Charpy impact testing

What the test establishes

Charpy results report energy absorbed by a specified notched specimen at a stated temperature. Verify the machine’s energy scale with certified reference specimens. NIST Charpy Machine Verification Program, updated October 8, 2026

Original acceptance-arithmetic example: assume a fictional contract requires three full-size specimens at −20 °C, a mean of at least 40 J and no individual value below 30 J. Results of 28, 45 and 53 J average 42 J, but the set fails the stated individual minimum. These limits are invented solely to demonstrate separate checks. They are not a grade requirement, a retest rule or an uncertainty allowance. Follow the actual specification for rounding, individual minima, permitted retests and subsize treatment.

For metallic materials, distinguish Charpy from Izod: a Charpy specimen rests on two supports and is struck opposite the notch; an Izod specimen is clamped at one end. ASTM E23 includes both arrangements, while ISO 148 addresses Charpy pendulum testing. Specify the method and edition required by the contract rather than treating all impact results as equivalent. This page addresses metallic specimens, not plastics impact methods. ZwickRoell, ASTM E23 overview

The principal result is absorbed energy in joules. It is not tensile strength, a component's allowable impact load, or a direct fracture-mechanics toughness value such as K or J. TWI cautions that Charpy results are used for comparison or against specification requirements, rather than directly calculating fracture toughness of parent metal or welds. TWI, what Charpy testing measures

Specimen identity matters

A standard full-size Charpy V-notch specimen is 55 mm long with a 10 × 10 mm section, a 2 mm-deep notch, and a 0.25 mm notch-tip radius. The specimen is supported at its ends and struck opposite the notch. These nominal dimensions describe the specimen; use the governing method for tolerances and detailed machining requirements. TWI, Charpy specimen geometry

Record the extraction location, orientation relative to the product, notch direction and material condition. TWI notes that rolled products can give different results parallel and transverse to the rolling direction. In welds, moving the notch changes the mixture of weld metal and heat-affected-zone microstructures sampled. “Weld impact test” is therefore incomplete without the notch location and specimen extraction details. TWI, notched-bar testing Part II

Subsize specimens may be necessary when full-size material cannot be extracted. They need their own permitted geometry and acceptance treatment: TWI specifically warns that a 10 × 5 mm specimen does not simply have half the notch toughness of a 10 × 10 mm specimen. Thickness also influences transition behaviour. Do not invent a full-size equivalent by multiplying measured joules by an area ratio unless the applicable product rule expressly requires that treatment. TWI, subsize specimens

V- and U-notches produce different stress concentrations and cannot be directly compared. Check notch quality, dimensions, position and surface condition before testing; machine capacity cannot compensate for a defective specimen. ZwickRoell, ISO 148-1 specimen preparation

Temperature and transfer

The specimen must reach the specified temperature throughout and retain the required condition until impact. Conditioning-medium temperature alone is insufficient evidence of specimen temperature. Handling, transfer equipment and time outside the conditioning environment can change the test condition. Use the selected method's conditioning and transfer provisions, with a documented procedure that the operator can actually execute. ZwickRoell, temperature control

A bounded example is NIST's instructional guidance for its −40 °C verification specimens, which explains the ASTM E23 transfer limit of 5 seconds or less from the cooling medium to the impact position. That is not permission to apply the same conditioning recipe to every specimen, temperature or standard. NIST verification instructional videos

Even “room temperature” needs a definition. NIST's June 20, 2023 investigation of room-temperature verification specimens found that temperature changes within a 10 °C range could significantly affect low-energy results and cause otherwise satisfactory machines to fail verification criteria. Use the reference material's certificate and the governing procedure, not an assumption that ordinary laboratory ambient conditions are interchangeable. NIST, room-temperature specimen study

Pendulum selection and trustworthy energy

Choose a machine with suitable verified energy range, striker/anvil geometry, impact velocity and specimen clearances. As a named example, ZwickRoell describes a 450 J HIT450P with an ISO upper absorbed-energy limit of 360 J, corresponding to 80% of pendulum capacity. This is a method-and-machine example, not a universal acceptance range. Low-energy work also requires attention to friction relative to measured energy. ZwickRoell, ISO 148-1 equipment selection

NIST distinguishes certified verification specimens for 8 mm and 2 mm strikers, as well as different energy levels and test temperatures. Match the reference material to the verification task; a successful check using one combination does not establish every machine configuration. NIST verification specimen families

NIST's SP 960-4, October 2000, explains why both physical machine checks and indirect verification matter. Worn anvils and excessive bearing friction can produce compensating errors that appear satisfactory at one energy level but do not cancel elsewhere. Inspect supports, striker and the specimen path, address friction and alignment, and follow current method requirements for verification frequency and limits. This older guide supports the error mechanisms, not current contractual intervals. NIST, installing, maintaining and verifying Charpy machines

Interpreting a set of results

Ferritic steels can show a ductile-to-brittle transition as temperature decreases. A temperature series can describe that transition; a single energy at one temperature cannot define the whole curve. Fracture appearance and lateral expansion provide additional information, with lateral expansion reported as a dimensional measure of deformation. Request these outputs when the product requirements call for them. TWI, fracture appearance and lateral expansion

Reporting and practical limits

Retain specimen IDs, product/heat and extraction locations, orientation, dimensions and notch type, test temperature and conditioning records, machine/striker configuration, verification status and individual energies. Preserve fracture observations, unusual test events and any required lateral-expansion measurements. Report the applicable acceptance rule separately from the measured results.

Instrumented Charpy testing adds force-time and derived force-displacement information; it is more than attaching extra decimal places to a pendulum energy reading. NIST provides a dedicated instrumented-test analysis suite associated with ISO 14556 and ASTM E2298. Specify that capability explicitly if the investigation requires it. NIST, Charpy Instrumented Test Suite

Related reference

Inspection and quality documents