In an impact test, the notch is where the specimen breaks, and it is designed to concentrate stress in a defined way. If the notch geometry varies, the absorbed energy varies, even when the material is identical. That is why labs that run Charpy and Izod tests in volume use a dedicated notch broaching machine rather than cutting notches by hand or on a general-purpose milling machine.
Why notch geometry changes results
The standard Charpy V-notch specimen, as defined in ISO 148-1 (IS 1757 Part 1) and ASTM E23, is a 10 mm × 10 mm × 55 mm bar with a 45° V-notch, 2 mm deep, with a 0.25 mm root radius. The root radius and depth control how sharply stress is concentrated:
- A larger root radius lowers stress concentration and tends to raise absorbed energy.
- A shallower notch leaves more material under the notch and raises energy.
- An off-centre or skewed notch changes the fracture path.
- Machining marks or heat damage at the notch root can create a premature crack starter or alter the local material.
Near the ductile-to-brittle transition temperature, results are especially sensitive, so small notch errors can push a result from pass to fail.
Broaching compared with milling
| Aspect | Broaching | Milling or grinding |
|---|---|---|
| Process | A tool with progressively deeper teeth cuts the full notch in one stroke | Form cutter or wheel removes material in passes |
| Consistency | Geometry fixed by the broach; highly repeatable | Depends on set-up and cutter wear |
| Speed | Fast, suited to batch work | Slower per specimen |
| Heat input | Low | Risk of heating if not cooled |
Both methods can produce compliant notches if controlled well. Broaching is popular in production and third-party labs because it removes much of the operator variation.
Checking notch dimensions
Whatever method you use, the notch must be verified. A notch projector or optical comparator shows the notch profile magnified against a template, so the angle, depth and root radius can be checked against the tolerance. Many labs check the first specimen of each batch and then at intervals, and keep a record. When the broach starts to wear, the root radius and surface finish drift; regular checks show when to resharpen or replace the tool.
Charpy and Izod notches
Charpy specimens are supported at both ends and struck behind the notch. Izod specimens are clamped vertically as a cantilever and struck on the notched face above the clamp. Notch forms are defined in the relevant standards for each test, and some products specify U-notches instead of V-notches. Check which broach profile you need for each specimen type and whether one machine can take interchangeable broaches for different notch types.
Matching the notcher to your impact tester
The notching machine and the impact tester work as a system. Consider:
- The specimen sizes your impact tester accepts, including sub-size specimens for thin products
- The notch types your customers specify
- Your daily specimen volume
- Whether you test at low temperatures, where notch accuracy matters most
Akuracy offers a notch broaching machine alongside its Charpy impact testing machine and the digital impact testing machine RI-300 E. See the impact testing machine collection for the full range.
Maintenance and broach life
- Keep the broach clean and lubricated as recommended.
- Clamp specimens firmly and squarely in the fixture before every stroke.
- Track the number of notches cut and the notch-check results to plan resharpening.
- Store spare broaches protected from damage.
Specimen preparation before notching
Good notches start with good blanks. Specimens should be machined to size with square, parallel faces, because the notch is located relative to these faces and the specimen must sit squarely on the anvils. The orientation of the specimen relative to the product, such as longitudinal or transverse to rolling direction, and the notch direction must follow the specification, since they can change results significantly. Mark each specimen clearly at an end away from the notch and fracture zone.
Recording notch checks
A simple log of each batch, broach ID, number of notches since resharpening and the measured notch depth and root radius gives traceability and shows when a broach is approaching the end of its useful life. If impact results ever come into question, this record lets you show that notch geometry was under control.
Safety
Keep hands clear of the broach path, use the guards supplied and switch off before clearing chips. Broach teeth are sharp; handle and store them carefully.
Key takeaways
- Notch depth, angle and root radius directly affect absorbed energy.
- Broaching gives repeatable notches with low heat input, suited to batch work.
- Check notches on a projector at the start of each batch and after tool changes.
- Match broach profiles to the notch types your standards require.
Frequently asked questions
Can one broaching machine cut both V and U notches?
Some machines accept interchangeable broaches for different notch forms. Confirm the profiles you need when you request a quote.
How often should notches be checked?
Check the first specimen of each batch and at regular intervals, and always after changing or resharpening the broach.
Does notching heat affect results?
It can. Excessive heat at the notch root may alter the local material. Broaching produces little heat, and milling should be cooled.
Request a quote
Akuracy (Ratnakar Enterprises) can advise on a configuration for your materials, standards and test volume. Share your specimen details and the standard you test to, and our team will reply with a suitable option. Request a quote online or message us on WhatsApp at +91 94038 91579.