13 Design Options

The preferential incorporation of reinforcing fibers into a polymer matrix opens up the design options available. At one end of the spectrum there are unidirectional fiber-reinforced materials that maximize the use of the available strength and stiffness of the fiber and produce a product that is highly directional in its properties. At the other end are random or multidirectional fiber materials whose properties approach isotropy.

1.3.1 Final Products

Today, advanced polymer composites find their greatest use in the aerospace sector where they were initially developed. Stealth aircraft such as the F-177 and the B-2 are only possible because of the unique properties of advanced polymer composites such as high strength and light weight. From helicopter blades to rocket motor casings to ballistic armor, these materials have fueled a revolution in new product applications. Initially, many projects attempted to replace a metal part with composite parts by direct substitution. This did not often work well. The unique properties of composites could not be incorporated in a part substitution and the resultant product frequently was more expensive than the original. Fortunately, as time passed, designers became more familiar with composite design methodologies and designed new products with composites in mind from the concept stage. The following section outlines the design approach.

1.3.2 Introduction to Methodology

The design of structures with advanced polymer composites proceeds through the application of classical lamination theory. Individual laminae, or plies, are stacked with the fibers oriented in various directions to build a laminate with the desired properties. Designers are used to working with materials such as plastics and metals that are described as homogeneous and isotropic. That is, the materials properties are not dependent upon the position or orientation in the material. For these classes of materials in a plane stress state, the relationship between stress and strain is described through the elastic constants Young's modulus E, and Poisson's ratio, v.

However, a laminated composite material cannot usually be accurately described this simply. Homogeneous orthotropic, homogeneous anisotropic, heterogeneous orthotropic, and heterogeneous anisotropic are additional descriptions that may be required to accurately analyze a laminated material. Fortunately, this complexity is not often required and, with the advent of modern software, is even manageable on desktop computers. For the balance of this section, we will make the assumption that a composite exhibits homogenous orthotropic behavior. We will also consider a special ply configuration that approaches isotropic behavior—quasi-isotropic.

1.3.3 Laminae

As indicated previously, a homogenous, isotropic material requires two independent constants to describe its stress-strain behavior. A homogeneous, orthotropic composite material has three perpendicular planes of material property. If the axes are chosen to coincide with the reinforcing filament direction, then this set is called the principal lamina direction. Dimensionally, these laminae are physically thin compared to their length and width. Although the thickness stresses are small and as applied in a laminated structure, a state of plane stress or plane strain is assumed. This leads to the need for four independent elastic constants in order to describe the stress-strain response: E11 and E22, Young's modulus; G12, shear modulus; and Vi2, major Poisson's ratio. This description of a lamina, or ply, is most common in the design of a laminated composite structure. Testing of ply materials is most oriented toward establishing these constants.

1.3.4 Laminates

When multiple layers of lamina are combined and act structurally as a single layer, a laminated composite is created. To analyze a laminated composite structure, the designer must know the properties of each layer and how the reinforcing fibers are oriented with respect to one another, that is the stacking sequence. For example, a laminate consisting of 16 individual layers may have the fibers oriented in the following fashion:

Two layers with fibers at 0° Two layers with fibers at 90° One layer with fibers at +45° Three layers with fibers at -45° Three layers with fibers at -45° One layer with fibers at +45° Two layers with fibers at 90° Two layers with fibers at 0°

This description is quite lengthy and shorthand methods have been developed to present the information:

[02/902/45/ - 45b/ - 45b/45/902/02]t or [02/902/45/ - 456/45/902/02]t or [02/902/45/ - 45b]s

Each of these methods describes the laminate. In the first method, each of the orientations is given along with the number of layers indicated by the subscript. The [ ]'s and the subscript T indicate that this is a description of the total laminate. The second method simply combines the two -45° layer groups into one. The third description, however, recognizes an important property of this particular lay-up sequence. It is symmetrical about the centerline of the laminate. Only one-half of the stacking sequence is explicitly listed, and the subscript T is replace by S to indicate the symmetry. While lamination theory can accurately analyze any stacking sequence, the condition of midplane symmetry is an important one for the designer of polymer composite structures. Nonsymmetrical lamina lay-up can result in out-of-plane bending and twisting under mechanical or thermal stress that must be considered.

In addition to midplane symmetry, there is one other design concept that is usually followed by designers. That is, the stacking sequence is usually "balanced." This means that there are an equal number of plies at angles of +0 and —0. Construction that follows this convention will avoid the shear coupling that is present in a single orthotropic lamina.

Lamination theory describes the stress-strain response of stacked orthotropic lamina. This behavior can be used to analyze the strength of the laminate if the assumption is made that the basic strength criteria for the lamina remain valid in the laminate. Under this assumption, a strength analysis proceeds by determining the individual ply stresses and/or strains in the laminate and comparing them to the allowable for the ply. Failure is often deemed to have occurred when one of the plies exceeds an allowable stress-strain limit. This first ply failure does not necessarily lead to complete failure of the laminate, as the failed ply may transfer some or all of the load it carried to another ply in the laminate and not exceed an allowable at that location. Procedures are available to analyze ply-by-ply failure sequences but are usually used as part of a failure analysis process rather than a design study.

A final word on composite laminate and ply failure. Since a fiber-reinforced lamina is modeled most frequently as an orthotropic material, the use of a failure criteria such as the maximum principal strain criteria used with isotropic materials is not applicable. A maximum strain criteria for an orthotropic material requires that the strains developed under load be referred to the lamina principal axes and evaluated against the tensile and compressive allowable for the lamina. This leads to the need for five failure strains; tensile and compressive limits in the fiber direction, tensile and compressive limits in the transverse to the fiber direction, and an in-plane shear limit. Other failure criteria, such as the Tsai-Wu criterion, are developed as yield surfaces that depend upon the interaction between the lamina principal direction and shear yield strengths. Commercial computer software for analyzing laminated composite structures is available. These packages can be customized to allow input of new materials, modified failure limits, and failure analysis methods.

1.4 TESTING/ANALYSIS 1.4.1 Mechanical Properties

The mechanical properties of polymer matrix composite materials depend upon the type of fiber and resin used, the relative percentages of each, the laminate lay-up, and the method of manufacture. The properties presented in this chapter are focused upon the fiber and resin materials that make up the composite. These are the product forms most often purchased by a user who combines them into a laminate. In this section, the methods used to determine the constituent and laminate properties commonly used in selecting materials will be reviewed.

The fiber properties presented in the previous tables are typical of what a potential buyer will encounter. The tensile strength, tensile modulus, and elongation are usually determined by the impregnated strand test. Over the past few years, industry standard test methods have been developed for determining these properties. The properties are, thus, reasonably comparable between manufactures in a general sense. The test is also useful for quality control purposes.

Fiber density is an important property. Often it is specific strength or modulus that controls the applicability, especially in weight and stiffness critical areas. There are also industry standards that can be used to measure this property.

The important properties of the polymer matrix resins used in advanced composites are both chemical and mechanical. For uncured thermoset resins, the important properties are related to the processing method to be employed. Viscosity, gel time, cure temperature, and the like all must be considered in order to properly process and cure the composite. The test methods used are common in the polymer manufacturing business and can be found in many references.

One of the most important properties of a cured thermoset resin is the glass transition temperature (Tg ). This parameter is both a measure of the completeness of the cure and an indication of the maximum service temperature of the composite. Differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) are two common techniques. DSC measures the amount of heat given off (or absorbed) in a resin sample as the temperature is increased. When the Tg no longer changes the resin is completely cured. With the DMA technique the response of the resin to mechanical stress is monitored with respect to temperature. The temperature at which a significant change to the elastic moduli is observed is the Tg. Since these methods measure two different parameters, they can give two different estimates of Tg. Care should be taken when reviewing supplier data as the method used is not always indicated.

As prepregs are an intermediate product from that combines fiber and resin in a specific ratio and partially processes the resin, it is important to know that the ratio and the "b-staged" resin are properly prepared. The fiber-resin ratio is measured by the aerial weight, or in grams per square meter, of fiber. Since this property is chosen by the application requirements, it is not a handbook type of quantity. A typical value for this parameter will place the fiber fraction at ~60% by volume.

In the cured laminate, the calculation of the relative amounts of fiber and resin is an important measure of the quality and proper processing history of the material. ASTM methods are available for determining these ratios and for determining the void content. Void content can have a detrimental effect on the properties of the composite and is usually limited to 1 or 2% of the material's volume. The technique involves burning (in the case of glass fiber) or chemically digesting (in the case of carbon fiber) the resin matrix. The relative weights (W) of fiber and resin, when combined with the densities (D) of the composite (C), fiber (F) and resin (R) will yield the void content:

The tensile properties of an advanced polymer composite material are usually measured with a flat coupon. ASTM D3039 is one test method standard that can be used. The test is applicable to unidirectional and oriented laminates. It differs in purpose from the impregnated strand test previously discussed. The structural fiber-resin ratios are more closely represented in the coupon test, and the results are more applicable to the actual planned use. The influences of the matrix and fiber-to-matrix interface are more evident. Testing at elevated temperatures and after exposure to other environmental conditions often use this specimen. ASTM methods also are available to govern the procedures used.

Compressive properties of polymer-matrix composites are difficult to measure. The ASTM provides a recommended method but many users develop their own. Again, the purpose of compressive testing is often to evaluate the performance of a fiber-resin combination to various service environments. Numerous tests for shear properties have been developed. A shear test is often used to measure the effectiveness the fiber-resin interface. The ASTM, again, provides methods to follow. A simple test such as ASTM 2344, apparent interlaminar shear strength, is often used for quality control and comparative purposes. ASTM D3518 is a procedure for measuring shear strength and modulus design data.

1.5 NONDESTRUCTIVE TESTS (QUALITY ASSURANCE)

Nondestructive tests of polymer matrix composites have received a great deal of attention. The difficulty and expense of performing destructive tests on actual structures have spurred the search for testing techniques that verify performance (quality assurance) without destroying the product. While no standard nondestructive tests for product quality exists, the use of ultrasonic techniques have become quite sophisticated. The ability to detect delaminations, inclusions, and voids on complicated geometries has made the test routine easier in many programs. Similarly, the use of infrared thermography to detect flaws or damage has developed recently.

1.6 ENVIRONMENTAL PERFORMANCE 1.6.1 Temperature

The service or operating temperature of a polymer matrix composite is probably the most important parameter considered in choosing the chemical nature of the matrix. In Table 1.5, the glass transition temperature, Tg, is an indication of the maximum service environment. The operating temperature is kept below the Tg. Polymer matrix composites are limited to 260°-316°C (500-600°F) applications.

Above these temperatures, metal or ceramic matrices are required. Testing for temperature effects is usually done by performing several of the mechanical tests previously described at elevated temperature. In general, tests that stress the matrix, such as shear and compression, will show the greatest effect. Temperature effects are generally reversible provided that the temperature exposure has not been high enough to cause physical damage to the matrix.

1.6.2 Moisture Exposure

Moisture tends to "plasticize" or soften the matrix. As with temperature effects, the composite properties are measured after exposure to water for varying times and at varying temperatures. Moisture effects, like elevated temperature effects, are generally reversible.

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