WO2009009207A2

WO2009009207A2 - Thermoplastic composite/metal laminate structures and methods of making thermoplastic composite/metal laminate structures

Info

Publication number: WO2009009207A2
Application number: PCT/US2008/061233
Authority: WO
Inventors: Rahul R. Kukharni; Kristan K. Chawla; Uday K. Vaidya
Original assignee: University Of Alabama At Birmingham
Priority date: 2007-04-23
Filing date: 2008-04-23
Publication date: 2009-01-15
Also published as: WO2009009207A3

Abstract

Long fiber thermoplastic composite/metal laminate structures, methods of making long fiber thermoplastic composite/metal laminate structures, and the like, are disclosed.

Description

Thermoplastic Composite/Metal Laminate Structures and Methods of Making Thermoplastic Composite/

Metal Laminate Structures

CROSS-REFERENCE TO RELATED APPLICATION This application claims priority to co-pending U.S. provisional application entitled "Thermoplastic Composite/Metal Laminate Structures and Methods of Making Thermoplastic Composite/Metal Laminate Structures," having ser. no. 60/925,739 filed on April 23, 2007, which is incorporated herein by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The U.S. government may have a paid-up license in this disclosure and the right in limited circumstances to require the patent owner to license to others on reasonable terms as provided for by the terms of Contract No. W911 NF04-2-018 awarded by the Federal Transit Administration.

BACKGROUND

There are several types of hybrid composite materials used in a wide range of applications. An example of a hybrid composite is the so-called fiber metal laminates (FMLs), which were developed as an aerospace material. FMLs include alternate layers of metal sheet and polymer matrix composites (PMC) reinforced with continuous fibers. The polymer used is generally a thermoset resin such as epoxy. FMLs have advantages such as high specific strength, good fatigue resistance, high damage tolerance capabilities, and good formability and machinability. New generation FMLs are being developed which include a thermoplastic matrix in the PMC layer instead of thermoset. Thermoset matrices are brittle and have low fracture toughness values. Also, the processing time for the laminates involving thermosets is long. Thermoplastics, on the other hand, have high toughness, short processing times, and are more environmentally friendly because of their recyclability. As an example, consider the FML system based on a thermoplastic composite including alternate layers of titanium (Ti) and glass fiber reinforced polyetherimide (GF/PEI). This FML system based on PEI can be used for high temperature applications because of its higher glass transition temperature compared to poly ether ether ketone (PEEK). In another study, the impact resistance of polypropylene based FMLs was found to be higher than that of thermoset based FMLs.

Some other examples of hybrid composites used in the automotive industries are SMCs (sheet molding compounds reinforced with continuous fibers), GMTs (glass mat thermoplastics reinforced with woven fabrics), and E-LFT (LFT composite reinforced with continuous fibers, also called as tailored LFT). In all the above examples, the properties of the baseline materials (such as SMC, GMT, and LFT, which contain discontinuous fibers, chopped or long), are enhanced by reinforcing them with the continuous fibers.

SUMMARY

Embodiments of the present disclosure include long fiber thermoplastic composite/metal laminate structures, methods of making long fiber thermoplastic composite/metal laminate structures, and the like.

Embodiments of the present disclosure include a laminate structure, comprising a layer of long fiber thermoplastic composite disposed between a pair of metal sheets.

Embodiments of the present disclosure also include a method for fabricating a laminate structure including: providing a long fiber thermoplastic source, wherein the long fiber thermoplastic source is selected from a long fiber thermoplastic pellet, a plasticized charge, or a combination thereof, providing a first metal sheet and a second metal sheet, disposing the long fiber thermoplastic pellets or plasticized charge between the first metal sheet and the second metal sheet, forming a long fiber thermoplastic layer by melting the long fiber thermoplastic source, and cooling the long fiber thermoplastic layer, the first metal sheet, and the second metal sheet to form the laminate structure.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the disclosure can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present disclosure. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views. FIG. 1 illustrates examples of LFT pellets and LFT plasticized/extruded charge.

FIG. 2 is a graph that illustrates tensile stress versus strain curves of LFT composite and LML.

FIG. 3 is a digital image that illustrates a failed section of an LML in a tension test showing delaminations between the plies. The extensive necking and ductile failure in the aluminum layers should be noted.

FIG. 4 is a digital image that illustrates the fracture surface of the LFT composite showing fiber breakage and pullout. The aluminum fracture surface can also be seen.

FIG. 5 is a graph that illustrates a stress versus apparent strain plot showing nonlinear behavior of the LML in a three-point bend test.

FIG. 6 (a) illustrates a schematic of a deformed laminate structure showing a crack on the tensile side of the aluminum ply. FIG. 6 (b) is a digital image that illustrates a SEM picture of a crack on the tensile side of the aluminum ply in the laminate structure. FIG. 6 (c) is a digital image that illustrates a higher magnification of the cracked surface in the aluminum ply. The portion of the aluminum ply below the crack is severely deformed, but not cracked.

FIG. 7 is a digital image that illustrates a failed three-point bend test specimen of the laminate structure showing no delaminations between the plies

FIG. 8 illustrates load versus displacement plots before and after sandblasting in short beam tests. The appearance and progression of interlaminar cracks should be noted. FIG. 9 is a digital image that illustrates a SEM picture of a failed specimen in a short beam test. One should note the clear interface separation between the upper aluminum and LFT layer while the interface between bottom layers is intact.

FIG. 10 illustrates load versus time plots at different impact energy values in LVI tests for LMLs (FIG. 10(a)) and LFT composites (FIG. 10(b)). One should note that LMLs show higher peak loads at all the energy levels as compared to LFT composites.

FIG. 11 shows six digital images that illustrate damage at various energy levels in LMLs (left) and LFT composites (right). FIG. 11 (a) is a digital image that illustrates the appearance of first crack at approximately 5 J. FIG. 11 (b) is a digital image that illustrates a crack opening takes place at 10 J. One should note that LML undergoes considerable plastic deformation. FIG. 11 (c) is a digital image that illustrates perforations at 20 J and 15 J in LML and LFT composite, respectively. One should note the extensive delaminations in LML.

FIG. 12 is a graph that illustrates a comparison of perforation energies of various FMLs and LML. Note that LML has a higher specific perforation energy compared to thermoset based FMLs. (CFRP = carbon fiber reinforced epoxy, GFPP = glass fiber reinforced polypropylene, and GFRP = glass fiber reinforced epoxy).

DETAILED DESCRIPTION

Before the present disclosure is described in greater detail, it is to be understood that this disclosure is not limited to particular embodiments described, as such may, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting, since the scope of the present disclosure will be limited only by the appended claims.

Where a range of values is provided, it is understood that each intervening value, to the tenth of the unit of the lower limit (unless the context clearly dictates otherwise), between the upper and lower limit of that range, and any other stated or intervening value in that stated range, is encompassed within the disclosure. The upper and lower limits of these smaller ranges may independently be included in the smaller ranges and are also encompassed within the disclosure, subject to any specifically excluded limit in the stated range. Where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included in the disclosure.

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. Although any methods and materials similar or equivalent to those described herein can also be used in the practice or testing of the present disclosure, the preferred methods and materials are now described.

All publications and patents cited in this specification are herein incorporated by reference as if each individual publication or patent were specifically and individually indicated to be incorporated by reference and are incorporated herein by reference to disclose and describe the methods and/or materials in connection with which the publications are cited. The citation of any publication is for its disclosure prior to the filing date and should not be construed as an admission that the present disclosure is not entitled to antedate such publication by virtue of prior disclosure. Further, the dates of publication provided could be different from the actual publication dates that may need to be independently confirmed.

As will be apparent to those of skill in the art upon reading this disclosure, each of the individual embodiments described and illustrated herein has discrete components and features which may be readily separated from or combined with the features of any of the other several embodiments without departing from the scope or spirit of the present disclosure. Any recited method can be carried out in the order of events recited or in any other order that is logically possible.

Embodiments of the present disclosure will employ, unless otherwise indicated, techniques of chemistry, materials science, physics, engineering, and the like, which are within the skill of the art. Such techniques are explained fully in the literature.

The following examples are put forth so as to provide those of ordinary skill in the art with a complete disclosure and description of how to perform the methods and use the compositions and compounds disclosed and claimed herein. Efforts have been made to ensure accuracy with respect to numbers {e.g., amounts, temperature, etc.), but some errors and deviations should be accounted for. Unless indicated otherwise, parts are parts by weight, temperature is in ⁰C, and pressure is at or near atmospheric. Standard temperature and pressure are defined as 20 ⁰C and 1 atmosphere.

Before the embodiments of the present disclosure are described in detail, it is to be understood that, unless otherwise indicated, the present disclosure is not limited to particular materials, reagents, reaction materials, manufacturing processes, or the like, as such can vary. It is also to be understood that the terminology used herein is for purposes of describing particular embodiments only, and is not intended to be limiting. It is also possible in the present disclosure that steps can be executed in different sequence where this is logically possible.

It must be noted that, as used in the specification and the appended claims, the singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to "a support" includes a plurality of supports. In this specification and in the claims that follow, reference will be made to a number of terms that shall be defined to have the following meanings unless a contrary intention is apparent.

Discussion

Long fiber thermoplastic composite/metal laminate structures (hereinafter "LML" or "laminate structure"), methods of making long fiber thermoplastic composite/metal laminate structures, and the like, are disclosed. In general, embodiments of the present disclosure utilize one or more layers of long fiber thermoplastic composite and metal sheets that can be optimized in terms of protection, weight, maintainability, service life, and/or cost.

Embodiments of the long fiber thermoplastic (LFT) composite/metal laminate are a hybrid composite having a layer of LFT composite disposed (e.g., positioned, deposited, or otherwise placed) between a pair of metal (e.g., aluminum) structures (e.g., sheets). In an embodiment, the LFT laminate structure includes a plurality of LFT composite layers, where each layer is between a pair of metal structures. LFT composites constitute a family of composites having a thermoplastic matrix such as, but not limited to, polypropylene, nylon, or polyurethane, and the like, that is reinforced with discontinuous fibers. An advantage of embodiments of LFT composites are their low cost. Another advantage is that embodiments of LFT composites can be processed using traditional plastic molding operations such as compression molding, extrusion, and/or injection molding.

Embodiments of the present disclosure have civilian and military applications in structural support use as well as the protection of personnel, vehicles, buildings, shelters, and the like. Embodiments of the present disclosure have high specific elastic modulus and strength. In particular, embodiments of the present disclosure are capable of withstanding the effects related to impact resistance, cyclic fatigue resistance, and damage tolerant structures that are used in the automotive sector, the military sector, the ground transportation sector, the aircraft sector, the marine sector, and the construction sector. In addition, embodiments of the present disclosure are advantageous because the thermoplastic composite is capable of being recycled. Furthermore, embodiments of the present disclosure are advantageous over alternatives such as thermoset composites in that the thermoplastic composite has better mechanical properties such as toughness, impact resistance, formability into shapes, short processing time, machinability, vibration and sound damping, and the like.

As mentioned above, an embodiment of the laminate structure utilizes alternating layers of long fiber thermoplastic composite and metal sheets. In particular, the laminate structure includes, but is not limited to, LFT layer disposed between two or more metal sheets. The laminate structure can be formed into a simple geometry {e.g., polygonal structures) or a complex geometry (e.g., irregular shapes). In particular, the laminate structure can be made into flat structures (e.g., panels for transportation vehicles, doors, covers, and the like), contoured and/or curved structures (e.g., body armor), or complex structures with multiple curves and/or contours (e.g., helmets, corner structures, and the like).

The term "metal sheet" may include sheets made of materials such as, but not limited to, metals, metallic alloys, metal matrix composites, metallic foams (solid metal containing a large volume fraction of gas-filled pores), and combinations thereof.

In particular, the metal sheet materials may include, but are not limited to, aluminum, magnesium, steel, titanium, nickel, copper, brass, zinc, alloys of each, composites of each, and combinations thereof. The aluminum alloy may include, but is not limited to, aluminum alloy 2014, aluminum alloy 2024, aluminum alloy 6061 , aluminum alloy 7075, and combinations thereof. These alloys are well known in the art and can include one or more of aluminum, chromium, copper, iron, magnesium, manganese, silicon, titanium, and/or zinc. The laminate structure may include metal sheets made of different materials or the same materials.

The thickness of the metal sheet depends, at least in part, upon the geometry of the final product. For example, a simple flat laminate structure can use metal sheets that are relatively thicker than complex or contoured laminate structures. In this regard, the metal sheets of simple laminate structures may have a thickness of about 0.1 to 5 mm. In an embodiment, the metal sheets of more complex laminate structures may have a thickness of about 0.1 to 1 mm. The length and width of the metal sheets depend upon the application. For example, a large metal sheet can be used and the laminate structure cut into appropriate dimensions for a particular application. The metal sheets may be about a few cm in length and width to about a few meters in length and width. The side(s) of the metal sheet in contact with the LFT can have a rough surface to increase mechanical interlocking of the LFT to the metal sheet. In an embodiment, the metal sheet can be roughened using techniques such as, but not limited to, sand blasting, milling, chemical treatments (e.g., etching), and the like.

In an embodiment, the LFT is in the form of pellets, where the pellets may have a length of about 3 to 500 mm, about 6 to 500 mm, about 6 to 25 mm, about 10 to 25 mm, about 6 to 15 mm, or about 10 to 15 mm.

Once the LFT is disposed on the metal sheet, the LFT can be formed into a layer. In an embodiment, the LFT layer may have a thickness of about 0.15 to 5 mm, about 0.3 to 5 mm, about 0.15 to 4 mm, about 0.3 to 4 mm, about 0.15 to 3 mm, or about 0.3 to 3 mm. The thickness of the LFT layer can be controlled by the amount of LFT pellets disposed between the metal layers. In an embodiment including multiple LFT layers, the thickness of the LFT pellets between each pair of metal sheets could be different.

The LFT includes materials such as, but not limited to, thermoplastic polyurethanes, polypropylene, nylon-based polymers, polystyrene, acrylonitrile butadiene styrene (ABS), styrene acrylonitrile (SAN), polyethersulphone (PES), polyetherimide (PEI), polyphenylene sulfide (PPS), polyetheretherketone (PEEK), polyetherketone (PEK), polyoxymethylene (POM), and combinations thereof, in conjunction with reinforcement fibers or materials such as aramid fibers (e.g., KEVLAR™, ZYLON™, TWARON™, and the like) polyethylene fibers (e.g., SPECTRA™, DYNEEMA™, and the like), polypropylene and nylon fibers, glass fibers or materials, carbon fibers, metallic fibers, and combinations thereof.

The fibers in the LFT are discontinuous fibers as opposed to continuous fibers. In discontinuous LFT fibers the aspect ratio (length to diameter ratio) can be about 1000 to 2000. In continuous fibers, the aspect ratio is much higher. In an embodiment, the LFT can have fiber lengths from about 3 to 50 mm, about 10 to 50 mm, about 3 to 25 mm, about 10 to 25 mm, about 3 to 15 mm, or about 10 to 15 mm. The LFT can include fiber loading that may be about 10 to 80 weight %, or about 20 to 40 weight %, loading of fiber in the LFT for any of these fiber lengths.

Characteristics of the LFT material include, but are not limited to, fiber length and aspect ratio (length/diameter) that can be varied to optimize the strength, modulus, and/or impact properties of the composite, as well as influence other mechanical and/or physical properties of embodiments of the composite structure. Embodiments of the present disclosure include LFTs with an aspect ratio of about 1000 to 2000. Embodiments of the present disclosure also include fiber filament diameters, depending upon the fiber type, of about 8 to 20 μm or about 10 to 20 μm.

Embodiments of the present disclosure can have high specific elastic modulus, specific strength, and/or specific impact resistance. Specific elastic modulus is defined as the ratio of modulus to density (or weight). Embodiments of the present disclosure can have specific elastic modulus of about 20 GPa/g cm^"3 to 80 GPa/g cm^'3. Specific strength is defined as the ratio of strength to density (or weight). Embodiments of the present disclosure can have specific strength of about 120 MPa/g cm^"3 to 300 MPa/g cm^"3. Specific impact resistance is defined as the ratio of impact to density (or weight). Embodiments of the present disclosure can have specific impact resistance of about 15 J/cm²/g to 50 J/cm²/g. The value range corresponding to the high specific elastic modulus, the specific strength, and the specific impact resistance value depends upon the components used. In an embodiment that includes E-glass fiber in a nylon matrix, the specific modulus is about 20 GPa/g cm^"3 +/- 10%, the specific strength is about 120 MPa/g cm^"3 +/- 10%, and the specific impact resistance is about 18 J cm²/g +/- 10%, which corresponds to an impact energy of about 5 J +/- 10%. A specific property is the physical property divided by density.

For the purposes of illustration, the following section describes a processing sequence proposed for the fabrication of an embodiment of a laminate structure. A non-limiting method for producing a laminate structure includes providing metal sheets and a plurality of LFT pellets and/or a plasticized LFT charge ("long fiber thermoplastic source" or "LFT source") obtained by extrusion. A plasticized LFT charge is the LFT material as it emerges from the plasticator. At this stage, the material is in the form of a viscous fibes containing melt, hence referred to as 'charge'. The LFT source is disposed between the first metal sheet and the second metal sheet. For example, the LFT source is disposed on a first metal sheet. A second metal sheet is disposed onto the LFT source, so that the first metal sheet and the second metal sheet are the top and bottom and the LFT source is between the first metal sheet and the second metal sheet. A heated structure (e.g., platen) is disposed on the first metal sheet and/or the second metal sheet. In addition to heating (e.g., about 100⁰C to 200⁰C, or about 300° C) the first metal sheet and the second metal sheet, the structures can be used to apply a pressure (e.g., about 0.1 to 0.2 MPa, or about 0.25 MPa) to the first metal sheet and the second metal sheet. The amount of temperature applied depends, in part, upon the LFT used. In general, the temperature is greater than the melting point of the LFT. One skilled in the art can select the appropriate temperature and/or pressure combination based on the LFT and the material of the metal sheets used. The heat and/or pressure cause the LFT pellets to melt or form a viscous charge and form a layer of LFT between the first metal sheet and the second metal sheet. Once the LFT layer cools, the combination of the first metal sheet, the second metal sheet, and the LFT layer disposed between the first metal sheet and the second metal sheet form the laminate structure.

It should be noted that the same or similar process could be used to form a plurality of layers of LFT between a plurality of metal sheets. For example, three LFT layers and five metal sheets can be used in an embodiment of the laminate structure. Each of the LFT layers can be made of the same or different LFT material and can be the same or a different thickness, where the thickness is controlled, at least in part, by the amount of LFT pellets used in a particular layer. Each of the metal sheets can be made of the same or a different material. It should be noted that the number of LFT layers and the metal sheets could be 1 to 101 , 1 to 51 , or 1 to 1 1.

It should be noted that ratios, concentrations, amounts, and other numerical data may be expressed herein in a range format. It is to be understood that such a range format is used for convenience and brevity, and thus, should be interpreted in a flexible manner to include not only the numerical values explicitly recited as the limits of the range, but also to include all the individual numerical values or subranges encompassed within that range as if each numerical value and sub-range is explicitly recited. To illustrate, a concentration range of "about 0.1% to 5%" should be interpreted to include not only the explicitly recited concentration of about 0.1 wt% to about 5 wt%, but also include individual concentrations (e.g., 1 %, 2%, 3%, and 4%) and the sub-ranges (e.g., 0.5%, 1.1%, 2.2%, 3.3%, and 4.4%) within the indicated range. The term "about" can include ±1 %, ±2%, ±3%, ±4%, ±5%, ±6%, ±7%, ±8%, ±9%, or ±10%, or more of the numerical value(s) being modified. In addition, the phrase "about 'x' to 'y'" includes "about 'x' to about 'y'".

It should be emphasized that the above-described embodiments of the present disclosure are merely possible examples of implementations, and are merely set forth for a clear understanding of the principles of this disclosure. Many variations and modifications may be made to the above-described embodiment(s) of the disclosure without departing substantially from the spirit and principles of the disclosure. All such modifications and variations are intended to be included herein within the scope of this disclosure and protected by the following claims.

Now having described the embodiments of long fiber thermoplastic composite/metal laminate structures and methods of making long fiber thermoplastic composite/metal laminate structures, in general, the examples describe some additional embodiments of the present disclosure. While embodiments of present disclosure are described in connection with the examples and the corresponding text and figures, there is no intent to limit embodiments of the present disclosure to this description. On the contrary, the intent is to cover all alternatives, modifications, and equivalents included within the spirit and scope of embodiments of the present disclosure.

Examples

In an embodiment, laminate structures were processed using nylon 6,6 LFT (12 mm long pellets and 23 volume % fibers) and aluminum alloy 2024. The laminate structures were processed by compression molding. The laminate structures were molded in 2/1 configuration, which included one layer of LFT composite between two aluminum plies. Mechanical behavior of the laminate structures was characterized by tensile, three-point bend, and low-velocity impact tests.

Tensile Testing

Tensile testing was performed on rectangular specimens of dimensions 15 x 140 x 1.4 mm. The testing was done on a TC-55 lnstron servohydraulic test frame. The crosshead velocity was 2 mm/min. A clip-on extensometer was used to measure the displacement of the gage length, which allowed values of Young's modulus of the laminate structures to be obtained. After failure, the fracture surfaces of samples were observed in a scanning electron microscope.

Three-Point Bend Tests

Three-point bend tests were done on a T-5000 Satec electromechanical test frame. Load versus displacement plots were obtained for each sample. The load- displacement plots were then converted to stress - apparent strain (obtained from the crosshead displacement) curves. Maximum flexure strength (σ_max) values were calculated based on the peak load value in the load versus displacement curves using the following relationship,

3V

(1) 2Bd² where P_max is the maximum load and S, S, and d are span length, breadth, and thickness of the sample, respectively. Short Beam Test

Short beam tests were done to estimate the interlaminar shear strength (ILSS) of the LML samples. The test was done on two sets of samples, one with interfacial surface roughness corresponding to the as-received condition of the metal and the other sand blasted to increase the surface roughness. The samples were cut according to ASTM standard D2344 from a plate of 2/1 configuration. Load vs. displacement plots were obtained from the three-point bend tests, and the ILSS was calculated from the following expression:

_ _ _ 3P max /o\

^{!LSS ~} 4Bd ^[ ' where P_max is the maximum load, S is the breadth, and d is the thickness of the sample.

Low Velocity Impact (LVI) Test

Low velocity impact tests on LMLs and LFT composites were done using a Dynatup 8250 drop weight impact testing machine. The impact tests were carried out on 2/1 laminates using 6.67 kg hemispherical impactor of 19.5 mm diameter. The energy of impact was varied by changing the release height of the impactor. A rectangular fixture having an opening of 75 x 75 mm was used to hold the samples. Square samples of 100 cm² were held between two aluminum plates of the fixture. Before impact, the weight of each sample was measured. The mass was divided by the area of the square plate to obtain an areal density (mass/area). Samples were tested in successive incremental energies until full perforation. Specific perforation energy (or specific perforation resistance) of LML and LFT composite samples were determined by normalizing the absorbed energy values with their respective areal densities. Similarly, specific peak load was obtained by normalizing the peak load by areal density.

Tensile Testing Results

Tensile stress-strain curves of the LFT composite/aluminum laminate and LFT composite are shown in Fig. 2. The curve for the LFT is practically linear until failure. The LML, however, showed a nonlinear behavior. The nonlinear behavior came mainly from the plastic deformation of the aluminum plies. There is an improvement in the properties of the LML over an LFT composite. Examination in a scanning electron microscope showed delaminations between the plies of the laminate, see Fig. 3. During the tensile loading of the LML, LFT composite failed first because of its low strain to failure. Within the LFT composite, various failure mechanisms were observed, including: nylon 6,6 matrix cracking, fiber/matrix interface debonding, fiber fracture, and pullout, Fig. 4. After the failure of the LFT composite, the load was taken by aluminum plies till the final failure of the LML by delamination at the LFT composite/aluminum interface and fracture of aluminum plies.

The average Young's modulus value of five samples determined was approximately 44.8 GPa. The average tensile strength of the laminate was found to be 244 MPa. The average values of specific modulus and strength of the LML were 20 GPa/gcnrf³ and 108.5 MPa/gcrrf³, respectively. There was a significant improvement in the modulus and the tensile strength of the LML compared to that of the LFT composite.

Rule-of-mixtures (ROM), based on volume fraction of the metal and composite layers in the laminate, was used to predict the laminate properties such as density, Young's modulus, and tensile strength of the laminate structure. The volume fraction of aluminum layers (VAI) was calculated as follows:

^VA, =

where t_Aι is the thickness of metal layer, p is the number of metal layers, and tuvn is the total thickness of the laminate structure. From this expression, the VAI for the laminate structure was calculated to be 0.57.

The density of a composite is given by the rule-of-mixtures. The expression for the density of the laminate structure (PLML) can be written as:

P₁ML = PAPA, + PLFT {1 - VΛ,) (⁴) where p_Aι and PLFT are density of the aluminum alloy and the LFT composite, respectively. The calculated density value was 2260 kg/m³, which agrees well with the experimentally determined value of 2250 kg/m³. The difference may be due to a small amount of porosity in the LFT.

In a similar way, to calculate the Young's modulus (E_LML) and tensile strength (OLML, at a particular value of strain) of the laminate structure, the following expressions were used:

E_LML = E_AIV_A, + E_Lri.{\ - V_Al) (5) σ_ML = ^_A1V_X, + σ_LFr(\ - V_AI) (6) where E_Aι and ELFT are the Young's moduli of the aluminum and the LFT composite, respectively, and σ'_Aι and σ'_LFτ are tensile strengths of the aluminum and the LFT composite at a particular value of strain.

The Young's modulus of the LML calculated from the Eq. 5 was 44.35 GPa, which compares well with the experimental value of 44.8 GPa. The tensile strength values of the LML at a strain value 0.005 from the ROM and experiment were 183 MPa and 165 MPa, respectively. The discrepancy in the strength values between ROM and experiment is likely due to two reasons: (1) reduction in the strength of the aluminum alloy 2024 after processing of the laminate and (2) because of variation in the strength of the LFT composite because of misorientation of discontinuous fibers. High strength in the aluminum 2024 stems from interaction between dislocations and the finely dispersed precipitates. During the processing of the laminate, the aluminum plies go through a heating and cooling cycle, which is likely to cause overaging of the alloy and hence coarsening of the finely dispersed precipitates and a reduction in strength. These coarse precipitates are not as effective as the fine precipitates in impeding the dislocation movement and hence a lower strengthening effect leading to a decrease in overall strength of the aluminum alloy occurs. In one study, it was found that the reduction in the strength values could be as large as 20% when the aluminum alloy 2024 was heated to 285 ⁰C. Taking into account the reduction in strength of aluminum only (ignoring the fiber misorientation), the ROM and experimental values of strengths are in a reasonable agreement. Predicted properties of the LML from the ROM and experimental values are summarized in the Table 1. Table 1. Comparison of the ROM and experimental properties of the laminate.

Three-Point Bend Test Results

Three-point bend tests also showed nonlinear behavior of the LML due to plastic yielding of aluminum plies, Fig. 5. The failure of the laminate structure was mainly by partial cracking of the aluminum ply on the tensile side of the sample, Fig. 6. No debonding was observed between the plies of the laminate, Fig. 7. The LFT layer in the middle and the top aluminum layer are in tension, while the bottom aluminum layer is in compression. The aluminum layer did not crack. The maximum flexure strength of the laminate structure was calculated based on the peak load reached in the flexure test. The LML showed significantly higher peak stress and strain to failure than that of LFT composite alone. The average maximum flexure stress of five LML samples was 683 MPa. The average maximum flexure stress values of the LFT composite in three-point bend test was 160 MPa. Unlike the laminate structure, the LFT composite failed in a brittle manner. Thus, the laminate structure showed improvement in flexure strength and damage tolerance.

Short Beam Test Results

Short beam flexure tests showed failure of the laminate due to delamination. Figure 8 shows the effect of surface roughness on load versus displacement plots obtained from the short beam tests. The curve with a higher peak load corresponds to a specimen where the aluminum surface was sandblasted to increase surface roughness, which in turn led to higher mechanical bonding between nylon 6,6 and the aluminum. The surface roughness measurements by profilometer showed that sandblasting increased the mean roughness from 0.4 μm to 3.2 μm. The first load drop in both the plots corresponds to the appearance of an interlaminar crack. After the first drop, load increases again, which corresponds to the sample bearing some load during progression of the delaminated crack. This is followed by the final failure. Figure 9 shows the interlaminar crack in the specimen. The average value of the interlaminar shear strength (ILSS) for sandblasted specimens was 34.4 MPa compared to that of 23.5 MPa for specimens not sandblasted. The results of the short beam tests are summarized in Table 2. Johnson reported interlaminar shear strength values of E-glass fiber/polyester sheet molding compound (SMC), polyester resin based chopped strand mat (CSM), and polyester resin based woven roving composite to be in the ranges 12-20, 22-30, and 22-30 MPa respectively. (Johnson AF (1986) Compos 17:233). Thus, the LMLs show an improvement in the ILSS over other composites.

Table 2. Comparison of ILSS values between no sandblasting and sandblasted specimens obtained by short beam tests.

Low Velocity Impact (LVI) Test Results

Figure 10 shows the load versus time plots for LML and LFT composite corresponding to different impact energies. The curve corresponding to a low impact energy level (3 J) showed ductile behavior of the LML, Fig. 10(a). The curves corresponding to 5 J impact energy showed appearance of a crack on the tensile side of the LML and LFT composite, Fig. 11 (a). The curves at impact energy levels 10 and 15 J in the case of LMLs correspond to cracking of compressive side of aluminum ply, crack propagation, and plastic deformation of aluminum plies while in the case of LFT composite, failure was mainly due to matrix cracking and fiber/matrix debonding, Fig. 11 (b). When perforated, LML showed failure in the form of extensive shear fracture of the top and bottom aluminum plies, delaminations between the LFT composite and aluminum plies, and fracture of the LFT composite was observed, Fig. 11 (c). All these failure mechanisms made LML more damage tolerant as compared to LFT composite, where fracture occurred by matrix cracking and fiber/matrix interface debonding. Specific absorbed energies corresponding to different impact energies, which were calculated by dividing absorbed energies by the respective areal densities of LML and LFT composite are shown in Table 3. Specific perforation energy was determined by dividing the perforation energy by the areal density. For LML, the average value of the perforation energy was 7.58 J/kg rrf², which was significantly higher as compared to the LFT composites, 1.72 J/kg m-².

Table 3. Comparison of LVI test results between LFT composites and LMLs. LML showed significant improvement in the impact properties over the LFT composite.

A comparison is made of the specific perforation resistance of the LML with the values of other hybrid composites such as thermoset based FMLs (Mg/carbon- epoxy and Al/glass-epoxy with volume fraction of composites, V_c = 0.57 and 0.53, respectively) and thermoplastic based FMLs (Mg/glass-PP and Al/glass-PP with volume fraction of composites, V_c = 0.53 and 0.52, respectively) in Table 3. Volume fraction of the LFT composite in the LML was approximately 0.43. Figure 11 shows that the LML showed improved perforation resistance compared to the thermoset based FMLs, which stems mainly from the higher toughness of thermoplastics.

Conclusions

Laminate structures, including layers of LFT composite and aluminum, were processed by compression molding. The laminate structure showed nonlinear behavior in tension and three-point bend tests. The Young's modulus of the laminate was found to be approximately 44.8 GPa, and the tensile strength was approximately 244 MPa. Failure mechanisms such as delaminations between the plies, fiber breakage and pullout, plastic deformation of aluminum plies, etc. were observed. Rule-of-mixtures (ROM) calculations of the laminate structure properties, such as density and modulus, matched well with the experimental results. Three- point bend tests showed a maximum stress of approximately 683 MPa. Failure took place by cracking of the aluminum ply on the tensile side without any ply delamination. lnterlaminar shear strength (ILSS) by short beam tests was found to be 34.2 MPa. An increase in interfacial roughness between LFT and aluminum resulted in higher ILSS. LVI tests showed that the LML had significantly improved specific perforation energy (7.58 J/kg rrf²) over the LFT composite (1.72 J/kg rrf²). This increase in the perforation energy was mainly because of the various failure mechanisms such as ply delaminations, bending and shear fracture of aluminum plies, and fracture of the LFT composite layer.

Claims

CLAIMS What is claimed is:

1. A long fiber thermoplastic composite/ metal laminate structure, comprising: a layer of long fiber thermoplastic composite disposed between a pair of metal sheets.

2. The laminate structure of claim 1 , wherein the long fiber thermoplastic includes: a material selected from thermoplastic polyurethanes, polypropylene, nylon-based polymers, polystyrene, acrylonitrile butadiene styrene (ABS), styrene acrylonitrile (SAN), polyethersulphone (PES), polyetherimide (PEI), polyphenylene sulfide (PPS), polyetheretherketone (PEEK), polyetherketone (PEK), polyoxymethylene (POM), or combinations thereof; and a fiber selected from an aramid fiber, a polypropylene fiber, a polyethylene fiber, a nylon fiber, a glass fiber, a carbon fiber, a metallic fiber, or combinations thereof.

3. The laminate structure of claim 2, wherein the fiber has a length of about 3 to 50 mm.

4. The laminate structure of claim 2, wherein the fiber has an aspect ratio of about 1000 to 2000.

5. The laminate structure of claim 2, wherein the fiber has a filament diameter of about 8 to 20 μm.

6. The laminate structure of claim 1 , wherein the metal sheet is made of a material selected from: metals, metallic alloys, metal matrix composites, metallic foams, or combinations thereof.

7. The laminate structure of claim 1 , wherein the metal sheet is made of a material selected from: aluminum, magnesium, steel, titanium, nickel, copper, brass, zinc, alloys of each, or combinations thereof.

8. The laminate structure of claim 1 , wherein the laminate structure has a characteristic, wherein the characteristic is a specific elastic modulus, wherein the specific elastic modulus is about 20 GPa/g cm^"3 to 80 GPa/g cm^"3.

9. The laminate structure of claim 1 , wherein the laminate structure has a characteristic, wherein the characteristic is a specific strength, wherein the specific strength is about 120 MPa/g cm^"3 to 300 MPa/g cm^"3.

10. The laminate structure of claim 1 , wherein the laminate structure has a characteristic, wherein the characteristic is a specific impact resistance, wherein, the specific impact resistance is about 15 J/cm²/g to 50 J/cm²/g.

11. The laminate structure of claim 1 , further comprising a plurality of layers of long fiber thermoplastic composite, wherein each layer is disposed between a pair of metal sheets.

12. The laminate structure of claim 2, wherein the fiber is nylon 6,6 and the metal sheet is made of aluminum.

13. The laminate structure of claim 1 , wherein the laminate structure is selected from: a flat structure, a contoured structure, a curved structure, a complex structure, or a combination thereof.

14. The laminate structure of claim 1 , wherein the thickness of the long fiber thermoplastic layer is about 0.15 to 5 mm.

15. A method for fabricating a laminate structure including: providing a long fiber thermoplastic source, wherein the long fiber thermoplastic source is selected from a long fiber thermoplastic pellet, a plasticized charge, or a combination thereof; providing a first metal sheet and a second metal sheet; disposing the long fiber thermoplastic source between the first metal sheet and the second metal sheet; forming a long fiber thermoplastic layer by melting the long fiber thermoplastic source; and cooling the long fiber thermoplastic layer, the first metal sheet, and the second metal sheet to form the laminate structure.

16. The method of claim 15, further comprising: applying pressure and heat to the first metal sheet and the second metal sheet.

17. The method of claim 15, wherein the metal sheet is made of a material selected from: metals, metallic alloys, metal matrix composites, metallic foams, or combinations thereof.

18. The method of claim 15, wherein the metal sheet is made of a material selected from: aluminum, magnesium, steel, titanium, nickel, copper, brass, zinc, alloys of each, or combinations thereof.

19. The method of claim 15, wherein the long fiber thermoplastic includes: a material selected from thermoplastic polyurethanes, polypropylene, nylon-based polymers, polystyrene, acrylonitrile butadiene styrene (ABS), styrene acrylonitrile (SAN), polyethersulphone (PES), polyetherimide (PEI), polyphenylene sulfide (PPS), polyetheretherketone (PEEK), polyetherketone (PEK), polyoxymethylene (POM), or combinations thereof; and a fiber selected from an aramid fiber, a polypropylene fiber, a polyethylene fiber, a nylon fiber, a glass fiber, a carbon fiber, a metallic fiber, or combinations thereof.

20. The method of claim 19, wherein the fiber is nylon 6,6 and the metal sheet is made of aluminum.

21. The method of claim 15, wherein a plurality of layers of long fiber thermoplastic between a plurality of metal sheets is formed.