# Why creep compliance and not just strain? (material science)

This is not about cooperating with frightening people. It’s about the concept of creep compliance, and why it’s of more interest than just the simple strain.

When you maintain a force on a solid object, like when you hang a weight on a rope to keep it under tension, generally it slowly gives way. This is called “creep”. The longer you wait, the more it changes. More force and higher temperature make it change more quickly.

Generally, if you double the force, the rate at which it changes does not double. These aren’t proportional.

While you might describe the change in dimension as a function of time, what is more commonly done is to describe the “creep compliance” as a function of time. Creep compliance is dimensional change divided by the force (per area). I want to understand why creep compliance is preferred over just ordinary dimensional change.

It would make sense if the rate of change was proportional to the force per area. Dividing it out would remove an entire degree of freedom and give a more generally useful result. But given that the dependence on force per area is typically far from proportional, and is often represented as a power law with an exponent far from 1, it seems like creep compliance is relatively obscure. Why shift it by just one order of force per area? What’s the advantage, the reasoning behind this choice?

It’s still possible to use. The time of day would still be workable if we divided it by Pi, too. I just want to grasp why we’d do so.

Thanks!!

Creep compliance is specified in terms of pressure.

Well thats like force, but its force per unit area.

The reason not to use force is that a small bit of a weak material will have little force on it , because force=pressure * area, but will still creep due to the pressure…
Your time example is a red herring - a strawman. Pi is dimensionless, so dividing time by pi still leaves it in the same dimensions as time. Besides you were just scaling by 3.1459 approx, not changing the meaning.

I was trying to stay away from more esoteric terms like stress and strain, though I guess there’s no point in that.

My question isn’t about force versus pressure, it’s why the strain would be divided by stress to the first power, as there isn’t anything very special about stress to the first power.

Any creep compliance function is still going to be linear locally. Adding a small amount of stress will add a small amount of strain equal to stress times a numerical factor. The units of the factor have to be strain/stress for that to be true.

Also, you need units of pressure in there somewhere to maintain the property that doubling the area doubles the force for constant creep.