Materials and Structure

Structural design is also concerned with material failure under operating conditions. If a light-strong structure is the target, the log-log plot of a failure strength against material density is of value in material selection. The colored zones represent the same material classes as before, and the red line represents a selected value of (s_f /r), where s_f is an appropriate failure mode. In the case of a ductile metal that should remain in its elastic range during use, s_f would be the yield stress of the material, s_y. If the material was a brittle ceramic, the fracture stress would be used. In a compression situation, either a yield stress or a buckling stress could be employed.

In addition to the (s/r) = C line two other lines are shown on the plot. These are used for selecting minimum weight materials for cantilever beam (s_y/r^3/2) or load bearing plate (s_y/r²) applications. The importance of the material density is seen from the power law dependence on this parameter.

From: Ashby, Materials Selection, Pergamon (1993)