Low-temperature Charpy impact testing measures the impact resistance of plastic materials in a simply supported beam configuration under sub-zero conditions, providing critical data on brittleness and cold-climate durability. Performed in accordance with ISO 179-1, GB/T 1043.1, and ASTM D6110, this test is essential for qualifying polymers used in automotive, pipeline, construction, and outdoor applications where low-temperature toughness directly determines product reliability and service life.
Technical Overview
The Charpy impact test, also known as the simply supported beam impact test, is one of the most widely used methods for evaluating the toughness and impact resistance of plastic materials. In this test, a rectangular specimen is placed horizontally across two supports and struck at mid-span by a pendulum hammer. The energy absorbed by the specimen during fracture is measured and used to calculate the impact strength, typically expressed in kJ/m².
At reduced temperatures, many polymers undergo a transition from ductile to brittle behavior, causing a dramatic drop in impact resistance. Low-temperature Charpy impact testing is therefore performed to assess how a material performs in cold service environments, to determine the ductile-to-brittle transition temperature (DBTT), and to support material selection and qualification for cold-climate applications. Testing is conducted in accordance with recognized standards, including ISO 179-1 (Plastics — Determination of Charpy impact properties — Part 1: Non-instrumented impact test), GB/T 1043.1 (the Chinese national standard technically equivalent to ISO 179-1), and ASTM D6110 (Standard Test Method for Determining the Charpy Impact Resistance of Notched Specimens of Plastics).
- Specimen configuration: simply supported beam, struck edgewise at mid-span
- Support span: 62 mm for ISO-type specimens, 101.6 mm for ASTM-type specimens
- Impact velocity: typically 2.9 m/s or 3.8 m/s depending on pendulum energy
- Result: impact strength in kJ/m², along with fracture mode classification
Testing Samples
ISO-Type Specimens (ISO 179-1 / GB/T 1043.1)
The preferred specimen dimensions are 80 mm × 10 mm × 4 mm, tested over a 62 mm support span. Specimens may be unnotched or notched. For notched testing, the most common configuration is the Type A notch with a 0.25 mm tip radius and a 2.0 mm notch depth, leaving 8.0 mm of remaining material under the notch. Type B (1.0 mm radius) and Type C (0.1 mm radius) notches are also defined for specific evaluation purposes. Specimens may be directly injection molded or machined from sheets, plaques, or finished parts.
ASTM-Type Specimens (ASTM D6110)
ASTM D6110 specifies longer and thicker specimens, typically 127 mm × 12.7 mm × 12.7 mm (or 12.7 mm × 6.4 mm), tested over a support span of 101.6 mm with a notch depth of 2.54 mm. Because specimen geometry strongly affects results, ISO-based and ASTM-based data should not be compared directly.
Specimen Preparation and Conditioning
Notches must be machined with a sharp, freshly prepared cutting tool, as notch quality significantly influences the measured result. For low-temperature testing, specimens are conditioned at the specified test temperature in a cryogenic chamber, a mechanically refrigerated cabinet, or a temperature-controlled liquid bath for a period sufficient to ensure thermal equilibrium throughout the specimen cross-section. In accordance with ISO 179-1, the time between removing a specimen from the conditioning environment and breaking it shall not exceed 10 seconds to prevent the specimen from warming.
Testing Items
Typical test items evaluated under this method include:
- Notched Charpy impact strength at low temperature (kJ/m²)
- Unnotched Charpy impact strength at low temperature (kJ/m²)
- Ductile-to-brittle transition temperature (DBTT) determined across a temperature series
- Fracture behavior classification: complete break (C), hinge break (H), partial break (P), or non-break (N)
- Temperature-dependence curves of impact strength for material characterization
- Comparative toughness evaluation of formulations, fillers, and recycled content at low temperature
Testing Methods
Apparatus
Testing is performed using a calibrated pendulum impact tester equipped with interchangeable hammers covering a range of energies (commonly 1 J, 2.75 J, 5.5 J, 7.5 J, 15 J, and 25 J). A low-temperature conditioning device, such as a mechanically refrigerated chamber or a liquid-nitrogen-cooled cryostat, maintains specimens at the target temperature. The instrument must be corrected for friction and windage losses, and energy calibration should be traceable to national or international standards.
Test Procedure
- Select a pendulum so that the absorbed energy falls between 10% and 80% of its nominal capacity
- Condition the specimens at the specified test temperature (for example, -20 °C, -30 °C, or -40 °C) until thermal equilibrium is reached
- Remove the specimen, place it on the supports with the notch facing away from the striker, and verify the support span and centering
- Release the pendulum within 10 seconds of removing the specimen from the conditioning environment
- Record the absorbed energy, apply friction corrections, and classify the fracture mode
- Repeat the test on multiple specimens (typically a minimum of 5 to 10) and calculate the arithmetic mean and standard deviation
Calculation of Results
For unnotched specimens, the Charpy impact strength is calculated by dividing the corrected absorbed energy by the cross-sectional area of the specimen. For notched specimens, the calculation uses the original cross-sectional area remaining beneath the notch. Results are reported in kJ/m² together with the test temperature, specimen type, notch type, striking orientation, and the applicable standard. When testing is performed at a series of temperatures, the impact strength or the percentage of brittle fracture is plotted against temperature to determine the ductile-to-brittle transition region.
Applications
Low-temperature Charpy impact data are used across many industries in which plastics are exposed to cold environments during processing, transport, installation, or long-term service:
- Automotive: bumpers, grilles, exterior trim, underbody shields, and battery enclosures requiring cold-climate durability
- Pipes and fittings: polyethylene gas and water pipelines, PVC drainage systems, and buried installations at low ground temperatures
- Construction: window and door profiles, roofing components, and facade elements used in cold regions
- Electrical and appliance housings intended for outdoor or refrigerated environments
- Cold-chain packaging and containers for frozen goods and cryogenic storage
- Research and development: evaluation of toughening modifiers, mineral fillers, copolymer grades, and recycled material content
- Quality control and material qualification programs, including verification testing by accredited third-party laboratories
FAQ
Why is Charpy impact strength tested at low temperature?
Many plastics become significantly more brittle as temperature decreases due to reduced molecular mobility and proximity to the glass transition. Low-temperature testing reveals weaknesses that are not apparent in tests performed at room temperature and is essential for verifying safe performance in cold climates, refrigerated environments, and winter service conditions.
What is the difference between the Charpy test and the Izod test?
In the Charpy test, the specimen is supported horizontally as a simply supported beam and struck at mid-span, whereas in the Izod test the specimen is clamped vertically as a cantilever and struck above the clamp. The two methods produce different results and are governed by different standards, with ISO 179-1 and GB/T 1043.1 covering Charpy testing and ISO 180 and ASTM D256 covering Izod testing.
What temperature range can be tested?
Common test temperatures include 0 °C, -10 °C, -20 °C, -30 °C, -40 °C, and -70 °C using mechanically refrigerated or liquid-nitrogen-cooled equipment. Temperatures as low as -196 °C can be achieved for special engineering assessments when liquid nitrogen immersion is used.
Why are notched specimens used?
The notch acts as a stress concentrator that promotes brittle fracture and simulates defects, sharp corners, and scratches that may exist in real components. Notched results reveal a material's notch sensitivity, which is critical for design purposes, while unnotched results reflect the inherent toughness of the material.
How long must specimens be conditioned before testing?
Specimens must be held at the test temperature long enough to reach thermal equilibrium throughout their thickness, which depends on the conditioning medium and specimen dimensions. After removal from the conditioning environment, the specimen must be broken within 10 seconds as specified by ISO 179-1 to prevent temperature drift.
How is the ductile-to-brittle transition temperature determined?
Specimens are tested at a series of decreasing temperatures, and the impact strength or the percentage of brittle fracture is plotted against temperature. The transition temperature is commonly identified as the temperature at which the fracture mode becomes predominantly brittle, or at the midpoint of the characteristic S-shaped or stepped curve.
What information is included in a test report?
A professional report should identify the test standard, specimen dimensions and preparation method, notch type, conditioning medium and duration, test temperature, pendulum energy and impact velocity, individual and mean impact strength values with standard deviation, fracture mode classifications, and a statement of measurement uncertainty where applicable.