Determination of Pendulum Impact Strength of Plastics: Principles, Methods, and Applications

Pendulum impact strength testing is one of the most widely used methods for evaluating the toughness and impact resistance of plastic materials under dynamic loading conditions. This article provides a comprehensive technical overview of pendulum impact strength determination for plastics, covering relevant international standards, specimen preparation, test procedures, and practical applications across industries.

Technical Overview

Pendulum impact testing measures the energy required to break a plastic specimen by a single swing of a pendulum hammer. During the test, a pendulum of known mass is released from a fixed height, striking a supported or clamped specimen at the lowest point of its arc. The difference between the initial potential energy and the residual energy after breaking the specimen represents the absorbed impact energy. This absorbed energy, normalized by the cross-sectional area of the specimen (for unnotched samples) or by the area behind the notch (for notched samples), yields the impact strength value, typically expressed in kJ/m².

Two principal configurations are used for plastics: the Charpy method, in which the specimen is supported horizontally at both ends and struck in the middle, and the Izod method, in which the specimen is clamped vertically as a cantilever and struck above the clamping point. The principal standards governing these tests include ISO 179-1 (Charpy), ISO 180 (Izod), ASTM D6110 (Charpy), ASTM D256 (Izod), and the corresponding Chinese national standards GB/T 1043 and GB/T 1843.

The test provides a comparative measure of a material's resistance to brittle fracture under impact conditions. It is highly sensitive to notch effects, temperature, molecular orientation, processing defects, and material degradation, making it valuable for both quality control and material research.

Testing Samples

Standard Injection-Molded Specimens

The most common specimen geometry for ISO-based testing is the 80 mm × 10 mm × 4 mm rectangular bar, typically produced by injection molding in accordance with ISO 294 or ISO 10724. These specimens may be tested unnotched or machined with a standardized notch. For ASTM-based testing, specimens of 63.5 mm × 12.7 mm × 3.2 mm to 12.7 mm thickness are commonly used, generally in the notched configuration.

Notched Specimens

Notches are machined into specimens to create a stress concentration that promotes a controlled, reproducible brittle-type fracture. ISO standards define three notch types: Type A (notch tip radius 0.25 mm), Type B (radius 1.00 mm), and Type C (radius 0.10 mm). Notch depth is typically 2 mm for A and B types and 0.8 mm for Type C. Proper notch machining with a profiled cutter and controlled cutting speed is critical, as notch geometry strongly influences test results.

Specimens Cut from Finished Products

Samples may also be cut from sheets, pipes, films, finished parts, or components for quality verification. Care must be taken to ensure the surfaces are flat and parallel, edges are free of cracks and chips, and the dimensions meet the tolerance requirements specified in the relevant standard.

Conditioning

Specimens must be conditioned before testing, typically at 23 °C ± 2 °C and 50 % ± 10 % relative humidity for a minimum of 16 hours (or as specified in ISO 291 / ASTM D618), unless the test is deliberately conducted at other temperatures to evaluate low-temperature or high-temperature impact behavior.

Testing Items

• Charpy impact strength, unnotched (ISO 179-1, ASTM D6110)
• Charpy impact strength, notched (ISO 179-1)
• Izod impact strength, notched (ISO 180, ASTM D256)
• Izod impact strength, unnotched (ISO 180)
• Tensile impact strength (ISO 8256) for materials too flexible to fracture in bending impact
• Instrumented impact analysis, providing force–displacement curves and fracture mechanics data (ISO 179-2)
• Temperature-dependent impact testing to determine ductile-to-brittle transition behavior
• Fracture classification according to standard designations: complete break, hinged break, partial break, or non-break

Testing Methods

Instrumentation and Calibration

Testing is performed on a calibrated pendulum impact tester equipped with interchangeable hammers of defined energy capacities, commonly 1 J, 2.75 J, 5.5 J, 11 J, and 22 J for ISO instruments. The machine frame must be rigid, the support anvils and striking edges must conform to the dimensional requirements of the standard, and the energy losses due to friction and air resistance must be verified. Annual calibration and periodic verification against reference materials are recommended.

Test Procedure

The specimen is measured for width and thickness (and residual width at the notch root for notched samples) using a calibrated micrometer or optical measuring device. For Charpy testing, the specimen is placed horizontally on the support anvils with the notch facing away from the direction of striker impact; for Izod testing, the specimen is clamped vertically with the notch facing the striker. The pendulum is released, and the absorbed energy is read from the scale, dial, or digital display. At least ten specimens are normally tested for each condition, and the mean, standard deviation, and fracture type are reported.

Hammmer Selection Criterion

The energy capacity of the pendulum must be selected so that the absorbed energy falls within an acceptable fraction of the capacity—typically between 10 % and 80 % for ISO tests and between 10 % and 90 % for ASTM tests. If specimens do not break completely, the result is reported as non-break, and a higher-capacity hammer or the tensile impact method should be considered.

Calculation of Results

Impact strength is calculated by dividing the corrected absorbed energy (in joules) by the cross-sectional area of the specimen or by the original cross-sectional area beneath the notch, expressed in kJ/m². Corrections for friction and windage losses are applied where required by the standard. The fracture behavior is classified and reported together with the numeric result, because ductile partial breaks and complete brittle fractures represent fundamentally different material responses.

Quality Assurance Considerations

Key factors affecting accuracy include specimen preparation method (molding versus machining), notch quality, clamping torque in Izod tests, alignment of the striker with the specimen centerline, and ambient temperature control. Interlaboratory comparisons and the use of certified reference specimens help verify measurement consistency in accredited third-party laboratories.

Applications

Pendulum impact strength data are used extensively across the plastics industry for material qualification, specification compliance, and failure analysis:

• Automotive: evaluating interior trim, bumpers, battery housings, and under-hood components for crashworthiness and cold-weather durability
• Packaging: verifying the drop-impact resistance of containers, caps, and closures
• Pipes and fittings: assessing impact toughness of PVC, PE, and PP piping systems per ISO 9854 and related product standards
• Electrical and electronics: confirming housing integrity of enclosures, connectors, and appliance parts
• Consumer goods: quality control of housewares, toys, sports equipment, and tools
• Medical devices: screening of polymer components requiring fracture resistance
• Material development: ranking of new resin formulations, impact modifiers, recycled content levels, and additive packages
• Process validation: detecting the influence of molding conditions, weld lines, moisture, and thermal degradation on part toughness

FAQ

What is the difference between Charpy and Izod impact tests?

In the Charpy test (ISO 179-1, ASTM D6110), the specimen lies horizontally on two supports and is struck at the center, with the notch facing away from the striker. In the Izod test (ISO 180, ASTM D256), the specimen is clamped as a vertical cantilever and struck above the clamp, with the notch facing the striker. The two methods produce different numeric values and are not directly interchangeable.

Why are notched specimens commonly used?

Notches create a well-defined stress concentration that promotes a sharp, reproducible brittle-type fracture. Notched impact strength reflects a material's notch sensitivity, which is critical because real parts almost always contain stress raisers such as sharp corners, holes, and gate marks.

What units are used for pendulum impact strength?

The result is normally expressed in kilojoules per square meter (kJ/m²) under ISO standards. ASTM standards historically report results in ft·lb/in, which can be converted to kJ/m² (1 ft·lb/in ≈ 53.38 J/m).

How many specimens should be tested?

Most standards require a minimum of five to ten specimens per condition, with ten being typical for ISO testing. Because impact results show significant scatter, reporting the mean, standard deviation, and fracture classification is essential.

What does a "non-break" result mean?

Non-break indicates the specimen did not completely fracture; it may bend, hinge, or only partially crack. This behavior usually indicates a very tough material at the test temperature, and the result should be reported as non-break rather than as a numeric value, or a higher-energy hammer should be used.

How does temperature affect impact strength?

Plastics generally exhibit lower impact strength at low temperatures due to the ductile-to-brittle transition. Testing at multiple temperatures (for example, −40 °C, −20 °C, 0 °C, and 23 °C) is common for outdoor, automotive, and cold-chain applications to establish the transition range.