Materials Selector for a catalyst

Pull on a material and its stress-strain curve tells its story: a straight elastic region with slope equal to Young's modulus, a yield point where permanent deformation begins, a peak at the ultimate tensile strength, and finally fracture. Ductile metals stretch far; brittle ones like cast iron snap with almost no warning.

Mohr's circle is a graphical way to transform a 2D stress state to any rotated axis. The circle's center is the average normal stress and its radius is the maximum shear. Where it crosses the horizontal axis gives the principal stresses — the orientation with zero shear. Educational tool.

Repeated loading well below the ultimate strength can still break a part — metal fatigue. The S-N curve plots stress amplitude against cycles to failure. Steels have an endurance limit: below it they last essentially forever, but aluminum has none and eventually fails at any stress. Cause of many catastrophic aircraft and bridge failures.

Heat a material and it expands: ΔL = α·L·ΔT, where α is the coefficient of linear expansion. Area grows at twice that rate and volume at three times. It is why bridges have expansion joints, why rails buckle in heat, and why Invar — with almost zero expansion — is prized for precision instruments.