This article specifies a method for determining the compressive 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.


Compression-testing machine, capable of maintaining a compression speed of 1 mm/min.

Two test jigs, for example as shown in Figure 1, each consisting of:

  • A baseplate (1).
  • An elastic compression pad (2) (made of polyamide, for example).
  • A support ring (3).

Parts 1 and 3 of the upper jig may be bolted together to prevent them falling.

The opening in the support ring shall fit snugly round the specimen in order to ensure consistent results. The maximum permitted opening diameter is 0.1 mm larger than the specimen diameter. If the specimen diameter is more than 0.1 mm smaller than that of the opening in the support ring, a support ring with a smaller opening shall be used. Deviations from nominal specimen dimensions can occur due to variations in the inner diameter of the mould or resin shrinkage.

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

The example drawing of  jig for testing specimens of diameter 6.0 mm.
Figure 1 — Example of jig for testing specimens of diameter 6.0 mm

Preparation and conditioning of specimens

The procedure specified hereafter is suitable for specimens of diameter 6 mm to be tested "as-moulded" and, if required, pre-treated. 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. Prepare a sufficient number of rods, using the method specified in ISO 3597-1, and cut them into specimens of length 22.5 mm. Take care to ensure that the cut ends of the specimens are parallel.

When rods of diameter other than 6 mm are used, the ratio of the rod length L to the rod diameter d shall satisfy the following equation:

L = 0.625d2


Before carrying out the compressive 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.

Conduct the compressive test in the same standard laboratory atmosphere as that used for specimen conditioning. Assemble a test specimen with a test jig at each end. Place the assembly vertically between the platens of the test machine. Compress the test specimen at a speed of 1 mm/min 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 2).

Four typical failure modes including crushing, splitting, bucking, and edge failure.
Figure 2 — Typical failure modes

Expression of results

For each specimen, calculate the compressive strength σC, expressed in megapascals, using the equation:

σC = (4 × F)/(πd2)


  • F is the maximum force, in newtons, that is recorded during the test;
  • d is the diameter, in millimetres, of the test specimen.

For each type of test, calculate the average of the results obtained on each specimen, discarding those results which are lower than 50 % of this average. Report the resulting value as the compressive strength of the material. If fewer than five results are available for this calculation, a further set of test specimens shall be prepared and tested. If this second set results in similar difficulties, record this situation in the test report.

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 specimens;
  • 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, compressive 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 compressive strength after pretreatment.

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