NCIE
NCIE
Cracks propagating faster than the speed of sound defy the laws of physics
Recent testing with elastic gels has shown that cracks propagate at rates exceeding accepted limits. New Science writes about this https://new-science.ru/treshhiny-rasprostranyajushhiesya-bystree-skorosti-zvuka-brosajut-vyzov-zakonam-fiziki/.
Forming at speeds exceeding the speed of sound and displacement waves, these so-called "super displacement" cracks appear to defy the laws of physics. This discovery could shed light on the mechanisms of powerful earthquakes and possibly lead to the development of more effective methods of preventing them. Stresses applied to a material under compression propagate from the crack tip along its entire length when the potential energy exceeds the breakdown energy of the material.
According to the basic mechanical model, the maximum speed of crack propagation usually does not exceed the speed of a Rayleigh wave, a type of wave that propagates along the surface of solids. However, experiments on plastic polymers in the 1970s showed that cracks propagated at supervelocities (exceeding the shear rate) under very low and moderate stresses.
Many theorists also assumed this phenomenon. However, these results were considered surreal and could not be transferred to real materials and were temporarily put aside. Nevertheless, earthquakes corresponding to this speed, the so-called superdisplacement earthquakes, have already begun to be recorded. In fact, the underlying seismic model corresponds to a rupture velocity equivalent to the propagation velocity of displacement waves (or S-waves) that oscillate at about 3 km/s.
Superdisplacement earthquakes destroy this model, reaching rupture velocities of 5 km/s. Since then, attempts have been renewed to decipher the physical mechanisms underlying this mysterious phenomenon. A discovery made by researchers at the Hebrew University of Jerusalem appears to support this superdisplacement model. In particular, they were able to experimentally confirm the presence of superdisplacement cracks that exceed the speed of displacement waves and accelerate to the speed of expansion waves (6 km/s).
To identify the superdisplacement cracks, the researchers used a porous hydrogel commonly used in molecular analysis. After tracking the cracks in all the samples with a high-speed camera, the researchers were extremely impressed by the speed at which they spread.
The fastest of these traveled at a speed 30% faster than the speed of sound.
