3-point bending test according to ISO 178

In this 3-point bending test according to Volkswagen standard VW TL 52660, the test piece is placed on the supports (bending support distance 60 mm) and bent using the pressure fin. The test piece must not break during this process (plastic tank for fuel in the automotive sector). The following parameters are determined in this test:

• maximum force
• deflection at force 50 N
• bending stress
• bending modulus of elasticity

Typically, the bending travel is measured using a crosshead travel measuring system. To increase the accuracy (especially for determining the bending modulus of elasticity), the use of a extensometer recommended. If the bending path is determined using a traverse path measuring system, the inherent deformation (elastic deformation of the load cell, test tool, machine frame) should be compensated using a correction curve (error curve of the inherent deformation).

If the crossbeam travel measuring system is used to measure the bending travel, the penetration path of the bending fin and the bending support are not taken into account in the measurement (less accurate - even when using a compensation error curve). However, this error can be neglected (especially in production monitoring) because comparative measurements are sufficient to monitor production fluctuations.

The tests are typically carried out with a support width of 64 mm (sample thickness h = 4 mm) at a test speed of 2 mm/min (creep speed to determine the E-modulus at 0,05 - 0,25% strain or true strain 0,0005 - 0,0025 mm/mm) and 5 mm/mm until sample breakage. If no sample breakage occurs with ductile (elastic, tough) materials, the test is terminated when a deflection of 6 mm is reached (with sample thickness h = 4 mm). The bending strength at sample breakage and that of non-breaking, elastic samples cannot be directly compared. To determine comparability, an alternative freely defined yield strength at x% can be used/compared to determine a bending stress σfx (derivation of this test from a tensile test).