NASA TECHNICAL NOTE NASATN D-7111 The frequency of stress-corrosion cracking has been high and the...

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  • NASA TECHNICAL NOTE

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    NASA TN D-7111

    APOLLO EXPERIENCE REPORT-

    THE PROBLEM OF

    STRESS-CORROSION CRACKING

    by Robert E. Johnson

    Manned Spacecraft Center

    Houston, Texas 77058

    NATIONAL AERONAUTICS AND SPACE ADMINISTRATION • WASHINGTON, D. C. • MARCH 1973

    https://ntrs.nasa.gov/search.jsp?R=19730011148 2020-04-15T20:43:24+00:00Z

  • !

    1. Report No. I 2. Government Accession No.

    NASA TN D- 7111 I 4. Title and Subtitle

    APOLLO EXPERIENCE REPORT

    THE PROBLEM OF STRESS-CORROSION CRACKING

    7. Author(s)

    Robert E. Johnson, MSC

    9. Performing Organization Name and Address

    Manned Spacecraft Center

    Houston, Texas 77058

    12. Sponsoring Agency Name and Address

    National Aeronautics and Space Administration

    Washington, D.C. 20546

    3. Recipient's Catalog No.

    5. Report Date

    March 1973

    6. Performing Organization Co'de

    8. Performing Organization Report No.

    MSC S-344

    10. Work Unit No,

    914-13-20-06-72

    11. Contract or Grant No.

    13. Type of Report and Period Covered

    Technical Note

    14. Sponsoring Agency Code

    15. Supplementary Notes

    The MSC Director waived the use of the International System of Units (SI) for this Apollo

    Experience Report because, in his judgment, the use of SI Units would impair the usefulness of the report or result in excessive cost.

    16, Abstract

    Stress-corrosion cracking has been the most common cause of structural-material failures in

    the Apollo Program. The frequency of stress-corrosion cracking has been high and the magni-

    tude of the problem, in terms of hardware lost and time and money expended, has been signifi- cant. In this report, the significant Apollo Program experiences with stress-corrosion

    cracking are discussed. The causes of stress-corrosion cracking and the corrective actions

    are discussed, in terminology familiar to design engineers and management personnel, to

    show how stress-corrosion cracking can be prevented.

    17. Key Words {Suggested by Author(s})

    • Stress Corrosion

    • Aluminum Alloys

    • Titanium Alloys

    18. Distribution Statement

    19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No. of Pages 22. Price

    None None 20 $3.00

    * For sale by the National Technical Information Service, Springfield, Virsinia 2215l

  • CONTENTS

    Section Page

    SUMMARY ..................................... 1

    INTRODUCTION .................................. I

    DISCUSSION .................................... 2

    Lunar Module Stress-Corrosion Failures ................... 2

    Pressure-Vessel Stress Corrosion ...................... I0

    CONCLUDING REMARKS ............................. 14

    REFERENCES ................................... 15

    iii

  • FIGURES

    Figure

    1

    2

    3

    4

    10

    11

    12

    13

    Lunar module structure .........................

    Crack in a swaged type 7075-T6 aluminum tube found on LTA-3

    (magnification: 3x) ..........................

    Closeup of the crack shown in figure 2 (magnification: 7x) ......

    Edge view of the crack shown in figures 2 and 3 (magnification: 3x). Cross sections of points A_ B_ and C are shown in figure 5 .....

    •Closeups of the cross-section points indicated in figure 4

    (a) Cross-section point A ........................ (b) Cross-section point B ........................ (c) Cross-section point C ........................

    Crack in a typical swaged type 7076-T6 aluminum tube (magnification: 10x) .........................

    Cross section of the crack shown in figure 6. The outlined area is thought to be a swaging lap. Closeup of the outlined area is

    shown in figure 8 ...........................

    Closeup of the outlined area shown in figure 7 .............

    Significance of gap-dimension errors on the resulting stress

    (a) Linear offset ............................. (b) Angular offset ............................

    Crack in the radius of a machined longeron made of type 7079-T652

    aluminum alloy ............................

    Crack in the radius of a machined tank-truss fitting made of

    type 7075-T6511 aluminum alloy ...................

    Crack in the radius of a stiffener made of type 7075-T6 aluminum

    alloy

    (a) Location of the crack in the radius of a stiffener (indicated by dashed line) ............................

    (b) Closeup of the crack shown in figure 12(a) ..... ........

    Interrivet cracking on a channel made of type 7075-T651 aluminum

    alloy ..................................

    Page

    3

    4 5 5

    7

    iv

  • Figure

    14

    15

    16

    17

    18

    19

    Cracking around the fastener holes as a function of the alloy grain direction ................... .........

    Cracking on a strap made of type 7075-T6 aluminum alloy. The failure origin can be seen at point A, and secondary cracks can be seen at points B and C ......................

    Intergranular cracking at the midthickness from stress-corrosion

    cracking and transgranular cracking at surfaces, which indicates high tension loads (magnification: 100x) ..............

    Cracking on a machined part. (Note the relationship of the crack to the alloy grain direction. ) .....................

    Stress corrosion of type 6A1-4V titanium alloy from a failed nitrogen tetroxide pressure vessel

    (a) Failed pressure vessel .......................

    (b) Magnified view of the inside surface shorting cracks ....... (c) Transgranular cracks ....................... (d) Electron fractograph of the failed surface .............

    (e) Cross section of the pressure-vessel wall showing a flat frac- ture with a shear lip .......................

    Origin of the failure that resulted from methanol in a type 6A1-4V titanium-alloy pressure vessel

    (a) Tank cross section at the fracture face (magnification: 10x) . (b) Microstructure of the tank inner surface

    (magnification: 50x) ....................... (c) Microstructure at the fracture face ................ (d) Microstructure at the fracture face (different location from

    fig. 19(c)) .............................

    Page

    I0

    11 11 11 11

    12

    13

    13 13

    13

    V

  • APOLLO EXPERIENCEREPORT

    THE PROBLEM OF STRESS-CORROSION CRACKING

    By Robert E. Johnson Manned Spacecraft Center

    SUMMARY

    Stress-corroSion cracking has been the most common cause of structural- material failures in the Apollo Program. The frequency of stress-corrosion cracking has been high and the magnitude of the problem, in terms of hardware lost and time and money expended, has been significant. In this report, the significant Apollo Pro- gram experiences with stress-corrosion cracking are discussed. The causes of stress- corrosion cracking and the corrective actions are discussed, in terminology familiar to design engineers and management personnel, to show how stress-corrosion cracking can be prevented.

    The basic conclusion of this report is that the environments and alloys used in the Apollo Program in which failures occurred were generally the same as those en- countered in past aircraft and missile failures. Two exceptions to this conclusion are the methanol-titanium and the nitrogen tetroxide-titanium incompatibilities, which were combinations unique to the Apollo Program. Better communications are needed be- tween designers and fabrication personnel, and a more thorough education on existing

    knowledge of stress-corrosion problems should significantly reduce problems in the construction of future spacecraft. A secondary conclusion presented in this report is that the use of certain aluminum alloys and high-strength steels and the use of certain heat-treatment procedures should be avoided.

    I NTRODUCTION

    The following definition of stress-corrosion cracking is found in reference 1. "When certain metal alloys are exposed to a corrosive environment while at the same time they are subjected to an appreciable, continuously maintained, tensile stress, rapid structural failure can occur as a result of stress corrosion. This is known as stress-corrosion cracking and is characterized by a brittle type failure in a material that is otherwise ductile. "

    The stress-corrosion susceptibility of an alloy is affected by the temperature, the

    grain direction (in certain alloys), the grain size, and the distribution of phases or pre- cipitates in the alloy, in addition to such obvious factors as alloy composition, environ- ment, and stress level. Although the problem of preventing stress-corrosion cracking

  • is difficult, the use of correct assembly procedures and the proper recognition during the designphaseof the factors affecting structural sensitivity canbe instrumental in avoiding this problem.

    The structural materials used in the Apollo spacecraft hardware are high-strength alloys that are widely used in aircraft construction. The stress-corrosion behavior of thesealloys is well establishedin certain environments. Therefore, it is important to examinesomeof the Apollo Program hardware failures to determine why they oc- curred andhow they were overcom