Determination of flexural fracture strength of plastics is a fundamental mechanical test that measures the maximum bending stress a plastic material can withstand before breaking. This article provides a comprehensive technical overview of flexural fracture strength testing, covering relevant international standards such as ISO 178 and ASTM D790, specimen requirements, test procedures, calculation methods, and typical industrial applications.
Technical Overview
Flexural fracture strength, also referred to as flexural stress at break, is the bending stress recorded at the moment a plastic specimen fractures under a bending load. It is one of the most widely measured mechanical properties for rigid and semi-rigid plastics because bending is a common loading mode in real-world service conditions, from snap-fit clips and structural brackets to piping, panels, and housings. In contrast to the maximum flexural strength, which simply reflects the highest stress reached during the test, the fracture strength specifically characterizes the point of failure and is therefore a key indicator of a material's brittleness or ductility.
The test is typically performed on a universal testing machine equipped with a three-point or four-point bending fixture. A rectangular specimen is supported on two parallel anvils, and a loading nose applies force at mid-span (three-point) or at two points (four-point) until the specimen breaks or reaches a defined limit. The principal international standards governing this test include ISO 178 (plastics — determination of flexural properties), ASTM D790 (flexural properties of unreinforced and reinforced plastics), ASTM D6272 (four-point bending), GB/T 9341, and JIS K 7171. Test results are highly dependent on specimen geometry, span-to-thickness ratio, testing speed, temperature, and humidity, which is why strict adherence to standardized conditions is essential for obtaining comparable and reproducible data.
Testing Samples
A wide range of plastic materials and product forms can be evaluated for flexural fracture strength, provided the material is sufficiently rigid to support bending without excessive deformation. Typical sample categories include the following:
Molded and Machined Laboratory Specimens
Standard rectangular bars are produced by injection molding, compression molding, or machining from sheets, plates, or finished parts. Under ISO 178, the preferred specimen is 80 mm × 10 mm × 4 mm, tested over a support span of 64 mm. Under ASTM D790, specimens are typically cut to a length at least 10 percent greater than the support span, and the span is set at 16 times the specimen thickness. Specimens must be flat, free of voids, sink marks, warpage, and edge defects, since surface flaws act as stress concentrators and can significantly lower the measured fracture strength.
Rigid and Semi-Rigid Thermoplastics
Common materials include polypropylene (PP), polyethylene (PE) of high density, polystyrene (PS), ABS, polyvinyl chloride (PVC), polyamide (PA), polycarbonate (PC), polyoxymethylene (POM), polyethylene terephthalate (PET), and polybutylene terephthalate (PBT). Brittle grades fracture cleanly and yield a clear break value, whereas ductile grades may deform extensively without fracturing.
Thermosetting Plastics
Phenolic resins, epoxy resins, unsaturated polyester molding compounds, melamine-formaldehyde, and urea-formaldehyde materials are frequently tested because thermosets are generally stiffer and more brittle, making flexural fracture strength a primary quality characteristic.
Fiber-Reinforced Plastics and Composites
Glass-fiber or carbon-fiber reinforced plastics, sheet molding compounds (SMC), bulk molding compounds (BMC), and pultruded profiles are tested to characterize reinforcement efficiency and interfacial adhesion. Because fracture behavior in composites depends strongly on fiber direction, specimens are prepared and oriented with respect to the processing direction.
Finished Products and Components
Sections cut from pipes, sheets, profiles, automotive interior parts, and electronic enclosures can be tested to verify conformity between raw-material data and end-product performance. Sampling from finished goods must account for curvature, thickness variation, and local features such as weld lines or gate marks.
Testing Items
A flexural test campaign normally reports a complete set of bending properties, of which fracture strength is one component. The principal measurable items include:
- Flexural fracture strength (flexural stress at break, σfB): the bending stress at the instant the specimen fractures.
- Maximum flexural strength (σfM): the highest stress recorded during the test, which may occur before fracture.
- Flexural stress at conventional deflection (σfc): the stress at the deflection corresponding to 3.5 percent outer-fiber strain, reported when ductile specimens do not break.
- Flexural strain at break: the nominal outer-fiber strain at the point of fracture.
- Flexural modulus (Ef): the ratio of stress to strain in the elastic (linear) portion of the force-deflection curve.
- Flexural offset yield stress: the stress at the intersection of the curve with a defined offset, used for materials exhibiting a gradual transition to plastic deformation.
- Force-deflection curve characteristics: fracture type (brittle or semi-ductile), energy indicators, and the consistency of failure behavior across replicate specimens.
Testing Methods
Standard Reference Methods
- ISO 178 / GB/T 9341 / JIS K 7171: three-point bending test for plastics, internationally harmonized procedures.
- ASTM D790: three-point bending of unreinforced and reinforced plastics and electrical insulating materials, with Procedure A (strain rate 0.01 min⁻¹) and Procedure B (strain rate 0.1 min⁻¹).
- ASTM D6272: four-point bending method, which subjects a longer region of the specimen to a uniform bending moment and is preferred for certain composites and materials sensitive to contact stresses.
Specimen Conditioning and Test Atmosphere
Specimens are conditioned prior to testing in accordance with ISO 291 or ASTM D618, commonly at 23 °C and 50 percent relative humidity. The test itself is normally conducted in the same controlled laboratory atmosphere. For special evaluations, specimens may be tested at low temperatures to assess embrittlement, or after heat aging, water immersion, or UV exposure to study environmental effects on fracture strength.
Test Apparatus and Setup
A calibrated universal testing machine is fitted with a bending fixture consisting of two parallel supports and one (three-point) or two (four-point) loading noses with defined radii of curvature. The support span is set to 16 times the specimen thickness as specified by the standards. Specimen width and thickness are measured at multiple points to the accuracy required by the standard, and load and deflection are recorded continuously by the machine's force transducer and crosshead displacement or a contact extensometer.
Test Procedure
- Measure and record specimen dimensions, verifying freedom from visible defects.
- Set the support span to 16 times the mean specimen thickness and align the loading nose centrally between the supports.
- Select the crosshead speed according to the standard; ISO 178 prescribes a testing speed of 2 mm/min for the standard 4 mm thick specimen, while ASTM D790 calculates speed from the target outer-fiber strain rate, span, and thickness.
- Apply the load at constant speed until the specimen fractures, or until the maximum strain or conventional deflection limit is reached.
- Record the force at break and the corresponding deflection, then calculate flexural stress and strain.
- Test at least five valid specimens and report the arithmetic mean, standard deviation, and, where relevant, the individual values.
Calculation of Results
For three-point bending, flexural stress is calculated as σf = 3FL / (2bh²), where F is the applied force, L is the support span, b is the specimen width, and h is the specimen thickness. The flexural fracture strength is obtained by inserting the force at break into this equation. Nominal flexural strain at the outer surface is calculated as εf = 6sh / L², where s is the mid-span deflection. For four-point bending under ASTM D6272, the stress equation becomes σf = 3FL / (4bh²). If a specimen does not fracture, the standards require reporting the flexural stress at the conventional deflection instead, with a clear statement that no break occurred, because reporting only a maximum stress without this qualifier can be misleading.
Quality Control of Results
Results are considered valid only when fracture occurs between the supports and is not initiated by fixture contact damage. Outliers caused by improper alignment, specimen defects, or slippage are rejected. Measurement traceability is assured through regular machine calibration, verification of load cell accuracy, and fixture geometry checks, typically within an ISO/IEC 17025-accredited third-party laboratory quality system.
Applications
Flexural fracture strength data support a broad range of engineering and commercial activities across many industries:
- Material selection and design: comparing candidate plastics for bent or loaded components such as lids, clips, levers, brackets, and load-bearing panels.
- Incoming material and production quality control: verifying resin lots, molding consistency, and batch-to-batch uniformity against datasheet values.
- Research and development: evaluating new formulations, filler and reinforcement content, impact modifiers, and processing parameters on fracture performance.
- Automotive engineering: validating interior trims, under-hood components, battery housings, and structural parts against mechanical load requirements.
- Electronics and electrical equipment: qualifying insulating parts, connectors, and enclosures for bending loads during assembly and service.
- Construction and infrastructure: assessing pipes, conduits, panels, and geogrid materials for bending capacity and long-term durability.
- Medical and packaging sectors: confirming the reliability of device housings, trays, and rigid containers.
- Environmental and aging studies: quantifying how temperature, humidity, UV exposure, chemical contact, and thermal cycling change fracture strength over time.
- Failure analysis: comparing the fracture strength of failed parts with reference material to determine whether material degradation, processing defects, or design stress caused the failure.
- Regulatory compliance: demonstrating conformity with product standards and customer specifications through testing at accredited third-party laboratories.
FAQ
What is the difference between flexural strength and flexural fracture strength?
Maximum flexural strength is the highest stress the specimen experiences during the test, while flexural fracture strength is the stress at the exact moment the specimen breaks. In brittle materials the two values are identical; in ductile or semi-ductile plastics the maximum stress often occurs before fracture, or the specimen may not fracture at all within the test range.
What happens if the plastic specimen does not break during the test?
For materials that bend without fracturing, the standards require reporting the flexural stress at the conventional deflection, which corresponds to 3.5 percent outer-fiber strain under ISO 178. The report must state clearly that no break occurred so the value is not mistaken for a fracture strength.
Which standard should be chosen, ISO 178 or ASTM D790?
The choice depends on the market, customer specification, or regulatory framework. The two standards are technically similar but differ in specimen dimensions, conditioning requirements, and data-reporting conventions, so results are not automatically interchangeable. When in doubt, follow the standard cited in the product specification or consult an accredited third-party laboratory.
How does the span-to-thickness ratio affect the result?
The standards specify a span of 16 times the specimen thickness to minimize shear effects and interlaminar stress errors in the calculated bending stress. Too short a span introduces shear-dominated failure and overestimates the stress, while an excessive span can cause instability; strict span control is therefore critical.
How many specimens are required for a valid test?
Both ISO 178 and ASTM D790 require testing a minimum of five valid specimens for a meaningful average, and additional specimens may be needed for heterogeneous materials such as fiber-reinforced composites or when statistical confidence intervals are required.
Why do test temperature and speed influence flexural fracture strength?
Plastics are viscoelastic materials whose stiffness and yield behavior are strongly time- and temperature-dependent. Lower temperatures and higher testing speeds generally increase stiffness and promote brittle fracture with higher fracture stress, while elevated temperatures and slower speeds encourage ductile deformation. All test conditions must therefore be documented alongside the results.
Can flexural fracture strength testing be performed on finished products?
Yes. Specimens can be cut from molded parts, sheets, pipes, and profiles, provided the cut surfaces are finished and the geometry meets the standard's requirements. Testing actual parts is valuable for confirming that processing conditions, fiber orientation, or part design have not degraded the material's intrinsic fracture performance.