materials-science

There are tens of thousands of engineering materials and no way to compare them by intuition. This lesson builds the material index, the single combined number that ranks candidates for a stated job, shows why steel, aluminium and titanium have almost identical stiffness per kilogram, and covers the environment and processing constraints that overrule the ranking.

Most catastrophic structural failures happen far below yield strength, because a crack was already there. This lesson builds the fracture mechanics that predicts the largest crack a part can carry, shows why raising strength shrinks that number, and works through the two disasters that turned fatigue from a curiosity into a design discipline.

A dislocation is a line of atomic misfit that lets a crystal slip a few bonds at a time, and its mobility decides whether a material bends or breaks. This lesson explains how it moves, why some crystal structures permit it and others do not, and the four standard ways engineers obstruct it to buy strength at the price of ductility.

A perfect copper crystal should yield at about 7 GPa. Real annealed copper gives way at 10 MPa, hundreds of times lower, and the gap took thirty years to explain. This lesson separates stiffness from strength from toughness, works the theoretical calculation, and shows why the answer turned out to be defects rather than arithmetic.