Fiber Metal Laminates Research Articles

Enhanced interfacial adhesion of CF/PEEK-titanium hybrid laminates via rare-earth coordination interaction

Improving the adhesion between metal sheet and resin is an important issue for the development of high-performance fiber metal laminates (FMLs). In this study, a novel rare-earth surface modification is applied for the first time to enhance the interfacial bonding strength of titanium sheets and polyetheretherketone (PEEK). Density functional theory (DFT) calculations show that cerium element could form coordination bonds at the interfaces between metallic titanium and resin. The surface wettability of titanium sheet is boosted, which is beneficial to PEEK infiltration and physical adhesion improvement. Meanwhile, with the existence of coordination bonds at the interface, the interfacial shear strength between titanium and PEEK, and the flexural strength and interlaminar shear strength of CF/PEEK-Ti laminates are enhanced significantly. Therefore, the cerium surface modification method proposed in this study has great potentials for FMLs, which provides a new strategy for generating chemical bond at interfaces between metal and resin.

Investigations on Guided Ultrasonic Wave Dispersion Behavior in Fiber Metal Laminates Using Finite Element Eigenvalue Analysis

AbstractComposite materials such as fiber metal laminates combine the advantages of metallic materials and fiber‐reinforced polymers. Hence, these materials are of great interest for thin‐walled structures in lightweight engineering. Due to the structure of these materials, damage to fiber metal laminate components occur more frequently inside the structure than with conventional materials. Since the detection of interlaminar damage is more complicated compared to external damage, it is one of the biggest challenges in the use of fiber metal laminates. One approach to detect this kinds of damage, is the use of guided ultrasonic waves, for example Lamb waves. To be able to perform such damage detection, knowledge about the propagation behavior of this kind of waves in fiber metal laminates is fundamental. Abrupt stiffness variations across the thickness of fiber metal laminates, resulting from the different material layers, lead to the question whether the known approaches for the propagation of guided ultrasonic waves in isotropic and transversely isotropic materials are applicable here. Therefore, the objective of this work is to investigate the propagation behavior of these guided ultrasonic waves in fiber‐metal laminates over large frequency ranges. For this purpose, dispersion relations from finite element simulations are compared with experimental data and numerical solutions based on the analytical framework. The investigations are carried out using a fiber metal laminate consisting of steel and carbon fiber‐reinforced polymers. Due to the orthotropy of the laminate, wave propagation in the fiber direction and perpendicular to it is considered. For the finite element simulations a linear two dimensional eigenvalue analysis is used. This method is especially suitable because it offers a very efficient modeling approach for this kind of application. The experimental data are based on measurements contained in previous publications by the authors. The comparison of the finite element simulations with the experimental data and the data from the analytical framework show that they are in good agreement. The results shown in this work serve to validate the numerical approach presented and allow for further, more complex simulations.

A two‐dimensional analysis of the influence of delamination on the wave propagation in a fiber metal laminate

AbstractStructural Health Monitoring (SHM) of composite materials using Guided Ultrasonic Wave (GUW) represent a central research topic with a large range of application fields. One major concern in this context is the detection of hidden inter‐laminar damage such as delaminations in composites, as these cannot be detected by visual inspection [1]. Fiber Metal Laminate (FML) consist of different layers of metal (e.g. steel) and Fiber Reinforced Polymer (FRP) stacked on top of each other and belong to a new type of material system that has been insufficiently explored in the context of damage detection using GUW. Following the objective of damage detection using GUW, two dimensional numerical simulations of the wave propagation in FML composed of layers made from steel and Carbon Fiber Reinforced Polymer (CFRP) are conducted. In order to properly identify and describe the different phenomena occurring when the GUW encounter a delamination three kind of numerical models were considered: undamaged plate, damaged plate with symmetrically and off‐center placed delamination. As far as the modeling of delaminations, the present work is restricted to a zero‐thickness‐delamination generated by de‐merging nodes at the interface of the layers. Mode‐selective excitation (fundamental symmetric and anti‐symmetric mode wave) of the models was then applied and the B‐scan representations of the in‐plane and out‐of‐plane displacement fields were generated and compared. Effects such as reflection, transmission and mode conversion of the main wave are observed and analyzed. Thus, a singular phenomenon as the trapped mode wave can be clearly observed and interpreted.

On the influence of the probabilistic microstructural characteristics of glass fiber‐reinforced composites on the wave propagation in GLARE

AbstractIn an attempt to reduce the crack propagation speed in structures made of fiber‐reinforced composites metal layers are added to the laminate forming a new group of materials, so‐called fiber metal laminates. GLARE, a combination of glass fiber‐reinforced polymer and aluminum, is well‐known and frequently used in the aeronautical industry. One major drawback of such materials is their susceptibility to impact damage, which is not detectable by visual inspections. Hence, an adequate structural health monitoring technique to detect interlaminar damage like delaminations is essential for components made from fiber metal laminates. One approach here is the use of guided ultrasonic waves because they travel long distances without substantial damping and cover the whole thickness of thin‐walled structures. However, the probabilistic microstructure causes a continuous mode conversion in wave guides made from fiber‐reinforced composites. This significantly influences the signal processing and as a consequence, the interpretation of captured time signals. This phenomenon is induced by the random spatial distribution of the fibers in the matrix material, leading to an ongoing excitation of new wave modes within the propagating S0 wave mode. In this research, the effect is studied for the wave propagation in GLARE 3/2‐0.4. Therefore, the spatial distribution of the transverse YOUNG's modulus for the glass fiber‐reinforced layers is represented by homogeneous second‐order GAUSSian random fields. Afterwards, the propagation of the guided ultrasonic wave is simulated numerically. The results reveal that the continuous mode conversion is not only observable within the glass fiber‐reinforced polymer layers but also at the top and bottom surface and hence, within the isotropic layers. This holds for the wave propagation both in and perpendicular to the fiber orientation of the glass fiber‐reinforced polymer layers.

Theoretical Modeling and Experimental Verification of the Bending Deformation of Fiber Metal Laminates.

Fiber metal laminates have been widely used as the primary materials in aircraft panels, and have excellent specific strength. Bending deformation is the most common loading mode of such components. An accurate theoretical predictive model for the bending process for the carbon reinforced aluminum laminates is of great significance for predicting the actual stress response. In this paper, based on the metal-plastic bending theory and the modified classical fiber laminate theory, a modified bending theory model of carbon-fiber-reinforced aluminum laminates was established. The plastic deformation of the thin metal layer in laminates and the interaction between fiber and metal interfaces were considered in this model. The bending strength was predicted analytically. The FMLs were made from 5052 aluminum sheets, with carbon fibers as the reinforcement, and were bonded and cured by locally manufacturers. The accuracy of the theory was verified by three-point bending experiments, and the prediction error was 8.4%. The results show that the fiber metal laminates consisting of three layers of aluminum and two layers of fiber had the best bending properties. The theoretical model could accurately predict the bending deformation behaviors of fiber metal laminates, and has significant value for the theoretical analysis and performance testing of laminates.

Assessment of failure mechanism of double-strap 3D-FML adhesively bonded joints under tensile and compressive loadings using cohesive zone modelling approach

Despite the widespread applications of Fiber Metal Laminates (FMLs), there is a clear lack of information regarding the performance of bonded joints mating 2D-FMLs and the recently-developed 3D-FMLs, particularly when considering their performances under compressive loading. Therefore, as a follow-up to the authors’ recent investigations, an extensive series of numerical analyses are conducted in the present study to simulate the tensile and compressive (i.e., buckling, and post-buckling) responses of double-strap adhesively bonded joints mating 3D-FMLs. The 3D-FMLs are joined using carbon fiber-reinforced plastic (CFRP) straps and structural epoxy resin. The damage initiation and evolution in the bonded regions are modelled by seven different FE models, which incorporated the mixed-mode trapezoidal Cohesive Zone Modelling (CZM) approach in conjunction with continuum elements. Firstly, the effects of different numbers of CZM layers and thicknesses on simulating the damage mechanism are investigated. Subsequently, the influence of CFRP straps’ thickness and length on the performance of the adhesive joints is parametrically studied. The distributions of peel and shear stresses along the length of the bonded regions are also systematically explored. Based on the results, the optimal joint configuration is established, and the most effective modelling framework is highlighted by comparing numerically predicted results against experimental results.

Soft impact of GLARE fiber metal laminates

Dynamic response of fully clamped GLARE fiber metal laminates (FMLs) subjected to closed-cell aluminum foam projectile impact has been investigated experimentally and numerically. FMLs with various layup angles are made of glass fiber prepreg and aluminum alloy with identical thickness among each type of layers. The active failure modes of FMLs, including the global deformation and local denting with cracks of the metals, fiber fracture, delamination and interlayer debonding are observed in the experiments. It is shown that the damage degree and deflections of FMLs decrease with increase of the thickness. FMLs of the oblique angle layups have similar deformation and failure modes as the orthogonal layups. Finite element (FE) simulations are performed and are in good agreement with the experimental results. Most kinetic energy of the metal foam projectiles is dissipated by the deformations of the metal layers, the deformations and fracture of the composite layers, debonding and foam compression. The impact resistance of FMLs can be enhanced by increasing the interlayer bonding strength.

Dynamic response and failure behaviour of thermoplastic fibre–metal​ laminates subjected to confined blast load

Due to the confinement effect, the blast load from a confined space explosion tends to cause more severe damage to structures. When subjected to this type of confined explosion, thermoplastic fibre–metal laminate (TFML) panels would experience repeated shock waves and relatively longer durations of quasi-static pressure. A better understanding of the dynamic response and damage mechanism of TFMLs under confined blast loading can greatly benefit the design of composite structures with improved blast resistance. In the present work, TFMLs with seven different configurations were designed and fabricated. Confined-blast experiments with various masses of trinitrotoluene (TNT) were performed on these TFMLs. The experimental results, including deflection time histories and X-ray computed tomography (CT) images, have been applied to the development of a method for predicting the dynamic response of laminates under confined blast loads. The findings of this work will assist in the rapid assessment of the deformation of fibre–metal laminates and assist in the pre-design of laminate structures in confined explosions.

The Correlation of LVI Parameters and CAI Behaviour in Aluminium-Based FML.

An experimental analysis of mechanical behaviour for aluminium-based fibre metal laminates under compression after impact was conducted. Damage initiation and propagation were evaluated for critical state and force thresholds. Parametrization of laminates was done to compare their damage tolerance. Relatively low-energy impact had a marginal effect on fibre metal laminates compressive strength. Aluminium-glass laminate was more damage-resistant than one reinforced with carbon fibres (6% vs. 17% of compressive strength loss); however, aluminium-carbon laminate presented greater energy dissipation ability (around 30%). Significant damage propagation before the critical load was found (up to 100 times the initial damaged area). Damage propagation for assumed load thresholds was minor in comparison to the initial damage size. Metal plastic strain and delaminations are dominant failure modes for compression after impact.

Design and optimization of the bumper beam with corrugated core structure of fiber metal laminate subjected to low-velocity impact

Fiber metal laminates (FMLs) exhibit good impact resistance performance with hybrid structures based on alternating stacking of metal alloy layers and fiber-reinforced composite plies. Therefore, FMLs become excellent candidates for bumper beams in automobiles with an enhanced energy absorption capacity. Herein, the design and optimization of the bumper beam consisted of FMLs corrugated sandwich structure subjected to low-velocity impact were investigated. Firstly, material characteristic parameters of aluminum alloy and carbon fiber reinforced thermoset polymer (CFRP) were obtained from the quasi-static and dynamic experiments respectively. Then, finite element analysis (FEA) of cap structures of FMLs was established to capture the deformation process of the low-velocity impact. The simulation results were well-validated using material parameters from the dynamic mechanical tests. Afterward, the influence of material type and stacking sequences on performance was numerically investigated under low-velocity impact loading, whereas the performance of aluminum alloy and CFRP were set as controls. A multi-objective optimization method was considered to realize the maximum energy absorption with minimum impact peak force. The non-dominated sorting genetic algorithm II (NSGA-II) was introduced to obtain the global optimum solution. The current work highlights the excellent mechanical performance of the FMLs bumper beam in low-velocity impact.

Blast analysis of efficient honeycomb sandwich structures with CFRP/Steel FML skins

The explosive blast is a major threat in the form of a terrorist attack to damage defense, aerospace, as well as civilian structures. This hazard of an explosive attack affects the lives of people and property. As a result, the modern era is searching for blast resistance structures to save humans and properties from these serious attacks. However, the current numerical analysis represents an effort in the novel design of blast-proof sandwich structures to meet the current era's demand. A stainless-steel sandwich structure using a square honeycomb core was designed to perform conventional weapons effects program (CONWEP) air-blast analysis on ABAQUS/Explicit. After a mesh sensitivity study and validation of the steel sandwich structure with experimental data published in the literature, the effect on blast characteristics of the aluminum (Al) foam-filled honeycomb and carbon fiber reinforced polymer (CFRP) with steel skins using fiber metal laminate (FML) for the front, back, or both skins of the sandwich structure was investigated. The entire finite element (FE) modeled sandwich structures were subjected to 1 to 10 kg of TNT air-blast loads at stand-off distances (SoD) ranging from 150 mm to 200 mm, and their blast resistance performance was evaluated using skin deflection and energy absorption. The damage initiation and evolution of composite laminates in the FML skins were investigated by the Hashin, Puck, and Singh failure criteria, respectively. These criteria were implemented via the VUMAT code. The obtained results showed that using FML skin for the sandwich structure diminished both the front and back skin deflection while improving specific energy absorption. A positive impact on blast mitigation by sandwich structures was observed with an increase in SoD. The lightweight FML front skin and steel back skin used bare square honeycomb hybrid sandwich structure had the smallest back skin deflection and the highest energy absorption up to 3 kg TNT. For the same combination of skins, the hybrid sandwich with foam-filled square honeycomb core represented a positive impact on the blast proof characteristics for high-intensity blasts ranging from 5 kg to 10 kg TNT. However, the novel designed hybrid sandwiches are recommended as a protective structure for defense, ships, automotive, etc.

A size-dependent analytical model to predict sound transmission loss of double-walled fiber metal laminated nanoplates

In the present study, the sound transmission loss (STL) through the air-filled rectangular double-walled cross-ply fiber metal laminated (FML) nanoplates under simply supported and clamped boundary conditions is studied using the nonlocal strain gradient theory (NSGT) and third-order shear deformation theory (TSDT). NSGT is complemented with hardening and softening material effects, which can significantly enhance the accuracy of small-scale results. The sound velocity potential and Hamilton’s principle are employed to derive the coupled size-dependent vibroacoustic equations. The Galerkin method is exploited to solve vibroacoustic equations and obtain the STL. The developed solution is examined in terms of its accuracy and precision via a comparison with other available data in existing research. The effects of different parameters such as boundary conditions, nonlocal and strain gradient parameters, lay-ups, incident angles, and acoustic cavity depth on the STL through the double-walled FML nanoplates are investigated.

Quasi‐static compression behavior and crashworthiness of GLARE laminate conical frustum with progressive circular and square cut‐outs

AbstractThe design of structures using advanced composite materials such as fiber metal laminate requires an in‐depth understanding of their deformation behavior and response to the discontinuities in the structures. In the present study, a GLARE kind of fiber metal laminate in 2/1 configuration was fabricated in the form of conical frustum. Three cut‐outs of two different shapes such as circular and square were intentionally made at the center of the frustum at an angular distance of 120°. The quasi‐static compression test was conducted to determine the crashworthiness characteristics of laminate frustums with cut‐out and the results were compared with perfect frustum. The study showed that the laminate with cut‐out undergone for inward buckling near the discontinuities. Apart from that, the delamination and outward buckling were noted in the edges of the cut‐outs. The energy absorption of laminate frustum with circular cut‐outs was observed to be better than square cut‐outs. The performance of laminate frustums with cut‐outs was ranked using Technique for Order Preference by Similarity to an Ideal Solution technique. A finite element analysis was carried out to simulate the deformation behavior of laminate frustum with cut‐outs and the numerical results of deformation mode closely represented the experimental results.

Comparison of the Low-Velocity Impact Responses and Compressive Residual Strengths of GLARE and a 3DFML

The impact performance and compression after impact characteristics of 2D and 3D fiber metal laminates (FMLs) are investigated both experimentally and numerically. Commercial-grade GLARE3A-3/2-0.3, and a recently developed FML, which incorporates a unique 3D glass fabric, are used in the study. Both FMLs have similar areal densities. The specimens are subjected to impact loading at three energy levels-low, intermediate, and high. The test results indicate that GLARE is slightly more resilient under impact compared to the 3DFML. However, since GLARE is much thinner than the 3DFML, the two-material systems exhibit very different failure modes. GLARE and 3DFML lost up to 62.6% and 41.5% of their original compressive load-bearing capacity, respectively. Robust and accurate finite element models are developed that can predict the damage evolution and failure modes of both FMLs. Knowing the level of reduction in the residual load-bearing capacity of a material resulting from an impact is of practical importance when assessing the service life of materials. However, further exploration would be required to determine how the information obtained through testing relatively small-sized specimens in a laboratory environment can be extrapolated to larger real-life structural components.

The investigation on dynamic damage evolution and energy absorption of FML pin joints with different ply stacking sequences

With fiber reinforced composites widely applied in aerospace, the crashworthiness aspects for aircraft have been an important part of the design. Fiber metal laminate (FML) is a kind of hybrid composite fabricated by fiber reinforced plastic composites and metal layers, showing a potential application on energy absorption. In this work, the damage evolution and energy absorption of FML pin joints are investigated comprehensively. The configurations of FML pin joints with constant geometric parameters are designed to avoid net-tension damage failure in the experiments. The damage evolution processes of specimens are captured by a high-speed camera, and the surface strain is calculated by Digital Image Correlation (DIC) method. Abaqus/Explicit solver is adopted to simulate the mechanical responses of FML pin joint considering ductile damage of metal, damage evolution of composites, and delamination of the interface. The results indicate that the initial dynamic ultimate bearing strength of FML incorporate notable contribution from inertia effects. The average bearing strength associated with energy absorption efficiency is related to the ply stacking sequence of composites. The fiber direction of laminates should be perpendicular to the movement direction of pin as much as possible under the premise that the initial ultimate bearing strength satisfies the design requirement. The results obtained in this work provide comprehensive design guidance for the improvement of crashworthiness in the composite fuselage.

Application of a new strategy for time-efficient laser treatment of galvanized steel substrates to improve the adhesion properties

Laser structuring to improve the adhesion properties of steel substrates in fiber-metal laminates offers many advantages that are highly suitable for modern industrial requirements. Maintenance and energy costs are relatively low, it is easy to automate, and there are no by-products such as chemicals or abrasives to dispose of or recycle. This makes laser structuring a particularly environmentally friendly process, which is nowadays more important than ever. On the other hand, the process time for laser structuring is much higher than for chemical pre-treatment, for example. In past studies, the time and cost efficiency of the laser structuring process has tended to play a minor role. However, there are approaches in which laser structured surfaces are adapted to the shear stress peaks occurring within the adhesive layer, thus requiring only partial structuring of the area to be bonded, potentially saving process time. In this experimental study, electrolytically galvanized steel substrates were partially laser structured to match the shear stress distribution and then bonded to a carbon fiber-reinforced plastic. The adhesion properties achieved were characterized using shear tensile tests and compared with the properties of the fully structured ones. With the partial laser structuring, a saving of 66 % of the conventional process time was achieved while maintaining 95 % of the same shear strength.

The Effect of Hybridizing Natural Fibers and Adding Montmorillonite Nanoparticles on the Impact and Bending Properties of Eco‐Friendly Metal/Composite Laminates

This study aims to fabricate eco‐friendly fiber metal laminates (FMLs) with acceptable flexural and impact properties. For this reason, the jute and basalt fibers are hybridized into the epoxy matrix and then sandwiched between 2024 aluminum layers. After that, various contents of montmorillonite nanoparticles are dispersed in these structures to enhance these mechanical properties. To characterize the effect of hybridizing fibers and incorporating nanoparticles, macrostructural and microstructural investigations are performed. In hybrid structures, the highest flexural and impact properties are achieved, when the basalt layers are placed as exterior layers of the composite core. However, in the FMLs containing nanoclay particles, the FMLs with the 1.5 or 3 wt% nanoclay have an acceptable improvement in the mechanical properties, which relates to the exfoliated or intercalated structure of nanoclay. Creating nanocomposite interlocks on the surface of aluminum layers, enhancing the adhesion bonding at the interface of metal/composite, and improving the adhesion between jute or basalt fibers with the epoxy matrix are the distinct improving mechanisms, which are characterized by macro‐ and microstructural methods.

A Critical Review on Fiber Metal Laminates (FML): From Manufacturing to Sustainable Processing

Composite materials such as Fiber Metal Laminates (FMLs) have attracted the interest of the aerospace and automotive industries due to their high strength to weight ratio, but to use them as structures it is necessary to master the manufacturing and wiring techniques of these materials. Therefore, this paper aims to address and summarize the drilling and milling processes in FMLs based on a literature review of papers published from 2000 to 2023. Parameters used in multi-material manufacturing and machining such as drilling and milling, tool geometry, tool coating, lubricants and coolants published by researchers were analyzed, compared and discussed. Machining process parameters related to sustainability were also analyzed. A SWOT analysis was carried out and discussed to identify opportunities for improvement in the machining process. There are opportunities to develop the surface treatment of aluminum alloys, such as testing other combinations than those already used, testing non-traditional surface treatments and manufacturing modes, and developing sustainable techniques during the FML manufacturing process. In the area of tooling, the opportunities are mainly related to coatings for tools and changing machining parameters to achieve an optimum finished part. Finally, to improve the sustainability of the process, it is necessary to test coated drills under cryogenic conditions to reduce the use of lubricants during the machining process.

Forming challenges of small and complex fiber metal laminate parts in aerospace applications—a review

Forming challenges of small and complex fiber metal laminate parts in aerospace applications—a review

Mechanical investigation of inverse FML under mode II loading using acoustic emission and finite element method

Fibre metal laminates (FML) are a specific type of material made of both metal and fibre-reinforced composite layers. In this paper, the authors analyse the behaviour of fibre metal laminates with unusual thermoplastic matrices in composite layers under mode II loading. A three-point bending setup was used along with end notched flexural specimen (ENF) to determine behaviour and of the interface between metal and composite layers under shear stress. Due to nonstandard layup configuration, mixed experimental–numerical method is proposed by authors to determine fracture energy. The finite element method is used to analyse conducted tests. A model with cohesive elements is applied to determine damage in the interface layer. For the purpose of understanding the damage process in particular layers, Acoustic Emission analysis was conducted for each specimen and parameters like amplitude, frequency, duration, root-mean-squared level of noise were obtained. As a result of conducted research critical energy in mode II was obtained GIIC=0.05Nmm as well as critical stress value in local shear direction (initiation criterion) tso=10MPa. The result obtained can be used as part of the material model in assessing the mixed-mode state of the researched material.