Mechanical Testing of Composites

As the demands for light-weight composite structures for aerospace, ground transportation, and environmentally sustainable energy systems develop, so do the mechanical testing requirements for composite materials, components and structures.

Anisotropic and inhomogeneous composite materials, for use in demanding structural applications, require a wide range of mechanical tests. Determination of properties requires tension, compression and shear tests. Qualification and materials development requires other test types to explore more complex properties, such as open hole tension/compression (OHT/OFC), inter-laminar fracture toughness, compression after impact (CAI) and fatigue. 

Tests need to be conducted over a range of temperatures on materials that may have been conditioned in a variety of environmental conditions, such as high humidity and immersion in fluids.

Composite tests have been standardized by a number of organizations. The main international composite testing standards are those maintained by ASTM, ISO, CEN (European Committee for Standardization).

The most widely used testing methods are listed below: 

TENSILE TESTING

In-plane tensile testing of plain composite laminates is the most common test.

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. 

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, such as the Tinius Olsen Hydraulic Tensile Grip.

Accurate alignment of the grips and specimen is very important when testing composite materials due to the fact that the anisotropic properties, such as the modulus and strength, of the material differ depending on the direction of the applied stress and are often brittle in nature. 

Adjustable alignment fixtures are available to ensure that testing systems meet the alignment criteria required for reliable composites testing. To be effective, alignment fixtures need to allow adjustment of both concentricity and angularity, while the machine load-string is under load. 

The accepted method of checking for alignment under load is to use a strain gaged “alignment cell or specimen.” The alignment cell should have dimensions that are as close to the specimens being tested as possible. Typically the alignment cell will be fitted with two or three groups of four strain gages. 

Additionally, Tinius Olsen Horizon software is available that will provide a display of both the bending and the concentricity and angularity errors. This software identifies the adjustments that are required and displays, in real time, the effect of the adjustments, enabling accurate machine set-up in minutes.

Strain measurement for composites tensile testing is usually performed using either an extensometer or a bonded strain gage. The use of an extensometer is generally preferred as strain gages involve additional labor and cost.

COMPRESSION TESTING

Utilising the Tinius Olsen ST and SL series of UTM’s, composite compression test methods need to provide a means of introducing a compressive load into the material while preventing it from buckling. Most composite materials are produced in the form of laminate panels and, hence, the material being tested will be in the form of a relatively thin and flat rectangular test specimen.

There are several different compression test methods that utilize all possible combinations of the above methods of load introduction and buckling prevention. Common composites compression test standards include: ASTM D695, ASTM D3410, ASTM D6641, ISO 14126, and prEN 2850.

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 gages or a biaxial extensometer. Standards for this test include ASTM D3518 and ISO 14129.

The inter-laminar shear strength test (ILSS), sometimes referred to as short beam shear, is a simple test performed using a small specimen loaded in a three point bend configuration.

The ratio of the specimen thickness to the support span is high; this helps generate large shear loads along the center line of the specimen. ILSS standards in common use include: ASTM D2344, EN2563 and ISO 14130.

COMPRESSION AFTER IMPACT TESTING

Damage tolerance is a major concern with composite laminates. Compression after impact (CAI) testing provides a measure of damage tolerance. The CAI test is usually conducted on a rectangular laminate panel. The test consists of two parts:

First, the panel is clamped around the periphery and then subjected to a controlled impact in the center of the panel (A drop tower is required to provide the impact),

Secondly, the panel is then placed in a jig and subjected to an edgewise compressive load until it fails. The failure load gives an indication of the residual strength of the panel after the impact damage. CAI standards in use include: ASTM D7136/D7137, ISO 18352, and prEN 6038.

FATIGUE TESTING

Compared to the large number of well-defined “static” tests on composite materials, fatigue testing of laminates is much more open. It’s 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 limited by 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.

Fatigue testing software is now available which is capable of monitoring the test specimen temperature and intelligently adjusting the test frequency to minimize the test duration, while ensuring that the specimen does not overheat. Finally monitoring damage and defining failure of a composite material in a fatigue test is not straightforward—a common means of tracking damage is by monitoring the change in specimen stiffness during the test, but de-lamination damage has little effect on the tensile stiffness.

TEST ENVIRONMENTS

The most common test environment for composite materials is temperature (generally in the range -80 to 250 C). Specimens are often pre-conditioned in different environments prior to testing. Pre-conditioning is often in hot/wet conditions; however exposure to fluids (e.g. water, fuel, and hydraulic fluids) are also used. 

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.

The mechanical testing of composite materials is complex, involving a range of test types, a plethora of standards, and the need to condition and test in a variety of different environments. Life is made easier with Tinius Olsen, well-aligned test machines and grips, interchangeable test fixtures, and test software with pre-configured test methods.

For further information contact Richard Coombs at richard.coombs@tiniusolsen.co.uk