Compression limits As we have elastic and plastic deformation within objects when we apply stress to an object is there a similar process for compression.
Is there a limit where we compress something where it will not return to its original shape?
Or will the electrostatic forces within the molecule repel other molecules so it returns to its original shape. However, if we compressed the molecules to the point where the strong nuclear forces take control would the molecules just fuse together?
 A: 
Is there a limit where we compress something where it will not return to its original shape? Or will the electrostatic forces within the molecule repel other molecules so it returns to its original shape. 

For any static pressures that we are currently able to generate in a laboratory by devices such as diamond anvil cells (i.e., pressures up to about 5 million atmospheres), the answer is "Yes", the material will generally return to its original shape and size if it doesn't undergo any structural phase transitions while it is being pressurized. For example, gold can be compressed in a laboratory to static pressures so high that its crystallographic unit cell volume is compressed to about half its size at zero pressure. But when that pressure is released, that unit cell springs back to its original zero pressure volume. 
Other materials may undergo structural phase transitions under high pressures (e.g., Si, Ge, Zr, Sn, etc., etc.). In such cases, the reverse transition back to its original, low-pressure structure may be sluggish and for some materials may not occur, so that the material is recovered in a metastable phase with a different structure and different unit cell volume than it originally started out with. Carbon is perhaps the best known example. It starts out with a graphitic 2-D structure but, after extreme pressurization and de-pressurization, is recovered in a completely different structure with a different unit cell volume (i.e., it transforms to diamond).
A: Although most engineering stress-strain curves are for tensile loading; the terminology also applies to compression.
The properties will not behave the same as in the tensile direction in general (i.e. compressing steel has a different curve than stretching it); but it can be described by the same terms.
There would be an "elastic limit" for compression as well; after which the deformations would become plastic and (partially) permanent.
On a molecular level it isn't much different for compression vs. tension.  When unstressed, the molecules are spaced and oriented in a state where they are stable; they do not want to be in any different orientation.  When you add a stress, you change these orientations.  If enough change is introduced from stress, the materials unloaded orientation may be different than before it was stressed in the first place.
