It can be seen from the graph that both the aluminium and the duralumin have the shape of the typical graph shown in Theory 1. However, the values for the yield stress and the ultimate tensile strength are very different for the two specimens.
The yield stress for aluminium is about a quarter that for duralumin, because there is far less resistance to the movement of the dislocations in pure aluminium. In the alloy the precipitates hinder the motion of the dislocations and a much higher stress is required to initiate slip.
Once slip has started the stress required for further plastic deformation increases a little, due to work hardening in the material. (For information on work hardening see Theory 3). However, as there is still greater resistance to dislocation movement in the duralumin due to the precipitates, the pure aluminium specimen necks for longer.
The ductility in the pure aluminium specimen is so high that the specimen only breaks once the neck has become very narrow, as seen in the photos below, the narrow neck allowing the load to drop to almost zero before failure occurs.
In contrast, the duralumin specimen begins to neck, but then fails with a brittle fracture, resulting in a cup and cone fracture surface - pictured below.
 |
 |
|
Duralumin (cone) |
Duralumin (cup) |
 |
 |
|
Pure aluminium |
|
Fracture surfaces from tensile tests. (Click on image to view larger version.)
The fracture in the duralumin is initiated at microvoids, which can be seen in the SEM image below. These coalesce, to form an internal crack. Final failure occurs when the shear stress causes the remaining cross section to tear. The shear stress is greatest at 45° to the applied load, and hence forms the angled walls, resulting in the distinctive cup and cone profile.
SEM image of the fracture surface of duralumin. (Click on image to view larger version.)
previous | next
© 2004-2013 University of Cambridge.