The following is what I read:
If, however, the elastic limit is exceeded, only the elastic strain
will be recovered, and the plastic strain will remain as permanent set. The stress in the material will unload along a path parallel to the elastic modulus.
What is the difference between Permanent set and Elastic limit?
It is given that the stress in the material will unload parallel to the elastic modulus. Why is this the case? Also, is yield point and elastic limit two different points or the same? From the graph below it seems like they both are the same point.
Best Answer
Permanent set is the deformation that stays in the material after the applied stress is removed. The elastic limit is the maximum stress that is still within the materials elastic range. If you apply a greater stress then the elastic limit, you get permanent set in the object - it gets deformed in a way that it doesn't fully recover when the load is removed.
For a well behaved material, the plastic deformation shouldn't change the Young's modulus. This means that the loading behaviour in the new elastic range will still have the same slope, but because it is permanently set, it now has a strain even when all the applied stress is removed (this strain is the permanent/plastic deformation).
They are synonyms in most contexts. Especially in simple engineering curves like this, yield is the point when plastic deformation begins to occur, and elastic limit is the limit where elastic deformation stops, so that makes them functionally identical.