The method for determining the flexural strength of composite rods of circular cross-section made of roving-reinforced resin. The test may be carried out on "as-moulded" rods, or on rods that have been pretreated by immersion in boiling water for a specified time. The test is intended for inspection and quality control of rovings or for evaluating their suitability for use in a resin system.


Flexural-testing apparatus, with the following additional characteristics:

  • The load applicator shall be a steel rod waisted in the middle to ensure a distribution of stresses. A load applicator suitable for a 6-mm-diameter test specimen is shown in Figure 1. It is important that the radius R of the waist is not more than 0.25 mm larger than the actual (measured) radius of the test rod specimen.
  • The span between the supports shall be 16 times the diameter of the test composite rods specimen.

Micrometer or vernier gauge, capable of measuring to the nearest 0.1 mm.

Preparation and conditioning of test specimens

Prepare and condition a suitable number of specimens. For each type of test, the minimum required number of specimens is eight. If specific statistical limits are agreed between interested parties, additional specimens may be tested to permit statistical analysis of the results. The required specimen length is 20 times its diameter, i.e. 120 mm for a rod of diameter 6 mm.

Drawing of load applicator for testing specimens of diameter 6 mm.
Figure 1 — Example of load applicator for testing specimens of diameter 6 mm.


Before carrying out the flexural-strength test, measure the diameter of each test specimen with a micrometer or vernier gauge to the nearest 0.1 mm at two points around the circumference located 90° apart at the centre of the specimen. Use the average of these two measurements in the calculation of the results.

Carry out the flexural-strength test on the specimens, modified as specified in 1.1, in the same standard laboratory atmosphere as that used for specimen conditioning. The speed of the load applicator shall be 5 mm/min. Polyethylene film may be placed between the support and the rod to avoid compressive failure. No information is available about the influence on the results when using such film.

The person requiring the test may require that the mode of failure be recorded (see Figure 2).

Six examples of possible failure modes
Figure 2 — Examples of possible failure modes.

Expression of results

For each test specimen, calculate the flexural strength σF, expressed in megapascals, using the equation:

σF =( 8 × F × l)/(πd3)


  • F is the breaking force, in newtons;
  • l is the span, in millimetres;
  • d is the diameter, in millimetres, of the test specimen.


The precision of this test method is not known because inter-laboratory data are not available. When interlaboratory data are obtained, a precision statement will be added at the following revision.

Test report

The test report shall include the following information:

  • A reference to this part of ISO 3597;
  • All information necessary for complete identification of the roving tested;
  • All information necessary for complete identification of the resin used;
  • The diameter of the mould used to prepare the test rods;
  • Whether a release agent was used and, if so, details of the agent used;
  • The curing and post curing conditions;
  • The test atmosphere;
  • The test results:
    • For "as-moulded" specimens, the glass content, diameter, flexural strength and, if required, mode of failure of each specimen tested and (except for the mode of failure) the average value;
    • If testing was also carried out after pretreatment:
      • The medium used;
      • The length of the pretreatment;
      • The individual and average values of the flexural strength after pretreatment.

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