NBSIR 87-3681

Application of Optical Image

Analysis to Quantitative

Microstructure Characterization of

Composite Materials

James F. Kelly

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Ceramics Division

Materials Building, A256

Gaithersburg, MD 20899

December 1987

Sponsored by:

U.S. Army Materials Technology Laboratory

Watertown, MA 02172

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NBSIR 87-3681

APPLICATION OF OPTICAL IMAGE

ANALYSIS TO QUANTITATIVE

MICROSTRUCTURE CHARACTERIZATION OF

COMPOSITE MATERIALS

James F. Kelly

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Ceramics Division

Materials Building, A256

Gaithersburg, MD 20899

December 1987

Sponsored by:

U.S. Army Materials Technology Laboratory

Watertown, MA 02172

U.S. DEPARTMENT OF COMMERCE, C. William Verity, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director
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October 1, 1987

CONTRACT REPORT TO: Dr. James McCauley

Army Materials Technology Laboratory

FROM: Dr. James F. Kelly

National Bureau of Standards

TIME PERIOD: This report covers work done under contract with the Army

Materials Technology Laboratory during FY1987.

Classification Code: 21702020 64-8929 P728102.13 2588 S19129

Cost Code: 7 TY00K5X475XD

PROJECT TITLE: Standards for Quantitative Microstructural Characterization

of Composites and Advanced Structural Ceramic Materials

PROJECT SCOPE: The ultimate objective of this project is to define standard

procedures for numeric description, by image analysis (optical and SEM) , of

the microstructural features relevant to mechanical properties of organic

matrix composites and advanced structural ceramic materials. The effort in

this initial year of the project at NBS concentrated on two aspects of

image analysis characterization: the system calibration procedure to

satisfy accuracy and precision requirements, and the specific details of a

microstructural analysis applied to a single composite specimen. For this

work, AMTL provided a calibration test slide containing a number of

geometric figures of specified dimensions and a prepared composite

specimen of unidirectional graphite fiber reinforced polymer, GFRP; Sample

I.D. No. 4-7 AS4-4 10A.

Report Title: Application of Optical Image Analysis To Quantitative

Microstructure Characterization of Composite Materials

Abstract

Proper specification of the micros tructure of organic matrix composites is

necessary for control of their mechanical properties. A study of the

requirements for quantitative microstructure characterization using an

image analysis(IA) system is reported. System calibration procedures are

described with a comparison between two commercial IA systems. The

requirements for a standard reference material for second phase area

fraction are presented and a numerical procedure is developed to describe

the inhomogeneity in second phase dispersion throughout the matrix.

IMAGE ANALYSIS SYSTEM CALIBRATION

There are two aspects of measurement accuracy using an optical microscope

or SEM based image analyzer.

1. Calibration of the pixel size. This can be done with either a

length or an area standard. This calibration standard must cover a

sufficient length(area) such that the pixel resolution is insignificant.

This limitation should be explicitly defined in any proposed handbook of

recommended procedures. e.g. A standard length extending at least 100

pixels would be required for 1% precision.
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2. Threshold setting. Each pixel in the image area is interpreted by

the computer as either off or on, based on whether the measured intensity

level at the pixel location exceeds the threshold intensity level.

Materials, such as the GFRP samples, exhibiting good optical contrast

between the separate phases, can be clearly separated between the bulk

matrix and reinforcing phase. The difficulty in setting the correct

threshold level arises from an uncertainty in precisely defining the

boundary regions or fiber perimeters. For large features, such as the

calibration disk, this isn't a problem since the number of perimeter pixels

are few compared to the total intersected in making either a diameter or

area measurement. However, for features the size of the graphite

fibers(approx. 7um) , the perimeter pixels may represent 20 to 25 per cent

of the fiber area. This large uncertainty means that the proper

calibration of pixel size only guarantees an accurate measure of large

features

These observations point out the need for an area fraction standard

reference material which is similar to the test specimen in the following

characteristics

1. Total second phase area fraction

2. Size distribution of second phase features

3. Contrast between phases

TEST SLIDE CALIBRATION

A comparison of the calibration between the NBS Omnicon system and the AMTL
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Quantimet system was obtained by analyzing the AMTL test slide after

calibrating with the Omnicon test slide.

The image analysis system utilized in these analyses was an Omnicon* 3500,

manufactured by Bausch and Lomb* . A calibration slide provided with this

unit was used to define the pixel dimensions. The procedure was to select

for imaging a circular disk of known diameter which covered a sufficient

number of pixels to achieve good precision in the calibration. At the

maximum system magnification of approximately 770X, and a screen size of

22cm by 16.5cm, a lOOum diameter disk extends over more than 300 pixels.

This calibration is checked each time the system is initialized. Typical

variation in calibration of about 1 % was observed on a day to day basis.

The area of each of the twelve circular discs on the AMTL test slide were

measured using the highest microscope magnification (40X objective lens).

It required six fields to cover the twelve disks. The analysis was

repeated twice at each of three threshold settings designated low, medium,

and high. The results of these analyses are given in Table I. The Feret

diameters of the 12 disks were measured in two perpendicular directions.

The 40X lens and a medium threshold setting were used for these

measurements

For comparison the results of AMTL measurements provided by John Ricca and

Rebecca Jurta are included as Table III. As can be seen in these tables,

the mean values of the NBS measurements differ by less than 1% from those

made by AMTL for both area and diameter. This should give us confidence

* OMNICON is a registered trademark of Artek Systems Corp.
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that an excellent calibration of the pixel size can be made using a similar

test slide specimen or a standard stage micrometer often provided with the

optical microscope.

MICROSTRUCTURE CHARACTERIZATION OF GFRP SAMPLE 4-7 AS4-4 10A

The unidirectional graphite fiber reinforced polymer composite specimen is

a rectangular section cut and polished by AMTL from a tensile specimen with

the graphite fiber axes perpendicular to the cutting plane. The composite

is approximately 25mm by 1.4mm. Figure 1 shows photomicrographs of the

sample which demonstrate two important features of the microstructure: 1.

There is little variation in the diameters of the graphite fibers and 2.

The area fraction or concentration of fibers is not homogeneous.

It seems unlikely that small variations in the fiber diameter distribution

would have any significant affect on mechanical properties, thus it should

be sufficient to measure a few fiber diameters as verification that the

mean diameter is within an acceptable range. The more critical issue is

how to accurately measure the mean fiber area fraction and obtain a

meaningful measure of the local area fraction distribution.

The mean fiber area fraction was measured by manually setting the threshold

for fiber detection to a level which gave a satisfactory visual display on

the Omnicon monitor. Namely, a setting such that the fiber image areas

appear intensified, but not the matrix areas. As discussed earlier, this

procedure is done without calibration since no appropriate area fraction

reference standard is available. However, the threshold setting is

reproducible and should yield results which differ by a constant factor
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from area fraction measurements made on the same material using a different

threshold.

The Omnicon 3500 was programmed to measure the fiber area fraction in the

current field of view, then translate the microscope stage to an adjacent

field for measurement until a matrix of 300 fields (75 X 4) were completed.

Each field covers an area of 295um by 220um, thus about 55% of the sample

is covered by the 300 fields. The program stores the 300 individual area

fraction results and calculates the minimum, maximum, mean and variance of

the data. These data are then output via the RS232 printer port to the

input port of an MS-DOS PC and stored on disk. This is a more convenient

format for printing, plotting, or further data analysis. The results for

two replicate 300 field analyses are presented in Table IV.

The most striking feature of the microstructure, the inhomogeneity in fiber

concentration, has no well defined measure. Measurement of area fraction

over a large section of the sample gives us a reproducible measure of the

fiber area fraction, but the variability from field to field is a function

of the field size. The occurrence of areas with fiber area fractions

significantly less than the mean are likely to be of critical importance in

the resultant mechanical properties of the composite material. One

measure, which I would propose for correlation with mechanical properties,

is the variance of the fiber area fraction distribution measured at several

field sizes. As the field size is decreased the increase of this variance

is an indicator of inhomogeneity of fiber distribution. Ultimately, as the

field decreases to encompass only a few fibers we would expect an increased

variance due simply to counting statistics. However, at intermediate field
mi

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sizes which still cover a large number of fibers, the variance in fiber

distribution will increase more rapidly with decreasing field area for an

inhomogeneous dispersion than for a homogeneous one. Measurements covering

the same region on the GFRP specimen were using three different field

sizes. The results are presented in Table V.

The change in variance seen in Table V expresses the increased variability

in area fraction from field to field for the reduced field areas. When the

measured field size is large enough to average over many of the fiber

depleted regions, the measured variance in area fraction is low. As the

field area is decreased to where a single depleted region greatly reduces

the area fraction, we see the significant increase in the variance of the

area fraction distribution, while the mean area fraction remains

essentially constant.

The distribution of fiber area fraction for measurements made with three

different field sizes( full field, 1/4 field, 1/16 field) are shown in

Figures 2-4. The total sample area analyzed is the same for each of the

three distributions.

SUMMARY

Consideration of the use of automated image analysis in the micros tructural

characterization of uniaxial graphite fiber reinforced polymer matrix

composites has led to the following observations:

1 . There is a need for a standard reference material for second phase

area fraction calibration of image analyzers. This reference material
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should be similar to the test materials in total area fraction, individual

feature area, and second phase/matrix contrast.

2. A measure is needed to quantify the inhomogeneity in fiber

distribution. One possible such measure, the change in variance of the

area fraction distribution with changing field size was demonstrated.

3. A coordinated program of mechanical testing and microstructural

analysis is needed to establish the correlation between mechanical

properties and any measure of microstructural inhomogeneity.
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TABLE I

MEASURED AREAS* OF 12 CIRCULAR DISKS ON QUANTIMET TEST SLIDE

THRESHOLD TEST MINIMUM MAXIMUM MEAN MEDIAN STND . DEV

LOW 1 8447 8548 8481 8460 24.6

LOW 2 8438 8534 8474 8460 25.5

MEDIUM 1 8326 8380 8349 8350 14.5

MEDIUM 2 8290 8404 8355 8350 28.5

HIGH 1 8235 8321 8273 8267 23.6

HIGH 2 8236 8302 8262 8254 14.5

* All areas are in um2

TABLE II

MEASURED FERET DIAMETERS* OF 12 DISCS1 ON QUANTIMET TEST SLIDE

FERET DIRECTION MINIMUM MAXIMUM MEAN MEDIAN STND. DEV

X AXIS 103.7 104.6 104.2 104.4 2.3

Y AXIS 103.0 104.6 103.9 104.0 2.3

* Diameters are in um.

X Axis is taken parallel to long direction of test slide.

TABLE III

AMTL MEASUREMENTS OF DIAMETERS AND AREAS OF THE 12 DISKS

PARAMETER MINIMUM MAXIMUM MEAN

FERET X 102.4 104.1 103.3

FERET Y 102.4 104.7 103.2

AREA 8270 8486 8362

TABLE IV

FIBER AREA PERCENT RESULTS FOR 300 FULL FIELD ANALYSES

RUN # MINIMUM MAXIMUM MEAN STND . DEV.

1 19.5 63.1 44.2 7.0

2 21.7 61.9 44.6 7.4

TABLE V

FIBER AREA PERCENT RESULTS FOR VARYING FIELD SIZE

FIELD FRACTION* FIELDS MINIMUM MAXIMUM MEAN VARIANCE

1 X 1 18 35.6 53.5 46.5 22.

1/2 X 1/2 72 25.6 57.0 43.6 61.

1/4 X 1/4 288 11.4 64.8 44.2 146.

* Full Field dimensions are 295um X 220um

Figure

1

.

Optical

Micrographs

of

GFRP

Specimen

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w

m

mx

c

2

10

9 --

6 --

5 --

4 -*

1

0

0

-t— 10 —(—

20

-I

: 30 60

MEAN AREA % = 46.5

VARIANCE = 22.

-I 1 1-

70 80 90

Figure 2. Area Fraction Distribution of Graphite Fibers

Field Area: Full

Number of Fields: 18

100

wotm-hT]

-ttjo

Figure 3. Area Fraction Distribution of Graphite Fibers

Field Area: 1/4 Full

Number of Fields: 72

cnotrtn-.^

^no

100

90 --

80 --

MEAN AREA % = 44.2

VARIANCE = 146.

Figure 4. Area Fraction Distribution of Graphite Fibers.

Field Area: 1/16 Full

Number of Fields: 288

100

NBS-H4A 'rev. 2-80

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

1. PUBLICATION OR

REPORT NO.

2. Performing Organ. Report No. 3. Publ i cation D ate

SHEET (See instructions) NBSIR87-3681 Dec emb e r 1987

4. TITLE AND SUBTITLE

Application of Optical Image Analysis to Quantitative Microstructure

Characterization of Composite Materials

5. AUTHOR(S)

James F. Kelly

6. PERFORMING ORGANIZATION (If joint or other than N BS. see in struction s) 7. Contract/Grant No.

NATIONAL BUREAU OF STANDARDS

U.S. DEPARTMENT OF COMMERCE 8. Type of Report & Period Covered

GAITHERSBURG, MD 20899 FY87

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State. ZIP)

U.S. Army Materials Technology Laboratory

Watertown, MA

10.

SUPPLEMENTARY NOTES

1 Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11.

ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bi bl iography or literature survey, mention it here)

Proper specification of the microstructure of organic matrix composites is necessary

for control of their mechanical properties. A study of the requirements for

quantitative micros true ture characterization using an image analysis (IA) system is

reported. System calibration procedures are described with a comparison between

two commercial IA systems. The requirements for a standard reference material for

second phase area fraction are presented and a numerical procedure is developed to

describe the inhomogeneity in second phase dispersion throughout the matrix.

12.

KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) area fraction; calibration; characterization; composite; graphite fiber; image

analysis; microstructure; optical

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