Fiber Metal Laminates Research Articles

Elaboration of a Multi-Objective Optimization Method for High-Speed Train Floors Using Composite Sandwich Structures

The transportation industry needs lightweight structures to meet economic and environmental demands. Composite sandwich structures offer high stiffness and low mass, making them ideal for weight reduction in high-speed trains. The objective of this research is to develop a method of weight and cost optimization for floors of high-speed trains. The studied sandwich floor structure consists of Fiber Metal Laminates (FML) face sheets and a honeycomb core. Different variations of FMLs were investigated to define the optimal sandwich structure for minimum weight and cost. The Neighborhood Cultivation Genetic Algorithm (NCGA) was used to search the design space, and the Finite Element Method (FEM) was used to construct the optimal design of the train car floor panel. The FEM and optimization results had a maximum difference about 11%. The study concluded that using face sheets made entirely of Fiber-Reinforced Plastic (FRP) or Fiber Metal Laminates (FMLs) resulted in significant weight savings of approximately 62% and 32%, respectively, compared to a sandwich structure made entirely of aluminum, but a lighter structure was associated with higher cost. The main contribution of this study is the elaboration of a multi-objective optimization method that utilizes a wide range of constituent materials and structural components in order to construct weight- and cost-optimized sandwich structures.

Correlation between toughness and metal volume fraction in carbon-glass fiber laminates tested under tensile loading

ABSTRACT The conducted work investigates the correlation between toughness characteristics and metal volume fraction in Fiber Metal Laminates (FMLs) subjected to tensile loads. Aerospace grade Al 7075-T6 was utilized as the varying metal alloy in the study. The metal alloy was combined with glass and carbon fiber layers to create different configurations of FMLs with varying stacking sequences. For better adhesion between metal and composite parts, aluminum layers were etched with sodium hydroxide and chromic-sulfuric acid solutions. The laminate sheets were prepared using the hand layup method which was followed by a vacuum bagging process. Specimens were cut and tested as per ASTM standard. Results of the experimentation showed that the aluminum volume fraction is directly related to the failure strain and modulus of toughness, however, inversely proportional to the stiffness characteristics of the laminates. The highest improvement of 111% in toughness parameter in comparison to pure composite laminate was reported. Finally, a comprehensive analysis on the type of fracture behavior in comparison to classical laminate theory was critically discussed.

The effect of thermal and cryogenic environments on the impact performance of aluminum-glass fibers/epoxy laminated composites

In this research work, the effect of various environments on the impact properties of fiber metal laminates was investigated. To do so, in the first step, the 2024 aluminum layers were surface modified with the etching method. Then, the various glass fibers/epoxy composites with the configurations of (0°/0°/0°/0°, 90°/90°/90°/90°, +45°/−45°/−45°/+45° and 0°/90°/90°/0°) were sandwiched between two aluminum sheets. In the following, these samples were aged under thermal cycling (between 25 and 100 °C), isothermal aging at the temperature of 100°C, cryogenic cycling (between −196 and 25 °C), and cryogenic isothermal aging at the temperature of −196°C. To understand the effect of various aging methods on the mechanical features of these structures, the Charpy impact test was performed. Also, the failure mechanisms and related phenomena were assessed by visual observations and microstructural investigations. In the first steps of thermal aging, the impact strength of FMLs was improved, which the highest of that being 46.9%. In the higher cycles or exposing times, the impact strength showed reducing trends, due to thermal degradations of the epoxy matrix. The cryogenic cycling in the high cycles reduced the impact strength of FMLs. But in the cryogenic isothermal condition, in some cases, the improvement in the impact strength was seen, which the highest of that being about 43.4%.

The energy dissipation mechanism of bi-metal Kevlar\ itanium fiber metal laminate under high-velocity impact

Fiber Metal Laminate (FML) is an impact resistant material widely used in marine and aerospace engineering structures that need to withstand impact damage, especially deck and bulkhead structures. To investigate the effect of titanium skin on energy dissipation, a series of ballistic impact experiments were performed on a bi-metal FML in this study. The FML were made of Ti–6Al–4V (TC4) skins and orthogonally woven Kevlar-129 fabric impregnated with E54-epoxy resin. Thus, this FML can be also called as bi-metal Kevlar/titanium FML. In addition, a numerical model based on stress wave theory is proposed to study the energy dissipation mechanism of the bi-metal FML under high-velocity ballistic impact. The residual ballistic velocity of the numerical model is verified by the experimental data in this paper and previous experimental data, showing that the numerical model is a more efficient method to analyze the energy dissipation mechanism of bi-metal FML. According to the numerical results, as the dominant energy dissipation, the perfectly inelastic collision of titanium skins dissipated 43%–62% and 17%–27% of total energy at the high-velocity penetration and ballistic limit, respectively. However, regardless of whether the FML has been penetrated, the compression-shear of Kevlar interlayer dissipated 24%–27% of total energy. Furthermore, the numerical calculations also show that the ballistic limit increases approximately linearly with the relative titanium skin thickness when the front titanium skin of the FML is thicker. Finally, this paper proposes a simple specific energy absorption prediction formula that helps to evaluate the energy dissipation capacity of FML.

Lateral Buckling of Glare for Aerospace Application

Glare (Glass Reinforced Aluminium) which consists of fibre metal laminate composite consisting of aluminium and glass is used aerospace structures are supposed to buckling and impact loads. Lateral buckling analyses were made to determine critical buckling loads, and results were compared to Al 2024-T3 in this paper. Weight and load carrying capacity of Glare grades were taken into consideration and the importance of weight to critical load was stated. Numerical works were carried out by starting with Glass and Aluminum then continued for Glare Grades of Glare 2A, Glare 2B, Glare 3A, Glare 3B, Glare 4A, Glare 4B, Glare 5A, Glare 5B, Glare 6A and Glare 6B to estimate buckling load values. Several comparisons were presented for Glare grades based on Al 2024-T3 through paper. Glare 2A, 2B, 3A, 3B, 6A and 6B Grades have lower weight and buckling load values compared to Al 2024-T3. Lower weight is essential for aerospace applications. But optimum weight and load carrying capacity can be selected for intended applications by taking weight and load into consideration at same time. Although Glare grades of 4A 2-1, 4B 2-1, 5A 2-1 and 5B 2-1 having closer weight (17.60g, 17.60g, 19.13g and 19.13g respectively) to Al 2024-T3 (17.31g), higher buckling loads were determined for Glare grades numerically. The best choice for Glare as an alternative to Al 2024-T3 under lateral buckling loading can be decided for point of views of less weight to critical load ratio.

Accurate simulation on the forming and failure processes of fiber metal laminates: A review

Fiber metal laminates (FMLs) are a kind of composite material prepared by alternately arranging fiber layers and metal sheets at a certain temperature and pressure. It has been widely used in aerospace and automobile transportation for its excellent combined mechanical properties. For the forming and failure processes of FMLs, the interfacial behavior and damage evolution of components are hard to be observed experimentally. Therefore, it is of great importance to simulate them accurately. In this article, the development and application of FMLs were first introduced. Then the comparison of constitutive models in FMLs simulation was given, especially the dynamic constitutive model applied to the metal layer. After that, the important aspects of damage evolution, interface behavior, and model optimization in the simulation on the forming and failure process of FMLs were analyzed, and the emphasis is on the nonlinear progressive damage model of different materials, the construction of cohesive zone model and superior meshing methods. Furthermore, the experimental verifications of FMLs simulation were given. It is shown that the deformation behavior and damage characteristics of various kinds of FMLs during forming and failure processing can be accurately predicted by reasonable numerical simulation. Eventually, the future outlooks for numerical simulation of FMLs was proposed. Through this review, scholars and engineers who are interested in FMLs can systematically understand the numerical simulation work of FMLs, which is helpful in improving the quality of research in this field.

A Review on Mechanical Properties and Response of Fibre Metal Laminate under Impact Loading (Experiment)

A Review on Mechanical Properties and Response of Fibre Metal Laminate under Impact Loading (Experiment)

A finite element-based continuum damage model for mechanical joints in fiber metal laminates under static and fatigue loading

In this study the theory and experimental validation of a novel continuum damage modeling framework for highly predictive strength and fatigue analyses of mechanical joints in fiber-reinforced polymer (FRP) composites with local metal hybridization is presented. In contrast to existing damage modeling approaches for mechanical joints in fiber metal laminates (FML), the herein presented framework is able to predict the joint’s failure mode both under static and fatigue loading, which is an indispensable feature for the identification of damage tolerant lightweight joint designs. For this purpose, the laminate’s constituent materials (i.e. the metallic inlays and the FRP plies) are simulated individually by physically motivated static-fatigue continuum damage models. Here, the elastic and plastic mechanical strain energy is utilized to compute the materials’ damage parameters. In this way, the models are (i) able to suppress any mesh-dependence during material softening, (ii) able to account for damage initiation and growth, and (iii) applicable both in the low- and high-cycle fatigue regime. The algorithms are implemented as user-material (UMAT) subroutine for the commercial finite element software ABAQUS. The damage modeling framework is validated for open hole tension and T-bolt joint setups both under static and fatigue loading.

Thin fiber metal laminates comprising functionally graded ballistic-grade fabrics subjected to mechanical and damping characterization

Fiber metal laminates (FMLs) intended for blast and ballistic resistance applications need to contain ballistic grade plies like aramid (Kevlar®), ultra-high molecular weight polyethylene (Spectra®) as supporting plies in addition to the metallic layers. The order of stacking varies depending on the functional grading of plies. In the current work, five sequences of fiber metal laminates made of AA6061-T6, aramid, ultra-high molecular weight polyethylene, and paperboard have been developed. The novelty of the work is the basis of the layering based on functional grading by the shock impedance of the respective plies. The FML sequences were subjected to mechanical characterization comprising flexural and tensile tests, as well as damping characterization for vibration characteristics. The key metrics of areal density, storage modulus, natural frequency, damping coefficient, tensile strength, tensile modulus, tensile strain-to-failure, flexural strength, flexural modulus, flexural strain-to-failure were considered for comparing the different layups of the FMLs. From the experimental studies, placing a low-impedance ply (paperboard layer) as the penultimate layer before the AA6061-T6 rear skin led to the best mechanical and damping performance as seen in the FML sequences BRSP-II and BRSP-IV.

Effects of Specimen Thickness and Non-Isothermal Process on Creep Behavior of AA2024 Aluminum Alloy

Fiber metal laminate (FML) is a kind of lightweight material that has garnered much attention in recent years due to its excellent properties under tensile, flexure, and impact conditions. However, little attention has been paid to the evolution of aluminum alloy during the FML forming process. In this paper, the effects of specimen thickness and the non-isothermal process on creep behavior of AA2024 aluminum alloy, which is generally used in FMLs, were systematically studied in terms of creep strain, mechanical properties, and microstructure. The results show that the microstructure and mechanical properties of the aluminum alloy layer are greatly affected by the creep age during the FML forming process, but the specimen thickness has little effect on the creep behavior under experimental conditions. During the non-isothermal process, the creep strain generated during the heating stage is about 40% of total strain. Compared with the isothermal process, the creep strain of the non-isothermal process increases by 20–40%, and the apparent activation energy Qc is larger. According to the creep behavior of AA2024 aluminum alloy during the non-isothermal process, a creep constitutive model was established to accurately simulate the deformation of FMLs.

Prediction of stress distribution and stiffness degradation in fiber metal laminates containing matrix cracking

Fiber metal laminates (FMLs) are hybrid composite materials in which several thin metal sheets and composite layers are joined. The aim of the present study was to provide an appropriate analytical method to predict damages, including matrix cracks in composite layers and plasticity in metal layers subject to static uniaxial loading. Using the proposed model, the stress distribution in the metal and composite layers in the unit cell enclosed between the two cracks was extracted. Effects of crack spacing, applied stress, and layer thickness on the stress distribution and plasticity of the metal layer were further investigated. Finally, the stiffness reduction due to damage and the energy release rate due to matrix cracking were also extracted. In order to validate the damage model, numerical simulations were performed and the results were in good agreement with the outputs of an analytical model.

Development of an Innovative Glass/Stainless Steel/Polyamide Commingled Yarn for Fiber–Metal Hybrid Composites

Fiber-metal hybrid composites are widely used in high-tech industries due to their unique combination of mechanical, toughness and ductile properties. Currently, hybrid materials made of metals and high-performance fibers have been limited to layer-by-layer hybridization (fiber-metal laminates). However, layer-by-layer hybridization lacks in fiber to fiber mixing, resulting in poor inter-laminar interfaces. The objective of this paper was to establish the fundamental knowledge and application-related technological principles for the development and fabrication of air-textured commingled yarn composed of glass (GF), stainless steel (SS) and polyamide-6 (PA-6) filaments for fiber-metal hybrid composites. For this purpose, extensive conceptual, design and technological developments were carried out to develop a novel air-texturing nozzle that can produce an innovative metallic commingled yarn. The results show that an innovative metallic commingled yarn was developed using fiber-metal hybrid composites with a composite tensile strength of 700 ± 39 MPa and an E-modulus of 55 ± 7. This shows that the developed metallic commingled yarn is a suitable candidate for producing metal-fiber hybrid composites.

Modelling fatigue behaviours and lifetimes of novel GLARE laminates under random loading spectrum

This paper aims to experimentally and numerically probe fatigue behaviours and lifetimes of novel GLARE (glass laminate aluminium reinforced epoxy) laminates under random loading spectrum. A mixed algorithm based on fatigue damage concepts of three-phase materials was proposed for modelling progressive fatigue damage mechanisms and fatigue life of fibre metal laminates (FML) under random loading spectrum. To validate the proposed modelling algorithm, fatigue tests were conducted on the GLARE 2/1 and GLARE 3/2 laminates subjected to random loading spectrum, and fatigue mechanisms were discussed by using scanning electron microscope (SEM) analysis. It is shown that predominant fatigue failure of the GLARE laminate depends on the reference load level of random loading spectrum. Specifically, dominant fatigue failure of the GLARE laminate is dependent on fatigue strength of fibre layer at a high reference load level, but metal layer at a low reference load level. Numerical predictions agree well with experimental results, demonstrating that the proposed mixed modelling algorithm can effectively simulate fatigue behaviours and lives of the GLARE laminate under random loading spectrum.

Reinforcing interlaminar interface of carbon fiber reinforced metal laminates by graphene

Fiber metal laminas (FMLs) are disadvantaged by weak interlaminar strength limiting their application where dynamic loads are expected. In this study, graphene platelets (GnPs) were employed to strengthen the interlaminar interface in carbon fiber (CF) metal laminates. GnPs within range of 0–1 wt% were added into epoxy resin via ultrasonication. GnPs-based FML samples were prepared by wet lay-up method and tested under flexural loading, short beam shear and single lap shear. The reinforcing mechanism and failure modes were investigated. The results show that 0.5 wt% of GnPs increases the flexural strength and interlaminar shear strength by 39.7% and 53.2%, respectively. In addition, molecular dynamics (MD) simulations concluded that the addition of low content of GnPs increases the elastic modulus of the matrix and improves the resin adsorption on the surface of carbon fiber and Al sheets. The results from both the experimental and MD simulation indicate that adding GnPs can effectively improve the interlaminar mechanical properties of FMLs. In principle, the research method in this paper can be applied to the study of FMLs with other nanomaterials.

INTERFACIAL FRACTURE TOUGHNESS OF UNCONVENTIONAL SPECIMENS: SOME KEY ISSUES

Laboratory specimens used to assess the interfacial fracture toughness of layered materials can be classified as either conventional or unconventional. We call conventional a specimen cut from a unidirectional composite laminate or an adhesive joint between two identical adherents. Assessing fracture toughness using conventional specimens is a common practice guided by international test standards. In contrast, we term unconventional a specimen resulting from, for instance, bimaterial joints, fiber metal laminates, or laminates with an elastically coupled behavior or residual stresses. This paper deals with unconventional specimens and highlights the key issues in determining their interfacial fracture toughness(es) based on fracture tests. Firstly, the mode decoupling and mode partitioning approaches are briefly discussed as tools to extract the pure-mode fracture toughnesses of an unconventional specimen that experiences mixed-mode fracture during testing. Next, we elaborate on the effects of bending-extension coupling and residual thermal stresses often appearing in unconventional specimens by reviewing major mechanical models that consider those effects. Lastly, the paper reviews two of our previous analytical models that surpass the state-of-the-art in that they consider the effects of bending-extension coupling and residual thermal stresses while they also offer mode partitioning.

Protection mechanism of underwater double-hull coated with UHMW-PE subjected to shaped charge

Shaped charge warhead which is widely utilized for underwater weapons has a severe threat to the survivability of naval ships. Therefore, it is of great significance to improve their anti-destructive capability. Three types of loads - shock wave (SW), shaped charge projectile (SCP) and bubble - are generated during the process of shaped charge associated with underwater explosion, which can cause different damage characteristics into double-hull structure. In this paper, these three loads characteristics and their combined damage into double-hull structure are analyzed. Besides, the protection of fiber metal laminates on this structure are discussed. Firstly, Coupled Eulerian-Lagrangian (CEL) is used to simulate the full physical process of double-hull subjected to shaped charge associated with underwater explosion. Secondly, the damage characteristics of double-hull are obtained under the combined damage of the shock wave, shaped charge projectile and bubble. After that, the steel plate/UHMW-PE laminate (Fiber Metal Laminate, FML) is designed according to the equal surface density theory. The blast and penetration resistance of ultra-high molecular weight polyethylene (UHMW-PE) material is analyzed and the optimized FML models are discussed. The results show that a small plastic deformation and hole of the outer and inner plates of the double-hull are generated by the shock wave and the shaped charge projectile, respectively. A large plastic deformation of the outer plate is caused by the subsequent bubble load, which is torn and pulled off. The hole size of plate coated with PE and the residual velocity of projectile reduced by 26.66% and 69.50% compared with those of the steel plate, respectively. Besides, for the coated cases the outer and inner plates is respectively not pulled off and penetrated by projectile. Additionally, it is worth noting that the bubble collapses and cannot cause the secondary damage into double-hull due to the protection of coated PE. It indicates that the PE has a great protection performance of the double-hull subjected to shock wave, bubble and projectile, which can provide a reference for the naval ships protection design.

Mechanical response of fibre metal laminates (FMLs) under low to intermediate strain rate tension

• Young’s modulus of FMLs was observed to be strain rate insensitive. • Tensile strength of aluminium-based FMLs was found to be sensitive to strain rate. • Specific tensile properties of FMLs were significantly improved over pure metals. • Titanium-based FMLs had higher specific strength but lower SEA than aluminium-based FMLs. Fibre metal laminates (FMLs) are increasingly employed in various industries for their exceptional mechanical properties. Quasi-static tensile and flexural properties, fatigue performance and impact resistance of FMLs have been extensively investigated in the past. However, research on the intermediate strain rate range is still limited. This paper experimentally investigated the influence of strain rate on the tensile failure and mechanical response of self-made titanium- and aluminium-based FMLs at strain rates up to 10 2 s -1 . The results showed that, aluminium-based FMLs showed three-stage failure patterns, similar to that of the quasi-static tension. For titanium-based FMLs, only two stages were found in the stress-strain curves for all the strain rates. The strain rate had negligible influence on the Young’s modulus of all FMLs, the tensile strength and energy absorption capacity of titanium-based FMLs. However, the tensile strength and energy absorption capacity of aluminium-based FMLs were slightly strain rate sensitive. It was also found that titanium-based FMLs had higher specific tensile strength but lower specific energy absorption than aluminium-based FMLs at all strain rates.

A novel experimental method and computational micromechanical model for in-situ damage detection and prediction of stiffness degradation in cross-ply FML

A novel experimental method for in-situ damage detection in symmetrical and balanced glass/epoxy laminates composed of unidirectional layers is introduced in this paper. A 3D finite-element micromechanical model accompanies this method to predict stiffness degradation in a cross-ply fiber-metal laminate caused by transverse cracks and metal damage. The model is developed in Abaqus. Two user-defined subroutines, UMAT and UHARD, are coded in FORTRAN and linked to the model. Using a Python script, periodic boundary conditions are applied to the model. The predictions are validated against conventional loading-unloading experimental tests. Validation shows excellent agreement between the proposed model and the experiment.

The Guided Ultrasonic Wave Oscillation Phase Relation between the Surfaces of Plate-like Structures of Different Material Settings

Lamb waves occur in thin-walled structures in two wave modes—the symmetric and the antisymmetric mode. Their oscillation on the structures‘ surfaces is either in phase (symmetric) or shifted by a phase angle of π (antisymmetric). In this work, a method is developed by which to compare the surfaces’ oscillation phase relation to answer the question of whether fiber metal laminates show the same surface oscillation behavior as described for metals. The evaluation of time signals regarding the instantaneous phase angle is performed by using the continuous wavelet transformation and the short-time Fourier transformation. Numerical simulations utilizing the finite element method provide time signals from the top and bottom surface of different thin-walled structures of different material settings and configurations. The numerically obtained time signals are evaluated by the developed methods with respect to the oscillation phase. Subsequently, the oscillation phase is evaluated experimentally for the wave propagation in a fiber metal laminate. It is shown that the method based on the continuous wavelet transformation is suitable for the evaluation of oscillation phase relations in time signals. Additionally, it is proven that fiber metal laminates show only two phase relations, which indicates the occurrence of Lamb waves.

Numerical and Experimental Investigation of Flexural Properties and Damage Behavior of CFRTP/Al Laminates with Different Stacking Sequence

Fiber Metal Laminates (FMLs) are hybrid materials that combine metal components with fiber-reinforced composites. The properties and failure modes of CArbon fiber Reinforced composites/Aluminum Laminates (CARALLs) composed of T700/PA6 unidirectional prepreg and 6061 aluminum alloy were studied using experimental and numerical simulation analysis. Through three-point bending experiments, the bending behavior of CARALLs with different composite/metal layer methods was examined. It was found that FMLs in the 2/1 patch form (one layer of aluminum and two layers of T700/PA6 unidirectional prepreg) show the highest bending modulus and strength compared with other stacking sequences. With the metal volume fraction increased, the bending properties of CARALLs decreased, suggesting the important role of the carbon fiber composite layer in the load-bearing capacity. Lastly, the Linde and Hashin failure criteria were employed to analyze the bending behavior and damage mechanism of CARALLs with different stacking sequences. The simulation results were in good agreement with the experimental results, which provides more insight into the prediction of the bending behavior of CARALLs hybrids.