The cupping test measures the deep drawing performance of materials. The test uses a spherical punch to drive the clamped sample into the die until penetration cracks appear. The determined cupping depth is the test result.
Usually, a cupping tester is used to do this test. After the cup test, the samples look like cups that have been punched (but only broken cups). If the steel plate doesn’t work well for deep drawing, it’s easy for the stamped part to break when it’s being made. The Erichsen cupping test is a way to find out how well sheet metal can be bent. It is the oldest and most common type of sheet formability test. During the test, a ball-end punch was used to push the metal sheet, whose edges were being pressed by the die and the blank holder, into the die until the top of the bulge cracked.
During the test, the punch pushed the metal sheet into a half-sphere-shaped bulge. Take the punching depth when cracks or necking appear at the top of the bulge as the test index. This is called the cupping value, or I value, and it is measured in millimeters. The maximum load is used to figure out the test index. If you can’t figure out the maximum load, you can use the punch’s penetration depth when visible (light-letting) cracks appear as a sign. But the value found by using the visible crack method is higher than the value found by using the maximum load method. This is because the method of looking for cracks is more accurate than the method of putting the most weight on it.
How does the automatic cupping tester work?
When starting the test, the instrument automatically clamps the test panel and uses a hemispherical junction. The punch pushes the test plate into the telescopic die at a consistent rate, forming a dome with the coating facing outward to measure the paint film’s elasticity and read the indentation depth from the running interface. On the magnifying glass display screen, watch as the coating on the sample plate cracks or falls off the substrate, and then analyze the results.
Using the cupping tester to discover metal fractures
The cupping test determines if metal can be produced without breaking. The cupping test fixes the sample’s edge with an extrusion die and an anti-wrinkle pressure plate. The metal is deformed mostly in the center, and just a little section of the edge glides into the extrusion die hole. Two-dimensional tensile stress on the plate’s surface can cause it to change shape, usually by making the area of the plate’s surface bigger in the middle.
Reduced center thickness can cause the sheet to shatter during forming. Many factors determine how far you can cup based on cupping test results.
The principle of metal cupping testing
The cupping test involves squeezing a metal plate or strip sample with a steel ball or spherical punch until it cracks. The plate or strip’s cupping is measured in millimeters. Metals’ plastic deformation properties are assessed using the cupping value.
Preparation of test samples for metal materials
- The thickness of the testing sample of metal is the same as the thickness of the real metal.
- When cutting the metal material test sample, the sample must be kept flat, and its edges must not have burrs and distortions.
- The metal material testing sample should be kept in its original state and should not be subjected to cold or hot processing. It should also not be hammered.
- The samples cut from the coil are tested without straightening.
What you need to know before the cupping test
- The requirements before the cupping testare:
- The operator must understand thecupping testing machine’s principles and structure.
- Learn the function of each knob and button
- The clamping force control valve must be set to the minimum pressure before each cupping test.
- The inner diameter of the pad die, the die, and the diameter of the punch are selected according to the table.
- Before the cupping test, the working part of the sample in contact with the punch and the top of the spherical surface of the punch must be coated with non-corrosive lubricating oil to reduce the influence of the friction between the sample and the punch on the test results.
- Install the sample between the die and the cushion die, and rotate the die to clamp the sample with a clamping force of about 10 kN. The die was rotated by 0.05 mm according to the dial, and then the test was carried out.
- During the cupping test, align the dial at the zero point, and then apply pressure evenly and slowly until the pressure-bearing part begins to rupture. The reading on the scale when the crack started to occur is the cupping value. Cupping value readings are accurate to 0.1mm.
- The forward speed of the punch should be controlled at 5-20 mm/min.
- The cupping testis carried out along the center line of the strip sample. The distance from the center of the cup to the edge of the sample is half the width of the sample. When you put cups next to each other, the distance between the centers of each cup should be equal to the width of the specimen.
- Two or more cupping depths shall be recorded for the same material, and the arithmetic mean value shall be taken as the cupping depth of the material.
An Analysis of Influencing Factors of the Cupping Test for Metal Material Detection
1) The effects of die size and pad die size. Practice has proved that the wear of the pressing die and the pad die on the cupping machine has a great influence on the cupping test machine results, which can increase the test results by 0.4-0.6mm, but generally speaking, the wear process is slow, and the die can be replaced every few years.
2) The effect of punch wear. The wear of the punch can reduce the value of the cup protrusion, so it must be checked frequently and replaced in time if the wear is found.
3) The impact of pressing force. The size of the pressing force can affect the cupping value. With the increase of the pressing force, the cupping value gradually decreases, which is mainly caused by the fluidity of the material during the test. The greater the pressing force, the greater the positive pressure on the sample placed between the die and the pad die, and the greater the friction between the sample and the die. There is frictional resistance that will make it hard for the material to be plastically deformed, so the sample’s deformation is limited to a small area between the cushion die and the die. This means that the measured cupping value is very small.
4) The impact of the punch’s increasing speed. During the test, the rising speed of the punch must be slowed down when it is close to breaking; otherwise, the rising speed of the punch will increase the cupping value. With different punch speeds, the length and width of the cupping cracks are very different. The punching speed is fast, and the cupping value increases due to inertia. Therefore, the speed of the punch must be slow. Generally, it should be controlled at 5–20 mm/min.
5) Lubricant’s effect on cupping value. Different lubricants are used on the same material, and the measured cupping values are also different.
How to use the cupping test machine in detail
- Understand the structure, principle, and method of operation of thecupping test machine before understanding the function of each button.
- Select the cupping testmold according to the following table and measure the width of the sample. Record the thickness in the table. The relationship between the cupping test mold and the size of the sample, the width of the sample, the thickness of the sample, the diameter of the punch, the hole diameter of the upper mold, the backing plate aperture, etc.
- Install the mold: install the concave mold on the concave mold seat of the test machine, the male mold seat on the center piston, place the blank holder on the blank holder piston, and the convex stem and blank holder on the blank holder ring. The groove on the piston is the positioning part of the blank holder.
- Sample preparation: The cupping testis only applicable to metal sheets with a thickness of 0.2 to 2mm. The cupping sample blank on BHB-80A is square, with a width of not more than 90×90±1mm. The sample blank should be flat without scratches, and there should be no burrs on the edges.
- Clean the sample, lubricate the side of the sample in contact with the punch and the punch’s spherical surface, and place the sample on the blank holder. The square groove on the blank holder is the positioning line of the sample blank, and the sample blank can be placed on the line.
- Place the die holder on the testing machine in the die cylinder. Press the blank holder button and adjust the blank holder hydraulic handle to make the blank holder hydraulic pressure reach 2.6MPa (the blank holder force = 100±1.0KN).
- Examine the sample by raising the middle piston. Stop the center piston if a light-transmitting crack appears near the sample’s dome. Turn the electro-hydraulic controller’s knob to halt the center piston and reset the ammeter. Stop the edge from pressing the button. Turn on the oil pumping return switch, lift the die base, and remove the test piece.
- Repeat steps 5 to 8. Each material should be tested more than twice, and the arithmetic mean of the obtained cupping depth is taken as the cupping depth value of the material.
- After the experiment, remove the mold.
The cupping test is the oldest and most popular sheet formability test. Cupping tests are used for sheet forming performance research, stamping process simulation and parameter research and evaluation, welding bead detection and welding process index evaluation, coating performance testing and packaging material quality evaluation, etc. It’s one of the most essential simulation test methods for sheet metal stamping.