The effect of thermal cycles on the mechanical properties of fiber–metal laminates

The effect of thermal-shock cycles on the mechanical properties of fiber–metal laminates (FMLs) has been evaluated. FML plates were composed by two AA2024 Al sheets (1.6 mm thick) and one composite ply formed by two layers of unidirectional glass fiber epoxy prepreg and two layers of epoxy adhesive tape of glass fiber reinforced epoxy adhesive. The set was manufactured by hand layup and typical vacuum bag technique. The curing cycle was in autoclave at 125 ± 5 °C for 90 min and an autoclave pressure of 400 kPa. FML coupons taken from the manufactured plate were submitted to temperature variations between −50 and +80 °C, with a fast transition between these temperatures. Tensile and interlaminar shear strength were evaluated on samples after 1000 and 2000 cycles, and compared to nonexposed samples. 2000 Cycles corresponds to typical C Check interval for commercial aircraft maintenance programs. It was observed that the thermal-shock cycles did not result in significant microstructural changes on the FML, particularly on the composite ply. Similarly, no appreciable effect on the mechanical properties of FML was observed by the thermal-shock cycles.     

The effect of thermal fatigue on the mechanical properties of the novel fiber metal laminates based on aluminum–lithium alloy

The effect of thermal fatigue on the mechanical properties of the novel fiber metal laminates (FMLs) based on aluminum–lithium alloy was investigated. The results indicated that no obvious delamination or defects were observed in the novel FMLs exposed to 1000 cycles. The samples treated with different cycles still exhibited stable and excellent interlaminar properties comparing with the as-manufactured ones. Furthermore, the tensile and flexural strength of the FMLs even increased with the thermal fatigue cycles owing to the positive age hardening behavior of aluminum–lithium layer. The homogeneous and fine precipitation of T₁ phases dominated the strengthening effect of aluminum–lithium alloy. Besides, the novel FMLs after thermal fatigue treatments still possessed the similar resistance to fatigue crack growth (FCG) when compared with the as-manufactured ones. The slight changes in the properties of aluminum–lithium layers had no detrimental effect on the FCG.     

Post-impact fatigue damage growth in fiber–metal laminates

Fiber–metal laminates (FMLs) are a family of hybrid materials currently being considered for use in airframe structural applications. Post-impact fatigue strength tests were carried out on several varieties of GLAss REinforced (GLARE) aluminum laminates. The panels were impacted in a drop weight impact tower located at the Institute for Aerospace Research of the National Research Council of Canada. Observations made by other researchers that the internal impact damage in FMLs is confined to the immediate impact site were confirmed. The impacted specimens were cycled in tension–tension fatigue until failure. Cracks developed along side the dent and also at the edges of the gauge section of the specimen. Aluminum baseline specimens had significantly lower fatigue lives than the FML specimens. The stress-state surrounding the dent is complicated and contributed to unusual fatigue crack initiation behavior in some GLARE variants.     

Diaphragm forming of continuous fibre reinforced thermoplastics: influence of temperature,pressure and forming velocity on the forming of Upilex-R® diaphragms

In the diaphragm forming process, the thermoplastic composite sheet is clamped between two high temperature thermoplastic diaphragms. In the present study, the influence of temperature, pressure and forming rate on the deformation of high temperature PI diaphragms (Upilex-R^®, Ube Industries) is described. At temperatures below 275 °C the upper diaphragm slides over the bottom diaphragm and shows a more global deformation, above 305 °C, the upper diaphragm cannot slide over the bottom diaphragm and deforms in the same manner. The region 275–305 °C is a kind of transition region between the previous two temperature ranges. A hydrostatic pressure of 1 bar turned out to be sufficient to deform the diaphragms, therefore, no influence of pressure was observed. The deformation of the bottom diaphragm is independent of forming rate, while the upper diaphragm showed some dependence.     

Simulation of the response of fibre–metal laminates to localised blast loading

The response of Fibre–Metal Laminates (FML) to localised blast loading is studied numerically in order to interpret the deformation mechanism due to highly localised pressure pulses causing permanent deformations and damage observed experimentally in FML panels comprising different numbers of aluminium alloy layers and different thickness blocks of GFPP material [Langdon GS, Lemanski SL, Nurick GN, Simmons MS, Cantwell WJ, Schleyer GK. Behaviour of fibre–metal laminates subjected to localised blast loading: part I – experimental observations and failure analysis. International Journal of Impact Engineering 2007;34:1202–22; Lemanski SL, Nurick GN, Langdon GS, Simmons MS, Cantwell WJ, Schleyer GK. Behaviour of fibre–metal laminates subjected to localised blast loading: part II – quantitative analysis. International Journal of Impact Engineering 2007;34:1223–45; Langdon GS, Nuric GN, Lemanski SL, Simmons MS, Cantwell WJ, Schleyer GK. Failure characterisation of blast-loaded fibre–metal laminate panels based on aluminium and glass-fibre reinforced polypropylene. Composite Science and Technology 2007;67:1385–405]. The influence of the loading and material parameters on the final deformation characteristics is examined. Particular attention is paid to the transient deformation process by using finite element and analytical models to analyse the panel behaviour. It is shown that the response of the FML panels is extremely sensitive to the spatial and temporal distribution variation of the pressure caused by the blast loading. The study reveals that the properties of GFPP in the through-thickness direction play an essential role in the velocity transfer, which influences considerably the failure and final deformed shape of the FML panel. Good agreement between the experimental and numerical results is observed. Comparisons between the responses of relatively thin FML panels, monolithic aluminium alloy plates of equivalent mass and a foam-core panel to localised blast are also presented and discussed.     

Reinforcement effects of aluminum–lithium alloy on the mechanical properties of novel fiber metal laminate

The novel fiber metal laminates based on aluminum–lithium alloy (NFMLs) were investigated to improve the stiffness and damage tolerance. The aluminum–lithium sheets were rolled from 2 mm to 0.3 mm by cold forming, aged to T3 state and anodized in phosphoric acid. Then, NFMLs were prepared by the optimized process. The mechanical properties of NFMLs were evaluated by floating roller, interlaminar shear, tensile, bending and fatigue crack growth (FCG) tests respectively. The results indicated that the aluminum–lithium alloy was mainly strengthened by δ′ phases at T3 state. The rough micro morphology was constructed on the surface of aluminum–lithium layer by anodizing process. NFMLs and conventional Glare presented similar density and quite excellent interlaminar properties. Compared with Glare, however, NFMLs exhibited slight strength increase and obvious elastic modulus improvement regardless of the fibers plies and sampling direction. A better resistance to FCG of NFMLs was also verified.     

Design and analysis of longitudinal–flexural hybrid transducer for ultrasonic peen forming

Ultrasonic peen forming (UPF) is an emerging technology that exhibits great superiority in both its flexible operating modes and the deep residual stress that it produces compared with conventional plastic forming methods.Although ultrasonic transducers with longitudinal vibration have been widely studied,they have seldom been incorporated into UPF devices for machining in confined spaces.To meet the requirements of this type of machining,a sandwich-type piezoelectric transducer with coupled lon...     

In-vitro forming of calcium phosphate layer on sol–gel hydroxyapatite-coated metal substrates

In-vitro deposition of calcium phosphate layer (CPL) on metallic substrate requires special surface preparation in order to provide an interfacial bond. In this work 316 stainless steel surface is modified through deposition of a thin film (0.5 m) of sol–gel hydroxyapatite (SG-HA). This well-bonded film acts as an intermediary and nucleation surface of the CPL film. The SG-HA films were annealed at 375 °C (samples coded 375-ACS) and 400 °C (400-ACS) to achieve different crystallinity of the films, and thus to affect and study the CPL nucleation process. The CPL growth was investigated in terms of deposition kinetics and microstructural development. A deposition rate of dense CPL of about 0.43 m/day was achieved on the crystallized film of 400-ACS, and 0.22 m/day of porous CPL on amorphous 375-ACS. A compositional variation of Ca/P ratio across the CPL film thickness (400-ACS) was observed. Lower Ca/P ratio of 1.2 was detected near the substrate-CPL interface and about 1.5 near the solution-CPL interface. Infrared analysis showed the CPL to be of apatitic calcium-deficient structure. Kinetic model explaining the advancement of the CPL upon the in-vitro immersion is proposed.     

Growth of aligned ZnO nanorods on woven Kevlar® fiber and its performance in woven Kevlar® fiber/polyester composites

ZnO nanorods were grown by a seeding treatment on surface-functionalized woven Kevlar® fiber (WKF), and the decorated WKF was used to prepare composites with polyester resin (PES) via vacuum-assisted resin transfer molding. Fourier transform infrared spectroscopy (FT-IR) confirmed the surface functionalization of the WKF. The characteristic peaks of ZnO observed by FT-IR and X-ray diffraction (XRD) indicated the growth of ZnO nanorods on the surface of the WKF. FT-IR analyses of the composites were consistent with an interaction between the WKF, and ZnO and PES. The intensity of the XRD peak for crystalline ZnO increased with increasing growth of the nanorods. The morphology of the nanorods was studied by scanning electron microscopy. The growth of the nanorods increased with increasing treatment time. Thermogravimetric analysis also supported the growth of nanorods. The presence of ZnO nanorods significantly improved the impact resistance of the WKF/PES composites; the penetration thresholds were also determined. The WKF/ZnO/PES composites had substantially higher tensile strengths and moduli than the ZnO-free composites.     

Influence of embedded gap and overlap fiber placement defects on the microstructure and shear and compression properties of carbon–epoxy laminates

This paper presents results from an experimental study of the influence of embedded defects created during automated fiber tape placement, on the mechanical properties of carbon/epoxy composites. Two stacking sequences have been examined, [(−45°/+45°)₃/−45°] and [90°₄/0°₃/90°₄], in which gaps and overlaps have been introduced during fiber placement. These materials have been cured in an autoclave either with or without a caul plate, then analyzed by ultrasonic C-scan. The microstructures were characterized by scanning electron microscopy. In-plane shear tests were performed on the ±45° laminates and showed that the use of a caul plate does not affect mechanical behavior of plies in the embedded defect region. Compression tests were performed on 0°/90° laminates and in this case the presence of a caul plate is critical during polymerization as it prevents thickness variations and allows defects to heal.     

Effect of hydrofluoric acid etching of glass on the performance of organic–inorganic glass laminates

Organic–inorganic glass laminates with polyurethane (PU) as an adhesive interlayer were prepared by a warm-pressing method. The hydrofluoric acid etching of glass surface was performed to investigate its effect on the mechanical behavior of glass laminates. Results show that the acidic etching treatment of glass seldom influences the transparency and haze of glass laminates when the etching time is below 30 min. The bonding strength and fracture stress of glass laminates firstly increase and then decrease with increasing etching time. This could be attributed to the formation of three-dimensional interface of glass laminates. The unique interface structure not only increases the contact area between glass and PU layer, leading to the improvement of interface bonding, but also modifies the stress distribution at the interfaces, which is favorable to prevent the crack propagation and delamination failure of laminates.     

An investigation into the flexural and drawing behaviors of GFRP-based fiber–metal laminate

Fiber–metal laminates (FMLs) are advanced composite materials that consist of bonded thin metal sheets and fiber-reinforced composite layers. In this article, mechanical behavior of a thermoplastic-based FML is investigated, which is composed of glass-fiber-reinforced polypropylene (GFRP) laminate and aluminum AA1200-O as the core and skin layers, respectively. Engineering constants of the composite laminate were achieved using Timoshenko's beam theory, flexural and tensile test results. Finite element simulations of the GFRP-based FML were performed to predict the behavior of this material in three-point bending and deep drawing tests. Some experimental verification tests were conducted to prove the reliability of results in the FE analysis of the FML. Comparison of the results shows an excellent correlation between the FE analysis and experimental tests.     

Buckling behavior of fiber reinforced plastic–metal hybrid-composite beam

It is known that the buckling is characterized by a sudden failure of a structural member subjected to high compressive load. In this study, the buckling behavior of the aluminum tubular beam (ATB) was analyzed using finite element (FE) method, and the reinforcing arrangements as well as its combinations were decided for the composite beams based on the FE results. Buckling and bending behaviors of thin-walled ATBs with internal cast polyamide (PA6) and external glass and carbon fiber reinforcement polymers (GFRPs and CFRPs) were investigated systematically. Experimental studies showed that the 219% increase in buckling load and 661% in bending load were obtained with reinforcements. The use of plastics and metal together as a reinforced structure yields better mechanical performance properties such as high resistance to buckling and bending loads, dimensional stability and high energy absorption capacity, including weight reduction. While the thin-walled metallic component provides required strength and stiffness, the plastic component provides the support necessary to prevent premature buckling without adding significant weight to the structure. It is thought that the combination of these materials will offer a promising new focus of attention for designers seeking more appropriate composite beams with high buckling loads beside light weight. The developed plastic–metal hybrid-composite structure is promising especially for critical parts serving as a support member of vehicles for which light weight is a critical design consideration.     

Effect of glass fiber sizing on the cure kinetics of vinyl–ester resins

The cure characteristics of thermosetting resins are affected by the presence of reinforcements as a result of surface–resin interactions. Surface treatments and sizing can significantly affect such interactions; hence, sizing or surface treatment selection may significantly affect resin cure characteristics. This is of particular concern in the processing of composite materials, since neat resin cure characteristics often will not provide the appropriate basis for predicting the cure behavior of the composite. In this work, the effect of several commercially sized S-2 glass systems on the cure of vinyl–ester resin was investigated. Generally, a significant increase in the cure rate of the glass-modified systems is observed. Furthermore, a relationship between the surface energy characteristics of the fibers and the degree of cure acceleration is established, and possible mechanisms for the effect are discussed. It is apparent that sizing selection can significantly affect cure processes for vinyl–ester systems.     

Effect of nano-metal particles on the fracture toughness of metal–ceramic composite

A theoretical model is proposed to study the influence of nano-metal particles (NMPs) on the fracture toughness of metal–ceramic composites (MCC). In the framework of the model, the crack tip intersects the grain boundary of the NMPs. Stress concentration at crack tip initiates edge dislocations which makes a shielding effect on the crack and leads to fracture toughness of the MCC. The dependence of critical crack intensity factors on grain size of the NMPs was calculated. The calculation suggested that the existence of the NMPs lead to an increase of critical crack intensity factors by 14%.     

Analysis of unsymmetric CFRP–metal hybrid laminates for use in adaptive structures

The potential of using multistable composite materials for adaptive structures is currently receiving interest from the aerospace community because they possess more than one single equilibrium configuration. Unsymmetric CFRP laminates are studied which have an inner isotropic metallic layer. These hybrid laminates are studied using analytical, finite element and experimental techniques. The thermal contraction of the isotropic layer upon cool down from cure induces large in-plane thermal loads which act remotely from the laminate’s neutral plane, increasing snap-through moments and out-of-plane displacements. The curvatures of the hybrid laminates can be doubled compared to pure unsymmetric CFRP laminates.     

Dynamic response of Kevlar®29/epoxy laminates under projectile impact-experimental investigation

This authors of this article investigated the dynamic response of woven Kevlar®29/epoxy laminates subjected to the impact loading. The cylindrical aluminum foam projectile and steel projectile were used to exert the impulse on the laminates. Deformation/failure modes, deflections, strain histories, and failure mechanisms were obtained and discussed. The results showed that with the high toughness of Kevlar fiber, the deformation modes of the laminates exhibited some characteristics similar to the metal panel, such as large global deformation. The failure mechanisms like matrix failure, fiber splitting, and fibrillation were observed. These micron failures led to the macroscopic delamination and fracture of the laminates.     

Influence of the fiber volume fraction and the fiber Weibull modul on the behavior of 2D woven SiC/SiC — a finite element simulation

Summary The behavior of two-dimensional woven SiC/SiC ceramic matrix composite (CMC) is studied by numerical simulations based on the finite element method (FEM). Starting point of the investigations is a micromechanical model regarding a three-dimensional unit cell, which takes damage and fracture of the single components—fiber bundles and inter yarn matrix—into account. The scattering of the strength values which is characteristic for ceramic material is involved using Weibull distribution. In a first step the unit cell regarded within the simulations is cooled down to consider the residual thermal stresses resulting from the fabrication process. In a second step the unit cell is subjected to tensile loading and its behavior—especially the influence of the scattering of the strength values—is studied. To be able to estimate the influence of important parameters on the behavior of the composite a macrostructure is built up using the results obtained for a large number of unit cell. Thus an averaging effect is reached and the behavior obtained for the macrostructure should be characteristic for the composite. Doing so, the influence of the fiber volume fractionv _f and the fiber Weibull modulM _f on the composite behavior can be studied.Dedicated to Prof. Dr.-Ing. Dr.-Ing. E. h. mult. Oskar Mahrenholtz on the occasion of his 70th birthday