial transfer by friction at fastener hole up to the aluminum substrates plasticization-but change the stress range, the extent of clearance compensation, and the cracking evolution. The last results on fatigue tests on double-lap BS joints highlight that the SiC-reinforced sealant enhances the fatigue stability in thick joint configuration.

Recent literature has introduced a novel quantitative approach for assessing critical stress and fracture toughness, utilizing a coupled stress energy criterion, commonly referred to as the coupled criterion (CC). However, evaluating the required parameters demands considerable effort using semi-analytical methods or finite element analysis techniques. The Boundary Element Method (BEM) provides an alternative to traditional approaches, as it eliminates the need for domain meshes and directly computes unknown field variables on the interfaces. The present study employs the CC and a 2D multi-zone BEM to access failure initiation and propagation in the adhesive interface of three-point bending test (3PBT's) with available experimental and numerical responses. The interface properties are then used in a progressive interface damage BEM analysis to evaluate the structural response of the tests and validate the values obtained for these properties. The study addressed both symmetric and asymmetric adhesive failure scenarios, indicating that the evaluation of fracture parameters in both cases is analogous because of the short crack length at the onset of failure. The results demonstrate that the use of the new quantitative methodology with BEM models, proposed herein, allows one to model complex geometries and interface debonding problems with greater accuracy and efficiency.

Bonded stepped repairs to aircraft composite structures offer many advantages, such as a smooth aerodynamic surface, high strength, and low mass addition. However, their design remains challenging due to the varying stiffness along the bondline in a thin laminate. This study investigates, numerically and experimentally, to what extent an “equivalent” stepped joint can be used to design a stepped repaired panel. In the proposed case, failure is driven by laminate fracture instead of patch disbonding. Tension tests on stepped repairs at the scale of coupons and panels were carried out in 11 different configurations. Specimens were obtained by hot-bonding as it would be done to perform in-situ repairs. Finite element modelling was performed with cohesive zone modelling to account for disbonding and delamination, and continuum damage mechanics to simulate composite failure. This multiscale experimental study showed that stepped repaired coupons have a similar behaviour to repaired panels in terms of damage mechanisms, failure onset location, and tensile strength. It supports the idea to use coupons instead of whole panels to carry out experimental testing of stepped repairs. A good agreement with 2D and 3D numerical simulations was also found. They predicted accurately the strength of the repairs and highlighted a failure location compatible with the experimental results. As a conclusion, an equivalent stepped joint can be representative for the strength of a stepped repaired panel, including when failure occurs inside the laminates.

In the context of enhancing the sustainability of aerospace vehicles, the structures must be designed to anticipate the possibility of disassembly. Mandatory certification in aeronautics and the robustness of satellites require structures to maintain their integrity to ensure safety during their life, which can come into conflict with the need for disassembling or demising. This is particularly the case for composite structures assembled though adhesive bonding. In this paper, a controlled disassembly device of the substrates of a component for keeping their integrity and surface states after debonding is proposed. The addition of thermally expandable particles (TEPs) and a metallic grid is investigated in various proportions, substrates, and adhesives. It is shown that the introduction of the disassembling system into a 100 µm adhesive bonding does not significantly affect the strength of the joint during life. Disassembly only happens when and if desired at a load level above the natural mode I and mode II ruptures of the assembly. The proposed solution helps to make functionalization of the bonded joint a good way to address sustainability regarding the preservation of resources, transforming the adhesive joint into a structural functional material.

Lightweight aeronautical structures and power generation structures such as wind turbines are fitted with protected external layers designed and certified to withstand severe climatic events such as lightning strikes. During these events, high currents flow through the structural protection but are likely to induce effects deeper in the supporting composite material and could even reach or perforate pressurized tanks. In situ measurements are hard to achieve during current delivery due to the severe electromagnetic conditions, and the lightning strike phenomenon on these structures is not yet fully investigated. To gain a better understanding of the physics involved, similarities in direct damage between lightning-struck samples and those subjected to pulsed lasers and an electron gun are analyzed. These analyses show the inability of a pure mechanical contribution to fully reproduce the shape of the delamination distribution of lightning strikes. Conversely, the similarities in effect and damage with the thermomechanical contribution of electron beam deposition are highlighted, particularly the increase in core delamination due to the paint and the apparent similarities in delamination distribution.

Design for composite material additive manufacturing is governed by multiple process variables that can be computationally expensive to optimize. This is especially true when considering discrete variables, such as the material type to be used, which lead to a lot of possible solutions that have to be evaluated. Here, we propose a workflow for optimizing topology and fiber placement of 3D volumetric structures based on mechanical performance under multiple load cases and environmental impact. An eco-informed material selection from a set fibers and polymers is followed by a methodology to optimize the manufacturing setting. By performing these two steps sequentially, the number of input parameter sets to be tested is reduced in a combinatorial scale, along with the computational cost. The framework can be easily extended by adapting the analyses and holds significant promise for the design of additive manufactured sustainable structures.

This paper presents the formulation of an enriched cohesive element (CE) accounting for the confinement and the viscoelasticity of the adhesive layer under pure mode I solicitation. This formulation follows the structure of the hybrid approach, which combines the use of solid bulk elements (BEs) and CEs. This leads to define the enriched CE as a zero-thickness interface core embedded within two solid bulk adhesive regions. The interface is modelled with a spring whose stiffness is degraded by a damage variable. The bulk adhesive regions are modelled with a standard linear solid (SLS) model with one Maxwell arm. This formulation is implemented using the ABAQUS finite element (FE) software through a custom UMAT material routine, and applied to a single enriched CE as well as to a double cantilever beam (DCB) bonded joint. It is also evaluated numerically against the respective equivalent hybrid models.

Rubber-like materials such as sealant with recent developments in formulation are widely used in various structural components, especially in aerospace industry with many applications such as the sealing of bolted joints on aircraft. The objective of this paper is to assess the load transfer distribution in bolted-sealed joints through an approach coupling both experimental and numerical tests. Quasi-static and relaxation tests under uniaxial and pure shear loading were carried out to determine the parameters of phenomenological hyperelastic laws and of the generalized Maxwell model for the sealant PR 1782 C2. The fastener is an alloy steel bolt with a protruding head when the substrates are made from aluminum 2024-T3. Experimental and numerical quasi-static tests were then performed on double-lap bonded and bolted-sealed joints under in-plane loading. The numerical tests are done using 2D and 3D Finite Element (FE) models; they involve the visco-hyperelastic behavior previously assessed for the sealant. Accounting for visco-hyperelasticity makes it possible to better estimate the load transfer between bolt and sealant layer. Typically, the bolt load transfer rate is derived from numerical output for different sealant thicknesses. In the considered geometrical joint configuration, the sealant load contribution is between 7% and 13% according to the sealant thickness. Methodology and results provide a solid basis for the fatigue strength prediction of bolted-sealed joints.

In this paper, damage produced by lightning strike, laser shock and electron beam deposition on a protected Carbon Fiber Reinforced Plastic composite laminate is studied in order to find analogies of effects and damage between these experimental means. As lightning strike physics on CFRP coated with a Lightning Strike Protection and paint is not fully understood, these analogies could be able to enhance lightning strike modelling by potentially uncoupling the physics at hand and having access to additional measurement instruments. The different experimental setups are briefly described before analyzing the damage response of the aeronautical CFRP protected using an Expanded Copper Foil and coated in aeronautical paint. Eventually the results are compared to build potential analogies able to enhance lightning strike modelling.

Determining adhesive failure in adhesively bonded joints is still challenging in the field. While the three-point bending test (3PBT) (ISO 14679:1997) has been helpful in identifying critical forces and displacements related to bond strength, it only allows for a qualitative assessment of the bond line. Recently, a new quantitative methodology has been developed to determine critical stress and fracture toughness using the coupled stress-energy criterion and the 3PBT. However, these assessments require a significant effort using semi-analytical or finite element (FE) analysis. Therefore, the present study proposes a reliable set of analytical equations to determine peel and shear stress distributions using a weak interface formulation and 1D Euler-Bernoulli approach. More precisely, a particular numerical method computes the integration constants from these equations. A new method has been proposed for calculating the interfacial stiffness in peel and shear mode, based on the material and geometrical parameters of the geometry. This method differs from the previous approach, where the interfacial stiffness was calibrated from the experimental behavior of the test. The whole approach has been validated through experimental and numerical analysis, including the costly 3D FE analysis. An analytical expression for interfacial energy release is suggested, developed following the works of Fraisse and Schmit on J-integral assessment of sandwich-type overlaps and depending on the applied force and a rotation, which could be experimentally measured. Therefore, this work is significant progress in determining bond strength using a simple mechanical test and equations applicable to various industries.

Composite stepped repairs can achieve high strength recovery without the addition of bolts or fasteners to the structure. They are therefore a major issue in the field of aerospace composite structure damage repair. However, there is no standardized method to design this type of repairs. Many analytical, semi-analytical and finite element models were proposed throughout the years to predict the strength of stepped repairs, using various hypotheses and simplifications. The aim of this paper is to investigate the influence of modelling hypotheses on stress distribution and strength prediction of composite stepped repairs. Five simplified stepped joint models using macro-element (ME) modelling and finite element method (FE) are compared to a full 3D FE model of a stepped repaired panel. The influence of step length and adhesive fracture toughness was investigated to determine the field of validity of each model. Among FE models, it was shown that modelling the equivalent joint under 2D generalized plain strain gives a very close strength prediction to the 3D stepped repair model while saving computation time. Simplified macro-element models under bar or beam hypotheses are fairly close to the results of FE modelling, but the deviation between those and FE is sensitive to step length and adhesive fracture toughness.

An extended formulation of the macro-element (ME) based models, representing for both adherends and adhesive along the entire overlap in only one four-node element, is presented. Compared to earlier modelling, continuum ME (CME) and discrete ME (DME) based models, the adherend parts are also modelled as plane continuum media, for which high order displacement fields are freely supposed. Both extended CME (ECME) and extended DME (EDME) based modelling can have their displacement field orders of each continuum layer be set individually, and can be enriched using springs for interfaces or boundary conditions modelling, allowing the stresses to vanish at the free edges. The methodology and formulation of the stiffness matrix for both the adherend without adhesive and the bonded overlap is presented. The assessment is performed for a single-lap joint geometry by comparison results from a plane strain finite element (FE) model with extended ME-based models. Good agreements are shown for both thin and thick adhesive case studies.

In the present work, a specific three-point bending test is applied to evaluate how the roughness can impact the bond strength (adherence) and the mechanism of interfacial failure initiation. The study is conducted using an aluminum alloy 2024-T3 as substrate and the DGEBA (polyepoxide bisphenol A diglycidyl ether) /DETA (diethylenetriamine) as adhesive, considering different abrasive surface treatments. An optimal roughness is reached to maximize the critical force during failure initiation; besides, the roughness impacted the failure propagation mechanism and the failure initiation area for each abrasive treatment. A power-law regression is considered to correlate the critical force and the failure initiation area, considering different average roughness. Local assessment using a mechanical profilometer and Scanning Electron Microscope (SEM) with Focused Ion Beam (FIB) are applied to measure the residual adhesive thickness at failure initiation and failure propagation zones as well as the initiation-to-propagation transitions. It is constated that the residual adhesive thickness is dependent on the average roughness of the substrate. Finally, Weibull’s analysis is undertaken to demonstrate that the mechanism of failure initiation for all substrate groups is essentially the same, although the failure propagation mechanism can be different.

Microscale residual stress may develop during the manufacturing of Carbon Fiber-Reinforced Polymer (CFRP) composites and negatively affect apparent macroscale mechanical properties. Accordingly, accurately capturing residual stress may be essential in computational methods used for composite material design. This work presents a new data-driven methodology for the evaluation of microscale residual stress in CFRPs using fiber push-out experiments with in situ scanning electron microscopy (SEM) imaging. SEM images reveal significant through-thickness matrix sink-in deformation in resin-rich areas after nearby fibers are pushed out, which is attributed to the release of microscale process-induced residual stress. The sink-in deformation is measured experimentally, and a Finite Element Model Updating (FEMU) method is used to retrieve the associated residual stress. The finite element (FE) analysis includes simulation of the curing process, test sample machining, and fiber push-out experiment. Significant out-of-plane matrix deformation larger than 1% of the specimen thickness is reported and associated with a high level of residual stress in resin-rich areas. This work emphasizes the importance of in situ data-driven characterization for integrated computational materials engineering (ICME) and material design.

A methodology for the formulation of the stiffness matrix of an enriched finite element, called continuum macro-element (CME), representing for the full length of a bonded overlap and both the adhesive and the adhesive in only one four-node elements, is presented. Compared to earlier macro-elements modelling the bonded overlap as beams on elastic foundation, the CME supposed the adherends and the adhesive as plane continuum media, for which higher-order displacement fields is supposed for the adhesive. The formulation of the stiffness matrix for the adherend outside of the overlap is presented as well to address the stress analysis of single-lap bonded joint for assessment purpose. The assessment is performed by comparisons with the results from (i) a plane strain finite element and (ii) two recent papers by Nguyen and Le Grognec (2021) and Methfessel and Becker (2022). Good agreements are shown.

We present the correlation of high energy proton and atmospheric neutron single-event test results on a recent 1200V SiC MOSFET, obtained with destructive and non-destructive methods. The correlation of the results is discussed, showing a good agreement between both methods. TCAD modelling is used to investigate the failure mechanisms.