The Definitive Guide to ASTM D790 Flexure Testing of Plastics

The 6800 Series test system is a more advanced and efficient option for users desiring increased repeatability and higher throughput. Though this system can also be used for Type 1 testing, it is most commonly used for Type 2 testing, which relies on an extensometer and deflectometer in order to provide a more accurate reporting of strain.

Type 2 testing requires the use of a strain measurement device, or extensometer. The first and most advanced option is a non-contacting video extensometer, Instron's AVE2. The AVE2 reports strain without ever physically contacting the specimen or entering the test space. The second option pictured below is an automatic contacting extensometer, the AUTOX750, which increases throughput by minimizing operator contact. Finally, a 2630 Series clip-on extensometer is a popular option for labs that do not require high throughput. For ASTM D790 testing, all three extensometers do need to be used in conjunction with a 2810-403 deflectometer.

For labs looking to increase their throughput, several modifications to the system setup can be made. Because D790 testing can be time consuming, Multi-Station 6800 series testing machines are a popular choice for labs with high throughput needs. Fully automated test systems are also available, and are designed to incorporate specimen measurement, specimen loading, testing, strain measurement, and specimen removal. These systems can run for hours without requiring any operator interaction and help reduce variability due to human error.

ASTM D790 specimens are rectangular in shape and are either molded, extruded, or cut from plates or sheets. The dimensions of ASTM D790 specimens depend on the thickness of the material, which the standard defines as being the depth of the specimen. Sheet materials with a thickness greater than 3.2 mm will have a width and span proportional to their thickness. Sheet materials with a thickness between 3.2 and 1.6 mm will have a width of 12.7 mm and a span defined as specimen thickness X 16. Sheet materials less than 1.6 mm in thickness will be 50.8 mm X 12.7 mm with a 25.4 mm support span.

Specimen dimensions for electrical insulating materials have a different set of metrics based on their nominal thickness. Molded materials are generally 3.2 mm deep, 12.7 mm wide, and 127 mm long. They use a support span equal to specimen depth X 16. High-strength reinforced composites should have dimensions as outlined in the standard to encourage failure to happen in the outer fibers of the specimen. When testing composites and other anisotropic materials, care should be taken to ensure that failure occurs only due to forces accrued during bending, and specimens should be cut to ensure that the strongest fiber direction is the one being tested.

Specimens must be measured with a micrometer that is in accordance with ASTM D5947. Three measurements of specimen width and thickness must be taken, with their average being reported. The Automatic Specimen Measuring Device (ASMD) feature in Bluehill Universal allows operators to connect up to two micrometers or measurement devices to the computer and have them input the mean of the measurements directly into the software. This eliminates the chances of input errors and increases efficiency.

A three-point bend fixture is used for ASTM D790. This fixture consists of a loading nose attached to the moving crosshead and a fixed member with two specimen supports, or anvils, which can be adjusted to fit the distance of the specimens' support span. The surface of the anvils and loading nose should be cylindrical and have a radii of 5 mm unless otherwise specified, and the length of the cylindrical member should be longer than the width of the specimen.

Instron’s bend fixture includes several key features to improve test accuracy. Graduated length units on the support beam allow for accurate positioning of the anvils and easy centering of the deflectometer when using strain devices. Instron bend fixtures also come with alignment arms that can be adjusted to fit the width of the specimen, because inconsistent specimen alignment can cause major variations in results and proper care must be taken to ensure that specimens are aligned consistently for each test.

The latest version of ASTM D790 was released in 2017. It differs from the previous version, 2015E2, in several ways:

• The verification requirement for Type 1 testing (crosshead displacement) was changed from a Class D system to a Class B system per E2309.
• The standard for Type 2 apparatus (deflectometer and extensometer) was changed from Class B to Class B-2.
• Note 10 and Appendix XI were added to the standard to explain what compliance correction is and how to apply it.