Why do tensile testing graphs show a negative slope at these regions?

[gikiian]

Do tensile testing machines reduce the value of load in these regions? If yes, why? If no, what's happening in the graph?

 

[afreiden]

Most of the time, the plots show engineering stress. You input the cross sectional area, A, of the specimen, and the machine will give you a plot of Force/(A)const for every incremental displacement. In reality, "A" is changing (decreasing, in a tensile test), rather than remaining constant. If you were to plot the "true stress" - Force/(A)actual, the stress should always increase.

 

[OldEngr63]

These are regions of dynamic elongation in which the transferred force level is falling even as the specimen continues to elongate. If this continues very far (as is the case at the right end), rupture occurs.

These are really scaled force plotted against scaled elongation curves. With convenient scaling, we can think of them as stress - strain curves, but in the dynamic regions, this is not strictly true.

 

[afreiden]

It's an issue of lateral contraction and whether or not it is being considered.

As OldEngr63 states, engineering stress vs. strain is just a scaled version of force vs. deformation. Either way, you'd get a plot that looks like the one the TS posted.

As I said before, however, true stress vs. strain would NOT look like the plot that the TS posted.

 

[256bits]

Do tensile testing machines reduce the value of load in these regions? If yes, why? If no, what's happening in the graph?

If you are asking what is happening to the sample, the graph shows the elastic and plastic deformatiom. As the stress is increased the material deforms elastically, which is the straight line region. At a value of stress the material begins to deform plastically and the material begins to neck down at the same or less level of stress. While this material is deforming plastically it is also work hardening, until the point in the graph where the curve swings upwards again ( workhardening has made the material stronger ). As you progress along the curve, more stress results in more plastic deformation, and more work hardening, until the ultimate tesile stress is reached, after which the specimen necks down and rupture occurs.

The curve appears to be that for some sort of soft metal, possibly a mild steel. Not all materials exhibit the same pattern in their stress strain curve - a brittle material shows a curve decisively different from that of a ductile materail.