Flexural strength testing is one of the most widely performed mechanical evaluations of plastic materials, providing essential data for material selection, quality control, and structural design. This article explains how the flexural strength of plastics is determined in accordance with ISO 178, ASTM D790, and equivalent national standards, covering specimen requirements, test procedures, calculation formulas, and typical industrial applications.
Technical Overview
Flexural strength, also known as bending strength, is the maximum stress sustained by the outer surface of a plastic specimen when it is loaded in bending until it breaks or reaches a specified strain limit. Because plastic components such as panels, housings, profiles, and trim parts are frequently subjected to bending loads in service, flexural strength is a fundamental parameter on virtually every plastic material datasheet.
The test is most commonly performed as a three-point bending experiment, in which a rectangular specimen rests on two supports and is loaded at mid-span by a loading nose. The surface in contact with the loading nose is placed in compression while the opposite surface is placed in tension; failure typically initiates on the tensile side. The principal standards governing the determination are:
- ISO 178 — Plastics — Determination of flexural properties
- ASTM D790 — Standard Test Methods for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials
- GB/T 9341 — Chinese national standard, technically aligned with ISO 178
- JIS K 7171 — Japanese standard, also aligned with ISO 178
- ASTM D6272 — Standard Test Method for Flexural Properties of Unreinforced and Reinforced Plastics by Four-Point Bending
For a three-point configuration with a mid-span load, the maximum flexural stress is calculated as σf = (3 × F × L) / (2 × b × h²), where F is the applied force, L is the support span, b is the specimen width, and h is the specimen thickness. The corresponding maximum outer-fiber strain is εf = (6 × s × h) / L², where s is the mid-span deflection. Because bending strength depends strongly on specimen geometry and test speed, strict adherence to the specified span-to-depth ratio and crosshead rate is essential for comparable results.
Testing Samples
Molded Multipurpose Specimens
The preferred specimen under ISO 178 is the ISO 3167 multipurpose test specimen, typically injection molded with a central testing section of 80 mm × 10 mm × 4 mm. The specimen is tested flatwise on a support span of 64 mm (a span-to-depth ratio of 16:1). Molded specimens should be free of flash, sink marks, voids, and gating stresses, as these defects can act as crack initiation sites and depress the measured strength.
Specimens Machined from Sheets and Finished Products
When testing sheet, plate, laminate, or samples cut from finished parts, specimens are machined to a rectangular bar and the support span is adjusted to maintain the required span-to-depth ratio (16:1 for most materials; 32:1 is used for highly anisotropic or very stiff fiber-reinforced materials to minimize shear effects per ASTM D790). Cut edges must be smooth, flat, parallel, and free of chips or saw marks, since surface flaws on the tensile side dramatically reduce the measured strength.
Filled and Fiber-Reinforced Compounds
For glass-fiber or mineral-filled compounds, flexural properties are direction-dependent due to fiber orientation during molding. Specimens should be tested both parallel and perpendicular to the flow direction where applicable, and the orientation must be clearly stated in the report. Reinforced materials generally require a larger span-to-depth ratio to reduce the influence of interlaminar shear on the apparent strength.
Specimen Conditioning
Prior to testing, specimens are conditioned in a standard atmosphere of 23 °C and 50% relative humidity in accordance with ISO 291 or ASTM D618. Conditioning duration is typically at least 24 hours (40 hours per ASTM D618 Procedure A) to ensure moisture equilibrium, because moisture content and temperature measurably affect the modulus and strength of hygroscopic plastics such as polyamides.
Testing Items
A complete flexural test program typically reports the following quantities derived from the force–deflection curve:
- Flexural strength (σfM) — the maximum flexural stress sustained during the test; for materials that do not break, ISO 178 defines the conventional flexural strength as the stress at 3.5% outer-fiber strain, while ASTM D790 reports the stress at 5% strain
- Flexural stress at break (σfB) and flexural strain at break — applicable to brittle or fiber-reinforced materials that fracture during the test
- Flexural modulus (Ef) — under ISO 178, a chord modulus calculated between 0.05% and 0.25% strain; under ASTM D790, typically the tangent modulus of the initial linear portion of the curve
- Flexural stress at specified strain — for example, stress at 3.5% strain, used to compare ductile thermoplastics that yield without fracturing
- Full force–deflection (stress–strain) curve, which characterizes the material behavior as brittle, semi-brittle, or ductile in bending
For quality control purposes, flexural strength and flexural modulus are the two most frequently cited results, as they respond sensitively to changes in formulation, molecular weight, filler content, and molding conditions.
Testing Methods
Apparatus and Fixture Setup
Testing is performed on a calibrated universal testing machine with load verification in accordance with ISO 7500-1 or ASTM E4. The bending fixture consists of two parallel supports and one (three-point) or two (four-point) loading noses with radii specified by the standard, typically around 5 mm for the loading nose, to avoid local indentation and premature surface damage. Deflection may be measured from the crosshead position after compliance correction, or directly with a deflectometer for maximum accuracy. The support span is set using gauge blocks to the required precision, and the specimen is centered with its long axis perpendicular to the supports.
Three-Point Bending Procedure (ISO 178 / ASTM D790 Procedure A)
The specimen is placed on the supports and the crosshead speed is set to produce a nominal outer-fiber strain rate of 1% per minute. For the standard ISO 178 specimen (4 mm thickness, 64 mm span), this corresponds to approximately 2 mm/min. A small contact preload is applied to seat the specimen, and the test then proceeds until the specimen breaks or the specified strain limit (3.5% per ISO, 5% per ASTM) is reached. Force and deflection are recorded continuously, and stress and strain are computed at each point using the standard equations.
Four-Point Bending Procedure (ASTM D790 Procedure B / ASTM D6272)
In four-point bending, the load is applied through two loading noses spaced symmetrically at half the support span, producing a region of pure, uniform bending moment between the loading points. The maximum stress for this geometry is σf = (3 × F × L) / (4 × b × h²). Because there is no stress concentration directly beneath a single central loading nose, four-point bending is preferred for brittle materials, high-modulus composites, and studies where failure must occur away from the loading contact.
Calculation and Reporting
Key calculations include:
- Flexural stress (three-point): σf = (3 × F × L) / (2 × b × h²)
- Flexural stress (four-point, load span = span/2): σf = (3 × F × L) / (4 × b × h²)
- Flexural strain: εf = (6 × s × h) / L²
- Flexural modulus: Ef = (σ2 − σ1) / (ε2 − ε1), using the strain limits defined by the applicable standard
The test report should state the standard used, specimen dimensions and preparation method, span and span-to-depth ratio, crosshead speed, conditioning conditions, number of specimens, individual and mean results with standard deviation, and whether specimens broke or the test was terminated at the conventional strain limit.
Factors Affecting Results
- Test speed — higher strain rates generally increase the measured strength and modulus of viscoelastic plastics
- Span-to-depth ratio — ratios below 16:1 introduce shear deformation that inflates the apparent deflection and reduces the apparent modulus
- Temperature and humidity — elevated temperature or absorbed moisture lowers stiffness and strength, particularly for polyamides and hygroscopic polymers
- Specimen quality — notch-like defects on the tensile surface act as stress concentrators and scatter the results
- Orientation and anisotropy — fiber-filled and oriented materials show markedly different values along versus across the flow direction
Applications
Flexural strength determination supports a broad range of industrial and engineering activities:
- Material datasheets and grade comparison for material selection in product design
- Incoming inspection and quality control of resins, compounds, sheets, and molded parts
- Research and development of filled, reinforced, recycled, and bio-based plastic formulations
- Verification of mechanical performance for automotive interior and exterior components, appliance housings, electrical enclosures, and construction panels
- Independent verification of supplier data by accredited third-party laboratories for procurement, regulatory submissions, and dispute resolution
- Failure analysis, batch traceability investigations, and benchmarking of competing materials
Because flexural testing requires only a simple bar specimen and produces rapid, reproducible results, it is often the most cost-effective method for routine mechanical characterization of rigid and semi-rigid plastics.
FAQ
Why does the test stop at 3.5% strain under ISO 178?
Many ductile thermoplastics can bend far beyond 3.5% outer-fiber strain without breaking, and at large deflections the standard beam equations are no longer geometrically accurate. The 3.5% limit (5% under ASTM D790) provides a conventional, reproducible endpoint so that ductile materials can be compared on a consistent basis.
What is the difference between three-point and four-point bending?
Three-point bending applies the load at mid-span, creating a maximum stress at a single point directly under the loading nose. Four-point bending distributes the load at two points, creating a uniform stress region between them, which avoids contact stress concentration and is preferred for brittle or high-strength reinforced materials.
Is flexural modulus the same as tensile modulus?
No. Flexural modulus is measured in bending and reflects a combination of tension and compression behavior across the specimen thickness, while tensile modulus is measured under uniaxial tension. For many plastics the values are numerically close, but they are not interchangeable, and the test standard must always be cited alongside the value.
What sample dimensions are required?
The standard ISO specimen is 80 mm × 10 mm × 4 mm tested on a 64 mm span. For other thicknesses, the support span is adjusted to maintain a span-to-depth ratio of 16:1, and the crosshead speed is recalculated to keep the strain rate at 1% per minute. A minimum of five specimens per material is typically tested.
Which plastics are suitable for this test method?
ISO 178 and ASTM D790 apply to rigid and semi-rigid thermoplastics, thermosets, and filled or fiber-reinforced compounds. Very flexible materials that cannot sustain bending without excessive deflection are outside the scope and are better characterized by tensile or flexural endurance methods.
How should results from different laboratories be compared?
Results are only comparable when the specimen preparation, span-to-depth ratio, test speed, conditioning, and temperature are identical and all testing is performed to the same standard. For regulatory or commercial purposes, testing should be performed by accredited third-party laboratories under documented, controlled conditions.