ISO 178 Plastics flexural / bending test

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ISO 178 Plastics - Determination of flexural properties

ISO 178 specifies a method for determining the flexural properties of rigid and semi-rigid plastics under defined conditions. A standard test specimen is defined, but parameters are included for alternative specimen sizes for use where appropriate. A range of test speeds is included.

ISO 178 test method is used to investigate the flexural behaviour of the test specimens and to determine the flexural strength, flexural modulus and other aspects of the flexural stress/strain relationship under the conditions defined. It applies to a freely supported beam, loaded at midspan (three-point loading test).

ISO 178 Plastics flexural / bending test

ISO 178 Test Principle

The core principle of the ISO 178 test is the three-point bending test. A test specimen with a rectangular cross-section is placed on two supports (freely supported beam) and is deflected by a central loading edge applied midway between the supports. The specimen is deflected at a constant rate until rupture occurs at the outer surface or until a maximum strain of 5% is reached. Throughout the test, the applied force and the resulting deflection at the midspan are continuously measured to generate a stress-strain curve.

Test Methods of ISO 178 bending test:

Defines different approaches based on the data required and the material's behavior:

Method A

Uses a constant flexural strain rate of 1%/min throughout the entire test.

Method B

Uses a slower strain rate (1%/min) for the initial linear portion to accurately determine the flexural modulus, and then switches to a higher strain rate (5%/min or 50%/min, depending on ductility) for the remainder of the stress-strain curve.

Test Types

(I to IV)

The standard categorizes tests into four types based on the required precision and accuracy.

Type I is for basic stress/strength data, while Type IV requires highly accurate and true modulus measurements using advanced deflection measurement tools.

Test Equipment required for ISO 178 Plastic Bending Test:

Recommend UnitedTest Universal Testing Machine and Bending fixtures.

Universal testing machine	Complies with ISO 7500-1 (force calibration) and ISO 9513 (deflection calibration)
Three-point bending fixture	The setup consists of two supports and one central loading edge. They must be parallel within ±0.2 mm over the specimen width. Loading edge radius R₁ = 5.0 ± 0.2 mm; Support radius R₂ = 2.0 ± 0.2 mm (specimen thickness ≤ 3 mm) / 5.0 ± 0.2 mm (thickness > 3 mm); Parallelism tolerance within ± 0.2 mm over specimen width.
Deflection measuring system	Type Ⅰ–Ⅱ: Crosshead displacement; Type Ⅲ: Crosshead displacement with compliance correction; Type Ⅳ: Direct measurement with a deflectometer.
Dimension measuring tool	Micrometer with accuracy ± 0.01 mm for specimen width and thickness.

Test Specimen Information

1, Preferred Specimen (most commonly use)

Dimensions: Length 80 ± 2 mm, width 10.0 ± 0.2 mm, thickness 4.0 ± 0.2 mm; support span L = (16 ± 1)h = 64 mm.

2, Alternative Specimens.

Width matches nominal thickness

3, Preparation & Quantity:

Prepared by compression molding (ISO 293/295), injection molding (ISO 294-1/10724-1) or machining (ISO 2818).

Minimum 5 specimens per test direction; discard results if rupture occurs outside the middle 1/3 of the span.

Anisotropic materials: Test in two principal directions if property difference > 20 %.

Key Test Parameters:

Span Length (L): The distance between the two supports. The conventional deflection (s_c=1.5h) corresponds to a flexural strain of 3.5% when the span-to-thickness ratio (L/h) is 16 .

Test Speed (v): The rate of relative movement between the supports and the loading edge. Speeds range from 1 mm/min to 500 mm/min, with specific tolerances (±20% for low speeds, ±10% for high speeds). The lowest speed (1 mm/min) is typically used for thin specimens (1 to 3.5 mm thick) .

Flexural Strain Rate: Defined as the rate at which flexural strain increases, usually set at 1%/min or 5%/min.

Details Test Procedures of ISO 178 plastic flexural/bending test:

Preparation: Condition the specimens in a controlled atmosphere (ISO 291) for a specified period. Measure the width and thickness at the center of the specimen .

Setup: Adjust the span length (L) on the testing machine. Place the specimen on the two supports, ensuring it is centered. Position the loading edge at the midspan .

Execution: Select the appropriate test speed based on the specimen thickness and the material's expected ductility. Start the machine to apply the load at a constant speed, continuously recording the force and deflection .

Termination: Stop the test when the specimen ruptures or reaches a maximum flexural strain of 5% .

Calculation: Calculate the flexural stress, flexural strain, and flexural modulus from the recorded force-deflection data using the formulas provided in Clause 9 of the standard.

Test Application (Industry & Material Scope)

Applicable Materials

Thermoplastics (molding, extrusion, casting; filled/reinforced/unfilled) and rigid thermoplastic sheets.

Thermosetting molding materials and sheets (filled/reinforced).

Short fiber-reinforced compounds (fiber length ≤7.5 mm before processing).

Industry Fields

Automotive parts, electronic & electrical enclosures, building profiles, plastic packaging, material R&D, quality control and batch consistency inspection for plastic products.

Related Standard:

ASTM D790	Standard Test Methods for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials
ISO 14125	Fibre-reinforced plastic composites -- Determination of flexural properties
ASTM D6272	Standard Test Method for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials by Four-Point Bending
ASTM D7264	Standard Test Method for Flexural Properties of Polymer Matrix Composite Materials
GB/T 9341	Equivalent to using ISO 178
JIS K 7171	Japan standard Basically consistent with the technical content of ISO 178. (プラスチック－曲げ特性の求め方)
ISO 1209-1	Rigid cellular plastics - Determination of flexural properties - Part 1: Basic bending test

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A single column tensile tester is a sophisticated material testing instrument designed for measuring mechanical properties of various materials under tension, compression, bending, shear, and other loading conditions. As the name suggests, it features a single vertical column supporting a moving crosshead that applies force to test specimens.

Three-point bending test fixture for fiber-reinforced plastic composites, designed in accordance with ISO 14125, provides a standardized method for determining the flexural properties of composite materials using a three-point bending test.

Q1: What is ISO 178?

A: ISO 178 is the international standard titled "Plastics — Determination of flexural properties."The 2019 edition (sixth edition) specifies a method for determining the flexural behavior, flexural strength, flexural modulus, and other aspects of the stress/strain relationship of rigid and semi‑rigid plastics using a three‑point bending test.

Q2: Why is the ISO 178 test important?

A: The test is important because it:

Characterizes key mechanical properties (flexural strength, modulus, strain at break) essential for understanding how plastics behave under bending loads.

Serves as a reliable quality control tool to ensure batch‑to‑batch consistency in manufacturing (e.g., injection molding, extrusion).

Provides standardized, comparable data for material selection and specification, even though it is not directly used for designing finished products.

Helps identify directional dependencies in anisotropic materials (e.g., fiber‑reinforced plastics).

Q3: What test speeds are used?

A: Test speeds range from 1 mm/min to 500 mm/min, with tolerances of ±20 % (lower speeds) or ±10 % (higher speeds). The speed is selected to achieve a target flexural strain rate:

1 %/min for modulus determination (e.g., 2 mm/min for the preferred 4 mm‑thick specimen with a 64 mm span).

5 %/min or 50 %/min for the remaining test (Method B), depending on material ductility.

Q4: How does ISO 178 handle anisotropic materials?

A: For materials with direction‑dependent properties (e.g., fiber‑reinforced plastics), specimens must be tested in both principal directions (parallel and perpendicular to orientation). The orientation relative to the product direction and force application must be recorded. If the difference in properties exceeds 20 %, testing in both directions is mandatory.

Q5: Why is compliance correction required in ISO 178?

A: To eliminate errors from machine/ fixture deformation, load cell compression and screw torsion. It ensures accurate deflection measurement for reliable flexural modulus results (mandatory for Type Ⅲ tests).

Q6: How many test specimens are needed for one test direction?

A: At least 5 valid specimens per direction; discard results if the specimen breaks outside the middle 1/3 of the support span.

Q7: What is the difference between flexural strength and flexural modulus?

A:Flexural strength: Maximum flexural stress the specimen withstands (MPa).

Flexural modulus: Stiffness of the material (ratio of stress to strain in the elastic region, MPa).

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