Ductility can be defined as the ability of a material to deform plastically before fracture. Many methods can be used for measuring the draw-ability of metal sheets, for example, bending, scratching, coining drawing, etc. One of them is the Erichsen cupping test for sheet metal.
Erichsen Cupping test for sheet metal includes placing the metal sheet in the machine and punching a cup into the sheet until a crack appears. Then the depth of the cup is measured and eventually, ductility is measure as a function of this depth.
Erichsen cupping test standards
There are two most commonly used Erichsen cupping test standards for reference purposes. One standard is ASTM E 643-78 and the other is ISO 20482. While performing a deep drawing test for research and standard testing purpose, it is in the best interest to follow standard guidelines given in standards.
Erichsen Cupping test Theory
The first thing to understand while searching for Erichsen cupping test theory in principle. The principal of the Erichsen cupping test for sheet metal is simple. Forcing a hemispherical punch into the metallic sheet until a crack appears and at that point measuring the depth of cup as the circular scale shows. Harder the metal, with fine grain structure, will result in lesser depth and fine shape of a cup. On the other hand, ductile metal, with coarse grain size, results in deeper indent with a rough cup shape.
The principle diagram of the Erichsen cupping machine is shown below. It consists of a die containing hollow space to move punch. A clamping system to clamp the metallic sheet or steel sheet in between two frames of a die. A mirror is placed behind the punch and sheet to observe crack and to know up to where the force is to apply on a punch. The punch is moved into the plain sheet by moving the wheel. The scale is attached to the wheel to measure the depth of indent in millimeters.
Working is as simple as described in the principal. First of all, a metallic sheet is placed between the die and is clamped. The thickness of the metallic sheet is important while performing the Erichsen cupping test for sheet metal according to international standards. Typically the thickness of the sheet is 1mm. when clamped, the wheel is moved until it touches the sheet. After that, the scale attached to the wheel is set to zero and then a force is applied on the sheet through cupping punch.
The punch deforms the metallic sheet. Indentation depends upon the ductility of material which is materials property. Punch is moved into the sheet until a crack is observed from the mirror behind the forcing unit. The movement of the wheel also moves the circular sliding scale. The units of scale are in mm. at the point of crack beginning. The measurement of the scale is noticed and then the sheet is unclamped. The measurement taken is the depth of indent which is then compared with the values given in the literature.
The properties that can be observed from this Erichsen cupping test can be,
Composition; composition of the metal is assumed from its drawability. Lesser the drawability, the more will be the addition of the alloying elements.
Mechanical properties; The strength of a metal is a strong component of its ductility. So strength can be assumed by this test in one sense.
Physical properties; for example, elastic modulus, dimensions, and requirements for maintaining shape.
How to perform an Erichsen Cupping test for sheet metal
For finding deep drawing ability of metal sheets, the Erichsen cupping test is used. Here, we have explained a simple procedure to perform the Erichsen cupping test.
For experimentation, following material and equipment is needed;
- Erichsen cupping machine
- Screw gauge
- Metal sheets of Aluminum, Brass, and Steel
We are going to give you a standard comparison of ductility among Aluminum, Brass, and Steel sheets.
1. Firstly, the thickness of the sheets was measured using a screw gauge.
2. Then the sheets were placed one by one at the place between ram and die.
3. The wheel was then rotated counterclockwise until the clearance of ram became 0.2mm.
4. Then the circular wheel was adjusted to zero.
5. The ram was locked by pushing the clutch to the right so that only die can move forward.
6. Then the wheel was rotated counterclockwise with a uniform velocity until the crack was appeared on the sheet, as seen from the mirror.
7. The cupping index was recorded from the circular scale.
8. The clutch was then pushed towards the left to unlock the ram.
9. Finally, the wheel was moved clockwise to proceed with other readings.
10. The ductility test of all three samples was carried out following the same procedure.
- The standard cupping indices for brass and aluminum are 10.06 and 7.98 respectively. The observed values of the samples deviate from the literature. The possible reason could be the personal error, alloying additions or impurity.
- The Erichsen cupping index for the Brass is higher because brass is an alloy of copper and zinc. And Zn has an FCC crystal structure which has greater drawability.
- The Erichsen cupping index for the Al is relatively high because it also has an FCC crystal structure. And FCC structure presents a high ability of deformation before fracture.
- The Erichsen cupping index for the steel (AISI 1040) is low because steel is an alloy of iron and carbon. And iron has a BCC crystal structure which shows less permanent deformation before fracture.