The circular argument behind spider and silkworm silk mechanical properties

• Spider silk is not inherently stronger or tougher than silkworm silk. • Cross-sectional area measurement is the main source of uncertainty in fiber mechanics. • Silk fibers follow a universal fracture mechanics law. Comparative studies of silk mechanical properties often highlight spider silk as superior to silkworm silk, but such claims are frequently based on inconsistent methodologies and limited species selection. This study revisits the comparison by emphasising the importance of accurate fibre cross-sectional area measurements, particularly given the irregular geometry of many silkworm silks. By correcting for overestimated cross-sectional areas, which were often assumed to be circular, we show that spider and silkworm silks exhibit comparable mechanical properties. The highest-performing natural silk in our dataset is not from a spider but from a bagworm. Additionally, we apply the theoretical framework based on fracture mechanics proposed by Porter, demonstrating that fibre strength scales with Young’s modulus and inversely with diameter, in line with Griffith-Irwin fracture theory. This scaling holds for both natural and synthetic fibres, suggesting a universal failure mechanism. Our findings advocate for broader consideration of non-model silks and a more physics-informed approach to understanding silk mechanics.