3-point bending test according to ISO 178 + ASTM D790

In this 3-point bending test according to ISO 178 (which Volkswagen specifies as internal standard VW TL 52660), the test piece is placed on the supports (bending support spacing: 60 mm) and bent using the compression fin. The test piece must not break during this process (plastic fuel tank for the automotive sector). The following parameters are determined during this test:

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

Typically, the bending displacement is determined using a crosshead displacement measuring system. To increase accuracy, especially when determining the flexural modulus of elasticity, the use of an extensometer that measures directly on the specimen is recommended. However, if the bending displacement is determined using a crosshead displacement measuring system, the inherent deformation (the elastic deformation of the load cell, test tool, and machine frame) should be compensated using a correction curve (error curve of the inherent deformation).

When the crosshead 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, leading to inaccuracies – even when using a compensation error curve. However, this error can be neglected, especially in production monitoring, since comparative measurements are sufficient to monitor production fluctuations.

The tests are typically carried out with a span of 64 mm (sample thickness h = 4 mm) and a test speed of 2 mm/min (creep rate to determine the Young's modulus at 0,05–0,25% strain or true strain 0,0005–0,0025 mm/mm). They are continued until the specimen breaks. If no specimen breaks with ductile (elastic, tough) materials, the test is terminated at a deflection of 6 mm (at a specimen thickness of 4 mm). The flexural strength at specimen break and that of non-breaking, elastic specimens cannot be directly compared. To ensure comparability, a freely defined yield strength at x% can alternatively be used or compared to determine a bending stress σ_(fx) (this test is derived from a tensile test).