Introduction

Note: Nothing in this standard supercedes applicable laws and regulations.

Note: In the event of conflict between the English and domestic language, the English language shall take precedence.

This document describes test procedures and conditions for determining the loss in counterforce of a standard specimen shape compressed at a constant deformation in specified media, under conditions of time and temperature. It is designed for testing of:

a. Specimens of vulcanized rubber, and thermoplastic elastomers with instantaneous Shore A hardness from 45 to 90 (ASTM D2240), molded button (ASTM D395) and,

b. Finished production articles of same hardness range (e.g., molded profile O-ring) per ASTM D3183.

Purpose.

This document is a supplement to ASTM D6147, SAE J2979 and ISO 3384 procedures for Compressive Stress Relaxation (CSR) testing.

Supplements. Supplemental features include:

a. Mandatory classification of test conditions and requirements using a “line callout” designation. See 8.2 and 8.3. In 2013, the SAE International Committee on Automotive Rubber Specifications added a SR suffix to the J200M classification standard allowing compression stress relaxation to be specified using this industry classification standard. Unfortunately, the use of alpha-numeric codes to identify all the test parameters is limited. GM will continue to use this standard for identifying all relevant test conditions.

b. Authorized Fixtures. See 3.2.3.

c. For referee purposes, a designated system with data acquisition to provide continuous counterforce measurement for the specified test duration. See 3.2.2.

d. Deviations from these industry procedures. See 1.3 through 1.3.3 and Appendix A, Table A1.

Applicability.

Specifying retained force for your compound will only reveal if it is capable of contributing to a sealing solution. It cannot evaluate sealing contribution in relation to design, construction/dimensional variation, and assembly. These contributions are evaluated in GMW3155 testing.

Under constant load (strain), rubber forms new crosslinks resulting in permanent physical deformation (change in shape). The intensity (stress) of the internal forces at constant deformation decreases over time and temperature. Stress relaxation is that measure of loss in force (stress) or the amount retained as a percent of the initial stress. This information is of practical value for studying materials for gaskets, hose clamped or with end fittings and other compressed applications.

The button and O-ring deflection is 15%, or 15% and 35% for manual and continuous measurement. Lower specimen deflection produces higher percent compression load loss. See 5.2.1.

Continuous CSR (Method B) runs the GMW3155 thermal cycle on elastomer compounds with a low temperature TR10 of -25 °C or colder to define their low temperature performance.

Remarks. This document uses two (2) methods to measure counterforce; discontinuous (manual) or continuous at the specified media and test temperature. The two (2) methods differ significantly from Method A and Method B of ASTM D6147 and ISO 3384. Both GM methods load and start at the same temperature, but the end temperature measurements differ.

Method A (Manual – (M) at 23 °C). The test specimen compression and the initial measurement of the counterforce take place at 23 °C ± 2 °C. The test specimen is then stored in a chamber controlled at the test temperature, but it is removed from the chamber (or lowered to temperature) for each of the subsequent force measurements, which are made at 23 °C ± 2 °C. This is the only method for manual measurement of counterforce subjected to isothermal or cyclic temperatures. It is the most common method used for generating CSR data.

Method B (Continuous at Temperature). The test specimen compression and the initial measurement of the counterforce take place at 23 °C ± 2 °C. The test specimen is then stored in a chamber controlled to the test temperature or cycle. Measurements are made on a continuous basis at the designated temperatures. When designating this method, continuous measurement equipment is required. Make sure the source has access to this equipment.

SAE J2979 describes counterforce measurement by the intermittent (manual) method and only applies to Method A. Follow this procedure for manual measurement with the following exceptions:

Subsequent Test Time Intervals. Fixture cool down time (2 h) and measurement time (1 h) are not to be included in the test time. The only exception is when a single end of test measurement is specified in Table 3, as a sequence 5.

Test Pressure Vessel. See 3.2.5 for each media.

Test Specimens. O-ring provisions are not part of SAE J2979. See 3.3.6.

Duration of Test. Use the appropriate Table 3 code.

Specimen Molding to Testing. The minimum time permitted from compound finish mixing, molding (includes post-cure if appropriate) of test specimens is 72 h, not 24 h. See 3.3.