Army Research Laboratory 

Aberdeen Proving Ground, MD 2 1005-5066 

ARL-TR-2399 February 2001 

Nondestructive Characterization of Impact 

Damage in Metallic/Nonmetallic 

Composites Using X-ray Computed 

Tomography Imaging 

William H. Green and Joseph M. Wells 

Weapons and Materials Research Directorate, ARL 

Aooroved for uublic release; distribution is unlimited. 
Abstract 

Characterizing internal impact damage in composites can be difficult, especially in 

structurally complex composites or those consisting of many materials. Many methods for the 

nondestructive inspection/nondestructive testing (NDIiNDT) of materials have been known and 

used for many years, including x-ray film, real-time and digital radiographic techniques, and 

ultrasonic techniques. However, these techniques are generally not capable of mapping threedimensional (3-D) complex damage patterns, which is necessary to visualize and understand 

damage cracking modes. Conventional x-ray radiography suffers from the loss of 3-D 

information. Structural complexity and signal dispersion in materials with many interfaces 

significantly effect ultrasonic inspection techniques. This makes inspection scan interpretation 

difficult, especially in composites containing a number of different materials (i.e., polymer, 

ceramic, and metallic). 

X-ray computed tomography (CT) is broadly applicable to any material or test object through 

which a beam of penetrating radiation may be passed and detected, including metals, plastics, 

ceramics, metallic/nonmetallic composites, and assemblies. The principal advantage of CT is 

that it provides densitometric (e.g., radiological density and geometry) images of thin cross 

sections through an object. Because of the absence of structural superposition, the images are 

much easier to interpret than conventional radiological images. The user can quickly learn to 

read CT data because images correspond more closely to the way the human mind visualizes 3-D 

structures than projection radiology. Any number of CT images, or slices, from scanning an 

object can be volumetrically reconstructed to produce a 3-D attenuation map of the object. The 

3-D attenuation data can be rendered using multiplanar or 3-D solid visualization. In multiplanar 

visualization, four planes of view can be defmed to be anywhere in an object. These 

visualization modes produce easily interpretable images with very good spatial resolution and 

excellent dimensional capability. This report discusses current applications of advanced CT 

imaging to characterizing impact damage in metallic/nonmetallic composites. Examples, 

including encapsulated ceramics in metal matrix composites, will be presented. 

ii 
Table of Contents 

List of Figures ..._.f..f _ . . . . . . . . . . . . . . . . . . ..f..._............~.........._...................ff.f..._................f..f. V 

1. Introduction. _____.. .._._.~ . . . . . . . . . _. ~.f.............~........~......ff...f..___....._........f...f.f.__....~.......... 1 

2. Principles of X-ray CT .f.f............ff_____.__..........,..f...__r____.__._............ _ .. . . . . ..__ ____......... _ 1 

2.1 Technique ............................................................................................................. 1 

2.2 Volume Reconstruction ....................................................................................... 3 

3. Characterization of Armor Materials __......_ . . . . . . . . .._............................ff___..___.......... 3 

4. Results . _ __. . _ ___ __. . _ ___. ___. . .. .._. . . . . . f....f...._......................~....._.__.__._,_........__..........fff.___. 4 

4.1 Encapsulated Ceramic Silicon Carbide Plate ....................................................... 4 

4.2 Impacted Ceramic Titanium Carbide Disk ........................................................... 6 

5. Conclusions ._...f.........................f + __..... . . . . . . . . . . . . . . ..~.____.... .__........I~. . . . . . . . . . ..f...__........... 10 

6. References _.__.___._ _ ______ _+..ff..........._f............................*................~...........~,.,*~.....~...... 11 

Distribution List .___..______.___________. .__~ ___.._._......_.... ____..___.__. ._.._._fI**_ f ___._______._.____..____ 13 

Report Documentation Page . . . . ..ff~.._.....~..........~...~..._.......................~........f..ff......... 15 

. . . 

111 
r 

iv 
List of Figures 

Figure 

1. 

2. 

1 3, 

4. 

5. 

6. 

7. 

8. 

9. 

Schematic of the RO X-ray CT Scan Technique .~......................._._................_.......... 

Schematic of SIC Plate in Encapsulating MMC Material _______ __..++_ ._. _.__+_.+ +.__ _.___.____ 

A CT Image at a Height of 97.2 mm in Sample MMC-03 _ _ ._+.+. . . .+..+.+. .+_. .. . . . . . _ .. . . . ._ 

A Multiplanar Visualization of the Middle 100 mm of Sample MMC-03 . . . . . . ..__..._.. 

A CT Image 18.18 mm From the Sectioned Surface _.._____._.__._..__._________.__.___.____.____

A Multiplanar Visualization of Impacted TIC Piece _._.__...__.______.__________________.______

Top Slice and Front Slice Views With Different Front Slice Distances From 

Impact Face . . . . . . .._................_....f...f _ _.......___ff_.ffff_ ..fffff...........f...f........ _...__._ _.............. 

Top Slice and Side Slice Views With Different Side Slice Distances From 

Axis (Centerline) ...f...._____.__.___._.____.__.........~..~.........~.......__.._._f a _..... + .. . . . . ...+ + .. . ...+.. + . . . . . 

3-D Solid Visualizations of Impacted TiC Piece __.___.___.______.f _ ...f....ff......f.f......f..f.....f 

& 

2 

5 

6 

6 

8 

8 

8 

9 

10 

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vi 
1. Introduction 

Characterizing internal impact damage in candidate armor materials can be difficult, 

especially in structurally complex materials. Many methods for the nondestructive evaluation 

(NDE) of materials have been known and used for many years, including x-ray film, real-time 

and digital radiographic techniques, and ultrasonic techniques. Conventional x-ray radiography 

suffers from the loss of three-dimensional (3-D) information. Structural complexity and signal 

dispersion in materials with many interfaces, including composite materials, can make ultrasonic 

inspection very difficult. X-ray computed tomography (CT) is broadly applicable to any material 

or test object through which a beam of penetrating radiation may be passed and detected, 

including metal, plastics, ceramics, metallic/nonmetallic composites, and assemblies. The 

principal advantage of CT is that it provides densitometric (e.g., radiological density and 

geometry) images of thin cross sections through an object. Because of the absence of structural 

superposition, the images are much easier to interpret than conventional radiological images. 

Any number of CT images, or slices, from scanning an object can be volumetrically 

reconstructed to produce a 3-D attenuation map of the object. The 3-D attenuation data can be 

rendered using multiplanar or 3-D solid visualization. This report discusses current applications 

of advanced CT imaging to characterizing impact damage in candidate armor materials, 

including metallic/nonmetallic composites and ceramics. 

2. Principles of X-Ray CT 

2.1 Technique. Figures la and lb schematically show the rotate-only (RO) x-ray CT 

technique. The x-ray source and detector remain stationary. The object remains stationary 

relative to the turntable. The collimated horizontal fan beam “scans” a slice of the object as the 

turntable rotates 360”. The height (h) and thickness (t) of the slice is known. A set of 

attenuation line integrals is generated from the scan, as in equation 1, 

I p(s)ds 7 -ln(I/IO), 
S - X-ray Source 

D - Detector T 

C - Collimator 

0 - Qbject c-3 t - slice thickness 

‘1’ - Turntable h - slice height 

a. side view 

*Collimator not sh0~1~ 

S - X-ray &wrce 

0 - Qbjcct 

D - Detector 

?‘ - turntable 

=‘-------~ /i 

--__ 

--- 

f - fan angle .----a/>’ 

ti 

b. top view 

Figure 1. Schematic of the RO X-ray CT Scan Technique. 

where I, is the intensity of the unattenuated radiation, p is the linear attenuation coefficient, and I 

is the intensity of the attenuated radiation over the integrated path length (Stanley 1986). The 

line integrals can be conceptually grouped into subsets referred to as “views.” Each view 

corresponds to a set of ray paths through the object from a particular direction. The views are 

also referred to as “projections” or “profiles,” while each individual datum within a given 

projection is referred to as a “sample,” or often just a “data point.” A state-of-the-art scanner 

routinely collects millions of measurements per scan, each one accurately quantified and 

precisely referenced to a specific line of sight through the object of interest. The views from the 

scan are passed to the reconstruction algorithm for processing (Stanley 1986). The CT 
reconstruction process yields a two-dimensional (2-D) array of numbers corresponding to the 

cross section of the object. The 2-D array of numbers (i.e., densitometric gray levels) is the CT 

image. 

2.2 Volume Reconstruction. The excellent dimensional accuracy and the digital nature of 

CT images allow the accurate volume reconstruction of multiple adjacent slices. The slices are 

stacked to provide 3-D information throughout the entire object or a section of the object. The 

two ways of visualizing volumetric data are multiplanar reconstruction (MPR) and 3-D 

reconstruction. Multiplanar reconstruction displays top, front, side, and oblique slices through 

the object. The orientation of the top slice is parallel to the cross-sectional image plane: The 

front slice is orthogonal to the top slice. The side slice is orthogonal to both the top and front 

slices. The oblique slice can be placed on any one of the other three slices. The MPR display is 

similar to an engineering drawing. However, each view (i.e., top, front, side, and oblique) is a 

slice with finite thickness through the object, not a 2-D projection. Thle top, front, and side slices 

can be moved anywhere in the reconstructed volume. The oblique slice can be rotated through 

360”. 

The volumetric data is displayed as a 3-D solid object in 3-D reconstruction, and the 

orientation of the solid in space can be changed to facilitate different views. The solid can also 

be “virtually’ sectioned by only displaying part of the reconstructed volume, which creates a 

“virtual” cutting plane on the solid that shows the x-ray density values on that plane. This plane 

may be orthogonal to the cross-sectional image plane. In effect, virtual sectioning shows the 

cutting plane as it would look if the object was actually destructively sectioned along that plane. 

3. Characterization of Armor Materials 

Characterizing opaque ceramic materials is required to provide both quality assurance control 

and damage assessment of candidate armor ceramics in either confined or applique 

configurations. Destructive characterization techniques are limited by the statistical sampling of 

selected items, which are then no longer functional for further ballistic testing or service use. 

3 
Nondestructive techniques avoid that limitation, but vary in their capability to detect and record 

the location, orientation, shape, and size of multiple flaw and damage types of interest in such 

applications, particularly where more than one type of material is involved. Using a 

nondestructive characterization method allows the comparison of “before” and “after” damage 

states in the target material. f 

An accurate, high-resolution, nondestructive inspection method is required to evaluate the 

state of damage in both “as-fabricated” and ballistically impacted candidate armor ceramic 

materials, including encapsulated materials. Earlier efforts utilizing ultrasonic techniques to 

inspect discontinuously reinforced aluminum metal-matrix-composite (MMC) materials did not 

indicate that ultrasonic inspection would do a suitable job. The method must be able to detect 

features in both an MMC encapsulating material and in the encapsulated monolithic ceramic 

with comparable sensitivity limits. The presence of structurally or physically different materials 

or their interfaces cannot interfere with detecting flaws or damage throughout the sample. 

Ultimately, the method should be capable of inspecting real-armor components in an 

encapsulated configuration. X-ray CT has these capabilities. 

4. Results 

4.1 Encapsulated Ceramic Silicon Carbide Plate. Encapsulating armor ceramics by cast 

aluminum metal-matrix-composites, Al MMCs, takes advantage of the intrinsic higher specific 

modulus of the MMC material and the potentially lower-cost casting fabrication method 

(Trevino et al. 1998). The impacted armor ceramic material must be recovered to examine the 

microstructural and fi-actographic features of the damaged material. Target systems using 

unconfined ceramic elements are normally incompatible with recovering the impacted ceramic 

fragments. Encapsulated ceramics have inherent post-impact recoverability, as long as the 

encapsulating material remains substantially intact. Also, the encapsulating material itself may 

play a significant role by preventing or minimizing bending in the ceramic rear face during 

ballistic impact. Reducing rear face bending of the brittle ceramic avoids high tensile bending 

stress, which can induce premature back surface crack initiation. Figure 2 is a schematic of 

. 

4 
Figure 2. Schematic of Sic Plate in Encapsulating MMC Material. 

sample MMC-03, in which the silicon carbide (Sic) plate is vertically and horizontally centered 

and closer to the front 6-in x 6-in (152.4 mm) face of the encapsulating MMC material. 

The sample was scanned perpendicular to its front and back faces in translate-rotate (TR) 

mode from a height of 22.2 mm to a height of 122.2 mm. The cross-sectional image plane was 

parallel to the 2-in (50.8 mm) through thickness direction. In TR mode, the object being scanned 

is translated through the fan beam between finite rotations of the turntable. The source-to-object 

distance (SOD) in the direction of the source-to-image distance (SID) does not change. The 

turntable rotates after each translation is finished. The TR scan data is reorganized into a set of 

equivalent RO views for the reconstruction process. The SOD and SID were 662.75 mm and 

930.00 mm, respectively. The slice thickness and increment were I .OO mm, resulting in 

contiguous scans. Each slice was reconstructed to a 1,024 x 1,024 image matrix using 1,726 

views. Scnn time WRS abo,ut 32 min/slice, with 100 slices rrquired to SCUI the: IOO-Ron dib:L;lrlcc. 

The scan configuration used the 420 keV x-ray tube with the linear detector array (LDA). The 

tube energy was 370 keV, the current was 2.25 mA, and the focal spot was .8 mm. Figure 3 is a 

CT image 97.2 mm from the bottom of the sample, where the front of the sample is at the top of 

the image. Cracking in both the MMC material and the Sic plate is evident. The plate is not 

horizontally centered and is tilted in respect to the front face of the MMC material. Lastly, the 

plate is only about 3.5 mm to 6.0 mm away from the front face of the MMC material. The plate 

was displaced during the MMC casting process. 
Figure 3. A CT Image at a Height of 97.2 mm in Sample MMC-03. 

Figure 4 is a multiplanar visualization of the sample over the scanned IOO-mm distance, with 

the top slice view parallel to the image plane. The top slice view is 49.5mm high. All the views 

show cracks and cavities in the MMC material, including flaws directly adjacent to the SIC plate. 

The side slice and oblique slice views show a few faint indications of cracking in the plate. The 

oblique angle is 25” from the horizontal in the top slice view. 

Figure 4. A Multiplanar Visualization of the Middle 100 mm of Sample MMC-03. 

4.2 Impacted Ceramic Titanium Carbide Disk. An impacted titanium carbide (TIC) disk 

was sectioned in half. The disk was impacted by a 90% tungsten-heavy-alloy short-rod 

penetrator approximately at the center of one side. The thickness and diameter of the disk were 

approximately 25 mm and 75 mm, respectively. The entire height of one piece was scanned 

perpendicular to the impact face in TR mode, with the sectioned surface resting on the turntable. 
The SOD and SID were 662.82 mm and 930.00 mm, respectively. The slice thickness and 

increment were 0.50 mm and 0.20 mm respectively, resulting in overlapping scans. Overlapping 

scans generally improve MPR and 3-D solid images because they result in better quality 

attenuation data (i.e., better photon statistics) in the overlapping regions. Each slice was 

reconstructed to a 1,024 x 1,024 image matrix using 1,238 views. The scan time was about 25 

mm/slice; 183 slices were required to scan the entire piece. The scan configuration used the 420 

keV tube with the LDA. The tube energy was 350 keV, the current was 2.5 mA, and the focal 

spot was 0.8 mm. 

Figure 5 is a CT image 18.18 mm from the sectioned surface; the impact face is at the bottom 

of the image. The image shows crack damage from left to right and near the back surface. 

Figure 6 is a multiplanar visualization of the entire piece, with the top slice view parallel to the 

image plane. The top slice view is 18.18 mm from the sectioned surface; all views show crack 

damage. The front slice and side slice views show the distribution of damage perpendicular to 

and in the through thickness direction, respectively, for those slices. The oblique slice view 

shows an area of concentrated damage in the immediate vicinity of the sectioned surface, and it 

shows damage shaped roughly like a ring between the top and sectioned surface of the piece. 

The oblique angle is 5” from the horizontal in the top slice view. 

Figures 7a, 7b, and 7c are top slice and front slice views with different front slice locations. 

The series of top slice and front slice views shows how damage perpendicular to the through 

thickness direction changes with distance from the impact face. Figures 8a, 8b, and 8c are top 

slice views with different side slice locations. The side slice locations in the figures are 

15.6 1 mm, 13.12 mm, and 5.64 mm to the left of the axis of the piece, respectively. The series 

of side views shows how damage in the thickness direction changes with distance from the line 

of impact. Figures 9a, 9b, and 9c are 3-D solid visualizations showing different virtual sections. 

Figure 9b shows the piece with approximately one-third of it virtually cut off, while in Figure 9c 

approximately one-half is cut off. The sectioned surfaces appear as if the piece was actually cut 

at those surfaces. This is an excellent way to visualize damage while viewing the entire piece. 

Side Slice View 
a. entire piece b. two-thirds of the piece c. one-half of the piece 

Figure 9. 3-D Solid Visualizations of Impacted TiC Piece. 

5. Conclusions 

X-ray CT is an acceptable nondestructive technique for characterizing “as-fabricated” and 

ballistically impacted candidate armor materials. Pre-existing flaws, including cracking, in both 

the MMC encapsulating material and the monolithic ceramic SIC plate in sample MMC-03 were 

detected by this technique. The size, location, and orientation of the flaws were recorded to 

establish a “pre-impact” baseline for subsequent examination following ballistic impact. The 

displacement of the SIC plate was significant in both the through thickness and planar directions. 

Such displacement is undesirable and can be avoided in subsequent casting attempts. The 

displacement was adequately characterized by the nondestructive CT method, to allow for 

appropriate adjustments in the sample support and alignment during planned subsequent ballistic 

testing. The state of damage in a sample from an impacted TIC disk was characterized 

throughout the sample. Multiplanar and 3-D solid visualization were used to successfully 

characterize 3-D damage patterns. 

This characterization method will be extended to other armor target configurations in the 

future. X-ray CT is capable of 3-D mapping complex damage patterns using multiplanar and 

3-D solid visualization. The capability to characterize the pre-impact and postimpact state of 

damage in armor materials, combined with 3-D mapping, can be used to better understand 

damage cracking modes. 

IO 
6. References 

Stanley, J. H. “Physical and Mathematical Basis of CT Imaging.” American Society for Testing 

and Materials (ASTM), ASTM CT Standardization Committee E7.01.07, ASTM Tutorial 

Section 3, Columbus, OH, 1986. 

Trevino, S., G. Hauver, E. Rapacki, 5. Wells, and P. Brand. “Stress Measurements in Sic Target 

Using Neutron Diffraction.” Presented at the 21 st Army Science Conference, Norfolk, VA, 

15-17 June 1998. 

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REPORT DOCUMENTATION PAGE Form Approved 

OMB No. 07040188 

Nondestructive Characterization of Impact Damage in Metallic/Nonmetallic 

Composites Using X-ray Computed Tomography Imaging 

William H. Green and Joseph M. Wells 

ARL-TR-2399 

12a. DlSTRlEUTlON/AVAllABlLlTY STATEMENT 12b. DISTRIBUTION CODE 

4pproved for public release; distribution is unlimited. 

13. ABSTRACT(Maxlmum 200 words) 

Characterizing internal impact damage in composites can be difficult, especially in structurally complex composites or 

hose consisting of many materials. Many methods for the nondestructive inspection/nondestructive testing (NDI/NDT) 

If materials have been lmown and used for many years, including x-ray film, real-time and digital radiographic 

echniques, and ultrasonic techniques. However, these techniques are generally not capable of mapping 

hree-dimensional (3-D) complex damage patterns, which is necessary to visualize and understand damage cracking 

nodes. X-ray computed tomography (CT) is broadly applicable to any material or test object through which a beam of 

lenetrating radiation may be passed and detected, including metals, plastics, ceramics, metallic/nonmetallic composites, 

tnd assemblies. The principal advantage of CT is that it provides densitometric (e.g., radiological density and 

geometry) images of thin cross sections through an object. Because of the absence of structural superposition, the 

mages are much easier to interpret than conventional radiological images. This report discusses current applications of 

advanced CT imaging to characterizing impact damage in metallic/nonmetallic composites. Examples, including 

ncapsulated ceramics in metal manix composites, will be presented. 

4. SUBJECT TERMS 15. NUMBER OF PAGES 

;-ray computed tomography, imaging, NDE (nondestructive evaluation), impact ballistic 17 

lamage, ceramics, composites, metal matrix composites 16. PRICE CODE 

7. SECURITY CLASSIFICATION 18. SECURITY CLASSIFICATION 19. SECURITY CLASSIFICATION 20. LIMITATION OF ABSTRACT 

OF REPORT OF THIS PAGE OF ABSTRACT 

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