Necking: A Comparative Study of deformation in Polymers and Metals

Location:

Concepts Shown:

deformation in polymers and metals

Equipment:

The polymer and metal tensile specimens. Six pack can holders that can serve the purpose of a 'brown bag' experiment.

Procedure:

Pass out the tensile specimens of the polyethylene and the polycarbonate so that the students can see how the neck propagates along the entire gage length of each polymer. Additionally, passing around the aluminum tensile bars will allow the students to compare this neck propogation in polymers to the thinning of the neck in the metal specimen.

Science:

Amorphous polymers preclude plastic deformation by slip. However, such deformation is the result of three distinct flow mechanisms. These are (1) homogeneous flow, (2) inhomogeneous shear band formation, and (3) crazing. Flow is caused by chain displacements and the conversion of the initially unaligned chains to a configuration in which the long axis of the molecule is preferentially aligned with the tensile axis. Strain hardening, or tensile strength is concurrent with the onset of the flow in which a neck forms and propagates along the gage length. If the neck traverses the entire gage length, there is additional hardening before eventual fracture. Unlike in a metal, once a neck is formed in a polymer, further deformation is not restricted to the necked region. In fact, the higher stress in the neck area accelerates the molecular alignment, which is accompanied by strengthening of the material in the neck in comparison to that material outside this volume. Strengthening results from the fact that as molecular alignment takes place, the stress in the neck region is supported by primary covalent bonds. The usual method of plastic deformation in metals is by the sliding of blocks of the crystal over one another along definite crystallographic planes, called slip planes. Crudely put, the slip of the crystal can be considered analogous to the distortion produced in a deck of cards when it is pushed from one end. Metals are different from polymers in that they are not long-chained macromolecules. Necking usually begins at maximum load during the tensile deformation of a ductile metal. An ideal plastic material in which no strain hardening occurs, would become unstable in tension and begin to neck just as soon as yielding took place. However, a real metal undergoes strain hardening, which tends to increase the load-carrying capacity of the specimen as deformation occurs. As opposed to alignment of molecules in polymers, deformation in metals takes place due to the movement of dislocations. When the metal begins to neck, dislocations line up in the neck region and hinder further dislocation movement. Such hindrance causes strain hardening. Following necking, the effective true tensile stress in the necked region is greater than without it. Furthermore, additional deformation is restricted to the neck region alone. As a result, there is a significant tapering of the neck witnessed. Remarks: This is a fairly inexpensive presentation. The tensile specimens do not have to be made. They can be found with Prof. Bigelow. In addition to passing out the tensile bars, one could also pass out a kind of 'brown bag' experiment using six pack holders. Each student would get one with the initial gage length marked. AT home he/she can see how necking propagates along the marked gage length. Six pack holders should be available at any grocery store. [eq].

References:

Author:
Rahul Pinto
Credits:


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