ISO 75-1:2020 pdf download – Plastics — Determination of temperature of deflection under load — Part 1: General test method.
4 Principle A standard test specimen is subjected to three-point bending under a constant load in the flatwise position to produce one of the flexural stresses given in the relevant part of ISO 75. The temperature is raised at a uniform rate, and the temperature at which the standard deflection, corresponding to the specified increase in flexural strain, occurs is measured. 5 Apparatus 5.1 Means of producing a flexural stress The apparatus shall be constructed essentially as shown in Figure 1. It consists of a rigid metal frame in which a rod shall move freely in the vertical direction. One end of the rod is fitted with a weight- carrying plate and the other end is equipped with a loading edge. The base of the frame is fitted with test-specimen supports. These supports and the vertical members of the frame are made of a material having the same coefficient of linear expansion as the rod. The test-specimen supports consist of metal pieces that are cylindrical in the contact area and with their lines of contact with the specimen in a horizontal plane. The size of the span, i.e. of the distance between the contact lines, is given in ISO 75-2 or ISO 75-3. The supports are fitted to the base of the frame in such a way that the vertical force applied to the test specimen by the loading edge is midway (±1 mm) between them. The contact edges of the supports are parallel to the loading edge and at right angles to the length direction of the test specimen placed symmetrically across them. The contact edges of the supports and loading edge have a radius of (3,0 ± 0,2) mm and shall be longer than the width of the test specimen. Unless vertical parts of the apparatus have the same coefficient of linear thermal expansion, the difference in change of length of these parts introduces an error in the reading of the apparent deflection of the test specimen.