How to Perform a Flexural Test on Plastics According to ASTM D790
Written by Kayla Thackeray
ASTM D790 is a testing method to determine the flexural (bending) properties of reinforced and unreinforced plastics, high-modulus composites, and electrical insulation materials. This guide is designed to introduce you to the basic elements of an ASTM D790 flexure test, and will provide an overview of the testing equipment, software, and specimens needed. However, anyone planning to conduct ASTM D790 testing should not consider this guide an adequate substitute for reading the full standard.
Is ASTM D790 the Right Standard for You?
ASTM D790 is one of several tests designed to measure the flexural properties of plastics. It is not designed to measure tensile properties, and anyone needing to quantify the tensile properties of plastic materials should refer to ASTM D638. This standard cannot determine flexural strengthMaximum fiber stress developed in a specimen just before it cracks or breaks in a flexure test. Flexural yield strength is reported instead of flexural strength for materials that do not crack in the flexure test. for materials that do not break or yield at or before 5% strain. Such materials may be more suitable for 4-point bend testing in accordance with ASTM D6272.
Differences between ASTM D790 and ISO 178
ASTM D790 is very similar to ISO 178, though it differs in several key points:
- ISO 178 requires the use of either a deflectometer or compliance correction in order to determine modulus. In ASTM D790 this is only a recommendation, and modulus can be calculated by crosshead displacement alone.
- Preferred specimen sizes are different, and because test speed is dependent on specimen depth, test speeds between the standards may vary. The preferred depth of ASTM D790 specimens is 3.2 mm. The preferred depth for ISO 178 specimens is 4 mm.
- ASTM D790 allows only one test speed, whereas ISO 178 allows a second (faster) test speed to be used after modulus is measured.
What Does it Measure?
ASTM D790 measures the flexural properties of a material while under a bending strain or deflection. This test is conducted on a universal testing system using a three-point bend fixture at a rate proportional to the depth of the specimen. ASTM D790 testing is used to determine the following mechanical properties:
• Tangent modulus - Also known as flexural modulus, this is the slope of the initial linear portion of the load deflection curve and is a measurement of the material's stiffness
• Secant modulus - The slope between the origin and a predefined point on the load deflection curve
• Chord modulus - The slope between two predefined points on the load deflection curve
• Flexural strength - The maximum flexural stress obtained during a bend test
• Flexural Stress at Break - The flexural stress at which a specimen breaks during a bend test. For some materials, the specimen breaks before a yield point, in which cases the flexural strength equals the flexural stress at break.
ASTM D790 describes two different test procedures intended for different types of material. Procedure A, which is the preferred method, employs a strain rate of 0.01 mm/mm/min. Procedure B employs a strain rate of 0.10 mm/mm/min and is intended for materials that may not break at 5% strain if tested at the lower rate. ASTM D790 allows strain measurement to be taken from either crosshead displacement or the readings of an extensometer, described as Type 1 and Type 2 testing respectively.
The required test speed for ASTM D790 is expressed as a function of specimen support span, specimen depth, and strain rate. Bluehill® Universal's Expression Builder allows users to input test speed as a function instead of as a static number. After the test operator enters the specimen measurements, the software will automatically modify the test speed according to the equation.
ASTM D790 testing can be done on either a tabletop or floor model universal testing machine with a variety of accessories that can be configured to optimize your testing. Because every laboratory has different needs, several different system configurations are available.
A sample basic configuration can be seen below. This test set up includes a 3400 Series test system with a 3-point bend fixture and no extensometer, meaning that strain in this case must be measured by crosshead displacement (Type 1 testing). Whenever strain is measured via crosshead displacement, compliance correction is recommended, though not required. 3400 series test frames can also be used in conjunction with extensometry for Type 2 testing.