Experimental investigation of the strength of …cradpdf.drdc-rddc.gc.ca/PDFS/unc67/p528755.pdf · Experimental investigation of the strength of damaged ... d’aluminium renforcés

Defence R&D Canada – Atlantic

DEFENCE DÉFENSE&

Experimental investigation of the strength

of damaged pressure hulls – Phase 2

J. R. MacKay

Technical Memorandum

DRDC Atlantic TM 2007-013

February 2007

Copy No. _____

Defence Research andDevelopment Canada

Recherche et développementpour la défense Canada

This page intentionally left blank.

Experimental investigation of the strength of damaged pressure hulls – Phase 2 Summary of Experimental Results

John R. MacKay

Defence R&D Canada - Atlantic Technical Memorandum DRDC Atlantic TM 2007-013 February 2007

DRDC Atlantic TM 2007-013 i

Abstract

This document presents a summary of results for Phase 2 of an experimental program to determine the collapse pressures of small-scale pressure hulls (ring-stiffened cylinders) with various amounts and patterns of simulated material loss due to corrosion. Six ring-stiffened aluminium cylinders were tested in the period from November 2006 to January 2007: two undamaged specimens, one specimen with corrosion damage to the stiffeners, and three specimens with various levels of shell corrosion. Initial analysis of the experimental data suggests eccentricity due to one-sided shell thinning has a strength-reducing effect that is in addition to the thinning itself. Further analysis of load-strain data will be performed and presented in a technical memorandum at a later date. An additional aluminium cylinder without stiffeners was fabricated in order to analyze a new experimental apparatus, which was recommended after the completion of Phase 1 testing [1]. The new test setup was implemented successfully, and was found to be suitable for the collapse testing of pressure hulls.

Résumé

Le présent document présente un résumé des résultats de la phase 2 d’un programme expérimental visant à déterminer les pressions d’écrasement de coque épaisses à petite échelle (cylindres renforcés à l’aide d’anneaux) avec diverses quantités et configurations de perte de matériau simulée causée par la corrosion. Six cylindres d’aluminium renforcés par anneaux ont été testés au cours d’une période comprise entre novembre 2006 et janvier 2007 : deux éprouvettes non endommagés, une éprouvette avec de la corrosion sur les raidisseurs de coque et trois éprouvettes avec des niveaux de corrosion variés. Une analyse initiale des résultats expérimentaux indique qu’une excentricité due à l’amincissement de la coque sur un côté a pour effet d’en réduire la résistance, en plus de l’effet de l’amincissement proprement dit. On procèdera à d’autres analyses des résultats sur les données de contrainte; les résultats seront présentés dans un document technique à une date ultérieure. Un autre cylindre en aluminium sans raidisseurs a été fabriqué afin d’analyser une nouvelle méthode d’expérimentation, qui a été recommandée à la fin des tests de la phase 1 [1]. Cette nouvelle méthode d’essai a été mise en œuvre avec succès, et on a constaté qu’elle convenait pour les essais de pression d’écrasement des coques épaisses.

ii DRDC Atlantic TM 2007-013


DRDC Atlantic TM 2007-013 iii

Executive summary

Background

DRDC Atlantic and the Netherlands Ministry of Defence have begun a collaborative project to study the structural behaviour of corroded pressure hulls. This document presents a summary of results for Phase 2 of an experimental program to determine the collapse pressures of small-scale pressure hulls (ring-stiffened cylinders) with various amounts and patterns of simulated material loss due to corrosion. Six ring-stiffened aluminium cylinders were tested in the period from November 2006 to January 2007: two undamaged specimens, one specimen with corrosion damage to the stiffeners, and three specimens with various levels of shell corrosion.

Principal Results

Simulated corrosion of the shell was found to decrease the collapse strength compared to similar undamaged models in all cases. The detrimental effects of corrosion thinning were associated with reduced structural stiffness and early onset of yielding in the corroded regions. Initial analysis of the experimental data suggests that: 1) the depth of thinning is more significant than the total volume of material removed, and 2) offsetting of the shell plating due to one-sided thinning has an additional strength-reducing effect.

Significance of Results

The current study reinforces the conclusions drawn after Phase 1 of the experimental program [1]; namely that: 1) corrosion, even in a very discrete and localized form, significantly reduces collapse strength, and thus the operational envelope, of pressure hulls, and 2) the mechanisms involved in the collapse of pressure hulls with corrosion damage are not well understood. The results of this study suggest that a simple universal “knock-down” formula, based on depth or volume of thinning, may be insufficient to ascertain the loss of pressure hull strength due to corrosion. The critical corrosion “patch” size must be determined for each particular pressure hull, taking into account the critical collapse mode, diving depth requirements and reserve strength.

Future Work

Further analysis of the experimental data, including comparison with design stresses and collapse pressures, will be performed and presented in a technical memorandum at a later date. Nonlinear finite element analyses of the specimens should also be undertaken. It is recommended that the new test setup, which was implemented successfully for Phase 2, be used for future collapse testing.

MacKay, J. R. (2007). Experimental investigation of the strength of damaged pressure hulls – Phase 2. DRDC Atlantic TM 2007-013. Defence R&D Canada - Atlantic.

iv DRDC Atlantic TM 2007-013

Sommaire

Introduction

RDDC Atlantique et le ministère de la défense des Pays-Bas ont entrepris un projet de collaboration pour étudier le comportement de la structure de coques épaisses corrodées. Le présent document présente un résumé des résultats de la phase 2 d’un projet expérimental pour déterminer les pressions d’écrasement des coques épaisses à petite échelle (cylindres renforcés à l’aide d’anneaux) avec diverses quantités et configurations de perte de matériau simulée causée par la corrosion. Six cylindres d’aluminium renforcés par anneaux ont été testés au cours d’une période comprise entre novembre 2006 et janvier 2007 : deux éprouvettes non endommagés, une éprouvette avec de la corrosion sur les raidisseurs de coque et trois éprouvettes avec des niveaux de corrosion variés.

Résultats

On a constaté que dans tous les cas, la corrosion simulée de la coque diminue la résistance à l’écrasement par rapport aux modèles similaires non endommagés. Les effets négatifs de la perte par corrosion étaient reliés à une réduction de la rigidité de structure et un fléchissement prématuré sur les zones corrodées. Une analyse initiale des résultats indique deux choses : 1) La profondeur de l’amincissement est plus importante que le volume total de perte de matériau; 2) Le déséquilibrage de la coque en raison de l’amincissement sur un côté a pour effet d’en réduire encore davantage la résistance.

Portée

L’étude actuelle confirme les conclusions tirées de la phase 1 du programme expérimental [1], à savoir : 1) La corrosion, même d’une forme très discrète et localisée, diminue de façon importante la résistance à l’écrasement, et par conséquent, les capacités opérationnelles des coques épaisses; 2) Les mécanismes contribuant à l’écrasement des coques épaisses endommagées par la corrosion ne sont pas bien compris. Les résultats de cette étude indiquent qu’une simple formule universelle consacrée qui se base sur la profondeur ou le volume de l’amincissement peut ne pas suffire à évaluer la perte de résistance d’une coque épaisse en raison de la corrosion. Il faut déterminer les dimensions des zones de corrosion critiques sur chaque coque épaisse individuelle, en tenant compte du mode d’écrasement critique, des exigences relatives à la profondeur de plongée et de la résistance de réserve.

Recherches futures

On procèdera à d’autres analyses des résultats, y compris une comparaison des contraintes prévues et des pressions d’écrasement; elles seront présentées dans un document technique à une date ultérieure. Des analyses par éléments finis de type non

DRDC Atlantic TM 2007-013 v

linéaire devraient être entreprises. Il est recommandé que la nouvelle méthode d’essai qui a été mise en œuvre avec succès pour la phase 2 soit utilisée pour les futurs essais d’écrasement.

MacKay, J. R. (2007). Enquête expérimentale sur la résistance de coques épaisses en-dommagées – Phase 2. DRDC Atlantic TM 2007-013. Defence R&D Canada - Atlantic.

vi DRDC Atlantic TM 2007-013

Table of contents

Abstract........................................................................................................................................ i

Executive summary ................................................................................................................... iii

Sommaire................................................................................................................................... iv

Table of contents ....................................................................................................................... vi

List of figures .......................................................................................................................... viii

List of tables .............................................................................................................................. ix

Acknowledgements .................................................................................................................... x

1. Introduction ................................................................................................................... 1

2. Test specimens............................................................................................................... 2

2.1 L510-Test ......................................................................................................... 2

2.2 L300-No1 ......................................................................................................... 4

2.3 L510-No3 ......................................................................................................... 5

2.4 L510-No5 ......................................................................................................... 8

2.5 L510-No7 ......................................................................................................... 9

2.6 L510-No9 ....................................................................................................... 11

2.7 L510-No11 ..................................................................................................... 14

3. Experimental Apparatus and Procedure ...................................................................... 18

4. Experimental Results................................................................................................... 21

4.1 L510-Test ....................................................................................................... 21

4.2 L300-No1 ....................................................................................................... 23

4.3 L510-No3 ....................................................................................................... 25

4.4 L510-No5 ....................................................................................................... 26

4.5 L510-No7 ....................................................................................................... 27

4.6 L510-No9 ....................................................................................................... 29

4.7 L510-No11 ..................................................................................................... 30

DRDC Atlantic TM 2007-013 vii

5. Discussion ................................................................................................................... 32

5.1 Effect of Corrosion Damage........................................................................... 32

5.2 Assessment of pressure testing apparatus....................................................... 34

6. Recommendations ....................................................................................................... 35

7. References ................................................................................................................... 36

List of Symbols, Acronyms and Initialisms ............................................................................. 37

Symbols ....................................................................................................................... 37

Acronyms and Initialisms............................................................................................ 37

viii DRDC Atlantic TM 2007-013

List of figures

Figure 1. Geometry of specimen L510-Test (mm)..................................................................... 2

Figure 2. Specimen L510-Test before testing............................................................................. 3

Figure 3. Strain gauge layout for specimen L510-Test .............................................................. 3

Figure 4. Geometry of specimen L300-No1 (mm)..................................................................... 4

Figure 5. Specimen L300-No1 before testing............................................................................. 4

Figure 6. Strain gauge layout for specimen L300-No1 .............................................................. 5

Figure 7. Basic geometry of specimen L510-No3 (mm)............................................................ 6

Figure 8. ‘Dog-bone’ flange corrosion on long cylinder ............................................................ 6

Figure 9. Specimen L510-No3 before testing............................................................................. 7

Figure 10. Strain gauge layout for specimen L510-No3 ............................................................ 7

Figure 11. Geometry of specimen L510-No5 (mm)................................................................... 8

Figure 12. Specimen L510-No5 before testing........................................................................... 8

Figure 13. Strain gauge layout specimen L510-No5 .................................................................. 9

Figure 14. Specimen L510-No7 with 42x42 mm patch of shell corrosion (inset) ................... 10

Figure 15. Strain gauge layout specimen L510-No7 ................................................................ 11

Figure 16. Strain gauge layout for specimen L510-No7 – showing central bay ...................... 11

Figure 17. Specimen L510-No9 with 100x100 mm patch of shell corrosion........................... 12

Figure 18. Strain gauge layout for specimen L510-No9 .......................................................... 13

Figure 19. Strain gauge layout for specimen L510-No9 – showing central bay ...................... 14

Figure 20. Strain gauge layout specimen L510-No9 – Plan view of corroded region.............. 14

Figure 21. Specimen L510-No11 with 100x200 mm patch of shell corrosion......................... 15

Figure 22. Strain gauge layout for specimen L510-No11 ........................................................ 16

Figure 23. Strain gauge layout for specimen L510-No11 – showing central bay .................... 17

DRDC Atlantic TM 2007-013 ix

Figure 24. Strain gauge layout for specimen L510-No11 – Plan view of corroded region...... 17

Figure 25. Pressure testing apparatus used in Phase 1 (left) and Phase 2 (right) testing.......... 18

Figure 26. Chord gauge device manufactured for measuring OOC ......................................... 20

Figure 27. Specimen L510-Test after initial collapse test, showing a single buckle................ 22

Figure 28. Specimen L510-Test after residual strength testing (n = 3 collapse shape) ............ 23

Figure 29. Results of chord gauge analysis for L300-No1 after initial collapse test................ 24

Figure 30. Specimen L300-No1 after residual strength test ..................................................... 25

Figure 31. Specimen L510-No3 after collapse test .................................................................. 26

Figure 32. Specimen L510-No5 after collapse (left) and residual strength (right) tests........... 27



Figure 35. Specimen L510-No11 after collapse test ................................................................ 31

List of tables

Table 1. Average measured material properties of experimental specimens.............................. 2

Table 2. Testing procedure for specimen L510-Test................................................................ 21

Table 3. Testing procedure for specimen L300-No1................................................................ 24





Table 8. Testing procedure for specimen L510-No11.............................................................. 30

Table 9. Summary of collapse and residual strength pressures for Phase 2 specimens............ 32

Table 10. Summary of collapse pressures for Phase 1 specimens............................................ 33

x DRDC Atlantic TM 2007-013

Acknowledgements

The author like to thank Mr. Theo Bosman of the Netherlands Ministry of Defence for his cooperation and advice on this Netherlands-Canada collaborative project, DRDC Atlantic engineering technologists Mr. Alex Ritchie and Mr. Dave Wright for their hard work and common sense advice in preparing and conducting this experimental program, the staff of the Mechanical Services and Prototype Development groups at DRDC Atlantic for their technical support, and Dr. Neil Pegg and Dr. Malcolm Smith of DRDC Atlantic for advice and guidance. Also, Dr. Fred van Keulen of the Delft University of Technology for his idea for an improved pressure testing apparatus.

DRDC Atlantic TM 2007-013 1

1. Introduction

DRDC Atlantic and the Netherlands Ministry of Defence are undertaking a multi-year experimental program to determine the effects of corrosion damage to the strength of pressure hulls. This study, which focuses on the strength-reducing effects of material loss due to corrosion, consists of the collapse testing of approximately 40 model pressure hulls. These small-scale ring-stiffened cylinders are fabricated from aluminium tubing and vary with respect to design geometry, geometric imperfections, and corrosion damage.

Phase 1 of the experimental program involved the testing of six ring-stiffened cylinders and was completed in 2005 [1]. Phase 2 saw the testing of seven cylinders in the period from November 2006 to January 2007. This technical memorandum presents an initial summary of the results of Phase 2 testing, including the implementation and analysis of a new experimental apparatus. Detailed analysis of the experimental load-strain data will be performed in the future and presented in a technical memorandum.

2 DRDC Atlantic TM 2007-013

2. Test specimens

Test specimens were machined from aluminium alloy tubing. Loss of material due to corrosion and subsequent grinding of a pressure hull was simulated by machining away shell or stiffener material on the experimental specimens. Coupon tests were performed to determine the material properties listed in Table 1. Poisson’s ratio was not measured, but was assumed to be 0.3.

Table 1. Average measured material properties of experimental specimens

YIELD STRESS (%0.2)

TENSILE STENGTH YOUNG’S MODULUS

SPECIMEN

(MPa) (MPa) (MPa)

L300-No1, L510-No3 302.4 373.7 71,350

L510-Test, L510-No5, L510-No7, L510-No9, L510-No11

333.0 387.6 71,300

The following sections describe the nominal geometry and instrumentation of the specimens tested in Phase 2 of the experimental program.

2.1 L510-Test

Specimen L510-Test was a cylinder with no stiffeners (Figure 1 and Figure 2), used to evaluate the new pressure testing apparatus and data acquisition system, which was recommended after Phase 1 [1].

60

510

108.5

14.5

4.5 20

Figure 1. Geometry of specimen L510-Test (mm)

Strain gauges were applied to the cylinder as shown in Figure 3, and described as follows:

• Outside gauges

• 12 uni-axial gauges about the circumference (30° increments), in both the axial and circumferential directions, on the outside of the shell at the


midpoint of the cylinder (2 x 12 = 24 gauges) – these gauges were in-tended to allow verification of the design formulae stresses, provide in-formation on the mode of failure, and allow the material behaviour to be monitored during testing

• Total of 24 gauges on the outside of the cylinder

• Inside gauges

• None

• Grand Total of 24 gauges

Figure 2. Specimen L510-Test before testing

Figure 3. Strain gauge layout for specimen L510-Test


2.2 L300-No1

Specimen L300-No1 was a short, externally stiffened cylinder with no corrosion damaged (Figure 4 and Figure 5).

20 60

80

300

15

45 50 45

2.5

110

16

2

3

10

Figure 4. Geometry of specimen L300-No1 (mm)

Figure 5. Specimen L300-No1 before testing

Strain gauges were applied to the cylinder as shown in Figure 6 and described as follows:

• Outside gauges

• 8 uni-axial gauges about the circumference (45° increments), in the circumferential direction, on the two central frame flanges (2 x 8 = 16 gauges) – these gauges were intended to detect any overall component to the collapse mode shape

• 8 uni-axial gauges about the circumference (45° increments), in both the axial and circumferential directions, on the outside of the shell at the midpoint of the central bay (2 x 8 = 16 gauges) – these gauges were in-


tended to allow verification of the design formulae stresses, as well as to provide information on the mode of failure

• 8 uni-axial gauges about the circumference (45° increments), in the circumferential direction only, on the outside of the shell at the butt of the T-section stiffener (2 x 8 = 16 gauges) – these gauges were intended to provide information on the mode of failure

• Total of 16 + 16 + 16 = 48 gauges on the outside of the cylinder

• Inside gauges

• 8 uni-axial gauges about the circumference (45° increments), in both the axial and circumferential directions, on the inside of the shell at the mid-point of the central bay (2 x 8 = 16 gauges) – these gauges were intended to allow verification of the design formulae stresses, as well as to provide information on the mode of failure

• Total of 16 gauges on the inside of the cylinder

• Grand Total of 64 gauges x 8 about

circumference

Uniaxial Strain Gauge – Axial Direction

Uniaxial Strain Gauge – Circumferential Direction

Figure 6. Strain gauge layout for specimen L300-No1

2.3 L510-No3

Specimen L510-No3 was a long, externally stiffened cylinder with ‘dog-bone’ corrosion on the central stiffeners (Figure 7, Figure 8, and Figure 9). Strain gauges were applied to the cylinder as shown in Figure 10, and described as follows:

• Outside gauges

• 12 uni-axial gauges about the circumference (30° increments, with 0° at centre of corrosion) in the circumferential direction, on the two central frame flanges (2 x 12 = 24 gauges) – these gauges were intended to detect an overall collapse mode shape

• 2 gauges per flange at 5° and 355° (2 x 2 = 4 gauges) – these gauges were intended to provide information on the effect of the corrosion.

• 1 uni-axial gauge about the circumference at 0° (centre of ‘dog-bone), in both the axial and circumferential directions, on the outside of the shell at


the midpoint of the central bay (1 x 2 = 2 gauges) – these gauges were in-tended to provide verification of the design formulae stresses, as well as provide information on the mode of failure

• Total of 24 + 4 + 2 = 30 gauges on the outside of the cylinder

• Inside gauges

• 12 uni-axial gauges about the circumference (30° increments), in both the axial and circumferential directions, on the inside of the shell at the mid-point of the central bay (2 x 12 = 24 gauges) – these gauges were in-tended to provide verification of the design formulae stresses, as well as provide information on the mode of failure



50 60 80

510

110

15

3 20

2

8

8

2

1 2 3 4 5 6 7 8

Figure 7. Basic geometry of specimen L510-No3 (mm)

Figure 8. ‘Dog-bone’ flange corrosion on long cylinder





2.4 L510-No5

Specimen L510-No5 was a long, internally stiffened cylinder with no corrosion damage (Figure 11 and Figure 12).

510

50 60

110

13

20 3

2

8

8

2

1 2 3 4 5 6 7 8

Figure 11. Geometry of specimen L510-No5 (mm)


Strain gauges were applied to the cylinder as shown in Figure 13, and described as follows:

• Outside gauges


• 12 uni-axial gauges about the circumference (30° increments), in both the axial and circumferential directions, on the outside of the shell at the midpoint of the central bay (2 x 12 = 24 gauges) – these gauges were in-tended to allow verification of the design formulae stresses, as well as provide information on the mode of failure


• Inside gauges

• 12 uni-axial gauges about the circumference (30° increments), in the circumferential direction, on the two central frame flanges (2 x 12 = 24 gauges) these gauges were intended to detect an overall collapse mode shape



Figure 13. Strain gauge layout specimen L510-No5

2.5 L510-No7

Specimen L510-No7 was a long, internally stiffened cylinder with shell corrosion. The basic geometry of the cylinder was the same as the undamaged specimen L510-No5 (Figure 11). The corrosion damage was in the form of a uniform patch of shell thinning, measuring 42x42 mm and 0.6 mm deep, in the central bay (Figure 14).

Strain gauges were applied to the cylinder as shown in Figure 15 and Figure 16, and described as follows:

• Outside gauges

• 12 uni-axial gauges about the circumference (30° increments, 0° at centre of corrosion), in both the axial and circumferential directions, on the out-side of the shell at the midpoint of the central bay (2 x 12 = 24 gauges) – these gauges were intended to allow verification of the design formulae stresses, as well as to provide information on the mode of failure and the effect of corrosion

• 1 uni-axial gauge about the circumference at 0°, in both the axial and circumferential directions, on the outside of the shell opposite the stiff-ener (2 x 2 = 4 gauges) – these gauges were intended to allow verification


of the design formulae stresses, as well as to provide information on the mode of failure and the effect of corrosion


Figure 14. Specimen L510-No7 with 42x42 mm patch of shell corrosion (inset)

• Inside gauges

• 12 uni-axial gauges about the circumference (30° increments), in the circumferential direction, on the two central frame flanges (2 x 12 = 24 gauges) – these gauges were intended to detect an overall collapse mode shape

• 5 uni-axial gauges in the circumferential direction, on the inside of the shell at the midpoint of the central bay, at the centre of corrosion (0°), at either edge of the corrosion patch (approx. 5.6° and 354.4°), and at 15° and 345°

• 1 uni-axial gauge in the axial direction on the inside of the shell at the midpoint of the central bay, at the centre of corrosion (0°)




Figure 15. Strain gauge layout specimen L510-No7

Figure 16. Strain gauge layout for specimen L510-No7 – showing central bay

2.6 L510-No9

Specimen L510-No9 was a long, internally stiffened cylinder with shell corrosion. The basic geometry of the cylinder was the same as the undamaged specimen L510-No5 (Figure 11). The corrosion damage was in the form of a uniform patch of shell


thinning, measuring 100x100 mm and 0.4 mm deep, centred over the central bay and spanning two bays of shell plating (Figure 17).

Figure 17. Specimen L510-No9 with 100x100 mm patch of shell corrosion

Strain gauges were applied to the cylinder as shown in Figure 18, Figure 19, and Figure 20, and described as follows:

• Outside gauges

• 12 uni-axial gauges about the circumference (30° increments, 0° at centre of corrosion), in the circumferential direction, on the outside of the shell at the midpoint of the central bay (12 gauges) – these gauges were in-tended to allow verification of the design formulae stresses, as well as provide information on the mode of failure and the effect of corrosion

• 2 additional uni-axial gauges in the circumferential direction at 15° and 345°, on the outside of the shell at the midpoint of the central bay (2 gauges) – these gauges were intended to provide information on the effect of corrosion

• 5 uni-axial gauges about the circumference (0°, 15°, 30°, 330°, 345°), in the axial direction, on the outside of the shell at the midpoint of the cen-tral bay (5 gauges) – these gauges were intended to provide information on the effect of corrosion

• Uni-axial gauges at 0°, in both the axial and circumferential directions, on the outside of the shell opposite the four central stiffeners (2 x 4 = 8


gauges) – these gauges were intended to allow verification of the design formulae stresses, as well as to provide information on the mode of fail-ure and the effect of corrosion



• Inside gauges


• Uni-axial gauges (0°, 15°, 345°), in both the axial and circumferential di-rections, on the inside of the shell at the midpoint of the central bay (2 x 3 = 6 gauges) – these gauges were intended to provide information on the mode of failure and the effect of corrosion





Figure 20. Strain gauge layout specimen L510-No9 – Plan view of corroded region

2.7 L510-No11

Specimen L510-No11 was a long, internally stiffened cylinder with shell corrosion. The basic geometry of the cylinder was the same as the undamaged specimen L510-No5 (Figure 11). The corrosion damage was in the form of a uniform patch of shell thinning, measuring 100 mm in the circumferential direction, 200 mm in the axial direction, and 0.4 mm deep. The corrosion patch was centred over the central bay and spanned four bays of shell plating (Figure 21).


Figure 21. Specimen L510-No11 with 100x200 mm patch of shell corrosion

Strain gauges were applied to the cylinder as shown in Figure 22, Figure 23, and Figure 24, and described as follows:

• Outside gauges

• 12 uni-axial gauges about the circumference (30° increments, 0° at centre of corrosion), in the circumferential direction, on the outside of the shell at the midpoint of the central bay (12 gauges) – these gauges were in-tended to allow verification of the design formulae stresses, as well as to provide information on the mode of failure and the effect of corrosion

• 2 additional uni-axial gauges in the circumferential direction at 15° and 345°, on the outside of the shell at the midpoint of the central bay (2 gauges) – these gauges were intended to provide information on the effect of corrosion

• 3 uni-axial gauges about the circumference (0°, 15°, 345°), in the axial direction, on the outside of the shell at the midpoint of the central bay (3 gauges) – these gauges were intended to provide information on the effect of corrosion

• Uni-axial gauges at 0°, in both the axial and circumferential directions, on the outside of the shell opposite the four central stiffeners (2 x 4 = 8 gauges), as well as on the outside of the shell at the midpoint of the two bays adjacent to the central bay (2 x 2 = 4) – these gauges were intended


to allow verification of the design formulae stresses, as well as to provide information on the mode of failure and the effect of corrosion


• Inside gauges


• Uni-axial gauges at 0°, in both the axial and circumferential directions, on the inside of the shell at the midpoint of the three central bays (2 x 3 = 6 gauges) – these gauges were intended to provide information on the mode of failure and the effect of corrosion






Figure 24. Strain gauge layout for specimen L510-No11 – Plan view of corroded region


3. Experimental Apparatus and Procedure

In Phase 1 of the testing program [1], specimens were loaded to collapse in the DRDC Atlantic pressure testing facility (Figure 25) by applying external hydrostatic pressure to the sealed and air-filled specimens. This method resulted in catastrophic failures with a high degree of material rupture, such that the immediate post-collapse shape was difficult to identify. A large amount of strain energy is stored in the pressure tank, specimen, and the testing fluid during pressurization, to be released upon collapse of the specimen. The catastrophic failures of the air-filled specimens were attributed to the lack of resistance to the large velocities resulting from the release of the stored strain energy [1].

Pump inlet

Fluid-release valve

Cross-over valve

Pressure transducers

Cables to data acquisition system

Air-filled specimen

Fluid-filled specimen

Pressure chamber

Lid or “breach”

0.9m

2.4m

Figure 25. Pressure testing apparatus used in Phase 1 (left) and Phase 2 (right) testing

A new experimental apparatus was implemented for Phase 2 testing, with the goal of preventing catastrophic specimen failures. The new test setup, as recommended after Phase 1, involved modifying the existing pressure tank as shown in Figure 25. The testing of a specimen is undertaken in the following steps:

1. Fill the specimen with testing fluid (mineral oil) and connect to the system.


2. Close the fluid-release valve and open the cross-over valve, and pre-pressurize the system to a load beyond the anticipated collapse value. This will result in a pressure differential between the inside and outside of the specimen of zero (i.e. a net load of zero on the specimen).

3. Close the cross-over valve and use the fluid-release valve to slowly release the fluid from the cylinder while monitoring the pressure differential and strains. The flow, and therefore the applied load, can be reduced or stopped altogether when approaching collapse, as indicated by the strain behaviour.

This method has another benefit in that the pump does not have to be used after step 2. This eliminates vibration and electronic noise associated with the operation of the pump.

Three types of tests were typically performed on each specimen:

• one or more loading cycles in the elastic range to check the instrumentation,

• loading up to the ultimate or collapse pressure, after which the specimen was unloaded and removed from the pressure tank for inspection of the collapse shape, and

• loading of the specimen to determine its ‘residual strength’ (i.e. the load it can resist after the collapse test has been performed).

The pre-pressure load on the system (i.e. a net load of zero on specimen) was approximately 16.5 MPa for all tests, except where noted. Collapse tests that were performed immediately after an elastic cycle generally used the same pre-pressure (i.e. Step 2 was not repeated). Chord gauge measurements, described below, and photographs were taken after collapse testing of each specimen to document the collapse shape before residual strength testing.

A chord gauge (Figure 26) is an instrument used to measure geometric imperfections, in the form of out-of-circularity (OOC), of cylindrical structures. The gauge length is based on the nominal circumference of the cylinder and the desired number of measurements. Displacement gauge readings, taken at equal intervals about the circumference, are subtracted from the zero gauge reading, taken on a flat surface, giving the chord height at each location. The chord heights are then converted to radial eccentricities, based on the average reading. Fourier analysis can be undertaken to determine the contribution of each mode, defined as n complete waves about the circumference, to the overall shape.

The transducers used to measure the tank and specimen pressures had ranges of 17.2 and 34.5 MPa, respectively. The transducers were accurate to within 0.25% of their range (0.043 and 0.086 MPa, respectively) for all errors. The net pressure on the specimen (i.e. tank pressure less the specimen pressure) is reported to three significant figures and is associated with the accuracy of the tank transducer (±0.09 MPa).

Experimental procedures for individual specimens are presented in tabular form in Section 4.


Figure 26. Chord gauge device manufactured for measuring OOC


4. Experimental Results

4.1 L510-Test

Specimen L510-Test underwent seven testing cycles in the elastic range of the structure over several days in order to analyze the new testing setup and data acquisition system (Table 2). The strain data indicated that the specimen did not yield during these initial tests.

Table 2. Testing procedure for specimen L510-Test

DATE NAME NOTES

10 Nov 2006 Trial 1 Elastic cycle (1.4 MPa), with 2.8 MPa pre-pressure. Maintained load on specimen for 0.5 hours to check for pressure loss due to leaks. A small loss of pressure was noted (0.03 MPa), but found to be acceptable.

10 Nov 2006 Trial 2 Elastic cycle (2.8 MPa), with 2.8 MPa pre-pressure, to check instrumentation.

10 Nov 2006 Trial 3 Aborted collapse test. Sudden loss of applied pressure at 7.1 MPa due to equipment malfunction, which was not identified immediately. Strain data indicated that no yielding occurred.

14 Nov 2006 Trial 4 Elastic cycle (3.4 MPa) to check instrumentation and testing apparatus. No problems were noted

14 Nov 2006 Trial 5 Aborted collapse test. Large increase in applied pressure at 5.5 MPa with no correspond-ing increase in strain. Tank pressure was stable, cylinder pressure dropped suddenly. It was determined that the hydraulic hose connecting specimen to outside of tank failed under net external pressure during Trial 3, and failed again at a lower pressure during this trial. Hose was replaced by stainless steel piping.

14 Nov 2006 Trial 6 Elastic cycle (3.4 MPa) to check new equipment. Observed time lag between applied pressure and strain response. Attributed to restricted fluid flow due to the small diameter of steel piping, which was subsequently replaced by larger diameter piping.

15 Nov 2006 Trial 7 Elastic cycle (3.4 MPa) to check new equipment. Pressure-strain time lag was signifi-cantly reduced at low loading rates.

15 Nov 2006 Trial 8 Collapse test. External pressure was applied gradually until collapse of the specimen. Instantaneous loss of applied pressure occurred at peak load, after which the specimen was able to resist a reduced pressure. Applied pressure was removed.

16 Nov 2006 Trial 9 Residual strength test. External pressure was applied gradually up to the initial load peak, after which the applied pressure dropped gradually as additional fluid was released from the specimen. Several local maxima and minima were followed in the post-peak load range. Test was stopped when rupture of the specimen was signified by a lack of strain increase with further fluid release from the specimen.

The first attempt at loading the specimen to collapse (Trial 3 in Table 2) was aborted due to the buckling of a flexible hydraulic hose connecting the specimen to the outside of the pressure chamber (Figure 25). The damaged hose was replaced by solid stainless steel piping, which was capable of withstanding the applied loads


during pressure testing, and was cold bent to connect the cylinder with the pressure tank lid.

The specimen was loaded to collapse, and failed at a pressure of 7.96 MPa. The applied pressure dropped to approximately 2.8 MPa immediately after peak load was reached, after which the specimen was able to carry the remaining pressure load. The specimen was removed from the test chamber and showed a single permanent buckle (Figure 27).

Figure 27. Specimen L510-Test after initial collapse test, showing a single buckle

The strain data showed that collapse was dominated by an n=4 mode, with a single lobe dominating the post-collapse behaviour, which agrees with the observed failure mode. The n=4 buckling mode was confirmed by chord-gauge measurement of the specimen after testing. The strain gauges indicated that yielding did not occur before collapse, and thus, the specimen buckled elastically.

The specimen was tested again to determine its residual strength after initial buckling. The maximum pressure resisted during this test was 4.16 MPa. The load-strain behaviour showed several peaks and valleys, roughly corresponding to the formation of additional buckling lobes. The final shape was strongly dominated by an n=3 mode (Figure 28), suggesting mode-shifting in the post-collapse region.


Figure 28. Specimen L510-Test after residual strength testing (n = 3 collapse shape)

4.2 L300-No1

Specimen L300-No1 underwent several elastic loading cycles (Table 3). The cylinder was then loaded to collapse, and failed at a pressure of 7.11 MPa. The applied pressure did not drop suddenly after the peak load was reached (as in the testing of specimen L510-Test), likely because the failure occurred at the shell without a global loss of stiffness of the combined shell-stiffener. The specimen was taken to a strain state somewhat beyond that at the peak pressure and then unloaded.

The specimen was removed from the test chamber and showed a single permanent interframe buckle. The dimple was almost imperceptible by visual inspection, but corresponded with the strain data, which showed yielding at the same point. Chord-gauge measurements were taken (Figure 29), which showed a mixed buckling mode, with significant contributions from modes up to n=8. The chord gauge measurements also confirmed the location of the buckle. The strain gauges indicated that the shell yielded at all measurement locations before collapse, and that the stiffeners did not yield.

The cylinder was tested again to determine its residual strength after initial collapse. The maximum pressure resisted (7.15 MPa) was slightly greater than the initial collapse load. Visual inspection showed that the final shape was in a mode corresponding to 6≤n≤8 (Figure 30). The specimen displayed a ‘classical’ interframe buckling mode, with lobes alternating between adjacent bays.


Table 3. Testing procedure for specimen L300-No1

DATE NAME NOTES

23 Nov 2006 Trial 1 Elastic cycle (2.8 MPa) to check instrumentation. Loss of signal from the strain gauges on the inside of the specimen was attributed to a loose connection at the watertight con-nectors in the specimen end-cap. Also, electronic noise was apparent in some strain gauges. Connectors were tightened.

23 Nov 2006 Trial 2 Aborted elastic cycle (no pressure was applied). Loss of strain gauge signals attributed to faulty watertight connector, which was replaced. Electronic noise was attributed to grounding of the specimen via the stainless steel piping. Plastic (homopolymer acetal, Delrin®) coupling was machined and used to connect piping to tank, in order to break the ground loop.

28 Nov 2006 Trial 3 2 elastic cycles (3.4 MPa) to check new equipment. No strain gauge problems occurred.

28 Nov 2006 Trial 4 Collapse test. External pressure was applied gradually until collapse of the specimen. Applied pressure did not drop immediately aft the peak load was reached. The specimen was taken to a strain state slightly beyond that at the peak pressure and then unloaded.

28 Nov 2006 Trial 5 Residual strength test. External pressure was applied gradually up to the initial load peak, after which the applied pressure dropped gradually as additional fluid was released from the specimen. Several local maxima and minima were followed in the post-peak load range. Test was stopped when further fluid release from the specimen did not result in an increase in strain.

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

0 60 120 180 240 300 360

Circumferential Angle (deg)

Radi

al S

hell

Ecce

ntric

ity (m

m)

Figure 29. Results of chord gauge analysis for L300-No1 after initial collapse test


Figure 30. Specimen L300-No1 after residual strength test

4.3 L510-No3

Specimen L510-No3 underwent a loading cycle in the elastic range before it was loaded to collapse (Table 4), failing at a pressure of 9.25 MPa. A significant loss of applied pressure occurred immediately after the maximum pressure was reached; the load stabilized at approximately 2.3 MPa.


DATE NAME NOTES

4 Dec 2006 Trial 1 Elastic cycle (2.8 MPa) to check instrumentation.

4 Dec 2006 Trial 2 Collapse test. External pressure was applied gradually until collapse of the specimen. Instantaneous loss of applied pressure occurred at peak load, after which the specimen was able to resist a reduced pressure. Applied pressure was removed.

The specimen was removed from the test chamber and showed a single permanent overall buckle (Figure 31). The visual evidence agreed with the strain gauge data, which indicated that the stiffener displacements showed a general n=3 mode at collapse, dominated by large strains at the location of the corrosion (i.e. a single lobe of the n=3 mode shape dominated post-collapse). Yielding occurred first in the shell nearest the corrosion, followed by yielding of the stiffener flanges at the corrosion.

This specimen was not tested to determine its residual strength, as it was decided that the initial collapse shape should be preserved


Figure 31. Specimen L510-No3 after collapse test

4.4 L510-No5

Specimen L510-No5 was loaded to collapse after an initial elastic loading cycle (Table 5), failing at a pressure of 9.08 MPa. The applied pressure dropped suddenly shortly after the maximum pressure was reached (although some post-peak strain occurred before the sudden loss of stiffness); the load stabilized at approximately 2.7 MPa.

The specimen was removed from the test chamber and showed a single permanent overall buckle (Figure 32). The strain gauges indicated that the stiffener displacements were dominated by an n=3 mode at collapse, and a single lobe of this mode shape dominated the post-collapse region. Yielding occurred first in the shell, followed by yielding of the stiffener flanges just before collapse.

The cylinder was tested again to determine its residual strength after initial buckling. The maximum pressure resisted was 3.64 MPa, and the final shape of the specimen was similar to the shape after the collapse test (single global lobe), but was enlarged, especially in the axial direction (Figure 32).



DATE NAME NOTES

8 Dec 2006 Trial 1 Elastic cycle (3.4 MPa) to check instrumentation. Some electronic noise in circumferen-tial gauges (approximately 20 με ‘jumps’ in strain) on the outside of the shell at 30° and 270°. Noise problem did not occur at full pre-pressure.

8 Dec 2006 Trial 2 Collapse test. External pressure was applied gradually until collapse of the specimen. Instantaneous loss of applied pressure occurred shortly after the peak load was reached. The specimen was then able to resist a reduced pressure before the applied load was removed.

8 Dec 2006 Trial 3 Residual strength test. External pressure was applied gradually up to the initial load peak, after which the applied pressure dropped gradually as additional fluid was released from the specimen. Test was stopped when further fluid release from the specimen did not result in an increase in strain.

Figure 32. Specimen L510-No5 after collapse (left) and residual strength (right) tests

4.5 L510-No7

Specimen L510-No7 underwent a complete loading cycle in the elastic range and was then loaded to collapse (Table 6), failing at a pressure of 7.07 MPa. The applied pressure dropped suddenly after the maximum pressure was reached (although significant post-peak strain occurred before the sudden loss of stiffness); the load stabilized at approximately 4.6 MPa.



DATE NAME NOTES


13 Dec 2006 Trial 2 Collapse test. External pressure was applied gradually until collapse of the specimen. Instantaneous loss of applied pressure occurred shortly after the peak load was reached. The specimen was then able to resist a reduced pressure before the applied load was removed.

13 Dec 2006 Trial 3 Residual strength test. External pressure was applied gradually up to the initial load peak, after which the applied pressure dropped gradually as additional fluid was released from the specimen. Test was stopped when further fluid release from the specimen did not result in an increase in strain.

The specimen was removed from the test chamber and showed a single permanent global buckle (i.e. both shell and stiffeners were deformed) centred at one side of the corrosion patch (Figure 33). The corrosion patch itself showed additional local deformation in the form of a half sine wave in both directions.


The strain gauges indicated that the stiffener displacements were dominated approximately equally by modes 3≤n≤6, at pressures up to and including the collapse pressure. Yielding occurred first at the shell at the centre of the corrosion patch, followed by yielding of the shell at the base of the stiffeners adjacent to the corrosion. The stiffener flanges did not yield until after collapse. It is possible that the large


deformation of the corroded shell caused lateral rotation of the stiffeners, which led to stiffener tripping and ultimate failure of the cylinder.

The cylinder was tested again to determine its residual strength after initial buckling. The maximum pressure resisted was 4.75 MPa, and the final shape of the specimen was similar to L510-No5, i.e. a single global lobe over the length of the cylinder, approximately centred on the corrosion patch (Figure 33).

4.6 L510-No9

Specimen L510-No9 was loaded to collapse after an initial elastic loading cycle (Table 7), and failed at a pressure of 7.68 MPa. The applied pressure dropped suddenly after the maximum pressure was reached (although significant post-peak strain occurred before the sudden loss of stiffness); the load stabilized at approximately 3.4 MPa.


DATE NAME NOTES


19 Dec 2006 Trial 2 Collapse test. External pressure was applied gradually until collapse of the specimen. Instantaneous loss of applied pressure occurred shortly after peak load was reached. The specimen was then able to resist a reduced pressure before the applied load was removed.

19 Dec 2006 Trial 3 Residual strength test. External pressure was applied gradually up to the initial load peak, after which the applied pressure dropped gradually as additional fluid was released from the specimen. Test was stopped when further fluid release from the specimen did not result in an increase in strains. Circumferential gauge on the outside of the shell in the central bay at 15° was not functional, as it was damaged in the collapse test.

The specimen was removed from the test chamber and showed a single permanent global buckle, spanning two stiffeners and their associated bays of shell plating, and located at the edge of the corrosion patch, at approximately 15 (Figure 34). The strain gauges indicated that the stiffener displacements were dominated approximately equally by modes n=3 and n=4, at applied pressures up to and including the collapse pressure.

Yielding occurred first at the shell at the edges of the corrosion patch (15° and 345°) in the central bay, followed by yielding of the shell at the centre of the corrosion. The stiffener flanges yielded at only a single location (330° on Stiffener #4), immediately preceding collapse.

The cylinder was tested again to determine its residual strength, which was found to be 3.97 MPa. The final shape of the specimen was similar to L510-No7, i.e. a single global lobe over the length of the cylinder, centred on the edge of the corrosion patch (Figure 34).



4.7 L510-No11

Specimen L510-No11 was loaded to collapse after an initial elastic loading cycle (Table 8), failing at a pressure of 7.58 MPa. The applied pressure dropped suddenly after the maximum pressure was reached (although some post-peak strain occurred before the sudden loss of stiffness); the load stabilized at approximately 3.7 MPa. The specimen was removed from the test chamber and showed a single permanent global buckle (Figure 35), spanning the central bay and one adjacent bay, located at the edge of the corrosion patch (approximately 15°).


DATE NAME NOTES

3 Jan 2007 Trial 1 Elastic cycle (2.8 MPa) to check instrumentation.

3 Jan 2007 Trial 2 Collapse test. External pressure was applied gradually until collapse of the specimen. Instantaneous loss of applied pressure occurred shortly after peak load was reached. The specimen was then able to resist a reduced pressure before the applied load was removed. The load-strain relationship at pressures approaching the collapse pressure was apparently altered by changes in the rate of fluid release from the specimen, which may be related to deformation of the steel piping.

The strain gauges indicated that the stiffener displacements were dominated by an n=3 mode at pressures up to and including the collapse pressure. Yielding occurred


first at the shell at the edges of the corrosion patch (15° and 345°) in the central bay, followed by yielding of the shell at 0° at mid-bay in the corroded bays. The stiffener flanges did not yield until after collapse.

This cylinder was not tested to determine its residual strength, as it was decided that the initial collapse shape should be preserved.

Figure 35. Specimen L510-No11 after collapse test


5. Discussion

The collapse and residual strength pressures for Phase 2 specimens are summarized in Table 9.

Table 9. Summary of collapse and residual strength pressures for Phase 2 specimens

Specimen Description Collapse Pressure1 (MPa)

Residual Strength1 (MPa)

L510-Test Test cylinder with no stiffeners 7.96 4.16

L300-No1 Short, externally stiffened cylinder (no corrosion)

7.11 7.15

L510-No3 Long, externally stiffened cylinder with ‘dog-bone’ corrosion on central flanges

9.25 N/A

L510-No5 Long, internally stiffened cylinder (no corrosion)

9.08 3.64

L510-No7 Long, internally stiffened cylinder with 42x42x0.6mm corrosion patch on shell

7.07 4.75


7.68 3.97


7.58 N/A

1. Net pressure on specimen, accurate to within ±0.09 MPa.

The cylinder without stiffeners, L510-Test, failed by elastic buckling in an overall n=4 collapse mode. The shape of the specimen changed to one dominated by an n=3 mode during residual strength testing (i.e. in the post-collapse region). Nonlinear finite element analysis of this specimen before testing predicted the collapse pressure within 3%, which is better than most previous predictions for stiffened cylinders using the same methods. This may indicate that material modeling (especially failure criteria) requires more investigation, since the numerical method was quite accurate for this non-yield-initiated collapse.

5.1 Effect of Corrosion Damage

The short cylinder (L300-No1) failed by interframe collapse at a pressure 10% lower than the nominally identical specimen from Phase 1, L300-No2 (Table 10). The mean shell thickness in the central bay of L300-No1 was slightly less (1.5%) than that of L300-No2. This may partially account for the discrepancy in collapse pressures; however, other factors, such as variability in material strength and the level of OOC, are likely involved. The collapse pressures of the undamaged specimens were both


greater than the companion specimens with corrosion (L300-No3 and L300-No4 from Phase 1).

Table 10. Summary of collapse pressures for Phase 1 specimens

Specimen Description Collapse Pressure1 (MPa)

L300-No2 Short, externally stiffened cylinder (no corrosion)

7.87

L300-No3 Short, externally stiffened cylinder with 34x34x0.625mm corrosion patch on shell

6.77

L300-No4 Short, externally stiffened cylinder with 34x34x0.625mm corrosion patch on shell

6.94

L510-No1 Long, externally stiffened cylinder (no corrosion)

9.05

L510-No2 Long, externally stiffened cylinder with ‘dog-bone’ corrosion on central flanges

8.59

1. Net pressure on specimen, accurate to within ±0.04 MPa.

The cylinder with ‘dog-bone’ corrosion (L510-No3) failed in a global collapse mode, with a single lobe of the pre-collapse n=3 mode shape dominating the post-collapse shape. This specimen had a collapse pressure that was almost 8% greater than its companion corroded specimen from Phase 1 (L510-No2), and more significantly, was over 2% stronger than a similar specimen with no corrosion (L510-No1). Cylinder L510-No3 was on average 2% thinner in the central shell bay, and had greater geometric imperfections, than the undamaged cylinder L510-No1.

Specimen L510-No1 was tested to failure after it had been loaded past its yield limit in a previous aborted test. This may have negatively impacted its collapse strength and is a possible explanation for the greater strength of specimen L510-No3. A second long cylinder without corrosion will be tested in the future, and may provide some added insight.

The long, internally stiffened cylinder without corrosion (L510-No5) failed in an overall n=3 collapse mode. The corroded cylinders L510-No7, L510-No9, and L510-No11 failed at collapse pressures 22.2%, 15.4%, 16.6% less than the undamaged cylinder L510-No5, respectively. All of the long, internally stiffened specimens with shell corrosion failed at or near the thinned shell in a single global collapse lobe (i.e. there was significant deformation of both the shell and stiffener, but a full overall collapse shape about the circumference was not apparent).

Several observations lead to the conclusion that both corrosion depth and the eccentricity effects of one-sided shell thinning are more significant than the actual volume of material lost:


• The two cylinders with the largest areas of corrosion (L510-No9 and L510-No11) showed buckling failures at the edge, rather than the centre, of the corrosion patch. Similar local deformations in the thinned area were observed in the short cylinders with shell corrosion from Phase 1 (L300-No3 and L300-No4).

• The volume of multi-bay shell corrosion in L510-No9 was double that of L510-No11, but the collapse pressure was only 1.3% less.

• A small patch of corrosion, 0.6 mm in depth, on the shell between stiffeners (L510-No7) was found to be more detrimental to strength than larger patches, 0.4 mm in depth, spanning two or more bays of shell plating (L510-No9 and L510-No11). This suggests that corrosion depth is more important than total volume of corroded material when considering loss of pressure hull strength.

5.2 Assessment of pressure testing apparatus

The new pressure testing setup was successfully implemented. It allowed the amount of testing fluid leaving the specimen, and thus the applied load, to be controlled up to the collapse of the cylinder. Precise control of the applied pressure in the post-collapse region was more difficult to achieve, especially for specimens that failed in a global collapse mode. The instantaneous drop in pressure after the ultimate load was reached for these specimens was likely due to the general loss of stiffness associated with global buckling. Shell failures allow the transfer of load to the adjacent stiffeners and thus a more stable post-collapse behaviour, as was observed for specimen L300-No1.

The new test apparatus eliminated the large, undesired post-collapse deformations that occurred in Phase 1 testing [1]. The collapse shape after initial collapse testing was preserved in all cases, and final plastic deformations after unloading were small enough such that the collapse mode could be measured with a chord gauge. Testing of the residual strength of the cylinders was possible, with tracking of several load peaks and valleys in the post-collapse region, which wouldn’t have been possible with the original test setup.

The improved data acquisition system allowed real-time visualization of the strain-pressure data, and tracking of out-of-circularity during testing. This further aided in precisely controlling the applied load.


6. Recommendations

Initial analysis of the experimental data suggests that the role of eccentricity due to one-sided shell thinning can have an equal or greater strength-reducing effect than the thinning itself. Further analysis of the experimental data, including comparison with design stresses and collapse pressures, will be performed and presented in a technical memorandum at a later date. Nonlinear finite element analyses of the specimens should also be undertaken.

The new test setup was implemented successfully, and it is recommended that this method be used for future testing. The method could be improved by mounting the transducer that measures pressure in the specimen directly on the cylinder end-cap. This would eliminate the uncertainty associated with time lag in the load-strain data due to impeded fluid flow from the cylinder. This would require a pressure transducer capable of withstanding high pressure loads on its protective casing.


7. References

1. MacKay, J.R. (2007). Experimental investigation of the strength of damaged pres-sure hulls – Phase 1. (DRDC Atlantic TM 2006-304). Defence R&D Canada – At-lantic.


List of Symbols, Acronyms and Initialisms

Symbols

n buckling mode or out-of-circularity circumferential wave number

Acronyms and Initialisms

DND Department of National Defence

DRDC Defence Research and Development Canada

OOC out-of-circularity

m metre

mm millimetre

MPa megapascal


DCD03 2/06/87-M/ DREA mod. 17 Dec 1997

DOCUMENT CONTROL DATA (Security classification of title, body of abstract and indexing annotation must be entered when the overall document is classified)

1. ORIGINATOR (the name and address of the organization preparing the document. Organizations for whom the document was prepared, e.g. Establishment sponsoring a contractor's report, or tasking agency, are entered in section 8.)

Defence R&D Canada – Atlantic PO Box 1012 Dartmouth, NS, Canada B2Y 3Z7

2. SECURITY CLASSIFICATION (overall security classification of the document

including special warning terms if applicable).

UNCLASSIFIED

3. TITLE (the complete document title as indicated on the title page. Its classification should be indicated by the appropriate abbreviation (S,C,R or U) in parentheses after the title).

Experimental investigation of the strength of damaged pressure hulls – Phase 2

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MacKay, J. R.

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February 2007

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Technical Memorandum

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Defence R&D Canada – Atlantic PO Box 1012 Dartmouth, NS, Canada B2Y 3Z7

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DRDC Atlantic TM 2007-013

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DCD03 2/06/87-M/ DREA mod. 17 Dec 1997

13. ABSTRACT (a brief and factual summary of the document. It may also appear elsewhere in the body of the document itself. It is highly desirable that the abstract of classified documents be unclassified. Each paragraph of the abstract shall begin with an indication of the security classification of the information in the paragraph (unless the document itself is unclassified) represented as (S), (C), (R), or (U). It is not necessary to include here abstracts in both official languages unless the text is bilingual).

This document presents a summary of results for Phase 2 of an experimental program to determine the collapse pressures of small-scale pressure hulls (ring-stiffened cylinders) with various amounts and patterns of simulated material loss due to corrosion. Six ring-stiffened aluminium cylinders were tested in the period from November 2006 to January 2007: two undamaged specimens, one specimen with corrosion damage to the stiffeners, and three specimens with various levels of shell corrosion. Initial analysis of the experimental data suggests eccentricity due to one-sided shell thinning has a strength-reducing effect that is in addition to the thinning itself. Further analysis of load-strain data will be performed and presented in a technical memorandum at a later date. An additional aluminium cylinder without stiffeners was fabricated in order to analyze a new experimental apparatus, which was recommended after the completion of Phase 1 testing [1]. The new test setup was implemented successfully, and was found to be suitable for the collapse testing of pressure hulls.

14. KEYWORDS, DESCRIPTORS or IDENTIFIERS (technically meaningful terms or short phrases that characterize a document and could be helpful in cataloguing the document. They should be selected so that no security classification is required. Identifiers, such as equipment model designation, trade name, military project code name, geographic location may also be included. If possible keywords should be selected from a published thesaurus. e.g. Thesaurus of Engineering and Scientific Terms (TEST) and that thesaurus-identified. If it not possible to select indexing terms which are Unclassified, the classification of each should be indicated as with the title). pressure hull ring-stiffened cylinder submarine collapse pressure corrosion structural analysis


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