The Charpy impact test measures the energy a notched steel specimen absorbs when fractured by a swinging pendulum — the standard screen for brittle fracture risk in structural steel, pressure vessels and weldments.
What the Test Measures
A 10×10×55 mm specimen with a 2 mm V-notch is struck behind the notch by a pendulum of known energy. The energy absorbed (joules) reflects toughness; tested across temperatures it maps the ductile-to-brittle transition of the steel.
Apparatus Required
- Pendulum impact machine (commonly 300/450 J) with 2 mm striker per ISO 148 or 8 mm per ASTM E23, verified to the standard
- Specimen anvils and centring tongs
- V-notch broaching machine or milling setup for specimen preparation
- Conditioning bath (e.g. −60 to +100 °C) with temperature control ±1 °C
Test Procedure
- Machine specimens with the notch axis as specified (through-thickness surface finish and notch tolerance are critical).
- Condition at the specified test temperature at least 10 min in liquid; transfer and strike within 5 s.
- Release the pendulum; record absorbed energy from the dial/encoder.
- Test in sets of three per temperature; note lateral expansion and percentage shear fracture where required.
Calculation & Reporting
Report individual and mean absorbed energies (J), test temperature, striker size, notch type and specimen orientation. Sub-size specimens (7.5/5/2.5 mm) must be identified — energies do not scale linearly.
Acceptance Criteria
Typical structural requirements: S355J2 ≥ 27 J at −20 °C; S355K2 ≥ 40 J at −20 °C; pressure equipment and offshore specs set project values. One result may fall below minimum (not below 70% of it) if the set average passes, per most product standards.
Frequently Asked Questions
Why does test temperature matter so much?
Ferritic steels lose toughness sharply below their transition temperature. A steel absorbing 150 J at 20 °C can absorb under 20 J at −40 °C — specifications therefore tie the energy to a design-relevant temperature.
ISO 148-1 or ASTM E23 — are results interchangeable?
Not strictly: the 2 mm (ISO) and 8 mm (ASTM) strikers give slightly different energies on some steels. Certify to the standard named in the material specification.
Recommended Apparatus
NL Scientific manufactures the Universal Impact Tester (CHARPY & Izod) 5J / 5.5J (TOUCH Screen) for this method. Browse the full Steel Testing Equipment range or request a quotation from our engineers.
