,what is the point of entering the material non-linearity if you know that the stresses are going to be lower than the yield strength of the material? We use material non-linearity when we want to observe the behavior and results of the structure, especially at the region which is manifesting higher stresses than the yield strength (since this asserts that this region is the place where the permanent plastic deformation will be very dominant).
Now, for the titanium alloy, the yield strength is already 930 MPa, which is quite higher than the stress of 620 MPa. This means that the point where you are seeing a stress of 620 MPa won't deform plastically (and its stress will not decrease after inserting a material non-linearity) at all, since the stress is already lower than the yield strength.
And also, if you want to know that why does the stress (which appears to be greater than the yield strength of that material) decreases when using material non-linearity, then the reason is that the plasticity results in loss of stiffness of the structure. This means that the stress needs to get relieved at that location, since the structure is no longer capable of bearing so high stresses.
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