Plastics Charpy impact testing is a widely used method for determining the impact resistance and toughness of plastic materials under high-speed dynamic loading. Governed by international standards such as ISO 179-1, ASTM D6110, and GB/T 1043, this test measures the energy absorbed by a simply supported specimen when fractured by a swinging pendulum. This article provides a comprehensive technical overview of the test principle, specimen requirements, testing procedures, and practical applications across industries.
Technical Overview
Charpy impact testing, also known as simply supported beam impact testing, is one of the most fundamental methods for evaluating the impact toughness and notch sensitivity of plastic materials. In this test, a rectangular specimen is placed horizontally on two supports, resembling a simply supported beam, and struck at the midpoint by a pendulum hammer. The energy absorbed during fracture is measured and used to calculate the impact strength of the material.
The test is standardized internationally by ISO 179-1 (Plastics — Determination of Charpy impact properties), in North America by ASTM D6110, and in China by GB/T 1043. These standards define specimen geometry, support span, pendulum energy, striking velocity, and the calculation of impact strength values. The test can be performed on both notched and unnotched specimens, allowing engineers to assess the material's sensitivity to stress concentrations, which is critical for real-world product performance.
Charpy test results help classify material failure behavior as brittle, ductile, or mixed-mode fracture, providing essential data for material qualification, quality control, and design validation. Compared with Izod impact testing (cantilever beam configuration), Charpy testing supports both edgewise and flatwise impact and is particularly well suited for rigid thermoplastics, thermosets, and fiber-reinforced composites.
Testing Samples
Unnotched Specimens
Unnotched specimens are used to evaluate the inherent impact toughness of a material without artificial stress concentration. The most common geometry is the type 1 specimen measuring 80 mm × 10 mm × 4 mm, supported over a span of 62 mm and struck edgewise by the pendulum. Unnotched testing is useful for materials such as unfilled polypropylene, polyethylene, and ABS, where the overall energy absorption capacity is of primary interest.
Notched Specimens
Notched specimens feature a machined V-shaped or U-shaped notch that acts as a stress concentrator, simulating surface defects, sharp corners, or scratches found in real products. Common notch types include type A (V-notch, 0.25 mm radius) and type B (U-notch, 1.0 mm radius), with a residual width of 8 mm for standard specimens. Notched Charpy testing reveals notch sensitivity and the tendency of a material to fail in a brittle manner, which is essential for engineering design decisions.
Injection-Molded and Machined Specimens
Specimens may be directly injection molded into the standard multi-purpose test bar (type 1A according to ISO 3167) or machined from sheets, pipes, finished parts, or compression-molded plaques. Machined specimens must be prepared with care to avoid introducing residual stresses, heat-affected zones, or rough surfaces that could distort the measured impact strength. When testing finished products, sampling location and orientation should be documented, as processing conditions and molecular orientation significantly influence results.
Conditioned Specimens
Plastic materials are moisture- and temperature-sensitive, so specimens must be conditioned before testing, typically at 23 °C and 50% relative humidity for a defined period according to ISO 291 or ASTM D618. For hygroscopic materials such as polyamide (nylon), dry-as-molded and moisture-conditioned states may both be evaluated because absorbed water acts as a plasticizer and dramatically increases impact toughness.
Testing Items
Charpy impact testing programs typically include the following measurement items:
- Charpy impact strength of unnotched specimens (kJ/m²)
- Charpy impact strength of notched specimens (kJ/m²)
- Charpy impact strength of notched specimens at low temperature or elevated temperature
- Instrumented Charpy impact testing with force–displacement curves
- Tensile impact strength for thin or flexible materials
- Failure mode classification: brittle fracture (C), ductile fracture (D), or partial ductile fracture
- Comparative evaluation of material formulations, fillers, and reinforcement content
Additional derived information includes the ductile-to-brittle transition behavior across a temperature range, the effect of notch radius on toughness, and statistical scatter analysis of replicate specimens.
Testing Methods
Test Principle and Instrumentation
The test is performed on a pendulum impact machine in which a hammer of known potential energy is released from a fixed height. The specimen rests on two anvils separated by the standard support span, and the pendulum strikes the specimen at the center at a velocity of approximately 3.8 m/s for ISO 179-1. After breaking the specimen, the pendulum continues to swing to a reduced height. The difference between the initial and residual potential energies, corrected for friction and windage losses, equals the energy absorbed by the specimen. Impact strength is calculated by dividing the absorbed energy by the cross-sectional area of the specimen at the fracture plane (or beneath the notch for notched specimens).
Standard Test Procedures
Typical test procedures follow these steps:
- Condition specimens at 23 °C / 50% RH until moisture equilibrium is reached
- Measure and record specimen dimensions, including residual width at the notch, with a precision of at least 0.02 mm
- Select a pendulum hammer with an energy capacity such that the absorbed energy is between 10% and 80% of the available energy
- Calibrate the machine and verify friction losses according to ISO 13802 or ASTM E23 procedures
- Position the specimen centered on the supports with the notch facing away from the pendulum striker
- Release the pendulum, break the specimen, and record the absorbed energy
- Classify the fracture mode as complete break, hinge break, partial break, or non-break
- Test at least five (preferably ten) specimens and report the mean, standard deviation, and individual values
Instrumented Charpy Impact Testing
Instrumented testing, covered by ISO 179-2, equips the striker with force and displacement sensors that record the entire force–time or force–deflection curve during fracture. This enables separation of the energy required for crack initiation and crack propagation, providing a much deeper understanding of failure mechanics than the single energy value obtained from conventional testing. Instrumented data are particularly valuable for research and development of toughened polymers, composites, and crash-relevant automotive components.
Key Factors Affecting Results
Several factors must be controlled to obtain reproducible and comparable results:
- Test temperature: impact toughness of many plastics drops sharply below their glass transition or ductile–brittle transition temperature
- Notch quality: notch radius, depth, and surface roughness strongly influence measured strength
- Hammer energy selection: an oversized hammer reduces measurement resolution, while an undersized hammer may not break the specimen
- Specimen orientation: edgewise (e) or flatwise (f) impact, and direction relative to molding flow, affect results for anisotropic materials
- Support span and specimen clamping alignment
- Environmental conditioning, especially moisture content for polyamides and other hygroscopic polymers
Applications
Material Qualification and Comparison
Charpy impact data are routinely used to compare candidate materials during product development, verify incoming resin quality, and confirm the effectiveness of impact modifiers, fillers, and reinforcements. Datasheet values from resin producers are typically generated using notched Charpy testing, making it a common language between suppliers and converters.
Automotive Industry
Interior trim, bumper systems, battery housings, and underbody components must withstand impact loads at both ambient and low temperatures. Charpy testing at −30 °C or −40 °C is frequently specified in OEM material standards to ensure that parts do not shatter in cold climates. Accredited third-party laboratories commonly provide low-temperature Charpy testing for automotive material approvals.
Construction and Pipes
Pressure pipes, fittings, drainage systems, and profiles made from PVC, PE, and PP are evaluated for impact resistance to ensure resistance to handling damage, installation impacts, and external shock loads. Standards in the EN and GB frameworks reference Charpy or falling-weight impact methods for construction product certification.
Electrical and Electronic Products
Housings, enclosures, connectors, and insulating parts require adequate impact resistance to survive drop, assembly, and in-service loads. Charpy testing supports conformity with IEC and UL safety requirements for electrical equipment enclosures and helps manufacturers validate flame-retardant formulations that often reduce impact toughness.
Packaging and Consumer Goods
Rigid packaging, appliances, tools, and sporting goods manufacturers use Charpy data to balance stiffness, cost, and drop resistance. Comparative testing of recycled content levels and bio-based polymers also relies heavily on notched and unnotched Charpy measurements.
FAQ
What is the difference between Charpy and Izod impact testing?
Charpy (simply supported beam) testing supports the specimen at both ends and strikes it at the center, while Izod (cantilever beam) testing clamps the specimen at one end and strikes the free end above the notch. Charpy testing per ISO 179-1 and ASTM D6110 supports edgewise and flatwise impact, whereas Izod testing per ISO 180 and ASTM D256 uses a clamped cantilever configuration. Both provide impact strength in kJ/m², but values are not directly interchangeable.
Why are both notched and unnotched results reported?
Unnotched results reflect the overall toughness of the material, while notched results reveal its sensitivity to stress concentrators such as sharp corners, scratches, and weld lines. A material with high unnotched impact strength but low notched impact strength is considered notch-sensitive and should be designed with generous radii and smooth surfaces.
What does a partial break or non-break result mean?
If the specimen does not separate completely, the standard classifies the result as hinge break, partial break, or non-break. These outcomes indicate very high toughness relative to the selected hammer energy, and results should be reported with the appropriate break classification. A higher-energy hammer may be required for full fracture characterization.
How many specimens should be tested?
ISO 179-1 recommends a minimum of five specimens, while ten specimens provide better statistical reliability, especially for materials with high scatter such as fiber-reinforced or highly filled compounds. Mean, standard deviation, and fracture classification should all be reported.
Can Charpy testing be performed at low temperatures?
Yes. Specimens are conditioned in a controlled-temperature chamber, typically using dry ice or liquid nitrogen cooling, and tested within seconds of removal to minimize temperature drift. Low-temperature Charpy testing is essential for automotive, outdoor, and cryogenic applications where ductile-to-brittle transitions are a design concern.
Which organizations publish the relevant standards?
The primary standards are ISO 179-1 and ISO 179-2 from the International Organization for Standardization, ASTM D6110 from ASTM International, and GB/T 1043.1 and GB/T 1043.2 from the Chinese national standards system. Accredited third-party laboratories perform Charpy impact testing in accordance with these frameworks to support product certification and regulatory compliance.