Fiber Metal Laminates Research Articles

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.

Crashworthiness analysis of a composite guardrail under impact loading

Road safety barriers prevent cars from deviating from the roads. Guardrails as common roadside safety barriers protect cars from dangers alongside the roads, such as steep slopes and trees, and redirect traffic from road hazards. Conventional guardrails are generally made of metal or concrete. Although steel guardrails absorb a high amount of crash energy by deformation, these types of guardrails have short lifecycles of less than 15 years. Due to their unique properties, composite materials are potential candidates to use as guardrails. The present study considers the three types of guardrails (steel, composite, and fiber metal laminate (FML)), and the crash model is developed according to standards. LS-DYNA software is employed to simulate and conduct the crash test In the first stage, impact simulation is performed on a carbon/epoxy composite plate. In the next step, impact simulation is performed on a metal fiber multilayer (FML) plate with carbon fiber and aluminum metal. Then, the crash test on the steel guardrail is completely simulated. These three stages are performed to verify the proposed model. In the next stage, the crash test for a carbon/epoxy composite guardrail is simulated, and the effect of different parameters is investigated. Finally, a crash test on the FML guardrail with carbon fibers and steel metal is simulated. Two FML samples with [CE/Steel/CE] layups are considered. The absorbed energy, specific energy, and acceleration severity index (ASI) for all different guardrails are obtained and compared to determine the optimum guardrail regarding energy absorption, mass, and ASI parameters. Results reveal that sample FML-B guardrail is the best of the three guardrail types, with the highest specific energy absorption and lower ASI parameter.

Hygrothermal dynamic behavior of fiber metal laminated structure under dual-ellipse distribution heat source

Refined plate theory could account for parabolic variation of transverse shear stress through the thickness, and satisfy the zero traction boundary conditions on the top and bottom surfaces of the plate without introducing shear correction factor, of which all displacement variables can be depicted in functions of shear and bending components. Based on three-dimensional transient thermal conduction theory and the refined plate theory, an analytical solution of fiber metal laminated (FML) plate under hygrothermal environment is presented. In present work, distinct from other literatures, the separate variable method and Newton-Cotes method is applied to solve the three-dimensional transient temperature field (subjected to moving dual-ellipse distribution laser heat source) firstly. Secondly, the constitutive equations and geometrical equations are derived. Subsequently, the governing equations in hygrothermal environment are obtained. Finally, the Newmark method and Galerkin method are used to solve the governing equations of which in terms of displacement functions. The results show that fiber orientation affects the bending stiffness, the selection of (45°/ 0°) fiber orientation is more resistance to deformation and normal stress, and bending part is more sensitive to shear factor between layers.

Simple stitch-assisted carbon fiber-aluminum laminates via out-of-autoclave process

Fiber–metal laminates (FMLs) are hybrid materials that provide the advantages of both metal and fiber-reinforced polymer composite (FRP) such as improved damage tolerance and impact strength, particularly in aerospace and automotive applications. However, the production of FMLs remains expensive because the manufacturing process is performed in an autoclave. In addition, delamination failure due to low interfacial adhesive strength between the metal and fiber layer is a problem that must be solved to improve the mechanical properties of FMLs. In this study, simple stitch-assisted carbon fiber-aluminum laminates were developed using vacuum-assisted resin transfer molding (VARTM). The FMLs produced through vacuum-assisted resin transfer molding were stitched with aramid fibers through holes on Al sheets for the resin flow associated with the VARTM. The mechanical properties of the FMLs were investigated by impact and compression after impact tests as well as double cantilever beam and flexural tests. The stitching effectively improved the interfacial strength and toughness owing to the anchoring between the aluminum and carbon fiber layers, thus preventing delamination crack propagation. As a result, the stitched FML with significantly improved interfacial adhesive and toughness was developed by the VARTM process, which will accelerate the application of FML to various lightweight structures.

Enhancing the mechanical properties of fiber metal laminate by surface treatment

AbstractFiber‐metal laminates with high toughness and high strength have become an important direction for the development of new materials in the future. During the production and service process of fiber‐metal laminates, brittle fracture and interlaminate damage are easily caused by low load, which leads to the instability and damage to the overall structure. This article mainly studies the effects of surface treatment process, and material combination on the mechanical properties of glass fiber/polypropylene (PP)/stainless steel mesh (SSM) composite laminates. It is shown that the adhesion force of the phosphating SSM sample is 1.73 times that of the etched sample, and the bonding effect with organic substances is relatively better. The laminate with the structure of “unetched SSM + maleic anhydride grafted polypropylene + aminosilane glass fiber fabric” had the best tensile load. The etched and phosphatized SSM enhanced the interface adhesive effect with PP‐g‐MAH, thereby improving the overall structural strength of the laminate.

Impact Resistance of a Fiber Metal Laminate Skin Bio-Inspired Composite Sandwich Panel with a Rubber and Foam Dual Core

This paper reports the development of a novel bio-inspired composite sandwich panel (BCSP) with fiber metal laminate (FML) face sheets and a dual core to improve the low-velocity impact behavior based on the woodpecker's head layout as a design template. The dynamic response of BCSP under impact load is simulated and analyzed by ABAQUS/Explicit software and compared with that of the composite sandwich panel (CSP) with a single foam core. The impact behavior of BCSP affected by these parameters, i.e., a different face sheet thickness, rubber core thickness and foam core height, was also reported. The results show that BCSP has superior impact resistance compared to CSP, with a lower damage area and smaller deformation, while carrying a higher impact load. Concurrently, BCSP is not highly restricted to any particular region when dealing with stress distributions. Compared to CSP, the bottom skin maximum stress value of BCSP is significantly reduced by 2.4-6.3 times at all considered impact energy levels. It is also found that the impact efficiency index of BCSP is 4.86 times higher than that of CSP under the same impact energy, indicating that the former can resist the impact load more effectively than the latter in terms of overall performance. Furthermore, the impact resistance of the BCSP improved with the increase in face sheet thickness and rubber core thickness. Additionally, the height of the foam core has a notable effect on the energy absorption, while it does not play a significant role in impact load. From an economic viewpoint, the height of the foam core retrofitted with 20 mm is reasonable. The results acquired from the current investigation can provide certain theoretical reference to the use of the bio-inspired composite sandwich panel in the engineering protection field.

Mechanical Characterization of Aluminum Sandwich Structures with Woven-Ply Pineapple Leaf/Glass Fiber-Reinforced Hybrid Composite Core

ABSTRACT Fiber-metal laminates consisting of alternating metal and fiber-reinforced polymer layers have displayed remarkable performance in several engineering applications. This work aims to identify the feasibility of incorporating pineapple leaf fiber to partially supersede glass fiber in thermoplastic-based fiber-metal laminates. Fiber-metal laminates made of pineapple leaf/glass/polypropylene/aluminum were fabricated using the hot press molding technique. The tensile, flexural, Charpy impact and quasi-static indentation tests were performed. The findings indicated that the hybridization of glass with pineapple leaf fibers improved the mechanical properties of the laminates. The results are particularly promising in [G/P/G] laminates in which their tensile and flexural strengths are 38.98% and 20.19% higher than [P/P/P] laminates. In addition, the Charpy impact strengths of [G/P/G] laminates are also 236.66% and 175.68% greater than those of [P/P/P] laminates. In terms of indentation properties, the maximum indentation forces of [G/P/G] laminates are 16.71% and 13.76% higher than those of [P/P/P] laminates at indenter diameters of 12.7 and 20.0 mm, respectively. Interestingly, in-plane and out-of-plane properties of [G/P/G] laminates were comparable to [G/G/G] laminates. Thus, it is anticipated that the hybridization concept could escalate the utilization of natural fibers as a potential reinforcement for engineering applications.

Detection of Debonding Area in Aluminium-GFRP Laminates by Electromagnetic Transducer

Electromagnetic Acoustic Transducer (EMAT) is applied to nondestructive testing of aluminum-GFRP laminates. Because of its high specific strength, laminated FRP came to be used in place of metal materials. However, once delamination is introduced by the impact force in the out-of-plane direction, the compressive strength in the in-plane direction is greatly reduced. To improve the tolerance for impact force, fiber metal laminates (FML) have been developed. Since debonding may occur at the interface of FRP and metal sheet, nondestructive detection of the debonding is required as health monitoring of the FML. In this study, GFRP laminates were molded on aluminum sheets, and we measured the distribution of standing wave amplitude in the aluminum sheets using EMAT for shear wave. In the bonded area, the shear wave is partially transmitted to the FRP on reflection at the boundary, which makes the attenuation coefficient larger than that in the debonded area. It was found that in scanning the EMAT, the standing wave amplitude starts to change when the center of the EMAT passes the boundary between bonding and debonding areas. It was also revealed that minimum size of the debonding area that EMAT can detect is comparable to its effective area. For accurate evaluation of the debonding area, resonant frequency measurement is recommended at each point.