Jonathan A. Salem

Glenn Research Center, Cleveland, Ohio

Mechanical Characterization of ZnSe

Windows for Use With the Flow Enclosure

Accommodating Novel Investigations in

Combustion of Solids (FEANICS) Module

NASA/TM—2006-214100

February 2006
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Jonathan A. Salem

Glenn Research Center, Cleveland, Ohio

Mechanical Characterization of ZnSe

Windows for Use With the Flow Enclosure

Accommodating Novel Investigations in

Combustion of Solids (FEANICS) Module

NASA/TM—2006-214100

February 2006

National Aeronautics and

Space Administration

Glenn Research Center
Acknowledgments

Available from

NASA Center for Aerospace Information

7121 Standard Drive

Hanover, MD 21076

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22100

Available electronically at http://gltrs.grc.nasa.gov

The author thanks NASA’s Human System Research and Technology Office for funding this work,

and Abhishek Singh, Summer High School Apprenticeship Research Program (SHARP), for measuring

the branch lengths and the hardness.
NASA/TM—2006-214100 1 

Mechanical Characterization of ZnSe Windows for Use With the 

Flow Enclosure Accommodating Novel Investigations in 

Combustion of Solids (FEANICS) Module 

Jonathan A. Salem 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Abstract 

Young’s modulus, Poisson’s ratio, equibiaxial fracture strength, inherent crack size, grain size, 

hardness and the branching constant of ZnSe windows to be used with the FEANICS (Flow Enclosure 

Accommodating Novel Investigations in Combustion of Solids) experiments were measured in order to 

determine design allowables. The average Young’s modulus, Poisson’s ratio, equibiaxial fracture 

strength, flaw size, grain size, Knoop hardness, Vicker’s hardness, and branching constant were E = 

74.3±0.1 GPa, v = 0.31, σ f = 57.8±6.5 MPa, a = 21±4 μm, l = 43±9 μm, HK0.5/15 = 0.97±0.02 GPa, 

HV0.5/15 = 0.97±0.02 GPa, and Ab = 1.0±0.1 MPa√m. The properties of current ZnSe made by chemical 

vapor deposition are in good agreement with those measured in the 1970’s. The hardness of CVD ZnSe 

windows is about one-twentieth of the sapphire window being replaced, and about one-sixth of that of 

window glass. Thus the ZnSe window must be handled with great care. The large grain size relative to the 

inherent crack size implies the need to use single crystal crack growth properties in the design process. In 

order to determine the local failure stresses in one of the test specimens, a solution for the stresses 

between the support ring and the edge of a circular plate load between concentric rings was derived. 

Introduction 

The International Space Station FEANICS (Flow Enclosure Accommodating Novel Investigations in 

Combustion of Solids) module is designed to perform various combustion experiments in zero gravity. 

The module fits into the Combustion Integrated Rack (CIR) of the Fluids and Combustion Facility (FCF) 

and is observed via eight CIR sapphire windows shown in figure 1. FEANICS intends to replace one of 

those windows with a 63-mm-diameter CVD (chemical vapor deposition) ZnSe (zinc selenide) window 

which will allow a laser beam of 10.6 μm wavelength to pass. Both FEANICS and CIR are being 

developed at the NASA Glenn Research Center, Cleveland, Ohio. 

ZnSe is a soft, weak ceramic that exhibits crack growth in the presence of water. Further, its strength 

depends on grain size. A recent review of the literature on CVD ZnSe indicated that much of the crack 

growth and fracture strength data was generated during the 1970’s (ref. 1) with commercial grades which 

no longer exist. The reported fracture strength was greater than 50 MPa with a Weibull modulus of 9 for 

well prepared test specimens. In order to verify the mechanical properties of the CVD ZnSe1

 window 

material being used for viewing the FEANICS module, fracture strength, grain size and other properties 

were measured and are archived in this report as part of the NASGRO2

 design process.

 

1

CVD Zinc Selenide, (Rohm and Haas, Woburn, Massachusetts) 2

NASGRO Fracture Mechanics and Fatigue Crack Growth Analysis Software Version 4.2, May, 2005. www.nasgro.swri.org. 

Also, see NASA SSP 30560. 
NASA/TM—2006-214100 2 

Figure 1.—Combustion Integrated Rack and ZnSe window assembly for use with 

the FEANICS module. 

Test Specimen Preparation and Inspection 

Circular plates of the CVD ZnSe to be used with the FEANICS module were prepared according to 

the drawing and specifications in figure 2, which were developed for the actual windows, with one 

exception: the thickness of the test specimens was reduced to 4 mm to comply with American Society for 

Testing and Materials (ASTM) C1499 (ref. 2). Optical inspection of the edges at ~10× indicated an 

excellent edge-finish with no apparent chips. Some details of the polishing procedure are given in the 

appendix. Additional tests on witness specimens to be manufactured along with the actual window are 

planned. 

Elastic Properties 

Young’s modulus and Poisson’s ratio were determined at room temperature in accordance with 

ASTM C1259 (ref. 3) by using an automated linear interpolation routine (ref. 4). The average modulus 

and Poisson’s ratio were E = 74.3±0.1 GPa and v = 0.31±0.001, respectively, as shown in table 1. This is 

slightly higher than some of the reported values (69 to 72.4 GPa (ref. 1)), likely due to the use of a sonic 

technique as compared to a strain gage. The average bulk density was required for estimation of the 

elastic properties. The measured value was 5.25±0.003 g/cc.
Figure 2.—Specification for biaxial strength test specimens and FEANICS window. 

mm 

NASA/TM—2006-214100 3
TABLE 1.—YOUNG’S MODULUS AND POISSON’S RATIO OF ROHM AND HAAS CVD ZnSe. 

Specimen number 

Specimen 

thickness, 

h 

mm 

Specimen 

diameter, 

D 

mm 

Specimen 

mass, 

g 

Flexural 

frequency, 

f2

Hz 

Torsional 

frequency, 

f1

Hz 

Density, 

ρ

g/cc 

Poisson’s 

ratio, 

v f2/f1 h/r 

Young’s 

modulus, 

E 

GPa 

Tag 27335: 

1RH 4.025 63.52 66.8689 6390 3790 5.243 0.31 1.686 0.127 74.4 2RH 4.036 63.50 67.0314 6430 3800 5.244 0.31 1.692 0.127 74.4 3RH 4.027 63.52 66.9133 6410 3790 5.244 0.31 1.691 0.127 74.5 4RH 4.002 63.53 66.5884 6350 3760 5.250 0.31 1.689 0.126 74.2 5RH 4.006 63.52 66.6160 6350 3760 5.248 0.31 1.689 0.126 74.0 6RH 4.057 63.55 67.5582 6433 3810 5.250 0.31 1.688 0.128 74.3 7RH 4.052 63.53 67.4034 6421 3810 5.248 0.31 1.685 0.128 74.3 8RH 4.037 63.55 67.1861 6393 3790 5.248 0.31 1.687 0.127 74.2 9RH 4.033 63.53 67.0277 6407 3790 5.243 0.31 1.691 0.127 74.2 

10RH 4.003 63.56 66.6818 6344 3760 5.250 0.31 1.687 0.126 74.3 11RH 4.037 63.39 66.8498 6430 3810 5.248 0.31 1.688 0.127 74.2 12RH 4.042 63.53 67.2197 6425 3800 5.246 0.31 1.691 0.127 74.4 13RH 4.022 63.53 66.8277 6403 3790 5.242 0.31 1.689 0.127 74.6 14RH 4.035 63.52 67.1737 6395 3790 5.253 0.31 1.687 0.127 74.2 15RH 4.005 63.53 66.6289 6356 3770 5.248 0.31 1.686 0.126 74.4 16RH 4.007 63.52 66.6571 6373 3770 5.250 0.31 1.690 0.126 74.4 

Tag 27460: 

17RH 3.938 63.53 65.5723 6250 3700 5.253 0.31 1.689 0.124 74.2 18RH 3.936 63.53 65.5447 6250 3700 5.253 0.31 1.689 0.124 74.2 19RH 3.937 63.52 65.4160 6270 3710 5.244 0.31 1.690 0.124 74.5 20RH 3.967 63.53 66.0247 6290 3730 5.250 0.31 1.686 0.125 74.2 21RH 3.926 63.55 65.3782 6230 3690 5.250 0.31 1.688 0.124 74.3 22RH 3.949 63.52 65.6696 6300 3720 5.248 0.31 1.694 0.124 74.6 23RH 3.963 63.56 66.0175 6283 3720 5.251 0.31 1.689 0.125 74.2 24RH 3.973 63.51 66.0248 6330 3740 5.246 0.31 1.693 0.125 74.4 25RH 3.935 63.53 65.4649 6263 3710 5.248 0.31 1.688 0.124 74.6 

Average = 74.3±0.1GPa 

NASA/TM—2006-214100 4
NASA/TM—2006-214100 5 

Figure 4.—Example failure origins in 63.5 mm diameter CVD ZnSe test specimens.

Fracture Strength 

Fracture strength of the CVD ZnSe to be used with the FEANICS module was determined by loading 

circular plates between two concentric rings in accordance with ASTM C1499 (ref. 2). The strength tests 

were performed in distilled, deionized water at an average stress rate of 10.2 MPa/s. A summary report on 

the testing details is given in the appendix. 

The measured mean equibiaxial fracture strength was σ f = 57.8±6.5 MPa, as shown in table 2. The 

Weibull modulus, m, and characteristic strength,σθ, as estimated with the maximum likelihood estimator 

were 9.6 and 60.6 MPa, respectively. The values as estimated by rank regression were 10.9 and 60.3 MPa, 

respectively. A Weibull plot of the data is shown in figure 3, and examples of failed test specimens are 

shown in figure 4. 

Figure 3.—Weibull plot of the equibiaxial fracture strength of CVD ZnSe. 

20 30 40 50 60 70 80 90 100 

Equibiaxial Fracture Strength, MPa 

σθ m

Probability of Failure, % 

n/s 

60.63 9.584 15/0

95 

80 

60 

40 

20 

10 

5 

2 

11RH 

σf =56.6 MPa 

Origin

14RH 

σf =63.0 MPa 

22RH 

σf =50.5 MPa 

Origin

Origin
These results compare well with the data generated by point loading of circular plates supported with 

three balls (BO3B) (refs. 1 and 5) and large four-point bend (4PB) tests, as shown in figure 5. For BO3B 

tests run in water at stress rates of 75 and 0.075 MPa/s, the mean strengths were 56.1±10.1 MPa and 

49.1±8.6 MPa, respectively (ref. 1). For 139 large 4PB tests in 45 percent RH (relative humidity) air, the 

mean strength was 52.3±6.9 MPa, the Weibull modulus was 9.23, and the characteristic strength was 

54.3 MPa (refs. 1 and 6). Small, poorly machined 4PB beams exhibited significantly lower strength as shown 

in figure 5 (ref. 1). 

The test specimen effective surface area, Ae, and characteristic fracture strength for the large 4PB and 

ROR tests were not correlated according to the usual formula 

m

e

e

A

A

1

1

2

2

1

⎟

⎟

⎠

⎞ ⎜

⎜

⎝

⎛ = σ

σ

θ

θ (1) 

despite the Weibull moduli being very similar. Based on equation (1) and the large 4PB data, the expected 

characteristic strength of the ROR tests was 48.0 MPa as compared to 60.3 MPa. Evidently, differences in 

manufacturing resulted in different strength levels despite the similar Weibull moduli. 

Crack Size 

Test Method and Environment

4PB BO3B ROR 4PB 4PB BO3B 

Mean Fracture Strength, MPa

20

30

40

50

60

70

80

90

100

20

30

40

50

60

70

80

90

100

CVD ZnSe Mean & Standard Deviation

Water Air ~45% RH

Dry N2

A (1265) (405) e

(mm) =

Figure 5.—Summary of published strength of CVD ZnSe (ref. 1). 4PB is four point bending; 

BO3B is point loading of a circular plate supported by 3 balls; ROR is concentric loading of a 

circular plate between two rings of different diameters; Ae is the Weibull effective area based 

on the Principal of Independent Action.

NASA/TM—2006-214100 6

small (This work) small large 

The measured fracture strengths can be used to estimate the flaws sizes of the strength test specimens by 

assuming that the maximum principal stress and single crystal fracture toughness result in mode I failures. 

Crack lengths corresponding to the maximum stress intensity for both a long shallow crack with a/c = 0.1 

and a half-penny crack (a = c) were estimated and are tabulated in table 2. The estimated crack lengths, with 

the exception of one value for a specimen that failed from an unusually large flaw, are invariably less than 

the average grain size, implying that single crystal properties will dominate crack growth. The test specimen 

that failed from an unusually large flaw (no. 11 RH) was not included in estimating an average inherent flaw 

size of a = 21±4 μm. Because the fracture strength measurements were made in water, the initial crack sizes 

were somewhat smaller. 
TABLE 2.—EQUIBIAXIAL STRENGTH OF ROHM AND HAAS CVD ZnSe. 

TEST SPECIMENS WERE LOADED AT A RATE OF 316 N/s. 

Estimated crack length, a Specimen 

number 

Specimen 

thickness, 

h 

mm 

Specimen 

diameter, 

D 

mm 

Young’s 

modulus, 

E 

GPa 

Poisson’s 

ratio, 

v 

Load 

diameter, 

DL 

mm 

Support 

diameter, 

DS 

mm 

Stress 

rate, 



σ

MPa/s 

Failure 

load, 

F 

N 

Equibiaxial 

fracture 

strength, 

f σ

MPa 

a = c 

mm 

a = 0.1 c 

mm 

Tag 27335: 

11RH 4.037 63.39 74.2 0.31 25.446 50.808 9.93 1801 56.6 0.110 0.049 12RH 4.042 63.53 74.4 0.31 25.446 50.808 9.91 1653 51.8 0.025 0.011 13RH 4.022 63.53 74.6 0.31 25.446 50.808 10.00 1976 62.5 0.017 0.008 14RH 4.035 63.52 74.2 0.31 25.446 50.808 9.93 2003 63.0 0.018 0.008 15RH 4.005 63.53 74.4 0.31 25.446 50.808 10.08 1942 61.9 0.018 0.008 16RH 4.007 63.52 74.4 0.31 25.446 50.808 10.08 1992 63.5 0.017 0.007 

Tag 27460: 

17RH 3.938 63.53 74.2 0.31 25.446 50.808 10.43 2155 71.1 0.013 0.006 18RH 3.936 63.53 74.2 0.31 25.446 50.808 10.44 1671 55.2 0.022 0.010 19RH 3.937 63.52 74.5 0.31 25.446 50.808 10.44 1629 53.8 0.022 0.010 20RH 3.967 63.53 74.2 0.31 25.446 50.808 10.27 1758 57.1 0.022 0.010 21RH 3.926 63.55 74.3 0.31 25.446 50.808 10.49 1960 65.1 0.018 0.008 22RH 3.949 63.52 74.6 0.31 25.446 50.808 10.39 1537 50.5 0.026 0.012 23RH 3.963 63.56 74.2 0.31 25.446 50.808 10.30 1472 48.0 0.028 0.013 24RH 3.973 63.51 74.4 0.31 25.446 50.808 10.26 1596 51.8 0.025 0.011 25RH 3.935 63.53 74.6 0.31 25.446 50.808 10.44 1655 54.7 0.022 0.010 

 Average strength = 57.8 MPa  Standard deviation = 6.5 MPa 

NASA/TM—2006-214100 7
NASA/TM—2006-214100 8 

Hardness 

The hardness of polished and etch specimens was measured by using Vickers and Knoop indentation 

with a 15 s load duration. Due to the softness of ZnSe, measurements were made both at the loads required 

by ASTM C1326 and C1327 (refs. 7 and 8) and at lower loads. Because of the large grain size, the low load 

indentations were usually smaller than a single grain. Thus, to provide consistency and investigate the effect 

of grain orientation, grains with and without twins visible were indented. At least three tests were run per 

indentation load, but typically five to ten. The results are summarized in table 3. The 0.5 kg Knoop and 

Vickers hardness is 0.97±0.02 GPa. 

In general, lower load indentation produced higher hardness values. Low load Vicker’s indentation 

within grains produced the highest hardness values and the most scatter. The high average value for single 

grains appears to be at least partially an artifact of the low load rather than a reflection of single crystal 

hardness anisotropy, whereas the scatter is a reflection of anisotropy. 

It should be noted that the hardness of CVD ZnSe is about one-twentieth of the sapphire being replaced 

(ref. 9), and about one-sixth of that of soda-lime-silicate glass. Thus the ZnSe window must be handled with 

great care. 

TABLE 3.—HARDNESS OF CVD ZnSe AT 23 °C AND 50 PERCENT RELATIVE HUMIDITY 

Load, 

g 

Knoop hardness, 

HK 

GPa 

Vickers hardness, 

HV 

GPa 

25 (no twins) ------------- 1.12±0.071 

50 (no twins) 1.10±0.102 1.11±0.051 

25 (twins)3 ------------- 1.18±0.041 

50 (twins) ------------- 1.15±0.041 

200 0.98±0.01 ------------- 

500 0.97±0.02 0.97±0.02 

1000 0.95±0.014 0.96±0.025 

1. Indentations were within a single grain. 

2. Indentations covered more than one grain. Both regions with twins and regions not displaying twins 

were indented. 

3. Indentions diagonals were place at both 45 and 0/90° to the twins. No measurable difference 

was detected. 

4. Slight cracking around indentation tips. 

5. Cracking from indentation tips. 

Grain Size 

The measured strength and fracture toughness of ZnSe is a function of the grain size (refs. 10 and 11). To 

compare the current materials strength and grain size to that published, the grain size was measured in 

accordance with ASTM E8 (ref. 12) by using the linear intercept method. Sections oriented parallel and 

perpendicular to the growth direction were polished and etched for several minutes by using a mixture of 

200 gms chromic acid, 15 gms Na2SO4, 50 ml HNO3, and 900 ml H2O. Figure 6 shows typical grain structure 

of the CVD ZnSe being used. The grain structure is complex, varies widely and heavy twinning is apparent. 

Regions of many grains with twins oriented toward the deposition direction were observed as well as regions 

containing more grains of an orientation that darkened upon etching, implying some heterogeneity. Although 

the average measured grain size is l = 43±9 μm, grains as large as 150 μm can be observed, again implying 

that single crystal properties should be used in design of structural components. A slight difference between 

measurements made parallel and perpendicular to the deposition plane was measured, ( tl = 42±2 versus pl = 

47±3 μm at 95 percent confidence). The results are compared to published values in figure 7. 
Figure 6.—Microstructure of CVD ZnSe.

Crack Branching Constants 

Crack branching constants are useful in determining the nominal failure stress of components and 

estimating the presence of residual stresses in test specimens. The crack branching length is usually 

described by 

−0.5 σ = f b b A r (2) 

0.25 mm 

21S 200×02 

0.125 mm 

7C 50×01 

Grain Size, m

400 100 45 25 16

Fracture Strength, MPa

10

20

30

40

50

60

70

Coors, Gould, Raytheon Materials 

Raytheon 

Rohm Haas (This work)

Figure 7.—Fracture strength as a function of grain size for CVD ZnSe 

made by various manufacturers. 

(ref. 10) 

 (ref. 11) 

H

and 

and 

NASA/TM—2006-214100 9
NASA/TM—2006-214100 10

where σf is the fracture stress and Ab is the empirical constant corresponding to the branching boundary with 

radius rb, as shown in figure 8. The exponent of –0.5 on rb is a generally accepted value, but does vary 

somewhat. The branch lengths were measured on each specimen by optical microscopy, and the branching 

constant, Ab, was calculated by using four methodologies: (1) point wise estimation with equation (2); (2) 

nonlinear curve fit to equation (2) with and without assuming an exponent of M = –0.5; (3) by taking 

logarithms of equation (2) with and without assumption of M = –0.5 and applying linear regression: 

f b Ab logσ = M log r + log ; (3) 

and (4) by linear regression of the formula (ref. 13) 

f b b th σ = A r + σ −0.5 (4) 

where σth has been interpreted as either a threshold stress for branching or a residual stress (refs. 13 and 14). 

Ideally, the branch length should be treated as the dependant variable rather than as the independent variable 

because it results from the applied load. However, the treatment is customarily done as shown in equations 

(2) to (4). 

Three of the test specimens failed just outside of the load ring and one just inside the support ring. The 

specimen failing just inside the support ring did not exhibit branching before the crack intersected the test 

specimen edge. Estimations of Ab were made with and without the inclusion of these test specimens to 

determine the affect of a non-uniform stress state. For the specimen failing just inside the support ring, an 

elastic solution to calculate the edge stress was not available in the published literature, so the following 

solution was derived (ref. 15): 

( )( )( )

2 2 2

2 2 2 2

4

3 1

h R r

F R R r R

D

L S D r π

ν − − − σ = (5) 

( )( )( )

2 2 2

2 2 2 2

4

3 1

h R r

F R R r R

D

L S D

π

ν − − +

σθ = (6) 

where σr and σθ are the radial and tangential stresses, respectively, between the test specimen edge and 

support ring, r is the radial failure position, RS the support radius, RL the load radius, RD the specimen radius, 

h the thickness, and F the failure load. Equations (5) and (6) were found comparable to finite element 

analysis. The stress at the edge of the specimen is 

( )( )

2 2

2 2

2

3 1

D

L S

h R

F R R

π

ν − − σθ = . (7) 

Figures 8 to 10 show the data plotted according to equations (2) to (4), and tables 4 and 5 summarize 

branch lengths and branch constants via the methods described. The figures indicate that the single specimen 

that failed at low local stresses (i.e., the stress at the point of failure) will influence the results particularly 

when curve fitting techniques are used. 

Equations (2) to (4) result in a branching constant of Ab ≈ 1.0 MPa√m when M is taken as –0.5, with 

estimates from equation (4) invariably being the lowest. If M is determined via fitting, Ab increases to 

~2 MPa√m, with an estimated M ≈ –0.4. Consideration of the test specimen failing just inside the support 

ring tends to increase Ab, particularly when M is estimated from curve fitting. Equation (4) gives a 

σth ≈ 9 MPa for all cases. 
rb, m

Figure 8.—Fracture strength as a function of crack branching length for CVD ZnSe. 

Figure 9.—Fracture strength as a function of crack branching length for CVD ZnSe. 

rb, m

0.0001 0.001 0.01 0.0002 0.00035 0.0006 0.002 0.0035 0.006

Stress at Failure Location, MPa

20

30

40

50

60

70

80

10

100

CVD ZnSe

Regression

f b Ab log σ = M log r + log

M = –0.37 

M = –0.5 

0.000 0.001 0.002 0.003 0.004 0.005

Stress at Failure Location, MPa

10

20

30

40

50

60

70

80

CVD ZnSe

Regression

2rb

5.0

bbf σ rA − =

M = –0.5 

M = –0.38 

NASA/TM—2006-214100 11
NASA/TM—2006-214100 12

Figure 10.—Fracture strength as a function of crack branching length 

to the –0.5 power for CVD ZnSe. 

The most sensitivity to the use of local stress as compared to equibiaxial stress for the specimens failing 

outside of the load ring was exhibited by equation (3) (e.g., when specimens 14, 20, and 21 are used with the 

equibiaxial stress, Ab = 1.9 MPa√m, whereas use of the local stress gives Ab = 2.4 MPa√m). 

The most agreement between equations (2) to (4) occurs when the three specimens that failed just 

outside the load ring are used with the equibiaxial stress. This also produces the exponent closest to –0.5. 

This likely occurs because the actual stresses near the load ring are slightly higher than those calculated with 

the standard elastic solution (ref. 17) due to contact stresses, thereby decreasing the branch length. The 

specimen failing well outside the load ring was subjected to a near uniaxial stress state and yields an 

individual branching constant of Ab = 1.7 MPa√m, in good agreement with the reported value of 1.7 MPa√m 

for uniaxial stresses (ref. 16). It should be noted, however, that Ab values derived from uniaxial and biaxial 

tests have shown agreement, implying a single value of Ab regardless of stress state (ref. 18). Whether the 

significance of the agreement between this data point and the reported value for uniaxial testing is a 

coincidence or an indication of a difference between biaxial and uniaxial branching for ZnSe is unclear due 

to a lack of data. However, it should be noted that the branching occurred in an asymmetric fashion: the 

branch length toward the specimen center was 4.7 mm while branching away from the specimen center did 

not occur before the crack propagated 7.4 mm and traversed the specimen edge. If Ab is calculated using the 

static edge stress, the branch stress, or the stress at the origin with the respective branch lengths, Ab values of 

1.5, 2.5, or 1.9 MPa√m result respectively (see table 4), implying a higher Ab for uniaxial states. However, 

the use of the static stresses calculated at the plate edge and the branch point are likely inappropriate because 

of the dynamics of fracture. Use of the stress at the point of failure and the branch length, which are the only 

well defined values, results in Ab = 1.7 MPa√m. 

Besides considering the equibiaxial stress and local stress in calculation of Ab, estimates were made 

using a mode I equivalent stress based on the Principal of Independent Action (PIA). This usually increased 

the estimated value of Ab and decreased the estimated value of M, but did not result in better agreement 

rb

-0.5, m-0.5

10 20 30 40 50 60 70 80

Stress at Failure Location, MPa

10

20

30

40

50

60

70

80

CVD ZnSe

Regression

2rb

th

0.5

f b b σ = A r + σ −
between the cases shown in table 5. Because only a single stress, rather than the stress components that 

makeup an equivalent stress, can be determined from a single equation such as (2), the use of an Ab value 

calculated with biaxial data and equation (2) may be the best approach when multiaxial stresses are 

considered likely. 

For fractographic analysis of CVD ZnSe components subjected to a biaxial stress state, the branching 

constant is Ab = 1.0±0.1 MPa√m. Better estimates of the branch constant and distinction of the best technique 

would require much more data covering a larger range of failure stresses. 

TABLE 4.—LOCAL STRESSES AND BRANCH LENGTHS FOR CVD ZnSe. 

Stress at failure locationa Specimen 

number 

Equibiaxial 

fracture 

stress, 

σf 

MPa 

Failure 

position, 

mm Radial, 

σr 

MPa 

Tangential, 

σθ

MPa 

Branch 

length, 

rb 

mm 

Branch 

constant 

Ab

MPa√m 

(via Eq. (2)) 

12RH 51.8 10.68 0.41 1.0 

13RH 62.5 12.11 0.27 1.0 

14RH 63.0 13.53 55.9 60.6 0.29 1.0 

15RH 61.9 10.68 0.32 1.1 

16RH 63.5 9.26 0.29 1.1 

17RH 71.1 10.68 0.21 1.0 

18RH 55.2 9.26 0.35 1.0 

19RH 53.8 12.11 0.48 1.2 

20RH 57.1 13.53 50.7 55.0 0.32 1.0 

21RH 65.1 14.25 51.8 60.3 0.27 1.0 

22RH 50.5 4.27 0.48 1.1 

23RH 48.0 11.40 0.43 1.0 

24RH 51.8 10.68 0.37 1.0 

25RH 54.7 9.97 0.29 0.9 

Calculations for no. 11RH

Edge Stress 56.6 24.36 0 17.5 7.3 1.5 

Local Stress “ “ “ “ –1.7 24.8 4.7 1.7 

Local Stress “ “ “ “ “ “ “ “ 6.0b 1.9 

Branch Stressc “ “ “ “ 15.8 36.9 4.7 2.5 a

Values are not shown for specimens that failed in the central region. 

b

Average of lengths from origin to branch point and from origin to specimen edge. 

c

The stresses calculated at the point of branching and at the specimen edge were based on static elastic analysis and thus are likely 

inappropriate because of the dynamics of failure. 

NASA/TM—2006-214100 13
NASA/TM—2006-214100 14

TABLE 5.—BRANCH CONSTANTS FOR CVD ZnSe AS CALCULATED WITH EQUATIONS (1) TO (3). 

Equationa M Ab 

MPa√m

σth 

MPa r

2 

Without no. 11RH, 14RH, 20RH, and 21RH

(2) point wise –0.5 1.0±0.1 ----- ---- 

(2) nonlinear –0.5 1.0±0.6 ----- 0.76 

(2) nonlinear –0.43±0.07 1.9±1.1 ----- 0.78 

(3) –0.5 1.0±0.1 ----- 0.72 

(3) –0.41±0.08 2.2±1.3 ----- 0.76 

(4) –0.5 0.9±0.2 9.1 0.79 

With no. 14RH, 20RH, 21RH, and equibiaxial stress 

(2) point wise –0.5 1.0±0.1 ----- ---- 

(2) nonlinear –0.5 1.0±0.6 ----- 0.88 

(2) nonlinear –0.44±0.06 1.7±1.9 ----- 0.89 

(3) –0.5 1.0±0.1 ----- 0.75 

(3) –0.43±0.07 1.9±1.0 ----- 0.78 

(4) –0.5 0.9±0.1 7.3 0.80 

With no. 14RH, 20RH, 21RH, and local stress 

(2) point wise –0.5 1.0±0.1 ----- ---- 

(2) nonlinear –0.5 1.0±0.6 ----- 0.85 

(2) nonlinear –0.41±0.07 2.1±1.1 ----- 0.87 

(3) –0.5 1.0±0.1 ----- 0.69 

(3) –0.39±0.06 2.4±1.2 ----- 0.75 

(4) –0.5 0.8±0.1 10.4 0.77 

With no. 11RH, 14RH, 20RH, 21RH, and local stress 

(2) point wise –0.5 1.1±0.1 ----- ---- 

(2) nonlinear –0.5 1.0±0.5 ----- 0.89 

(2) nonlinear –0.39±0.04 2.6±0.9 ----- 0.92 

(3) –0.5 1.0±0.1 ----- 0.83 

(3) –0.37±0.02 3.0±0.5 ----- 0.96 

(4) –0.5 0.9±0.1 9.4 0.93 

a

Equation (2) is −0.5 σ = f b b A r ; Equation (3) is f b Ab logσ = M logr + log ; Equation (4) is f b b th σ = A r + σ −0.5 ; 

r

2

 = Pearson coefficient squared. 

Summary and Conclusions 

The properties of CVD ZnSe were measured in order to determine design allowables for the FEANICS 

module. The measured values were in good agreement with values reported in the 1970’s. The average 

Young’s modulus, Poisson’s ratio, equibiaxial fracture strength, flaw size, grain size, Knoop hardness, 

Vicker’s hardness, and branching constant for the CVD ZnSe were E = 74.3±0.1 GPa, v = 0.31, σ f = 

57.8±6.5 MPa, a = 21 ± 4 μm, l = 43±9 μm, HK0.5/15 = 0.97±0.02 GPa, HV0.5/15 = 0.97±0.02 GPa, and 

Ab = 1.0±0.1 MPa√m. The hardness of CVD ZnSe is about one-twentieth of sapphire, and thus the ZnSe 

window must be handled with care. Due to the occasional presence of very large grains and the small 

inherent flaw size, it is recommended that the single crystal properties be used in design of structural 

components made from CVD ZnSe. In order to calculate the stresses between the support ring and the edge 

of circular plates loaded between concentric rings, a stress solution was derived. 

Recommendations 

Test witness coupons that are manufactured along with the flight window(s). Development of a fine 

grained transparent ZnSe would enhance window fracture toughness, fracture strength, and slow crack 

growth resistance. 
References 

1. J.A. Salem, “Estimation of ZnSe Crack Growth Properties for Design of the FEANICS Windows,” 

NASA/TM—2005-213359 (2005). 

2. ASTM C1499–01 “Determination of Monotonic Equibiaxial Flexural Strength Advanced Ceramics,” in 

Annual Book of ASTM Standards, vol. 15.01, American Society for Testing and Materials, West 

Conshohocken, Pennsylvania (2004). 

3. ASTM C1259–94, “Standard Test Method for Dynamic Young’s Modulus, Shear Modulus, and Poisson’s 

Ratio for Advanced Ceramics by Impulse Excitation of Vibration,” in Annual Book of ASTM Standards, 

vol. 15.01, American Society for Testing and Materials, West Conshohocken, Pennsylvania, (2004). 

4. J.A. Salem and A. Singh, “Polynomial Expressions for Estimating Elastic Constants From the Resonance 

of Circular Plates,” NASA/TM—2005-213843, (2005). 

5. A.G. Evans and H. Johnson, “A Fracture-Mechanics Study of ZnSe for Laser Window Applications,” J. 

Am. Ceram. Soc., vol. 58, no. 5–6, pp. 244–249 (1975). 

6. G.A. Graves, D.E. McCullum, and J.M. Wimmer: Exploratory Development and Investigation of the 

Thermal, Electrical, Mechanical, and Physical Properties of Infrared Laser Window and I–R 

Transmitting Materials; Final Technical Report. AFML−TR−77−23, UDRI−TR−77−06, 1977. Available 

from the NASA Center for Aerospace Information. 

7. ASTM C1326, “Standard Test Methods for Knoop Indentation Hardness of Advanced Ceramics,” in 

Annual Book of ASTM Standards, vol. 15.01, American Society for Testing and Materials, West 

Conshohocken, Pennsylvania (2005). 

8. ASTM C1327, “Standard Test Methods for Vickers Indentation Hardness of Advanced Ceramics,” in 

Annual Book of ASTM Standards, vol. 15.01, American Society for Testing and Materials, West 

Conshohocken, Pennsylvania (2005). 

9. J.A. Salem, “Slow Crack Growth and Fracture Toughness of Sapphire for the International Space Station 

Fluids and Combustion Facility,” NASA/TM—2006-214023 (2006). 

10. P. Miles “High Transparency Infrared Materials⎯A Technology Update,” Opt. Eng., vol. 15, no. 5, 

pp. 451−459 (1976). 

11. J.C. Wurst and T.P. Graham: Thermal, Electrical, and Physical Measurements of Laser Window 

Materials; Final Report. AFML−TR−75−28, UDRI−TR−75−06, 1975. Available from the NASA Center 

for Aerospace Information. 

12. ASTM E112, “Standard Test Methods for Determining Average Grain Size,” in Annual Book of ASTM 

Standards, vol. 03.01, American Society for Testing and Materials, West Conshohocken, Pennsylvania 

(2005). 

13. J.B. Quinn, “Extrapolation of Fracture Mirror and Crack-Branch Sizes to Large Dimensions in Biaxial 

Strength Tests of Glass,” J. Am. Ceram. Soc., 82 [8], pp. 2126–32 (1999). 

14. J.J. Mecholsky, Jr., S.W. Freiman, and R.R. Rice, “Fractographic Analysis of Ceramics,” pp. 336–379 in 

Fractography in Failure Analysis, Special Technical Publication 645, B.M. Strauss and W.H. Cullen, 

eds., American Society for Testing and Materials, West Conshohocken, PA, (1977). 

15. J.M. Manderscheid, private communication, NASA Glenn Research Center, June 20, 2005. 

16. J.J. Mecholsky, Jr., S.W. Freiman, and R.W. Rice, “Fractographic Analysis of Ceramics,” J. Materials 

Science, 11, pp. 1310–1319 (1976). 

17. J.A. Salem and L.M. Powers, “Guidelines for the Testing of Plates” pp. 357–364 in Proceedings of the 

27th International Cocoa Beach Conference on Advanced Ceramics and Composites: B, Ceramic 

Engineering and Science Proceedings, vol. 24, no. 4, Waltraud M. Kriven and H.T. Lin, editors 

(January, 2003). 

18. J.J. Mecholsky, Jr., and R.W Rice, “Fractographic Analysis of Biaxial Failure in Ceramics,” pp. 185–193 

in Fractography of Ceramic and Metal Failures, Special Technical Publication 827, American Society 

for Testing and Materials, West Conshohocken, Pennsylvania (1984). 

NASA/TM—2006-214100 15

Appendix—Equibiaxial Fracture Strength Testing Report 

1. Test Date: July 9th and 12th, 2004. 

2. Rohm Haas CVD ZnSe produced in April of 2004. Material may have been produced in two lots 

(Tag no. 27355 and no. 27460 delivered on 4/07/05 and 4/16/05, respectively). 

3. Polished according to the specifications in figure 2. Rohm and Haas declined to disclose all stages of 

polishing as it is proprietary. Diamond grinding followed by polishing with an alumina slurry was 

indicated. A 10 min soak in water was performed prior to testing. 

4. Instron 8521 servo-hydraulic test frame with Instron 10KN load cell no. 1462. 

5. Test fixtures were marging steel hardened to 50 RC. The load ring diameter was 25.44 mm and the 

support ring diameter was 50.81 mm. The cross section radius of the ring contact points were 4 mm. 

6. Two 0.005 in. grafoil layers were used between the load fixtures and the test specimen. Also, the 

compression sides of the test specimens were covered with cellophane tape. 

7. Test environment was distilled, deionized water from the distillation unit in building 49. Ambient 

temperature was 76 °F with 45 percent RH. 

8. Test mode was load control with a load rate of 316 N/s resulting in a nominal stress rate of 10 MPa/s. 

9. The measured Poisson’s ratio and Young’s modulus are summarized in table 1. 

10. The average diameter and thickness of each test specimen is summarized in table 2. 

11. The preload applied to each test specimen was 10 N. The resultant pre-stress was 0.3 MPa. See table 2. 

12. The breaking load and equibiaxial flexural strength of each test specimen is summarized in table 2. 

NASA/TM—2006-214100 17
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E–15435

WBS–22–400–35–40–07

22

Mechanical Characterization of ZnSe Windows for Use With the Flow

Enclosure Accommodating Novel Investigations in Combustion of Solids

(FEANICS) Module

Jonathan A. Salem

Zinc selenide; flexural strength; Modulus of elasticity; Poisson ratio; Hardness; Stress

corrosion; Weibull; Structural reliability; Microstructure; Grain size; Branching constant;

Laser window; Pressure chambers

Unclassified -Unlimited

Subject Category: 27

Responsible person, Jonathan A. Salem, organization code RXL, 216–433–3313.

Mechanical and physical properties of ZnSe windows to be used with the FEANICS (Flow Enclosure Accommodating

Novel Investigations in Combustion of Solids) experiments were measured in order to determine design allowables.

The average Young’s modulus, Poisson’s ratio, equibiaxial fracture strength, flaw size, grain size, Knoop hardness,

Vicker’s hardness, and branching constant were 74.3 ± 0.1 GPa, 0.31, 57.8 ± 6.5 MPa, 21 ± 4 mm, 43 ± 9 μm, 0.97 ±

0.02 GPa, 0.97 ± 0.02 GPa, and 1.0 ± 0.1 MPa√m, respectively. The properties of current ZnSe made by chemical vapor

deposition are in good agreement with those measured in the 1970’s. The hardness of CVD ZnSe windows is about onetwentieth of the sapphire window being replaced, and about one-sixth of that of window glass. Thus the ZnSe window

must be handled with great care. The large grain size relative to the inherent crack size implies the need to use single

crystal crack growth properties in the design process. In order to determine the local failure stresses in one of the test

specimens, a solution for the stresses between the support ring and the edge of a circular plate load between concentric

rings was derived.