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    Literature Review

    2.1. Introduction

    Literature review is made on the characterization of FMLs to know

    the information on the basic characterization of Fiber Metal Laminated

    composites. The characterization reported in the literature covers

    behavior of the laminates under static, fatigue and impact loads and

    shows the wide range of applications and the flexibility in design of

    FMLs. The basic survey related to identification of the area and broad

    definition of the problem is presented here and detailed discussion on

    specific points is presented in subsequent chapters along with the

    evolution of the solution. The reviewed papers herewith-presented in

    order of the publishing year.

    2.2. Review

    An experimental study is conducted on a laminate consisting of

    monolithic thin aluminum plates alternating with unidirectional

    carbon/epoxy (Fiberite) prepeg tapes (Dov Sherman et al. [19]). Enhanced

    strain energy dissipation caused by multiple fracture mechanisms led

    the FML to exhibit pseudo ductile behavior. It is also observed that a

    minimum volume fraction of the reinforcing layers is required to exhibit

    this behavior. The mechanical behavior of the laminate is explored. The

    influence of number of layers, volume fraction on transverse properties

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    are also investigated. The loss of stiffness with increase of the

    applied strain is estimated using a simple shear lag theory.

    Static indentation and low and high velocity impact tests are

    conducted on specimens with a circular clamped test area (Vlot [20]).

    Monolithic Al 2024-T3 and 7075-T6, various grades of FMLs and

    composites are tested. Fiber critical behavior is observed in ARALL and

    CARE laminates, due to low strain to failure. It is observed that GLARE

    laminates will show a fiber critical or aluminum critical failure mode

    depending on the lay up of the laminate. The dent depth after impact on

    FML is approximately equal to that of the monolithic aluminum alloy.

    The results from impact tests also showed that the damage zone in the

    FMLs is smaller than observed for fiber reinforced composite materials.

    Statistical analysis, stress analysis and failure characteristics

    analysis of two types of tension specimens (ARALL) are made in (Wu et

    al. [21]). The specimen geometries considered are straight sided and dog

    bone specimens. It was found that the tensile yield strength, tensile

    modulus and tensile ultimate strength are independent of specimen type.

    Results from both experimental and analytical studies are compared.

    Analysis is made (Johnson et al. [22]) on a material system

    consisting of thin sheets of titanium bonded together with a polymer

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    composite prepeg consisting of a high temperature resin reinforced

    with high or intermediate modulus fibers useful for aerospace

    applications. It is observed to have beneficial performance for this FML,

    the polymer composite layers should have a higher modulus than that of

    titanium. More isotropic behavior is observed in HTCL than a

    unidirectional composite and this behavior is attributed to transverse

    property contribution of titanium layers. This work includes analytical

    studies as well as experimental results. Many combinations of

    constituents in various proportions were analytically studied by

    predicting the complete tensile stress-strain curve up to failure. Effect of

    fiber volume fraction, orientation, titanium percentage on stress strain

    behavior is studied.

    Features of spliced laminates are discussed in (Asundi et al. [6]).

    Spliced concept is used to manufacture much wider panels (> 4mts) and

    to retain the benefits of smaller panels of FMLs used in the construction.

    The increased width capability can results in a significant reduction in

    manufacturing cost. The author in his discussion concluded that spliced

    laminates are promising candidates for fuselage and lower wing materials

    for the next generation of very large civil transport aircraft and the ultra

    high capacity aircraft for 600 to 800 passengers.

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    Inelastic behavior and strength of fiber metal hybrid composite,

    GLARE 2 is studied under static loading conditions in (Kawai et al. [23]).

    The classical lamination plate theory is applied for describing the off-axis

    inelastic behavior of GLARE 2 laminate. It has been shown that the

    anisotropy for the tensile fracture strength of the GLARE 2 laminate can

    be predicted using Tsai-Hill theory. A CLT based model, which takes into

    account the transverse failure in the GFRP layer to cause an

    instantaneous degradation of transverse and shear elastic module is

    used to describe the characteristic deformation behavior of the GLARE 2

    laminate. Influence of degradation methods on modeling of the GFRP

    failure is investigated by adopting three different methods (i) no-ply

    fracture method, (ii) complete-ply fracture method and (iii) incomplete ply

    fracture method. Stress-strain diagrams till the ultimate failure of the

    laminate are drawn and compared with experimental results. The no-ply

    fracture method predicted much larger tangent moduli and higher

    strengths than those of the experimental results after yielding of the

    aluminum layers. The complete-ply fracture method overestimated the

    stiffness reduction and reported less strengths. The incomplete-ply

    fracture method that retains the stiffness in the fiber direction after

    GFRP layers have satisfied the Tsai–Hill criterion, yielded good

    approximations when compared with experimental results. The off-axis

    influence of young’s modulus and poisons ratios is also discussed.

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    Numerical models for delamination in FMLs are discussed in

    (Hashagen et al. [24]). Intensive experimental analyses have been carried

    out to assess the possible application of new design methods that make

    use of the layered structure of FMLs. To support the experimental

    analyses, numerical models have been developed to describe

    delamination in FMLs. To achieve this goal, a special continuum element

    and corresponding interface elements are introduced. Loading functions

    have been derived to account for delamination. Using this methodology

    the impact of delamination on spliced FMLs has been studied

    numerically and has been compared to experimental results.

    Results from the investigations carried out on a material consisting

    of carbon-epoxy prepreg and aluminum alloy layers are reported in

    (Klement et al. [25]). Mechanical properties like tensile strength, tensile

    modulus, shear strength, bending strength and modulus are determined.

    The results of formability testing are described. It is observed under

    fatigue loading the crack propagation rate is greatest immediately after

    crack initiation and decreases with the number of loading cycles. The

    material formability results showed that forming techniques can be used

    to limited extent for manufacture of parts by using this material.

    Analytical formulations to predict energy absorption and the

    ballistic limit of fully clamped GLARE panels subjected to impact by a

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    blunt cylinder were formulated (Fatt et al. [26]). The ballistic limit was

    found through an iterative process such that the initial kinetic energy of

    the projectile would equal the total energy dissipated by panel

    deformation, delamination/debonding and fracture. The transient

    deformation of the panel as shear waves propagate from the point of

    impact was obtained from an equivalent mass–spring system, whereby

    the inertia and stiffness depend on the shear wave speed and time. The

    formulation results are within 13% margin of the results reported by

    experimentation.

    The tensile and fatigue properties of carbon fiber reinforced PEEK-

    titanium FMLs are investigated in (Cortes et al. [27]). It has been

    observed during the in-plane on-axis tensile tests on unidirectional un-

    notched laminates, that their mechanical properties follow the

    predictions offered by a simple law of mixtures approach. Tension-

    tension fatigue tests on notched unidirectional FMLs has shown that

    these laminates offer fatigue lives up to fifty times greater than those

    preferred by a notched monolithic titanium alloy. The variation of

    modulus, un-notched tensile strength and notched tensile strength with

    volume fraction of composite is studied. The presence of small quantities

    of titanium has shown a lot of improvement in the notched tensile

    strength. It has also been shown that delamination is more widespread

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    in FMLs based on thick composite layers than in laminates containing

    thin composite layers.

    The high velocity impact response of a range of novel aluminum

    foam sandwich structures has been investigated (Reyes et al, [28]) using

    a nitrogen gas gun. Tests were undertaken on sandwich structures based

    on plain composite and FML skins. High velocity impact tests on the

    sandwich structures resulted in a number of different failu