Scanning Ultrasound Holography Allows Nanoscale Subsurface Imaging
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the wide range of experimental and computational results, and many models contradict each other. By means of large-scale atomistic simulations, Buehler and Gao show that hyperelasticity, the elasticity at large strains, can play a governing role in dynamical crack tip instabilities in the fracture of brittle materials. They report a modified instability model that treats the dynamical fracture instability as a competition between different mechanisms controlled by local energy flow and local stress field or atomic forces near the crack tip. Their results suggest that the fracture instability not only appears in materials with defects; it is an intrinsic phenomenon of dynamical fracture. See Figure 1 (p. 167). “Our new theory reduces to existing models in limiting cases, but allows for a unified treatment of the instability problem applicable to a much wider range of materials,” said Buehler. “We have discovered that the key to understanding the discrepancies in the literature is to consider the material behavior close to the breaking of bonds, rather than the material properties at small strains.”
Most existing theories of fracture assume a linear elastic stress–strain law by only considering small strain deformation. However, the relation between stress and strain in real solids is strongly nonlinear due to large deformation near a moving crack tip, a phenomenon referred to as hyperelasticity or nonlinear elasticity. The scientists have made another surprising discovery. “We find that elastic stiffening materials behavior, as found in rubber-like materials, can dramatically change the instability dynamics of cracks,” Buehler said. Rubber is soft at small deformations, and becomes harder as the stretch is increased. “In such elastically stiffening materials, stable intersonic crack motion is possible,” he said. These results are in contrast to existing theories, in which the speed of elastic waves is considered the limiting speed of fracture, analogous to the speed of light. “We discovered that the classical theories of instability dynamics are only valid in a small range of material behavior,” said Buehler. “In most real materials, the softening or stiffening close to bond breaking leads to a fundamental change
in the instability dynamics, because energy flow is reduced or enhanced due to change in local wave speed.” These results represent a breakthrough in understanding how cracks propagate in brittle materials, and the findings could have wide impact in many scientific and engineering disciplines, said the researchers.
Scanning Ultrasound Holography Allows Nanoscale Subsurface Imaging The characterization of deeply buried or embedded structures and features with lateral resolution of under 100 nm is critical in a number of science and engineering arenas. Subsurface features can be imaged with techniques such as acoustic microscopy, but with limited spatial resolution. More recently, scanning probe techniques, such as ultrasonic force microscopy, have been used for nanomechanical mapping of elastic and viscoe
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