Charpy V-notch testing: the essentials

The Charpy V-notch (CVN) test is one of the most common toughness tests on steel. A notched specimen is broken by a pendulum impact at a specified temperature, and the machine measures the energy absorbed during fracture.

That simple description hides several procurement variables. A Charpy requirement is incomplete unless you know the temperature, specimen orientation, specimen size, acceptance energy and test frequency.

What the test measures

ISO 148-1 specifies the Charpy pendulum impact test method for metallic materials. The result is typically reported as absorbed energy in joules (J).

Higher absorbed energy generally indicates greater resistance to brittle impact fracture under the test conditions, but Charpy energy is not a direct structural design property and should not be treated as interchangeable with fracture-toughness parameters such as CTOD.

Test temperature matters

Steel toughness can change substantially with temperature. Ferritic steels in particular can show a transition from relatively ductile behaviour to brittle behaviour as temperature decreases.

This means:

A test result of 120 J at -20 °C does not prove compliance with a requirement of 50 J at -40 °C.

Even when the -20 °C result is far above the required energy, the material has not been tested at the specified -40 °C condition. Extrapolating to a lower temperature requires an accepted material-specific basis or transition-curve evidence; it cannot be assumed from one warmer-temperature test.

This is why suffixes such as JR, J0 and J2 matter in EN structural grades. See JR, J0, J2 and K2: impact-toughness suffixes explained.

Longitudinal or transverse?

The test-piece axis can be taken longitudinal or transverse to the principal direction of working/rolling. The required orientation comes from the applicable product standard or project specification.

Transverse and longitudinal results should not be compared as if the sampling direction were irrelevant. For a focused explanation, see Longitudinal vs transverse Charpy orientation.

Specimen size matters

The familiar full-size Charpy specimen is 10 mm × 10 mm in cross-section, but thin products may not physically permit a full-size specimen. Sub-size specimens can therefore be used where the governing standard allows them.

Do not simply scale the required energy in proportion to specimen area unless the applicable product/test standard explicitly tells you to do so. Acceptance rules for sub-size specimens are specification-dependent.

Why results are often reported as three specimens

A Charpy test set commonly consists of multiple specimens from the defined test unit, and the governing material/product standard normally specifies how individual and average values are assessed.

The exact number, averaging rule and permitted low individual value should therefore be taken from the applicable specification, not invented from a generic Charpy rule.

Test frequency is not defined by “Charpy” itself

ISO 148-1 explains how to perform the test. It does not decide how many plates, heats, lots or pipe lengths must be tested for your purchase.

Test frequency is established by the material/product standard and any project supplementary requirement. Depending on the product, it can be based on heat, heat-treatment batch, test unit, thickness range, rolling unit, lot or another defined population.

This distinction is important when reviewing an MTC: a valid Charpy result still needs to represent the product you are buying under the correct sampling rules.

Sampling location and preparation

ISO 377 provides general requirements for the identification, location and preparation of samples and test pieces for mechanical testing of steel products. The product standard then defines the relevant sampling location and orientation.

Moving the sample location can materially change the measured toughness, especially in thick products or weldments.

Charpy vs CTOD

Charpy is a relatively simple impact-toughness test using a machined notch. CTOD uses a fatigue-precracked fracture-mechanics specimen and is intended to quantify crack-tip resistance much more directly.

A high Charpy result is therefore not a substitute for a specified CTOD test. See CTOD testing explained.

What to check on an MTC

For a Charpy line on an MTC, verify:

• test temperature;
• longitudinal/transverse orientation;
• specimen size;
• individual results and reported average;
• applicable required minimum;
• heat/test-unit traceability;
• whether the test frequency matches the product/project requirement.

A certificate that merely states “Impact: OK” gives far less useful evidence than one that reports these details.

References and verification sources

• ISO 148-1:2016 — Charpy pendulum impact test, Part 1
• ISO 377:2017 — Location and preparation of samples and test pieces