Bending test of plastics

Bending test of plastics according to EN ISO 178

Specimens for bending test of plastics according to EN ISO 178

The basis of any well-conducted test is a quality test machine (accurate, stable, reliable and user-friendly) and a properly prepared specimen.

For this reason, EN ISO 178 for flexural testing also addresses the definition of a well-prepared specimen, which should preferably have dimensions of 80 x 10 x 4 mm (L x W x H). In the event that a specimen of these dimensions cannot be produced, the specimen should have the following dimensions: l/h = 20.

The width in relation to the thickness should then have the following parameters:

3 < h < 5 b = 10 mm

5 < h < 10 b = 15 mm

Before starting the bending test

The bending test is carried out on smaller testing machines because the forces are not as high as in the tensile test - depending on the material, of course. However, while tens of kN are achieved in the tensile test of plastics/metals, hundreds of N to lower units of kN are achieved in the bend test of plastics.

Before the test, the supports of the bending jig must be set to the correct support distance, where the standard says L = (16 +/- 1) * h.

Of course, it is important to insert the specimen correctly on the bending fixture afterwards, as the result (elastic limit value) depends on the deflection and the force measured. The deflection and the measured force can only be influenced by the correct positioning of the specimen - parallelism, perpendicularity...

Subsequently, it is advisable to set the preload during the test - the specimens may be slightly uneven and it is important that no flattening occurs for the subsequent deflection values. However, it is important not to overload the specimen and to follow the recommendations of the standard.

Bending test speed

The test speed is set according to the requirements of the material standard - most commonly a loading rate of 1% per minute is chosen. This achieves a deflection of the test specimen of 1% within one minute, and means that it is possible to set a loading speed of 2 mm/min on the machine.

This test speed remains stable throughout the test.

Requirements for the bending test according to EN ISO 178

the standard does not directly require the use of a strain gauge for the determination of the E-modulus (modulus of elasticity), but it does require the sensing of the path and force with some accuracy
if direct sensing with a deflection sensor is not possible, the standard allows for a so-called correction curve. A single rigid body test measurement is then performed on the test machine, the measurement data is stored and the correction curve is then subtracted from the force-track curve in a test on a real specimen. The Trapezium software with the Shimadzu machine makes this very intuitive.
If a deflection sensor is used, the measured values are x% more accurate

Bending test results EN ISO 178

The output of the bend test is a graph and a set of results describing the important events in the bend test.

Similarly to the tensile test, the most important results are obtained at the beginning and at the end of the test.

E - modulus of elasticity; E-modulus; Young's modulus - this is a calculated value but based on measurements. The two most commonly used points are those that can be constructed sectionally. The intercept intersects the curve at the point where the elongation reaches the chosen value. This method is very simple, but it is sensitive to the data obtained - because it works with only two points. Therefore, the use of a deflection sensor is recommended.

σfC - stress at contract deflection - the bending stress at contract deflection , which is defined as a deflection equal to 1,5 times the thickness h of the test body.

σfM - flexural strength - as the maximum bending stress that the test piece can withstand during the bending test