e. 

I’ 

NASA Technical Memorandum 10 1478 

Ultrasonic Imaging of 

Textured Alumina 

David B. Stang 

Sverdrup Technology, Inc. 

NASA Lewis Research Center Group 

Cleveland, Ohio 

and 

1 Jonathan A. Salem and Edward R. Generazio 

Lewis Research Center 

Cleveland, Ohio 

I National Aeronautics and Space Administration 

i 

May 1989 

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( N AS A- T Y- 10 147 d 1 UL T 9 A dN I C T M A G I Nq rl F 

TEXTURE0 ALUMINA (NA5A. t~\rris Research 

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ULTRASONIC IMAGING OF TEXTURED ALUMINA 

David B. Stang 

Sverdrup Technology, Inc. 

NASA Lewis Research Center Group 

Cleveland, Ohio 44 135 

and 

Jonathan A. Salem and Edward R. Generazio 

National Aeronautics and Space Administration 

Lewis Research Center 

Cleveland, Ohio 44135 

Summary 

Ultrasonic images representing the bulk attenuation and 

velocity of a set of alumina samples were obtained by a pulseecho contact scanning technique. The samples were taken from 

larger bodies that were chemically similar but were processed 

by extrusion or isostatic processing. The crack growth resistance and fracture toughness of the larger bodies were found 

to vary with processing method and test orientation. The results 

presented here demonstrate that differences in texture that 

contribute to variations in structural performance can be 

revealed by analytic ultrasonic techniques. 

Introduction 

There is much interest in ceramics in the materials and structures research community. Researchers anticipate that many 

heat engine components will be manufactured from ceramics 

in the near future. One complication which is currently being 

addressed is variation in mechanical properties. Component 

or specimen properties can vary because of anomalies in the 

material such as voids, inclusions, and large grains (refs. 1, 

2, and 3). Nondestructive evaluation is an important research 

tool for studying these anomalies since it contributes to 

processing optimization and to the understanding of mechanical 

properties (refs. 4 and 5). 

Alumina is often used to study the effect of microstructure 

on mechanical properties since its various microstructural 

characteristics can be controlled in specimen fabrication. One 

characteristic that can be controlled is texture. Texture occurs 

in extrinsic and intrinsic forms. Intrinsic texture refers to a 

directionality in crystallographic and grain arrangement. 

Extrinsic texture refers to macroscopic features: that is, the 

distribution and shape of anisotropic pores and impurities. 

Sample preparation through isostatic processing generally 

results in little intrinsic or extrinsic texture. However, textures 

will result from extrusion processing. Salem et al. (ref. 1) 

reported the measured effects of texture on the fracture 

toughness of alumina. They found that crack growth resistance 

was influenced by the orientation of the test sample with 

respect to the principal direction of the extrusion texture. 

This report presents a relationship between specimen textures 

and ultrasonic measurements. Ultrasonic scans have been used 

for many years in a multitude of nondestructive testing applications. The measurements involved here, however, are more 

precise than those used for routine flaw detection purposes. 

Ultrasonic waves interact with voids, pores, inclusions, and 

grain boundaries of materials. The variations in fracture 

characteristics are governed by these factors and by texture 

differences that are more subtle (ref. 6). This report describes 

precision ultrasonic velocity and attenuation scans that reveal 

these subtle microstructural differences. 

Specimens 

Material Processing and Characteristics 

The specimens used in this study were made from 96 percent 

alumina powder. Two sets of specimens were produced. One 

set was isostatically pressed, and the other was extruded 

(ref. 1). Both were then fired at 1630 "C. The effects of extrusion can be seen in the tabulation of grain and pore dimensions 

1 
(table I). For both materials, the pores and grains were similar 

in size, but there was a slight orientation parallel to the 

extrusion axis in the extruded material. This orientation can 

be seen in the photomicrographs (fig. 1). The long axes of 

the pores and grains tended to be parallel to the axis of 

extrusion. This slight elongation and alignment is evidence 

of the texture of the material. 

Toughness and Crack Growth Characteristics 

The alumina fracture specimens were cut and chevronnotched for toughness and crack growth tests. The geometry 

and orientation of the specimens are illustrated in figure 2. 

The longitudinal L axis, the long axis of the specimen blanks, 

was the axis of extrusion. The isopressed as well as the 

extruded specimens were made such that notches were of three 

different orientations. 

Fracture testing was performed by loading the specimens 

at a stroke rate of 0.05 mm/min (Bubsey, R.T.; Pierce, W.S.; 

and Shannon, J.L. : Generalized Wide Range Expressions for 

the Short Bar and Short Rod Chevron-Notched Fracture 

Toughness Specimens. NASA TM-83796, 1989, to be published and ref. 7). The fracture toughness KIc was calculated 

with the equation 

2 

PmYjnin K -- IC - S( W’”) 

where P,, is the maximum test load, S and Ware specimen 

thickness and width (fig. 2(b)), and Ykn is the stress intensity 

coefficient, which is found from the experimental compliance 

relations of Bubsey et al. (NASA TM-83796). 

Crack growth resistance tests were performed by gradually 

loading the specimen until a small increment of stable crack 

extension occurred. Then the specimen was unloaded. This 

was repeated until a crack extension of 10 mm occurred, which 

was the instability point. Crack growth resistance, KIR was 

found by using the maximum load sustained and the stress 

intensity coefficient (Bubsey et al. NASA TM-83796). 

Both fracture toughness and crack growth resistance of the 

extruded samples were found to vary with test orientation 

(table II). The toughest samples (greatest Klc) were the RC 

samples-those oriented such that the extrusion direction was 

perpendicular to the direction of crack growth. The sample 

designation refers to its orientation with respect to the 

longitudinal L, circumferential C, and radial R dimensions of 

the original body (fig. 2). The toughness of the RC extruded 

samples was roughly 40-percent greater than that for the 

isopressed material. The crack growth resistance KIR was also 

e #* 

-EXTRUSION DIRECTION - 

(a) Pore structure of isopressed alumina. 

(c) Grain structure of isopressed alumina. 

(b) Pore structure of extruded alumina. 

(d) Grain structure of extruded alumina. 

Figure 1 .-Microstructure for radial-circumferential RC test orientations. 

ORIGINAL PAGE 

BLACK AND WHITE PHOTOGRAPH 
ISOPRESSED 

(a) 

sL +25 _JII 

EXTRUDED 

i W 

(b) 

(a) Removal patterns for isopressed and extruded alumina specimens. All dimensions are given in millimeters. 

(b) Geometry of alumina specimens. 

Figure 2.-Geometry and orientation of alumina fracture specimens. 

3 
found to be greatest in the RC samples. The greater toughness 

resulted from interaction between the crack front and the 

aligned pores and grains. The crack front deviated because 

of the alignment, resulting in energy absorption. 

Photographs of fracture surfaces clearly illustrate the texture 

of the extruded material (fig. 3). Ridges and valleys parallel 

to the extrusion direction occur across the surfaces. The 

surfaces of the isopressed material have no outstanding 

features, implying similar fracture behavior for all three 

orientations. The samples that were fractured such that the 

extrusion axis was perpendicular to the direction of crack 

propagation (RC) were tougher than those fractured in other 

directions. 

Ultrasonic Technique 

Velocity and Attenuation 

Ultrasonic pulse-echo procedures are well known (refs. 4, 

5, 8,9, and 10). The microstructure of a material influences 

the characteristics of a pulse as it travels through it. Pores and 

4 

grain boundaries act as scattering sites, removing energy from 

the pulse. Porosity also influences the speed at which the pulse 

travels (refs. 5 and 10). A schematic of the procedure is shown 

in figure 4. A broadband pulse is sent by a transducer through 

its buffer rod. Reflections of the pulse return from the front 

surface (echo ISI), the back surface (echo B,), and the back 

surface again (echo B2) after a reflection off the front surface. 

The Fourier transform of each waveform represents the 

amplitude and phase as a function of frequency. The 

attenuation as a function of frequency is given by 

where 

(d) 

(a) Isopressed specimen. 

(c) Radial-circumferential RC orientation. 

(b) Longitudinal-circumferential LC orientation 

Extrusion axis perpendicular to surface. 

(d) Radial-longitudinal RL orientation. 

Figure 3.-Typical fracture topography. Arrows indicate extrusion axis 

.+ 

ORIGiNAL PAGE 

BLACK AND WHITE PHOTOGRAPH 
:BACK SURFACE 

I 

FRONT 

SURFACEPIEZO- // I 

I 

X 

T 

-GLYCERIN 

COUPLANT 

I, 

(b) TIME 

(a) Schematic of contact pulse-echo system. 

(b) Pulse echoes shown as voltage as function of time. 

Figure 4.-Pulse-echo system. Echoes from front surface, FS,, FS,; echoes 

from back surface, B,, B,, B3. 

and 

X sample thickness 

FSI buffer rod echo without sample in contact 

FS2 buffer rod echo with sample in contact 

The velocity of the pulse can be found through the time 

interval between echoes B1 and B2. The cross correlation 

velocity (refs. 9 and 11) is given by 

2x 

70 

- 

where 7o is the time shift for which 

reaches a maximum. This is a mathematical analogy of the 

“pulse overlap” method of finding velocity. It may also be 

referred to as mean or group velocity. 

Ultrasonic Scanning 

The alumina samples used for scanning were cut from eight 

halves of fractured specimens. They were machined into 10- 

by 20- by 4-mm blocks with parallel sides. One 10- by 20-mm 

face on each was polished using a 3-pm diamond slurry. This 

surface condition gave a reflection coefficient which allowed 

for good attenuation measurements (ref. 8). 

A precision acoustic scanning system, described in greater 

detail elsewhere (ref. lo), was used to scan the samples. Each 

sample was mounted on a micropositioning table with a 

transducer fixed above in contact with the sample surface. The 

transducer had a center frequency of 50 MHz and an 

approximate beam width of 1 mm. All scans were done over 

an array of 35 by 15 points; the points were separated by 0.5 

mm. The three waveforms, FS2, B,, and B2, were acquired 

at each point for later calculation of spectra and images. 

Results and Discussion 

Velocity Characteristics 

The effects of texture can be clearly observed in the composite image of velocity of all eight samples (fig. 5). The 

averages of velocity and attenuation are found in table 111. 

Averages were determined by using all 525 array points from 

both samples of each type. Although bulk density is similar 

for all samples (ref. l), their structural directionality (texture) 

is revealed. The samples with notably higher velocity (LC) 

are those for which the direction of sound propagation was 

the same as the direction of extrusion (longitudinal). Extrusion 

aligned the grains and crystal axes so that the velocity in the 

longitudinal direction increased. The elastic modulus was 

greater than in the isopressed samples (ref. l), as might be 

expected. The pore alignment can be observed in the photographs of the fracture surfaces (fig. 3). The pores were also 

ellipsoidal (table I). The ultrasonic wave traveling in the 

direction of the extrusion axis “sees” more continuous 

material, so on the average the energy traveled with higher 

velocity. There was some variation within each of the LC 

images (around 3 percent) where the central region was slightly 

more porous. 

The velocity of both of the isopressed samples ranged only 

from 0.981 to 0.989 cm/psec, resulting in featureless images. 

This is not surprising since isopressed samples are expected 

to be structurally homogeneous and isotropic. The images of 

the extruded samples were more heterogeneous. The RC and 

RL samples (sound propagation perpendicular to extrusion 

direction) show variations due to extrusion-caused irregularities. Note that figure 6 includes the same data as figure 5 

with a different scale. The maximum value on the scale reference is smaller on figure 6, so the contrast was enhanced. The 

velocity over these samples ranged from 0.985 to 1.005 cdpse~. 

The lower velocities indicate lower densities, or more porosity. 

These samples have bands of varying velocity oriented in the 

same direction as the extrusion. These bands illustrate grouping 

of pores during the processes of extrusion and sintering, where 

differential shrinkage can take place. Grain and pore alignment 

with the extrusion axis are strikingly apparent in figure 6. 

5 
ULTRASONIC 

VELOCITY. 

CH/~SEC 

(a) Isopressed sample 1. 

(c) Radial-circumferential RC sample 1. 

(el Radial-longitudinal RL sample 1. 

PFi (h) 

(b) Isopressed sample 2. 

(d) Radial-circumferential RC sample 2. 

(f) Radial-longitudinal RL sample 2. ,, I 

(g) Longitudinal-circumferential LC sample 1. Extrusion (h) Longitudinal-circumferential LC sample 2. 

axis parallel to direction of ultrasound propagation. 

Figure 5.-Velocity images of alumina samples. Arrows indicate axis of extrusion. 

- 1.04 

- 1.02 

1 

-1.00 

- -98 

6 
J 

U 

1 MM 

ULTRASONIC 

VELOCl TY 

CM/~SEC 

(a) Radial-circumferential RC sample 1. 

(b) Radial-circumferential RC sample 2. 

(c) Radial-longitudinal RL sample 1. 

(d) Radial-longitudinal RL sample 2. 

Figure 6.-Velocity images of extruded alumina samples. Arrows indicate axis of extrusion. 

Attenuation Characteristics 

Attenuation images of the samples for 50 MHz are shown 

in figure 7. The isopressed materials exhibited extremely low 

and uniform attenuation, as seen in figure 7(a) and (b). The 

extruded samples, however, exhibit a larger range of high 

attenuation. Within this wide range of attenuation, small 

regions of uniform attenuation are aligned parallel to the 

extrusion direction in the RC and RL orientations (fig. 7(c) 

to (0). These variations indicate regions of varying pore size 

developed during the extrusion and sintering processes. 

Extrusion caused agglomerations of powder particles which 

densified during sintering. This densification occurred at the 

expense of the surrounding less dense regions. The ultrasonic 

energy was scattered by these large pores. Among the three 

types of extruded samples, the degree of attenuation did not 

significantly differ. The extrusion caused an increase in 

velocity along the longitudinal axis but did not cause an 
I ATTENUATION, 

NP/CM 

r 

I‘ 

-2.5 

-2.0 

-1.5 

-1.0 

(a) Isopressed sample 1. 

(c) Radial-circumferential RC sample 1. 

(e) Radial-longitudinal RL sample 1. 

(g) Longitudinal-circumferential LC sample 1. 

(b) Isopressed sample 2. 

(d) Radial-circumferential RC sample 2. 

(f) Radial-longitudinal RL sample 2. 

(h) Longitudinal-circumferential LC sample 1. 

Figure 7.-Attenuation images of alumina samples 

attenuation effect dependent on the axis of propagation. These 

images are consistent with the observation by Generazio (refs. 

10 and 12) that often there is high attenuation in areas of 

velocity gradients. (While the LC samples had greater velocity 

values, velocity variance was similar for all of the extruded 

samples.) 

Concluding Remarks 

Ultrasonic images representing bulk attenuation and velocity 

of alumina samples were obtained using a pulse-echo contact 

scanning technique. The samples were taken from larger 

bodies which varied in that some were isopressed and others 

were extruded. The larger bodies were mechanically tested, 

and crack growth resistance was found to vary with test 

specimen orientation. 

Regions of texture and porosity contribute to the fracture 

properties in high-temperature ceramic and composite 

materials (ref. 13). The identification of these regions is 

therefore of great importance. 

The results presented here demonstrate that differences in 

texture which contribute to variations in structural performance 

can be revealed by analytic ultrasonic techniques. The scanning 

technique found ultrasonic velocity and attenuation variations 

that were related to subtle microstructural differences that 

controlled fracture properties. Velocity was significantly 

higher in the samples in which the wave traveled along the 

axis of extrusion. Areas of high attenuation were found in all 

of the extruded samples, but no anisotropy similar to the 

velocity results was observed. 

8 
~ ~ ~~~ 

References 

Specimen 

processing 

Isopressed 

Extruded 

RC 

RL 

LC 

1. Salem, J.A.; Shannon, J.L., Jr.; and Bradt, R.C.: The EffectofTexture 

on the Crack Growth Resistance of Alumina. NASA TM-100250, 1987. 

2. Manderscheid, J.M.; and Gyekenyesi, J.P.: Fracture-Mechanics Concepts 

in Reliability Analysis of Monolithic Ceramics. Nondestructive Testing 

of High-Performance Ceramics, A. Vary and J. Snyder, eds., American 

Ceramic Society, 1987, pp. 59-72. (NASA TM-100174.) 

3. Singh, J.P.: A Review of the Effect of Flaws on the Fracture Behavior 

of Structural Ceramics. ANLIFE-86-3, Sept. 1986. 

4. Baaklini, G.Y.; Generazio, E.R.; and Kiser, J.D.: High Frequency 

Ultrasonic Characterization of Sintered SIC. NASA TM-100825, 1987. 

5. Klima, S.J.; and Baaklini, G.Y.: Ultrasonic Characterization of Structural 

Ceramics. Analytical Ultrasonics in Materials Research and Testing, 

A. Vary, ed., NASA CP-2383, 1986, pp. 117-126. 

6. Davidge, R.W.: The Texture of Special Ceramics with Particular 

Reference to Mechanical Properties. Proc. Br. Ceram. SOC., no. 20, 

June 1972, pp. 364-378. 

7. Shannon, J.L., Jr.; and Munz, D.: Specimen Size and Geometry Effects 

on the Fracture Toughness of Aluminum Oxide Measured with ShortRod and Short-Bar Chevron-Notched Specimens. Chevron-Notched 

Toughness, Crack growth 

KIc, resistance,a 

MPa-m''* 

MPa-ml" KIR* 

3.67h0.05 2.70 

4.94*0.37 4.13 

3.99*0.12 3.15 

3.63+0.21 2.47 

Specimens, Testing and Stress Analysis, ASTM STP-855, J.H. 

Underwood, S.W. Frieman, and F.I. Baratta, eds., ASTM, Philadelphia, 

8. Generazio, E.R.: The Role of the Reflection Coefficient in Precision 

Measurement of Ultrasonic Attenuation. Mater. Eval., vol. 43, no. 8, 

July 1985, pp. 995-1004. 

9. Hull, D.R.; Kautz, H.E.; and Vary, A,: Measurement of Ultrasonic 

Velocity Using Phase-Slope and Cross-Correlation Methods. Mater. 

Eval., vol. 43, no. 11, Oct. 1985, pp. 1455-1460. 

10. Generazio, E.R.; Roth, D.J.; and Baaklini, G.Y.: Imaging Subtle 

Microstructural Variations in Ceramics With Precision Ultrasonic 

Velocity and Attenuation Measurements. NASA TM-100129, 1987. 

11. Bracewell, R.N.: The Fourier Transform and Its Applications. McGrawHill, 1974, p. 46. 

12. Generazio, E.R.; Stang, D.B.; Roth, D.J.: Dynamic Porosity Variations 

in Ceramics. Mater. Eval. vol. 46, no. 10, 1988, pp. 1338-1343. 

13. Becher, P.F.; and Wei, G.C.: Toughening Behavior in Sic-WhiskerReinforced Alumina. J. Am. Ceram. SOC., vol. 67, no. 12, Dec. 1984, 

1984, pp. 270-280. 

pp. C267-CZ69. 

Specimen 

processing 

[sopressed 

Extruded 

TABLE 1.-AVERAGE PORE TABLE 11.-TOUGHNESS AND CRACK TABLE 111.-AVERAGE ULTRASONIC 

Parallel to Perpendicular 

long axis to long axis 

Average pore dimension, 

pm 

8.8 8.8 

9.0 8.0 

GROWTH RESISTANCE VELOCITY AND ATTENUATION WITH 

STANDARD DEVIATION 

cmlpsec Nplcm 

Extruded 

.996 f 0.004 

0.77 *0.40 

.98+0.56 

.76 f 0.42 

alntercept of KIR versus crack extension curve. aProbable values ranged from 0.0 to 0.23. 

9 
Natlonal Aeronautics and I Space Administration 

1. Report No. 2. Government Accession No. 

NASA TM-101478 

4. Title and Subtitle 

Ultrasonic Imaging of Textured Alumina 

Report Documentation Page 

3. Recipient's Catalog No. 

5. Report Date 

May 1989 

7. Author@) 

David B. Stang, Jonathan A. Salem, and Edward R. Generazio 

6. Performing Organization Code 

8. Performing Organization Report No. 

E-4600 

17. Key Words (Suggested by Author($) 

Ultrasonics; Imaging; Alumina; Texture; 

Nondestructive evaluation; Ultrasonic velocity 

9. Performing Organization Name and Address 

National Aeronautics and Space Administration 

Lewis Research Center 

Cleveland, Ohio 44135-3191 

18. Distribution Statement 

Unclassified - Unlimited 

Subject Category 38 

12. Sponsoring Agency Name and Address 

National Aeronautics and Space Administration 

Washington, D.C. 20546-0001 

110. Work Unit No. 

535-07-01 

11. Contract or Grant No. 

13. Type of Report and Period Covered 

Technical Memorandum 

15. Supplementary Notes 

David B. Stang, Sverdrup Technology, Inc., NASA Lewis Research Center Group, Cleveland, Ohio 44135; 

Jonathan A. Salem and Edward R. Generazio, NASA Lewis Research Center. 

16. Abstract 

Ultrasonic images representing the bulk attenuation and velocity of a set of alumina samples were obtained by a 

pulse-echo contact scanning technique. The samples were taken from larger bodies that were chemically similar 

but were processed by extrusion or isostatic processing. The crack growth resistance and fracture toughness of 

the larger bodies were found to vary with processing method and test orientation. The results presented here 

demonstrate that differences in texture that contribute to variations in structural performance can be revealed by 

analytic ultrasonic techniques. 

Security Classif. (of this report) 20. Security Classif. (of this page) 21. No of pages 22. Price' 

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