This part of ISO 75 specifies three methods, using different values of constant flexural stress, that can be used for the determination of the temperature of deflection under load of plastics (including filled plastics and fibre-reinforced plastics in which the fibre length, prior to processing, is up to 7,5 mm) and ebonite: - method A, using a flexural stress of 1,80 MPa; - method B, using a flexural stress of 0,45 MPa; - method C, using a flexural stress of 8,00 MPa. The standard deflection Δs used to determine the temperature of deflection under load corresponds to a flexural-strain increase Δ ε f defined in this part of ISO 75. The initial flexural strain due to the loading of the specimen at room temperature is neither specified nor measured in this part of ISO 75. The ratio of this flexural-strain difference to the initial flexural strain depends on the modulus of elasticity, at room temperature, of the material under test. This method is therefore only suitable for comparing the temperatures of deflection of materials with similar room-temperature elastic properties. NOTE The methods give better reproducibility with amorphous plastics than with semi-crystalline ones. With some materials, it may be necessary to anneal the test specimens to obtain reliable results. Annealing procedures, if used, generally result in an increase in the temperature of deflection under load (see 6.6). For additional information, see ISO 75-1, clause 1.
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