01. V-notch Specimen
The commonly used Charpy V-notch features a sharp notch root, with a typical included angle of 45° and a root radius of 0.25 mm. The sharp root creates intense stress concentration, making the material more prone to brittle fracture.
Application scenarios: Evaluating ductile-brittle transition characteristics of materials. It is specified for acceptance inspection of most steel products and low-temperature impact tests. Many structural steels and pressure vessel steels in Chinese standards require V-notch specimens.
Features: Highly sensitive to material brittleness. It demands high machining precision for the notch. Minor deviation of the root fillet will cause significant fluctuation of impact energy values.
02. U-notch Specimen
The Charpy U-notch has a rounded notch base with a standard root radius of 1 mm. It produces weaker stress concentration compared with the V-notch, and the specimen absorbs higher impact energy before fracture.
Application scenarios: Mostly applied for materials with good ductility, such as some non-ferrous metals and cast steel. U-notches were widely adopted in early impact testing.
Features: Test results show relatively lower scatter, yet it is insensitive to variations in material brittleness and cannot adequately indicate cold brittleness tendency.
Important Reminder Many users confuse U-notch specimens with large-radius curved impact specimens, and mistakenly label large-radius specimens as U-notch specimens - this is incorrect.
Although both have curved notch bottoms, they belong to two completely different specimen types:
U-notch specimen(Left Figure): A test specimen defined in GB/T 229, with a standard root radius of 1 mm. It measures the impact toughness of metallic materials for product acceptance.
Large-radius standard impact reference block(Specimens 3, 4, 5 and 6 from the left in the right figure): A calibration standard for metrological verification. Its notch radius is far larger than that of the U-notch. It is NOT used for material testing; it is specially designed to calibrate dynamic force and force-displacement curves of instrumented impact testers.
| Item | V-notch / U-notch Specimen | Large-radius Standard Impact Reference Block |
|---|---|---|
| Purpose | Test the inherent impact toughness of materials (product acceptance) | Calibrate the impact tester itself (metrological verification, not material testing) |
| Notch geometry | Sharp V-notch / rounded U-notch | Extra-large curved recess, no sharp root |
| Test subject | Submitted material samples | Impact testing machine |
| Machining & sourcing | Notch broached on the raw material of the test piece; custom-made specimens | Ready-made certified sets available for purchase, no on-site machining required |
| Interchangeability | V-notch and U-notch cannot replace each other; results are non-convertible | Shall not be used as test specimens for product inspection |
Supplementary note: V-notch reference blocks can be used for simple energy calibration of impact testers. Large-radius reference blocks are required for calibrating dynamic force measurement and impact waveform. The two types serve different calibration purposes. The root radius tolerance of V-notches is extremely strict. V-notches are harder to machine and cost more per specimen.
03. Summary of Core Differences
Stress concentration: V-notch > U-notch. V-notches are more likely to initiate brittle cracking.
Impact energy: For the same material at the same temperature, U-notch generally yields higher impact energy than V-notch.
Machining difficulty: V-notches have stricter requirements, with the root fillet as the critical controlled dimension.
Standard compliance: Test results from the two notch types cannot be converted or substituted. The notch type specified in the standard must be adopted.
For material testing, a set of 5 specimens is commonly used for ductile-brittle transition tests to calculate the average value. Routine product acceptance impact tests normally use 3 specimens per set. A single specimen carries high randomness and shall NOT be used to judge material conformance, which may lead to biased conclusions. Avoid testing only one specimen for convenience.
Though the notch seems to be a minor machining detail, it directly determines whether impact test data is valid.
