Necking and Failure

When the loading is continued beyond the ultimate stress, the cross-sectional area decreases rapidly in a localized region of the test specimen. Since the cross-sectional area decreases, the load carrying capacity of this region also decreases rapidly. The load (and stress) keeps dropping until the specimen reaches the fracture point.

Exercise: Two of the movie files of tensile specimens are shown below. The specimen on the left is steel, the one on the right aluminum. Scroll to just before the end of the left movie, and play the last few seconds. You should notice the extreme necking that occurs, as well as the final fracture surface. What shape does the fracture surface take? How do things change for the Aluminum sample?