The Different Term of Displacement, Deflection and Deformation

As a structural engineer, one of the most terms used is displacement, deflection, and deformation. these terms that are discussed frequently in structural engineering. I have a problem related to this term, since working as structural engineering in consultant or main contractor. the term for deflection quite near with elastic strain, whilst for deformation and displacement usually use for a plastic strain. 

Explanation about elastic and plastic strain will be posted in the next post. the simple way thinks about elastic in (structural engineer) is a condition where a node, joint of the beam could be back in normal form after got load on the beam. for plastic is a condition where a node, joint of the beam couldn't back to original form after got load. Plastic strain phase going through elastic strain.  During my explanation, I have a clear opinion from other netizens about this case. 

These are best answer (I think) from Quora.com talk about displacement, deflection, and deformation  :

In the field of structural engineering these are closely related terms, and sometimes used interchangeably, though if you want to get specific there are cases where one term is more appropriate than another.
These are how I understand the terms, though I'll be curious to see if anyone disagrees.

Displacement is the distance from which one node or element (beam, column, frame, etc) moved from its original location. The movement could be from a beam deflecting, but it could also be the result of the entire object moving, un-distorted, like a box sliding on a surface with friction. Displacement can both be measured in terms of distance and in terms of rotation.

Deflection is the distance that an object bends, twists from its original position. I would generally assume that an objects deflection does not include rigid movement of the object.

Deformation is the actual distortion that occurs to a structural member. We most commonly discuss elastic and plastic deformations. Elastic deformations are those deformations that, when the loading is removed, will return to the original undeflected shape. Plastic deformations are those that will remain even after the loading is removed.

Because most structures are securely attached to their foundation, there is general no rigid body motion, and displacement and deflection are particular similar concepts. I would say the outlier is deformations. Displacements and Deflections are generally measures of distance (or ratios of that distance compared to a meaningful value, such as the member length) while deformation is what happened to the member to cause the displacements/deflections.

So lastly that in my working stage, I prefer using deflection term in my analysis related to elastic strain, linear static analysis. for plastic stage of strain, dynamic, blast, drop object and others, I prefer chose displacement and deformation term. 

Let me know that you have a good idea to share. Cheers