TABLE OF CONTENTS 
Most decisions you make in life involve trade-offs. Sometimes the trade-off is to cope with conflicting constraints: I must pay this bill but I must also pay that one you pay the one that is most pressing. At other times the tradeoff is to balance divergent objectives: I want to be rich but I also want to be happy and resolving this is harder since you must balance the two, and wealth is not measured in the same units as happiness.
So it is with selecting materials and processes. The selection must satisfy several, often conflicting, constraints. In the design of an aircraft wing-spar, weight must be minimized, with constraints on stiffness, fatigue strength, toughness, and geometry. In the design of a disposable hot-drink cup, cost is what matters; it must be minimized subject to constraints on stiffness, strength, and thermal conductivity, though painful experience suggests that designers sometimes neglect the last. In this class of problem there is one design objective (minimization of weight or of cost) with many constraints, a situation we have already met in Chapter 5. Its solution is straightforward: apply the constraints in sequence, rejecting at each step the materials that fail to meet them. The survivors are viable candidates. Rank them by their ability to meet the single objective, and then explore the top-ranked candidates in depth. Usually this does the job, but sometimes there is an extra twist. It is described in Section 9.2.
A second class...
