S. J. Bless

Impact‐induced failure waves in glass bars and plates

Glass bars and plates were subjected to impact loading. Failure waves were observed to propagate behind the compression waves. Material traversed by the failure wave suffers total loss of tensile strength and substantial drop in shear strength. Failure wave propagation velocities exceed the maximum crack propagation speed, but are not constant. In bars, failure wave speed range from 2.3 to 5.2 mm/μs, increasing with increasing impact velocity; in plates, the wave speed is about 2 mm/μs. The failure is ‘‘explosive’’ in nature, leading to radial expansion in bars and an increase in mean stress in plates.

Spall strength of shock‐loaded glass

The spall strength of soda lime glass was measured with Manganin stress gauges, in planar impact experiments, below and above the Hugoniot elastic limit of the glass. It was found that while in its elastic range the glass can withstand very large tensions (over 30 kbars), its spall strength reduced to zero when shocked just above the HEL. This confirms the assumption that the elastic limit marks the onset of fracture in glass upon shock loading.

Dynamic high‐pressure properties of AlN ceramic as determined by flyer plate impact

The dynamic properties of hot‐pressed aluminum nitride ceramics were determined in a series of plate impact experiments using longitudinal and transverse in‐material manganin gauges. The Hugoniot curve for hot pressed specimens was determined in the range of 0–190 kbar with a value of 94±2 kbar for the Hugoniot elastic limit (HEL). Using both gauge orientations, the stress deviator in the specimens was determined as the difference between longitudinal and transverse stresses. It was found that the stress deviator remains relatively constant above the HEL, and is about 10% higher than the value at the HEL point. The inferred Hugoniot converges to the extrapolation of the hydrostat. Since the convergence is not due to loss of strength, it may be due to a phase change in the AlN from low‐pressure (wurtzite) structure to high‐pressure (rocksalt) structure.

Shear strength of shock‐loaded alumina as determined with longitudinal and transverse manganin gauges

The shear strength of shock‐loaded commercial alumina (AD‐85 manufactured by Coors) is determined in the 0–140‐kbar range of shock stresses. Longitudinal and transverse manganin gauges were used to determine the principal stresses in the shocked specimens. Shear strengths were determined from the difference between the longitudinal and lateral stresses. It was found that the shear strength remains essentially constant at about 27 kbar for shock stresses between 60 kbar (the Hugoniot elastic limit) and the maximum shock amplitude tested in this series (142 kbar). The source for the high shear strength is attributed to the confining pressures that strengthen the comminuted ceramic. Evidence for this interpretation is obtained by considering the release profiles as recorded by the longitudinal gauges when the free‐surface rarefactions reach gauge location.

On the influence of the loss of shear strength on the ballistic performance of brittle solids

Abstract The shear strength of solids correlates with penetration resistance in ballistic experiments. The present study was conducted in order to demonstrate the relation between the loss of shear strength of shock loaded brittle solids and ballistic penetration resistance. Soda lime glass and alumina were chosen as the test materials because their properties under planar shock wave experiments were recently measured. Ballistic experiments using the thick backing configuration demonstrated a correlation between loss of shear strength at high shock pressures and decreased ballistic performance.

Study of spall and recompaction of ceramics using a double-impact technique

A new plate impact technique for studying the dynamic properties of shock‐damaged materials has been developed. The technique is based on impacting the specimen with two flyer plates which are separated by a small gap. Impact of the first plate causes spall in the target. Impact of the second plate closes the spall. The transmitted shock wave is monitored with a VISAR and/or manganin stress gauge. Analysis of the waves gives the properties of the damaged region. Preliminary results for copper and polycrystalline aluminum oxide have been obtained.

APPLYING STEINBERG'S MODEL TO THE HUGONIOT ELASTIC LIMIT OF POROUS BORON CARBIDE SPECIMENS

Plate‐impact experiments were performed on boron carbide specimens, having different porosities, in order to measure their Hugoniot elastic limit (HEL) values. The measurements were performed with commercial manganin gauges embedded at the back surface of the specimen and backed by a thick Plexiglas disk. The measured values show an almost linear decrease in the HEL values between 194 kbar (For the fully dense material) to 96 kbar for a specimen with 16.3% porosity. These values were compared with a theoretical model [suggested by D. Steinberg (LLL report LLL‐UCID‐16946, 1975)] which accounts for the dependence of the HEL on porosity, and the agreement is shown to be good.

SHEAR STRENGTH OF TITANIUM DIBORIDE UNDER SHOCK LOADING MEASURED BY TRANSVERSE MANGANIN GAUGES

The shear strength of hot pressed TiB2 was measured under shock compression to about 3 times the HEL (75 kbar) by using transverse manganin gauges. The shear strength is taken as one-half the difference between the longitudinal (shock) and transverse stress. The shock stress was computed by impedance matching with the copper impactor. The strength of TiB2 at a shock level of 240 kbar increases to twice its initial value at the HEL. This pressure hardening may explain the superior ballistic performance of this material. These results are compared with similar data on other ceramics.

Shock Fracture and Recompaction of Ceramics

A new double impact technique for studying the dynamic properties of shock damaged materials is presented. Two flyer plates are separated by a small gap. The first plate causes spall in the target. The second plate recompacts the material and allows determination of the properties of the damaged zone. Two polycrystalline commercial aluminas were studied using VISAR and manganin gauges. A theoretical model that treats the spall zone homogeneously was used in a modified SWAP code.

Dynamic unloading behavior of soda lime glass

An experimental study was conducted to see if the reverberation plate technique, using commercial manganin stress gauges, could be used to determine the dynamic unloading behavior of brittle materials. Using a 50‐mm powder gun, plate impact experiments were conducted on soda lime glass, providing unloading data from stress states from below the Hugoniot elastic limit to 17.0 GPa. The slopes of the unloading paths were found to increase as the peak compressive stress was increased. Such unloading behavior is similar to that of quartz, suggesting that the technique is viable.