Plastic Charpy Impact Testing: Principles, Standards, Methods, and Applications

Plastic Charpy impact testing, also known as simply supported beam impact testing, is a widely used method for evaluating the impact toughness and notch sensitivity of plastic materials under a single rapid strike. Governed by international standards such as ISO 179-1, ASTM D6110, and GB/T 1043.1, this test measures the energy absorbed by a specimen when fractured by a swinging pendulum and is essential for quality control, material selection, and product certification. This article provides a comprehensive technical overview of the test principle, specimen requirements, testing procedures, and typical industrial applications.

Technical Overview

The Charpy impact test, referred to in Chinese engineering practice as the simply supported beam (jianzhi liang) impact test, is a standardized pendulum-based method for determining the impact resistance of rigid plastics. In this test, a rectangular specimen is laid horizontally across two support anvils so that it behaves as a simply supported beam, and a pendulum hammer equipped with a striking edge is released from a fixed height to strike the specimen at mid-span, directly opposite any machined notch. The difference between the initial potential energy of the pendulum and the residual energy after breaking the specimen represents the energy absorbed by the material during fracture.

The principal international and national standards covering this method include ISO 179-1 (Plastics — Determination of Charpy impact properties — Part 1: Non-instrumented impact test), ASTM D6110 (Standard Test Method for Determining the Charpy Impact Resistance of Notched Specimens of Plastics), GB/T 1043.1 (the Chinese national standard, technically aligned with ISO 179-1), and JIS K 7111 for the Japanese market. Results are typically expressed as impact strength in kJ/m², calculated by dividing the absorbed energy by the cross-sectional area of the specimen at the fracture plane. The test distinguishes between ductile and brittle failure behavior and is one of the most frequently performed mechanical tests in plastics laboratories.

• Test principle: single-blow pendulum fracture of a simply supported specimen
• Primary standards: ISO 179-1, ASTM D6110, GB/T 1043.1, JIS K 7111
• Reported result: impact strength in kJ/m² or absorbed energy in joules
• Failure modes classified as complete break, hinged break, partial break, or non-break
• Applicable to rigid, semi-rigid, and fiber-reinforced thermoplastics and thermosets

Testing Samples

Standard Multipurpose Specimens

The preferred specimen for ISO 179-1 testing is the type 1A multipurpose test body defined in ISO 3167, with dimensions of 80 mm × 10 mm × 4 mm. These specimens are normally produced by injection molding under controlled conditions specified in ISO 294, ensuring consistent crystallinity, orientation, and internal stress levels. ASTM D6110 commonly uses specimens of 127 mm × 12.7 mm × 3.2 mm or machined equivalents, so dimensional compliance with the governing standard must be confirmed before testing.

Notched Specimens

Notched specimens are used to assess notch sensitivity and to promote brittle, reproducible fracture. ISO 179-1 defines three notch geometries cut into the narrow face of the specimen: type A (notch tip radius 0.25 mm), type B (radius 1.0 mm), and type C (radius 0.10 mm). The remaining ligament width beneath the notch is 8.0 mm for type 1A specimens. Notches must be machined with a sharp, freshly prepared cutter because notch quality strongly influences measured impact strength.

Unnotched Specimens

Unnotched specimens measure the total energy absorbed by the material in the absence of stress concentration. This variant is frequently specified for materials such as unplasticized PVC, glass-fiber-reinforced compounds, and highly ductile thermoplastics in which notched testing may lead to non-break behavior. For unnotched ISO 179 testing, the specimen is typically struck flatwise or edgewise depending on the material and product specification.

Machined Specimens from Finished Products

When evaluating pipes, sheets, profiles, injection-molded parts, or failed components, specimens are machined directly from the finished article. Orientation must be documented: for pipes, specimens are taken both longitudinally (axial) and circumferentially (hoop direction) because extrusion-induced orientation can produce markedly different impact responses. All machined surfaces should be smooth and free of saw marks, which act as artificial notches and reduce measured strength.

Testing Items

Accredited third-party laboratories offering Charpy impact services typically report the following parameters:

• Notched Charpy impact strength (kJ/m²), by notch type A, B, or C
• Unnotched Charpy impact strength (kJ/m²)
• Absorbed impact energy (J) for the individual specimen and the arithmetic mean
• Standard deviation and coefficient of variation across the specimen set
• Fracture classification: complete break (C), hinged break (H), partial break (P), or non-break (N)
• Direction of loading (edgewise or flatwise) and specimen orientation relative to processing direction
• Impact strength as a function of test temperature, including low-temperature brittleness studies

Testing Methods

Testing is performed on a calibrated pendulum impact machine in accordance with the selected standard. The general procedure is as follows:

• Conditioning: specimens are conditioned per ISO 291 or ASTM D618, normally at 23 °C and 50% relative humidity for at least 40 hours, unless otherwise specified.
• Specimen measurement: thickness and width (or remaining ligament width for notched specimens) are measured at the fracture plane with a precision of 0.02 mm or better, and notch depth is verified with optical or tactile gauges.
• Machine setup: the support span is set to 62 mm for ISO 179 type 1A specimens (span-to-thickness ratio defined by ASTM D6110 for ASTM geometries), and the pendulum capacity is selected so that the absorbed energy falls between approximately 10% and 80% of the nominal energy of the hammer.
• Impact execution: the specimen is placed with the notch facing away from the pendulum, the pendulum is released from its latched height (striking velocity typically 2.9 m/s to 3.8 m/s), and the energy absorbed is read from the scale or digital transducer after fracture.
• Calculation: impact strength equals absorbed energy divided by the cross-sectional area of the specimen (thickness × width for unnotched; thickness × ligament width for notched), expressed in kJ/m².
• Verification: at least five valid specimens are tested per condition, and any specimens that fail outside the support line, slip during impact, or show non-representative defects are discarded and replaced.

For low-temperature evaluations, specimens and anvils are conditioned in a controlled-temperature chamber or liquid bath, and testing is completed within a short, standardized window (typically five seconds) after removal from the conditioning medium to prevent warming of the specimen surface. Instrumented pendulum machines equipped with force–displacement transducers can additionally provide force–time curves, crack initiation energy, and propagation energy for advanced fracture analysis.

Applications

Charpy impact data support a broad range of engineering, regulatory, and commercial decisions:

• Material qualification and comparison of competing resins for automotive interior and exterior components, where low-temperature toughness is a key requirement
• Quality control in extrusion and injection molding, detecting degradation, moisture contamination, or poor fusion lines
• Verification of pressure pipes and fittings for water, gas, and drainage systems, where circumferential and axial impact behavior is assessed against EN, ISO, and GB product standards
• Construction materials such as PVC window profiles, sheets, and panels requiring certified impact performance
• Packaging, appliance housings, electrical enclosures, and consumer products subject to drop and impact loading
• Research and development, including the effect of fillers, modifiers, recycled content, and post-processing conditions on toughness
• Failure analysis, where comparison of the impact strength of a failed part against virgin material helps identify embrittlement caused by UV exposure, chemical attack, or thermal aging

FAQ

What is the difference between Charpy and Izod impact testing?

Both are pendulum impact tests, but in the Charpy configuration the specimen is supported horizontally as a simply supported beam and struck at mid-span opposite the notch, whereas in the Izod test the specimen is clamped as a vertical cantilever and struck above the clamp on the notched side. ISO 179 governs Charpy testing while ISO 180 governs Izod testing; ASTM D6110 covers Charpy and ASTM D256 covers Izod. The two methods yield different numeric values for the same material and are not interchangeable.

Why do notched and unnotched results differ so strongly for the same plastic?

The notch concentrates stress at a sharp tip, triaxializing the stress state and suppressing ductile yielding. Materials that appear tough in unnotched testing, such as polycarbonate or ABS, may show dramatically lower notched impact strength. The ratio between the two values is itself a useful indicator of notch sensitivity in design.

Which notch type should be specified?

Type A (0.25 mm tip radius) is the default and most widely referenced geometry in ISO 179-1 and GB/T 1043.1. Type B is preferred for brittle materials such as highly filled or fiber-reinforced compounds that otherwise shatter unpredictably, while type C with its sharper 0.10 mm radius is used for very tough, notch-insensitive materials to promote complete fracture.

How many specimens are required for a valid result?

ISO 179-1 requires a minimum of five specimens per condition, with more specimens recommended for partial-break or non-break outcomes. When more than half of the specimens do not break completely, the material is reported as tough under that condition and an unnotched or sharper-notch test may be substituted.

What does a low Charpy impact strength indicate?

A low value indicates brittle behavior, which may stem from inherent material characteristics, low ambient or test temperature, excessive notch sharpness, molecular degradation during processing, or environmental embrittlement in service. Because Charpy results are sensitive to specimen preparation and test conditions, comparisons are only valid when geometry, notch type, orientation, temperature, and conditioning are identical, which is why testing at accredited third-party laboratories operating under ISO/IEC 17025 quality systems is recommended for certification and dispute resolution.