U-Notch Impact Toughness Test: Principles, Standards, Specimens, and Methods

The U-notch impact toughness test is a standardized pendulum impact method that measures the energy absorbed by a notched metallic specimen when fractured by a single hammer blow. Using the classic 10 mm × 10 mm × 55 mm Mesnager specimen with a 5 mm deep U-shaped notch, the test evaluates a material's resistance to brittle fracture over a wide temperature range. This article explains the test principle, applicable standards, specimen requirements, test procedures, and typical industrial applications.

Technical Overview

The U-notch impact toughness test is a notched-bar pendulum impact test in which a swinging hammer fractures a standardized specimen containing a U-shaped notch, and the energy absorbed during fracture is recorded as a measure of impact toughness. The test derives from the early twentieth-century work of Charpy and Mesnager, and the U-notch specimen is therefore often called the Mesnager specimen. Because the U-notch is a relatively blunt stress concentrator with a 1 mm root radius, it produces a less severe triaxial stress state at the notch root than the sharp V-notch, and the absorbed energy values for the same material are correspondingly higher.

The principal standards governing the test include ISO 148-1 (Metallic Materials — Charpy Pendulum Impact Test — Part 1: Test Method), ASTM E23 (Standard Test Methods for Notched Bar Impact Testing of Metallic Materials), and GB/T 229 in China, which is technically aligned with ISO 148. Older European specifications based on EN 10045-1 and DIN 50115 also refer to U-notch testing, and many legacy or regional product standards still specify U-notch acceptance values. The absorbed energy is reported as KU, frequently with a subscript such as KU2 that identifies the 2 mm radius pendulum striker geometry defined in ISO 148-2.

Although the Charpy V-notch test has become the dominant method for modern structural steels, the U-notch test remains widely used for castings, forgings, thick weldments, and heterogeneous materials, and for products specified under standards inherited from earlier European and Eurasian practice. The results are comparative indicators of notch toughness under dynamic loading rather than direct design properties, and they are strongly influenced by specimen geometry, temperature, and material homogeneity.

Testing Samples

Standard U-Notch (Mesnager) Specimen

The standard specimen is 55 mm in length with a 10 mm × 10 mm square cross section. A U-shaped notch 5 mm deep with a 1 mm root radius is machined in the center of one face, leaving an unbroken ligament of 5 mm beneath the notch. The notch must be centered within close tolerances, and the machined surfaces must be free of grooves, burns, and markings that could act as unintended stress raisers.

Sub-Size Specimens

When the product thickness is insufficient for a full-size specimen, sub-size bars of 10 mm × 7.5 mm × 55 mm or 10 mm × 5 mm × 55 mm may be used, with the notch geometry maintained on the reduced section. Because absorbed energy depends on the ligament area, sub-size results cannot be compared directly with full-size values unless the applicable product standard defines reduced acceptance levels or scaling provisions.

Sampling Orientation and Location

Specimens are extracted at the locations and orientations defined by the product specification, commonly longitudinal or transverse to the principal working direction. For weldments, samples are cut so that the notch can be positioned in the weld metal, at the fusion line, or at defined points within the heat-affected zone, with the exact notch location verified by etching or macroscopic examination before testing.

Machining and Surface Quality

The notch is produced by milling, broaching, or grinding with sharp tooling and adequate coolant to avoid overheating, overheated or work-hardened layers, and inaccurate notch profiles. Dimensional verification of notch depth, root radius, and ligament thickness is performed before testing, and specimens are marked only on surfaces that do not participate in the fracture process.

Testing Items

Typical test programs based on U-notch specimens include the following items:

• Room-temperature U-notch absorbed energy (KU or KU2) as a routine quality-control value.
• Low-temperature impact toughness at specified temperatures such as 0 °C, −20 °C, −40 °C, −50 °C, or −60 °C for cryogenic and outdoor service assessments.
• Elevated-temperature impact tests for components operating at high service temperatures.
• Ductile-to-brittle transition curves obtained by testing a series of specimens across a temperature interval, from which transition temperature criteria are derived.
• Percentage of ductile (fibrous) fracture appearance on the broken surfaces, determined by visual comparison with reference charts.
• Lateral expansion measured behind the notch as an indicator of ductile deformation capacity.
• Instrumented impact parameters, including force–displacement curves, crack initiation energy, and crack propagation energy.

Product specifications typically require the average of three specimens to meet a minimum absorbed energy, for example 27 J, 39 J, or 47 J, at a designated test temperature, with defined limits for individual values and provisions for retesting.

Testing Methods

Equipment and Calibration

Testing is performed on a pendulum impact machine with capacities commonly ranging from 150 J to 500 J. The specimen is supported on two anvils spaced 40 mm apart and struck opposite the notch by the pendulum striker, which has a 2 mm nose radius in standard ISO practice. Machines are verified at regular intervals in accordance with ISO 148-2 or ASTM E23 using certified reference specimens, and friction and windage losses are corrected as part of the energy determination.

Temperature Conditioning

Specimens are conditioned to the test temperature in a liquid bath, a heated medium, or a gas environment, with the temperature controlled within approximately ±2 °C. For 10 mm thick specimens, a soaking time of at least 5 minutes in a liquid medium or about 30 minutes in a gaseous medium is applied. After removal from the conditioning medium, the specimen must be positioned and broken within 5 seconds so that the notch region does not drift from the target temperature.

Test Execution

The procedure follows a defined sequence of steps:

• The specimen is placed symmetrically on the anvils with the notch centered in the support span and facing away from the direction of striker travel.
• The pendulum is released from its raised position without initial velocity and fractures the specimen in a single blow.
• The absorbed energy is read from the machine scale or digital indicator, and any friction or windage correction is applied.
• The result is checked against machine capacity; readings above roughly 80 percent of capacity, or at the extreme low end of the scale, are treated with caution and retested on a higher or lower capacity machine when necessary.

Post-Test Evaluation

After fracture, the two specimen halves are examined to determine the fracture character. The proportions of fibrous ductile and crystalline brittle fracture are estimated, lateral expansion is measured with a suitable gauge, and any anomalies such as casting defects, inclusions, or unbroken ligaments are documented because they can invalidate the energy value.

Instrumented Impact Testing

An instrumented striker fitted with strain gauges records the force–displacement history during the impact event. This allows the total absorbed energy to be separated into elastic, crack-initiation, and crack-propagation components, which is particularly valuable for research on weldments, thick sections, and materials exhibiting mixed fracture behavior.

Applications

The U-notch impact toughness test supports a broad range of industrial and research objectives:

• Acceptance testing of pressure vessel plates, boiler components, and large forgings under legacy and regional specifications that specify KU values.
• Quality control of cast steels and cast irons, where the blunt notch averages the behavior of heterogeneous microstructures.
• Weld procedure qualification and production weld testing, including targeted sampling of weld metal, fusion line, and heat-affected zone.
• Structural, pipeline, and bridge components specified under standards derived from earlier European and Eurasian practice.
• Fabrication validation, including verification of heat treatment, normalization, quench-and-temper, and stress-relief conditions.
• Research into temper embrittlement, strain aging, hydrogen embrittlement, and ductile-to-brittle transition behavior.
• Screening of materials for low-temperature and cryogenic service environments.

Because the U-notch imposes a milder stress concentration than the V-notch, it is often preferred when the intent is to characterize the bulk toughness of coarse or non-uniform material rather than to detect small differences in notch sensitivity. Conversely, when the objective is to rank modern structural steels for resistance to brittle fracture initiation, the sharper V-notch generally offers greater discrimination.

FAQ

What is the difference between the U-notch and V-notch impact tests?

Both tests use the same 10 mm × 10 mm × 55 mm pendulum setup, but the V-notch is 2 mm deep with a sharp 0.25 mm root radius and a 45° included angle, whereas the U-notch is 5 mm deep with a blunt 1 mm root radius. The V-notch creates a more severe stress concentration and lower, more discriminating energies; the U-notch yields higher energies and is gentler on heterogeneous materials such as castings and thick welds.

What does a KU2 result mean?

KU2 denotes the absorbed energy measured on a U-notch specimen using a pendulum striker with a 2 mm nose radius, as standardized in ISO 148-2. The value is expressed in joules and typically represents the average of three specimens tested at the specified temperature.

Why is the U-notch preferred for castings and thick welds?

The blunt 1 mm radius notch is easier to machine reproducibly and is less sensitive to local microstructural variations, inclusions, or minor defects. This makes the U-notch well suited to materials whose toughness varies locally, allowing a more representative measure of average material behavior.

Over what temperature range can the test be performed?

With suitable conditioning equipment, tests can be carried out from cryogenic temperatures achieved with liquid nitrogen down to about −196 °C, through ambient, and up to several hundred degrees Celsius for elevated-temperature evaluation, subject to the limits of the conditioning medium and handling time.

How many specimens are required for one test condition?

Most standards and specifications require a set of three specimens per condition, with both individual values and the average reported. Retest provisions apply when a single low value occurs while the average still satisfies the specified minimum.

Can U-notch results be converted to V-notch values?

No universally accepted conversion exists between KU and KV energies because the two notch geometries produce different stress states and ligament sizes. Some empirical correlations exist for specific steel grades and strength levels, but they must not be used for specification compliance without explicit agreement between the purchaser and supplier.