Composites Testing for Aerospace Applications

Composite materials are changing the face of manufacturing and product development, and no industry has seen this more than aerospace. Aerospace designers are incorporating composite materials to help make their vehicles lighter, faster and more fuel-efficient. Boeing and Airbus, two leaders in the aviation industry, are heading the composite charge. Half of the Boeing 787 and the Airbus A350 aircrafts are constructed of composite materials. Other manufacturers are increasingly using composites for a variety of aircraft sections and components. The aviation giants, as well as aerospace-focused organizations like SpaceX and NASA, are drawn to composites for their very high stiffness-to-weight ratio and their resistance to fatigue and corrosion. In the broadest sense, a composite material is a material made from two or more constituent materials with different properties. When these materials are combined, they produce a material with improved characteristics from the individual components. The many types of composite materials used in aerospace applications include thermoset and thermoplastic composites, laminates, fiber-reinforced composites, sandwich-core materials, resins, films and adhesives. Thanks to materials scientists, these materials are evolving and improving at an incredible rate. The future of materials science appears to involve a heavy focus on composites. Mechanical Testing of Composites Composite materials used in aerospace applications will face incredibly harsh conditions and must be thoroughly tested to ensure safety and reliability. Because composite materials are anisotropic and inhomogeneous, full characterization of the material properties must be conducted if they are to be used in structural aerospace situations. Determination of bulk properties requires tension, compression and shear tests. In qualification and materials development, other test types – such as open-hole tension/compression, interlaminar fracture toughness, compression after impact and fatigue – are used to explore more complex properties. Tests need to be conducted over a range of temperatures on materials that may have been conditioned in a variety of environmental conditions (e.g., high humidity and immersion in fluids). Tensile Testing In-plane tensile testing of laminates is one of the most common mechanical tests completed on composite materials. Other tensile-tested composite materials include resin-impregnated bundles of fibers and sections of sandwich-core materials. Examples of common standards for the tensile testing of laminates are ASTM D 3039, EN 2561, EN 2597, ISO 527-4 and ISO 527-5. The specimens are parallel-sided with bonded tabs to prevent the grip jaws from damaging the material and causing premature failures. Gripping mechanisms include manual and hydraulic wedge grips. Compression Testing In composite compression test methods, a compressive load is introduced into the material while preventing it from buckling. Composite materials are often laminate panels, and the test specimens are frequently in the form of relatively thin and flat rectangles (Fig. 1). Compressive loads are introduced into a test specimen by the following methods. End loading: All of the load is introduced into the flat end of the test specimen. Shear loading: The load is introduced into the wide faces of the test specimen. Combined loading: A combination of shear and end loading is used. Shear Testing In-plane shear properties can be measured on a tensile test specimen with a ±45-degree fiber orientation. The specimen’s axial and transverse strain is measured using either strain gauges or a biaxial extensometer. Standards for this test include ASTM D3518 and ISO 14129. The interlaminar shear-strength test, sometimes referred to as short-beam shear, is a simple test performed using a small specimen loaded in a three-point-bend configuration (Fig. 4). The ratio of the specimen thickness to the support span is high. This helps generate large shear loads along the centerline of the specimen. Interlaminar shear-strength standards include ASTM D2344, EN2563 and ISO 14130. Fatigue Testing Compared to the large number of well-defined “static” tests on composite materials, fatigue testing of laminates is much more open. It is important to have accurate alignment and correct gripping to avoid failures near the grip jaws. Also, high lateral stiffness is paramount to prevent buckling in tests that include compressive loading. It should be noted that some of the anti-buckling guides used in “static” testing are problematic if used in cyclic testing due to friction effects. When conducting fatigue tests on polymer composites, the maximum test frequency is restricted by the need to limit the temperature rise in the test piece (e.g., the maximum temperature rise recommended by the ISO 13003 fatigue standard is 10°C). Other Mechanical Tests A variety of other standardized mechanical tests on composite materials include: flexure testing; tension and compression tests on specimens with open and closed holes; bearing-strength tests (Fig. 2); and interlaminar fracture-toughness tests. Thermal Analysis and Testing Thermal analysis covers a range of techniques used to determine the physical or chemical properties of a substance as it is heated, cooled or held at constant temperature. Typical thermal-analysis tests for aerospace composites include dynamic mechanical analysis (DMA), thermomechanical analysis (TMA), differential scanning calorimetry (DSC) and thermogravimetry (TGA). DMA measures the mechanical and viscoelastic properties of materials such as thermoplastics, thermosets, elastomers, ceramics and metals. It involves the performance on glass transition tests on larger materials and increases the number of variables for those tests. DMA’s other highlights include a -190°C to 600°C (-310 to 1112°F) temperature range, as well as humidity control and fluid-bath options. Similar to a mechanical test frame, DMA can perform bend, shear and tension tests. The TMA instrument, which can reach temperatures ranging from -80°C to 1600°C, tests for coefficient of thermal expansion. It also performs glass transition tests on smaller samples. In addition to performing glass transition tests on homogenous materials, DSC measures enthalpy internal energy changes in samples due to variations in their physical and chemical properties as a function of temperature or time. It can also determine heat flow. DSC’s temperature range is from -80°C to 550°C (-112 to 1022°F). The TGA device measures the change in weight of a sample as it is heated, cooled or held at a constant temperature. It is primarily used to characterize material composition and has a temperature range of 23-1600°C (73-2912°F). One key aspect of the TMA and DSC machines is their ability to be purged with an inert, dry gas such as helium, argon or nitrogen. This feature is key when tests are taken to subambient temperatures. The gas keeps the furnace and specimens as dry as possible. Physical-Properties Testing Physical properties testing of composites helps ensure that the material complies with industry specifications and meets safety standards. Common physical-properties tests include resin, fiber and void content. The constituent content of a composite material must be known in order to analytically model its material properties, which are affected by the reinforcement or matrix. Other physical-properties tests include hardness, water absorption, density and specific gravity, and moisture content. Test Environments The most common test environment for composite materials is temperature (generally in the range of -80 to 250°C). Specimens are often pre-conditioned in different environments prior to testing. Pre-conditioning is often in hot/wet conditions, but exposure to fluids (e.g., water, fuel and hydraulic fluids) is also used. The time taken for polymer composite materials to achieve equilibrium with a conditioning medium is usually a few days or weeks. So, short-duration testing, including tensile testing of pre-conditioned composite materials, can generally be conducted in a temperature-only environment. Chambers designed for testing at low and high temperatures are generally equipped with forced convection for heating and liquid-nitrogen injection systems for cooling.

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