Two Charpy bars cut from the same plate can absorb noticeably different energy, and the reason is often the notch rather than the steel. Impact testing is unusually sensitive to how the specimen is made, because the whole test is built around a small notch held to tight tolerances. Prepare that notch loosely and the number moves.
The stakes are why the method is so controlled. Impact energy is often used to guard against brittle fracture, to qualify a steel or a weld for low-temperature service, or to sign off a heat before it ships. A specimen prepared outside tolerance can fail a good material or pass a poor one, and neither outcome is cheap to unwind.
Impact Testing in Brief
An impact test measures how much energy a notched bar absorbs when a swinging pendulum breaks it. In a Charpy test the bar sits as a simply supported beam and is struck behind the notch. In an Izod test the bar is clamped as a cantilever and struck above the notch. Both report absorbed energy, and both depend on a notch prepared to standard.
The number that comes out is absorbed energy, reported in joules or foot-pounds. A tough, ductile material bends and tears and soaks up a lot of energy, while a brittle one snaps and absorbs little. Some specifications also record the lateral expansion at the back of the bar and the percentage of shear on the fracture face, which describe the same behavior in other terms. All of those readings assume the bar and its notch were made to the method.
Steels in particular do not give a single impact number but a curve. Energy is low and the fracture brittle at cold temperatures, high and ductile when warm, with a transition region in between. That is why a specification usually fixes a test temperature, or asks for a set of bars across a range, and why preparing a matched set is what lets that curve be built from comparable specimens.
The Notch Is What You Are Testing
Unlike a tensile coupon, an impact specimen concentrates the whole test at one feature. The notch sets the stress state at its root, so the depth, the angle, and the root radius shape how the crack starts and how much energy the break takes. A root radius that is slightly too large, or a notch slightly shallow, shifts the result, so the standards place tight tolerances on the notch rather than on the bar as a whole.
That is what makes impact work notch-sensitive in a way tensile testing is not. A tensile specimen spreads its load over a gauge length, so a small surface feature is one factor among many. An impact bar puts almost everything at the notch root, so the geometry there is not a detail, it is the measurement. For that reason the notching step is treated as a controlled operation with its own tolerances and inspection.
Charpy V-Notch, Charpy U-Notch, and Izod
The common impact specimens differ in the notch and in how they are gripped and struck. The table lays them out.
| Specimen | Notch | How It Is Loaded | Where It Is Used |
|---|---|---|---|
| Charpy V-notch | A 45 degree V, nominally 2 mm deep with a small root radius | Simply supported, struck behind the notch | The most common metals impact test |
| Charpy U-notch or keyhole | A rounded or keyhole notch, cut deeper than the V | Simply supported, struck behind the notch | Older and specification-specific work |
| Izod | A V-notch on a bar clamped upright | Cantilever, struck above the notch | Some metals and many plastics |
Choosing among them is rarely a free choice. The governing specification usually names the specimen type and the notch, and the test machine and fixtures have to match. The Charpy V-notch dominates metals work today, U-notch and keyhole forms appear in older or specific specifications, and Izod is common where a clamped cantilever setup or a plastics method is called for.
What ASTM E23 Controls
ASTM E23 is the reference for notched-bar impact testing of metallic materials, and ISO 148-1 is its international counterpart. Between them they fix the bar dimensions, the notch geometry and its tolerances, the test setup, and how the pendulum machine itself is verified. The tolerances are the point of the method, since a notch only works as a controlled feature when every bar carries the same one. The governing numbers come from the current edition of the standard rather than from a handbook figure, so preparation is written to the standard in force.
The standard reaches past the specimen. It also covers how the pendulum machine is set up and verified, including checks against reference specimens so that two labs breaking identical bars should read close to the same energy. For steel products, ASTM A370 often points to E23 for the impact method while the product specification sets the acceptance values, the test temperature, and the specimen details. Reading the specification and the method together is how the preparation gets pinned down.
Cutting the Notch
The notch is cut, not molded, and making it repeatable is where preparation equipment earns its place. Blanks are first machined to the bar section and length, and then the notch is cut to the standard profile. The TensileMill CNC Classic Upgrade prepares flat tensile and impact blanks and pairs with a compatible notching setup to cut U- and V-notches associated with ASTM E23, while the MICRO handles impact blanks in a compact footprint. A dedicated impact clamping fixture holds a set of bars so the same notch is cut across a batch rather than one at a time.
Order matters in the sequence. The bar is brought to size and squareness first, since a notch cut into an out-of-square blank inherits that error, and the notch is cut last so nothing distorts it afterward. Cutting a full set in one fixture, rather than one bar at a time, is what keeps the notch consistent across the specimens that will be compared, which is the point of a batch of impact bars.
How the notch is cut also shapes the root. Broaching pulls a shaped tool through the notch in one pass and is common for production sets, while milling forms it with a rotating cutter, and either can meet the profile when the tooling and setup are right. What matters for the result is a clean root at the specified radius with no tearing, since the finish at the very bottom of the notch is where the crack starts.
Orientation, Location, and Sub-Size Bars
Where the bar comes from matters, since impact properties vary with rolling direction and position, and the specification states the orientation and the sampling location. When the material is too thin for a full-size bar, the standard allows sub-size specimens, and the reported energy is tied to the specimen size that was used. Marking the orientation and the size on each bar keeps the result traceable to how it was made.
Notch orientation is part of that record. The specification states which way the notch faces relative to the surface and the rolling direction, and a through-thickness or a surface notch can answer different questions about the same product. Getting the orientation wrong is not a small dimensional miss, since it can change what the test is measuring, so it is fixed before cutting rather than judged afterward.
Testing Cold and Hot
Many impact specifications call for a test temperature, since the ductile-to-brittle behavior of steels depends strongly on it. That does not change how the notch is prepared, but it does mean the bars are conditioned and transferred quickly, which puts a premium on having a full set prepared in advance so the temperature soak and the test stay on schedule.
Because the bar starts warming or cooling the moment it leaves the bath or oven, the standard limits the time between conditioning and impact, usually to a matter of seconds. That timing is a testing-side concern, but it feeds back into preparation, since a lab moving through a temperature series needs its full set of bars notched, marked, and sorted before the first one is soaked.
Common Notching Mistakes
Most impact preparation problems trace back to the notch or to an inconsistent set.
- A root radius outside tolerance, which changes the stress at the notch and the measured energy
- A notch cut off-depth or not square to the bar axis
- Burrs or tool marks left at the notch root
- Mixing specimen orientation or size within a set that should match
- Cutting one bar at a time, so the notch drifts across a batch
- Preparing the wrong specimen type or notch for the governing specification
Where Preparation Ends and Testing Begins
Preparation stops at the notched bar. The impact test itself needs a verified pendulum impact machine with the correct striker and capacity, which is a separate instrument from the preparation equipment. And the notch profile, its tolerances, and the test temperature come from the standard and the specification, not from what is convenient to cut.
Verification is part of that separation. ASTM E23 sets out how the impact machine is checked, and a lab relies on that verification rather than on the preparation equipment to trust the energy reading. The two sides meet at the specimen, so a bar that is correct and consistent is the most useful thing preparation can hand to the test.
Bringing Impact Preparation In House
For a lab that runs Charpy or Izod work, preparing bars in house keeps the notch under your control and the schedule in your hands. Choosing the setup starts with the specimen type, the material and thickness, and the standard the work follows. We build the TensileMill CNC machines that prepare flat tensile and impact blanks, so the useful help we can give is matching a preparation workflow to your specimens rather than pushing a single system.
The payoff is control and turnaround. A set of bars notched the same way removes one of the larger sources of scatter in impact data, and preparing them on your own schedule keeps a temperature series or a weld qualification from waiting on an outside shop. For labs with steady Charpy or Izod volume, that combination is usually what justifies bringing the step in house.
Send us your specimen type, material, thickness, and the standards you follow, and we will point you to the right setup. Request a quote or talk to a specialist to start.