The samples made out of spring steel are taken from same lot. The specimens were shaped following heating to forging temperature - Forming- Hardening -Tempering methods. After storage of almost 6 months one of the specimens has been subjected to fatigue loading in it's elastic limit. The modulus of elasticity (slope of stress vs strain curve) and yield point of the fatigue loaded specimen increased significantly. The unusual behavior happened only after 25000 cycles. Could someone please explain it?
Thanks in advance
if your spring steel is made by cold forming, it means that it has some work hardening and hysteresis energy from the previous fabrication process. Also, you can check the hardness of different test samples and find out what is their equivalent toughness and it would be a first examination to validate your result. The time of storage does not affect the material if an environmental effect doesn't observe.
@Khokon: Was this material cold rolled or hot rolled? Are the samples just raw material such as bar stock/wire/sheet metal or are the samples "parts" which have been formed into a geometric shape such as a coil spring? If the latter is true, the act of spring coiling (for example) will impart residual stresses which will remain in the material until it is either annealed or exposed to a load cycle of sufficient magnitude to "cancel" it. If you are comparing the material's modulus during it's very first load application with it's modulus after 25000 cycles, I suspect that your very first test produced an erroneous value due to residual stress in the material which resulted from cold forming. In my experience with testing compression springs, it was common to put the spring through a series of load cycles before measuring it's rate and free length. Otherwise the residual stresses in the material could add or subtract from the externally applied stress which would affect the resulting strain (and therefore affect the calculated modulus). If the samples have not been exposed to any cold forming operation (such as a hot-rolled bar stock), the increase in yield strength after 25000 cycles may be attributed to strain-hardening which could have occurred while the applied cyclic loads approached the proportional limit. I know that you mentioned that the loading was "in it's elastic limit" but you must keep in mind that the specified elastic/proportional limit for a given metal is typically measured once the material has yielded by .2%. That would likely be a sufficient enough load to induce some degree of strain hardening. Especially if it happened 25000 times. However, one would expect the modulus of elasticity to remain the same with the only changes being reduced elongation, increased yield strength and UTS.
Ali Farokhi Nejad and Jason Raasch , thank you for your kind answers. FIY, the specimens were shaped following heating to forging temperature - Forming- Hardening -Tempering methods.
I totally agree with @Jason Raasch. I think the problem is related to the fabrication process and unequal grain size due to rolling process.
You need to consider strain rate effects which arise due to dynamic loading..
If cycles were of different frequencies and there was an increase around 25 000 cycles, it could be kinematic strain hardening above yield. But then it was not within it's elastic limit, such that the point of yield must somehow have been lowered over time. Will storage and testing under sunlight vary the temperature by +/-70degC every day?
- Badges
- Science topic
- Similar topics
- Biological Science
- Physiology
- Physiological Processes
- Stress