Gerald L. Nutt

Comments on the bond strength measurements of Gupta and co-workers

Abstract The use of shock-induced spallation is emerging as a promising technique to determine bond strength at bi-material interfaces. With shock waves it is theoretically possible to apply the stress to an interface so rapidly that collective atomic bond separation occurs rather than nucleation and propagation of cracks. Gupta and co-workers have deduced from threshold spall experiments and finite element calculations, the bond strength of the interface between SiC and Si, and between SiC and various forms of carbon. A number of critical assumptions are used by the authors to arrive at their results, several of which can be examined by finite element calculations. We perform the necessary calculations and find the assumptions are incorrect, causing them to report bond strengths much higher than can be justified. Possible solutions to this problem are discussed.

Reactive flow lagrange analysis in plastic bonded explosives

Abstract A description of Lagrange gauge measurements in PBX-9404 and RX-26-AF is given. The data are used to study the progress of reaction in these explosives. The results are discussed along with the underlying theortetical assumptions. Emphasis is given to the practical problems of constructing a description of the chemical reaction from gauge data.

Use of Shock Waves to Measure Adhesion at Interfaces

The central problem in the study of composite materials is the adhesive strength of the electronic bond between reinforcement and matrix. We have introduced a unique method of measuring the interface bond strength of a wide variety of engineering interfaces (e.g. metal/ceramic, semiconductor/metal, metal/polymer). Specimens are composed of a relatively thin overlayer on a thick substrate. The specimens are shocked using a magnetic hammer which accelerates a thin metal flyer onto the substrate. The shock, upon reflection at the free surface, is incident on the bonded interface as a tensile wave, spalling the overlayer. The method is unique in using free surface velocity measurements to determine the interface stress at the instant of separation. The debonding process is sufficiently rapid (on the order of 1.0 ns) that debonding occurs by the simultaneous breaking of atomic bonds, rather than by propagation of cracks nucleating at stress concentrations near existing flaws.

Direct measurement of interface bond strengths

Research on the bond strength of Ni films on sapphire was conducted. Ni was deposited on a 1.27 cm diameter piece of sapphire 0.32 cm thick. The substrate was impacted with a Cu flyer 0.0064 cm thick at 20kV producing an axial stress of over 6 GPa in a pulse /approximately/32 ns long. The yield strength of Ni is nominally 1.2 GPa and the yield strength of sapphire is >10 GPa. The free-surface velocity of the Ni film was measured using a VISAR argon-ion laser interferometer. The light reflected from the surface of the film is split into two portions one of which is set through a delay. The two portions of reflected light are then mixed. The interference fringes are recorded using photomultipliers. The photomultiplier data are recorded and stored digitally. A five point running average is made of the raw data in order to smooth the photon noise. The velocity history of the free surface of the film is calculated using the time delay of the etalon block, its index of refraction, and the laser frequency. 8 refs., 4 figs.

Measurement of Bond Strength at Metal/Ceramic Interfaces

We report on a novel method for measuring the bond strength of metal ceramic interfaces. Test specimens are created by vapor depositing a metal film on a ceramic substrate. The specimen is impacted with a thin metallic flyer sending a short planar shock pulse into the ceramic. If the shape and amplitude of the wave is properly controlled the interface will spontaneously disbond creating new free surfaces. Measurements indicate the disbonding process occurs in less than 1.0 ns, which we believe is too short for crack propagation along existing flaws. Therefore, we conclude that simultaneous breaking of atomic bonds rather than propagation and coalescence of cracks is the means by which the film and substrate are separated. The free surface velocity of the metal overlayer is monitored during spall by laser interferometry. The data constitute a direct measurement of the bond strength. The measured bond strengths are reproducible and do not show a dependence on shock amplitude for identically prepared specimens. 9 refs., 6 figs., 1 tab.

REACTIVE FLOW LAGRANGE ANALYSIS IN RX-26-AF

A series of Lagrange gauge records, taken in RX-26-AF are analyzed. The explosive is initiated at 27 Kb sustained shock pressure. The resulting reaction runs through approximately 1.5 cm of the PBX, while pressure and velocity measurements are taken at various Lagrange coordinates. Using the equation of state for reactant, and product gases, in the J.W.L. form, a reactive flow Lagrange analysis (RFLA) is used to calculate the reaction progress coordinate. The results show an interesting two-stage reaction possibly associated with the different reaction rates of HMX and TATB. Problems with consistency and error are discussed.

Explosions on a gas-vacuum interface

A finite‐difference computer code is used to calculate the time development of an explosion on a gas‐vacuum interface. An analytic theory of the shape of the shock wave produced in the explosion is compared with the results of the computer simulation. The assumptions used in obtaining this analytic solution are verified, and the degree to which the variables describing the explosion are self‐similar is examined. Finally, certain consistency relations among the similarity exponents are tested.

A reactive flow model for a monomolecular high explosive

The thermodynamics of a reacting fluid element is discussed in a context appropriate to a hydrodynamic computer code. This is followed by a treatment of reaction initiation and reaction growth with reference to global reaction rates measured by reactive flow Lagrange analysis. The reactant and product gases are represented in the equation of state of the fluid element as two unmixed phases. The parameters in a common phenomenological representation of the global reaction rate are fit to the measured rate, and fine tuned by iterative code calculations of measured flow variables. We discuss changes in the form of the rate equation which would improve the agreement with experiment.

Shape of shock wave produced by a concentrated impact on a surface

An approximate similarity solution, derived by Raizer, of a concentrated impact (or intense explosion) at the boundary of a semi‐infinite volume of a perfect gas is used to determine the propagation velocity of the shock front as a function of its position. This velocity function is then used to obtain the shape of the propagating shock wave. It is shown that dish‐shaped shock fronts are formed when the movement of the gas at the surface is into the gas region and that cup‐shaped shock fronts are formed when the movement is out of the gas region. Comparison of these results with the shapes of explosions and meteorite craters are discussed.