Wave steadiness in aluminas

Direct measurements of strain in shock-loaded glass specimens

“…This interpretation can also explain the zero spall strength recorded behind the front 14 and the dark areas of damage nucleation seen in high-speed pictures. 15 Since this failure wave was recorded close to the measured threshold of 4 GPa this is also in accord with the 4 GPa threshold found for permanent refractive index change in shocked glass by Gibbons and Ahrens. 21…”

“…Further, the failure wave speed does not approach that of the shock wave at stresses in excess of the HEL but rather appears to approach the shear wave velocity. 11,15 It was found that the high spall strength diminishes to zero behind this front and a substantial decrease in shear strength, as evidenced by the lateral stress measurements, results when the failure front traverses the specimen. More recent work by Raiser et al, 16 using VISAR, showed that the failure wave phenomenon does not depend on surface roughness, and Bourne et al 15 have shown successively these waves in soda lime and Pyrex glasses using high-speed photography.…”

“…11,15 It was found that the high spall strength diminishes to zero behind this front and a substantial decrease in shear strength, as evidenced by the lateral stress measurements, results when the failure front traverses the specimen. More recent work by Raiser et al, 16 using VISAR, showed that the failure wave phenomenon does not depend on surface roughness, and Bourne et al 15 have shown successively these waves in soda lime and Pyrex glasses using high-speed photography. In order to account for these waves, Clifton 17 proposed a model, in the context of nonlinear wave theory, according to which the failure front is a propagating phase boundary which separates comminuted from intact material with a transformation shock.…”

Failure in a shocked high-density glass

“…10,11 An interesting feature of precursor decay in borosilicate glass was that it could be reduced by lowering the temperature of the specimen from ambient to 77 K. 10 It is a matter of some debate whether such an effect occurs in aluminas, although it appears that the phenomenon is pronounced in low ͑85-95%͒ alumina content materials and much less obvious in almost pure polycrystalline aluminas. 12,13 The formation of failure waves in materials was first noted in a K9 glass ͑similar to soda-lime͒. 14 The distinguishing feature of such a wave is the loss of tensile strength across the front and the reduction of shear strength that is also observed.…”

“…[15][16][17] Since the initial observation of these waves, high-speed photography has revealed their form as a front resembling propagating fracture following some distance behind the shock. 18,19 Some authors have claimed that the wave travels at constant speed while others have contended that it slows as it moves in from the impact surface. 20,21 The nature of the mechanism that gives rise to the failure front remains a matter of debate.…”