Charpy and Izod Impact Tests: Method, Specimens and the Ductile-to-Brittle Transition

An impact test is a mechanical test that measures the energy a notched bar of metal absorbs when it is broken by a single blow from a swinging pendulum. The Charpy test supports the bar at both ends as a simple beam and strikes it behind the notch; the Izod test clamps the bar upright at one end as a cantilever and strikes its free end on the notched face. The result is the absorbed energy in joules, a measure of notch toughness: the resistance of the metal to fracture under a sudden load in the presence of a stress concentration. Because the energy of many steels falls steeply below a certain temperature, an impact result is always stated with the temperature at which it was measured. The Charpy V-notch test, defined by ISO 148-1 and ASTM E23, is the impact test that steel product standards specify. The Izod test survives in a few older metals standards and is now used mainly for plastics.

Impact toughness and absorbed energy

A tensile test pulls a smooth bar slowly. It omits three conditions that favour brittle fracture: a sharp notch, a high rate of loading and a low temperature. The impact test combines all three. The notch concentrates stress and creates a triaxial stress state at its root, which restrains plastic flow. The pendulum loads the bar at a strain rate far above that of a tensile test, which raises the stress needed for plastic flow. The test temperature is then varied to find where the metal stops yielding before it breaks. A metal that absorbs a high energy under these conditions tears in a ductile manner; one that absorbs little cracks with almost no deformation.

The absorbed energy is the potential energy of the pendulum before release minus the energy remaining at the top of its swing after it has broken the bar, corrected for bearing friction and air resistance. ISO 148-1 writes it as KV for a V-notch specimen and KU for a U-notch specimen, with a subscript giving the radius of the striker, so that KV2 is a V-notch result with the 2 mm striker. The figure depends on the specimen size, the notch, the striker and the orientation of the specimen in the product, and it cannot be converted directly into the fracture toughness used in fracture mechanics. Its value lies in comparison: between heats, between temperatures, and against the minimum a product standard sets.

The Charpy test: ISO 148-1 and ASTM E23

The Charpy test is defined internationally by ISO 148-1, in North America by ASTM E23, and in India by IS 1757 (Part 1), the Indian adoption of ISO 148-1:2016. The standard specimen is a bar 55 mm long and 10 mm square with a notch machined across one face at mid-length. It lies horizontally against two anvils 40 mm apart, with the notch facing away from the pendulum and centred within 0.5 mm of the mid-plane between the anvils. The striker hits the face opposite the notch, at mid-span, and bends the bar open at the notch until it breaks or is pushed through the anvils.

The striker has an edge radius of 2 mm, the traditional ISO form, or 8 mm, the traditional ASTM form. Both standards now admit either; because the two can give different results on the same steel, the product standard states which applies and the result names it. In ISO practice the pendulum strikes at 5 to 5.5 metres per second. If the absorbed energy exceeds 80 percent of the pendulum's initial potential energy, ISO 148-1 requires the result to be reported as approximate, because the pendulum slows significantly while breaking the bar. Tests are conventionally made in sets of three specimens at each temperature, and the average is compared with the requirement, often with a lower limit on any single value.

Unless another temperature is specified, ISO 148-1 tests at 23 degrees Celsius plus or minus 5 degrees. For a test at a specified temperature, the specimen is conditioned to within 2 degrees of it: in a liquid bath held within 1 degree for at least 5 minutes, or in a gas chamber held within 1 degree for at least 30 minutes. No more than 5 seconds may pass between removing the specimen from the bath and the blow, so that it does not warm or cool toward room temperature before it is struck.

The V-notch and U-notch specimens

Results are comparable only between specimens of the same form and dimensions. The V-notch, with its sharp 0.25 mm root, concentrates stress more severely than the blunt U-notch and is the form steel standards specify.

ISO 148-1 holds the specimen width to plus or minus 0.075 mm and the V-notch root radius to plus or minus 0.025 mm, because a small change in the notch changes the energy. Where the product is too thin for a full-size specimen, subsize specimens 7.5 mm, 5 mm or 2.5 mm thick are machined with the same notch, and their energies are not compared with full-size results. A specimen taken from heat-treated material is finish-machined and notched after the final heat treatment. Its orientation in the product is stated as well, using the codes of ISO 3785: in a rolled bar, inclusions and grain are drawn out along the axis, and a crack running across them meets more resistance than a crack running along them.

The Izod test

In the Izod test the specimen stands vertically, gripped in a vice so that the root of the notch lies in the plane of the top face of the grips, with the notch facing the pendulum. The hammer strikes the projecting end on the notched face, 22 mm above the notch, and breaks the bar as a cantilever. IS 1598, the Indian standard for the Izod test of metals, sets a striking energy of 165.6 joules and a hammer speed of 3 to 4 metres per second.

BS 131-1 and IS 1598 still define the Izod test for metals, and ASTM E23 covers it beside the Charpy test. Its practical drawback is temperature. The specimen must be clamped in the vice before it is struck, which makes testing below or above room temperature impractical: a cooled specimen warms during clamping, and the 5-second transfer that keeps a Charpy result valid cannot be met. Steel standards that require impact testing at a temperature therefore specify the Charpy test. For plastics the Izod test continues in wide use under ISO 180 and ASTM D256, which are separate methods with their own specimens and are not used on metals.

Charpy against Izod

The two tests use specimens of the same section and, for the V-notch, the same notch, but the loading differs and so does the energy. No general conversion between an Izod value and a Charpy value exists, and a requirement written in one test is met only by that test.

The ductile-to-brittle transition

The ductile-to-brittle transition is the fall in absorbed energy that ferritic, martensitic and other body-centred cubic steels show over a band of falling temperature. In these steels the stress needed to move dislocations rises steeply as the temperature falls, while the stress needed to split the crystal along its cleavage planes changes little. Above the transition the steel yields first and fails by ductile tearing, absorbing a high energy and leaving a dull, fibrous fracture surface. Below it the steel cleaves before it can yield, absorbing little energy and leaving a bright, crystalline surface. Plotted against temperature, the energy forms an S-shaped curve with an upper shelf, a transition zone and a lower shelf.

No single temperature defines the transition, so ISO 148-1 lists several criteria and leaves the choice to the product standard: the temperature at which the energy reaches a stated value, commonly 27 joules; the temperature at half the upper-shelf energy; the temperature at which the fracture surface is half shear and half crystalline; and the temperature at which the lateral expansion, the widening of the specimen on the compression side opposite the notch, reaches 0.9 mm. The transition temperature rises with grain size and section thickness, with embrittlement from tempering in the wrong temperature range in alloy steels, and with phases that embrittle the matrix, such as the intermetallic phases that form in duplex stainless steel held between roughly 600 and 1,000 degrees Celsius.

Face-centred cubic metals have many close-packed slip systems that remain active at low temperature, and they do not cleave. Austenitic stainless steels such as 304 and 316, nickel alloys and aluminium therefore show no ductile-to-brittle transition and stay tough down to cryogenic temperatures. Ferritic and martensitic stainless steels do show one. Duplex 2205, ferrite and austenite in roughly equal proportion, sits between: its ferrite phase undergoes a transition below about minus 50 degrees Celsius. The iron-carbon phase diagram sets out where body-centred ferrite and face-centred austenite occur in steel.

Impact requirements in steel product standards

A product standard states an impact requirement as a minimum absorbed energy at a stated temperature, usually as an average of three specimens with a lower limit on any single value, and names the specimen orientation and the striker. Structural steel to EN 10025-2 is graded by it: subgrades JR, J0 and J2 require 27 joules at plus 20, 0 and minus 20 degrees Celsius, and K2 requires 40 joules at minus 20 degrees Celsius. Low-temperature bolting to ASTM A320 Grade L43 requires 20 foot-pounds, about 27 joules, at minus 101 degrees Celsius. Duplex stainless steel for offshore service under the NORSOK M-630 material data sheets is impact tested at minus 46 degrees Celsius.

The Charpy test also serves as a screen for microstructural damage rather than as a toughness measurement. ASTM A923 Method B detects intermetallic phases in duplex stainless steel by breaking specimens at minus 40 degrees Celsius; for UNS S32205 and S31803 base metal the acceptance minimum is 54 joules. Material that has spent too long in the intermetallic range can fail the test even though its composition and tensile properties still meet the product specification. The duplex stainless steels article describes those phases and the fabrication limits they impose.

Impact testing in the Laxcon Steels laboratory

The Laxcon Steels laboratory at Sanand is accredited by NABL to ISO/IEC 17025:2017 (certificate TC-12337) and includes an automatic impact testing machine alongside its mechanical testing, chemical analysis and metallography equipment, as shown on the quality page. Laxcon Steels holds a NORSOK M-650 qualification as a manufacturer of special materials for hot rolled bar in ASTM A479 grade 316 and in UNS S32205 and S31803. It supplies hot rolled round bar from 16 to 125 mm and duplex steel bar made to order in the grades of its grade reference.

Frequently asked questions

What is the difference between the Charpy and Izod impact tests?

The Charpy test supports the specimen horizontally at both ends and strikes it at mid-span on the face opposite the notch. The Izod test clamps the specimen vertically at one end and strikes its free end on the notched face, 22 mm above the notch. Charpy specimens can be cooled or heated and struck within seconds, so Charpy is the test steel standards specify; Izod is now used mainly for plastics.

What does a Charpy impact test measure?

It measures the energy, in joules, that a notched bar absorbs when it is broken by one blow from a pendulum. The energy indicates notch toughness at the test temperature.

What is the ductile-to-brittle transition temperature?

It is the temperature band in which the absorbed energy of a ferritic or martensitic steel falls from a high upper shelf to a low lower shelf as the fracture changes from ductile tearing to cleavage. Product standards define it by a criterion, most often the temperature at which the Charpy V-notch energy falls to 27 joules.

Why do austenitic stainless steels show no ductile-to-brittle transition?

Austenite has a face-centred cubic structure with many slip systems that stay active at low temperature, and it does not cleave. Grades such as 304 and 316 therefore keep a high impact energy down to cryogenic temperatures, unlike ferritic and martensitic steels, whose body-centred cubic structure cleaves below the transition.

Sources

• ISO 148-1:2016, Metallic materials, Charpy pendulum impact test, Part 1: test method: specimen dimensions and tolerances, notch geometry, strikers, test temperatures, transfer time, the 80 percent capacity limit and the transition criteria of Annex D.
• ISO 148-2, Metallic materials, Charpy pendulum impact test, Part 2: verification of testing machines.
• ASTM E23, Standard Test Methods for Notched Bar Impact Testing of Metallic Materials.
• ASTM A370, Standard Test Methods and Definitions for Mechanical Testing of Steel Products.
• IS 1757 (Part 1):2020, Metallic materials, Charpy pendulum impact test, Part 1: test method.
• IS 1598:1977, Method for Izod impact test of metals: specimens, striking distance, hammer speed and striking energy.
• BS 131-1, Methods for notched bar tests, Part 1: the Izod impact test on metals.
• ISO 3785, Metallic materials, designation of test specimen axes in relation to product texture.
• EN 10025-2, Hot rolled products of structural steels, Part 2: technical delivery conditions for non-alloy structural steels.
• ASTM A320/A320M, Standard Specification for Alloy-Steel and Stainless Steel Bolting for Low-Temperature Service.
• ASTM A923, Standard Test Methods for Detecting Detrimental Intermetallic Phase in Duplex Austenitic/Ferritic Stainless Steels.
• NORSOK M-630, Material data sheets and element data sheets for piping.