I think "underextending" is it.

As a Materials Science graduate; I will just point out the oft missed distinction between "stress" and "strain".

"Stress" is what is applied to something.

"Strain" is what it does in consequence of the stress.

So you apply "stress" to a spring by attacing a weight.

It "strains" by extending.

This has two phases; elastic and inelastic. From the first; when the stress is removed the strain is entirely reversed. One you stress beyond the "elastic limit" inelastic strain occurs and the spring is permanently deformed. That is "overstressing".