Sheet and strip metals are pressed, drawn and stretched into everything from kitchen sinks to car panels and electrical enclosures. A tensile test tells you strength and elongation, but it does not fully represent how a sheet behaves when it is stretched over a punch in two directions at once. The Erichsen cupping test is a simple, fast way to assess that behaviour, and it is widely used for incoming material checks and production control.
What the test measures
The Erichsen test measures the ability of a sheet to undergo plastic deformation by stretch forming. A hemispherical punch is pushed into a clamped sheet until a crack appears through the thickness. The depth of the cup at the moment of cracking, in millimetres, is the Erichsen index (IE). A higher index means the material can be stretched further before it fails.
The method is specified internationally in ISO 20482. Indian standards for sheet products may also reference cupping tests; check the specific product standard or customer specification you are working to for the edition and requirements.
Test principle
- A test piece is clamped firmly between a die and a blank holder so that it cannot draw in from the edges.
- A punch with a hardened, polished spherical end is driven against the sheet, stretching it into a dome.
- The punch advances steadily until a crack appears through the sheet, visible as a sudden drop in force or observed directly through a mirror or camera on the machine.
- The punch travel at that point is read as the Erichsen index.
Because the edges are clamped, the material in the dome is stretched biaxially. This is closer to the deformation in many pressing operations than a uniaxial tensile test.
Specimens
- Test pieces are flat, clean and free of burrs, and wide enough to be clamped with sufficient margin around the punch, as the standard requires for the tooling size used.
- The standard tooling suits a defined thickness range; thinner or thicker sheets and narrow strip need the alternative tooling sizes defined by the standard.
- Indentations should be spaced far enough from each other and from the edges that one test does not affect the next.
- The punch and the sheet surface are lightly lubricated as specified, because friction changes the result.
Factors that affect the result
| Factor | Effect |
|---|---|
| Sheet thickness | The index generally changes with thickness, so compare like with like |
| Lubrication | Less friction usually increases the index |
| Clamping force | Insufficient clamping lets material draw in and inflates the index |
| Punch speed | Must stay within the range set by the standard |
| Crack detection | Late detection overstates the index |
Reading and reporting results
Report the Erichsen index as the mean of the valid tests, with the number of tests, sheet thickness, tooling used, lubrication and any deviation from the standard. It is good practice to keep the tested cups. The shape and orientation of the crack can reveal directional properties of the sheet: a straight crack often indicates strong anisotropy, while a circular crack around the dome suggests more uniform properties.
Results are most useful as comparisons: between batches of the same grade and thickness, between suppliers, or before and after a change in annealing or rolling practice. Comparing the index of different thicknesses or very different materials directly can be misleading.
Industries that use it
- Steel and non-ferrous sheet producers, for production control
- Automotive and appliance pressing shops, for incoming material checks
- Utensil and hollowware manufacturers working with stainless steel and aluminium
- Coating and paint labs, where the test reveals how well a coating survives deformation
Choosing an Erichsen testing machine
When selecting a machine, consider the sheet thickness range and materials you test, whether you need manual or motorised punch drive, how cracks are detected and how the index is read, and whether you need data recording. Akuracy offers an Erichsen testing machine; the Erichsen testing machine collection shows the current range. Tell us your materials and thicknesses so we can recommend a configuration.
A typical test sequence in the lab
A simple routine keeps results comparable from day to day. Cut test pieces from the same location in each coil or sheet, label them with the batch and rolling direction, and deburr the edges. Measure thickness with a micrometer at the test location. Check that the punch surface is clean and undamaged, because scratches on the ball act like added friction. Apply the specified lubricant to both the punch and the sheet in the same way every time.
Clamp the test piece with the specified holding force, then advance the punch at a steady speed within the standard's range. Watch for the first through-thickness crack, using the mirror or camera, or the force drop if your machine records force. Stop the punch immediately and record the depth. Repeat for at least the number of tests your specification requires, and discard any test where the crack started at a defect or edge.
Keeping control charts of the Erichsen index for each grade and thickness turns a single test into a useful process monitor, showing gradual changes in annealing or rolling before they cause splits in the press shop.
Frequently asked questions
What does a higher Erichsen value mean?
A higher Erichsen index means the sheet stretched further before cracking, indicating better stretch formability under the same test conditions.
Can Erichsen results replace a tensile test?
No. The two tests measure different things. Tensile tests give strength and elongation; the Erichsen test assesses biaxial stretch formability. Many specifications ask for both.
Why must the sheet be clamped firmly?
If the sheet draws in from the edges, the dome forms partly by drawing rather than stretching, which raises the index and makes results incomparable.
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.