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RESEARCH PAPERS

Thermomechanically Controlled Erosion in Army Cannons: A Review

[+] Author and Article Information
John H. Underwood

US Army Armament Research, Development and Engineering Center Benét Laboratories, Bldg 115, Watervliet, NY 12189junder@pica.army.mil

Gregory N. Vigilante, Christopher P. Mulligan, Mark E. Todaro

US Army Armament Research, Development and Engineering Center Benét Laboratories, Bldg 115, Watervliet, NY 12189

J. Pressure Vessel Technol 128(2), 168-172 (Jan 11, 2006) (5 pages) doi:10.1115/1.2175022 History: Received November 15, 2005; Revised January 11, 2006

Metallographic characterization is presented of thermal damage of Cr-coated steel in a fired cannon; Cr and Ta coated steel in a vented-erosion-simulator; and bulk Si3N4 in laser heating. Common features of rapid crack-induced erosion are noted. (i) Cracks form normal to the surface, often permanently open, indicating tensile stress was present at some point during thermal damage. (ii) Softening of Cr and Ta coatings and Si3N4 occurs near the heated surface, verified by metallography and hot hardness. The transformation of steel beneath the coatings is used as an in-situ verification of temperatures that cause thermal damage. (iii) Crack-induced under-cutting of thermal-damage islands is observed for coatings and bulk Si3N4. A thermomechanical analysis of rapid crack-induced erosion observed in severe cannon firing and firing simulation suggests the following key failure mechanisms common to metals and Si3N4. (i) High near-bore transient temperatures increase thermal expansion compression and concurrently decrease the elevated temperature strength. For metals, the thermal compression stress greatly exceeds strength, to depths of about 0.5mm. (ii) Thermal stress exceeding strength produces compressive yielding, which, upon cooling, causes tensile residual stress and cracking. The near-bore residual tension is high enough to cause one-cycle cracking of both Cr and Ta coatings; hydrogen from combustion enters via the cracks and causes cracking in the steel below the coating. For Si3N4, cracks are encouraged by the low fracture toughness of Si3N4. (iii) Repeated thermal cycles deepen and widen cracks to form islands that can be undercut, leading to island removal and rapid erosion failure of the cannon. For Cr and Ta coatings, undercutting is by hydrogen cracking in the steel and degradation of the coating interface by combustion products that enter via the cracks. For Si3N4, undercutting is by direct thermomechanical cracking.

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Copyright © 2006 by American Society of Mechanical Engineers
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Figures

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Figure 1

Metallographic section of the electroplated Cr-on-steel 120mm tube following 118 cannon firings

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Figure 2

Metallographic section of the electroplated Cr-on-steel sample following 100 VES firings

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Figure 3

Metallographic section of the sputtered Ta-on-Cr-on-steel sample following 100 VES firings

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Figure 4

Metallographic section of bulk Si3N4 sample following a single laser heating cycle

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Figure 5

Transient surface temperature for cannon firing and three material/ firing simulation conditions

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Figure 6

Peak temperature versus depth for cannon firing and three material/ firing simulation conditions

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Figure 7

Compressive stress versus strength for Cr on steel in cannon and VES firing

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Figure 8

Compressive stress versus strength for Ta on steel in VES firing and bulk Si3N4 in laser heating

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Figure 9

Shear failure mechanism of a crack-formed island on a thermally damaged surface

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Figure 10

Transient thermal shear stress at the base of an island upon subsequent firing; L∕h=3

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