ASTM D790 3-Point Flexure Test on Plastics
The ASTM D790 standard describes a test method used to determine the flexural properties of unreinforced and reinforced plastics, including high modulus composites and electrical insulating materials. Typical test results according to ASTM D790 include the flexural modulus, stresses and strains at the yield point, the flexural strength and the flexural strain at break. The tests are used for rigid and semi-rigid plastics, however only up to a 5% strain limit.
Objective & characteristic values of ASTM D790
The 3-point flexure test to ASTM D790 is a traditional standardized characterization method for rigid and semi-rigid plastics. The flexural properties determined with these tests are of great importance to designers, engineers and material manufacturers to ensure that the plastic material used meets the requirements for its intended application.
Flexural modulus: The flexural modulus is a measure of the stiffness of the material and indicates how well it can resist flexural loads. The ASTM D790 standard provides three calculation methods for the flexural modulus, each of which leads to different results:
Tangent Modulus (Modulus of Elasticity): The ratio, within the elastic limit, of stress corresponding to strain
Secant Modulus: Ratio of stress to corresponding strain at any selected point on the stress-strain curve
Chord Modulus: Slope between two specified points on the stress-strain curve
Flexural strength: Flexural strength provides information about the maximum load that the material can withstand in flexure before it breaks.
Flexural stress at a specified flexural strain
Flexural stresses and flexural strains at the yield point and at specimen break.
Specimens
Test specimens that are cut from sheets, plates, or molded or extruded shapes, or molded to the desired finished dimensions are acceptable. The actual dimensions used shall be measured in accordance with Test Methods D5947. The depth of the specimen shall be defined as the thickness of the material. The depth shall not exceed width. The cross section of the specimens shall be rectangular with opposite sides flat and parallel(60.2mm) and adjacent sides perpendicular along the full length of the specimen.
To test plastics according to ASTM D790, the specimens are produced by injection molding or machined from sheets, plates or molded parts by mechanical processing.
According to ASTM D790, test specimens with a cross-section of 3.2 mm x 12.7 mm and a length of 127 mm are usually used for plastic molding compounds. The specified thickness to support ratio of 16 leads to a support span of 51 mm. In addition, the standard defines a series of additional requirements for specimens with a thickness greater than 3.2 mm and less than 1.6 mm.
To test laminated thermosets and sheet materials for electrical insulation, the specimens may have to be machined to a thickness of 25.4 mm or 12.7 mm. If the shear strength is very low compared to the tensile strength, the thickness-to-support ratio must be increased to 32 or even 40 to avoid shear fractures.
MEASURING AND SETTING SPAN
For flexural fixtures that have adjustable spans,it is important that the span between the supports is maintained constantor the actual measured span is used in the calculation of stress, modulus,and strain.and the loading nose or noses are positioned and aligned properly with respect to the supports. Some simple steps as follows can improve the repeatability of your results when using these adjustable span fixtures.
MEASUREMENT OF SPAN
This technique is needed to ensure that the correct span; not an estimated span,is used in the calculation of results.
Scribe a permanent line or mark at the exact center of the support where the specimen makes complete contact. The type of mark depends on whether the supports are fixed or rotatable (see Figs.A2.1 and A2.2).
Using a vernier caliper with pointed tips that is readable to at least 0.1mm(0.004 in.), measure the distance between the supports,and use this measurement of span in the calculations.
Setting the Span and Alignment of Loading Nose(s)— To ensure a consistent day-to-day setup of the span and ensure the alignment and proper positioning of the loading nose, simple jigs should be manufactured for each of the standard setups used.An example of a jig found to be useful is shown in Fig.A2.3.
English 
Spanish 
Russian