Scott I. Jackson

Detection of high explosive detonation across material interfaces with chirped fiber Bragg gratings

Measuring detonation wavefront position and velocity changes across material interfaces has been demonstrated using an all-optical chirped fiber Bragg grating approach. An experiment was conducted with a cylindrical rate stick consisting of multiple high explosive (HE) segments. A measurement of detonation position across interfaces of different HE formulations was made for both low and high spatial resolution gratings. The results show the accuracy of measured velocities is increased with the higher resolution gratings. A decaying shock driven into the polymethyl methacrylate (PMMA) segment provided a measure of the minimum internal fiber pressure required for prompt grating destruction and accurate wavefront position measurement.

Fiber Bragg grating sensing of detonation and shock experiments at Los Alamos National Laboratory

An all optical-fiber-based approach to measuring high explosive detonation front position and velocity is described. By measuring total light return using an incoherent light source reflected from a fiber Bragg grating sensor in contact with the explosive, dynamic mapping of the detonation front position and velocity versus time is obtained. We demonstrate two calibration procedures and provide several examples of detonation front measurements: PBX 9502 cylindrical rate stick, radial detonation front in PBX 9501, and PBX 9501 detonation along a curved meridian line. In the cylindrical rate stick measurement, excellent agreement with complementary diagnostics (electrical pins and streak camera imaging) is achieved, demonstrating accuracy in the detonation front velocity to below the 0.3% level when compared to the results from the pin data. In a similar approach, we use embedded fiber grating sensors for dynamic pressure measurements to test the feasibility of these sensors for high pressure shock wave research in gas gun driven flyer plate impact experiments. By applying well-controlled steady shock wave pressure profiles to soft materials such as PMMA, we study the dynamic pressure response of embedded fiber Bragg gratings to extract pressure amplitude of the shock wave. Comparison of the fiber sensor results is then made with traditional methods (velocimetry and electro-magnetic particle velocity gauges) to gauge the accuracy of the approach.

PROTON RADIOGRAPHY OF PBX 9502 DETONATION SHOCK DYNAMICS CONFINEMENT SANDWICH TEST

Recent results utilizing proton radiography (P‐Rad) during the detonation of the high explosive PBX 9502 are presented. Specifically, the effects of confinement of the detonation are examined in the LANL detonation confinement sandwich geometry. The resulting detonation velocity and detonation shock shape are measured. In addition, proton radiography allows one to image the reflected shocks through the detonation products. Comparisons are made with detonation shock dynamics (DSD) and the reactive flow model Ignition and Growth ( I&G) for the lead detonation shock and detonation velocity. In addition, predictions of reflected shocks are made with the reactive flow model.

Fiber Bragg sensing of high explosive detonation experiments at Los Alamos National Laboratory

An all optical-fiber-based approach to measuring high explosive detonation front position and velocity is demonstrated. By measuring total light return using an incoherent light source reflected from a fiber Bragg grating sensor in contact with the explosive, dynamic mapping of the detonation front position and velocity versus time is obtained. We demonstrate two examples of detonation front measurements: PETN detasheet test and detonation along a multi-HE cylindrical rate stick containing sections of PBX 9501, Comp B, TNT, PBX 9407, PBX 9520, and inert PMMA. In the PETN detasheet measurement, excellent agreement with complementary diagnostics (electrical pins) is achieved, with accuracy in the detonation front velocity at the 0.13% level when compared to the results from the pin data.

Determination of the velocity-curvature relationship for unknown front shapes

Detonation Shock Dynamics (DSD) is a detonation propagation methodology that replaces the detonation shock and reaction zone with a surface that evolves according to a specified normal-velocity evolution law. DSD is able to model detonation propagation when supplied with two components: the normaldetonation- velocity variation versus detonation surface curvature and the surface edge angle at the explosiveconfiner interface. The velocity-curvature relationship is typically derived from experimental rate-stick data. Experimental front shapes can be fit to an analytic equation with an appropriate characteristic shape to examine detonation velocity-curvature variation computed from that analytic expression. However, in some complex explosive-confiner configurations, an appropriate functional form for the detonation front shape may be difficult to construct. To address such situations, we numerically compute the velocity-curvature variation directly from discrete experimental front-shape data using local rather than global fitting forms. The results are then compared to the global method for determining the velocity-curvature variation. The possibilities and limitations of such an approach are discussed.

ANFO RESPONSE TO LOW-STRESS PLANAR IMPACTS

Ammonium Nitrate plus Fuel Oil (ANFO) is a non-ideal explosive where the mixing behavior of the mm-diameter prills with the absorbed fuel oil is of critical importance for chemical energy release. The large-scale heterogeneity of ANFO establishes conditions uniquely suited for observation using the spatially- and temporally-resolved line-imaging ORVIS (Optically Recording Velocity Interferometer System) diagnostic. The first demonstration of transmitted wave profiles in ANFO from planar impacts using a single-stage gas gun is reported. Major observations including an extended compaction precursor, post-shock particle velocity variations and between-prill jetting are reported.

Predicting Runaway Reaction in a Solid Explosive Containing a Single Crack

This work predicts the critical conditions required for the onset of reaction runaway in a narrow high‐explosive slot intended to simulate a crack. We review ongoing experiments where flames propagated through such slots at velocities up to 10 km/s, reaching pressures in excess of 1 kbar. A model is developed where slot pressurization is attributed to gas‐dynamic choking at the slot exit. The combination of choking and a pressure‐dependent reaction rate is shown to be capable of runaway reaction for a range of slot dimensions and pressures. This model agrees with experimental pressure measurements of reaction runaway in slots and provides a mechanism for the erratic burning observed with some explosives under high pressure.

Experimental measurement of the scaling of the diameter- and thickness-effect curves for ideal, insensitive, and non-ideal explosives

Numerous two-dimensional high-explosive slab tests were fielded for explosives that exhibit ideal (PBX 9501), slightly non-ideal (PBX 9502), and highly non-ideal (ANFO) detonation. Detonation velocity versus slab thickness t (thickness-effect curves) are compared to previous diameter-effect measurements obtained by varying the diameter d of cylindrical rate sticks. The scale factors d/t necessary to overlay the diameter- and thickness-effect curves were computed for each explosive formulation. We observe that the scale factor varies with detonation velocity (or level of detonation ideality). The measured scale factors range from 1.89–2.20, 1.25–1.87, and 1.79–1.05 for PBX 9501, PBX 9502, and ANFO formulations, respectively, as detonation velocity varies from the (near failure) critical velocity to the Chapman-Jouguet velocity. These results support our previous theoretical prediction that the scale factor relating the diameter- and thickness-effect curves will increasingly deviate from two as the detonation structure becomes increasingly non-ideal.

Deflagration Phenomena in Energetic Materials: An Overview

In this chapter, we review the basic behavior that occurs in deflagrating high explosives. Consideration of the deflagrating characteristics of high explosives is often neglected as they are primarily intended to detonate. However, explosives can exhibit a wide range of deflagrative behavior. Propagation rates can range from slow and steady propellant-like burns at millimeters per second to rapid gas-pressure-driven burns approaching detonation velocities, depending on the burning pressure, the explosive porosity, and the level of confinement. In the most extreme cases, the pressure waves generated by rapid burn rates can induce a detonation in the high explosive.

A SIMPLE LINE WAVE GENERATOR USING COMMERCIAL EXPLOSIVES

We present a simple and inexpensive explosive line wave generator which has been designed using commercial sheet explosive and plane wave lens concepts. The line wave generator is constructed using PETN‐ and RDX‐based sheet explosive for the slow and fast components, respectively, and permits the creation of any desired line width. A series of experiments were performed on a 100‐mm design, measuring the detonation arrival time at the output of the generator using a streak camera. An iterative technique was used to adjust the line wave generators slow and fast components, so as to minimize the arrival time deviation. Preliminary tests achieved a wavefront simultaneity of 100 ns with a 7.0 mm/μs detonation wave. Designs, test results, and concepts for improvements are discussed.