NBSIR 74-572

Torsional Buckling of Composite

Cylindrical Shells

D. E. Marlowe and G. F. Sushinsky

Engineering Mechanics Section

Mechanics Division

Institute for Basic Standards

National Bureau of Standards

Washington, D. C. 20234

September 1974

Final

Prepared for National Aeronautics and Space Administration

Langley Research Center

Hampton, Virginia 23365

NBSIR 74-572

TORSIONAL BUCKLING OF COMPOSITE

CYLINDRICAL SHELLS

D. E. Marlowe and G. F. Sushinsky

Engineering Mechanics Section

Mechanics Division

Institute for Basic Standards

National Bureau of Standards

Washington, D. C. 20234

September 1974

Prepared for National Aeronautics and Space Administration

Langley Research Center

Hampton, Virginia 23365

Final

U. S. DEPARTMENT OF COMMERCE. Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts. Director

CONTENTS

1 . INTRODUCTION

2. NOMENCLATURE

2.1 Units

2.2 Designations

3. ANALYSIS

3.1 Torsional Buckling

3.2 Design of an Optimum Strength Laminate

4. SPECIMENS

4.1 Composite Materials

4.2 Compo si te-Reinforced-Aluminum-Al loy Specimens

4.3 Composite-Reinforced-Titanium-Alloy Specimens

4.4 Material Properties

4.5 Specimen Fabrication

5. TESTING PROCEDURE

5.1 Instrumentation

5.2 End Fixtures

5.3 Compression Test Procedure

5.4 Torsional Test Procedure

6. RESULTS FROM TESTS

6.1 Elastic Behavior of Specimens

6.2 Shear Stress-Shear Strain Curves

6.3 Buckling Torque

7. DISCUSSION

7.1 Elastic Buckling

7.2 Effect of Direction of Twist

7.3 Linear Scaling

7.4 Optimum Layup

7.5 Composite-Reinforced-Metal Specimens

7.6 Post-Buckling Failure Behavior
of these materials in complex structures requires a thorough knowledge

of the behavior of these materials when subjected to various mechanical

loadings. Considerable data already exist on the tensile and com- pressive behavior of these materials, whereas shear data are not

abundant. The need for shear data prompted experimental investigations

(refs. 3 and 4) of the torsional behavior of thin-walled cylindrical

shells laminated with unidirectional composites and aluminum cylindrical

shells reinforced with unidirectional composites.

The primary purpose of the investigation reported herein was

to investigate the elastic torsional buckling strength of all -composite

and composite-reinforced metal shells in which the composite laminate

is not unidirectional. Test results on 39 specimens are reported.

Boron/epoxy composite specimens in three lengths and two diameters,

and graphite/epoxy composite specimens in two lengths and two diameters were tested. In addition, three configurations of composite-reinforcedaluminum-al loy and titanium-alloy specimens were studied. The effects

of stacking sequence, direction of loading, and linear scaling

of specimen dimensions were studied and an investigation o-f a stronger

or "more optimum" stacking sequence was analytically determined

and subsequently investigated experimentally.

This investigation was conducted at the National Bureau of

Standards under the sponsorship and with the financial assistance

of the National Aeronautics and Space Administration, Langley Research

Center.

; ; 2. NOMENCLATURE

2.1 Units

The units for physical quantities used in this report are given in both the U. S. Customary Units and the International System

of Units (Si). Measurements and calculations were made in U. S.

Customary Units. Conversion factors relating the two systems are presented in Appendix A3 of reference 5.

2.2 Designations

The system for designating specimens which identifies both

the specimen material and the specimen length is given in table

1. In the following tables a sample number is appended to this

designation.

The stacking sequence or ply orientation for a laminate is

designated by (ot| , 0.2, 013, —)^ where the is the ply angle of

the outermost lamina, a2 is the ply angle of the second lamina,

etc. The subscript "s" indicates cases where the stacking sequence

is symmetric about the mid-thickness of the shell. Positive ply

angle is measured clockwise from a generator line on the cylindrical

2
surface as shown in figure 1.

3. ANALYSIS

3,1 Torsional Buckling

The computer program "Buckling of Generally Orthotropic Cylinders,"

which was developed by Chao (ref. 6), was used to predict the buckling

torques for each specimen. These were compared with the experimental

results. In the program, buckling loads are calculated for a multilayered cylindrical specimen loaded in combinations of torsion,

axial compression and radial pressure. Chao ' s theory treats orthotropic

layers of composite material whose principal material axes can

be oriented in any direction. Specimen buckling is analyzed using

Timoshenko's general equations of equilibrium (ref. 7). Using the

concept of reduced flexural rigidity, the bending and membrane

forces are uncoupled. The specimen is then assumed to have orthotropic elastic properties. The program seeks the solution with

the lowest buckling torque by iterating on the number of circumferential

buckling waves.

3,2 Design of an Optimum Laminate

Wu investigated the torsional buckling strength of all-composite

four-ply cylindrical shells using two specific stacking sequences

(ref. 8, fig. 27). Using this as a point of departure, several

similar curves were produced using the Chao analysis in an effort

to determine an "optimum" four-ply laminate configuration for both

al 1 -composite and composite-reinforced-metal cylinders. A comparison

of some of these curves, including the stacking sequences chosen

by Wu", is shown in figure 2, Similar curves for boron/epoxy-reinforcedtitanium cylinders are shown in figure 3,

4, SPECIMENS

The experimental program consisted of room- temperature torsional testing of 39 cylindrical shells designed to fail by elastic

torsional buckling. Replicate samples were tested for each specimen

configuration. Table 2 presents the geometry, laminate thicknesses,

filament volume fractions, and stacking sequence for each of the

individual specimens. Composite specimens were tested in three

lengths and two diameters, and reinforced-metal specimens were

"The buckling torques of these stacking sequences are computed for

a negative loading according to Chao ' s sign convention.

3
tested in one Length and one diameter. The dimensions for the allcomposite specimens wese chosen such that two diameter-to-thickness

(D/t) ratios and three length-to-radius (L/r) ratios were tested.

4.1 Composite Materials

The boron/epoxy material used in this investigation was purchased

in 3.0-in (7.6-cm) wide tape. The tape contained 0.004-in (0.01-

cm) diameter filaments pre-impregnated with a 350 °F (180 °C)-curing

epoxy resin system. This material was supplied with a 0.001-in

(0.003-cm) fiberglass scrim cloth to facilitate handling.

The graphite/epoxy material used was a high-modulus graphite

fiber pre-impregnated with a 350 °F (180 °C)-curing epoxy resin

system. The pre-impregnated material was supplied by the manufacturer

in the form of 3.0-in (7.6-cm) wide tape.

All uncured resin materials were stored at 0 °F (-18 °C) prior

to use.

4.2 Composite-Reinforced-Aluminum-Alloy Specimens

The aluminum-alloy cylinders, which were to be reinforced

with composite material, were machined from a single lot of 6061-

T6 seamless drawn tubing. These cylinders had a nominal wall thickness

of 0.022 in (0.056 cm) and a nominal outer diameter of 6.0 in (15

cm). The specimens tested had a gage length of 10.0 in (25.4 cm).

The thickness of the composite reinforcement was approximately

equal to the thickness of the aluminum- alloy cylinder. The dim,ensions of these specimens are shown in table 2.

4.3 Composite-Reinforced-Titanium-Alloy Specimens

The titanium- alloy cylinders, which were to be reinforced

with composite material, were rolled from 6A1-4V titanium-alloy

sheet and butt welded along the seam. The finished cylinders had

a nominal wall thickness of 0.025 in (0.064 cm), a nominal outer

diameter of 6.0 in (15 cm), and a gage length of 10.0 in (25.4

cm). The dimensions of these specimens are shown in table 2.

4.4 Material Properties

Longitudinal elastic modulus (E^), transverse elastic modulus

(E12) and Poisson's ratio (^12^ were determined for all materials

used in this program. The preparation of and testing procedure

for these specimens are described in the appendix to reference

3. The in-plane shear modulus (G12) was determined for each material

as the average value obtained from torsional tests on the 0-degree

unidirectional composite specimens and the all-metal specimens

tested in this program and in the program reported in refs. 3 and

4, The average material properties measured on the aluminum- alloy.
titanium-alloy, boron/epoxy, and graphite/epoxy materials are shown

in table 3,

The values of elastic moduli for the composite materials,

^11» ^12» ^12» were corrected for thickness variation when they

were used in the analysis. A rul e-of -mixtures equation.

t E + (t, - t )E^

c c k c' m

was used where

E, = the corrected modulus,

k

t = nominal thickness of the composite,

ct^ = measured thickness of the composite,

E^ = measured modulus of the composite,

and E = nominal modulus of the epoxy matrix material.

4,5 Specimen Fabrication

Boron/epoxy and graphite/epoxy-composite cylinders were fabricated

in a manner similar to that described in refs. 3 and 9, The composite

material was laid-up over a mandrel and covered by one ply of treated

release cloth and the number of fiberglass bleeder plies required

to absorb the excess of epoxy resin from the curing sample. Pressure was applied by enclosing the layup in FEP heat-shrinkabl e fluorocarbon

tubing. A thermocouple was embedded in the laminate and the specimen

was cured using the manufacturer's suggested time- temperature cycle.

The aluminum- alloy and titanium- alloy cylinders which were reinforced on the outer surface with composites were chemically

cleaned and primed immediately prior to the layup process. A single

layer of film-epoxy adhesive was applied to the metal cylinder

followed by the required number of plies of prepreg material, glass

release cloth, bleeder, and heat-shrinkable tubing. With the boron/epoxy

prepreg, it was necessary to add an additional layer of 0.001-in

(0,003-cm) fiberglass scrim cloth between the film-epoxy adhesive

and the first ply of boron/epoxy in order to balance the total

laminate. The composite layup techniques for these cylinders were

similar to those used for the all -composite specimens, except that

the metal cylinder served as a non-removable mandrel.

5
5. TESTING PROCEDURE

5.1 Instrumentation

Each of the cylindrical test specimens was instrumented on

the outer surface of the shell at the midlength. Five 45° rosette

foil strain gages having a gage length of 0.25 in (0.63 cm) were

equally spaced around the specimens. These were oriented to measure

strains in the axial direction and at ±45° to the axial direction.

Several specimens were tested with additional strain gages in an attempt to detect the earliest indication of buckling and to check

the uniformity of strain distribution.

' 5.2 End Fixtures

The end fixtures used for the all-composite specimens are shown in figure 4a, They are close-fitting end plugs which are bonded to the inner surface of the cylinder with an epoxy adhesive.

The end fixtures used to test the composite-reinforced-metal specimens

are shown in figure 4b. In these fixtures, both the inner and outer

surfaces of the cylinders are bonded into the end fixtures.

5.3 Torsional Testing Procedure

Thirty-three specimens were tested in the Engineering Mechanics

Laboratory of the National Bureau of Standards in a 40,000-lbfin (4,500-N-m) capacity torsional testing machine (ref. 10). Six

other specimens were tested at the NASA-Langley Research Center,

Structures and Materials Laboratory. These included two 40-in (101.6-

cm) long all-composite specimens tested in a 60, 000-lbf-in (6,800-

N-m) capacity torsional testing machine and four composite-reinforced

metal specimens tested in torsion in a 100, 000-lbf-in (11,300-Nm) capacity triaxial testing machine. A typical test setup in each

of these machines is shown in figure 6. Torque was transmitted

from the testing machine heads to the specimen fixtures by means

of rectangular keys inserted through slots in the testing machine

heads and the specimen end fixtures. All tests were performed at room temperature and at a uniform twisting rate of 0.005 rad/min.

During the torsional tests performed at the National Bureau

of Standards, strain measurements were made at discrete values

of torque. The magnitude of the torque increment between successive

sets of strain readings was determined by the specimen behavior

during the previous increment. During tests performed at the NASALangley Research Center, torque and strain were recorded at a virtually

continuous rate in NASA-Langl ey ' s central digital -data recording

facility.

All specimens were first loaded with a negative (counterclockwise)

torque (fig. 1). This direction of twist caused a tensile stress

in the filaments of cylinders fabricated with positive ply-orientation

6
angles. Several specimens were loaded in both the positive and

negative directions to determine the effects of the direction of

twist on the buckling strength.

6. TEST RESULTS

6.1 Elastic Behavior of Specimens

The elastic properties of the torsional specimens tested are shown in table 4. This table gives the effective values of E;^;^ and Gi2 which were predicted by the laminate analysis of the Chao

program and measured during tests. The experimental moduli were

determined from a least squares fit to the data of the first six

load increments applied in the linear elastic range of each test.

6.2 Shear Stress-Shear Strain Curves

The shear stress-shear strain curves from all specimens tested

are shown in figure 7. The shear strains were averaged from the

measurements made at the mid-length of the specimens with the five

rosette gages. Curves for aluminum- alloy specimens similar to the

20-in (50.8-cm) specimjns reported here are given in reference

3.

6.3 Buckling Torque

The maximum measured torques for the specimens tested in this

investigation are given in table 5. These represent experimental

buckling torques except where noted. Included in this table are the values of the buckling torque predicted by the Chao analysis

and the ratio (correlation factor) of the maximum measured torque

to the torque predicted by the analysis. The elastic material properties

used in the analysis to predict the buckling torques are those

presented in table 3 corrected for thickness variations.

Photographs of typical buckled specimens are shown in figure

8.

7. DISCUSSION

7.1 Elastic Buckling

The ability of the Chao theory to predict the buckling load

of composite cylinders loaded in torsion can be estimated by analysis

of the data given in table 5. For the specimens which fail by elastic

buckling, the correlation factor between analysis and experiment

is .81 with a standard deviation of 13 percent. This is approximately

the same degree of agreement as that generally attributed to Donnell's

treatment of isotropic cylinders (ref. 11). It is interesting to

7
note that the analysis appears to be in better agreement with experiments

for boron/epoxy specimens than for graphite/epoxy specimens. An

evaluation of the effects of fabrication variables can be made

by comparing the measured buckling torques of replicate specimens.

The standard deviation of the differences in measured buckling

torques between replicate specimens was five percent for all specimens

tested which failed elastically. The difference between 13 percent

and 5 percent reflects the "modeling error" in using the prediction

equation. There appears to be a tendency for higher strength specimens

fo fail by mechanisms other than elastic buckling.

7.2 Effect of Direction of Twist

The effect of direction of twist in specimens with unidirectional,

off-axis layups is shown by the results on specimens BL03, (ref.

4, Table 4) BL05, BL06 and BL07 (table 5). Specimens BL03 (Predicted

Buckling Torque, 3,480 Ibf-in (393 N-m), Experimental Buckling

Torque 3,400 Ibf-in (384 N-m)) and BL07 with the +45-degree ply

orientation failed by elastic buckling near the predicted value.

However, the two specimens with -45-degree orientation, where principle

tensile stresses were normal to the fibers, failed in the matrix

or matrixfiber interface at torques well below the expected elastic

buckling torques.

A more complete understanding of the effects of direction

of twist and stacking sequence is obtained from the behavior of

specimens BOl through B04 and C04, C06, C07 and COS. These specimens

were symmetric, balanced ±45-degree laminates. They were twisted

in both the positive and negative directions. In all cases, the

direction of twist which loaded the outer ply fibers in compression

resulted in the higher buckling torques (figs. 9 and 10). In general,

these torques were approximately twice the torques of the specimens

twisted in the direction in which the fibers in the outer ply were loaded in tension.

7.3 Linear Scaling

The effect of a linear scaling of cylinder dimensions was

examined. Specimens BOS and B06 were half as long, half as thick

and had one-half the diameter of specimens BL08 and BL09. This

scaling resulted in the same L/r and D/t ratios. As predicted by

the analysis, and as can be seen in table 6, the buckling shear

stresses for the two sizes tested are nearly equal for both sizes

of cylinder (fig. 11). Similar results were expected on specimen

pairs C09, CIO and CLOl , CL02. However, specimens CLOl and CL02

failed at loads well below the expected buckling torques. These

failures were catastrophic without evidence of prior buckling.

These premature failures may have been caused by local irregularities

in the cylinder wall. Such irregularities could have occurred during

the fabrication of the shell.

8
Specimens were also tested in which the effect of scaling

only the specimen length was examined (fig. 12), These specimens

were fabricated with a nominal 6-in (15-cm) diameter and 4 ply

laminate thickness in three lengths. As can be seen from table

2, specimens BOl and B02 were 10 in (25.4 cm) long, specimens BLIO

and BLll were 20 in (50.8 cm) long, while specimens BLLOl and BLL02

were 40 in (101.6 cm) long.

7.4 Optimum Layup

Using ^the results of the computer study shown in figure 2,

specimens were fabricated and tested with a stacking sequence of

(-82.5, 30, 20, -82.5) degrees. As predicted by the analysis and

as can be seen in table 7, the buckling torque for the optimum

layup is twice that for a (-45, +45 )g degree 1 ayup and seven times

the buckling torque for a (0,0)^ degree layup.

7.5 Composite-Reinforced-Metal Specimens

Tests were performed on specimens in which composite reinforcement,

in accordance with the "optimum" ply stacking sequence discussed

above, had been applied to the outer surface of thin-walled metal

cylinders. This was a further evaluation of a concept for the economic

use of composite materials which had been explored previously (refs.

3 and 4), These results are shown in table 8. The buckling strengths

of the equivalent -weight aluminum-al loy and titanium-alloy cylinders

have been calculated. The aluminum- alloy cylinder having a thickness

of 0,038 in (0.096 cm), a length of 10 in (25.4 cm) and a diameter

of 6 in (15 cm) has a predicted buckling strength of 40,200 Ibfin (4,500 N-m), The similar titanium alloy cylinder, with a thickness

of 0.034 in (0.076 cm) has a predicted buckling strength of 50,100

Ibf-in (5,700 N-m). The analysis predicts a considerable strengthto-weight savings for a reinforced-metal specimen over the equivalentweight metal specimen. The actual strength-to-weight savings is

somewhat less than predicted, due to the relatively low load at

which the composite material debonded from the metal or endcap

failure occurred, for the specimens tested. The results of the

tests on specimens Cll and C12 indicate that the ultimate shear

strain of the graphite material in this ply configuration is about

0.0150. At failure, the shear strains in specimen AC03 were about

0.0075. The buckling failure loads, therefore, would be expected

to be somewhat higher and the strength-to-weight savings would

improve. Nevertheless, the strength-to-weight ratios which were obtained are larger than those obtained with unidirectional ly-reinforcedmetal specimens (ref. 3).

9
7,6 Post-Buckling Failure Behavior

As the specimen wall is loaded in torsion near the buckling

load, it begins to assume the buckled wave pattern particular to

that specimen configuration. At initiation of buckling and during

post-buckling loading it was observed that the failure behavior

of the two composite materials was different. The specimens fabricated

with boron/epoxy continued to carry load in the post-buckling regime,

as indicated by curves of figure 7. Deep elastic buckles, as shown

in figures 8 d, e, f and h can be twisted into the boron/epoxy

specimens without permanent damage. Specimens fabricated with graphite/

epoxy, however, failed catastrophically soon after the maximum

torque was reached. An example of this type of failure is shown

in figures 8c and g.

' ' 8. CONCLUSIONS

Based on the investigation of elastic torsional buckling in

thin-walled-composite cylindrical shells reported herein, the following

conclusions are drawn:

1. An available computer analysis has been exercised

extensively and its usefulness has been established

in predicting buckling torques for several laminate

configurations. The effect of uncoupling the

bending and extension forces for the thin laminate,

which was assumed by the analysis, has been

experimentally tested and the validity of the

assumption verified by the resulting agreement

between analysis and experiment. Correlation

factors in excess of 0.80 for experiment in

relation to analysis were not uncommon for these

tests. There appears to be a modeling error

of approximately 8 percent between the analysis

and experiment.

2. Torsional buckling strengths which differ by

as much as a factor of two may result from reversing

the direction of twist of a thin-walled-composite

cylinder. This is of potential importance in

applications where reversals of loading may

occur and in selection of a lay-up configuration

for a single direction of torque.

3. The buckling results obtained by reversing the

stacking sequence in a laminated thin-walled

cylinder loaded in torsion have been shown to

be equivalent to the buckling results obtained

by reversing the direction of twist on the original

stacking sequence.

10
4. Shear stress-shear strain curves computed from

the results of tests on large composite, and

composite-reinf orced-metal torsional specimens,

and the principal elastic moduli (Ej^ )j ^.nd in-plane shear moduli (G;^2) computed from these

curves, have been given.

5. A 2:1 linear scaling of cylinder dimensions,

while maintaining constant L/r and D/t ratios,

resulted in buckling torques which differed

by about an order of magnitude, while the shear

stresses at failure were approximately equal.

6. An "optimum" stacking sequence which produced

significant increases in the predicted and measured

buckling loads for a four-ply cylinder was determined. This results in considerable increases

in the strength-to-weight ratio over other se- quences examined herein and in reference 3,

7. The "optimum" stacking sequence was also used

for aluminum- alloy and titanium-alloy cylinders

reinforced on their outer surfaces with composite

materials. This sequence resulted in significant

increases in the strength-to-weight ratio over

several other ply sequences tested. There is

a tendency for these higher strength specimens

to fail by mechanisms other than elastic buckling.

8. Boron/epoxy cylinders which fail by elastic

torsional buckling have considerably more postbuckling strength than similar cylinders fabricated

from graphi te/epoxy

9. REFERENCES

1. Dexter, H. B., Flight Service Evaluation of Composite Structural

Components, Proceedings of the Conference on Fibrous Composites

in Flight Vehicle Design (Dayton, September 1972 AFFDL-TR72-130, pp 369-407 December 1972.

2. Davis, J. G., Jr., Evaluation of Metal Structures Selectively

Reinforced with Filamentary Composites for Space Shuttle Application, ibid., pp 445-489.

3. Marlowe, D. E., Torsional Instabilities in Composite and CompositeReinforced Aluminum-Alloy Thin-Walled Cylinders, NBS Technical

Note No. 728, 1972.

4. Marlowe, D. E., Sushinsky, G. F., and Dexter, H. B., Elastic

11
Torsional Buckling of Thin-Walled Composite Cylinders, Composite

Materials: Testing and Design (Third Conference) Williamsburg,

Va. , March 1973 ASTM STP 546, Amercian Society for Testing

and Materials, pp 84-108, 1974.

5. ASTM Designation E380-72, Standard Metric Practice Guide, American

Society for Testing and Materials, Philadelphia, Pennsylvania.

6. Chao, Tung-Lai, Minimum Weight Design of Stiffened Fiber Composite

Cylinders, AFML TR 69-251, September 1969.

7. Timoshenko, S. P., and Gere, J. M., Theory of Elastic Stability,

McGraw Hill Book Co., Inc., New York, 1961.

8. Wu, Chi-Hwa, Buckling of Anisotropic Circular Cylindrical Shells,

Doctoral Dissertation, Case-Western Reserve Univ., June 1971.

9. Davis, J. G,, Tube Fabricating Process, United States Patent

No. 3,607,495, September 1971.

10. Chwirut, D. J., Coordinator, Research and Testing Facilities

of the Engineering Mechanics Section, National Bureau of Standards,

Washington, D, C, NBS Special Publication No. 370, January

1973.

11. Batdorf, S. B., Stein, M., and Schildcrout, M., Critical Stress

of Thin Walled Cylinders in Torsion, NACA TN 1344, 1947.

12
Table 1

- Specimen Designations

Designation Material Specimen length

in cm

T titanium alloy 10.0 25.4

B boron /epoxy 10.0 25.4

BL boron/epoxy 20.0 50.8

BLL boron/epoxy 40.0 101.6

C graphite /epoxy 10.0 25.4

CL graphite /epoxy 20.0 50.8

AC graphite/epoxy reinforced 10.0 25.4

aluminum alloy

TB boron/epoxy- reinforced 10.0 25.4

titanium alloy

TC graphite/epoxy- reinforced 10.0 25.4

titanium alloy

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NBS-114A (REV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. I'LIIM.K A riON OR Ri;POR T NO. NBSIR 74-572

2. Cov't Accessit)n No.

4. TITLK AND SUBTn i.l'

Torsional Buckling of Composite Cylindrical Shells

5. Publication Date September 1.974

6» t' r f or ni I n ^ Or^iin i Ziit ion (-odc

7. aU 1 HUK(J>) D. E. Marlowe and G. F. Sushinsky

8, l^crlOrming Organ. Report No. NBSIR 74-57?

9. PKRFORMINc; ORGANIZATION NAMF AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON D C "70914

10. Project/Task/Work Unit No. 21.30445

11. (Contract /Grant No.

L-48. 826

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

National Aeronautics and Space Administration

Langley Research Center

Hampton, Virginia 23365

13. Type of Report & Period

ov crc d Final

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

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

bibliography or literature survey, mention it here.)

The elastic buckling strength has been determined for thin-walled composite and

composite-reinforced-metal cylindrical shells. Tests were performed on boron/epoxy and

graphite/epoxy-all-composite specimens, on boron/epoxy-reinforced-titanium specimens

and on boron/epoxy and graphite/epoxy-reinforced aluminum specimens. Cylinders were

tested with several unidirectional-ply and cross-ply layups.

The results of the tests were compared with the buckling strengths predicted by the

torsional buckling analysis of Chao. For the cylinders which fail by buckling, the

experimental buckling torques were approximately 81 percent of the torques predicted by

the analysis.

The experimental results of tests on 39 specimens are presented. Torsional bucklin

strengths which differ by as much as a factor of two may result from reversing the

direction of twist of a thin-walled cross-ply composite cylinder. This has been shown

to be equivalent to reversing the stacking sequence of the laminate. This is of po- tential importance in applications where reversals of loading may occur. An "optimum"

stacking sequence which produced significant increases in the predicted and measured

buckling loads was determined. Cylinders fabricated with this stacking sequence exhibit

consxderable increases in the strength-to-weight ratio over other sequences examined.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons

Aircraft structures; buckling; composite materials; metal reinforcement; stability; stacking sequence; thin shells; torsion.

18. AVAILABILITY [x Unlimited

I

' For Official Distribution. Do Not Release to NTIS 0 Order From Sup. of Doc, U.S. Government Printing Office Washington, D.C. 20402, SD Cat. No. CH

1

' Order From National Technical Information Service (NTIS)

Springfield, Virginia 22151

19. SECURITY CLASS

(THIS REPORT)

UNCL ASSIFIED

21. NO. OF PAGES

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

22. Price

USCOMM-DC 29042-P74