ISO 178:2019 pdf download – Plastics — Determination of flexural properties.
4 Principle A test specimen of rectangular cross-section, resting on two supports, is deflected by means of a loading edge acting on the specimen midway between the supports. The test specimen is deflected in this way at a constant rate at midspan until rupture occurs at the outer surface of the specimen or until a maximum strain of 5 % (see 3.8) is reached, whichever occurs first. During this procedure, the force applied to the specimen and the resulting deflection of the specimen at midspan are measured. This document specifies two methods: method A and method B. Method A uses a strain rate of 1 %/min throughout the test. Method B uses two different strain rates: 1 %/min for the determination of the flexural modulus and 5 %/min or 50 %/min, depending on the ductility of the material, for the determination of the remainder of the flexural stress-strain curve. NOTE 1 The strain rates mentioned above are to be interpreted as nominal ones. Nominal test speeds are calculated using Formula (4). For the machine settings the best fitting ones are selected from Table 1. NOTE 2 For materials exhibiting nonlinear stress/strain behaviour, the flexural properties are only nominal. The formulae given have been derived assuming linear elastic behaviour and are valid for deflections of the specimen that are small compared to its thickness. With the preferred specimen (which measures 80 mm × 10 mm × 4 mm) at the conventional flexural strain of 3,5 % and a span-to-thickness ratio, L/h, of 16, the deflection is 1,5h. Flexural tests are more appropriate for stiff and brittle materials showing small deflections at break than for very soft and ductile ones. 5 Test machine 5.1 General The machine shall comply with ISO 7500-1 and ISO 9513 and the requirements given in 5.2 to 5.4.
5.4  Force- and deflection-measuring systems 5.4.1  Introductory remarks Flexural tests, according to the specific requirements on the data to be obtained, can be differentiated in several classes, comprising different complexity and requirements on accuracy. This starts with simple tests for obtaining flexural strength only on the one hand and on the other hand necessitates the use of a deflectometer to obtain the deflection accurately and free of compliance effects of the machine. The compliance of flexural testing machines has several possible sources (play and deformations in fixtures, deformations in the load train, and deformations of the load cell). Precise and true determination of deflection is especially important for the determination of the flexural modulus, for which the use of uncorrected crosshead displacement is not suitable. For a repeatable determination of flexural modulus results a compliance correction shall be applied or, preferably, a deflectometer shall be used.