Composites Testing for Aerospace Applications

8/20/2019 Composites Testing for Aerospace Applications

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CompositesTesting for Aerospace Applications

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, f iber-reinforced composites, sandwich-core materials,

resins, fi lms 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, ful l characterization of the

material properties must be conducted if they are to be used instructura l aerospace situations.

Determination of bulk properties requires tension,

compression and shear tests. In qualif ication 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 oftemperatures on materials that may have been conditioned in

Westmoreland Mechanical Testing & Research;Youngstown, Pa.

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.

MATERIALS

CHARACTERIZATION

& TESTING

Fig. 1. Compression test

Fig. 4. Short-beam shear test



MATERIALS

CHARACTERIZATION

& TESTING

a variety of environmental conditions (e.g., high humidity and

immersion in f luids).

Tensile Testing

In-plane tensile testing of laminates is one of the most commonmechanical tests completed on composite materials. Other

tensile-tested composite materials include resin-impregnated

bundles of f ibers 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 isintroduced 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 f lat

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 biaxia l 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 suppor t span is high.

This helps generate large shear loads along the centerline of thespecimen. 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


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Fig. 2. Bearing-strength test

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 fatiguetests on polymer composites, the maximum test frequency is

restricted by the need to l imit 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: f lexure testing; tension and compression tests

on specimens with open and closed holes; bearing-strength tests

(Fig. 2); and interlaminar fracture-toughness tests.

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MATERIALS

CHARACTERIZATION

& TESTING

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 mechanicalanalysis (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 f luid-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 t ransition 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 t ime. It

can also determine heat f low. 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 specif ications and meets safetystandards. Common physical-properties tests include resin,

fiber and void content. The constituent content of a composite

material must be known in order to analytical ly 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, butexposure to f luids (e.g., water, fuel and hydraulic f luids) 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.

For more information: Contact Westmoreland Mechanical Testing &

Research, P.O. Box 388, 221 Westmoreland Drive, Youngstown,

PA 15696; tel: 724-537-3131; fax: 724-537-3151; e-mail:

[email protected]; web: www.wmtr.com

References1. McEnteggart, Ian, “Mechanical Testing of Composites,” Quality

Magazine, July 2014

2. Yancey, Robert, “How Composites are Strengthening the AviationIndustry,” Industry Week, June 2012

Fig. 3. Flatwise tension test

Documents

Composites Testing for Aerospace Applications