Flexural strength determination is one of the most widely performed mechanical tests for plastic materials, quantifying their resistance to bending failure under standardized laboratory conditions. This technical guide explains the principles of plastic flexural strength testing in accordance with ISO 178, ASTM D790, GB/T 9341, and related standards, covering specimen preparation, test procedures, data calculation, and industrial applications. Whether for quality control, material qualification, or research and development, this article provides the essential knowledge needed to understand and correctly interpret plastic bend test results.
Technical Overview
Flexural strength, also known as bending strength or modulus of rupture, is the maximum stress a plastic specimen can withstand before failure occurs when loaded in bending. It is one of the fundamental mechanical properties used to characterize rigid and semi-rigid plastics, and it is frequently more critical than tensile strength for evaluating materials used in beams, panels, housings, and structural components that experience bending loads during service.
The determination of flexural strength is typically performed using a universal testing machine equipped with a three-point or four-point bending fixture. In the most common three-point configuration, a rectangular specimen rests on two parallel supports and a loading nose applies force at mid-span until the specimen breaks or reaches a defined strain limit. The test records the force–deflection curve, from which flexural stress, flexural strain, flexural modulus, and flexural strength are calculated.
The principal standards governing plastic flexural strength determination include:
- ISO 178 — Plastics — Determination of flexural properties
- ASTM D790 — Standard Test Methods for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials
- ASTM D6272 — Standard Test Method for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials by Four-Point Bending
- GB/T 9341 — Plastics — Determination of flexural properties (Chinese national standard, technically equivalent to ISO 178)
- JIS K 7171 — Plastics — Determination of flexural properties (Japanese industrial standard aligned with ISO 178)
Because bending behavior combines tensile and compressive stresses on opposite surfaces of the specimen, flexural testing provides a practical and economical assessment of overall structural performance. It is widely specified in material datasheets, procurement specifications, and regulatory submissions, and accredited third-party laboratories routinely perform this test for certification and compliance purposes.
Testing Samples
Rigid and Semi-Rigid Thermoplastics
Most commodity and engineering thermoplastics are well suited to flexural strength determination, including polypropylene (PP), polyethylene (PE), polyvinyl chloride (PVC), polystyrene (PS), acrylonitrile butadiene styrene (ABS), polycarbonate (PC), polyamide (PA), polyoxymethylene (POM), polymethyl methacrylate (PMMA), polyethylene terephthalate (PET), and high-temperature grades such as polyetheretherketone (PEEK) and polyphenylene sulfide (PPS). Specimens are usually produced by injection molding, compression molding, or machining from sheets and rods.
Thermosetting Plastics
Thermoset materials such as phenolic resins, epoxy resins, unsaturated polyester resins, melamine-formaldehyde, and urea-formaldehyde molding compounds are commonly evaluated by flexural testing. For these brittle materials, the flexural test is often preferred over tensile testing because specimen preparation is simpler and gripping effects are eliminated, resulting in lower scatter of results.
Fiber-Reinforced Plastic Composites
Plastic Sheets, Panels, and Finished Products
Rigid plastic sheets, building panels, drainage components, electrical insulating parts, and cut sections from finished goods can also be tested, provided that suitable rectangular specimens can be obtained and the thickness meets the requirements of the selected standard. For very thin or highly flexible materials, tensile testing is generally more appropriate than flexural testing.
Testing Items
A complete flexural strength determination typically reports the following parameters:
- Flexural strength (maximum) — the maximum flexural stress sustained by the specimen before fracture or before the conventional deflection limit is reached
- Flexural stress at break — the stress recorded at the moment of specimen fracture, applicable when fracture occurs before the strain limit
- Flexural stress at 3.5% strain — the conventional strength value reported per ISO 178 when the specimen does not break, corresponding to the outer-fiber strain limit used to define failure for ductile materials
- Flexural modulus — the ratio of stress to strain within the elastic region, calculated in ISO 178 as a secant modulus between 0.05% and 0.25% strain
- Flexural offset yield stress — the stress at the intersection of the force–deflection curve and a defined offset line, used for materials that yield without a clear break point
- Deflection at break and strain at break — deformation values recorded at specimen failure
Testing Methods
Specimen Preparation and Conditioning
The preferred specimen geometry per ISO 178 is a rectangular bar of 80 mm length, 10 mm width, and 4 mm thickness, tested over a support span of 64 mm. ASTM D790 permits a range of specimen sizes, with a common preferred specimen of 3.2 mm thickness and 12.7 mm width. In all cases, the support span-to-thickness ratio is maintained at 16:1 to ensure that failure occurs in bending rather than shear. Specimens must be free of visible defects, flash, and residual stresses, and each face should be flat and parallel.
Prior to testing, specimens are conditioned in a standard laboratory atmosphere of 23 °C ± 2 °C and 50% ± 10% relative humidity in accordance with ISO 291 or ASTM D618, typically for a minimum of 16 hours. A minimum of five valid specimens is tested for each material and direction, and the arithmetical mean, standard deviation, and coefficient of variation are reported.
Three-Point Bending Procedure
In the three-point bending configuration specified by ISO 178, GB/T 9341, and ASTM D790 Procedure A, the specimen is placed on two cylindrical supports of 5 mm radius, and a loading nose with a 5 mm radius applies force at mid-span. The crosshead speed is calculated to produce a nominal outer-fiber strain rate of approximately 1% per minute using the relationship: speed = (strain rate × span²) / (6 × thickness). For the standard ISO specimen, this corresponds to approximately 2 mm/min. The test proceeds until the specimen breaks or the conventional deflection is reached, while force and deflection data are continuously recorded.
Four-Point Bending Procedure
ASTM D6272 describes a four-point loading configuration in which the load is applied through two loading noses symmetrically positioned within the span. This arrangement produces a region of constant bending moment between the loading points, eliminating the stress concentration directly under a single loading nose. Four-point bending is particularly useful for materials that fail under the loading nose in three-point testing or when a larger uniformly stressed region is required.
Calculation of Results
For the three-point configuration, flexural stress is calculated as: σ = 3FL / (2bh²), where F is the applied force, L is the support span, b is the specimen width, and h is the specimen thickness. Flexural strain at the outer surface is calculated as: ε = 6hs / L², where s is the mid-span deflection. The flexural modulus is determined from: E = L³m / (4bh³), where m is the slope of the linear portion of the force–deflection curve. All results are reported in megapascals (MPa), together with specimen dimensions, span, speed, and the observed failure mode.
Equipment and Calibration
Testing is performed on a calibrated universal testing machine fitted with a properly aligned flexural fixture, a calibrated load cell appropriate to the expected force range, and a system for measuring deflection, typically the crosshead position or, for higher accuracy, a dedicated deflection transducer. Fixture alignment, support radius verification, and regular calibration traceable to national standards are essential for valid and reproducible results.
Applications
Flexural strength determination supports a broad range of engineering and commercial activities:
- Quality control and incoming material inspection for resin producers, compounders, and molders
- Material comparison and selection for automotive interior and exterior components, appliance housings, and consumer products
- Qualification of glass-fiber and carbon-fiber reinforced composites for structural and semi-structural applications
- Verification of electrical insulating materials and laminates for the electronics and power distribution industries
- Construction product compliance, including plastic panels, profiles, and piping components
- Research and development of new polymer formulations, additives, and processing parameters
- Failure analysis, batch-to-batch consistency studies, and long-term aging or heat-deflection evaluations
- Regulatory submissions and certification testing performed by accredited third-party laboratories
Because flexural data appear on virtually every commercial plastic datasheet, consistent and standardized flexural strength determination is indispensable for global material specification and supplier qualification.
FAQ
What is the difference between flexural strength and tensile strength?
Tensile strength measures resistance to a pulling force applied along the specimen axis, while flexural strength measures resistance to bending, which simultaneously places one surface in tension and the opposite surface in compression. For many rigid plastics, the measured flexural strength is higher than the tensile strength because only the outer fibers reach the maximum stress during bending.
Which standard should be used: ISO 178 or ASTM D790?
ISO 178 and GB/T 9341 use a 80 mm × 10 mm × 4 mm specimen with a 64 mm span, whereas ASTM D790 typically uses thinner and narrower specimens with a 16:1 span-to-depth ratio. The choice depends on the specification, the customer requirement, or the regional market. Results from the two standards are similar in principle but should not be directly compared without qualification.
Why is the span-to-thickness ratio fixed at 16:1?
The 16:1 ratio ensures that the specimen fails primarily in bending rather than in interlaminar shear. Too short a span produces excessive shear stress, while too long a span leads to excessive deflection and invalid stress calculations.
What happens if the specimen does not break during the test?
Ductile plastics often bend without fracturing. In such cases, ISO 178 defines flexural strength as the stress at a conventional outer-fiber strain of 3.5%, which corresponds to a mid-span deflection of approximately 6 mm for the standard specimen. ASTM D790 similarly allows the test to be stopped at 5% strain if no break occurs.
How many specimens are required for a valid test?
Both ISO 178 and ASTM D790 require a minimum of five specimens per material condition and direction. Additional specimens are recommended for anisotropic composites or when scatter is high, and outlying results should be investigated rather than simply discarded.
Can flexible plastics and films be tested for flexural strength?
Flexural strength determination is intended for materials that break or reach the defined strain limit within the usable deflection range of the fixture. Very flexible plastics and thin films usually do not fail in bending and are more appropriately characterized by tensile testing per ISO 527 or ASTM D882.
What factors most commonly cause invalid or scattered results?
Frequent causes include incorrect span setting, poor fixture alignment, non-parallel specimen surfaces, inadequate conditioning, testing outside the calibrated force range, and specimens containing sink marks, voids, or residual molding stresses. Careful specimen preparation and strict adherence to the standard's dimensional and conditioning requirements are the keys to reliable data.