The design analysis techniques, used to determine vulnerability to attack or lethality of an explosive detonation, have been tested against a large database of experimental and test results. Building explosive testing chambers presents a whole new set of challenges, since the purpose of such tests is not the survival or destruction of the vessel, but gaining a better understanding of the explosion's dynamics. Los Alamos and Lawrence Livermore National Laboratory have chosen to combine aboveground testing using mock materials, advanced X-ray and proton radiography, and advanced computing capabilities for complex simulations. These vessel systems, when used with diagnostics such as flash or proton radiography, provide important data that help our weapon's designers validate design codes and support the certification of the weapon systems. The Atomic Weapons Establishment in the United Kingdom has similar activity under way. Testing has shown that within a millisecond, the stresses within the pressure vessel shift from a sharp, uniform impulse to a 1 kHz vibration. The amount of stress, or excitation, that the impulse places on the structure can be figured as the integral of the load over time of duration.
Bang. After years of preparation, the whole event is over in less than a hundredth of a second. It's too quick to see, a blink of an eye, dividing the day into anticipation and aftermath.
The equivalent of a 40-pound charge of high explosives is detonated inside a massive metal tank; the detonation itself lasts only a fraction of a millisecond. The rest of the split second consists of the steel confinement vessel responding to the shock waves from the detonation. The waves reach the vessel's inner surface, with multiple reflections, vibrations, and eventual energy absorption.
We do this to understand the nature of massive explosions and the effects on surrogate materials. Much of this work, which simulates to some degree the forces touched off by a nuclear detonation, is classified. But as we go about this work in an era of nuclear test moratoriums, the importance of the confinement vessels grows, and the government oversight body has required that these vessels be constructed to a consensus standard.
At the moment, that standard is the ASME Boiler & Pressure Vessel Code, but that standard has never dealt explicitly with the kinds of impulsive loads experienced in these tests. That will soon change.
Engineering has traditionally dealt more with static loads than with impulsive ones: Think of the ancient engineers designing block and tackle systems or building an aqueduct. Explosions, when they were engineered for, were forces to be channeled internally (as inside a cannon barrel) or to be resisted by protective structures.
The latter type of force has been of particular importance in recent decades. Nuclear submarines are designed to withstand external blast pressures from enemy nuclear or non-nuclear munitions. Or, from the opposite perspective, defensive missile warheads are tested to guarantee destruction of an incoming target. The design analysis techniques used to determine vulnerability to attack or lethality of an explosive detonation have been tested against a large database of experimental and test results.
Ships or submarines are subjected to shock tests, for instance, by setting off a large explosive charge a good distance from the beam of the ship. (The ships are invariably crewed; close-range explosions might cause casualties.) The charges provide a side-on shock similar to what might be created by a distant underwater nuclear explosion. Since the vessels aren't meant to be destroyed, they are tested at only a fi-action of their design capability. But the tests help to give crewmen live fire experience and enable designers to better understand how ships perform under shock conditions.
Understanding the Dynamics
Building explosive testing chambers presents a whole new set of challenges, since the purpose of such tests isn't the survival or destruction of the vessel, but gaining a better understanding of the explosion's dynamics. A pressure vessel under internal high explosive detonation pressure is typically subjected to two distinct types of loading. Immediately following detonation of the high explosive, the shock wave inside the vessel imparts a transient impulsive pressure loading to the vessel wall. Then a long-term quasi-static pressure buildup occurs in the vessel as' a result of expanding reaction gas products. It is the early-time impulsive loading that is limiting for containment vessel design.
To make such a vessel, then, one needs to use high-strength steels that can withstand the impulse loads without significant amounts of yielding and distortion. One also must form and weld the high-strength steel plate and forgings into shapes (such as spherical shells) that resist internal pressure in an optimal manner.
But it goes beyond the obvious. Into this steel chamber, one must also design windows using materials that are essentially transparent to test instrumentation and diagnostics, but that also provide confinement equivalent or nearly equivalent to the high-strength steel-no small order. And one must protect the internal surface of the vessel from excessive penetration by fragments from the detonation. For vessels that are intended for reuse, the fragmentation damage issue and the potential for low-cycle fatigue failure from multiple detonations are major considerations
Dozens of vessels have been designed, fabricated, and used over the past several decades, even prior to the cessation of underground nuclear testing in 1992. An early Los Alamos National Laboratory design consists of a containment vessel with a 6-foot inside diameter and three access ports. The high explosive charge is placed inside the vessel at the geometric center of the spherical shell. The United Kingdom Atomic Weapons Establishment design is a cylindrical vessel with a hemispherical end-cap and a heavy reinforcement closure connection.
These vessels were built using rational design criteria. In neither case were the design rules of the ASME Boiler & Pressure Vessel Code used directly. For the Los Alamos designs, the basic approach used for nuclear power plant equipment subjected to extreme and unlikely forces was adapted to include limits on strain that provide protection against plastic instability failure. For the British designs, the stresses caused by the explosive forces have been classified in a different manner than used by the ASME Code, to take into account the difficulty of creating failure mechanisms in vessels by impulsive loads.
Imagine yourself, for a few tenths of a millisecond, in one of these pressure vessels immediately following the detonation of 40 pounds equivalent of high explosives. The early-time loading on the inside surface has a peak pressure amplitude of almost 12,000 psi. If this happened to be a sustained pressure amplitude, the vessel would be torn apart by the associated tensile stresses, with shards of steel flying off in every direction.
But that doesn't happen. That's because the loading takes place in a much shorter time than that in which the pressure vessel can respond structurally. The sustained internal pressure from this high explosive detonation is only on the order of 1,000 pounds per square inch, well within the design basis.
If the initial load is impulsive rather than sustained, the response to the shock wave by the vessel is initially only inertial: It begins to move, but the loading time for the impulse is insufficient to cause stretching and bending. The impulse time scale is very short, after all, when compared to the periods of natural vibration of the pressure vessel, especially the natural vibration bending modes of the vessel.
Failure can occur only after the vessel stretches and bends-that is, after the initial impulse has disappeared and the absorption of the impulsive energy begins. The intensity of the pressure within the vessel after the initial impulse shows a recurring and slightly decaying pattern that has a period of about 1 millisecond per cycle, or a frequency of about 1,000 cycles per second. It is during this time of decaying vibration that a potential failure can occur, such as the formation of multiple plastic hinges and associated plastic instability. If the vessel withstands this vibration without excessive distortion, and provided no brittle failure mechanism or any low-cycle fatigue failure mechanism forms, the eventual quasi-static pressure inside the vessel can be readily withstood.
In contrast to the brief detonation impulse, months and years are spent preparing for a single test. And the tests cost millions of dollars each. The instrumentation used to look through the confinement boundary in order to observe the event is sophisticated and state-of-the-art. But what is seen by this advanced instrumentation and the interpretation of the observations is classified information, because these tests have critical national security implications.
Since the limited underground nuclear weapons test ban went into effect, a number of these controlled experiments have been conducted to help model the behavior of explosions and their effects on surrogate materials. And with the current moratorium on underground nuclear testing, the nuclear weapons laboratories of the U.S. Department of Energy are faced with great challenges in certifying the nation's nuclear weapons stockpile.
Los Alamos and Lawrence Livermore National Laboratory have chosen to combine aboveground testing using mock materials, advanced X-ray and proton radiography, and advanced computing capabilities for complex simulations. These vessel systems, when used with diagnostics such as flash or proton radiography, provide important data that help our weapon's designers validate design codes and support the certification of the weapon systems. The Atomic Weapons Establishment in the United Kingdom has similar activity under way.
These high-pressure vessels have other security-related uses as well. U.S. Department of Defense programs on explosive ordinance and demolition, such as the program at Sandia National Laboratories, are faced with an ever increasing need to dispose of and demolish unused, aged, and sometimes unknown munitions in a safe and contained environment using similar vessel designs. The types of impulsive dynamic loads to which these vessels are subjected are also similar to the loads generated by weapons stockpile certification tests.
The confinement vessels used for these tests have been designed and constructed with a very high level of safety, based on requirements specified by experts in material behavior, pressure vessel design, and dynamic analysis. As a result, no confinement vessel to date has failed to perform its intended function. But new requirements are emerging. Even with this demonstrated safety record, the Defense Nuclear Facilities Safety Board, the regulatory oversight body for such facility design and construction, has recommended that, wherever possible, the construction rules of the ASME Boiler & Pressure Vessel Code be used, as applicable.
But existing ASME code rules for such vessels may not be appropriate. Although various vessel designs have been used effectively and safely over the past four decades for testing and demolition activities, explicit design criteria for impulsively loaded vessels from the ASME code have never existed. The absence of a consensus to standardize and adopt those design practices has been a barrier.
Things are about to change. The U.S. and British labs are currently working with ASME Boiler and Pressure Vessel Code, Section VIII Division 3 (High Pressure Vessels) to develop design criteria directly applicable to impulsively loaded vessels. A task group chaired by Robert Nickell, a former ASME president and a designer of similar vessels, started meeting in January this year to begin developing the various design requirements with the intention of submitting an ASME Code Case for approval. The task group reports to the Special Working Group on High Pressure Vessels, the consensus ASME code body responsible for developing and maintaining Section VIII Division 3. The Pressure Vessel Research Council has also played an important role in reviewing and approving Welding Research Council bulletins that will serve as the basis for the task group's work.
The evaluation of the vessel's structural response to the sustained pressure loading is carried out in identical fashion to traditional ASME Section VIII Division 1 design procedures. And any cyclic fatigue considerations associated with the vibration response of the vessel can be carried out in a similar manner to the traditional fatigue evaluation procedures of ASME Section VIII Division 2. However, accounting for the early-time impulsive loading goes beyond standard practices embodied in the ASME code.
In order to extend the standard practice to include impulsive loads, data and information from the past four or five decades of national defense- related technology development must be used. Nuclear weapons effects testing in the 1960s and 1970s involved impulsive loads on aerospace structures. One of the important findings from the tests of that era was that failure of the structure did not correlate simply with the size of the load. Instead, the impulse was the deciding factor. The failure of a structure was governed by the ability to withstand stress and deformation over the duration of load. Failure, in this case, could be penetration, buckling, plastic collapse, or any of a variety of structural failure modes.
FAILURE OF THE STRUCTURE DID NOT CORRELATE SIMPLY WITH THE SIZE OF THE LOAD; INSTEAD, THE IMPULSE WAS THE DECIDING FACTOR
Even with this wealth of data, codifying a design procedure for impulsive loads is fraught with difficulties. There's the problem of the time lag between the impulsive load-complete within a few tenths of milliseconds- and the onset of structural failure, often at two to 10 milliseconds. Even though the load has long since disappeared, there's a causal connection between the load and the failure.
The amount of stress, or excitation, that the impulse places on the structure can be figured as the integral of the load over time of duration. Initially, this excitation is resisted by inertial forces, which are gradually transformed into structural deformation and internal stresses. If the excitation is sufficiently large, the structural deformation and internal stresses can cause failure.
The very fact that the impulsive load has disappeared at the time the internal stresses are fully developed also leads to stress classification ambiguities. One school of thought treats all of the internal stresses as either secondary or peak, since they were never in equilibrium with external forces. Others want to bypass stress classification entirely and seek design margin instead on the global failure mode, whether that global failure mode is plastic collapse or buckling.
The end result will provide credible design criteria approved by a highly recognized international standards development organization. Businesses will benefit from establishing this code, but the real benefit will accrue to national security. For the first time, we will have a standard for designing these vessels, and that will help in analyzing mock weapons tests. As long as we maintain the nuclear test ban, those mock tests will be a key source of strategic weapons data.