materials selection
Choosing a material is like choosing the right tool for a job: you do not automatically grab the strongest thing on the shelf, you match the material to what the job actually needs. Michael Ashby's systematic selection method turns the question 'what must this part do?' into engineering language, and then lets you sift the best candidates out of a hundred thousand possible materials.
The recipe has four steps. First state the function (for example, support a load). Then list the constraints (must not break, must be light enough, must fit the space). Then set the objective (minimize mass, or minimize cost). Finally identify the free variables you may choose (the material and the cross-section). You use the constraints to screen out anything that fails outright, then use a performance index to rank the survivors. A bicycle frame, for instance, must be stiff, strong, and as light as possible.
This matters because there are well over a hundred thousand usable materials and no human can compare them all by intuition. Be honest about the catch: real selection is almost always multi-objective (you want light AND cheap AND durable), so there is often no single winner, only a set of sensible trade-offs to choose between.
A bicycle frame must be stiff and strong but as light as possible. The designer translates this into: function = light stiff beam; constraints = required stiffness and strength, must fit the frame geometry; objective = minimize mass. Screening removes brittle glass and floppy low-stiffness foams; ranking by the index E^(1/2)/rho then pushes the choice toward aluminum, titanium, and carbon-fiber composite — exactly the frame materials you see in bike shops.
Turning 'make a good bike frame' into constraints and an objective, then letting the numbers narrow the field.
Selection is not 'find the best material' but 'find the best material for THIS job under THESE constraints' — change the objective from lightest to cheapest and the winner often changes completely.