Flexural phonon instability defines intrinsic van der Waals elastic limits in the interlayer direction

Frenkel’s cohesive model predicts an ideal elastic strain limit of ~10% for strong solids, a bound upheld in conventional materials. We demonstrate that such consistency breaks down in van der Waals (vdW) solids, challenging established strength theories. In situ tensile tests, combined with first-principles calculations, reveal unexpected localized decohesion failure at only ~3% strain in graphite and ~2% in h-BN along the interlayer direction—well below defect-controlled limits—defining their intrinsic elastic limits. This localization phenomenon is absent in MoS2, GaSe, and 3D crystals. We identify flexural phonon instability as the trigger for a cascade of strain localization, creating periodic nanogaps to release elastic strain energy. This dynamic instability redefines the intrinsic elastic limits of vdW crystals and opens pathways to tailor their structural/transport properties via strain engineering. Textbook strength theories break down for van der Waals crystals. We show that flexural phonon instability triggers interlayer localized decohesion in graphite and h-BN, unmasking their unexpectedly low intrinsic van der Waals elastic limits.