ISO 178: Perform bending tests on plastics according to standards
The ISO 178 standard specifies the Method for determining the bending properties of plastics in the 3-point bending test This standard is central to materials testing in quality assurance, incoming goods inspection, and development. Here you will find a concise overview of the standard and learn how to conduct tests in accordance with it in practice.
the essentials in brief
- DIN EN ISO 178 describes the 3-point bending test on plastics.
- This includes, among other things, Flexural modulus, flexural strength and flexural strain Determined under defined test conditions.
- For results to comply with standards and be comparable, the following must be observed: Sample geometry, support span, test speed and deflection measurement precisely defined. Depending on the accuracy requirements, the standard distinguishes between test types I to IV.
- For a precise determination of the Young's modulus, a direct deflection measurement required. Test type IV uses a separate displacement measuring system for this purpose. EXTENSOMETER and provides the best basis for results that are comparable across laboratories.
Fundamentals of ISO 178
The DIN EN ISO 178 standard is an internationally recognized standard for determining the mechanical bending properties of plastics. It forms the basis for comparable test results in development, quality assurance, and incoming goods inspection.
What does the standard regulate?
ISO 178 describes the procedure and requirements for the 3-point bending test on dimensionally stable plastic specimens. The standard specifies:
- how the experimental setup should be designed,
- which sample geometries are permissible,
- which characteristic values such as flexural modulus, flexural strength or flexural strain need to be determined,
- and what tolerances must be observed during measurement.
It applies to thermoplastic, thermoset and filled plastics in the solid state and is internationally established as a testing standard (often also in conjunction with ASTM D790).
Test principle overview
In the 3-point bending test, a plastic sample is positioned on two parallel supports and loaded by a bending fin acting centrally. The measured forces and deflections provide information about the bending behavior of the material.
Implementation & Requirements in Practice
For reliable and standard-compliant results according to ISO 178, the test setup, specimen geometry, and measurement procedure must be precisely aligned with the requirements of the standard. The following section explains the most important framework conditions for the successful execution of the 3-point bending test.
Experimental setup and test parameters
The 3-point bending test is performed with a centrally applied force via a bending fin between two parallel supports. Important test parameters are:
- Support span: usually 16 times the sample height (e.g. 64 mm for a 4 mm sample height),
- Testing speed: typically 2 mm/min,
- Bending angle: less than 5° (quasi-linear behavior),
- Final criterion: breakage of the specimen or defined deflection (e.g. 6 mm at h = 4 mm).
For ductile materials, the test ends at a defined deformation, not at fracture. For comparability, a defined bending stress at x% strain (σ fx ) can be determined.
Sample shapes and dimensions
The standard specifies test specimens from the middle section of the general-purpose test specimen according to ISO 20753. Common dimensions are:
- Length (l₃): ≥ 80 mm,
- Width (b₁): 10 ± 0,2 mm,
- Thickness (h): 4,0 ± 0,2 mm (preferred).
The dimensional accuracy of the samples is important: Even a deviation of 0,1 mm in thickness can distort the calculated stress value by up to 5%.
Measurement methods for deflection (test types I–IV)
DIN EN ISO 178 distinguishes four types of testing – graded according to accuracy requirements:
| Test type | Objective | Measurement method |
|---|---|---|
| I | Maximum tension or force | No deflection measurement |
| II | Strains > 1% | Traversweg |
| III | Reproducible modulus of elasticity | Traverse path + correction curve |
| IV | More precise, correct modulus of elasticity | Direct displacement measuring system (extensometer) |
Depending on the test objective, a simple measurement of the crosshead travel may suffice, or a precise, load-free deflection measurement according to ISO 9513 may be required. The latter is particularly important for comparative measurements between different laboratories.
Testing machines & automation
Valid test results according to ISO 178 require machines that are both mechanically precise and compliant with the standard in terms of software. In addition to manual benchtop and floor-standing machines, automated testing systems are becoming increasingly important in serial testing.
Suitable machine models
GALDABINI's universal testing machines are ideally suited for ISO 178-compliant 3-point bending tests. They offer:
- Load ranges from 2,5 kN to 2000 kN,
- high frame rigidity for precise force measurement,
- free configuration of support spans and bending units,
- Constant speed truss control,
- Integration of extensometers or traverse displacement sensors.
The machines meet the requirements of ISO 7500-1 (accuracy class 1 or better) and can be calibrated with DAkkS-certified force sensors.
Automated bending test according to ISO 178
For high sample throughput, the use of automated testing procedures is worthwhile. Solutions like the roboTest system from GALDABINI make this possible:
- Automated insertion, testing and evaluation of samples,
- Operator relief during routine checks,
- Consistent testing conditions without manual influences,
- direct connection to LIMS or QS systems.
Frequently asked questions about DIN EN ISO 178
When is recalibration of the testing machine required for ISO 178 tests?
Testing machines should be calibrated regularly according to ISO 7500-1 – usually annually. Especially for sensitive bending tests, an additional check is recommended after modifications, software updates, or unusual results.
What should you do if a sample does not break during a bending test?
If a specimen does not break, the test ends at a defined deflection (e.g., 6 mm with a 4 mm specimen thickness). The bending stress determined in this way is not directly comparable to fracture values; here, the approach using a defined yield strength (σ fx ) is helpful.
Is DIN EN ISO 178 also suitable for fiber-reinforced plastics?
Yes, the standard can also be applied to glass or carbon fiber reinforced plastics, as long as the material is dimensionally stable. However, it is important to consider the anisotropy during sample preparation and evaluation.
How can measurement results be compared across multiple laboratories?
For high comparability, ISO 178 recommends test method IV with direct deflection measurement (e.g., with an extensometer). Additionally, samples, test parameters, and measuring instruments should be clearly documented and, if necessary, validated through interlaboratory studies.
