Drag the section, slide the force, or swap materials — stiffer material means less strain for the same stress.
Steel deforms under load — always. The question is how much. δ = PL/AE tells you everything: more load means more stretch, a longer member stretches more, a bigger cross-section or stiffer material fights back. That's why swapping from lumber to steel for the same member shrinks the deformation by 17×. The deformation is elastic — release the load and the member springs back. Structural engineers only design in this range.
When you apply stress, the material deforms. Strain is the measure of that deformation: ε = δ / L (change in length divided by original length). It's dimensionless — typically a very small number. Steel at yield: ε ≈ 0.0017 (0.17%). In the elastic range, stress and strain are proportional: σ = Eε (Hooke's Law), where E is the elastic modulus (29,000 ksi for steel). Remove the load and the deformation disappears.
The stress-strain curve tells the complete story of a material. Steel: linear elastic up to Fy (yield strength), then a flat yield plateau where it deforms without additional stress, then strain hardening until it reaches Fu (ultimate strength), then necking and fracture. This curve explains why steel is ductile — it can absorb enormous deformation after yielding before it breaks. Concrete's curve is very different: it peaks and drops quickly, which is why concrete is brittle without reinforcement.
Every term in δ = PL/AE is a design lever. Increase any numerator term, get more deformation. Increase any denominator term, get less. They're all linear — except that L compounds quickly because it governs both force and geometry.