John S. Rinehart

Some Quantitative Data Bearing on the Scabbing of Metals under Explosive Attack

Fracturing, or scabbing, of a material near a free surface as the result of a transient compressional stress wave of high intensity impinging on that surface has been observed for many years; however, little quantitative data that relate the fracture to the nature of the stress wave and the physical properties of the material seem to exist. The phenomenon has been investigated for five metals, 1020 steel, 4130 steel, 24S‐T4 aluminum alloy, brass, and copper, by using an explosive charge to induce a high intensity stress wave in the metal. The distribution of pressure within the wave was determined by a modified Hopkinson‐bar type of experiment.Scabbing has been found to be governed principally by the spatial distribution of pressure within the wave and a critical normal fracture stress σc that is characteristic of the material and perhaps the state of stress. Numerical values of σc were obtained for each of the five metals.

Scabbing of Metals under Explosive Attack: Multiple Scabbing

Further quantitative data bearing on the scabbing of metals under explosive attack are presented in this paper. In particular, the mechanics of the generation of multiple scabs is established. Experimental data have been obtained that show that multiple scabbing will occur whenever the maximum stress σ0 in the transient wave that impinges on a free surface is more than double the critical normal fracture stress σc of the material. The number of scabs that will be formed will be equal to the first whole number smaller than the quotient σ0/σc. The thickness of each scab is governed by the shape of the stress wave.

Phenomena Associated with the Flight of Ultra‐Speed Pellets. Part I. Ballistics

Small metallic pellets have been fired at velocities up to 6 km/sec by means of modified shaped charges. Previous work in this field has been confined largely to observations of meteors and conventional shaped charges. Data obtained on ballistic phenomena verify the conventional drag force determinations extrapolated to large Mach numbers. Steel pellets at velocities of 2.5 km/sec lose little mass in flight. Pellets composed of lighter metals, such as aluminum at a velocity of 5 km/sec, burn vigorously with the emission of much light and heat. It has been found that the motion of both nonburning and burning pellets can be represented within experimental error by the empirical equation v=v0e−αs, where v0 is the initial velocity of the pellet, v is its velocity after it has traveled a distance s through the air, and α is a constant.

Hardness Plateaus and Twinning in Explosively Loaded Mild Steel

Hardness measurements were made along radii of circular cross sections of an annealed thick‐walled low‐carbon steel cylinder internally loaded with an explosive charge. The degree of work‐hardening in the cylinder wall was found to decrease nonuniformly with an increase in distance from the surface in contact with the explosive. Each hardness versus distance curve exhibited a series of plateaus along which the hardness remained constant. The plateaus appear to be related to Neumann bands which are present in the steel. The maximum number of twin directions in a single grain increases with the magnitude of the stress. The presence of the hardness plateaus and the increase in the number of twin directions with hardness and stress level point to the existence of critical stresses for twinning.

Deformation and Fracturing of Thick‐Walled Steel Cylinders under Explosive Attack

Tests have been conducted on annealed heavy‐walled cylinders of low carbon (1020) steel internally loaded by explosive charges. The purpose of these tests was to obtain basic information on the manner and type of fracturing and plastic flow obtained by extremely high pressures acting for short durations. It was observed that all of the cylinders tended to fracture in long fragments in the same basic pattern, but with variations because of wall thickness and manner of loading. Experimental results indicate that the radial cleavage type of fracture is initiated within the cylinder wall and propagated to the surface, while the shear type fracture associated with the inner portion of the cylinder wall appears as an independent energy relieving process.Microstructure analysis of the cylinder fragments shows a definite relationship between the type of fracture and the amount of distortion of the grain boundary. Considerable shock twinning is present in the cylinder fragments, and the grain structure near the in...

Conical Surfaces of Fracture Produced by Asymmetrical Impulsive Loading

Conical surfaces of fracture generated in thick‐walled cylinders that were asymmetrically and impulsively loaded through the use of internal explosive charges are presented. It is deduced from qualitative considerations that the conical fractures result from the interaction between transient tensile stress waves. The two tensile stress waves that are responsible for the fracture arise from a single compressional stress wave that is reflected from two inclined free surfaces. The angle of fracture associated with these conical surfaces has been used to measure the velocity of propagation of the stress waves in low‐carbon steel, brass, copper, lead, and 24ST aluminum alloy.

Phenomena Associated with the Flight of Ultra‐Speed Pellets. Part III. General Features of Luminosity

Ultra‐speed pellets of the lighter metals such as aluminum and magnesium generate intensely luminous trails when fired through air. A pronounced characteristic of the luminous trails is the intermittent manner in which the light is emitted. This effect has also been observed in meteors and in studies of conventional shaped charges. Photographs indicate that flashing is caused by yawing of the pellet.The brilliant flashes of light are associated with the ablation and subsequent burning of material from the pellet. There is evidence that the material is removed from the pellet in the form of small droplets.The brightness, luminous output, and temperature of the trail generated by an aluminum pellet have been measured. The brightness is approximately 25,000 lamberts, the luminous output is 30,000 lumen‐seconds, and the color temperature is 2900°K.

Some Experimental Indications of the Stresses Produced in a Body by an Exploding Charge

The effects produced by small cylindrical charges that were detonated on the surfaces of heavy steel plates are discussed. Particular attention is paid to (a) shapes of crater, (b) changes in hardness, (c) flow patterns, (d) fractures, and (e) changes in microstructure. The distribution of stress appears in some respects to correspond to that which might be set up by a static load. The mechanisms of the inelastic deformation, however, differ very greatly between the static case and the dynamic case. Dynamic loading produces numerous shock twins and fractures that undoubtedly occur because of the very high strain rates involved.

Surface Motion Associated with Obliquely Incident Elastic Waves

Well‐known laws which govern the reflection of elastic waves that strike free surfaces obliquely are used to deduce particle motion at the free surface of a body. The surface particle motion is presented in the form of diagrams and graphs for the case of an incident longitudinal wave. Considerations indicate that, for oblique incidence, the particle motion at the surface will not, in general, be perpendicular to the surface but will depend on the angle of incidence and the Poissons ratio of the material. The data are expected to be of value in the solution of problems connected with impulsively loaded bodies such as metal‐explosive systems.

Temperature Dependence of Young's Modulus and Internal Friction of Lucite and Karolith

The Youngs modulus and specific energy loss of Lucite and Karolith were measured at different frequencies in the neighborhood of 50 kc/sec. as a function of temperature. The temperature range for Lucite was from −55°C to 65°C, and for Karolith, 25°C to 110°C. It was found that the reciprocal of Youngs modulus versus temperature curves showed positive curvatures at higher temperatures for both materials. At room temperature the Youngs moduli of Lucite and Karolith were 4.72 and 6.14×1010 dynes/cm2, respectively. The specific loss was found to vary with frequency for both materials and varied in a linear fashion for Lucite at room temperature. Absorbed water seemed to have a pronounced effect on the Youngs modulus of Karolith.