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Formability Test: Friction Test

The basic guidelines to select the friction test method are simplicity and similitude. Simplicity means the separation of frictional effects from other effects and similitude means how much the friction test is similar to the actual operation. The main two friction tests for sheet metal are: pinch type and draw bending type.


Pinch Style

In a pinch type friction test, a metal strip is pulled while applying the pressure. The tangential force divided by normal force is the friction coefficient.

In this method:

  • The interpretation is simple.
  • Similitude to the sheet metal forming is less.

 

Draw Bending Type

The other is a draw bending type.

In this method:

  • The interpretation is not so simple.
  • Similitude to sheet metal forming is more.

 

Capstan Friction Test

The capstan friction test is one of the draw bending type friction tests. The general procedures of this test are as follow:

1. Both ends clamping to grips;
2. Inlet end fix, outlet end move to desired tension;
3. Both ends move, measure friction.

 

From the measured inlet and outlet tension forces, the friction coefficient is calculated. The difference between these two forces comes from two sources, plastic bending and friction.

The bending force FB is calculated first according to Swift from the geometry and yield stress data. Here: s y is the yield stress, t is the strip thickness, small w is the width of the strip, r is the pin radius, and theta is the angle of wrap. Then the friction coefficient m is calculated by the equation under it. This equation was derived from the assumption that the strip behaves like a rope around a pulley except the bending force FB.

 


Surface of mill finished aluminum sheets

Draw bending friction test results

 

The mill finished aluminum sheets have the oriented roughness and the magnitude of friction is quite dependent on the sliding directions. For example, at this friction test at Alcoa, the friction coefficient in longitudinal direction was more than double of the friction coefficient in transverse direction. The capstan friction test is commonly used to measure the strain dependent friction of a metal strip because it can control the strain during the test.


Investigation of Strain Distribution During the Capstan Friction Test

Starting with uniform distribution, as the strip slides over the pin, the strip shows the non-uniform and transient strain distributions, and finally reaches the uniform and steady distribution. It can be said from the results that the steady condition was reached at time 1.01 sec. So, the steady condition was reached after the strip slid the distance, which is 1.5 times to the quarter circumference of the pin.