July 2011 

NASA/CR–2011-217161

Edgewise Compression Testing of STIPS-0 

(Structurally Integrated Thermal Protection 

System)

Amy R. Brewer 

Analytical Services and Materials, Inc., Hampton, Virginia 
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Hampton, Virginia 23681-2199 under Contract NNL09AM18T 

 

July 2011 

NASA/CR–2011-217161

Edgewise Compression Testing of STIPS-0 

(Structurally Integrated Thermal Protection 

System)

Amy R. Brewer 

Analytical Services and Materials, Inc., Hampton, Virginia 
Available from: 

NASA Center for AeroSpace Information 

7115 Standard Drive 

Hanover, MD 21076-1320 

443-757-5802 

The use of trademarks or names of manufacturers in this report is for accurate reporting and does not 

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National Aeronautics and Space Administration. 
i 

TABLE OF CONTENTS 

TABLE OF CONTENTS ................................................................................................................................ I

LIST OF TABLES .........................................................................................................................................II

LIST OF FIGURES .......................................................................................................................................II

ACKNOWLEDGEMENTS ............................................................................................................................III

NOMENCLATURE .......................................................................................................................................IV

ABSTRACT................................................................................................................................................... 1

1. INTRODUCTION .................................................................................................................................. 1

2. SITPS-0 TEST ARTICLE ..................................................................................................................... 2

3. EDGEWISE COMPRESSION TEST SETUP ....................................................................................... 4

3.1. SPECIMEN POTTING ....................................................................................................................... 4

3.2. SPECIMEN INSTRUMENTATION......................................................................................................... 5

3.2.1. DCDTs (Out-of-Plane Displacements) .................................................................................... 6

3.2.2. Strain Gages (In-Plane Strain and Poisson’s Ratio) ............................................................... 7

3.2.3. Photogrammetry (In-Plane Strain, Out-of-Plane Displacements) ........................................... 8

3.2.4. Acoustic Emission Sensors (Damage Indicator) ..................................................................... 9

3.3. TEST STAND ................................................................................................................................10

3.4. TEST SEQUENCE..........................................................................................................................11

4. TEST RESULTS ................................................................................................................................. 12

4.1. COMPRESSIVE ELASTIC MODULUS................................................................................................12

4.2. PROPORTIONAL LIMIT...................................................................................................................13

4.3. POISSON’S RATIO ........................................................................................................................14

4.4. BENDING / OUT-OF-PLANE DISPLACEMENTS..................................................................................15

4.4.1. DCDTs ...................................................................................................................................15

4.4.2. Photogrammetry ....................................................................................................................18

4.4.3. Acoustic Emission .................................................................................................................19

4.5. IN-PLANE STRAINS.......................................................................................................................20

4.5.1. Longitudinal Strain Gages .....................................................................................................20

4.5.2. Photogrammetry ....................................................................................................................22

4.6. POST-TEST NON-DESTRUCTIVE EVALUATION ................................................................................22

4.6.1. Thermography Image of Delamination ..................................................................................22

4.6.2. Computed Tomography (CT) ................................................................................................23

5. CONCLUSIONS ................................................................................................................................. 25

REFERENCES ............................................................................................................................................26

APPENDIX A: IML OUT-OF-PLANE DISPLACEMENTS (PHOTOGRAMMETRY) ................................. 27

APPENDIX B: POST-TEST CT IMAGES ..................................................................................................31

B.1 VOIDS AT OVERWRAP INTERSECTIONS ................................................................................................32

B.2 THROUGH PANEL THICKNESS - CT IMAGES OF SITPS-0 ......................................................................33

B.3 THROUGH PANEL DEPTH - CT IMAGES OF SITPS-0 .............................................................................40
ii

LIST OF TABLES 

TABLE 1. EDGEWISE COMPRESSION LOADING SEQUENCE ..............................................................................11

TABLE 2. COMPRESSIVE ELASTIC MODULUS ..................................................................................................13

TABLE 3. POISSON'S RATIO...........................................................................................................................15

LIST OF FIGURES 

FIGURE 1. SITPS TEST ARTICLE..................................................................................................................... 2

FIGURE 2. AETB ORIENTATION OF SITPS-0 AETB BARS ................................................................................ 3

FIGURE 3. SURFACE ANOMALIES FROM APPLIED INSTRUMENTATION FOR THERMAL TESTING.............................. 4

FIGURE 4. REQUESTED POTTING GEOMETRY (DIMENSION IN INCHES) ............................................................... 5

FIGURE 5. INSTRUMENTED SITPS-0 TEST ARTICLE IN TEST LOAD FRAME........................................................... 6

FIGURE 6. INSTRUMENTATION LAYOUT............................................................................................................. 6

FIGURE 7. DCDT LAYOUT FOR BENDING AND OUT-OF-PLANE DISPLACEMENTS................................................. 7

FIGURE 8. STRAIN GAGE LOCATIONS FOR IN-PLANE STRAINS........................................................................... 8

FIGURE 9. PHOTOGRAMMETRY SYSTEM SETUP................................................................................................ 9

FIGURE 10. ACOUSTIC EMISSION SENSOR LOCATION .....................................................................................10

FIGURE 11. SOUTHWARK EMERY 120-KIP TEST STAND ..................................................................................11

FIGURE 12. SITPS-0 STRESS-STRAIN CURVES (PSI) .....................................................................................13

FIGURE 13. SITPS-0 PROPORTIONAL LIMIT (OFFSET = -5.0 ) .....................................................................14

FIGURE 14. ROSETTE STRAIN GAGE RESPONSE USED TO CALCULATE POISSON'S RATIO FOR EACH FACESHEET . 15

FIGURE 15. DCDT OUT-OF-PLANE DISPLACEMENT SIGN CONVENTION...........................................................16

FIGURE 16. OUT-OF-PLANE DISPLACEMENT (DCDT) - 10K LOADING SEQUENCE ............................................16

FIGURE 17. OUT-OF-PLANE DISPLACEMENT (DCDT) - 20K LOADING SEQUENCE ............................................17

FIGURE 18. OUT-OF-PLANE DISPLACEMENTS (DCDT) – 37K LOADING SEQUENCE .........................................17

FIGURE 19. OVERVIEW OF OUT-OF-PLANE DISPLACEMENTS @ 37K LBS.........................................................18

FIGURE 20. OUT-OF-PLANE DISPLACEMENTS AT P = 37K LBS (PHOTOGRAMMETRY SYSTEM) ..........................19

FIGURE 21. ACOUSTIC EMISSION - FIRST ARRIVAL OCCURRENCES ................................................................. 20

FIGURE 22. IN-PLANE STRAINS (STRAIN GAGE DATA) ....................................................................................21

FIGURE 23. IN-PLANE STRAINS AT P = 37K LBS (PHOTOGRAMMETRY SYSTEM) ...............................................22

FIGURE 24. THERMOGRAPHY IMAGE OF IML SHOWING DELAMINATION.............................................................23

FIGURE 25. POST-TEST CT IMAGES ..............................................................................................................24

FIGURE A-1: PHOTOGRAMMETRY IMAGE (FRAME) WITH RESPECT TO STRESS-STRAIN CURVE..........................27

FIGURE A-2. PHOTOGRAMMETRY IMAGES ......................................................................................................28

FIGURE B-1. VIEWING ORIENTATION FOR CT IMAGES....................................................................................31

FIGURE B-2. VOIDS AT AETB OVERWRAP INTERSECTIONS............................................................................32

FIGURE B-3. THROUGH PANEL THICKNESS - CT IMAGES OF SITPS-0 ............................................................33

FIGURE B-4. THROUGH PANEL DEPTH - CT IMAGES OF SITPS-0 ...................................................................40

Hi-Nicalon is a trademark of COI Ceramic, Incorporated, 7812 West 4100 South, 

Magna, Utah 84044. 
iii

ACKNOWLEDGEMENTS 

 The author would like to thank the following groups for their contributions to the 

SITPS Program.

 The Hypersonics Program of Fundamental Aeronautics for their contribution to the 

SITPS program, 

 The SITPS Team at NASA for oversight and participation in the development of the 

SITPS Concept and 

 The Structural Mechanics and Concepts Branch test facility at Langley Research 

Center for test resources and oversight. 

In particular, the author would like to thank the following persons for their contributions 

to the testing and evaluation of the SITPS-0 panel. 

 Jeff Gragg of NASA Langley Research Center (LaRC-D326) for his participation in 

the testing of the SITPS test article, 

 Matt Moholt of NASA Dryden Flight Research Center (DFRC-RS) for his 

participation in testing of the SITPS-0 test article, 

 Patrick McNeil of Analytical Sciences & Material, Inc. at NASA Langley Research 

Center (LaRC-D312) for the Photogrammetry work, and 

 The NDE Group at NASA Langley Research Center (LaRC-D313): Patty Howell for 

Thermography, Eric Burke for Computed Tomography, Eric Madaras and Michael 

Horne for Acoustic Emission, 

 Kim Bey and Sandra Walker of NASA Langley Research Center (LaRC-D312) for 

their participation in editions of this document. 
iv

NOMENCLATURE 

8HS ............. eight Harness Satin weave 

A .................. cross-sectional area (in2) 

AETB ........... Alumina Enhanced Thermal Barrier 

CT ............... Computed Tomography 

DCDT .......... Direct Current Displacement Transducer 

E .................. elastic modulus (psi) 

IML .............. Inner Mold Line (M55J/954-3 Cyanate Ester) 

OML ............ Outer Mold Line (S200Hm) 

P .................. compressive load (lb) 

PIP .............. Polymer Impregnation and Pyrolsis 

S200Hm ...... modified S200H facesheet 

SG............... Strain gage 

SITPS .......... Structurally Integrated Thermal Protection System 

................... engineering strain (in/in) 

L ................ longitudinal strain (in/in) 

T ................ transverse strain (in/in) 

prop ........... engineering strain (in/in) at proportional limit 

.................. engineering stress (psi) 

prop ........... engineering stress (psi) at proportional limit 

.................. Poisson’s ratio
1 

ABSTRACT 

 The Structurally Integrated Thermal Protection System (SITPS) task was initiated by 

the NASA Hypersonics Project under the Fundamental Aeronautics Program to develop 

a structural load-carrying thermal protection system for use in aerospace applications. 

The initial NASA concept for SITPS consists of high-temperature composite facesheets 

(outer and inner mold lines) with a light-weight insulated structural core. (1) 

 An edgewise compression test was performed on the SITPS-0 test article at room 

temperature using conventional instrumentation and methods in order to obtain panellevel mechanical properties and behavior of the panel. 

 Three compression loadings (10, 20 and 37 kips) were applied to the SITPS-0 panel. 

The panel behavior was monitored in-siti using standard techniques and non-destructive 

evaluation (NDE) methods such as photogrammetry and acoustic emission. The elastic 

modulus of the SITPS-0 panel was determined to be 1.146x106

 psi with a proportional 

limit at 1039 psi. Barrel-shaped bending of the panel and partial delamination of the 

IML occurred under the final loading. The delamination was verified post-testing using 

thermography. The panel was also imaged with Computed Tomography (CT) posttesting. 

1. INTRODUCTION 

 The Structurally Integrated Thermal Protection System (SITPS) task was initiated by 

the NASA Hypersonics Project under the Fundamental Aeronautics Program to develop 

a structural load-carrying thermal protection system for use in aerospace applications. 

The initial NASA concept for SITPS consists of high-temperature composite facesheets 

(outer and inner mold lines) with a light-weight insulated structural core. (1) 

The Structurally Integrated Thermal Protection System (SITPS) concept provides 

an integrated structural component designed to carry both aerodynamic and thermal 

loads. The inner and outer walls carry the aerodynamic loading. The outer wall 

operates at high temperatures typical of hypersonic re-entry. The thermally-insulative 

inner core will provide thermal protection through the thickness of the system which 

maintains the inner surface at below acceptable maximum temperatures. 

 An edgewise compression test was performed on the SITPS-0 test article at room 

temperature using conventional instrumentation and methods in order to obtain panellevel mechanical properties and behavior of the panel. Of particular interest were the 

basic mechanical properties of the test article including the elastic modulus, 

proportional limit and Poisson’s Ratio.
2 

2. SITPS-0 TEST ARTICLE 

The as-fabricated test article, SITPS-0 panel, is shown Figure 1. The SITPS-0 panel 

measured approximately 11.5” x 11.5” x 2.25” thick and the recorded area weight is 5.8 

lbs/ft2

.

 The SITPS-0 core consists of two orthogonal layers of eleven 1-inch x1-inch AETB16 (alumina enhanced thermal barrier) bars. The AETB-16 bars were individually spiral 

wrapped with Hi-Nicalon/SiNC (Hi-Nic, 2D 8HS weave composite, 30, 1-ply, 0.0125-

inch). The AETB and Hi-NicalonTM/SiNC fabrication protocol are not available at this 

time (proprietary). 

Figure 1. SITPS Test Article 

 

 The outer mold line (OML) is S200Hm SiC/SiC 2D 8HS ceramic matrix composite 

([0](6), 0.075”). The layers of fabric forming the OML were green and were adhered to 

the AETB bar core prior to densification; therefore, densification of the OML occurred 

from the free face side only. The Polymer Impregnation and Pyrolysis (PIP) 

methodology was used for densification of the OML and AETB-16 bar assembly (9-

cycles) with painting of the exposed surfaces between PIP cycles. 

The inner mold line (IML) is a M55J/954-3 Cyanate Ester tape (8-ply, [0/45/-45/90],

0.096-inch). The IML was fabricated and cured by COI Ceramics, Inc. The cured IML 

was bonded to the SITPS-0 panel with FM-300 adhesive after completion of the 

densification process of the OML and AETB bar assembly. 

 The orientation of each AETB bar in the SITPS-0 panel is shown in Figure 2. The 

thermal and mechanical properties of AETB are different in the through-thickness and 

in-plane directions, therefore orientation is important. The exposed edges of each 

AETB bar (Edges #1 through #4) were visually inspected for the AETB orientation, the 

through-thickness direction is orthogonal to the fiber direction. Edges #1 and #3 are 

shown and the arrows indicate the AETB through-thickness direction. The bottom figure 

shows AETB bar 8, 9 and 10 of Edge #1. 
3 

Figure 2. AETB Orientation of SITPS-0 AETB Bars 

(Magnification of Edge #1 - AETB bars 8, 9, and 10) 

(Preferred orientation is ↕ through thickness) 

 

 Prior to mechanical testing, the SITPS-0 test article was subjected to thermal 

testing.(2) The thermal testing required that several thermocouples be inserted into the 

AETB-16 material which resulted in gouges in the AETB-16 shown in Figures 2 and 3

(Figure 3 designated by ○). The white residue on the overwraps and the OML in 

Figures 2 and 3 is residue from glue that held the thermocouple wires in place. 

2 4

3 1

Edge Number

Edge #1

Edge #3

AETB Bar 1 2 3 5 4 9 6 10 11 7 8

Edge 1

AETB Bars

8,9,10
4 

Figure 3. Surface Anomalies from Applied Instrumentation for Thermal Testing 

3. EDGEWISE COMPRESSION TEST SETUP 

3.1. Specimen Potting 

 The panel ends were potted using a 1-inch form and Unisorb V100 potting material. 

The specimen and forms were positioned such that the potting enclosed the ends of 

specimen and the potting material was to extend outside the 1-inch form. After potting, 

the top and bottom surfaces of the potting medium were machined or ground such that 

the potted surfaces were flat and parallel to within ±0.001-in. of each other, and potting 

surfaces were perpendicular to the specimen. The requested potting layout is shown in 

the Figure 4. 
5 

Figure 4. Requested Potting Geometry (Dimension in Inches) 

The ends to be potted are designated by the arrow (↕) and were in the warp direction 

of the OML facesheet. Longitudinal AETB bars, running along OML facesheet, are in 

the loading direction. Transverse AETB bars, running along the IML facesheet, are 

perpendicular to the loading direction. 

 Note: The panel was embedded in the potting material ~1/2 inch at the top of the 

specimen and ~1 inch at the bottom. Although the specimen was not potted 

symmetrically on the top and bottom of the SITPS-0 panel, no detrimental effects were 

expected. 

3.2. Specimen Instrumentation 

 The SITPS-0 panel was heavily instrumented with redundant systems due to the fact 

that only one panel existed and panel-level mechanical properties were of the utmost 

importance. The panel was instrumented with the following systems. 

 DCDTs for out of plane displacements 

 Stain gages for in-plane strains 

 Photogrammetry for out of plane displacements 

 Photogrammetry for in-plane stains 

 Acoustic Emission sensors as a damage locator 

The instrumented panel shown in the test load frame is presented in Figure 5. 
6 

Figure 5. Instrumented SITPS-0 test article in test load frame 

(View of OML Facesheet) 

 The instrumentation layout on the OML and IML are identical as shown in the 

mirrored images in Figure 6. The vertical and horizontal lines indicate the 1-inch AETB 

bars to provide the instrumentation location relative to the AETB bars. The DCDTs (	)

are located near the same edge of the specimen. A detailed discussion of each type of 

instrumentation is presented in the following subsections. 

Figure 6. Instrumentation Layout 

3.2.1. DCDTs (Out-of-Plane Displacements) 

 Three 
0.25-inch DCDTs (DC Displacement Transducers) (	) were placed 

perpendicular to the OML and IML to monitor specimen out-of-plane displacement 
7 

during loading and to distinguish between specimen bending deflections and facesheet 

disbonding. As shown in Figure 7, the DCDTs were placed roughly two inches from the 

left-hand side of the specimen (as viewed from the OML) and equidistant from the top of 

the specimen. The OML DCDTs were located at the intersection of AETB bars 2 and 3 

(numbered left to right) and at the mid-span of AETB bars 3, 6 and 9 (numbered top to 

bottom) on the IML. The DCDT data was acquired at 10Hz (samples per second) using 

the Micro-Measurements Smart Strain (V3.10) data acquisition system and the Model 

5100B scanners. 

Figure 7. DCDT Layout for Bending and Out-of-Plane Displacements 

3.2.2. Strain Gages (In-Plane Strain and Poisson’s Ratio)

 Twelve longitudinal strain gages ( ) and one rectangular rosette strain gage (  )

were installed on both the OML and IML facesheet at mid AETB bar locations as shown 

in Figure 8. The longitudinal strain gages are universal general purpose Measurements 

Group, Inc. CEA-06-187UW-350 (gage length = 0.187-inch) and the center strain gages 

are CEA-06-250UR-350 (gage length = 0.250-inch) placed in a rectangular rosette 

configuration (0, 45 and 90). 

 All strain gages were oriented for longitudinal strain with the exception of the center 

strain gages. The center strain gage are rectangular rosettes providing longitudinal and 

transverse strain to determine Poisson’s effect under loading. The upper and lower 

corner strain gages on the OML and IML were used for platen alignment to ensure 

proper load introduction into the panel. 

DCDT 

Position

OML 1 IML 1

OML 2 IML 2

OML 3 IML 3
8 

Figure 8. Strain Gage Locations for In-Plane Strains 

 The strain gage data was acquired at 10Hz using the Micro-Measurements Smart 

Strain (V3.10) data acquisition system and the Model 5100B scanners. 

3.2.3. Photogrammetry (In-Plane Strain, Out-of-Plane Displacements) 

 Full field strain and out-of-plane deformations were monitored by photogrammetric 

methods using the VIC 3D (Video Image Correlation) System by Correlated Solutions, 

Inc. The VIC 3D System employs three-dimensional image correlation allowing the 

measurement of all three (X, Y and Z) surface displacement fields simultaneously. The 

OML and IML of the test article were spray-painted with white Krylon enamel paint and 

then sputtered with black Krylon enamel paint for use in the correlation algorithm. 

 The VIC 3D setup for the edgewise compression test of the SITPS-0 test article is 

shown in Figure 9. The setup consists of four 5-MByte monochrome cameras; two 

cameras each viewing the OML and IML simultaneously. The camera images were 

recorded at a rate of 2 frames per second. VIC 3D reports in-plane displacement with 

OML SG 10 8 6 4 2

IML SG 9 7 5 3 1

OML SG 14 26 12

IML SG 13 25 11

OML SG 24 22 20 18 16

IML SG 23 21 19 17 15
9 

accuracies up to 0.02 pixels and out-of-plane displacement accuracies up to 0.04 pixels; 

strain with accuracies up to 0.01% locally and 0.005% globally.

Figure 9. Photogrammetry System Setup 

3.2.4. Acoustic Emission Sensors (Damage Indicator) 

 Four Acoustic Emission (AE) sensors (●) were mounted to the OML and IML 

facesheets (total of 8 sensors). The AE sensors monitor the AE activity (energy release 

from introduction of damage) in the specimen during loading. The locations of the AE 

sensors are shown in Figure 10 (as viewed from the OML). 

Camera 1

OML IML

Camera 2

Camera 3

Camera 0
10

Figure 10. Acoustic Emission Sensor Location 

 

 The AE sensors were B1025 Sensors with a frequency bandwidth of 50 kHz - 2MHz. 

Tape was adhered to the specimen surface at the desired AE sensor location and the 

AE sensors were glued to the tape using an epoxy. The sensors were connected via 

the Digital Wave PA0 preamp/line to the Digital Wave FM1 signal conditioning 16 

channel amplifiers. The AE activity was captured on a portable computer running the 

Wave Explorer software. 

3.3. Test Stand 

 The test stand used was a 120-kip Southwark-Emery Test machine with a 6-inch 

maximum displacement (Figure 11). The test stand has a 23-inch x 24-inch leveling 

platen for maintaining proper load introduction. The leveling platen was instrumented 

with three additional 
1.00-inch DCDTs to monitor the trueness of the upper platen 

during the specimen loading. The platen DCDTs were placed equidistant (3-inch) from 

the platen edge. Platen alignment was accomplished using a pre-load on the specimen 

of 1000 lbs while monitoring strain gages at the corners of the specimen and adjusting 

the leveling screws on the platen until minimal strain deviation existed between the 

strain gages at the corners of the specimen. 

OML AE1 AE3

IML AE2 AE4

OML AE5 AE7

IML AE6 AE8
11

Figure 11. Southwark Emery 120-kip Test Stand 

3.4. Test Sequence 

 The objective of the edgewise compression was to reach the proportional limit of the 

SITPS-0 panel in incremental loading steps. The proportional limit is considered to be 

the so-called first matrix cracking stress, i.e., the point where - curve becomes nonlinear. The test sequence consisted of the three individual loadings listed in Table 1. 

Table 1. Edgewise Compression Loading Sequence 

Sequence Target Load Notes

1 10000 lbs

(~390 psi)

 Zero all instrumentation prior to load 

introduction.

 Acquire instrumentation data and 

photogrammetry.

2 20000 lbs

(~780 psi)

 Do not re-zero instrumentation

 Acquire instrumentation data and 

photogrammetry.

3 Proportional 

Limit

 Do not re-zero instrumentation.

 Acquire instrumentation data and 

photogrammetry.

Stop loading at proportional limit.

 The SITPS-0 edgewise compression tests were run under displacement control at 

~0.002 inches/min. Once the proportional limit was reached, the load was removed and 

Leveling Platen
12

the specimen was inspected by NDE techniques – thermography and computed 

tomography. 

4. TEST RESULTS 

 A summary of the results from the loading sequence of the edgewise compression 

test is shown in Table 2. A maximum of 37439 lbs (1459 psi) was attained at which 

time the loading was terminated. The proportional limit had been reached, and a 

suspected delamination was seen on the IML and bending was seen on the OML. The 

following subsections deals with the mechanical properties of the SITPS-0 panel 

loadings. 

4.1. Compressive Elastic Modulus 

The engineering stress,  (psi) is defined as 

(1) ࡭Ȁࡼ ൌ 

where P is the compressive load (lb) and A is the cross-sectional area (A = 25.66 in2).

 The linear strain gages were used to monitor the strain in the specimen. 

 The computation of the elastic modulus was restricted to the linear portion of the -

curve between -100 and -600 and  was the average of SG 15 through 24. The 

elastic modulus, E (psi), is 

ࡱ ൌ Ȁ (2)

where / is the slope of the linear region of the - curve. 

 The compressive elastic moduli resulting from the three loadings are shown in Table 

2 and graphically presented in Figure 12. Over the three loadings, a minor increase in 

modulus was seen. Moduli ranged from 1.095 x10-6 to 1.146 x10-6 psi. 
13

Table 2. Compressive Elastic Modulus 

Loading

Sequence

Maximum

Compressive 

Load (lbs)

Stress(psi)

Line Load

(lbs/in)

Compressive

Strain ()

Compressive

Elastic Modulus 

(psi)

10K 10094 lbs

432 psi

878 lbs/in 360  1.095 x106 psi

20K 20045 lbs

781 psi

1743 lbs/in 691  1.128 x106 psi

37K 37439 lbs

1459 psi

3256 lbs/in 1405  1.146 x106 psi

Figure 12. SITPS-0 Stress-Strain Curves (psi) 

4.2. Proportional Limit 

 The proportional limit is determined using the offset method at a strain offset of -

5.0. Using the elastic modulus (slope of the linear region of the - curve between -

100 and -600 and the strain offset of -5.0  the offset line is drawn. The 

proportional limit (prop and prop) is the point at which the offset line intersects the -

curve. The proportional limit occurred at prop = 1039 psi at a prop = -914 

corresponding to a total load of 26671 lbs and a line load of 2319 lbs/in. The graphical 

representation is shown in Figure 13. 
14

Figure 13. SITPS-0 Proportional Limit (Offset = -5.0 ) 

4.3. Poisson’s Ratio

Poisson’s ratio, , for each facesheet (OML and IML) was determined using the data 

from the rectangular rosette strain gages at the center of the specimen on each of the 

facesheets. Poisson’s ratio uses the slope of the linear region of the T -L curve 

ൌȀ (3)

TL is the slope of the linear region of the T -L curve where T is the transverse 

strain and L is the longitudinal strain. 

The transverse vs. longitudinal strain for each loading is presented in Figure 14. 

prop = -1039.5 psi

prop = -914.2 

-5.0  Offset Line
15

Figure 14. Rosette strain gage response used to calculate Poisson's ratio for each Facesheet 

Poisson’s ratio (37K loading) was determined as the slope of the linear portion of the 

37K lbs loading step occuring at longitudinal strains between -300 and -600.

Poisson’s ratio for the OML and IML is presented in Table 3.

Table 3. Poisson's Ratio 

Facesheet Poisson’s Ratio,  (37K)

OML (S200Hm) 0.064

IML (M55J/954-3) 0.343

4.4. Bending / Out-of-Plane Displacements 

 The DCDTs, Photogrammetry System, and the AE sensors were monitored for 

bending and out-of-plane displacements. See Specimen Instrumentation (Section 3.3) 

for the locations of the instrumentation. 

4.4.1. DCDTs 

 A total of six DCDTs were positioned perpendicular to the OML and IML facesheets 

(three DCDTs on each facesheet) during the loading sequence. The sign convention for 

the DCDT displacement data is shown in Figure 15. A positive displacement on the IML 
16

facesheet indicates an outward displacement away from the centerline of the specimen. 

A negative displacement on the OML facesheet indicates an outward displacement 

away from the centerline. The converse is true in both cases. 

Figure 15. DCDT Out-of-Plane Displacement Sign Convention 

 The DCDT displacements resulting from the three loading sequences are shown in 

Figures 16 – 18. It should be emphasized that the data presented in the following 

figures represent the panel response where the DCDT are located. 

Figure 16. Out-of-Plane Displacement (DCDT) - 10K Loading Sequence 

OML IML

- +


17

Figure 17. Out-of-Plane Displacement (DCDT) - 20K Loading Sequence 

 The displacements for the 10K and 20K loading sequence indicates the initiation of 

bending of the OML beginning at the onset of loading. The displacements on the OML 

are somewhat larger than the IML. 

Figure 18. Out-of-Plane Displacements (DCDT) – 37K Loading Sequence 
18

 The DCDT displacements resulting from the 37K loading sequence are shown in 

Figure 18. The OML displacements show a somewhat symmetrical bending about the 

centerline of the specimen. The displacement at the center of the specimen (OML 2) 

has the largest out-of-plane displacement with respect to the upper (OML 1) and lower 

(OML 3) DCDTs. OML1 and 3 exhibit somewhat similar behavior although differ in 

magnitude. 

 The displacements for the IML indicate a much different behavior. The upper (IML 

1) and lower (IML 3) DCDTs remain somewhat stable whereas the behavior at the 

center of the IML (IML 2) shows a divergence from IML1 and 3 starting at 800 psi. At 

1000 to 1200 psi, IML 2 shows a very rapid increase in out-of-plane displacement 

indicating a delamination of the facesheet. 

 A rough depiction of the out-of-plane displacement at the DCDT locations shown in 

the Figure 19 illustrating the bending of the OML facesheet and delamination on the 

IML. 

Figure 19. Overview of Out-of-Plane Displacements @ 37K lbs 

4.4.2. Photogrammetry 

 The photogrammetry data provides a global view of the panel response. The 

photogrammetric results from the VIC System Z-Displacement output (out-of-plane 

displacements) for the OML and IML at 37K lbs load are shown in the Figure 20. The 

photogrammetry images do not contain the full field image due to obstruction of DCDTs, 

strain gage wires and AE instrumentation. Also note that the accuracy of out-of-plane 

displacements was sacrificed for accuracy of full field strain measurements. The center 

of the specimen is marked with rectangular strain gage (  ). 

OML IML

- +


19

Figure 20. Out-of-Plane Displacements at P = 37K lbs (Photogrammetry System) 

 The Z-Displacements recorded by the photogrammetry system indicate bending on 

the OML as shown by the symmetrical lateral bands about the center of the specimen. 

The highest Z-Displacement occurs about the center of the specimen with lower ZDisplacement occurring at the top and bottom of the specimen. The photogrammetry 

data corresponds well to the out-of-plane displacement recorded by the DCDTs. 

 The Z-Displacements recorded on the photogrammetry system indicates slight 

bending of the IML with a pronounced Z-Displacement occurring near the location of 

DCDT IML2 which indicative of a delamination. The photogrammetry data corresponds 

well to the out-of-plane displacement and delamination recorded by the DCDTs. 

 A loading history (37K - curve) and the photogrammetry system images for out-ofplane displacements on the IML is shown in Appendix A. No evidence of the 

delamination on the IML was apparent in the first two loading sequences. The 

delamination initiated at approximately 1000 psi of the third loading sequence followed 

by a rapid increase in out-of-plane displacement as supported by the SITPS-0

instrumentation. 

4.4.3. Acoustic Emission 

 The Acoustic Emission (AE) sensors were installed on the OML and IML facesheets 

to monitor the energy release associated with the introduction of damage. See 

Specimen Instrumentation (Section 3.3) for the locations of the AE sensors. The AE 
20

data were filtered to remove low amplitude events (threshold = 50 mV) and only 

includes events that were received by at least two sensors. 

The AE activity for the 37K loading is shown in Figure 21. The histogram of First 

Occurrence Arrival indicates which sensor first detected an AE event. 

Figure 21. Acoustic Emission - First Arrival Occurrences 

 The odd-numbered AE sensors, located on the OML, show an even distribution of 

first arrival occurrences. However, the even numbers sensors, located on the IML, 

show a disparity in the distribution of first arrival occcurences with AE6 and AE2 having 

the greatest number of first arrival occurrences whereas AE 4 and AE8 have much 

fewer occurrences. Hence, the majority of AE activity was occurring in the region of 

sensors AE6 and AE2 on the IML. 

 The AE activity on the IML also corroborates the expected delamination shown by 

the out-of-plane displacements from the DCDT data, strain gage data and 

photogrammetry Z-Disp images on the IML. 

4.5. In-Plane Strains 

4.5.1. Longitudinal Strain Gages 

 The strain gage output for the OML and IML from the 37K loading sequence is 

shown in Figure 22 (See Figure 8 for the locations of the strain gages). OML strains 

were fairly linear throughout the 37K loading whereas the IML strains were non-linear. 

~37K
21

SG 11, 13 and 25 were located at the site of the expected delamination and showed a 

deviation from the linear response starting at ~1000 psi. The behavior of SG 11, 13 and 

25 are a classic behavior for delamination. 

Figure 22. In-Plane Strains (Strain Gage Data) 
22

4.5.2. Photogrammetry 

 The magnitude of the in-plane strains as reported by the photogrammetry system 

correlate reasonably well with the in-plane strains from the strain gage data reported in 

the previous section. The in-plane strains (eyy) from the photogrammetry system near 

the end of the test are shown in Figure 23. The high strain region on the right-hand side 

of the IML also indicates a delamination of the IML facesheet. 

Figure 23. In-Plane Strains at P = 37K lbs (Photogrammetry System) 

4.6. Post-Test Non-Destructive Evaluation 

4.6.1. Thermography Image of Delamination 

 As a follow-up to the delamination issue as indicated by the out-of-plane 

displacements (DCDT and photogrammetry system), the in-plane strains (strain gages 

and photogrammetry system) and the AE activity, a post-test thermography image was 

taken of the IML of the unloaded SITPS-0 panel. The IML of the specimen was coated 

with GE Aerocoat-C spray prior to thermography inspection to increase the emissivity of 

the surface image and the thermography viewing was extended to two minutes. The 

thermography image (Figure 24) confirms the delamination on the right-hand side of the 

IML and is indicated by the light to white region of the image. 

Y

X

Z

Y

X

Z
23

Figure 24. Thermography Image of IML showing delamination 

indicated by region within the dashed oval 

4.6.2. Computed Tomography (CT) 

 Computed Tomography (CT) images were taken of the SITPS-0 specimen prior to 

and after the edgewise compression test for damage inspection. In both sessions, the 

CT images were taken under no load. It was assumed that the out-of-plane 

deformations on the OML and IML may have caused damage to the SITPS-0 specimen. 

 Damage could include fiber fracture, matrix cracking, fiber breakage, etc. on the 

OML and fracture of the IML. However, no damage was apparent in the CT images 

when comparing pre- and post-test CT images. Figure 25 shows the viewing aspect and 

highlights the constituents and thermocouple insertion points in the SITPS-0 panel. See 

Appendix B for additional post-test CT images. 
24

 

Figure 25. Post-Test CT Images 

Transverse AETB Bars IML (M55J/954-3CE)

Longitudinal AETB Bars OML (S200Hm)

Hi-Nic 30 Spiral-Overwrap
25

5. CONCLUSIONS 

 The Structurally Integrated Thermal Protection System (SITPS), was initiated under 

the NASA Hypersonics Program to develop a structural load-carrying thermal protection 

system for use in aerospace applications. The SITPS panel consists of hightemperature composite facesheets and a light-weight insulation structural core. 

 An edgewise compression test at room temperature was performed on the SITPS-0

specimen in order to obtain mechanical properties and behavior of the panel. The 

specimen was fully instrumented for out-plane displacements (DCDTs and 

photogrammetry) and in-plane strains (strain gages and photogrammetry) in addition to 

test frame load. 

 The specimen was potted and loaded such that the OML (S200Hm facesheet) was 

in contact with the longitudinal AETB bars and the IML (M55J/954-3 facesheet) was in 

contact with the transverse AETB bars. The SITPS-0 test article was subjected to a 

sequence of loadings; 393 psi (Load = 10090 lbs, Line Load = 877 lbs/in), 781 psi (Load 

= 20045 lbs, Line Load = 1743 lbs/in) and 1459 psi (Load = 37439 lbs (Line Load = 

3256 lbs/in). 

 The compressive elastic modulus was determined to be E = 1.146x106

. The 

proportional limit was determined using a -5.0  offset at prop = 1039 psi (Load = 

26671 lbs, Line Load = 2319 lbs/in) @  prop = -914.2. Poisson’s ratio (37K loading) 

was calculated to be OML = 0.064 and IML = 0.343. 

 Out-of-plane displacements (DCDTs and photogrammetry) indicated bending of 

OML and bending/partial delamination on the IML which resulted in a barrel-shape 

displacement about the center of and at the edge of the specimen. The AE First Arrival 

Occurrences also indicated high activity on the IML at the location of the suspected 

delamination. The delamination of the IML was corrobrated with Thermography 

inspection. 

 OML strains were linear during loading whereas the IML strain were non-linear 

throughout the loading. There was reasonable correlation between in-plane strain (yy) 

and the photogrammetry system. 

 The SITPS-0 specimen incurred bending of the OML and delamination of the IML 

under lower than expected loads. Further compression testing of the panel would not 

yield additional information such as ultimate load and strain. 

 Investigations of the SITPS-0 test article will continue to assess and confirm the 

actual size and location of the IML delamination, i.e., IML/glue layer, glue/overwrap 

layer, overwrap/AETB separation, etc. Also, the material properties of the individual 
26

constituents of the as-built test article are being investigated and are required to aid in 

the development of the analytical models predictions for future testing. 

 REFERENCES 

1. Easler, Tim; Plunkett, Rich; “Structurally Integrated Thermal Protection System”, 

Presented at National Space & Missile Materials Symposium, Henderson NV, 24 

June 2009. 

2. Darybeigi, Kamran; Knutson, Jeffrey; Martin, Kim, “Thermal Testing for Validation of 

Thermal Modeling of Structurally Integrated Thermal Protection System”, Presented 

at the 34th Annual Conference on Composites, Materials and Structures, Cocoa 

Beach FL, January 25-28, 2010. 

3. Standard Test Method for Monotonic Compressive Strength Testing of Continuous 

Fiber-Reinforced Advanced Ceramics with Solid Rectangular Cross-section Test 

Specimens at Ambient Temperatures”, ASTM C1358-05. 
27

APPENDIX A: IML OUT-OF-PLANE DISPLACEMENTS 

(Photogrammetry) 

 The following images show the loading history (loading to 37K lbs (maximum load 

occurs at Frame 866) and unloading) and the out-of-plane displacement 

photogrammetry images of the IML facesheet. The photogrammetry frame numbers are 

indicated on the - curve shown in Figure A-1. The photogrammetry images do not 

contain the full field image due to obstruction of DCDTs, strain gage wires and AE 

instrumentation. The accuracy of out-of-plane displacements was sacrificed for 

accuracy of full field strain measurements. The delamination emanates from the right 

side of the IML (Frame 600) and travels inwards toward the center of the specimen. 

The delamination was confirmed by Thermography. Overall, the IML facesheet 

experienced bending. 

 

Figure A-1: Photogrammetry Image (Frame) with respect to Stress-Strain Curve 
28

Figure A-2. Photogrammetry Images 

Frame 100

Frame 200 Frame 300

Frame 400 Frame 500 
29

Note: Initiation of delamination begins at 

right side of specimen, mid-pane

Figure A-3. Photogrammetry Images (continued) 

Frame 600 Frame 700

Frame 750 Frame 800

Frame 825 Frame 850
30

Out-of-Plane displacement continued to

increase at right side of specimen, midpanel and progressed to the left side of the 

specimen

Unloading begins at Frame 875 and outof-plane displacements decrease.

Figure A-4. Photogrammetry Images (continued) 

Frame 866  Max Load Frame 875

Frame 900 Frame 925

Frame 950 - Unloaded
31

APPENDIX B: Post-Test CT IMAGES 

 Computed Tomography (CT) images were taken of the SITPS-0 panel after 

completion of the edgewise compression testing. The following images are taken under 

no load. The images were taken in quadrants and comparisons should be limited to 

within the quadrant. In general, the density can be related to the pixel intensity in the 

images; lower density is darker, higher density is lighter to white. 

 The CT images are viewed from the IML facesheet to the OML and are denoted as 

Through Thickness. Next, images as viewed from the top of the specimen to the bottom 

of the specimen and are denoted as Through Depth. The viewing orientations are 

shown below and the location of the image is marked by the red line on the specimen 

layout , the frame number of each image is shown to mark the progression of CT 

images through the specimen. 

CT Image

Through Panel Thickness Images

Through Panel Depth Images

Figure B-1. Viewing Orientation for CT Images 

OML IML

Mid AETB Bar IML

OML

IML

OML

AETB Bar Overwrap
32

B.1 Voids at Overwrap Intersections 

 Voids at the intersection of the AETB overwrap were prevalent throughout the 

SITPS-0 panel. These voids are interstitial spaces and are not to be confused with 

cracks. The voids were present prior to and after both the thermal cycling and the 

edgewise compression testing. 

Examples of voids follow in Figure B-2. 

Figure B-2. Voids at AETB Overwrap Intersections 

Through-Thickness

Overwrap on

Lateral AETB Bars

Through–Thickness

Overwrap on

Longitudinal AETB Bars

Voids at 

Intersection of AETB 

Overwraps

IML

OML

Through-Depth 

Intersection of Lateral and 

Longitudinal AETB Bars 
33

B.2 Through Panel Thickness - CT Images of SITPS-0 

Note: Distortion at the top and bottom of the images are due to interference with the 

potting frame. The through-panel thickness CT images are shown in Figure B-3.

Figure B-3. Through Panel Thickness - CT Images of SITPS-0 

Inside IML

Frame 69

OML IML

Glue Joint between IML and CMC overwrap

Frame 82

OML IML
34

Partial View of Panel

Figure B-3. Through Panel Thickness - CT Images of SITPS-0 (continued) 

OML IML
35

Figure B-3. Through Panel Thickness - CT Images of SITPS-0 (continued) 

OML IML

OML IML
36

Partial View of Panel 

Figure B-3. Through Panel Thickness - CT Images of SITPS-0 (continued) 

OML IML
37

Figure B-3. Through Panel Thickness - CT Images of SITPS-0 (continued)) 

OML IML

OML IML
38

Figure B-3. Through Panel Thickness - CT Images of SITPS-0 (continued) 

OML IML

OML IML
39

Figure B-3. Through Panel Thickness - CT Images of SITPS-0 (continued) 

OML IML

OML IML
40

B.3 Through Panel Depth - CT Images of SITPS-0 

Figure B-4. Through Panel Depth - CT Images of SITPS-0 
41

Figure B-4. Through Panel Depth - CT Images of SITPS-0 (continued) 

Frame 1115

Frame 981

L

Frame 1248

IML

OML
42

Figure B-4. Through Panel Depth - CT Images of SITPS-0 (continued) 

 The voids along the intersection of the overwrap and the lower AETB bars were 

seen both pre- and post- compression testing. 

Frame 1921

Frame 1785

L

Frame 2055

IML

OML
REPORT DOCUMENTATION PAGE Form Approved

OMB No. 0704-0188

2. REPORT TYPE 

Contractor Report

 4. TITLE AND SUBTITLE

Edgewise Compression Testing of SITPS-0 (Structurally Integrated 

Thermal Protection System)

5a. CONTRACT NUMBER

NNL09AM18T

 6. AUTHOR(S)

Brewer, Amy R.

 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

NASA Langley Research Center

Hampton, VA 23681-2199 

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National Aeronautics and Space Administration

Washington, DC 20546-0001

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NASA

13. SUPPLEMENTARY NOTES

Langley Technical Monitor: Kim S. Bey

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Unclassified - Unlimited

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Availability: NASA CASI (443) 757-5802

19a. NAME OF RESPONSIBLE PERSON

STI Help Desk (email: help@sti.nasa.gov)

14. ABSTRACT

The Structurally Integrated Thermal Protection System (SITPS) task was initiated by the NASA Hypersonics Project under the 

Fundamental Aeronautics Program to develop a structural load-carrying thermal protection system for use in aerospace 

applications. The initial NASA concept for SITPS consists of high-temperature composite facesheets (outer and inner mold 

lines) with a light-weight insulated structural core. An edgewise compression test was performed on the SITPS-0 test article at

room temperature using conventional instrumentation and methods in order to obtain panel-level mechanical properties and 

behavior of the panel. Three compression loadings (10, 20 and 37 kips) were applied to the SITPS-0 panel. The panel behavior 

was monitored using standard techniques and non-destructive evaluation methods such as photogrammetry and acoustic 

emission. The elastic modulus of the SITPS-0 panel was determined to be 1.146x106 psi with a proportional limit at 1039 psi. 

Barrel-shaped bending of the panel and partial delamination of the IML occurred under the final loading. 

15. SUBJECT TERMS

compression, mechanical testing, STIPS, structurally integrated thermal protection, TPS

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1. REPORT DATE (DD-MM-YYYY)

01- 07 - 2011