This part specifies a method for determining the apparent interlaminar shear strength of composite rods of circular cross-section made of roving-reinforced resin.


  • 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. 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 specimen.
    • The span between the supports shall be 5 times the diameter of the test specimen.
  • Micrometer or vernier gauge, capable of measuring to the nearest 0.1 mm.

Preparation and conditioning of test specimens

Prepare and condition a given 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 8 times its diameter, i.e. 48 mm for a rod of diameter 6 mm.


Before carrying out the interlaminar-shear 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 interlaminar-shear test on the specimens, in the same standard laboratory atmosphere as that used for specimen conditioning. The speed of the load applicator shall be 1 mm/min. Continue the test until the specimen fails. Record the maximum force, in newtons, that occurred during the test. The person requiring the test may require that the mode of failure be recorded (see Figure 1).

Acceptable modes (type X) and unacceptable failure modes (type Z and type Y).
Figure 1 — Acceptable and unacceptable failure modes (Only Type X is acceptable)

Check that a type X failure mode is obtained with each specimen. For other failure modes, repeat the test on further specimens. Continue testing until at least five valid results are obtained.

If five valid results are not obtained, prepare and test a further set of specimens. If this second set results in similar difficulties, record this situation in the test report.

Expression of results

For each test specimen, calculate the apparent interlaminar-shear strength τ, expressed in megapascals, using the equation:

τ = (8 × F)/(3πd2)


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

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, apparent interlaminar-shear 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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