3-point bending test on plastics according to ISO 178

The 3-point bending test according to ISO 178 is one of the most important test methods for determining the mechanical properties of plastics. Hegewald & Peschke offers a standard-compliant test system for determining the bending properties of thermoplastics and thermosets, such as the flexural modulus, flexural strength, and elongation at fracture. The test procedure is performed on a flexure testing machine such as the inspekt solo, inspekt blue or the inspekt table in combination with a 3-point bending device.

The 3-point bending test according to ISO 178

The standard DIN EN ISO 178 “Plastics – Determination of flexural properties” specifies the standardized test conditions under which plastics are tested in the 3-point bending test. The aim is to obtain reliable characteristic values for design, material selection, and quality assurance. In the 3-point bending test, a flat plastic sample is positioned on two supports and loaded at a specific point in the middle. The force and deflection are recorded continuously. Important material characteristics such as flexural modulus (material stiffness), flexural strength, and elongation at fracture can be determined from the resulting stress-strain diagram. If a high degree of comparative precision is required for deflection measurement, measuring probes can be integrated into the test system to determine the deflection under the specimen without applying a load. In most cases, however, deflection measurement via the crosshead travel of the testing machine is sufficient according to standard specifications.

The bending tests are typically performed with a span of 64 mm and a specimen thickness of h = 4 mm. A test speed of 2 mm/min has become established; in special cases, 5 mm/min is also used.

The test continues until the specimen breaks or until a maximum force is reached. If neither breakage nor a clear maximum force occurs, the test is stopped at a conventional deflection of 6 mm (at h = 4 mm).

The characteristic values determined in this way at specimen breakage and the values from non-breaking, ductile specimens are not directly comparable. To enable comparability, a freely defined proof strength at x percent can be specified, from which a bending stress σfx is derived. This procedure is based on practice from the tensile test, but is not generally standardized for bending tests.

The evaluation of the bending test is performed using the LabMaster test software and the corresponding module for bending tests.

Features of the 3-point bending bench:

• Suitable for test procedures according to ISO 178
• Capacity / load direction: 20 kN / compression
• Temperature range: room temperature
• Specimen dimensions / properties: strip-shaped specimens according to standard
• Dimensions / design: Support distance 10-150 mm, manually adjustable, width of support/fin 50 mm, radius of support 2 and 5 mm, radius of fin 5 mm, support/fin mounting fixed, height 100 mm, width 280 mm, depth 50 mm

Features of the universal testing machines:

• Universal application: Suitable for tensile, compression, bending, and peel tests on a wide variety of materials
• High-precision control: High accuracy in force and displacement application – ideal for demanding bending tests
• Efficient handling: Ergonomic design, brushless drive, and optional integration of external devices enable easy operation in laboratory and production environments

Conclusion: The 3-point bending test according to ISO 178 provides essential characteristic values for the material testing of plastics. With the universal testing software LabMaster and flexible equipment, the universal testing machines from Hegewald & Peschke enable efficient, precise, and standard-compliant plastic testing.

Make an inquiry now