ISO 178-2019 pdf download.Plastics – Determination of flexural properties.
5.3 Supports and loading edge
Two supports and a central loading edge shall be arranged as shown in Figure 2. The supports and the loading edge shall be parallel to within ±0,2 mm over the width of the test specimen.
The radius, R1, of the loading edge and the radius, R2, of the supports shall be as follows:
R1 = 5,0 mm ± 0,2 mm;
R2 = 2,0 mm ± 0,2 mm for test specimen thicknesses s3 mm;
R2 = 5,0 mm ± 0,2 mm for test specimen thicknesses >3 mm. The span, L, shall be adjustable.
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. Fora repeatable determination of flexural modulus results a compliance correction shall be applied or, preferably, a deflectometer shall be used.
5.4.2 Definition of precision and accuracy requirements
Table 2 defines objectives of testing in increasing order of test complexity and appertaining need for accuracy. A good precision without absolute accuracy as indicated in type Ill-tests can be sufficient in many quality control environments when properties are to be supervised over periods of time only. Accurate, meaning true and precise, results as indicated in type lV-tests are needed if the results are to be compared between laboratories. Different types of deflection measurement and different accuracy requirements for the deflection measurement are therefore defined, based on the needs on precision and trueness of the test results.ISO 178 pdf download.