American Institute of Aeronautics and Astronautics1
Nondestructive In-Situ Examination of Solid Rocket Motor Propellant Using Induced Positron Technology
Douglas W. Akers* and Curtis A. Rideout†
Positron System, Inc., Boise, ID, 83713
Failure of solid rocket motor propellants due to structural or chemical breakdown from manufacturing or aging effects can result in failure of the rocket and may require significant surveillance and maintenance requirements to assure operational readiness. Defects and aging effects in rocket propellants are significant issues particularly for new propellants, which may suffer from short experiential histories, frequently unknown lifecycle parameters, and the inability of current non-destructive inspection techniques to detect damage (other than gross cracking) in these materials. Grain cracking and dewetting are significant propellant problems, which result from strain-induced damage in the solid rocket motor propellant and are undetectable by current nondestructive inspection methods. An inspection technology that is capable of accurately, reliably, and nondestructively measuring strain and dewetting effects in advanced propellants through the outer rocket casing is needed for the purpose of assessing operational readiness for all solid rocket motors. New technologies that induce positrons inside the material to be characterized have demonstrated capabilities in nondestructively measuring material damage in many material types. Damage assessment of solid rocket fuel propellant through the outer missile shell has been demonstrated and indicates that accurate and reliable evaluations of component readiness, either at the manufacturing level or during field inspections can be performed. The feasibility assessment discussed here demonstrated that empirical data on solid rocket propellant strain buildup and grain cracking can be generated using the induced positron technologies, which can be used with confidence for assessing the damage state and mission readiness of advanced propellant or similar materials.
I. IntroductionFor many missile systems, solid propellants are utilized at a considerable weight savings and with minimized
maintenance requirements as compared to liquid propellant alternatives. However, failure of solid rocket motor propellants due to structural or chemical breakdown from either manufacturing or aging effects can result in strain aging and failure of the rocket. In many cases, rocket propellants are subjected to extensive maintenance or removed from service long before the end of actual life due to reliability concerns. Defect and strain aging effects in rocket propellants are a significant issue, particularly for new propellants which may suffer from short experiential histories and frequently unknown lifecycle parameters; including undetectable manufacturing or combined manufacturing/operational damage which may result in early failure and directly impact mission success. A related, critical problem is that current nondestructive inspection (NDI) techniques such as computed x-ray tomography provide limited or no capability to detect damage in propellants other than gross, large-scale cracking. Consequently, limits on inspection sensitivity impacts the application of these techniques to propellants, thereby increasing maintenance requirements and the risk that the missile system will fail and/or suffer unexpected, catastrophic failure during operation.
Propellant grain cracking and dewetting are significant problems which are induced by strain in the solid rocket motor propellant and are undetectable by current nondestructive inspection methods. An inspection technology that is capable of accurately, reliably, and nondestructively measuring strain and dewetting effects in advanced rocket fuel propellants through the outer rocket casing is needed for the purpose of assessing operational readiness for all solid rocket motors. New technologies, Photon and Neutron Induced Positron Annihilation (PIPA/NIPA) that induce positrons inside the material to be examined and allow inspection through the outer casing have demonstrated the
* Technology Director, 6151 N. Discovery Way, Boise, ID 83713, Member at Large.† Marketing and Sales Manager, 6151 N. Discovery Way, Boise, ID 83713, Member at Large.
Space 2004 Conference and Exhibit28 - 30 September 2004, San Diego, California
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capability to nondestructively characterize material damage, including defect formation and strain effects for many material types. This capability results in the potential for accurate and reliable assessment of propellant operability,both at the manufacturing stage and during field inspections. These advanced NDI techniques can be used as reliable rocket propellant damage inspection tools with the capability to characterize the buildup of manufacturing and operational damage without rocket disassembly and at detection limits such that initial manufacturing damage can be measured. Portable equipment makes the technology suitable for field environments and on site maintenance inspections.
The Induced Positron Annihilation (IPA) technologies represent new nondestructive material characterization technologies which can provide data on chemical structure changes and the buildup of defects in solid propellant fuels at levels well below those detectable by other techniques. This real-time measurement information is provided via computer screen/print-out at the completion of each measurement and provides a method of quantifying failure progression and damage growth at any point in life. Further, this technique can be used to provide early detection of chemical instability that may result in near term failure of the rocket propellant. Consequently, IPA provides suitable data for remaining life assessments based on in-situ measurements of propellant damage through the rocket casing.
II. Induced Positron Annihilation TechnologyThe IPA technology used in this research was recently developed by the Idaho National Engineering and
Environmental Laboratory (INEEL). The technology is both a volumetric and near surface damage measurement method that is a new addition to material characterization technologies and that has specific applications to almost all materials industries. IPA extends the current limited use of traditional, surface positron annihilation measurement technology to a much broader range of applications, allowing the process to be used as a more general-purpose, nondestructive assay technique. PIPA, the Photon Induced technology, has shown remarkable potential in the identification and measurement capabilities for material assessment that include:
- Identify atomic lattice defects <10 microns in size. - Measurement uncertainties on the order of less than 1%.- Multi-layer defect detection in metals and composites.- Cross-sectional analysis. - Assess lattice structure change/damage <1%, where crack initiation = 100%.
The induced positron annihilation (IPA) process generates positrons deep within the bulk material through the application of high-energy X-ray or neutron bombardment of the target material. Positrons are formed when the X-rays or neutrons cause a neutron to be ejected from a material’s atom (photo-neutron/neutron-two neutron reaction) and the resultant atomic isotope decays into a more stable material through positron decay. This is a revolutionary advancement over previous positron beam spectroscopy where the positron penetration and defect detection depths were significantly limited and impacted by surface characteristics. The positrons created by the IPA process are formed throughout the bulk material, achieving a better sensitivity and accuracy level of defect detection than positron beam spectroscopy. The depth of defect detection is only limited by the attenuation of the annihilation gammas to be measured by the germanium detector; related to the material density. This depth can be up to 2 inches in iron, 3.5 inches in titanium, and up to 4 inches in aluminum, which can be doubled with two detectors and access to both sides of the material/component. Figure 1 shows the PIPA processes for volumetric measurements and the creation of a probe for near-surface measurements. Figure 2 depicts the formation and subsequent thermalization of the positron as it travels through the lattice structure, searching for a lower charge density region (defected area), where it becomes trapped and then annihilates with an electron. The energy level of the annihilation gammas released is indicative of the level of damage. The positrons created by the IPA process are formed throughout the bulk material, achieving better sensitivity and accuracy of defect detection than surface positron beam spectroscopy. Additional information on the measurement processes used can be obtained from a number of references.1-7
The IPA technologies developed to date indicate that derivative methods of inducing positrons into materials and components are capable, leading to smaller, more portable and integrated designs; including probe designs capable of inspections through borescope holes. Improvements in miniaturization of electronics, faster computer processing speeds and development of smaller detectors have made it possible to build and operate an in-situ or small portable inspection capability. IPA’s capability to use a small, robust neutron generator or neutron source to generate the positrons within the bulk material results in the potential design of small, field portable units. The measurement
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response would be captured by a standard handheld detector(s) and fed to an output device, either a laptop or a configured PDA device.
III. Data and Life Remaining AnalysisPositron annihilation measurement data can be analyzed by several methods that provide different types of
information concerning the extent of damage and changes in the microstructure. These data can then be converted to remaining life estimates through standard regression and statistical methods that can be used to relate operational and integrated off-normal damage effects to the measured positron annihilation response. Measurements on a range of components from new through end-of-life can be used to define statistical bounds that identify the likelihood of failure for the component being examined. The following sections define the methods used to analyze the positron annihilation results and the statistical methods used to convert these data to remaining life estimates.
A. Data Analysis MethodologyThe positron data analysis is based on characterization of the distribution shape of the annihilation gamma
energies in the 511 keV peak. The annihilation energy (511 keV) of the annihilating positrons is incrementally affected by the momentum of the electron with which the positron annihilates. Defects contain a higher ratio of free electrons to core electrons than non-defected materials. This phenomenon can be explained by the tendency of free (conduction) electrons to lose energy and slow down or stop in the defect. Core electrons have a much higher linear momentum than do free electrons. Thus, gamma rays from annihilation events involving free electrons are more likely to approximate the energy (511 keV). This characteristic makes it possible to detect the presence of defects from the energy spectrum of the gamma ray emission when the positron and electron interact.
The primary measurement technique used is measuring the Doppler broadening of the 511 keV gamma ray peak. Although in principle, the Doppler broadened gamma ray spectrum can be deconvoluted to extract the electron momentum distribution, a simple shape parameter is commonly used to characterize the annihilation peak. Two parameters, S (for shape) and W (wings) are usually employed. The S-parameter is defined as the ratio of the counts (i.e., area) in a central region of the spectral peak to the total counts (area) in the peak, and W-parameter is defined as the ratio of counts in the wing region of the peak to the total counts in the peak. The S and W parameters have a simple relationship to the Doppler broadening (e.g., if the annihilation peak is narrow, which results when positrons predominately annihilate with slow moving electrons, the S-parameter is large and with fast moving core electrons the S parameter is small). These annihilations with the valence electrons are reflected in the S parameter and those with core electrons in the W-parameter. This “line shaping parameter” or “S” factor for the material is compared to known “S” factors for similar “as manufactured” and failed materials to quantitatively determine defect density and lifecycle percentage.
Figure 3 shows the change in the S parameter response as a function of the change in the shape of the 511 keV gamma ray peak shape and Fig. 4 provides an example of the measurement response for damaged and undamaged
η
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.5ll MeV
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material. As indicated in the figure, the peak relationship changes as a function of the defect levels in the material. Through the detection and quantification of damage build-up in structural materials and composites, a calibration process can be developed to include calibration curves that correlate the PIPA measurement response for propellant dewetting damage from the “as manufactured” to failure condition.
IV. Rocket Propellant Damage DetectionAn initial feasibility study was conducted to evaluate the ability of the volumetric Photon Induced Positron
Annihilation (PIPA) technique to detect and quantify operational damage in H19 simulated rocket propellant test specimens to assess dewetting and grain cracking. For the strain measurement tests used to induce dewetting and grain cracking, dog bone test specimens were obtained from ATK Thiokol Propulsion (ATK), a rocket propellant manufacturer, who provided technical support on both the test specimen development and on the test protocols for the simulated propellant tests. Figure 5 shows representative dog bone test specimens that were prepared by ATK and Fig. 6 shows an example of the fatigue test system used to produce a given stress/strain condition for the H19 simulated propellant samples. This test device, which was provided by ATK, was modified to incorporate an Entran load cell with a measurement range from 2-100 lbs. In addition, standard 1% increment strain gauges were obtained and attached to the coupons during testing to measure strain changes and for direct comparison with some ATK testing methods.
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The PIPA test protocol developed for these initial tests was based on information from ATK that indicated the strain range for the H-19 simulated material dog bone specimens ranges from about 2% up to failure (nominally 10%) with dewetting occurring at 4-6%. Consequently, the primary emphasis of the initial measurements was on the strain range between 0 and 10%. Another factor that can affect propellant failure is strain rate, nominally 0.2 in./min. (a change of 2% strain over a period of 15 seconds was used for the 2.6 inch gauge section of the ATK specimens). The strain rate, strain and load were monitored at each change in load.
A. H19 Propellant Coupon Strain MeasurementsStrain testing was performed on H19 simulated rocket propellant specimens under increasing levels of stress to
assess the change in the volumetric PIPA S parameter response as a function of the strain induced into the dog bone coupons and to assess the change in response when the coupon failed. For these measurements, a load cell was incorporated into one of the two test systems used (Fig. 6). For the tests performed with the load cell instrumented test assembly, the instantaneous load/stress was monitored throughout the entire measurement series at various points to assess both elastic (recoverable) and plastic (permanent) strain effects.
PIPA measurements were performed on specimens of the H19 simulated propellant to assess 1) coupon homogeneity and measurement reproducibility for these dog bone specimens, 2) the effect of increasing strain on the dog bone test specimens through failure and 3) the capability of the PIPA process to measure propellant characteristics through the outer composite shell of the missile assembly. In addition, as samples of composite shell material with a range of damage levels were available, measurements were performed to assess PIPA’s ability to detect damage in the outer composite shell.
Figure 7 shows the S parameter measurement data correlated with the strain as calculated using measured elongation for each specimen at 2% strain increments. These data clearly indicate that although there are initial specimen-specific variations in the S parameter response prior to introducing strain (possibly curing variations), the S parameter response monotonically increases for each sample and clearly quantifies the increase in strain to failure.
S Parameter Correlation with 0.2in./min. Strain Rate
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Figure 7. Correlation of S Parameter Response with 0.2 in./min. Strain Rate Coupons
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In addition to the correlations with the basic strain data and specimen elongation, good correlation was achieved between the PIPA response and the load/stress to which the specimens were subjected. The load on the specimens ranged from 6-20 lbs prior to the measurements and these results correlate well with the S parameter response. Figure 8 shows the correlation between load, strain and PIPA S parameter response.
Strain/ S parameter Correlation
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Figure 8. S Parameter Response Correlation with Load/Stress
B. Propellant Damage Measurement through the Composite Material Casing and Casing DamagePIPA measurements were performed to assess the capability of the PIPA technology to perform strain
measurements in the propellant through an outer rocket casing configuration. In addition, PIPA measurements were performed on specimens of good and damaged casing material to determine whether casing damage could also be detected. Figure 9 shows the method used to measure the propellant through the composite casing material.
Figure 9. Defect Detection through Outer Casing Shell
For PIPA measurements performed to assess propellant damage through the rocket casing material, the initial activation is performed at about a 30 degree angle from the measurement geometry depending on the thickness of the casing so that the measurement process is conducted through a section of the casing that is not affected by the
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Measuring the positron annihilation response by angling the detector from the side of the rocket casing can eliminate the “noise” of the outer casing and insulation, resulting in the measurement of only the solid rocket propellant.
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output of the linear accelerator. This method thereby eliminating the effect of any casing damage response on the PIPA measurement of damage accumulation in the propellant. Results of these tests demonstrated the ability of the PIPA process to obtain quantitative accurate measurement data through the casing similar to that performed without the rocket motor casing present.
Subsequent PIPA measurements were performed to quantify damage in the rocket motor casing as several samples were provided by ATK that had known propellant damage. Examination of the casing PIPA S parameter measurement results in Table 1 indicates that the reference sample and the no-damage specimen produce exactly the same S parameter response. These data clearly indicate the directly comparability of the two specimens and verifies that the no damage casing and reference are directly comparable and suitable for quantifying small damage increments in the casing material. In contrast to the data on no-damage specimens, the high damage sample produced a much higher S parameter result (0.0029) than the reference specimens. These data indicate that although there is no information on the extent or depth of the casing damage, the casing damage is detectable using the PIPA technology. Further measurements could have been performed to assess the depth of damage. The PIPA measurement data and response times for the rocket casing and propellant measurements indicate that PIPA is a robust measurement technique for performing propellant damage measurements through outer rocket casings. Further, the data indicate that characterization of the propellant at different depths inside the rocket assembly and damage in the casing itself is highly feasible
Table 1. MDA Propellant and Casing Measurements
Measured Coupon How Measured/Reference S ParameterPropellant Coupon (Failed) Through Casing 0.5511Casing Sample Q Reference Blank 0.5338Casing Sample P No Damage 0.5338Casing Sample H High Damage 0.5357
V. Conclusion
PIPA and related technologies are non-contact, nondestructive examination techniques that have successfullydemonstrated their capability to measure strain induced damage in propellants and the subsequent dewetting and grain cracking that occurs. Initial testing was performed on simulated rocket motor propellant (H19) test specimens, which were subjected to increasing levels of stress/strain that eventually resulted in grain cracking and specimen failure. PIPA measurement data clearly indicated the capability of this technique to quantify strain damage in the H19 propellant beginning at the as-manufactured condition through failure (nominally 12% strain for the H19 simulant). This ability to detect the strain damage that may result in propellant fracture and failure is important for evaluating the reliability of the propellant and to assess aging effects. Additional work needs to be performed to assess the ability of the PIPA technology to quantify chemical degradation and other damage mechanisms that affect rocket propellants. Additionally, initial feasibility for detection of damage in rocket propellant through the outer casing provides an in-situ measurement capability that can be used to assess rocket propellant and casing damage in the field.
References1Hughes, A. E. 1980, "Probing Materials with Positrons," Materials Engineering, Vol. 2, September, pp. 34-40.2Hautojarvi, P. 1979, Ed., “Positrons in Solids,” Topics in Current Physics No. 12, Springer-Verlag, Berlin. 3Schultz, P. J., and C. L. Snead, Jr., 1990. "Positron Spectroscopy for Materials Characterization,” Metallurgical
Transactions, 21, 4, May, pp. 1121-1131. 4Shultz, Peter. and Lynn K. G., “Interaction of Positron Beams with Surfaces, Thin Films, and Interfaces,”
Reviews of Modern Physics, Vol. 60, No. 3 July 1988.5Gauster, W. B., et al. 1978. “A Study of Deformation Fatigue of 316 Stainless Steel at Room Temperature by
Positron Annihilation”, NUREG CR/0118.6Nishiwaki, K., et al. 1979. "The Study of Fatigued Stainless Steel by Positron Annihilation," Proceedings of the
Fifth International Conference on Positron Annihilation Yamanashi, Japan, R. Hasiguti and K. Fujiwara, (eds.), Japan Institute of Metals, pp. 177-180.
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7United States Patent Number 6,178,218 B1, “Nondestructive Examination Using Neutron Activated Positron Annihilation,” January 23, 2001.
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