Ordnance Hermeticity Problems, Detection and Escapes

George R. Neff and Jimmie K. Neff 1soVac Engineering, Inc., Glendale, California, 91201

IsoVac Engineering has been engaged in research and failure analysis of ordnance device hermeticity problems for over thirty years. The findings show that there are distinct misapplications of various test methods in attempting to verify the hermeticity of ordnance devices. There is also a lack of advancement in the test methods being applied, which is not accompanying the changes in design toward "zero-cavity" devices such as initiators. The leak rate requirements for most ordnance devices cover the range of leakage involving very large or gross leakage only. That is also the range where most leak detection methods encounter difficulty. This paper is addressing those difficulties, technological advancements in testing, and presents some of the findings resulting from those leak test shortcomings and misapplications. The authors present some of the critically needed and little understood technology governing the tracer gas 'Seal Testing'.

I. Introduction

The hermeticity of ordnance devices must be viewed in proper perspective. This paper addresses the hermeticity of the ordnance devices from the point of manufacture, through the field firing of the device. The devices are considered to be of a quality that meets military standards, and are required to meet the design criteria that includes a 'hermetic seal' to at least 5 x 10-6 std ccisec. Military products are expected to meet that design requirement from the point of manufacture throughout their life, in order to protect the contents of that package. Any seal test failure from manufacture through field storage is considered to be a 'failure to meet design criteria'

In reviewing hermeticity problems associated with ordnance devices, it is important to: (a) categorize the devices by the size of their internal "free-volume" or "cavity", (as this is a most critical factor affecting the reliability of most leak detection methods); and (b) recognize the size of leaks that are being evaluated. The internal free-volume controls the amount of leak test tracer-gas that can be deposited or stored within the device through a leak passage. The size of the leak that is being evaluated or measured regulates the rate of tracer gas introduction as well as the rate of loss of that tracer gas after pressurization and during measurement. These factors have been carefully reviewed and are found to be commonly overlooked by many of those who are applying the most widely used leak testing methodology: "tracer-gas back pressurization" ( I ) . For clarification purposes, it is an accepted fact that the ordnance industry prefers not to use hot liquid immersion bubble testing methods. That limits the leak test methods primarily to dry gas back pressurization leak testing procedures such as helium mass spectrometry or radioisotope leak test.

11. Technology

The two leak test methods reviewed in the leak testing of ordnance devices are the Helium Mass Spectrometry method, and the Radioisotope Test Method. These two methodologies are reviewed to evaluate their respective pros and cons when applied to small or zero cavity ordnance devices, as that is clearly where the greatest challenge is found and the most non-hermetic 'escapes' are being found.. The major shortcoming is encountered when the leakage of a device is within the gross leak test range, i.e. 'visible' through ' 5 x 10-6 std cc/sec', (the measurement range applied to most military ordnance devices). This is seriously complicated when the device internally has a 'small cavity' or 'zero-cavity'. Most ordnance materials are either loaded as a 'loose-pack', or they are 'compressed' into the internal cavity of the device. Most loosely packed powders completely fill the cavity, leaving a very small free-space as inter-particulate voids that can entrap tracer gases in a leak test, while the con~pressed ordnance usually contains a binder that virtually eliminates almost all of the cavity.. How these conditions affect a tracer gas leak test is discussed below, along with the detailed technical discussion of both of these 'Leak Test Methods".

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In recent designs, many devices have been reduced in size, while maintaining a substantial charge, which is achieved by compressing the charge into the device housing. Organic binders are also used that are mixed with the ordnance materials and which virtually eliminate any interstitial or inter particulate cavities. Devices with defective

(2) seals using this type of design are found to escape leak detection when conventional leak test methods are used . Devices that use compressed ordnance with binders are found to have internal free-volumes as small as cubic centimeters, with most of that cavity lying under the bridge-wire. Such micro cavities cannot store sufficient amounts of tracer gas to make a conventional leak test reliable. It is also not an appropriate assumption that "if a

device has no cavity, it cannot leak". Ordnance devices are commonly found to have cavities as small as cc, with that micro-cavity usually found between the glass header and the bridge-wire. These devices are commonly termed as "Zero-Cavity" devices. The most critical zone in a small initiator or squib is the Bridge-wire zone. The bridge-wire typically lies across the glass-to-metal seal, which is also where most of the leaks are found. That leakage provides corrosive contaminates directly to the bridge-wire zone, where we find two 'dissimilar-metal' junctions, (ideal for galvanic corrosion).

I11 The Military Standards

Most of the US Military Standards from which the resultant electronics, space, aerospace, and commercial specifications have been derived for the 'Seal Test' or 'Leak Test Procedures' during the past 40 years, do not allow the conventional 'tracer-gas leak test methods to be used for the "Gross-Leak Rate Range", without very special conditions proven to be met. A quick review of the most common of those specifications seems warranted here:

-1 -6 10 ................................................... 5 x 10 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 -1 0

[t Bubble or Dye Penetrant +I[+ Helium or Radioisotope +I

-1 -6 10 ........................................ 5 x 10 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 -1 0

[t Bubble or Dye Penetrant +I[+ Helium or Radioisotope +]

L

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[t "No Cavity" Dye Penetrant +I[+ Helium or Radioisotope +I

MIS- 13474~ "Missile Inspection Systems-Squibs"

The leak rate ranges called out for these specifications, (lo-'--- 5 x 10-~---10- '~ std cclsec), are chosen to provide a generally accepted range for most of these specs. The 5 x mid-point is arbitrarily chosen as a commonly used point where the transition is made from "Gross Leaks7' to "Fine Leaks". It is recognized that great controversy exists in the acceptance of the type of gas flow and resultant characteristics of leakage that is seen in the 'Gross-Leak' range of leakage reviewed here, namely 'Viscous-Flow' versus 'Molecular-Flow'. Considerable research has been devoted to understanding this gas-flow theory, and much more is currently in process. The primary difficulty lies in the lack of confirmation of leak path size, length of the leak passage, and geometric configuration. However, with small cavity devices and gross leaks of the size most commonly encountered, either flow model will confirm a very rapid loss of tracer gas in a leak test. It should also be noted from most of these specifications, (as well as others), that the creators of these specifications found it necessary to limit the "Dry Tracer-Gas" test procedures to the "Fine- Leak" ranges, unless specific capabilities could be verified. Those specification limits were the result of years of studies and reviews by the electronics manufacturers and the users, who jointly proof tested the methods.

IV Test Methods

Application of conventional tracer gas leak test methods is not reliable for the testing of small cavity devices unless the procedures are modified to accommodate the device anomalies (3), otherwise the test method is being 'miss-applied'. Leaking-devices may escape without detection, which is resulting in devices being found, mostly with 'gross-leaks'. The study of small cavity ordnance devices indicates that perhaps as high as 80 % of the leaks are greater than std CC/S leak rate. Such large leaks in small cavity devices makes it difficult for any conventional tracer gas leak test method to reliably detect the leaker.

Helium Mass Spectrometry is commonly applied to fine-leak test devices to sensitivities of to as low as

10-1°std cclsec. As applied to most ordnance devices it involves the placement of devices in a pressure vessel where they are pressurized for a period of time to introduce enough helium tracer gas into the device through a leak, and then be detected when the device is evacuated into the mass spectrometer. The devices are placed into the test chamber of the mass spectrometer, (MIS). The test chamber is evacuated, (usually to a vacuum of -0.0007 atm), and the chamber atmosphere then sampled into the MIS in order to measure any helium tracer gas that may be leaking out of the device with a defective seal. Surveys of many users of this method state that they require from 20 to 45 seconds for the 'test chamber' to be evacuated to a low enough vacuum level to allow the 'test chamber' to then be safely opened into the MIS where it is quickly measured for traces of helium. This appears to be one of the major causes for the escape of leakers. The tracer gas in a gross-leaker is too quickly lost during 'test-chamber' pump- down, or its quantity greatly reduced before the MIS can sample for any helium leaking back out of the device. This can result in an escape or a leak rate indication to be much smaller than it is. For reference: a device with a 0.00 1 cc internal void and a 1 x 10-4 std cclsec leak, will loose about 64% of its tracer gas in 10 seconds when evacuated into the mass spectrometer. Larger leaks and smaller cavities will exacerbate the problem even more.

Most ordnance materials studied do not have much solubility for helium tracer gas. The helium is only 'entrapped' within cavities, interstitially, or in interparticulate spaces. If the gas is interstitially or interparticulately trapped, the size of the actual leak becomes critical. When the sample part is placed in the MIS 'test-chamber', a gross leak will allow a vacuum to be rapidly achieved within the leak path up to the surface of the ordnance material. At that point in time, the supply of tracer gas drops off exponentially and the MIS then sees only the

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diffusion of helium tracer-gas from within the powder. Even less helium is seen of the tracer-gas that may have dissolved into and must now evolve from the organic binders. Realizing that 60-80% of the leaks detected are large 'gross-leaks', it is easy to see that the helium tracer gas available for measurement within the MIS drops to only trace quantities. Pressurization of the device with helium tracer gas may dissolve helium into the binders, however the 'desorption' of that dissolved helium is extremely slow. Studies show that the result is an indication of a "much smaller leak", often less than the specification limit, and the device is passed. It should also be noted that the MIS measurement process is usually completed in less than one minute, thus lengthy diffusion equilibrium measurements are i~sually not practical. In simple terms, the 100% helium tracer gas within the leakage path may be at atm press after removal from the pressurization tank, but it is lost instantly when vacuum is applied. Studies show that the dissolved helium concentration in the organic binders is very low. When it is slowly released from the organic material, it is in such low concentration the M/S must be on a 'high sensitivity scale' to detect it, which in turn commonly produces "indicated-leak-rates" of or less, (well below the specification limit).

Radioisotope Test Method is normally applied using a "Back-pressurization" procedure, similar to the helium -10 test. Like the helium method, it is commonly used to detect fine leaks to levels less than 10 std cdsec. However,

it is also commonly used to detect gross leaks. The devices to be tested are placed in a pressurization chamber and pressurized with a mixture of Krypton85 gas and nitrogen or 'air'. The required concentration of Kr85 is about 1 part in ten thousand parts of air. To avoid diluting the Kr85 gas mixture, the pressurization chamber is first evacuated and then the Kr85 is transferred into the chamber to a desired pressure. Ordnance devices are pressurized at 30-75 psia for periods as short as 36 seconds to achieve sensitivities to 5 x atm ccls. The Kr85 is fully recovered back into the storage tank, and the devices are removed from the chamber and taken to a scintillation crystal detector. Here, the devices are measured for any traces of Kr85 gas within the part.

The primary difference between this method and the helium MIS method is that the Kr85 gas is measured in place, within the cavity of the device without the necessity to evacuate the Kr85 back out through the leak. The Kr85 gas emits both gamma and beta radiation. The beta radiation is a weak particle and will rarely penetrate the wall thickness of an ordnance device. The gamma ray does penetrate the wall and allows the detection of any Kr85 that may have leaked into the device through a leak. This measurement is made through the walls of the device, without the requirement to suck the tracer gas back out of the device through the leak.

DETECTION SYSTEM 7 I

,- Crystal Detector

The drawing shows an example of a device that has been bombed in Kr85 and then placed in the scintillation crystal for measurement of the gamma ray emission from the Kr85 trapped within a leaking device. The gamma rays emitted to the sides and down into the sodium iodide crystal are detected as counts per minute. The detection

process requires about 100 milliseconds. The number of counts per minute is an indication o f the number o f Kr85 molecules within the part. This is a direct measurement of the

"absolute leak rate" of the device.

The use of Kr85 tracer gas for the detection of gross leaks is allowed by Military Standards, if specific test requirements can be satisfied. The detection of Kr85 in small cavity devices is quite reliable if the cavity is large

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enough to allow a "detectable-quantit " of Kr85 to enter the device. The quantity of Kr85 that is normally accepted for the minimum detectability is - molecules of Kr8S That would be seen as approximately 1,000 counts per minute above the normal 600-800 clm ambient background. Comparing to the helium back-pressurization of a 0. lcc cavity device, where a MIS test normally assumes a 10% He in the device for measurement purposes That would

represent a helium concentration in the range of lo2" molecules of helium. An equivalent test would require a partial pressure of only loll th molecules of Kr85. The very high detectability of the Kr85 moIecule results in relatively short bombing times

The use of a radioactive tracer gas provides other unique tools in the understanding of leakage. Once a leaking device has been pressurized in Kr85 tracer gas, the number of Kr85 molecules within the device can be determined. The actual leak rate of the device can be calculated and that leak rate can then be verified very accurately by measuring the rate of loss of the radioactive gas from within the device (j). This is accomplished by placing the device in vacuum, and remeasuring the device for Kr85 content at time intervals to determine the exhaust rate of the Kr8.5 tracer gas through the leak. The partial pressure of the Kr85 gas is being tracked as a function of time as it leaks back out of the device through the leak, which allows a calculation of the "absolute leak rate" of the device. The gas flow regime can also be determined from this 'vacuum-decay' test procedure. Leak rate 'reference standards' using these actual devices are the result of this procedure.

An equation used for "Vacuum Confirmation of Leaks" (5)

Where: Pt = partial pressure, (or counts per minute) of Kr85 a t time "t" P, = original partial pressure, (or counts per minute) of Kr85

Leak rate (cchec) k =

cavity vol. (cc) t = time in vacuum (sec)

A new technology: The Kr8.5 method faces the same inability to detect a gross leak in very small cavity or 'zero-cavity' devices as does the helium method. There is not enough space to hold sufficient Kr85 to be detected. The Kr85 tracer-gas is also leaking out of the device very rapidly through a gross leak, reducing the detection potential and limiting the time for measurement. There is a new technology being applied to testing small and zero- cavity devices for gross leakage using a "gettering material" to collect and hold the Krypton85 tracer gas. The concept of gettering is not new, but the gettering of specific tracer-gases for leak detection has led to the patented

(6) (7) process of using 'steam-activated coconut-shell charcoal' for the gettering of Kr85 tracer gas in leak detection . This material has a unique ability to "Adsorb" Kr85 gas and holds it for periods of 15-30 minutes after bombing, even with a visibly 'wide-open' leak. The Kr85 is held very strongly to the charcoal by van der Waals forces on the

3 unusually high surface area of the charcoal. A piece of charcoal - 0.003" diameter has a volume of - 4 x cm ,

2 weighs 0.243 pgms, and provides 133 mm of surface area within the carbon lattice. This charcoal is a very efficient Kr85 gettering material, but, unfortunately, does not getter the helium molecule. This technology is being used in millions of ordnance devices for the assurance of detection of gross leaks. It also allows the devices to be tested for both gross and fine leaks in a single 'dry-gas' leak test. Leaking devices may be reworked, subjected to qualification tests and retested repeatedly in compliance with the military standards, which do not allow a dry tracer gas leak testing after a liquid bubble test has been applied. An additional feature of the charcoal is that it will adsorb up to 27% by weight of water, should a part develop a leak during its field life.

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V Conclusions

There needs to be an expanded effort to update the specifications applicable to the leak testing of ordnance devices. Recent studies show that there are misapplications of seal test methods being applied to devices that are for critical life threatening circumstances. Non-hermetic devices are escaping through the 'seal-testing' process, and are being frequently found through the application of other test methods or system functional detection. It is also recognized that some non-hermetic devices are fired in application before they can be compromised by the environment.

In addition to the author's studies, there are other studies being reported that are evaluating some of the causes of

hermetic seal failures in ordnance devices (lo). Such studies are also pointing to a need for specification

review. The specifications must be evaluated by physical application to devices, by comparison of methods, and confirmed by failure analysis to isolate the flaws in the methods and establish specific limits and/or restrictions to be incorporated into revisions of those specifications. The far too commonly heard approach is to continue to use the procedure, because "we have always used it", "we haven't seen any failures", "the procedure is allowed in our contract", etc. These are flawed, and do not focus on the reality of what the purpose of the specification is: to detect any "non-hermetic devices in the lot". If we misapply a test method, or use an inappropriate test method, we likely will not detect the non-hermetic device.

( 1 ) "Pyrotechnic Initiator Research at the University of Idaho", Rink et al, 1 9 ' ~ International Colloquium on the Dynamics of Explosives and Reactive Systems, Hakone, Japan, July 27,2003.

(2) "Testing of Non-Hermetic Initiators for Improved Airbag System-Issues and Needs", GR Neff, & JK Neff, lsoVac Engineering, SAE 2005 World Congress, April 14, 2005.

(3) ASNT Non-Destructive Testing Handbook, "Leak Testing", Third Edition, Volume 1, 1998

(4) "Fine and Gross Leak Testing of Hermetically Sealed Devices", Prisco A. Panza, Grumman Aircraft Engineering, Report FSR-AD9-01-68.2, 1968

(5)ASNT Non-Destructive Testing Handbook, "Leak Testing", Third Edition, Volume 1, Chapter 14, p.569, 1998

(6) U.S. Patent 5,452,661, "Hermetically Sealed Devices for Leak Detection", G.R. Neff, & J.K. Neff, 1995

(7) U.S. Patent 5,929,367. "I-lermetically Sealed Devices for Leak Detection", G.R. Neff, & J.K. Neff, 1999

(8) "Electro-Explosive Device Research at the University of Idaho", D.V. Gunter; A.C. DuBuisson; K.K. Rink, Proceedings of the 42"* SAFE Symposium, 2004, pp. 326-332.

(9) "Failure Mode Investigations Related to Non-Hermetic Behavior in Bridge-Wire Initiators", K.K. Rink, Proceedings of the 5'h Cartridge-Propellant Actuated Device (ChDIPAD) Technical Exchange Workshop, Naval Surface Warfare Center, Indian Head Maryland, 2004.

(10) "Thermal Stresses and Potential Failure Mechanisms in Airbag Initiators", L. Thompson; K.K. Rink; D. Blackketter, Proceedings of the 6"' International Congress on Thermal Stresses, Vienna, Austria, May 2005, pp301-304.

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