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i

NASA/TM—2011–216477

A Damage Tolerance Comparison of Composite

Hat-Stiffened and Honeycomb Sandwich 

Structure for Launch Vehicle 

Interstage Applications

A.T. Nettles

Marshall Space Flight Center, Huntsville, Alabama

December 2011

National Aeronautics and

Space Administration

Marshall Space Flight Center • Huntsville, Alabama 35812
ii

Available from:

NASA Center for AeroSpace Information

7115 Standard Drive

Hanover, MD 21076 –1320

443–757–5802

This report is also available in electronic form at

<https://www2.sti.nasa.gov/login/wt/>
iii

TABLE OF CONTENTS

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

2. MATERIALS AND TESTING .......................................................................................... 4

2.1 Sandwich Structure ....................................................................................................... 4

2.2 Hat-Stiffened Structure ................................................................................................. 4

3. RESULTS ........................................................................................................................... 7

3.1 Hat-Stiffened Structure ................................................................................................. 7

3.2 Sandwich Specimens ..................................................................................................... 31

4. DISCUSSION .................................................................................................................... 35

4.1 Co-Cured Versus Precured Hat-Stiffened Specimens .................................................... 35

4.2 Size of Impactor ........................................................................................................... 35

4.3 Location of Impact ....................................................................................................... 35

4.4 Comparison With Sandwich Panels .............................................................................. 36

5. CONCLUSIONS ................................................................................................................ 37

REFERENCES ....................................................................................................................... 39
iv
v

LIST OF FIGURES

 1. Concepts for carrying large compressive loads: (a) Sandwich and (b) hat stiffened ..... 1

 2. Double hat-stiffened specimen .................................................................................... 5

 3. Example of an IRT indication on an impacted specimen ............................................ 5

 4. Compression test of hat-stiffened specimens: (a) Front view and (b) rear view ........... 6

 5. Hat stiffener/face sheet disbond and total failure of specimen double 1 ...................... 7

 6. Location of impact on specimen double 2 ................................................................... 8

 7. IRT for specimen double 2 .......................................................................................... 8

 8. Hat stiffener/face sheet disbond growth on specimen double 2 .................................... 9

 9. Failure of specimen double 2 ...................................................................................... 9

10. Location of impact on specimen double 3 ................................................................... 10

11. IRT for specimen double 3 .......................................................................................... 10

12. Location of impact on specimen double 4 ................................................................... 11

13. IRT for specimen double 4 .......................................................................................... 11

14. Hat stiffener disbond of specimen double 4 ................................................................ 12

15. Delamination across entire face sheet of specimen double 4 ........................................ 12

16. Location of impact on specimen double 5.................................................................... 13

17. IRT for specimen double 5 .......................................................................................... 13

18. Dimpling at impact site and face sheet failure of specimen double 5 ........................... 14

19. Location of impact on specimen double 6 ................................................................... 14

20. IRT for specimen double 6 .......................................................................................... 15
vi

LIST OF FIGURES (Continued)

21. Hat stiffener failure of specimen double 6 ................................................................... 15

22. Buckling and catastrophic failure of specimen double 6 .............................................. 16

23. Location of impact on specimen double 7 ................................................................... 16

24. IRT for specimen double 7 .......................................................................................... 17

25. Buckling and catastrophic failure of specimen double 7 .............................................. 17

26. Location of impact on specimen double 8 ................................................................... 18

27. IRT for specimen double 8 .......................................................................................... 18

28. Failure of face sheet of specimen double 8 .................................................................. 19

29. Specimen double 8 showing intact hat stiffeners after face sheet failure ...................... 19

30. Location of impact on specimen double 9 ................................................................... 20

31. IRT for specimen double 9 .......................................................................................... 20

32. Specimen double 9 showing intact hat stiffeners after face sheet failure ...................... 21

33. Location of impact on specimen double 10 ................................................................. 21

34. IRT for specimen double 10 ........................................................................................ 22

35. Failure of face sheet of specimen double 10 ................................................................ 22

36. Specimen double 10 showing failed hat stiffeners and face sheet ................................. 23

37. Location of impact on specimen double 11 ................................................................. 23

38. IRT for specimen double 11 ........................................................................................ 24

39. Location of impact on specimen double 12 ................................................................. 24

40. IRT for specimen double 12 ........................................................................................ 25

41. Dimpling at impact site and face sheet failure of specimen double 12 ......................... 25
vii

LIST OF FIGURES (Continued)

42. Face sheet buckling and catastrophic failure of specimen double 12 ........................... 26

43. Location of impact on specimen double 13 ................................................................. 26

44. IRT for specimen double 13 ........................................................................................ 27

45. Face sheet failure of specimen double 13 ..................................................................... 27

46. Location of impact on specimen double 14 ................................................................. 28

47. IRT for specimen double 14 ........................................................................................ 28

48. Dimpling at impact site and face sheet failure of specimen double 14 ......................... 28

49. Residual strength versus impact energy for hat-stiffened specimens ............................ 30

50. Residual strength versus impact energy sandwich specimens ....................................... 32

51. Comparison of hat-stiffened and sandwich specimens ................................................ 32

52. Comparison of hat-stiffened and sandwich specimens with mass 

of structure included ................................................................................................... 33

LIST OF TABLES

1. Summary of CAI results on hat-stiffened specimens ............................................................ 30

2. CAI results for sandwich specimens used in this study ........................................................ 31
viii
1

TECHNICAL MEMORANDUM

A DAMAGE TOLERANCE COMPARISON OF COMPOSITE HAT-STIFFENED 

AND HONEYCOMB SANDWICH STRUCTURE FOR LAUNCH VEHICLE 

INTERSTAGE APPLICATIONS

1. INTRODUCTION

An interstage structure on a launch vehicle is a cylindrical structure that separates various 

stages of the vehicle. By utilizing carbon fiber composites instead of aluminum, it is hoped that the 

mass of the structure can be reduced. These structures are dominated by compression stresses and 

can either be stiffness (buckling) or strength critical. Two methods of obtaining a strong and stiff 

cylinder are by using sandwich construction or stringer-stiffened structure. Sandwich construction 

consists of using a lightweight core between two composite face sheets. The composite face sheets 

are designed to carry the structural compressive load and the core serves to distribute stresses via 

shear to the two face sheets such that the cross section of the structure has a high moment of inertia and thus high buckling load.

The stringer-stiffened concept utilizes one face sheet that has long, narrow sections with 

a constant prismatic cross section placed in the direction of loading at discreet intervals along the 

width of the part. The stringer adds stability to the face sheet to help prevent buckling. A common 

shape for the prismatic cross section of the stiffeners is a trapezoid whose cross section resembles 

a hat and thus the term ‘hat stiffener.’ These two concepts are shown schematically in figure 1.

Load-Bearing

Crown

Hat Stiffener

Lightweight Core Load-Bearing

Face Sheets

Webs

Compression Load

View A-A ́

(a) (b) View A-A ́

Compression Load

A

A A ́

A ́

Face Sheet

Feet

Figure 1. Concepts for carrying large compressive loads: (a) Sandwich and (b) hat stiffened.
2

The compressive failure mechanism differs in these two structures in that the hat-stiffened 

panel can have its face sheet buckle and still carry significant load through the hat stiffeners. Once 

the face sheet of a sandwich structure begins to buckle, catastrophic failure typically occurs. Thus, 

some of the load-carrying capability of the hat-stiffened structure is in the face sheet postbuckling 

regime.

For most structural launch vehicle applications, the composite part must be designed under 

damage tolerance considerations. The two types of structural concepts shown in figure 1 can have 

greatly different damage tolerance characteristics. The sandwich structure is uniform across its 

acreage and thus any impact from a foreign object will have essentially the same response regardless of where the impact takes place. The hat-stiffened concept, on the other hand, is not uniform 

across its acreage and will have a different response if impacted by a foreign object, depending on 

where the impact takes place. For example, an impact on the face sheet between hat stiffeners will 

behave differently than an impact directly on one of the hat stiffeners. This is very important as the 

hat stiffeners are on the outside of the cylinder for interstage structure and are thus more prone to 

accidental impacts from foreign objects.

There are two major modes of failure for compression-loaded composite structures—global 

buckling of the structure and ply level microbuckling (material failure). The effects of damage to 

global buckling have been examined and are mostly in the form of debonded stringers.1 However, 

impact damage is also a concern to a composite structure since such damage can cause the composite material to lose much of its load-carrying capability; i.e., the laminate will experience microbuckling at a lower load than had it not been impacted.

Impact damage to composite sandwich structures has been reviewed in detail elsewhere;2

however, the literature is sparser with regards to impact damage of hat-stiffened composite structures. One of the first studies on the effects of impact damage on hat-stiffened panels3 showed 

that impacts in the high stiffness regions—crown and face sheet behind crown—of the panel could 

cause a significant drop in compressive load-carrying capability for a structure designed to carry 

a line load of 9,000 lb/in. It was determined that impact damage in the low stiffness regions—the 

web and face sheet between hats—did not cause a significant decrease in compressive load-carrying 

capability. It was also shown that hat-stiffened panels designed for lower load-carrying capabilities 

(3,000 lb/in) did not suffer compressive strength degradation, even when impacted in the high stiffness regions.

Foam-filled, hat-stiffened panels designed for a load-carrying capability of 3,000 lb/in 

were evaluated in reference 4 and it was determined that impact to the crown had little effect on 

residual compressive load-carrying capability, but impact to the face sheet behind the crown, or 

at the crown face sheet interface region caused drops in compressive load-carrying capability. 

A related study using carbon fabric and resin transfer molding to manufacture hat-stiffened panels 

was examined in reference 5. The damage tolerance of the hat-stiffened panels in this study was 

increased by stitching the hat stringers to the face sheet. Results showed that damage tolerance was 

increased over panels made from preimpregnated, unidirectional tape, mainly due to the throughthickness reinforcements (stitching) at the hat/face sheet interface, preventing the stringer from 

debonding from the face sheet.
3

It has been noted that for a skin-stringer structure, the failure mechanism consists of 

a series of buckles in the face sheet which result in the load being transferred to the stiffeners which 

eventually leads to a material compressive failure (ply level microbuckling) in the stiffeners.6 This 

reference also suggests that a three-stringer panel be used for compression after impact testing 

since a stringer and two adjacent skin bays would represent the structure and the two stringers at 

the ends of the panel would replicate the boundary conditions of the actual structure. However, 

by having to test a panel of this size, more material and a larger capacity loading frame would be 

needed. It would be simpler to test a smaller specimen, such as a single stringer, and extrapolate the 

data to multistringer panels. This has been attempted with some success7 but finite element analysis 

is needed to size the single stringer specimen to replicate its behavior in a multistringer panel.

In this current study, two stringer panels were tested since that was the only structure available for damage tolerance testing. While certainly not ideal for design purposes, the two stringer 

panels were deemed sufficient to use for a simple comparison with sandwich structure.

It is the intent of this study to ascertain the criticality of impact damage to hat-stiffened 

composite panels that are designed to be used as a launch vehicle interstage. The major thrust of 

this study will focus on the criticality of the location of impact rather than the severity of impact. 

These data are then compared to compression after impact data from sandwich specimens also 

designed for use for a launch vehicle interstage.
4

2. MATERIALS AND TESTING

2.1 Sandwich Structure

The sandwich specimens consisted of composite panels that were manufactured by a co-cure 

process. The honeycomb core used was perforated 5052 aluminum with a density of 3.1 lb/ft3 and a 

thickness of 1.125 in. The face sheets consisted of IM7/8551-7 carbon/epoxy and the film adhesive used 

to bond the face sheet to the core was FM-300k with an areal density of 0.08 lb/ft2. The layup of the 

face sheet was [45, 0, –45, 0, 90, 0, 0, 90, 0]S. Each face sheet had a thickness of 0.10 in. The layup of 

the face sheet is the same as that scheduled to be used on full-scale hardware. The panels were manufactured as 24- by 2-in-square sandwich plates. All panels were subjected to nondestructive evaluation 

(NDE) using infrared thermography (IRT) testing to ensure no flaws were present before specimens 

were machined from the panels. From the 24- by 24-in panels, 4- by 6-in specimens were machined for 

impact and subsequent compression testing. This gave each specimen a load-bearing, cross-sectional 

area of 0.80 in2 which was used to calculate the breaking stresses.

After impact, the specimens were inspected with IRT after which the specimens were prepared 

for end-loaded compression testing. In order to avoid premature failures due to end brooming, the loaded 

ends of the specimen were potted with epoxy in an aluminum frame and machined to a tolerance of 

±0.001 in parallelism. Details of the specimen geometry and testing methodology are published elsewhere.8

2.2 Hat-Stiffened Structure

The hat-stiffened structure evaluated in this study was made from T40-800/5320 carbon/

epoxy unidirectional prepreg and consisted of two hat stiffeners. The hat-stiffened panels where 

manufactured utilizing out-of-autoclave techniques. The layup of the face sheet portion of the 

hat-stiffened panels was 17-ply [902, 0, 90, 45, –45, 02, 90bar]

S which gives the face sheet a layup 

with 35% 0° plies. The webs and feet had a layup of [45, –45]S and the crown consisted of [04, 45, 

04,–45, 0, 0bar]

S (83% 0o plies). The web, crown, and feet all had an outer ply of fabric placed at 

±45°. A gap inevitably forms between the inner two radii of each hat stiffener and the face sheet. 

A preformed epoxy rod was used as a gap filler and is referred to as a noodle. Some of the hat stiffeners were precured and then co-bonded to the face sheet in a secondary bonding operation. Most 

of the hat-stiffened specimens were manufactured with the hat stringer co-cured to the face sheet. 

Figure 2 shows a schematic of the type of specimen and the nomenclature that is used to describe 

them throughout this Technical Memorandum.

The hat-stiffened specimens were impacted using a drop-weight instrumented impact test 

apparatus. The locations of the impact on the hat-stiffened specimens were varied with the incident 

impact energy being held fairly constant. The impact energy used was similar to those used for 

comparable sandwich structure specimens. This was done to determine the criticality of location of 

impact on the hat-stiffened specimens. All of the impactors had spherically shaped ends, most with

a 0.5-in diameter.
5

Crown

Webs

Noodle

Face Sheet

Foot

0.875 in

2 in 1.375 in 2 in

0.875 in 0.875 in 0.875 in

2.5 in

4 in

Thickness = 0.15 in

Thickness = 0.11 in

1 in 1 in

8.875 in

Figure 2. Double hat-stiffened specimen.

After impact testing, the specimens were assessed for damage using IRT. This NDE technique utilizes a burst of heat from a quartz lamp and a very sensitive infrared camera monitors the 

dissipation of heat from the specimen. The heat will emit at a different rate at locations where damage exists and the infrared camera shows these areas (termed ‘indications’) on a digital computer. 

An example of an IRT indication is shown in figure 3. 

Indication

Figure 3. Example of an IRT indication on an impacted specimen.
6

The impacted specimens were mounted with strain gauges at the front and rear of each 

specimen as shown schematically in figure 4. These gauges were to ensure even loading of the specimen during testing. If the gauges differed by more than 10%, the test was stopped and shims were 

placed between the upper platen and the load cell until uniform loading was achieved.

Strain Gauges

Front View Rear View

Precision Ground

Steel Platens

Load Cell

Shims

Inserted if

Needed

(a) (b)

Figure 4. Compression test of hat-stiffened specimens: (a) Front view and (b) rear view.

The edges of the specimen were free and not supported. A 100-kip load frame was used 

with a test rate of 0.05 in/min. Each test was recorded with a digital video camera for subsequent 

analysis. Any face sheet buckling that occurred was only assessed visually. Ideally, back-to-back 

strain gauges would be placed at the center of the face sheet and monitored for divergence from 

each other; however, limited strain gauges for this study did not allow for this. Since the main measurement of concern was ultimate load-carrying capability, the buckling of the face sheets was not 

assessed in detailed.
7

3. RESULTS

3.1 Hat-Stiffened Structure

The results will be presented individually since each specimen—with a few exceptions—had 

a different impact location and/or energy that resulted in some differing compression-after-impact 

(CAI) results. The specimen nomenclature used was ‘double #’ where ‘double’ indicated two hat 

stiffeners per specimen and ‘#’ was a numerical designation for each specimen.

3.1.1 Double 1

This specimen had hat stiffeners that were precured and then bonded to the face sheet in 

a secondary bonding operation. This specimen was not impacted. As it was loaded, buckling of 

the face sheet was noted beginning at ≈40 kips. At 55 kips, there appeared to be a disbond between 

a foot of a hat and the face sheet. Catastrophic failure occurred at 66.6 kips. Figure 5 shows the hat 

stiffener/face sheet disbond and the specimen just after catastrophic failure.

Load = 65 kips Load = 66.6 kips

Hat Stiffener

Disbond

Figure 5. Hat stiffener/face sheet disbond and total failure of specimen double 1.

3.1.2 Double 2

This specimen had hat stiffeners that were precured and then bonded to the face sheet in 

a secondary bonding operation. The specimen was impacted on the foot of a web, towards the 

center of the specimen, with a 0.5-in-diameter impactor at impact energy of 5.5 ft-lb as shown 

schematically in figure 6.
8

Impactor

Diameter = 0.5 in

Impact Energy

= 5.5 ft-lb

 

Figure 6. Location of impact on specimen double 2.

A photograph of an IRT image of the impacted specimen is presented in figure 7. It appears 

that a 3.4-in length (‘length’ defined as in the direction of loading) of the foot has disbonded from 

the face sheet. This disbond does not span the entire width of the foot and has a maximum width 

of 0.6 in (the foot bond has a width of 0.875 in). Thus, about 70% of the width of one of the feet 

on one of the hat stiffeners has disbonded due to the impact event. 

Foot/Face Sheet

Disbond

Figure 7. IRT for specimen double 2.

Upon loading, the disbond between the foot and face sheet grew steadily, in a width-wise 

direction, starting at 30 kips. The disbond grew across the width of the foot, and at 46 kips, an 

audible pop occurred. Photographs of the specimen near the beginning of the test and at 46 kips 

are shown in figure 8. 
9

Disbond

Disbond

Growth Over

Entire Foot

Load = 30 kips Load = 46 kips

Figure 8. Hat stiffener/face sheet disbond growth on specimen double 2.

With further loading, catastrophic failure occurred at 58.1 kips. Catastrophic failure is 

defined in this study as total collapse of the face sheet and both hat stringers with no discernable 

time between collapse of these components of the structure. Catastrophic failure implies that the 

specimen could no longer carry any load. A photograph of the specimen just after failure is shown 

in figure 9.

Hat Stiffener

Failure

Face Sheet

Failure

Load = 58.1 kips

Figure 9. Failure of specimen double 2.

3.1.3 Double 3

This specimen had hat stiffeners that were precured and then bonded to the face sheet in 

a secondary bonding operation. This specimen was impacted at the center of the specimen, directly 

onto the face sheet between the two hats, with a 0.5-in-diameter impactor with an impact energy of 

5.6 ft-lb as shown schematically in figure 10.
10

Impactor

Diameter = 0.5 in

Impact Energy

= 5.6 ft-lb

 

Figure 10. Location of impact on specimen double 3.

A photograph of an IRT image of the impacted specimen is presented in figure 11. A relatively small indication suggests that there are one or more circular delaminations within the face 

sheet with an overall size of ≈0.5-in diameter.

Small Indication

Figure 11. IRT for specimen double 3.

The specimen was loaded to 58.7 kips at which catastrophic failure occurred. This is the 

same load magnitude at which specimen double 1 failed, indicating that the foot/face sheet disbond 

did not greatly lower the compressive load-carrying capability of the structure, compared to when 

the face sheet has slight damage.

3.1.4 Double 4

This specimen had hat stiffeners that were precured and then bonded to the face sheet in 

a secondary bonding operation. The specimen was impacted on the foot of one of the webs with 

a 1.5-in-diameter impactor with an impact energy of 6.3 ft-lb as shown schematically in figure 12. 

This is a similar impact to that on specimen double 1 except that a larger impactor with 14% more 

energy was used.
11

Impactor

Diameter = 1.5 in

Impact Energy

= 6.3 ft-lb

 

Figure 12. Location of impact on specimen double 4.

A photograph of an IRT image of the impacted specimen is presented in figure 13. The 

indication shows damage about 1.6 inches in length and spanning 80% of the width of the foot. 

Although this specimen was hit with more impact energy than specimen double 2, the length of 

the disbond between the foot and face sheet was about one-half that of specimen double 2.

Indication

Figure 13. IRT for specimen double 4.

The specimen exhibited disbonding of the impacted foot section from the face sheet at 

40 kips. This disbond appears to go beyond the foot of the hat stiffener and extends into the face 

sheet of the specimen. Figure 14 shows a photograph of the specimen just after this disbond has 

occurred. Apparently the disbond seen in the IRT image is in the face sheet itself rather than at 

the hat/face sheet bond as it was in specimen double 2. Thus, the larger diameter impactor perhaps 

changed the damage morphology.
12

Hat Stiffener

Failure

Load = 41 kips

Figure 14. Hat stiffener disbond of specimen double 4.

At 44.4 kips, the load fell suddenly to ≈20 kips as the large delamination propagated across 

the face sheet of the specimen from the original delamination that was seen at 40 kips (fig. 14). 

Testing was stopped at this point. A photograph of the specimen after the large delamination has 

formed across the entire face sheet is shown in figure 15.

Load = 44.4 kips

Drop to 20 kips

Large

Delamination

Figure 15. Delamination across entire face sheet of specimen double 4.
13

3.1.5 Double 5

This specimen had hat stiffeners that were precured and then bonded to the face sheet in 

a secondary bonding operation. This specimen was impacted on the face sheet directly behind 

a crown with a 0.5-in-diameter impactor with an impact energy of 10 ft-lb as shown schematically 

in figure 16.

Impactor

Diameter = 0.5 in

Impact Energy

= 10 ft-lb

 

Figure 16. Location of impact on specimen double 5.

A photograph of an IRT image of the impacted specimen is presented in figure 17. 

The impact caused a circular area of damage ≈1 inch in diameter.

Indication

Figure 17. IRT for specimen double 5.

The specimen exhibited dimpling of the face sheet at the impact location beginning at 

≈45 kips. At a load of 51.2 kips, the face sheet failed from the impact zone but the hat stiffeners 

remained intact. After failure, the specimen was able to carry a load of 25 kips. The face sheet 

dimpling and face sheet failure are shown in figure 18.
14

Dimpling of

Face Sheet at

Impact Location

Load = 50 kips Load = 51.2 kips

Drop to 25 kips

Figure 18. Dimpling at impact site and face sheet failure of specimen double 5.

3.1.6 Double 6

This specimen, and all subsequent specimens, had hat stiffeners that were co-cured to 

the face sheet in a single cure/bonding operation. This specimen was impacted at the center of 

one of the crowns with a 0.5-in-diameter impactor with an impact energy of 5.9 ft-lb as shown 

schematically in figure 19.

Impactor

Diameter = 0.5 in

Impact Energy =

5.9 ft-lb

 

Figure 19. Location of impact on specimen double 6.

A photograph of an IRT image of the impacted specimen is presented in figure 20. 

The image suggests damage about 1 in long and spanning 60% of the width of the crown.
15

Indication

Figure 20. IRT for specimen double 6.

The specimen exhibited a loud pop at 57.9 kips which consisted of failure of the impacted 

crown as shown in figure 21.

Hat Stiffener

Failure

Load = 58 kips

Figure 21. Hat stiffener failure of specimen double 6.
16

Buckling of the face sheet was apparent at ≈70 kips and the specimen failed in a catastrophic manner at 84.7 kips. Photographs of these two events are shown figure 22.

Face Sheet

Buckling

Hat Stiffener

Failure

Load = 84.7 kips

Face Sheet

Failure

Load = 80 kips

Figure 22. Buckling and catastrophic failure of specimen double 6.

3.1.7 Double 7

This specimen was impacted at the intersection of the web/crown area of one of the hat 

stiffeners with a 1.5-in-diameter impactor with an impact energy of 7.7 ft-lb as shown schematically in figure 23. This was to determine if the location of impact on the crown was a critical 

parameter by comparison to specimen double 6.

Impactor

Diameter = 1.5 in

Impact Energy 

= 7.7 ft-lb

 

Figure 23. Location of impact on specimen double 7.

A photograph of an IRT image of the impacted specimen is presented in figure 24. The 

indication is difficult to see due to the web of the hat stiffener being under the impact site. This 

changes the damage morphology as well as gives a different IRT signature as can be noted on 

all of the other specimens that show ‘nonuniformity’ at the edges of the crown.
17

Indication

Difficult to See

Figure 24. IRT for specimen double 7.

The specimen exhibited buckling of the face sheet beginning at 70 kips and failed catastrophically at 96.4 kips. Pictures of these two events are shown in figure 25. Since the crown did 

not fail as it did in specimen double 6, it is surmised that an impact at the crown/web interface 

is not as critical as an impact in the center of the crown. This is probably due to the high stiffness 

at the crown/web interface.

Face Sheet

Buckling

Load = 75 kips Load = 96.4 kips

Face Sheet and 

Hat Stiffeners Failed

Figure 25. Buckling and catastrophic failure of specimen double 7.
18

3.1.8 Double 8.

This specimen was impacted on the face sheet from the rear at the midpoint between hat 

stiffeners with a 0.5-in-diameter impactor with an impact energy of 9.9 ft-lb as shown schematically in figure 26.

Impactor

Diameter = 0.5 in

Impact Energy

= 9.9 ft-lb

 

Figure 26. Location of impact on specimen double 8.

A photograph of an IRT image of the impacted specimen is presented in figure 27. The 

impact produced a circular area of damage ≈1 inch in diameter. This is similar to the damage 

produced by impacting the face sheet under a hat stiffener (specimen double 5).

Figure 27. IRT for specimen double 8.

At a load of 69.3 kips, the face sheet of the specimen failed as shown in figure 28. 

Despite having a similar NDE signature as specimen double 5, this specimen demonstrated more 

residual compressive load-carrying capability. This is due to the co-cured, hat stiffener specimens 

demonstrating superior compression strength over the specimens with precured hat stiffeners.
19

Load = 69.3 kips

Figure 28. Failure of face sheet of specimen double 8.

The specimen was still able to carry a load of 36 kips as the hat stiffeners did not fail as 

shown in the post-testing picture in figure 29.

Face Sheet

Failure

Hat Stiffeners Intact

Figure 29. Specimen double 8 showing intact hat stiffeners after face sheet failure.
20

3.1.9 Double 9

This specimen was impacted on the face sheet behind a foot of one of the hat stiffeners with 

a 0.5-in-diameter impactor with an impact energy of 10.8 ft-lb as shown schematically in figure 30.

Impactor

Diameter = 0.5 in

Impact Energy

= 10.8 ft-lb

 

Figure 30. Location of impact on specimen double 9.

A photograph of an IRT image of the impacted specimen is presented in figure 31. The 

damage is about twice as long as it is wide, indicating that perhaps the impact caused a disbond 

of the foot from the face sheet (like specimen double 2).

Indication

Figure 31. IRT for specimen double 9.

The specimen exhibited debonding of the foot from the face sheet at the impact site beginning at 50 kips. This event was not captured on video since all views during loading were from the 

impacted side. At a load of 54.1 kips, the specimen face sheet failed and the specimen held a residual load of 31.9 kips. As with specimen double 2, the residual load-carrying capability is due to the 

fact that the hat stiffeners did not fail and could thus carry load. A post-testing picture of specimen 

double 9 is shown in figure 32.
21

Face Sheet

Failure

Hat Stiffeners

Intact

Figure 32. Specimen double 9 showing intact hat stiffeners after face sheet failure.

3.1.10 Double 10

This specimen was impacted on the face sheet directly behind one of the ‘noodles’ with 

a 0.5-in-diameter impactor with an impact energy of 10 ft-lb as shown in figure 33.

Impactor

Diameter = 0.5 in

Impact Energy 

= 10 ft-lb

 

Figure 33. Location of impact on specimen double 10.

A photograph of an IRT image of the impacted specimen is presented in figure 34. The 

damage is approximately circular with a diameter of 0.5 in. This damage size is smaller than other 

impacts of similar energy. This is due to the face sheet being most stiff (transversely) where the hat 

stiffener meets the face sheet.
22

Indication

Figure 34. IRT for specimen double 10.

At a load of 91.7 kips, the specimen failed catastrophically. The face sheet did demonstrate 

buckling at ≈70 kips, but this was not as detectable by video when viewed from the face sheet side 

of the specimen. A photograph of the specimen just after failure is shown in figure 35. Unlike 

specimens that could carry a residual load after face sheet failure, this specimen demonstrated 

catastrophic failure as the hat stiffeners broke at the same time as the face sheet. A post-test picture 

of the specimen is shown in figure 36, showing the failed hat stiffeners in addition to the failed face 

sheet.

Load = 91.7 kips

Figure 35. Failure of face sheet of specimen double 10.
23

Face Sheet

Failure

Hat Stiffener

Failure

Figure 36. Specimen double 10 showing failed hat stiffeners and face sheet.

3.1.11 Double 11

This specimen was impacted identically to specimen double 10. Impact was on the face 

sheet directly behind one of the ‘noodles’ with a 0.5-in-diameter impactor with an impact energy 

of 10 ft-lb as shown in figure 37.

Impactor

Diameter = 0.5 in

Impact Energy 

= 10 ft-lb

 

Figure 37. Location of impact on specimen double 11.

A photograph of an IRT image of the impacted specimen is presented in figure 38. 

As expected, this IRT image is similar to the IRT image of specimen double 10.
24

Indication

Figure 38. IRT for specimen double 11.

Buckling was seen at ≈75 kips and localized failure at the impact zone was noted at 83 kips. 

At a load of 85.7 kips, the specimen failed catastrophically. The only difference between the failure 

modes of specimen double 10 and specimen double 11 was that specimen double 11 exhibited preliminary failure before catastrophic break. The ultimate load was 7% lower in specimen double 11.

3.1.12 Double 12

This specimen was impacted on one of the crowns with a 0.5-in-diameter impactor with an 

impact energy of 10 ft-lb as shown schematically in figure 39. This impact was similar to the impact 

on specimen double 6, the main difference being that the impact energy used on specimen double 

12 was 70% larger than that used on specimen double 6.

Impactor

Diameter = 0.5 in

Impact Energy

= 10 ft-lb

 

Figure 39. Location of impact on specimen double 12.

A photograph of an IRT image of the impacted specimen is presented in figure 40. As 

expected, this IRT image shows more damage in the crown than the impacted crown on specimen 

double 7. The damage covers the width of the crown and is 2.8 in long compared to 60% of the 

width of the crown and a length of 1 in.
25

Indication

Figure 40. IRT for specimen double 12.

The impacted crown demonstrated a compressive failure at 50 kips. This event is shown in 

figure 41. The face sheet began a noticeable buckle at ≈60 kips and at a load of 77.8 kips, the specimen failed catastrophically. Photographs of these two events are shown in figure 42.

Hat Stiffener

Failure at

Impact Site

Load = 50 kips

Figure 41. Dimpling at impact site and face sheet failure of specimen double 12.
26

Face Sheet

Failure

Load = 77.8 kips

Face Sheet

Buckling

Hat Stiffener

Failure

Load = 69 kips

Figure 42. Face sheet buckling and catastrophic failure of specimen double 12.

3.1.13 Double 13

This specimen was impacted on the face sheet between the two hats with a 0.5-in-diameter 

impactor with impact energy of 10 ft-lb as shown schematically in figure 43. This impact is similar 

to that on specimen double 3, however, that specimen was manufactured with precured face sheets 

and hit with 44% less impact energy.

Impactor

Diameter = 0.5 in

Impact Energy

= 10 ft-lb

 

Figure 43. Location of impact on specimen double 13.

A photograph of an IRT image of the impacted specimen is presented in figure 44. The IRT 

image for this specimen is larger than that on specimen double 3, which is expected since this specimen was hit in the same location but with more impact energy.
27

Indication

Figure 44. IRT for specimen double 13.

At a load of 64.4 kips, the face sheet failed but the hat stiffeners stayed intact and the 

specimen carried a residual load of 35 kips. A photograph of specimen double 13 after face sheet 

failure is shown in figure 45.

Face Sheet

Failure

Hat Stiffeners

Intact

Load = 64.4 kips

Figure 45. Face sheet failure of specimen double 13.

3.1.14 Double 14

This specimen was impacted on the face sheet from the rear, directly between the two hat 

stiffeners with a 0.5-in-diameter impactor with impact energy of 10 ft-lb as shown schematically 

in figure 46. This impact was very similar to the impact inflicted upon specimen double 8.
28

 

Impactor

Diameter = 0.5 in

Impact Energy

= 10 ft-lb

 

Figure 46. Location of impact on specimen double 14.

A photograph of an IRT image of the impacted specimen is presented in figure 47. As 

expected, this IRT image is similar to the IRT image of specimen double 8, though slightly larger. 

The degree of damage seen in specimen double 14 is 1.1 in wide compared to a 0.9-in width for 

specimen double 8.

Localized dimpling at the impact zone was noted at 45 kips. At a load of 51.2 kips (26% 

lower than specimen double 8), the specimen failed, but continued to carry a load of 29 kips. 

Pictures of these two events are shown in figure 48.

Indication

Figure 47. IRT for specimen double 14.

Dimpling of

Face Sheet at

Impact Location

Load = 51.2 kips

Drop to 29 kips

Load = 50 kips

Figure 48. Dimpling at impact site and face sheet failure of specimen double 14.
29

Upon continued loading, the specimen then reached 37 kips before the load dropped 

again. This specimen demonstrated a similar failure mode but had less compressive load-carrying 

capability than specimen double 8. Upon closer inspection of the video recordings, it was noted 

that specimen double 14 had an audible cracking sound at the onset of the formation of the dimple 

at the impact site (45 kips). When specimen double 8 failed, there were no audible sounds before 

the face sheet failure. This indicates that specimen double 14 had less interlaminar strength in its 

face sheet than specimen double 8. This hypothesis was supported upon continued loading of 

specimen double 14 in which further delaminations formed in the face sheet rather than the load 

being effectively transferred to the hat stiffeners.

Later in the program it was confirmed that specimen double 14 came from a batch of specimens that all exhibited early face sheet delaminations, so much that the results of the CAI tests on 

this batch of specimens could not be included in this study since there was an obvious processing 

problem.

A summary of the hat-stiffened specimens is given in table 1. The impact energy for each 

specimen has been divided by the thickness of the laminate at the section of panel that was 

impacted. For example, all impacts on the crown portion of a specimen had their energies divided 

by 0.15 in, and all impacts on the face sheet portion of a specimen had their energies divided by 

0.11 in.

The CAI strength is given as load per unit length; i.e., the failure load divided by 8.875 in. 

The specimens in bold were co-cured.

The data in table 1 are represented graphically in figure 49. A curve fit or other type of data 

comparison is not possible since the impacts were in different locations on the hat-stiffened panel, 

which completely change the damage mechanics involved. The data of the co-cured, hat-stiffened 

panels on the plot are noted as to what part of the specimen the impact took place at.
30

Table 1. Summary of CAI results on hat-stiffened specimens.

Specimen

Impact 

Energy

CAI Strength

(in-lb/in)

Double 1 0 7,500

Double 2 602 6,600

Double 3 609 6,600

Double 4 682 5,000

Double 5 1,094 5,800

Double 6 472 9,500

Double 7 617 10,900

Double 8 1,082 7,800

Double 9 1,178 6,100

Double 10 1,091 10,300

Double 11 1,091 9,700

Double 12 800 8,800

Double 13 1,088 7,300

Double 14* 1,091 5,800

*Poorly processed specimen

11,000

10,000

9,000

8,000

7,000

6,000

5,000

4,000

400 500 600 700

Double 7

Double 6

and 12

Double 2

and 4

Double 3

Double 10

and 11

Double 8

and 14*

Double 5

Double 9

*Poorly Processed Specimen

Double 13

800

Impact Energy (in-lb/in)

CAI Strength (lb/in)

900

Precured Stiffeners

Co-Cured Stiffeners

1,000 1,100 1,200

Figure 49. Residual strength versus impact energy for hat-stiffened specimens.
31

 3.2 Sandwich Specimens

The CAI test results of the sandwich panels to be compared to the hat-stiffened panel are 

summarized in table 2. The impact energy has been divided by the thickness of the impacted face 

sheet (0.10 in) in order to produce more comparable numbers for comparison with the hat-stiffened 

specimens. The CAI strength is given as a line load; i.e., the total failure load was divided by the 

specimen width.

Table 2. CAI results for sandwich specimens used in this study.

Specimen

Impact 

Energy

CAI 

Strength 

(lb/in)

Sandwich 1 1,140 9,900

Sandwich 2 876 11,300

Sandwich 3 648 12,900

Sandwich 4 636 13,200

Sandwich 5 1,140 9,700

Sandwich 6 1,224 11,000

Sandwich 7 1,536 9,500

Sandwich 8 1,536 10,200

Sandwich 9 876 12,600

Sandwich 10 432 13,900

Sandwich 11 468 13,800

Sandwich 12 528 12,100

Sandwich 13 720 12,600

Sandwich 14 768 12,600

Sandwich 15 840 11,700

Sandwich 16 324 15000

Sandwich 17 1,008 11,500

Sandwich 18 648 13,800

Sandwich 19 852 12,100

Sandwich 20 1,572 10,900

Sandwich 21 1,992 9,000

Sandwich 22 1,116 11,700

Sandwich 23 864 12,300

Sandwich 24 768 12,900

The data in table 2 are represented graphically in figure 50. A power curve is fitted to the 

data since the impacts for these specimens were independent of location; i.e., the sandwich structure is continuous.

As a comparison of the CAI strength versus damage severity for the hat-stiffened panels 

and the sandwich panels, the plots in figures 49 and 50 are combined and presented in figure 51.
32

15,000

14,000

13,000

12,000

11,000

10,000

9,000

8,000

400 600 800 1,000 1,200

Impact Energy (in-lb/in)

CAI Strength (lb/in)

1,400 1,600 1,800 2,000

Figure 50. Residual strength versus impact energy sandwich specimens.

Since the hat-stiffened panels had a smaller areal weight than the sandwich panels, the data 

in figure 51 can be normalized for a better comparison of strength-to-weight ratios of the two types 

of panel construction. The hat-stiffened CAI data are divided by 1.76 lb/ft2 and the sandwich CAI 

data are divided by 2.34 lb/ft2.

14,000

12,000

Precured Stiffeners

Co-Cured Stiffeners

Sandwich Structure

10,000

8,000

6,000

4,000

400 600 800 1,000 1,200

Impact Energy (in-lb/in)

CAI Strength (lb/in)

1,400 1,600 1,800 2,000

Figure 51. Comparison of hat-stiffened and sandwich specimens.
33

A comparison of the sandwich and hat-stiffened CAI data, taking mass of the structure into 

consideration (strength-to-weight ratio), is shown in figure 52.

Precured Stiffeners

Co-Cured Stiffeners

Sandwich Structure

400 600 800

6,500

6,000

5,500

5,000

4,500

4,000

3,500

3,000

2,500

1,000 1,200

Impact Energy (in-lb/in)

CAI Strength (lb/in)

1,400 1,600 1,800 2,000

Figure 52. Comparison of hat-stiffened and sandwich 

specimens with mass of structure included.
34
35

4. DISCUSSION

4.1 Co-Cured Versus Precured Hat-Stiffened Specimens

Specimens double 1–5 were manufactured with the hat stiffeners procured and bonded onto 

the face sheet in a secondary bonding operation. All other hat-stiffened specimens had the stiffeners and face sheet co-cured in one bonding/curing operation. The co-curing of the hats to the 

face sheet obviously results in a higher load-carrying structure as relatively undamaged specimens 

carried 84.7, 96.4, 91.7, and 85.7 kips while an undamaged specimen manufactured by secondarily 

bonding the hats to the face sheet (double 1) failed at 66.6 kips.

4.2 Size of Impactor

Although only two specimens where impacted with an impactor of size other than 

0.5 inches in diameter, some observations can be made. For the procured, hat-stiffened specimens, 

double 2 and double 4 were both impacted on a foot of a hat stiffener. Specimen double 2 was 

impacted with an impactor of 0.5-in diameter and specimen double 4 was hit with a 1.5-indiameter impactor. When the impactor size was increased to 1.5 in diameter (and impacted with 

slightly more impact energy), an impact on the foot of a hat caused the residual load-carrying 

capability of the structure to be reduced from 58 to 44.4 kips. The IRT results show that the larger 

impactor caused a larger disbond between the impacted foot and the face sheet and compression 

testing revealed that the disbond caused from the larger impactor was actually within the face sheet 

rather than at the hat stiffener/face sheet interface. In the only other test utilizing an impactor size 

of 1.5-in, co-cured specimen double 7 was impacted at the edge of a crown. This impact did not 

reduce the residual compression load-carrying capability of the specimen, thus no valid comparisons regarding impactor size can be made.

4.3 Location of Impact

For the co-cured, hat-stiffened specimens, impacting the face sheet directly behind one of 

the noodles did not cause much strength reduction in the specimen. This is probably due to this 

area being very transversely stiff and thus the impact consists almost solely of contact stresses, 

which did not cause damage that was detrimental to the residual load-carrying capability of the 

specimen. For an impact on the edge of a hat stiffener, another highly transversely stiff area, the 

residual load-carrying capability, was also not greatly diminished. Impacting a hat stiffener in the 

center of the hat did cause a modest reduction in the load-carrying capability of the specimen when 

hit with sufficient energy; however, the failure mode was in two distinct phases. First, the impacted 

hat stiffener failed but the specimen continued to carry load until ultimate failure. Impacts on the 

face sheet or face sheet/hat stiffener interface apparently caused the greatest reduction in residual 

compressive load-carrying capability.
36

4.4 Comparison With Sandwich Panels

In order to better compare these hat-stiffened specimens to sandwich structure specimens, 

the mass per unit area of each type of structure was assessed. The double hat specimens had an 

average mass of 0.65 lbm. Since these specimens were 6 in high and 8.875 in wide, this gives an areal 

density of 1.76 lb/ft2. Sandwich panel used in this study had an areal density of 2.34 lb/ft2. The 

hat structure is thus 25% lighter than the sandwich structure that it will be compared to. Figure 49 

shows compressive line load that the structures could carry versus impact energy. The double hat 

specimens are divided out between the secondarily bonded hat type (CAI Pre) and the co-cured hat 

type (CAI Co). 

Note that the hat specimens are much more dependent upon where on the structure the 

impact occurs. The sandwich structure is uniform across its surface so all impacts on the surface 

are identical.

Once the residual compressive load-carrying capability of the specimens is normalized by 

the areal weight of the specimens, it appears that the hat specimens can carry more load after an 

impact event of a given energy with a spherical impactor as shown in figure 52.
37

5. CONCLUSIONS

From the data generated in this study, it appears that the compressive load-carrying ability of hat-stiffened and sandwich structure of the type considered for a launch vehicle interstage is 

comparable for a well-made structure when damage tolerance is considered. However, other factors 

such as inspectability and failure mode will also need to be addressed in a full damage tolerance 

study.

Since the hat-stiffened structure demonstrated redundancy in its design (load redistribution after a localized failure), it would be classified as a more ‘damage tolerant’ structure by most 

standards. For example, if this study was examining airplane fuselage structure, then the step-wise 

failure mode of the hat-stiffened structure would be most desirable as the fuselage can still carry 

a significant amount of ultimate load after initial localized failure, allowing for the airplane to land 

using ‘get home loads.’ However, for a launch vehicle, this step-wise failure mode has no benefits 

since the rocket cannot shut down once launched, thus the ultimate load-carrying capability is of 

interest.

With the limited data thus far, it is difficult to draw definitive conclusions about the damage 

tolerance aspects of the hat-stiffened structure. However, it does appear that, in general, an impact 

on the face sheet of a hat structure is more detrimental than an impact on the hat stiffener of the 

structure (as was found in ref. 4). An exception is when the impact is over the noodle of a hat, in 

which case, little detrimental effect to compression strength was seen at 10 ft-lb of impact energy. 

Further testing over a broader range of impact energies is planned for the next round 

of testing.
38
39

REFERENCES

1. Gadke, M.; Geier, B.; Goetting, H.; et al.: “Damage Influence on the Buckling Load of CFRP 

Stringer-Stiffened Panels,” Composite Structures, Vol. 36, pp. 249–275, 1996.

2. Abrate, S.: “Localized Impact on Sandwich Structures With Laminated Facings,” Applied 

Mechanics Reviews, Vol. 50, pp. 69–83, 1997.

3. Rhodes, M.D.; Williams, J.G.; and Starnes, J.H.: “Effect of Low-Velocity Impact Damage 

on the Compressive Strength of Graphite-Epoxy Hat-Stiffened Panels,” NASA TN D-8411, 

1997.

4. Ambur, D.R.: “Design and Evaluation of a Foam-Filled Hat-Stiffened Panel Concept for Aircraft Primary Structural Applications,” NASA TM 109175, NASA Langley Research Center, 

Hampton, VA, 1995.

5. Ambur, D.R.; and Dexter, B.H.: “Structural Performance of a Compressively Loaded FoamCore Hat-Stiffened Textile Composite Panel,” NASA TM-111451, NASA Langley Research 

Center, Hampton, VA, 1966.

6. Rousseau, C.Q.; Baker, D.J.: and Hethcock, J.D.: “Parametric Study of Three-Stringer Panel 

Compression-After-Impact Strength,” Composite Structures: Theory and Practice, ASTM STP 

1383, P. Grant and C.Q. Rousseau (eds.), American Society for Testing and Materials, West 

Conshohocken, PA, pp. 72–104, 2000.

7. Bisagni, C.; Vescovini, R.; and Davila, C.G.: “Assessment of the Damage Tolerance of Postbuckled Hat-Stiffened Panels using Single-Stringer Specimens,” Proceedings 51st AIAA/ASME/

ASCE/AHS/ASC Structures, Structural Dynamics and Materials Conference, Orlando, FL, USA, 

April 12–15, 2010.

8. Nettles, A.T.; and Jackson, J.R.: “Developing a Material Strength Design Value Based on 

Compression After Impact Damage for the ARES I Composite Interstage,” NASA/TP—2009–

215634, Marshall Space Flight Center, AL, 2009.
40

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A Damage Tolerance Comparison of Composite Hat-Stiffened

and Honeycomb Sandwich Structure for Launch Vehicle 

Interstage Applications

A.T. Nettles

George C. Marshall Space Flight Center

Huntsville, AL 35812

National Aeronautics and Space Administration

Washington, DC 20546–0001

Unclassified-Unlimited

Subject Category 24

Availability: NASA CASI (443–757–5802)

Prepared by the Materials & Processes Laboratory, Engineering Directorate

M–1326

Technical Memorandum

NASA/TM—2011–216477

impact, compression-after-impact (CAI), skin-stiffened, honeycomb sandwich, damage tolerance, 

impact damage

01–12–2011

U U U UU 52

In this study, a direct comparison of the compression-after-impact (CAI) strength of impact-damaged, hat-stiffened 

and honeycomb sandwich structure for launch vehicle use was made. The specimens used consisted of small substructure designed to carry a line load of ≈3,000 lb/in. Damage was inflicted upon the specimens via drop weight 

impact. Infrared thermography was used to examine the extent of planar damage in the specimens. The specimens were prepared for compression testing to obtain residual compression strength versus damage severity curves. 

Results show that when weight of the structure is factored in, both types of structure had about the same CAI 

strength for a given damage level. The main difference was that the hat-stiffened specimens exhibited a multiphase 

failure whereas the honeycomb sandwich structure failed catastrophically.

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