Gehman test

When testing the effect of low temperatures on elastomers it is rarely sufficient merely to enclose the standard apparatus in a refrigerated chamber special procedures are necessary. The relevant standard is BS 903 Part A13. Method for determination of stiffness at low temperatures (Gehman test). 

Rigidity in torsion Clash Berg, Gehman tests... 

ISO 812 1991 Rubber, vulcanized-Determination of low-temperature brittleness ISO 974 2000 Plastics - Determination of the brittleness temperature by impact ISO 1432 1988 Rubber, vulcanized or thermoplastic - Determination of low temperature stiffening (Gehman test)... 

ISO 1432 1988 Rubber, vulcanized or thermoplastic - Determination of low temperature stiffening (Gehman test)... 

The Gehman test is also standardised in BS903 Part A1318 and ASTM D105319. The British Standard is identical to the international method but the ASTM has a rather different layout as it covers coated fabrics as well as rubbers and a single point procedure is added for routine inspection. It only specifies a step temperature change procedure. 

Modulus changes at low temperatures can be monitored by the torsional modulus or Gehman test of ISO 1432 (BS903, Part A13) and by measurement of hardness. 

Gehman test is an alternative method to determine the extent of low temperature stiffening.The temperature effect can be followed from room temperature to -150°C. The sample used in this method has the following dimensions 40 x 3 x 2 mm. Test samples can be used for testing at least 16 h and less than 4 weeks after vulcanization. Samples are clamped in such a manner that at least 25 mm is covered by a cooling liquid. Specimen should be under zero torque due to the positioning of clamp stud. 

Hardness increase, differential scanning calorimetry (DSC), differential thermal analysis (125-127), low temperature compression set, and Gehman torsional stiffness are tests normally employed to measure crystallization properties. The heat of fusion of the crystalline phase of polychloroprene homopoljuner is approximately 96 kJ/kg (23 kcal mol ) and the activation energy for crystallization is 104 kJ/mol (25 kcal mol ). The extent of crystallization can be calculated from... 

Torsional Stiffness - Torsional stiffness tests measure the modulus of rigidity of an elastomer composition over a broad temperature range. Two widely-used tests are ASTM D-1043, which uses the Clash-Berg Tester, and the ASTM D-1053, which uses the Gehman Torsional Stiffness Tester. In both tests, the sample is chilled to a preset temperature, then twisted with a known force. The amount of twist is measured and related to the stiffness (modulus of rigidity) of the sample. The test temperature is then changed and the test is repeated, until a complete curve of stiffness vs. temperature is plotted. The temperature which produces a stiffness of 69 MPa (10000 psi) is sometimes taken as the Stiffness Temperature. 

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