ASTM D7264 – Specimen Size

The ASTM D7264 standard describes two test procedures to determine the flexural properties of composites:

• Procedure A: Three-point loading system using center loading on a simply supported beam.
• Procedure B: A four-point loading system with two load points evenly spaced from their adjacent support points and one-half the support span between each load point.

Since the two test processes typically yield slightly different property values, tests may be undertaken according to any test procedure for comparison purposes, provided that the same procedure is utilized for all tests. These processes can be utilized to determine flexural qualities that can be applied to design as well as quality control and specification needs. These processes can help evaluate a variety of environmental factors to identify which are design drivers and may call for additional testing. These techniques can also be used to assess a structure’s flexural characteristics.

Test Specimen Preparation

The ASTM D7264 standard support span-to-thickness ratio is designed for flexural strength so that failure occurs at the specimens’ outer surface and is only caused by the bending moment. The standard span-to-thickness ratio is 32:1, the standard specimen thickness is 4 mm (0.16 inches), and the standard specimen width is 13 mm (0.5 inches) with the specimen length extending by roughly 20% beyond the support span. The width of the specimen for fabric-reinforced textile composite materials must be at least two unit cells, as specified in Guide D6856. The support span-to-thickness ratio (32:1) and specimen width must be maintained when using an alternative specimen thickness if the standard specimen thickness cannot be attained in the given material system. If specified in the report, alternative support span-to-thickness ratios of 16:1, 20:1, 40:1, and 60:1 may also be employed. Additionally, the results of one test using one support span-to-thickness ratio and another test utilizing a different support span-to-thickness ratio cannot be compared.

When tested at low support span-to-thickness ratios, shear deformations can dramatically diminish the apparent modulus of highly orthotropic laminates. For flexural modulus calculations, a high support span-to-thickness ratio is suggested. In rare circumstances, it can be necessary to determine modulus and strength using different sets of specimens.

Number of Test Specimens

Unless valid findings can be obtained by using fewer specimens, such as in the case of a specified experiment, test at least five specimens for each test condition. The steps specified in Practice E122 should be consulted for statistically significant data. Report the method of sampling.