Astronuc said:
The picture is not wrong, but that is only one cycle. One would have to repeat the test on the same specimen, which would show a greater yield (YS) and ultimate tensile strength (UTS), then repeat the test. Note that there is some permanent strain (deformation) once a material exceeds the proportional or yield strength. Above yield, the material is considered to 'flow' rather than creep. The close to the ultimate tensile strength, the greater the permanent deformation and subsequent YS and UTS.
I'm having so much trouble understanding why the ultimate tensile strength (UTS) should increase due to the repetition of cycles...
I propose the following mental experiment. Imagine an extruded metal bar and cut it in half so both specimens can be assumed to have the same properties.
Let's use one of the specimens to determine the behavior of the metal on a tensile test. This is the result from the experiment (simplified so I can draw it more easily but with some resemblance to reality).
The interpretation would be:
- The specimen is suffering elastic deformation. If the load was released, it'd come back to its original shape.
- The elastic limit (yield limit) ##\sigma_y## is reached. Beyond this point, the specimen will acquire a permanent deformation. The crystalline structure of the specimen is changed by sliding planes (dislocations) which also introduces more defects in the mesh during the process which increases the yield point of the specimen. The process goes on until the UTS ##\sigma_{UTS}## is reached.
- The UTS is reached because necking starts. I was wondering if I'd add necking to this idealization since necking is the result of a weak point somewhere in the specimen. In an absolute ideal world, the specimen would keep increasing the load uniformly along its length until failure. Since necking is a constructive defect (the reduction in the area causes a local increment in stress) as soon as it starts the maximum axial load is reached. I propose we ignore necking and just imagine the specimen snaps once ##\sigma_{UTS}## is reached.
Now, for the second specimen, I'll repeat the tensile test but I'll do it in steps/cycles as you suggested. My claim is that those cycles will increase ##\sigma_y## but not ##\sigma_{UTS}##. I'll plot first on top of the previous graph.
- Cycle A→B→A (red). The specimen didn't reach its elastic limit so it comes back to its original shape without changes.
- Cycle A→C→D→F (orange). Yield has occurred. A new yield limit is established at D. F is the new resting length of the specimen.
- Cycle F→G→F (green). The specimen was permanently deformed in the previous cycle but this new stress cycle doesn't produce additional permanent deformation even if the stress level is above C which originally defined the yield strength. As mentioned in the previous cycle, the new yield limit is at D.
- Cycle F→D→H (purple). The specimen is loaded until the ultimate strength is reached (either it breaks or necking starts, I propose we ignore what happens beyond that point).
I don't see how adding more cycles would affect the UTS (ignore more complex things such as fatigue please). The yield point increases which has been established and accepted from early in the thread but the increment in UTS is still something I can't understand.
The load cycles could be shown starting from 0 but that changes nothing since the information is still the same. I consider the previous graph to be clearer for what we are discussing but I'll add these additional drawings because it's how it's usually shown in material's datasheets and a paper shared in the thread.
I'll show what I consider the relevant paths. This is A→C→H and F→D→H.
Am I misunderstanding something in what I showed above?
Astronuc said:
As the page suggests this is a complex topic so I could be misunderstanding what it says but, doesn't it only talk about when plastic deformation occurs? In other words, it's talking about how cold working increases the yield point with which I fully agree. However, it doesn't reference how the increment in UTS happens.
Astronuc said:
See some examples of strain hardening in this paper. Note the plot (Fig. 3) represents three separate tests.
Mechanical Behavior of 316L Stainless Steel after Strain Hardening
https://www.matec-conferences.org/articles/matecconf/pdf/2017/28/matecconf_2mae2017_02003.pdf
I read the paper and it doesn't explain the increment in UTS either. In fact, in the introduction I read the following:
Tensile tests of 316H stainless steels with strainhardening up to 4%, 8% and 12% at room temperature in Ref. [4] found that the values of Young'smodulus and the ultimate strengths are similar for different strain hardening, but the yield strengthsincrease with the magnitudes of strain hardening at different temperatures.
That coincides with what was my previous understanding of how metals work. The origin of this thread is that statement collides with the huge variations in UTS shown in screws that are supposed to be made of the same material as shown in the OP.
The previous statement is also shown in Section 3 with the following graphs and table where you can see the relevant change in yield strength but the basically same UTS.
Even if there is a positive relation between strain hardening and UTS, the effect and number of tests from that paper are so small it'd be argued if the change is relevant at all. Even if it is, it is nowhere close to the huge change in properties I shared in the OP.
So far, the explanation I think covers the increment in UTS is related to this picture:
Source
Cold working could be any mechanical stress on the specimen such as a tensile test. Tensile tests will increase the yield point if the specimen is taken beyond the elastic region but the UTS will remain the same. However, if the specimen is cold worked in a specific way such as being rolled or extruded/pulled so the crystalline structure changes in a relevant way without breaking the parent material, the UTS can be increased.
For example, steel wire usually shows greater mechanical properties compared with a billet of the same material and according to what I found,
screws are made of steel wire.
This of course would make the specimen more brittle just as shown earlier in this post but I think there is another relevant difference. The material would no longer show isotropic behavior because the crystalline mesh is no longer uniformly random. The UTS might have been increased in a particular direction but I feel it must have decreased in others.
Would you say this is correct?
