The U-notch impact test is a standardized pendulum impact method used to determine the impact toughness and absorbed energy of metallic materials, especially steels, castings, and welded joints. This guide explains the specimen geometry, applicable standards such as ISO 148 and GB/T 229, testing procedures, and typical industrial applications of U-notch (Mesnager) impact testing.
Technical Overview
The U-notch impact test, also known as the Charpy U-notch or Mesnager impact test, is a dynamic fracture test in which a notched bar specimen is broken by a single swing of a pendulum striker. The test measures the energy absorbed by the specimen during fracture under impact bending, expressed in joules (J) as KU2 or KU5, depending on the notch depth. Because the U-shaped notch has a blunt root radius of 1 mm, it produces a less severe stress concentration than the sharp V-notch, making it particularly suitable for evaluating coarse-grained materials, steel castings, forgings, and products specified under legacy Chinese and Eastern European standards.
The test is governed by several international and national standards, including ISO 148-1 (Metallic materials — Charpy pendulum impact test), GB/T 229 (Chinese national standard, which retains U-notch specimens), DIN EN ISO 148-1, and GOST 9454. ASTM E23 primarily specifies V-notch specimens but is frequently referenced for pendulum machine calibration and general impact testing practice. Test results are used to assess a material's resistance to brittle fracture, to screen heat treatment quality, and to determine the ductile-to-brittle transition behavior of ferritic steels at reduced temperatures.
Testing Samples
Standard U-Notch Specimens
The standard specimen measures 10 mm × 10 mm × 55 mm and is machined with a U-shaped notch of 1 mm root radius. The two principal variants are the KU2 specimen, with a 2 mm notch depth leaving an 8 mm ligament, and the KU5 specimen, with a 5 mm notch depth leaving a 5 mm ligament. The absorbed energy is reported accordingly as KU2 or KU5. The notch must be machined perpendicular to the specimen faces, free of grinding burns, scratches, and machining marks parallel to the notch root.
Metallic Materials
- Carbon steels, low-alloy steels, and high-strength structural steels
- Steel castings, forgings, and heavy sections where coarse microstructures are present
- Stainless steels and heat-resistant steels, particularly duplex and ferritic grades
- Cast irons and nonferrous alloys including aluminum, copper, titanium, and nickel alloys
- Fasteners, bars, plates, tubes, and profile sections sampled per product specifications
Welded Joints
Specimens may be removed from welded test panels with the notch located in the weld metal centerline, the fusion line, or the heat-affected zone (HAZ) at defined offsets. U-notch specimens are sometimes preferred for weld procedure qualification of thick sections and cast weldments because the blunt notch averages toughness over a larger volume than a sharp V-notch.
Sub-Size Specimens
When product thickness does not permit full-size bars, sub-size specimens such as 10 mm × 7.5 mm × 55 mm or 10 mm × 5 mm × 55 mm may be tested, and the absorbed energy requirements are reduced proportionally in accordance with the governing specification.
Testing Items
The following parameters are typically measured and reported for U-notch impact testing:
- Absorbed energy KU2 or KU5, in joules, corrected for friction and windage losses
- Impact toughness value (ak, in J/cm²), calculated as absorbed energy divided by the net cross-sectional area at the notch, as required by some product standards
- Ductile-to-brittle transition temperature (DBTT), determined from a test series across a temperature range
- Fracture appearance, including the percentage of fibrous (ductile) versus crystalline (brittle) fracture surface
- Lateral expansion and, in instrumented testing, the separation of crack initiation energy and crack propagation energy
- Temperature of the specimen at the moment of fracture
Testing Methods
Equipment and Calibration
Testing is performed on a pendulum impact machine with capacities commonly ranging from 150 J to 450 J or higher, featuring an 8 mm radius striker and support anvils with a 40 mm span, verified in accordance with ISO 148-2 or ASTM E23. The machine must be periodically verified using reference specimens and direct verification of key dimensions, and the absorbed energy must fall within 80 percent of the machine capacity to ensure acceptable resolution.
Specimen Preparation
Notches are produced by milling, broaching, or grinding with the notch root carefully finished to a 1 mm radius. Specimen marking must avoid the notch and contact surfaces. Orientation and location of sampling, such as longitudinal or transverse relative to the rolling or forging direction, are defined by the product specification or the purchaser.
Temperature Control
For tests below or above room temperature, specimens are conditioned in a bath or furnace until the target temperature is uniform throughout the section. Common media include liquid nitrogen-cooled alcohol for temperatures down to approximately -180 °C, ice-water mixtures at 0 °C, dry ice baths around -70 °C, and heated oil or air furnaces for elevated temperatures. Specimens must be broken within 5 seconds of removal from the medium to avoid temperature drift.
Test Procedure
- Verify machine calibration status and select a pendulum capacity appropriate to the expected energy
- Condition the specimen to the specified temperature and record the medium temperature
- Center the specimen on the anvils using a centering gauge so the notch faces the striker and is aligned with the midpoint of the span
- Release the pendulum from the locked position and allow it to strike and fracture the specimen in a single blow
- Read and record the absorbed energy, apply friction and windage corrections, and inspect the fracture surface
- Repeat for the specified number of specimens, commonly three per condition, and report individual and averaged values
Instrumented Impact Testing
Instrumented strikers fitted with strain gauges record the force-deflection curve throughout the impact event, allowing separation of crack initiation and propagation energies, determination of general yield force, and detailed characterization of transition behavior valuable for research and failure analysis.
Applications
- Pressure vessels, boilers, and piping: verifying notch toughness of plates, forgings, and weldments for safety-critical service
- Steel castings and heavy forgings: acceptance testing under specifications where the U-notch geometry is standard for coarse-grained products
- Shipbuilding and offshore structures: evaluating hull steels and welded joints for low-temperature service
- Railway components: assessing rails, axles, couplers, and bogie castings for resistance to brittle fracture
- Mining, cement, and heavy machinery: quality control of wear-resistant and high-manganese steel castings
- Welding procedure qualification: testing weld metal and HAZ toughness across specified temperature ranges
- Material research: mapping ductile-to-brittle transition curves and studying effects of composition, heat treatment, and embrittlement phenomena
FAQ
What is the difference between U-notch and V-notch impact tests?
The main difference is notch geometry. A V-notch has a 45° included angle with a 0.25 mm root radius and 2 mm depth, creating a sharp stress concentration, while a U-notch has a 1 mm root radius and a depth of 2 mm or 5 mm, creating a blunter, less severe constraint. V-notch tests generally yield lower absorbed energies and are dominant in Western specifications, whereas U-notch tests remain common in Chinese, Russian-influenced, and casting-related specifications.
Why choose a U-notch specimen instead of a V-notch?
The U-notch is preferred when the material is coarse-grained or heterogeneous, such as steel castings and thick forgings, where a sharp V-notch may sample too little material volume or cause premature, non-representative fracture. It is also required when the governing product standard or contract specifies KU2 or KU5 values, particularly for legacy designs and specifications originating from Chinese and Eastern European standards systems.
What do KU2 and KU5 mean?
KU denotes absorbed energy for a U-notch specimen, and the number indicates the notch depth in millimeters. KU2 corresponds to a 2 mm deep U-notch with an 8 mm ligament, while KU5 corresponds to a 5 mm deep U-notch with a 5 mm ligament. Both are reported in joules.
How is impact toughness in J/cm² calculated?
Some specifications require the impact toughness value ak, obtained by dividing the absorbed energy in joules by the net cross-sectional area of the specimen at the notch in square centimeters. For a standard 10 mm × 10 mm KU2 specimen, the net area is 0.8 cm².
What temperature range can be tested?
U-notch impact tests are routinely performed from approximately -196 °C, using liquid nitrogen cooling, up to several hundred degrees Celsius using heated furnaces, enabling full characterization of the ductile-to-brittle transition behavior of ferritic steels.
How many specimens are required per test?
Most specifications require three specimens per test condition, with individual values and the average reported. Additional specimens are tested when transition curves are being established or when retesting is permitted after an outlier result.