Charpy impact toughness testing, also known as simply supported beam impact testing, is a standardized method for evaluating the impact resistance and notch sensitivity of plastic materials under dynamic loading conditions. This article provides a comprehensive technical overview of the Charpy test principle, specimen requirements, testing procedures, applicable standards such as ISO 179-1, ASTM D6110, and GB/T 1043, and typical industrial applications. Whether you are a materials engineer, quality control professional, or product designer, this guide will help you understand how to properly conduct and interpret plastic Charpy impact tests.
Technical Overview
Charpy impact testing is one of the most widely used methods for characterizing the toughness and impact resistance of plastic materials. In this test, a rectangular specimen is mounted horizontally on two supports in a simply supported beam configuration, and a swinging pendulum strikes the specimen at its midpoint, directly opposite the supports. The energy absorbed by the specimen during fracture is measured by comparing the initial and final heights of the pendulum, providing a quantitative indicator of the material's ability to resist sudden impact loads.
The test is governed by several international and national standards. ISO 179-1 specifies the non-instrumented Charpy impact test for plastics, while ISO 179-2 covers the instrumented version, which records force-displacement data during fracture. ASTM D6110 defines the equivalent method for determining the Charpy impact resistance of notched plastic specimens, and the Chinese national standard GB/T 1043.1 is technically aligned with ISO 179-1. These standards ensure consistency in specimen geometry, support spacing, pendulum velocity, and energy calculation, enabling reliable comparison of results across laboratories.
Charpy impact strength is typically expressed in kilojoules per square meter (kJ/m²), calculated by dividing the absorbed energy by the cross-sectional area of the specimen. The test can be performed on unnotched specimens to evaluate overall material toughness, or on notched specimens to assess notch sensitivity, which is critical for components containing sharp corners, ribs, or fastener holes that act as stress concentrators in service.
Testing Samples
Standard Specimen Geometry
The preferred specimen for Charpy testing per ISO 179-1 is 80 mm in length, 10 mm in width, and 4 mm in thickness. Specimens are usually injection molded, compression molded, or machined from sheets, plates, or finished parts. For materials that cannot be produced in the standard thickness, specimens of alternative dimensions may be used, provided that the actual dimensions are measured accurately and reported with the results.
Notch Types
Notched specimens are prepared with a milled notch at the midpoint of the specimen face that will face away from the striking pendulum, placing the notch root in tension during impact. ISO 179-1 defines three notch geometries:
- Type A notch: V-notch with a 0.25 mm notch root radius, the most commonly used configuration
- Type B notch: V-notch with a 1.0 mm notch root radius, resulting in lower stress concentration
- Type C notch: V-notch with a 0.1 mm notch root radius, used for materials requiring sharper notch acuity
The remaining thickness under the notch is 8 mm for the standard 10 mm wide specimen, and the notch must be machined with a sharp, clean cutting tool free from machining marks that could influence the results.
Specimen Conditioning
Prior to testing, specimens must be conditioned to a standard atmosphere, typically 23 °C ± 2 °C and 50 % ± 10 % relative humidity, for a period of at least 16 hours for specimens up to 4 mm thick, extending to 88 hours for thicker samples. Conditioning ensures that moisture content and temperature, both of which strongly influence the impact behavior of hygroscopic polymers such as polyamides, are consistent and representative.
Testing Items
Charpy impact testing encompasses several distinct measurement configurations, each providing different insights into material behavior:
- Unnotched Charpy impact strength: measures the total energy required to break a smooth specimen, reflecting the overall toughness of the material
- Notched Charpy impact strength: measures the energy to break a notched specimen, quantifying notch sensitivity and resistance to crack initiation
- Edgewise impact (symbol e): specimen struck on the narrow longitudinal face, the standard orientation for notched specimens
- Flatwise impact (symbol f): specimen struck on the broad face, often used for unnotched specimens
- Temperature-dependent testing: performed at sub-ambient, ambient, or elevated temperatures to characterize the ductile-to-brittle transition behavior of polymers
- Instrumented impact analysis: records force-time and energy-time curves to separate initiation and propagation energy components
The test also classifies the fracture behavior of each specimen as complete break, hinged break, partial break, or non-break, which provides valuable qualitative information about the ductility of the material.
Testing Methods
Test Apparatus
The test is conducted on a pendulum impact machine equipped with interchangeable hammers of defined energies, commonly 1 J, 2.75 J, 5.5 J, 11 J, 22 J, and 50 J. The machine features two parallel support anvils spaced 62 mm apart for the standard specimen, a striking edge with a defined radius, and an energy indicating mechanism. Friction and windage losses must be determined and corrected according to the standard.
Test Procedure
- Select a pendulum hammer such that the energy absorbed by the specimen falls between 10 % and 80 % of the hammer capacity, ensuring measurement accuracy
- Verify the impact velocity, typically 2.9 m/s or 3.8 m/s depending on the machine geometry
- Measure the width and thickness of each specimen at the impact point to the nearest 0.02 mm using calibrated instruments
- Place the specimen on the supports with the notch facing away from the pendulum, aligned so the strike occurs at the midpoint
- Release the pendulum from its raised position and record the absorbed energy indicated after fracture
- Classify the break type and repeat the test on a minimum of ten specimens, or as specified, to obtain statistically valid results
Calculation of Results
The Charpy impact strength is calculated by dividing the corrected absorbed energy by the cross-sectional area of the specimen. For unnotched specimens, the area is the product of specimen width and thickness. For notched specimens, the area is the product of the width and the remaining thickness beneath the notch root. The individual results, the arithmetic mean, and the standard deviation are reported along with the notch type, impact direction, break classification, and test temperature. Results from notched and unnotched specimens must never be averaged together, and complete breaks should not be averaged with partial or non-break results.
Applications
Charpy impact testing plays a vital role across the entire plastics value chain, from raw material qualification to finished product verification. Typical applications include:
- Quality control of incoming resins and compounds against supplier specifications and material datasheets
- Material selection and comparison during product design, particularly for components exposed to impact, dropping, or accidental abuse
- Verification of the effect of additives, fillers, reinforcements, and impact modifiers on toughness
- Assessment of processing parameters, such as mold temperature and injection speed, on the impact performance of molded parts
- Evaluation of long-term aging, UV exposure, thermal cycling, and chemical resistance effects on material embrittlement
- Cold-temperature brittleness screening for automotive, packaging, construction, and outdoor applications
- Compliance verification for regulatory and customer specifications in industries such as automotive interiors, electrical enclosures, pipes and fittings, household appliances, and consumer goods
Manufacturers, research institutions, and accredited third-party laboratories routinely perform Charpy testing to support product certification, failure analysis, and research and development programs.
FAQ
What is the difference between Charpy and Izod impact testing?
The Charpy test supports the specimen horizontally at both ends in a simply supported beam configuration and strikes it at the center, while the Izod test clamps the specimen vertically as a cantilever beam and strikes the free end above the clamp. Charpy testing per ISO 179 is generally preferred in Europe and Asia, whereas Izod testing per ASTM D256 is more common in North America. The two methods produce different numerical results and are not directly interchangeable.
Why are notched specimens tested?
Machining a notch introduces a controlled stress concentration at a known location, ensuring consistent crack initiation and enabling evaluation of the material's sensitivity to sharp geometrical features. Notched results are essential for predicting the behavior of real components containing corners, ribs, snap-fits, and fastener holes.
How many specimens are required for a valid test?
ISO 179-1 recommends testing at least ten specimens to obtain a reliable mean value, though the exact number may be adjusted based on the statistical requirements of the product specification or the end customer.
What factors can influence Charpy test results?
Key factors include specimen preparation method, notch machining quality, test temperature, conditioning history and moisture content, hammer energy selection, and clamping or support alignment. Consistent control of these variables is essential for reproducible results.
Can Charpy tests be performed at low temperatures?
Yes. Testing is frequently conducted at temperatures down to -40 °C or lower using a cooled chamber, with specimens transferred to the machine and broken within a defined short time window after removal from the cooling medium. Low-temperature testing reveals the ductile-to-brittle transition that many polymers, particularly polypropylene and other polyolefins, undergo in cold service environments.