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Defence Research and Development Canada External Literature (P) DRDC-RDDC-2018-P014 February 2018

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Utilization of Micro X-Ray Computed Tomography for the Quality Assurance of the Gun Propellants

Paul HunglerCatalin-Florin Petre

DRDC – Valcartier Research Centre

Bibliographic information: International Journal of Energetic Materials and Chemical Propulsion, 2016,

Date of Publication from External Publisher: December 2016

15 (4): 325–338.

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© Her Majesty the Queen in Right of Canada (Department of National Defence), 2016© Sa Majesté la Reine en droit du Canada (Ministère de la Défense nationale), 2016

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IMPORTANT INFORMATIVE STATEMENTS

Disclaimer: This document is not published by the Editorial Office of Defence Research and Development Canada, an agency of the Department of National Defence of Canada, but is to be catalogued in the Canadian Defence Information System (CANDIS), the national repository for Defence S&T documents. Her Majesty the Queen in Right of Canada (Department of National Defence) makes no representations or warranties, expressed or implied, of any kind whatsoever, and assumes no liability for the accuracy, reliability, completeness, currency or usefulness of any information, product, process or material included in this document. Nothing in this document should be interpreted as an endorsement for the specific use of any tool, technique or process examined in it. Any reliance on, or use of, any information, product, process or material included in this document is at the sole risk of the person so using it or relying on it. Canada does not assume any liability in respect of any damages or losses arising out of or in connection with the use of, or reliance on, any information, product, process or material included in this document.

This document was reviewed for Controlled Goods by Defence Research and Development Canada (DRDC) using the Schedule to the Defence Production Act.

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UTILIZATION OF MICRO X-RAY COMPUTED TOMOGRAPHY FOR THE QUALITY ASSURANCE OF GUN PROPELLANTS

Paul Hungler1*, Catalin Florin Petre2

1 Royal Military College of Canada (RMCC), Kingston, Ontario, Canada

2 Defense Research and Development Canada (DRDC), Valcartier, Québec, Canada

*Address all correspondence to Paul Hungler E-mail: paul.hungler@rmc.ca Micro X-ray computed tomography (CT) was used to perform quality assurance tests on gun propellant grains to identify defects and measure critical internal geometries. Multiple batches of low-vulnerability ammunition (LOVA) were manufactured and high resolution three dimensional (3D) models of the propellant grains were examined. Defects such as shrinkage, cracking and non-uniform processing were found. Cracks in internal webs were located and the size of the crack was quantified by looking at axial and orthogonal slices of the propellant grain. Micro CT proved capable of providing precise measurements of internal features such as web thickness, which offered information on deformation. Using orthogonal slices the exact thicknesses of individual webs were measured, providing an indication of which areas within the grain would reach sliver point first. Results of multiple web measurements showed that well manufactured grains had normal distribution curves for web thickness with a mean close to the intended design dimension, while poorly manufactured grains do not conform to normal distributions. The study showed that micro X-ray CT is a very effective tool for the non-destructive evaluation of propellant grains and could be used effectively for research and development work on novel propellants.

KEY WORDS: gun propellant, X-ray computed tomography, quality assurance, processing and manufacturing of energetic materials, LOVA, non-destructive testing

1. INTRODUCTION

X-ray computed tomography (CT) is used extensively for medical and industrial applications to non-destructively image the interior structure of materials. The CT process involves the acquisition of numerous two dimensional (2D) projections at regular angular intervals, as specimens are rotated through 180o from their initial position. The 2D projections are used to develop a three dimensional (3D) model which provides vital information on defects/anomalies in the specimen. From the 3D model, axial and orthogonal slices of the specimen can be examined to measure critical internal dimensions along the entire height of the specimen. The

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technology utilized in CT continues to develop and micro X-ray CT is now capable of providing very high-resolution 3D models with spatial resolutions on the order of micrometers (Marton, Z et al., 2015).

X-ray imaging has been previously used to perform quality assurance (QA) checks on propellants, but mainly for solid propellant grains of rocket motors. For example, Grose and Kankane (2008) developed a technique to estimate the location of defects inside rocket propellant using multiple 2D X-ray radiographs. Ravindran et al. (2008) demonstrated the use of 3D X-ray CT for the inspection of solid rocket propellant systems and provided examples of the various defects and manufacturing anomalies that can be found in a typical system. Their study illustrated the capability of X-ray CT, but had limited information regarding quantification of the anomalies. Micro X-ray CT has also been applied to study the microstructure of propellants. Gallier and Hiemand (2008) used micro X-ray CT to examine and subsequently model the packing of solid particles in composite solid propellants. Despite this, the majority of literature on X-ray CT of propellants has been focused on space vehicles and rocket motors whilst little attention has been paid to the QA of gun propellants.

The main function of a gun is to convert the stored chemical energy contained in a propellant and transform it into kinetic energy which is transferred into a projectile. The performance characteristics of a propellant are directly related to its composition, as well as its geometrical design (Carlucci and Jacobson, 2008). According to Piobert’s Law, the burning of non-porous propellants is a surface phenomenon that proceeds layer by layer with the burning front always parallel to the surface. Therefore, the distance between surface areas (referred to as the web) and the web fraction (f) (the amount of web remaining during the burning process), are critical dimensions.

To determine the overall fraction of a propellant burnt(∅) the web fraction is utilized as well as a shape function(𝜃𝜃) which takes into account the complex geometries of various grains. Recently a novel approach was developed to predict burn rates in gun propellants without the use of shape functions (Xiao et al., 2015) but the traditional calculation outlined in Equation (1) is still utilized in the majority of internal ballistic calculations (Kulkarni and Maik, 2000):

∅(𝑡𝑡) = [1 − 𝑓𝑓(𝑡𝑡)][1 + 𝜃𝜃𝑓𝑓(𝑡𝑡)] (1)

The fraction of propellant burnt as a function of time ∅(𝑡𝑡) has a direct effect on the pressure curve of a gun and its muzzle velocity (Zadeh et al. 2011). Since Equation 1 is dependent on grain geometry in relation to both the web fraction and shape function, it is important that defects and geometrical abnormalities that would affect the guns performance are properly identified and, when possible, quantified.

In addition to the above, the ballistic performance of conventional and experimental propellants have been shown to depend on the mechanical response of the grains to the high stress and strain rate environment experienced during the interior ballistics cycle (Douchant, 1988; Lieb and

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Rocchio, 1983; Varga, 1990, Petre et all, 2011a). Ideally, during the ballistic cycle, propellant grains are not expected to fracture; they are assumed to be hard, incompressible cylinder-shaped (generally) entities. However, certain factors such as unwanted variations of the chemical composition of the grains, processing faults, the temperature of the environment, etc. can cause some of the grains to break under the high pressures produced during the ignition phase of the gun. This will result in an uncontrolled (therefore undesired) increase of total burning surface area of the grain, which in turn will increase the rate of produced gases. This can evolve into unexpected accidents such as gun breech blow, if the increase in pressure within the gun chamber exceeds the mechanical limits of the gun. In addition, it has been demonstrated that the mechanical properties of a propellant have a strong influence on its vulnerability, as a grain that is brittle and fractures upon exposure, for example, to a shape charge jet (SCJ), reacts more violently than a tougher, less brittle grain (Petre et al, 2011a) .

This study focuses on using micro X-ray CT to provide details on the various defects and anomalies that can be found in composite gun propellants. The high spatial resolution 3D models acquired using micro CT will be leveraged to quantify any defects and accurately measure internal dimensions such as web thickness and to assess adherence to intended design parameters.

2. EXPERIMENTAL

2.1 Propellant Manufacturing

Three different low-vulnerability ammunition (LOVA) composite propellant grains were produced for this study to illustrate the capacity of micro CT to identify the various defects/anomalies that can be found in granular gun propellants. The first propellant used here was a Canadian version of XM39 propellant developed in the 90’s (Huang et al, 1995; Hsieh and Li, 1998), a propellant that was well characterized in the past. This propellant contains two different crystal sizes (45 microns and 300 microns) of RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine), which was previously shown to be key in yielding good mechanical properties for this propellant (Petre et al, 2011a). Therefore, the XM39 served in this work as an example of a composite high quality propellant.

The other two propellants used in this study (LOVA 325 and LOVA 326) were HMX (1,3,5,7-tetranitro-1,3,5,7-tetrazocine) based LOVA propellants, which used one HMX crystal size (5 microns) and were developed at Valcartier for a previous study (Petre et al. 2011b). Additional information regarding the mechanical and combustion properties of propellants used can be found in the work of Petre et al. (2011b). HMX grains manufactured with one particle size are known to be susceptible to cracking during processing (Trumel et al., 2012). For this CT study the two HMX LOVA grains were used as an example of low quality composite propellants. The

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rest of the main ingredients of the propellants were nitrocellulose (NC), cellulose acetate butyrate (CAB) and acetyl triethyl citrate (ATEC), Table 1.

Table 1: Propellant Composition

Name RDX (%)

HMX (%)

CAB (%)

NC (%)

ATEC (%)

Ethyl Centralite (%)

LOVA XM39 76.0 0.0 12.0 4.0 7.6 0.4 LOVA-325 0.0 72.0 11.6 7.7 8.3 0.4 LOVA-326 0.0 72.0 3.7 15.0 8.9 0.4

The small scale propellant processing facility at Defence Research and Development Canada (DRDC) in Valcartier was used to manufacture all the propellant grains. The processing facility consists of a sigma-blade mixer, an extrusion press and cutting apparatus. A detailed description of the facility was provided by Ramachandran and Drolet (1986) as well as Petre et al. (2011b). The LOVA-type formulations produced used a typical solvent-based gun propellant processing, as described in detail by Beaupre and Durand (1990). In an attempt to obtain propellant grains with varying QA requirements, one batch of the XM39 grains and a total of eight different LOVA-HMX processing runs were produced by varying the manufacturing process, Table 2. The detailed geometry of the 7 perforation propellant grain used in this study is illustrated in Figure 1.

Table 2: Variations in Manufacturing Process

Name Number of Perforations

Extrusion Speed (mm/s)

LOVA XM39 7 0.2 LOVA-325-1 7 0.1 LOVA-325-2 7 0.3 LOVA-325-3 0 0.1 LOVA-325-4 0 0.3 LOVA-326-1 7 0.1 LOVA-326-2 7 0.3 LOVA-326-3 0 0.1 LOVA-326-4 0 0.3

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Figure 1: Geometry of 7 perforation propellant grain

2.2 Micro X-ray CT Procedure

A micro X-ray computed tomography instrument, at Queen’s University in Kingston, ON (Xradia Micro XCT400), was used to acquire all of the 3D images in this study. During the image acquisition the X-ray source had a voltage of 40 kV and a power setting of 8.8 W. The propellant grain under investigation was placed on the high precision rotary stage and rotated through 180o from its initial position. A total of 1000 2D projections were obtained for each scan with sampling taking place at an angular interval of 0.180o. The exposure time for each projection was 2.5 seconds and each complete tomographic acquisition took 1 h 45 min to complete. The detector used in the set-up was an Andor DV-436 Charge Coupled Device (CCD) cooled to -600C with a macro lens. Figure 2 illustrates the set-up and sample positioning used for the 3D imaging.

Following image acquisition, the stack of 2D projections was input into the Xradia reconstruction software and two image artifact reduction techniques were applied. A centre shift correction was calculated and applied to each set of data to align the images and reduce blurring caused by poor axis alignment. A beam hardening correction was also employed, which reduces streaking from the attenuation of low energy X-rays in the polychromatic beam utilized in this study. The final tomographic reconstruction was completed using a filtered back-projection algorithm which provided a 3D model in 16-bit gray scale form with 1024 x 1024 pixels. A total of 1014 axial slices were obtained for each data set with each slice having a thickness of 15.2 +/- 0.5 µm. Following reconstruction the 3D model’s critical dimensions were measured using ImageJ, an open source image processing program designed for scientific multidimensional

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images. For image segmentation and 3D feature extraction, Avizo a state-of-the-art image data processing and feature analysis software was employed.

Figure 2: Micro X-ray CT Set-Up

3. RESULTS AND DISCUSSION

As detailed in section 2.1, the LOVA 325 and 326 compositions and the manufacturing processes were varied to intentionally produce propellant grains with a range and variety of manufacturing qualities that could be then identified and evaluated using micro X-ray CT. For each of the eight different runs of LOVA 325 and 326 produced (Table 2), several samples of each propellant were examined. There were common traits for all LOVA produced such as the white colour and surface texture, however, many of the characteristics varied considerably from sample to sample. The XM39 grains had a more uniform quality and had a more uniform distribution of web thicknesses. The results presented in this section are intended to show examples of the different types of defects that can be identified with micro X-ray CT and various methods for quantification. It was not possible from the data obtained to create a standardized table that could summarize the amount and types of defects found in each formulation from Table 2. Instead, appropriate examples of each type of were chosen. If this technique were to be used for propellant manufacturing quality assessment, an experimental and testing approach could be developed to allow summarizing the fidings For geometrical evaluations, the LOVA 325 and 326 were compared to the XM39 grains to show deformation and differences between high and low quality manufactured grains.

3.1 Cracks

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The propellant grains for many of the runs showed evidence of shrinkage cracking occurring during the extrusion, cutting and drying processes. The grains with no perforations showed the most significant evidence of cracking. This cracking was sometimes evident on the outside of the grain by visual inspection but often the magnitude and size of the interior cracks could only be fully shown by looking at axial slices obtained by CT scanning. As an example, Figure 3 shows shrinkage cracking of LOVA-326-3 propellant. Figure 3(a) is a photographic image of the propellant grain and is similar in appearance to all the other grains produced. Examining this image it is very difficult to identify cracks through visual inspection. Figure 3 (b) is an axial slice of the end of the grain and shows some minor surface cracking. The size and extent of shrinkage cracking for this grain and many of the others produced was not evident until medial axial slices were viewed. Figure 3 (c) is a mid-axial slice of LOVA-326-3 and shows severe internal cracking in the centre of the grain.

Figure 3: LOVA-326-3, Shrinkage Cracking: (a) propellant grain; (b) top axial slice; (c) mid-axial slice.

The majority of the 7 perforation grains showed very little evidence of shrinkage cracking. The grains that did have internal cracking were readily identified and the size of the crack was accurately measured using the 3D model. For example, Figure 4 depicts a pellet from the LOVA-325-1 batch, which has a crack in one of the internal webs as highlighted by the circle. Using the orthogonal slice in Figure 4 (a) the horizontal length of the crack was measured to be 486 +/- 3.7 µm. The length of the crack in the vertical direction was calculated by looking at the axial slices. Figure 4 (b) is axial slice 426 located at the bottom of the crack, Figure 4 (c) is axial slice 447 located in the middle of the crack and Figure 4 (d) is axial slice 460 located at the top of the crack. If the thickness of each slice is 15.2 +/- 0.5 µm then the length of the crack in the vertical direction was calculated to be 516.8 +/- 8.4 µm.

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Figure 4: LOVA-325-1, Web Crack: (a) orthogonal Slice; (b) axial slice 426; (c) axial slice 447; (d) axial slice 460.

3.2 Processing

Several of the batch runs of LOVA showed evidence of poor processing. The micro CT technique can be used to show any processing inconsistencies inside the grains, which are not evident by visual inspection and often difficult to distinguish using only 2D X-ray imaging. Figure 5 clearly shows differences in the spatial distribution of components at different locations within a single grain. Figure 5 (a) depicts a medial axial slice with areas of voids caused from poor processing. Taking an orthogonal slice of the YZ plane at the centre of the far right perforation in Figure 5 (a) gives us Figure 5 (b). The grain consistency in the portion on the left hand side of the perforation is clearly poor. The right hand portion of the grain has a better consistency which is easily observed from the 3D CT model.

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Figure 5: LOVA-326-2, Mixing Inconsistencies: (a) medial axial slice; (b) orthogonal slice.

3.3 Deformation and Web Thickness

One of the most powerful characteristics of micro X-ray CT is its ability to accurately measure internal dimensions of objects. This is especially significant for materials such as propellant grains where geometry is directly related to performance. Some of the LOVA batches produced in this study showed evidence of deformation when their axial slices were examined and internal reference dimensions measured. As such, Figure 6 shows observed deformations in LOVA-326-1. Figure 6 (a) is axial slice 353 located at the bottom of the grain, Figure 6 (b) is axial slice 507 located midway along the height of the grain and Figure 6 (c) is axial slice 679 located at the top of the grain. The XZ and YZ planes were placed in the centre of the middle perforation in Figure 6 (a). From Figures 6 (b) and 6 (c), it is obvious that the middle perforation does not remain in the same location along the height of the grain. The distance that the middle perforation has moved vertically in relation to the XZ axis in Figure 6 (c) is 434.2 +/- 3.3 μm. It is believed that this is very quick and easy way to determine and quantify deformation in a propellant grain.

Figure 6: LOVA-326-1, Deformation: a) axial slice 353; (b) axial slice 507; (c) axial slice 679.

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The degree of deformation in a propellant grain is important as it affects the web thickness between surface areas. Burning rate is proportional to the surface area of a propellant grain resulting in three possible types of burning. If the overall surface area of the grain decreases during burning, as is the case for spheres and cylinders, it is considered regressive. Neutral burning occurs in tubular grains, while grains with multiple perforations experience progressive burning as the surface area increases during the burning process (Baily and Murray, 1989). In a progressive burning propellant the level of progression is relative to the diameter and number of perforations. As the size of the web decreases eventually burning reaches the stage when the surface areas of the perforations meet and the propellant grain disintegrates into longitudinal slivers which is called the sliver point. (Shimpi and Krier, 1975) In a propellant grain with multiple perforations the burning turns from progressive to regressive at the sliver point.

Using axial CT slices of propellant grains with 7 perforations it was possible to measure all 18 webs. Ten different slices equally distributed along the height of the grain were examined and web thicknesses were measured, resulting in 180 measurements for each grain. Many of the LOVA 325 and 326 showed significant variation from the intended geometry provided in Figure 1. Instead of all web thicknesses being uniform throughout, there was deviation both above and below the intended dimension as seen in Figure 7 (a) for LOVA-325-1. One possible explanation of this acute imperfection in web distribution could be attributed to quality of the extrusion die used to produce these propellants.

Figure 7: LOVA-325-1, Web Thicknesses: a) axial slice 436 with web dimensions; (b) frequency distribution of all web thicknesses.

The average of the 180 web thickness measurements for LOVA-325-1 was 1139 μm, which is very close to the intended web thickness of 1111 μm. However, a frequency distribution plot of the web thicknesses for the deformed LOVA was produced in Figure 7 (b) using bin sizes of 50 μm. It is evident form the plot that the web thicknesses are not normally distributed. For well manufactured grains all the web thicknesses should be close to the intended design dimensions

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resulting in a normal distribution curve with a mean value near the design thickness and a small standard deviation. In line with this observation, the XM39 grain shown in Figure 8 has an average web thickness of 1133 μm with a normal distribution and standard deviation of +/- 47 μm. The measurements provided in Figure 8 are intended to show the relatively equal thicknesses between internal, external and inter-perforation webs, generally characteristic of high quality 7 perforation grains.

Figure 8: XM39 Grain, Web Thicknesses

Variations in web thickness as seen in the deformed grains will affect the burning rate of the propellant as areas with smaller web thicknesses will reach the sliver point faster. After the sliver point occurs these locations will prematurely change from areas of progressive burning to areas of regressive burning.

One of the main benefits of CT is the ability to isolate and measure the thickness of internal structures along their full height. Figure 9 illustrates the variation in web thickness along the height of one of the webs in LOVA-326-2. In Figure 9 (a) the majority of the propellant grain has been removed to just show an orthogonal view of one of the internal webs. A total of 63 measurements of the web thickness were taken along the full height of the grain, approximately every fifth slice. A frequency distribution plot of the measurements was produced in Figure 7 (b) using bin sizes of 20 μm. The plot shows that the web thickness is not normally distributed and instead is left modal with the highest number of occurrences centred around the 1280 μm bin. The increased number of occurrences on the right side of the plot is a result of increased web thickness at the top and bottom of the grain.

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Figure 9: Web Thickness Variation LOVA-326-2: (a) internal web; (b) frequency distribution plot of web thickness

The information obtained regarding the internal web thickness has implications on performance. The larger than intended web thickness means this web will reach sliver point later than calculated resulting in a longer burn time. This deduction is only applicable to this single web, since a larger web thickness in one portion of the grain often results in a smaller than intended web thickness in another location within the grain. What is also important to realize is that since the web is thinner in the middle, the entire web will not reach its sliver point at the same time. The middle of the web will burn though faster resulting in two fragments that are no longer burning progressively. This non-uniform burning, resulting from deviation from the design geometry will slow the burn rate and affect the gun pressure curve.

4. CONCLUSIONS

Micro X-ray CT was shown to be capable of finding and quantifying a number of defects and geometrical inconsistencies discovered in some of the LOVA propellants produced for this study. Shrinkage cracking from the manufacturing process was discovered in several of the batches investigated here. The presence and extent of the cracking was not detectable during visual inspection, but could be easily seen and quantified by looking at axial CT slices. Inconsistent mixing within individual grains was also easily detected using the 3D model. Micro X-ray CT was capable of providing exact measurements of internal grain structures which affects burn rate and the gun’s expected pressure curve. Several quantitative methods for evaluating deformation and variations in web thickness were proposed. Differentiation between a high and a low quality manufactured grain was seen in the frequency distribution plots of their web thicknesses. Well manufactured grains had normal distribution curves while deformed grains resulted in non-standard distribution plots.

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Micro X-ray CT proved to be a very effective non-destructive technique for evaluating the interior structure of propellant grains and identifying and quantifying defects. Once the 3D model was acquired, quantitative measurements of important internal features were easily obtained. This technique could have immediate applications in research and development work for advanced propellants and could be adapted to look at other important propellant features such as non-apparent porosity.

REFERENCES

Bailey, A. and Murray, S.G., (1989) Explosives, Propellants and Pyrotechnics, London: Brassey’s, pp. 79-91. Beaupre, F. and Durand, R., (1990) Processing Studies of LOVA-Type Gun Propellants, Technical Report No. DREV-R-4603/90, Defence Research Establishment Valcartier, Canada. Carlucci, D.E. and Jacobson, S.S., (2008) Ballistics Theory and Design of Guns and Ammunition, New York: CRC Press, pp. 34-42. Douchant, A., (1988), A Drop-Weight mechanical property tester for gun propellants, Technical memorandum, DREV M-2906/88.

Gallier, S. and Hiernard, F., (2008) Microstructure of Composite Propellants Using Simulated Packings and X-Ray Tomography, J of Pro. and Power, 24(1), pp. 147-150. Ghose, B. and Kankane, D.K., (2008) Estimation of location of defects in propellant grain by X-ray radiography, Non-Destructive Testing and Condition Monitoring, 50(10), pp. 564-568. Huang, TH, Thynell, ST, Kuo, KK, (1995), Hot fragment conductive ignition of nitramine-based propellants, J of Prop. and Power, Vol. 11(4), pp 781-790 Hsieh, WH, Li, WY, 1998, Combustion behavior and thermochemical properties of RDX-based solid propellants, PEP, Vol 23(3), pp 128-136. Kulkarni, U.P. and Naik, S.D., (2000) Modelling of Heat Loss in Closed Vessels during Propellant Burning, Defence Sci J, 50(4), pp. 401-409. Lieb, R.J., Rocchio, J.J., (1983), Standardization of a Drop-Weight mechanical property tester for gun propellants, Technical report, ARBRLTR-02516, Aberdeen proving ground.

Marton, Z., Miller, S.R., Brecher , C., Kenesei, P., Moore, M.D., Woods, R., Almer, J.D., Miceli, A. and Nagarkar, V.V. , (2015) Efficient high-resolution hard x-ray imaging with transparent Lu2O3:Eu scintillator thin films, Proc. of SPIE, 9594, 95940E1-7. Petre, C.F., Tanguay, V., Brousseau, P., and Brochu, S., (2011a) Use of Drop Weight and Hot Fragment Conductive Ignition Tests to Characterize New Green and Insensitive Gun Propellants,

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42nd International Annual Conference of the Fraunhofer ICT, Karlsruhe, Germany, pp. 37-1 to 37-12.

Petre, C.P., Paquet, F., Nicole, C., Brochu, S., (2011b) Optimization of the Mechanical and Combustion Properties of a New Green and Insensitive Gun Propellant Using Design of Experiments, Int. J. of Enegetic Mat. And Chem. Prop., 10(5) pp. 437-453.

Ramachandran, P. and Drolet, J.F., (1986) A Users Guide for the DREV Gun Propellant Processing Facility, Technical Report No. DREV-M-2782/86, Defence Research Establishment Valcartier, Canada.

Ravindran, V.R., Sreelakshmi, C., Vibinkumar, S., (2008) Digital radiography-based 3D-CT imaging for the NDE of solid rocket propellant systems, Insight: Non-Destructive Testing and Condition Monitoring, 50(10), pp. 564-568.

Shimpi, S.A., Krier, H., (1975) Closed Bomb Test for the Assesment of Solid Propellant Grains Utilized in Guns, Combustion and Flame, 25(2), pp. 229-240.

Trumel, P., Lambert, P. and Biessy, M., (2012) Mechanical and microstructural characterization of a HMX-based pressed explosive: Effects of combined high pressure and strain, EPJ Web of Conferences, 26, pp. 020051-6.

Varga, L., (1992), Impact Testing Methods for Gun Propellants, Propellants, Explosives and Pyrotechniques, 17(3), pp 126-130.

Xiao, Z., Ying, S., He, W., and Xu, F., (2015) Interior ballistic prediction of gun propellants based on experimental pressure-apparent burning rate model in closed vessel, Sci. and Tech. of Energetic Mat., 76(1), pp. 1-7.

Zadeh, M.R.S., Kazemi, D, Amiri, H., (2011) Experimental analysis of the influence of length to diameter ratio on erosive burning in a solid tubular propellant grain, App.Mech. and Mat., 110-116, pp. 3394-9.

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Utilization of Micro X-Ray Computed Tomography for the Quality Assurance of the Gun Propellants

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Micro X-ray computed tomography (CT) was used to perform quality assurance tests on gun propellant grains to identify defects and measure critical internal geometries. Multiple batches of lowvulnerability ammunition (LOVA) were manufactured and high-resolution three-dimensional (3D) models of the propellant grains were examined. Defects such as shrinkage, cracking, and nonuniform processing were found. Cracks in internal webs were located and the size of the crack was quantified by looking at axial and orthogonal slices of the propellant grain. Micro CT proved capable of providing precise measurements of internal features such as web thickness, which offered information on deformation. Using orthogonal slices the exact thicknesses of individual webs were measured, providing an indication of which areas within the grain would reach sliver point first. Results of multiple web measurements showed that well manufactured grains had normal distribution curves for web thickness with a mean close to the intended design dimension, while poorly manufactured grains do not conform to normal distributions. The study showed that micro X-ray CT is a very effective tool for the nondestructive evaluation of propellant grains and could be used effectively for research anddevelopment work on novel propellants.

13. KEYWORDS, DESCRIPTORS or IDENTIFIERS (Technically meaningful terms or short phrases that characterize a document and could be helpful in cataloguing the document. They should be selected so that no security classification is required. Identifiers, such as equipment model designation, trade name, military project code name, geographic location may also be included. If possible keywords should be selected from a published thesaurus, e.g., Thesaurus of Engineering and Scientific Terms (TEST) and that thesaurus identified. If it is not possible to select indexing terms which areUnclassified, the classification of each should be indicated as with the title.)

Nanothermite, Friction, Resonant Acoustic Mixing, Nano Aluminium, Additives