Science of Engineering Materials

Equipment required:

Rolling mill

Hardness tester

Calipers

Materials required:

Annealed copper sheet

Suggested reading:
Textbook - Pg. 163-166

Introduction

Cold work means to mechanically deform a metal at a temperature below the recrystallization temperature. While serving to produce a stronger product, an important feature of cold-working is that the metal becomes more difficult to deform as the extent of deformation increases, i.e., the material is work hardened or strain hardened.

Plastic (or permanent) deformation is caused by the slip of atomic planes over each other. Slip is greatly facilitated by dislocations, which are one-dimensional or line defects in the crystal structure of the material. On the other hand, a dislocation hinders the motion of another dislocation. Cold working generates dislocations that serve as such obstacles. In fact, cold working generates so many dislocations that the configuration is referred to as a forest of dislocations. Thus, although dislocations cause a metal to be malleable, the increase in the density of dislocations makes it more difficult to deform the metal, which therefore becomes harder.

The amount of cold work is defined relative to the reduction in cross-sectional area or thickness of the metal by processes such as rolling or drawing. The hardness and strength of metals are increased with increasing amount of cold work, a process termed strain hardening. The density of dislocations in metal that has not been cold worked (annealed state) can be as low as with a correspondingly low hardness. A heavily cold worked metal can have a dislocation density of with a significantly higher hardness and strength.

The objective of this experiment is to determine the increase in hardness as a function of the amount of cold work. (In another experiment we do the reverse and undo the effects of cold work by annealing the metal.)

References

Background information on this experiment can be found in the following books:

Shackelford, Introduction to Materials Science for Engineers, Chapter 6, (2nd Ed.).

Van Vlack, Elements of Materials Science and Engineering, Chapter9, (6th Ed.) or Chapter 5 (5th Ed.).

Smith, Principles of Materials Science and Engineering, Chapter 6, (1st Ed.).

THE EXPERIMENT

The experiment is to be performed on the copper sheet that is provided to you. You should start out by measuring accurately the dimensions of the sheet and its hardness. The length and the width of the sheet can be measured with a ruler, but the thickness should be determined with calipers. The hardness should be measured with a Rockwell hardness tester. Be sure to read the instructions of the hardness tester before using it. Also read the section on hardness testing in these instructions before taking hardness data.

The thickness of the metal sheet should be reduced to about 40% of its original value in 5 steps of rolling. If the original thickness is d mm, the thickness should be reduced to about 0.95d mm in the first step. and to 0.9d, 0.8d, 0.65d, 0.5d and 0.4d mm in the subsequent steps. After each cold rolling step the hardness of the material is to be determined. At least three measurements should be made for each hardness determination.

The amount of cold work can be calculated as follows.

where Ao is the original cross-section of the sheet, A1 the cross sections after one step of cold work, d0 the original thickness of the sheet, d1 the thickness after one step of cold work.

In this experiment, the width, w, of the sheet is assumed constant (w=wo=w1= ......), then coldwork can be calculated by the change of thickness of the sheet during cold working as follows:

The hardness should be plotted as a function of the amount of cold work.