Hello everyone. I just finished performing a tensile testing lab in my materials course and was itrigued by the particular behaviour of steel.

My doubt regards the Yield Point Phenomenon for steel. This phenomenon is related to the spike in the stress strain curve right after the daparture from the elastic region. The curve spikes to a max( upper yield point) then lowers down to a plateau (lower yield point), then continues to increase. One of the questions asks us to explain this phenomenon. I'd guess it has something to do with the microscopic stucture of the material (perhaps slip). So if somebody could give me a brief explanation of what is causing this that would be greatly appreciated.

Thanks

This behaviour completely depends on the particular material you test. So a good response shouldn't be general, or at least should be adaptable enough...

The drop in the force is generally explained by the smaller cross section as the material is elongated. Then, the force may increase again as the material gets cold-drawn, which may increase its yield strength.

The curve you've seen is not the most common. Steel alloys people like to use show a consistently rising force rather.

By the way, be very cautious with anything material books and teachers affirm. Among the 1000s sorts of steel, at least 10% behave for any parameter in the opposite way than explained, and then specialists find an even more complicated explanation.

Consider material science as a pre-scientific theory where knowing each special case is more productive than any general explanation.