Z. Rosenberg

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.

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.

Spall Zones in Polycrystalline Ceramics

The nature of spall in ceramics was studied by plate impact experiments using VISAR and manganin gauge measurements and a new recovery technique. Three polycrystalline ceramics were included: 97.7% alumina, 85% alumina, and 99.9% beryllia. Spall strengths were determined to be a few kbar below and negligible above the HEL. Analysis of spall signals and recovered targets shows that spall occurs over finite regions. A “spall zone” model is proposed.

Determination of the dynamic response of W-2 tungsten by commercial manganin stress gauges

The dynamic response of a tungsten alloy (W‐2, manufactured by Kennametal) was determined by using commercial manganin gauges in planar shock‐wave and long‐rod experiments. The gauges were used both as in‐material stress transducers and as time of arrival sensors in the back surface configuration. These two techniques enabled us to determine the Hugoniot curve for W‐2 in the 0–500 kbars range. The spall strength and the Hugoniot elastic limit (HEL) of this material indicates brittle behavior which is different from most other metals. The long‐rod experiment yielded a value for the dynamic uniaxial stress experiment which is in excellent agreement with the static compressive strength and the dynamic one, as deduced from the measured HEL. The three values of compressive strength fall within the range of 12–14.4 kbars, indicating that the yield strength of W‐2 is relatively insensitive to strain rate.

Accounting for the response of dynamically loaded annealed manganin gauges

The piezoresistive response of annealed manganin stress gauges is analyzed in terms of an elastoplastic model which was used to describe the behavior of nonannealed gauges. It is shown that the measured resistance changes can be accurately accounted for if we choose the right values for the yield strength of the annealed gauge and its variation with shock stress. This good agreement between predicted and measured values enhances our confidence in the analytical model which was used recently to explain various features of the response of piezoresistance gauges to shock loading.