Why true strain is always smaller than nominal strain in tensile test?

[HongTechnology]

TL;DR

Why true strain is always smaller than nominal strain in tensile test?

Is that mean that actual length increase is smaller than theoretical length increase?

Is it related to atomic forces?

Hi everyone.

I'm studying Mechanics of Materials and I got some question about 'nominal/true strain'.

First of all, I agree that in tensile test, nominal stress is always smaller than true stress

σt > σn

cause when the material specimen is being stretched, then the actual area of the specimen at the cross section would be decreased.

However, I wonder why nominal strain is always larger than true strain in tensile test. εt < εn

If I'm right, I guess that statement implies that actual length increase would be smaller than theoretical length increase.

cause ε=δ/L, am I right?

If I'm right, what factor is the main reason for that truth? Is it related to the atomic forces?

p.s. I just started to use this PhysicsForums, so I wonder if it is okay to upload questions here.

If I did wrong, I would appreciate if you tell me where should I upload questions. I'm not very good at english TT.

Anyway, thanks for reading my question.

 

[HongTechnology]

Ah, I think I misunderstood the definition of each of them. It was a simple thing haha;