Pendulum Impact Strength Testing for Plastics: Principles, Standards, Methods, and Applications

Pendulum impact strength testing is the most widely used method for evaluating the toughness and impact resistance of plastic materials, measuring the energy a specimen absorbs when fractured by a single blow from a swinging hammer. Governed by internationally recognized standards such as ISO 179, ISO 180, ASTM D256, ASTM D6110, and the corresponding GB standards, this test is essential for material qualification, quality control, and product design across the automotive, electronics, packaging, and construction industries. This article explains the test principle, specimen requirements, test methods, and practical applications of pendulum impact testing for plastics.

Technical Overview

Pendulum impact strength testing is a standardized laboratory procedure used to determine the resistance of plastics to failure under high-speed impact loading. A pendulum hammer of a defined energy is released from a fixed height, strikes a supported or clamped specimen, and fractures it. The difference between the initial potential energy of the pendulum and its residual energy after impact represents the energy absorbed by the specimen during deformation and fracture.

The absorbed energy, corrected for friction and windage losses, is used to calculate the impact strength of the material, typically expressed in kJ/m² (energy per unit cross-sectional area) or J/m of notch width, depending on the standard applied. Two principal configurations are used for plastics:

• Charpy impact test – the specimen lies horizontally on two supports and is struck at mid-span on the face opposite the notch (ISO 179, ASTM D6110, GB/T 1043).
• Izod impact test – the specimen is clamped as a vertical cantilever and is struck on the notched face above the clamp (ISO 180, ASTM D256, GB/T 1843).

Both notched and unnotched specimens may be tested. Notched results reflect the material's sensitivity to stress concentrators and its notch toughness, while unnotched results provide a broader measure of overall impact energy absorption. Impact behavior of plastics is strongly temperature-dependent, so tests are frequently conducted at standard laboratory conditions as well as at elevated or sub-ambient temperatures relevant to the intended service environment.

Testing Samples

Standard Unnotched Specimens

ISO-type specimens are typically machined or injection-molded rectangular bars of 80 mm × 10 mm × 4 mm (length × width × thickness), in accordance with ISO 179 and ISO 180. ASTM-type specimens are generally 63.5 mm × 12.7 mm × 3.2 mm, with thicker 6.4 mm specimens also permitted for Izod testing under ASTM D256. Specimens must be free from visible defects, sink marks, flash, and internal voids, as these can cause non-representative scatter in the results.

Notched Specimens

For notched testing, a V-notch of defined geometry is machined into the specimen under controlled conditions to avoid introducing residual stresses or heating. The most commonly used ISO notch is the Type A notch with a radius of 0.25 mm; Type B (1.0 mm radius) and Type C (0.1 mm radius) notches are used for specific evaluations. A similar 0.25 mm radius notch is standard in ASTM practice. Notch cutting quality—particularly the radius accuracy and surface finish—has a significant influence on the measured impact strength and must be verified with calibrated tooling.

Specimen Conditioning

All specimens shall be conditioned in a standard laboratory atmosphere, typically 23 °C and 50 % relative humidity, until temperature and moisture equilibrium is reached before testing. For hygroscopic materials such as polyamides, additional drying or moisture-state conditioning may be required, since moisture content can dramatically alter impact performance. A minimum of five to ten specimens per condition is recommended to obtain statistically meaningful results.

Testing Items

Typical pendulum impact test items for plastic materials and products include:

• Charpy impact strength, notched and unnotched (ISO 179-1, ASTM D6110, GB/T 1043)
• Izod impact strength, notched and unnotched (ISO 180, ASTM D256, ASTM D4812, GB/T 1843)
• Absorbed impact energy (J) and normalized impact strength (kJ/m² or J/m)
• Fracture classification, including complete break, hinge break, partial break, and non-break behavior as defined in ISO 179
• Temperature-dependent impact strength at low temperature (for example −40 °C or −30 °C) or elevated temperature
• Instrumented impact analysis of force–time and force–deflection curves to separate initiation and propagation energies

These items can be performed on raw materials, compounded resins, molded parts, extruded profiles, films, sheets, pipes, and pipe fittings, depending on the product specification and applicable standard.

Testing Methods

Charpy Impact Method

In the Charpy configuration, the specimen is placed horizontally across two anvils and struck at the center by the pendulum. For notched specimens, the notch faces away from the striker so that the bending stress concentrates at the notch root and propagates through the remaining ligament. The absorbed energy is corrected for friction, and the Charpy impact strength is calculated as the absorbed energy divided by the cross-sectional area of the specimen (or of the remaining ligament under the notch). ISO 179 defines standard pendulum energies ranging from approximately 0.5 J to 50 J and impact velocities of 2.9 m/s or 3.8 m/s.

Izod Impact Method

In the Izod configuration, the specimen is clamped as a vertical cantilever beam with the notch facing the pendulum. The hammer strikes the notched side above the clamp, loading the notch root in tension. ISO 180 specifies an impact velocity of 3.5 m/s, while ASTM D256 uses approximately 3.5 m/s with pendulum energies selected to match the expected toughness of the material. Results under ASTM D256 are conventionally reported in J/m of notch, whereas ISO 180 results are expressed in kJ/m².

Valid Test Range and Instrument Calibration

To ensure valid results, the energy absorbed by the specimen should fall within an appropriate fraction of the pendulum's nominal energy—ISO standards generally require the absorbed energy to be between 10 % and 80 % of the pendulum capacity. If a specimen does not break, a higher-energy hammer is selected, or the result is reported as non-break. Pendulum testers must be periodically verified for friction losses, velocity, center of percussion, and anvil or clamp geometry against the requirements of the applicable standard.

Instrumented Pendulum Testing

Instrumented impact testers equipped with force and displacement transducers record the full force–deflection curve of the impact event, as described in ISO 179-2. This allows the total absorbed energy to be separated into crack initiation energy and crack propagation energy, providing deeper insight into ductile-versus-brittle failure mechanisms, particularly useful for research and failure analysis.

Applications

Pendulum impact strength testing supports material selection, design validation, and quality control across many industries:

• Automotive – evaluating interior trim, bumpers, battery housings, clips, and exterior components for crashworthiness and low-temperature durability.
• Electronics and electrical appliances – verifying housings, connectors, switch components, and enclosures against drop and impact loads.
• Pipes and fittings – assessing PVC, PE, and PP piping systems for resistance to handling damage and external impact in water, gas, and drainage networks.
• Construction – qualifying window profiles, door panels, sheets, and cladding materials for impact safety requirements.
• Packaging and consumer goods – comparing toughened grades of PP, PE, PET, ABS, and PC for containers, caps, and household products.
• Material development – screening impact modifiers, comparing recycled-content compounds, and studying the effects of temperature, aging, and environmental conditioning on toughness.

Accredited third-party laboratories routinely perform these tests as part of material datasheet verification, supplier qualification, regulatory compliance, and failure investigation programs.

FAQ

What is the difference between the Charpy and Izod impact tests?

The Charpy test supports the specimen horizontally on two anvils and strikes it opposite the notch, while the Izod test clamps the specimen vertically as a cantilever and strikes it on the notched side. The two methods apply different stress states and produce different numerical results, so the test method and standard must always be specified when reporting impact strength.

Why are specimens notched before testing?

The notch acts as a controlled stress concentrator that promotes brittle fracture under triaxial stress. Notched results reveal how sensitive a material is to scratches, sharp corners, and design features that concentrate stress in real parts, which is critical information for safe product design.

Which standards govern plastic pendulum impact testing?

The principal standards are ISO 179 (Charpy), ISO 180 (Izod), ASTM D6110 (Charpy), ASTM D256 (Izod), and the equivalent Chinese national standards GB/T 1043 and GB/T 1843. The choice depends on the target market, customer specification, and the product standard referenced.

What does a low impact strength value indicate?

A low value generally indicates brittle behavior, poor notch sensitivity resistance, possible material degradation, excessive moisture in some polymers, incorrect processing, or testing at a temperature below the material's brittle–ductile transition. Analysis of the fracture surface and comparison with unnotched results help identify the root cause.

How does temperature affect the results?

Most plastics become measurably more brittle as temperature decreases. Testing at low temperatures, such as −40 °C, is common for outdoor, automotive, and cold-chain applications, while elevated-temperature testing is used for parts exposed to heat in service.

What happens if the specimen does not break completely?

Standards define categories such as hinge break, partial break, and non-break. Results are recorded with the appropriate designation, and a higher pendulum energy may be used so that the absorbed energy falls within the valid measuring range of the instrument.