Light enough or go lighter?

Highlights: • A novel concept for a more objective judgement of lightweight designs, named the true lightweight degree, is established. • Limits, which help finding the mass-minimizing material choice for a stiffness-oriented component design, are derived. • These analytically determined limits could be validated by numerical topology optimizations. • It is found that traditional parameters for the lightweight degree only represent a lower limit. • The achievable true lightweight degree depends on the possible design freedom. -- Abstract: A novel concept for evaluating the lightweight design of structural components, named the true lightweight degree, is developed. It is found that traditional lightweight parameters just constitute a lower bound for stiffness-oriented designs. This can be attributed to the low underlying design freedom. However, this is often not suitable anymore as today's manufacturing processes and materials typically allow for an increased design freedom. With a simplified analytical model, it is shown that a combination of a requirement-based equivalent strain, the specific Young's modulus, and the yield strength gives an upper bound. Numerical topology optimizations prove that this theoretical upper bound can serve as a good qualitative criterion for the mass-minimizing material choice. The investigations reveal that the right material choice can be quite sensitive to the degree of design freedom. For example, under lower-bound design constraints, an aluminum alloy having a yield strength of 280 MPa enables a slightly lighter component mass than a steel alloy with a yield strength of 800 MPa. In contrast, the steel alloy yields a considerably lower mass if full geometric design freedom is assumed. Yet, the concept derived within this work is only valid for elastically loaded, stiffness-oriented components made of isotropic material.