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Research Papers: Materials Technology

The Sensitivity of Overall Collapse of Damaged Submarine Pressure Hulls to Material Strength

[+] Author and Article Information
John R. MacKay

Defence R&D Canada—Atlantic,
9 Grove Street, P.O. Box 1012,
Dartmouth, Nova Scotia, B2Y 3Z7,
Canada
e-mail: john.mackay@drdc-rddc.gc.ca

Fred van Keulen

Faculty of Mechanical, Maritime and Materials Engineering,
Delft University of Technology,
Mekelweg 2, 2628 CD Delft,
The Netherlands
e-mail: a.vankeulen@tudelft.nl

Contributed by the Ocean Offshore and Arctic Engineering Division of ASME for publication in the JOURNAL OF OFFSHORE MECHANICS AND ARCTIC ENGINEERING. Manuscript received July 18, 2011; final manuscript received April 3, 2012; published online February 25, 2013. Assoc. Editor: Pingsha Dong.

J. Offshore Mech. Arct. Eng 135(2), 021403 (Feb 25, 2013) (9 pages) Paper No: OMAE-11-1066; doi: 10.1115/1.4007051 History: Received July 18, 2011; Revised April 03, 2012

Budget and schedule restrictions sometimes require naval submarines to be operated with unrepaired corrosion damage to the pressure hull. It is important to understand the effects of corrosion wastage on the structural capacity of the hull, so that appropriate diving depth restrictions can be imposed if necessary. The current paper presents an experimental study of the interaction of material behavior with corrosion defects, especially with respect to their effect on overall elasto-plastic collapse of pressure hulls. Twenty ring-stiffened cylinders, representative of submarine pressure hulls failing by overall collapse, were machined from high- and low-grade aluminum alloy tubes. Artificial general corrosion damage was introduced in selected specimens by machining away material from the outside of the cylinder shell in rectangular patches of uniform depth. The cylinders were monotonically loaded to collapse under external hydrostatic pressure. One corroded cylinder was repeatedly loaded past the yield limit before the collapse test in order to study the effect of cyclic plastic loading on its ultimate collapse strength. Overall collapse pressures for corroded cylinders with a variety of patch sizes and depths and material strengths were reduced by, on average, 0.85 times the depth of thinning divided by the original shell thickness. The collapse strength of corroded cylinders was found to be more sensitive to the shape of the stress-strain curve than for intact specimens. Higher levels of strain hardening and ductility were found to improve the performance of damaged cylinders. Permanent deformations in the cyclically loaded cylinder, as measured with strain gauges, grew with each constant-amplitude load cycle; however, the additional deformations tended towards zero with increasing number of cycles, and a subsequent collapse test indicated that the cyclic loading did not affect the collapse pressure. The sensitivity of overall collapse to material strength is related to not only the yield stress, but also the plastic reserve of the material; higher levels of strain hardening and ductility increase overall collapse strength of hulls, especially those with general corrosion damage. The effect of a given level of corrosion thinning is less severe for cylinders with relatively greater levels of strain hardening. It is unlikely that cyclic plastic loading of corroded hulls will lead to premature collapse at a load level below the monotonic collapse pressure.

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Figures

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Fig. 1

Photographs of typical test specimens. Clockwise from middle-left: L510-No10A, with patch C general corrosion, after collapse testing; L510-No12A, with patch D general corrosion, before testing; the intact specimen L510-No6A, showing the internal ring-stiffeners and strain gauge wiring, before testing; and L300-No7A, with patch A corrosion damage, before testing.

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Fig. 2

Nominal axisymmetric geometry of L510 series of test specimens. All dimensions are in millimeters.

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Fig. 3

Measured out-of-circularity and shell thickness at the center of the central bay of L300-No6A

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Fig. 4

Average measured engineering stress-strain curves for tensile coupons taken from the axial and circumferential directions of an AA-6082-F28 cylinder

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Fig. 5

Typical engineering stress-strain curves for aluminum alloys and a high-strength naval quality steel, showing measured data for coupons taken from the circumferential direction of aluminum test cylinders, and the average curve for coupons taken from the rolling and transverse directions of a Q1N steel plate

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Fig. 6

Measured pressure-strain curves for strain gauges near the collapse sites of geometrically identical intact specimens machined from AA-6082-T6 (L300-No6) and AA-6082-F28 (L300-No6A); also showing photographs of the specimens after collapse testing

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Fig. 7

Measured pressure-strain curves for strain gauges near the collapse sites of geometrically identical corroded specimens (patch A) machined from AA-6082-T6 (L300-No7) and AA-6082-F28 (L300-No7A); also showing photographs of the specimens after collapse testing

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Fig. 8

Measured pressure-strain curves for strain gauges near the corrosion patch and the collapse site of specimen L510-No8A; also showing photographs of the specimen after collapse testing

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Fig. 9

Incremental residual circumferential strain at the outside of the corroded shell after each load cycle that was applied to test specimen L300 No8A; also showing the maximum applied pressure load for each cycle

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Fig. 10

Collapse pressure as a function of circumferential yield stress for cylinders machined from high- and low-grade aluminum

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Fig. 11

Normalized collapse pressure, Pc *, taken as the collapse pressure divided by the boiler pressure, as a function of the magnitude of corrosion thinning, δc, for cylinders machined from high- and low-grade aluminum

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Fig. 12

Corrosion knockdown factor, λc, as a function of the magnitude of shell thinning, δc, for cylinders machined from high- and low-grade aluminum. The experimental curve labeled “All Data” is based on results for all T6 and F28 cylinders in Tables 1 and 2, respectively, except for L510-No8A and L300-No8A.

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Fig. 13

Normalized engineering stress-strain curves for aluminum alloys and a high-strength naval quality steel. The measured engineering stress and strain data have been normalized by dividing by the yield stress and yield strain of the material, respectively.

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