MATERIALS TESTING SYSTEM
ASTM D3039 testing is performed on a table top or floor model universal testing machine. A 30 kN or 50 kN system may be sufficient for testing glass fiber composites, but a 100 or 250 kN system is needed for testing carbon fiber composites.
The grips used to hold composite specimens must provide a sufficiently strong and even pressure to prevent the specimen from slipping during testing. The jaw face patterns should be suitable for the material and be in good condition. The alignment of the grips should limit the bending strain to within 3-5% at moderate strain levels (>1000µɛ).
Suitable grip designs for ASTM D3039 include the Instron® precision manual wedge grips (2716-028 / 2716-030) and hydraulic wedge grips (2742-401 / 2742-501). Both of these grips use a moving body design and incorporate specimen location stops to provide reliable gripping of composites and other materials while achieving the required alignment.Extensometers
A number of different devices are available for measuring strain during the test. The most common are extensometers, which are available in a variety of different options depending on the needs of your laboratory. The simplest is a fixed gauge length 2630 clip-on extensometer to measure axial strain. An operator must clip this directly onto the specimen at the beginning of each test and remove it before the specimen breaks.
If testing for Poisson’s ratio, a transverse extensometer must also be added to measure the change in width throughout the elastic region of the specimen. A standalone transverse extensometer can be used to supplement an existing clip-on or automatic extensometer, or a biaxial device can be used to measure both axial and transverse strain simultaneously.
The AutoX750 automatically attaches to the specimen without interference by the test operator. This extensometer is useful in labs that have high throughput needs, as it eliminates the time-consuming need for manual manipulation by the operator and also provides more consistent placement on a large number of specimens. Consistent placement results in more repeatable modulus values.
Oftentimes, the composites being tested are ultimately destined to be used under non-ambient conditions. To simulate these end-use applications, ASTM D3039 is performed inside a temperature chamber where heating or cooling (LN2 or CO2) can be used. Strain gauges or clip-on extensometers can be used up to a maximum temperature of 200 °C. Alternatively, a non-contacting advanced video extensometer (AVE 2) can be used. The AVE 2 is mounted outside of the temperature chamber and uses a camera to track deformations in the specimen throughout the test, with the advantage that test operators do not need to open and close the chamber door during testing.
Electrical resistance strain gauges are also useful in determining strain during ASTM D3039 testing. Unlike extensometers, strain gauges are consumable items that can be used to measure strain at failure. These gauges typically consist of a thin metal foil grid which is bonded to the specimen with an adhesive. Strain gauges can be used in environmental conditions from cryogenic temperatures to over 200°C, but require conditioning in order to generate a useful electrical signal. An easy-to-use adapter is available for use with the standard electronics in an Instron test machine. Because composite materials do not substantially deform before failure, the extreme precision of a bonded strain gauge can sometimes be preferable to the use of an extensometer for ASTM D3039 testing.
Testing composite materials under non-ambient conditions is normally performed inside a temperature chamber. These chambers use forced air convection along with resistive heating elements to achieve high temperatures and liquid nitrogen or carbon dioxide cooling to achieve low temperatures. The Instron range of 3119-600 series environmental chambers provides extensive temperature testing capabilities for evaluating material properties under non-ambient testing conditions. A full range of complementary grips, pullrods, and extensometers is available.
Instron 6800 Series Universal Testing Systems provide unparalleled accuracy and reliability. Built on a patent-pending Operator Protect system architecture with an all-new Smart-Close Air Kit and Collision Mitigation features, the 6800 Series makes materials testing simpler, smarter, and safer than ever before.
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Instron’s universal testing machines are designed to perform the most common types of testing used in the composites industry, including but not limited to in-plane tensile, through thickness tensile, interlaminar shear, compression in end, shear, or combined loading, and compression after impact.