= Solution
Check stiffness and tip deflection using <Young's modulus>, failure from existing cracks using <fracture toughness> or the supplied critical energy-release rate, local crushing at supports, shear failure, buckling, fatigue and joint design. In a simple linear-elastic crack model, $K_c^2\simeq E'G_c$, so <toughness> cannot be inferred from strength alone. Wood also has grain-direction dependence, moisture sensitivity, natural defects, property scatter and time-dependent creep; the design must control grain alignment and compression-side stability. Metals raise issues of corrosion, <mass>, joining, machining, residual <stress> and heat-treatment state. Cost, availability, reproducibility, service <temperature> and end-of-life treatment can decide between otherwise feasible designs. A high merit index is a screening result, not a complete safety specification.
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