30 Oct 2026
7 min read
A Charpy result is usually reported as the energy absorbed when a pendulum breaks a notched specimen. To understand how to calculate Charpy impact test energy, start with the machine readings, then check that the specimen, test method and reporting units match the applicable standard. Charpy and Izod tests both assess impact behaviour, but their setups differ, so their results are not automatically interchangeable.
What Charpy impact energy measures
In a Charpy impact test, a pendulum strikes a specimen supported at both ends and breaks it. The machine determines the energy absorbed during fracture from the difference between the pendulum’s starting energy and its remaining energy after impact.
Absorbed energy is reported in joules (J). It is not the same as impact strength, which expresses energy relative to a specimen dimension or area when the selected method requires normalization. A result in joules should not be compared directly with a normalized value unless the units and calculation are made consistent.
For example, two specimens may absorb the same energy but have different dimensions or remaining ligament areas beneath their notches. Their raw energy readings may be equal even though a normalized result differs. Follow the reporting rules in the test method rather than applying a conversion by default.
The distinction matters when comparing the difference between Charpy and Izod impact tests. Both methods measure resistance to impact under specified conditions, but specimen support, clamping and impact direction differ. A comparison of Charpy and Izod testing can help clarify the method-level differences before you compare results.
How to calculate Charpy impact test energy
The basic calculation is:
Absorbed energy = initial pendulum energy − energy remaining after impact
Many machines calculate and display absorbed energy from the pendulum’s movement. If you are working from readings, use the energy values indicated by the instrument or the prescribed calculation for that machine and method. Do not subtract angles as if they were energy values; the machine’s geometry and calibration determine how pendulum position relates to energy.
For example, if the initial energy is 300 J and the indicated energy remaining after fracture is 255 J, the absorbed energy is:
300 J − 255 J = 45 J
This example illustrates the subtraction only. Use the machine’s recorded readings and account for corrections required by the applicable test method or equipment documentation.
If the method calls for normalization, divide absorbed energy by the specified specimen dimension or area. For example, when the reporting basis is the remaining ligament area, the calculation is:
Normalized impact value = absorbed energy ÷ remaining ligament area
The result’s units then reflect both quantities, such as J/cm². Use the exact area definition and unit convention required by the selected standard. Do not apply normalization if the method calls for reporting energy alone.
Before finalizing a Charpy test result, check:
- Whether the display shows absorbed energy or another value.
- Whether the test method requires a correction or normalized result.
- Whether specimen dimensions and notch measurements are recorded.
- Whether the reported units match the calculation.
- Whether the machine’s calibration status and setup are appropriate.
These checks also help when reviewing an Izod impact test or comparing results from a Charpy and Izod test: first confirm that each result was calculated and reported according to its own method.
Match specimen preparation to the test method
A correct calculation cannot compensate for a specimen that does not meet the required geometry or setup. Dimensions, notch shape, notch position, orientation and impact direction must follow the selected standard and test procedure.
Record the specimen’s dimensions and notch details before testing. Inspect the notch and machined surfaces for defects that could affect fracture. If the material, specimen size or notch geometry differs from the specified method, document the variation and follow the applicable procedure for deciding whether the result is valid.
The support and striker arrangement must also match the method. A Charpy specimen is supported at both ends, while an Izod specimen is clamped as a cantilever. The specimen must be seated or clamped in the correct orientation, with the notch and impact face positioned as prescribed.
When selecting a machine, verify that its configuration suits the required method and specimen. Ask how the supports, striker and any method-specific accessories are configured, and how those details will be documented. For an overview of available equipment, see Akuracy’s impact testing equipment information.
Charpy and Izod tests: key setup differences
The difference between Charpy and Izod impact tests begins with how the specimen is held:
| Criterion | Charpy test | Izod impact test |
|---|---|---|
| Specimen support | Supported at both ends | Clamped at one end |
| Specimen position | Typically horizontal | Typically vertical |
| Notch position | Positioned according to the selected Charpy method, commonly facing away from the striker | Positioned according to the selected Izod method, commonly facing the striker |
| Impact direction | Strikes the specimen across its span | Strikes the free end of the clamped specimen |
| Result basis | Energy absorbed; normalization only when required by the method | Energy absorbed; reporting basis follows the selected Izod method |
These are general distinctions, not a substitute for the exact method requirements. Standards and material-specific procedures define the details, including specimen geometry and orientation.
Typical applications depend on the material, product specification and governing test procedure. A laboratory may use Charpy testing to evaluate impact behaviour of metals or other materials, while Izod testing is common in specified applications such as plastics. The required method comes from the relevant specification—not from a preference for one test.
Because setup affects fracture behaviour, a Charpy result should not be treated as equivalent to an Izod result simply because both are expressed in energy units. For further background on material testing equipment and applications, identify the method and specimen requirements first.
Report results and select a suitable Charpy machine
A clear report lets another person understand what was tested, how the result was obtained and whether the method was followed. Include, as applicable:
- Material identification and specimen identification.
- Test standard and method, including any permitted deviations.
- Specimen dimensions, notch type and relevant notch measurements.
- Test temperature and conditioning details when required.
- Machine identification and calibration status.
- Individual absorbed-energy readings, units and any required calculation or correction.
- Fracture observations or other method-required notes.
When choosing a Charpy Impact Testing Machine, compare application fit and verifiable configuration—not just the purchase price. Ask the supplier to confirm the method, specimen sizes, striker and support arrangement, energy range, calibration process, installation requirements and operator training in writing. Request documentation that connects the delivered configuration to the agreed specification.
Also clarify how calibration, repair, spare parts and technical support are handled. Ask what response time to expect, which parts are available, and how service or software updates are documented. If the equipment is intended for a laboratory operating under a quality system, confirm the evidence and procedures the lab needs for its own requirements; do not assume that a machine’s description alone establishes compliance.
A manufacturer’s track record and engineering capability are useful evaluation factors, but they do not replace checks on the specific machine. Verify references, service arrangements, training and the acceptance checks that will be performed at installation. For context on the manufacturer and its equipment range, visit the Akuracy homepage.
Charpy Testing FAQs
What is a Charpy test used for?
A Charpy test measures the energy absorbed when a pendulum fractures a notched specimen under a specified setup. It is used to assess impact behaviour, with interpretation governed by the material specification and test method.
What is the angle of the hammer in an Izod impact test?
There is no single hammer start angle that applies to every Izod impact tester; the machine and applicable method determine the prescribed setup. Check the equipment instructions and test standard rather than assuming an angle.
How to interpret Charpy impact test results?
Interpret the absorbed energy alongside the material, specimen geometry, notch, test temperature and method. Compare results only when those conditions and reporting units are sufficiently consistent.
How to calculate impact test value?
Subtract the pendulum’s post-impact energy from its initial energy to obtain absorbed energy. Normalize by the specified specimen dimension or area only when the applicable method requires it.
What is the standard sample size for a Charpy impact test?
A commonly used full-size Charpy specimen is 10 × 10 × 55 mm, but the required size depends on the governing standard and material specification. Confirm the permitted dimensions and notch geometry before testing.
Recommended next steps
Before testing or purchasing equipment, identify the required standard, specimen dimensions, notch type, temperature conditions and reporting basis. Then confirm that the machine configuration, calibration process and support arrangements fit those requirements.
If comparing Charpy and Izod results, treat them as method-specific measurements and check the setup and units before drawing conclusions. A qualified testing professional should confirm the procedure for the material and application in question.
Raghav Menon writes about material testing, hardness measurement, balancing equipment, and laboratory quality control for manufacturing teams in India. His background includes reviewing test methods, calibration requirements, machine specifications, and service considerations such as spare parts, operator training, and repair response time. He focuses on evidence-based comparisons that distinguish rated capability from practical performance, including suitability for standards such as IS 1608 Grade 1. His articles are structured for engineers and quality managers who need clear guidance on equipment fit, operating limits, lifecycle cost, and supplier support.