Charpy (Simply Supported Beam) Impact Strength Testing of Plastics

Charpy impact strength testing, also known as simply supported beam impact testing, is the internationally recognized method for evaluating the impact toughness and notch sensitivity of plastic materials. Conducted in accordance with ISO 179-1, ASTM D6110, and GB/T 1043.1, the test measures the energy a pendulum hammer absorbs when fracturing a horizontally supported specimen, generating critical data for material selection, quality control, and product design. This article explains the test principle, specimen requirements, testing procedure, result calculation, and typical industrial applications of Charpy impact testing for plastics.

Technical Overview

Charpy impact testing, referred to in Chinese technical practice as the simply supported beam impact test, is a high-strain-rate mechanical test in which a rectangular plastic specimen is laid horizontally across two parallel supports and struck at its midpoint by a swinging pendulum. Because the specimen rests freely on two supports and behaves as a simply supported beam, the test measures the energy absorbed during fracture under a defined impact velocity. This absorbed energy, corrected for friction and air-resistance losses, is normalized by the cross-sectional area of the specimen to yield the impact strength, expressed in kJ/m².

The Charpy method is one of the most widely performed plastic impact tests because it is fast, repeatable, and highly sensitive to the brittleness of a material. It is particularly valuable for distinguishing brittle failure from ductile failure, ranking the toughness of competing materials, and detecting degradation caused by processing conditions, environmental aging, or formulation changes. The test complements the Izod (cantilever beam) method and is governed by an internationally harmonized family of standards, including ISO 179-1 and GB/T 1043.1.

Test Principle

A pendulum of known rated energy is raised to a fixed starting angle and released. The striker edge contacts the specimen at midspan, and the specimen deforms, initiates a crack (at the notch, if present), and fractures. The difference between the pendulum energy with and without a specimen in place, after applying friction and windage corrections, equals the energy absorbed by the specimen. For standard testing, the impact velocity is approximately 2.9 m/s to 3.5 m/s, simulating a moderately high rate of loading.

Testing Samples

Standard Injection-Molded Specimens

The preferred specimen for ISO 179-1, ASTM D6110, and GB/T 1043.1 is the multipurpose specimen measuring 80 mm × 10 mm × 4 mm, normally produced by injection molding under standardized conditions. When testing edgewise, the 10 mm dimension serves as the width (height) of the beam and the 4 mm dimension as the thickness, with the pendulum striking the 10 mm face. This geometry ensures good comparability of data between laboratories and between materials.

Machined Specimens from Finished Products

Specimens may also be machined from sheets, compression-molded plaques, pipes, fittings, or actual molded parts. In these cases, dimensions are adapted to the available material and the direction of machining relative to the flow direction or orientation must be recorded, since anisotropy from molecular or fiber orientation can significantly influence results. Where the nominal thickness is limited, sheet may be tested flatwise with an appropriate change of striker and support geometry.

Notched Specimens

To assess notch sensitivity, a V-notch is machined into the specimen, typically by milling in accordance with ISO 2818. ISO 179-1 and GB/T 1043.1 define three notch types:

• Type A notch: included angle 45°, tip radius 0.25 mm — the default notch for most thermoplastics.
• Type B notch: included angle 45°, tip radius 1.00 mm — a blunter notch used to reduce the severity of stress concentration.
• Type C notch: included angle 45°, tip radius 0.10 mm — a sharper notch for very tough materials.

The notch depth is 2.0 mm for all types, leaving a remaining ligament width of 8.0 mm beneath the notch.

Specimen Conditioning

Unless otherwise specified, specimens shall be conditioned in accordance with ISO 291 at 23 ± 2 °C and 50 ± 10 % relative humidity for a minimum of 16 hours prior to testing. Testing is then performed at the same temperature, typically 23 °C, unless a specific service temperature is being investigated.

Testing Items

Accredited third-party laboratories and in-house quality departments commonly perform the following determinations under the Charpy method:

• Unnotched Charpy impact strength (acU) — impact resistance of the plain specimen.
• Notched Charpy impact strength (acN) — impact resistance with Type A, B, or C notches.
• Edgewise (preferred) and flatwise impact determinations for different specimen geometries.
• Instrumented Charpy impact testing per ISO 179-2, recording force–deflection curves to separate crack initiation and propagation energies.
• Temperature-dependent impact testing (for example at −40 °C, −20 °C, 0 °C, and 23 °C) to determine the brittle–ductile transition behavior.
• Comparative evaluation of materials after heat aging, UV exposure, chemical contact, or recycled-content incorporation.

Testing Methods

Applicable Standards

• ISO 179-1: Plastics — Determination of Charpy impact properties — Part 1: Non-instrumented impact test.
• ISO 179-2: Instrumented impact test for detailed force–displacement analysis.
• ASTM D6110: Standard Test Method for Determining the Charpy Impact Resistance of Notched Specimens of Plastics.
• GB/T 1043.1 and GB/T 1043.2: Chinese national standards technically equivalent to ISO 179-1 and ISO 179-2.
• JIS K 7111: Japanese industrial standard for Charpy impact testing of plastics.

Equipment and Setup

Testing is performed on a calibrated pendulum impact machine verified in accordance with ISO 13802. For the standard 80 mm × 10 mm × 4 mm specimen, the support span is set to 62 ± 0.5 mm, and the specimen is positioned centrally with the notch facing away from the striker on the tension side. Machines are equipped with interchangeable pendulum hammers of rated energies typically ranging from 0.5 J to 50 J, allowing brittle and very tough materials alike to be measured within the required energy window.

Test Procedure

• Select a pendulum so that the corrected absorbed energy falls between 10 % and 80 % of the pendulum's rated energy; change hammers if the reading falls outside this range.
• Condition the specimens and verify the test temperature of 23 °C (or the specified temperature).
• Measure the specimen thickness and width (and the remaining ligament width of notched specimens) to the nearest 0.02 mm at the fracture location.
• Place the specimen symmetrically on the supports in the edgewise (or specified) orientation, with the notch aligned with the midpoint of the span.
• Release the pendulum, record the absorbed energy, and apply the friction and windage correction.
• Classify the failure mode as complete break (C), hinge break (H), partial break (P), or non-break (N).
• Test a minimum of five specimens per condition; ten specimens are recommended for fiber-reinforced or highly anisotropic materials.

Calculation and Expression of Results

The Charpy impact strength is calculated by dividing the corrected absorbed energy by the original cross-sectional area of the specimen. For unnotched specimens, acU = Ec / (b × h), where Ec is the corrected absorbed energy, b is the thickness, and h is the width; for notched specimens, the remaining area beneath the notch is used, acN = Ec / (b × hN), where hN is the remaining ligament width of 8.0 mm. With energy in joules and dimensions in millimeters, results are expressed in kJ/m². The test report shall state the standard used, notch type, test orientation, temperature, individual and mean values, standard deviation, and the failure classification codes.

Applications

Charpy impact data underpin material datasheets, incoming inspection specifications, and design allowables across virtually every sector of the plastics industry. Typical applications include:

• Quality control and lot release of thermoplastic resins, compounds, and masterbatches.
• Comparative material selection and toughness ranking of competing grades of PP, PE, ABS, PC, PA, PVC, POM, PS, and their blends.
• Research and development of formulations containing impact modifiers, mineral fillers, glass fibers, flame retardants, or recycled content.
• Verification of low-temperature performance for outdoor, automotive, and cold-chain applications, including the determination of brittle–ductile transition behavior.
• Automotive components such as interior trims, panels, underbody shields, and battery housings.
• Electrical and electronic equipment housings, appliance cabinets, and switchgear enclosures.
• Pressure and drainage pipes, fittings, and valves in PVC-U, PP, and PE systems referenced in EN and ISO product standards.
• Transport packaging, crates, pallets, and returnable containers subjected to repeated handling shocks.
• Support of failure analysis by comparing the impact strength of failed parts against virgin material benchmarks.

FAQ

What is the difference between Charpy and Izod impact testing?

In the Charpy (simply supported beam) test per ISO 179-1 and GB/T 1043.1, the specimen rests horizontally on two supports and is struck at midspan. In the Izod (cantilever beam) test per ISO 180 and ASTM D256, the specimen is clamped vertically at one end and struck above the clamp. The stress distribution, clamping influence, and specimen orientation differ, so Charpy and Izod values are not directly interchangeable.

Why are notched specimens used in Charpy testing?

The machined notch creates a controlled stress concentration and a triaxial stress state at its tip, which accelerates crack initiation and reveals the notch sensitivity of the material. This is important because real plastic parts inevitably contain stress raisers such as sharp corners, weld lines, gates, and scratches, and unnotched data alone can overstate practical toughness.

How is the correct pendulum energy selected?

The corrected energy absorbed by the specimen must lie between 10 % and 80 % of the pendulum's rated energy. Below this window, friction and resolution errors dominate; above it, insufficient energy remains for a valid measurement. The operator changes to a higher or lower capacity hammer and retests accordingly.

What does a "non-break" (N) result mean?

A non-break result indicates the specimen bent or partially deformed but did not separate; this is common for very ductile materials such as toughened PP, HDPE, or PC/ABS. The result is reported as "non-break" and should not be treated as a numeric impact strength. Where quantitative data are needed, instrumented Charpy testing per ISO 179-2 or testing at a lower temperature may be appropriate.

How many specimens should be tested?

A minimum of five valid specimens per condition is required by ISO 179-1, and ten specimens are recommended when testing fiber-reinforced or anisotropic materials, where scatter is inherently greater. Individual values, the arithmetic mean, and the standard deviation should all be reported.

How does temperature affect Charpy impact strength?

Impact strength is strongly temperature dependent. As temperature decreases toward or below the glass transition region, many plastics shift from ductile to brittle failure and the impact strength drops sharply. For this reason, automotive, outdoor, and cold-climate products are frequently qualified with Charpy tests at temperatures such as −30 °C or −40 °C in addition to the standard 23 °C condition.