Know more about the Charpy Impact Test
What Is the Charpy Impact Test?
The Charpy impact test is one of the most widely used methods for measuring the toughness of a material—its ability to absorb energy under sudden, high-speed loading (impact). A standardised notched specimen is struck by a swinging pendulum hammer, and the energy absorbed during fracture is calculated from the pendulum’s height before and after impact.
The simplicity and reproducibility of the Charpy test, combined with its sensitivity to material embrittlement and transition temperature behaviour, have made it a fundamental test in structural steel qualification, pressure vessel codes, pipeline specifications, and weld procedure qualification in the steel, oil and gas, and defence industries for over a century.
Charpy Test Setup and Procedure
Equipment
A Charpy pendulum test machine consists of a rigid frame, a swinging pendulum hammer of defined mass (typically 20 kg for a standard machine), two anvil supports for the specimen, and a dial or digital energy readout. Machine capacity is typically 300 J (ASTM) or 450 J (some ISO configurations).
Specimen Geometry (ASTM E23)
The standard Charpy specimen is a rectangular bar: 55 mm × 10 mm × 10 mm with a 2 mm deep, 45° V-notch at the midpoint. The notch concentrates stress, ensuring fracture initiates at the notch root rather than in an uncontrolled location.
Sub-size specimens (7.5 mm, 5 mm, and 2.5 mm thick) are used when the material thickness is insufficient for the standard geometry.
Test Procedure
- Machine the specimen to standard dimensions and notch geometry.
- Cool or heat the specimen to the specified test temperature and transfer to the machine within 5 seconds.
- Position the specimen with the notch facing away from the pendulum.
- Release the pendulum and record the absorbed energy (J or ft-lbf).
- Examine the fracture surface to determine percent shear fracture.
- Measure lateral expansion at the notch root.
What Charpy Test Results Tell You
Absorbed Energy (Joules or ft-lbf)
The primary output. Higher absorbed energy = greater toughness. Many structural codes specify minimum CVN energy requirements at the lowest design service temperature.
Fracture Appearance
- Shear fracture (fibrous, grey): Ductile fracture—high energy absorption
- Cleavage fracture (bright, crystalline): Brittle fracture—low energy absorption
- Mixed fracture: Transitional behaviour between ductile and brittle
Lateral Expansion
The permanent expansion of the specimen at the notch root is measured in millimetres. Indicates the degree of plastic deformation accompanying fracture. ASME Boiler and Pressure Vessel Code specifies minimum lateral expansion requirements for pressure vessel steels.
Ductile-to-Brittle Transition Temperature (DBTT)
Testing at multiple temperatures from above to below the transition range generates a CVN energy-vs-temperature curve. The DBTT is the temperature at which the material transitions from ductile (high-energy) to brittle (low-energy) fracture behaviour. Structural codes require that the DBTT be below the lowest service temperature with an adequate safety margin.
Charpy Testing Standards
- ASTM E23: Standard test methods for notched bar impact testing of metallic materials
- ISO 148-1: Metallic materials—Charpy pendulum impact test
- ASME Boiler and Pressure Vessel Code: Charpy testing requirements for pressure vessel steels
- API 5L: CVN testing requirements for line pipe steels
Conclusion
The Charpy impact test is a critical method for evaluating the toughness of materials under high-speed loading conditions, providing essential insight into their resistance to sudden fracture. By measuring absorbed energy, fracture behaviour, and transition temperature, it helps engineers assess material suitability for low-temperature and high-stress applications. Widely used in structural, pipeline, and pressure vessel industries, the test serves as a key tool for ensuring safety, preventing brittle failure, and meeting stringent engineering standards.
Why Choose Infinita Lab for Charpy Impact Testing?
Infinita Lab offers comprehensive Charpy impact testing per ASTM E23 and ISO 148 standards. Our accredited laboratory network is equipped with calibrated Charpy machines for testing at temperatures from +200°C to −196°C, supporting material certification, weld qualification, and failure analysis programs.
Looking for a trusted partner to achieve your research goals? Schedule a meeting with us, send us a request, or call us at (888) 878-3090 to learn more about our services and how we can support you. Request a Quote
Frequently Asked Questions (FAQs)
What is the difference between Charpy and Izod impact tests? Both tests measure absorbed impact energy, but differ in specimen orientation: Charpy specimens are supported horizontally at both ends with the notch facing away from the striker; Izod specimens are clamped vertically with the notch facing the striker. Charpy is universally used in engineering codes; Izod is common in plastics testing and some older metal specifications.
Why is the specimen transfer time limited to 5 seconds in Charpy testing? The specimen begins to warm (or cool) back toward ambient temperature as soon as it is removed from the conditioning bath. The 5-second transfer time limit (per ASTM E23) ensures the specimen is at the required test temperature when struck, preventing inaccurate results from temperature change during transfer.
What is the upper shelf energy (USE) and why does it matter? The upper shelf energy is the CVN energy plateau at temperatures well above the DBTT, where fully ductile fracture occurs. Higher USE indicates more inherent toughness at the microstructural level. Nuclear reactor pressure vessel steels are monitored for USE reduction due to radiation embrittlement throughout service life.
Can Charpy specimens be extracted from weld heat-affected zones? Yes. Charpy specimens are routinely machined from weld metal (WM), heat-affected zone (HAZ), and base metal (BM) positions in welded joint qualification testing. HAZ toughness is often the limiting factor in welded structural steel.
How does grain size affect Charpy impact energy? Finer grain sizes generally provide higher Charpy absorbed energy and lower DBTT. Grain refinement strengthens grain boundaries and reduces the length of cleavage facets, requiring more energy to propagate fracture. This is why normalizing or thermomechanically controlled rolling (TMCP) of structural steels improves low-temperature toughness.
