a metal in tensile test goes upto yield point,but breaks at lower point than yield pt at U T S.Why?

Answer 1

Sounds like it is brittle, check the brinell. Asian bolt manufacturers have been known to limit the alloys to scant minimums and process in such a way as to case harden to "confuse" diamond perpetrator brinell tests.

Answer 2

There are 3 general classes of influences acting on your test:
Object Geometry, Object Material Properties, and External Application to object.

When the object fails, it does so at its weakest point. That sounds intuitive. There are potential weanesses due to geometry, materials, and applied loads.

Options (not exhaustive)
-There could be a burr on the surface, acting as a crack nucleation point and a stress concentrator.
-There could be an concentricity, or an eccentricity error in the formation of the shape at the point.. again acting as a stress concentrator.
-There could be an internal void that acted as a stress concentrator.
-There could be a metallurgical defect due to cooling or inhomogenity of alloy at that point, acting as a stress concentrator.
-The lathing process that formed the piece could have work-hardened the section, making its boundary a stress-concentrator.
-The load could have been applied slightly off of parallel to the axis, and when the material deformed slightly in compression, the point at which it happened would have failed first.
-You could have tested at a different temperature than the UTS was performed. This could have made it more ductile, or more brittle.
-The UTS is an average.. and you should take into account its uncertainty when determining how far from the UTS your failure was.

Theres a lot of options.

The best practice is to take some rigorous measurements of the geometry before and after. You can determine if the initial geometry provided the stress concentration. You can also look at the failure surface and determine the crack nucleation point and failure mode.

Answer 3

A metal piece under tensile test goes upto yield point means after that plastic deformation will start which is irreversible.After that less load is required than yield load to fracture it as the cross-sectional area is reducing drastically.So it is evident that matal piece breaks at a lower point than yield at UTS.

Answer 4

Its actually an artifact of testing, but it's important for all practical purposes.
Once the yield point is surpassed, the sample starts to deform. as it gets longer, the cross section decreases so effectively it ultimately fails under less load than it yielded under. The test methods most commonly used (as well as most applications) do not compensate for the change in cross sectional area so the stress appears to be less.