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Research Papers: Gas Turbines: Ceramics

Effects of Target Size on Foreign Object Damage in Gas-Turbine Grade Silicon Nitrides by Steel Ball Projectiles

[+] Author and Article Information
Sung R. Choi1

 Naval Air Systems Command, Patuxent River, MD 20670sung.choi1@navy.mil

Zsolt Rácz

 FACC AG, Ried im Innkreis, Austria A-4910

1

Corresponding author.

J. Eng. Gas Turbines Power 134(5), 051301 (Mar 05, 2012) (8 pages) doi:10.1115/1.4004738 History: Received June 22, 2011; Revised June 23, 2011; Published March 05, 2012; Online March 05, 2012

Foreign object damage (FOD) phenomena of two gas-turbine grade silicon nitrides (AS800 and SN282) were assessed at ambient temperature applying impact velocities from 20 to 300 m/s using 1.59-mm diameter hardened steel ball projectiles. Targets in a flexural configuration with two different sizes (thicknesses) of 1 and 2 mm were ballistic-impacted under a fully supported condition. The severity of impact damage, as well as the degree of post-impact strength degradation, increased with increasing impact velocity, increased with decreasing target size, and was greater in SN282 than in AS800 silicon nitride. The critical impact velocity where targets fractured catastrophically decreased with decreasing target size and was lower in SN282 than in AS800. Overall, FOD by steel projectiles was significantly less than that by silicon-nitride ceramic counterparts, due to much decreased Hertzian contact stresses. A correlation of backside cracking velocity versus target size was made based on a simplified elastic foundation analysis.

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Copyright © 2012 by American Society of Mechanical Engineers
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Figures

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Figure 1

Microstructures of (a) AS800 and (b) SN282 silicon nitrides used in this work

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Figure 2

Post-impact flexural strength (σf ) as a function of impact velocity (V) for (a) AS800 and (b) SN282 silicon nitrides with different target thicknesses, impacted by 1.59 mm-diameter hardened steel ball projectiles at ambient temperature. The data for a target thickness of t = 3 mm, determined previously [25], are included for comparison. The “AsR” represents as-received strength with no impact.

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Figure 3

Critical impact velocity (Vc ) as a function of target specimen thickness (t) for AS800 and SN282 silicon nitrides impacted by 1.59 mm-diameter hardened steel ball projectiles at ambient temperature. The data for t = 3 mm [25] were included for comparison.

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Figure 4

Probability of catastrophic failure upon impact as a function of impact velocity for different target thicknesses for AS800 and SN282 silicon nitrides impacted by 1.59 mm-diameter hardened steel ball projectiles at ambient temperature. The data for t = 3 mm [25] were included for comparison.

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Figure 5

Weibull plots of post-impact strengths (σf ) of (a) AS800 and (b) SN282 silicon nitrides for different target thicknesses, impacted by 1.59 mm-diameter hardened steel ball projectiles at ambient temperature. The data for t = 3 mm [25] were included for comparison. P = Failure probability; “CF”: Catastrophic failure.

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Figure 6

Overall shape (a) and fracture surface (b) of a hardened steel ball projectile split into two upon impact on an AS800 target with t = 2 mm at 250 m/s, showing an evidence of tensile failure from the impact site

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Figure 7

Typical appearances of backside cracking occurring in (a) AS800 and (b) SN282 targets with a thickness of 1 mm, impacted at 40 m/s (above) and 65 m/s (below) by 1.59 mm-diameter hardened steel ball projectiles at ambient temperature. Values of corresponding post-impact strength are also given for each of targets.

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Figure 8

Typical fracture surfaces of target specimens impacted at 200 m/s by 1.59 mm-diameter hardened steel ball projectiles at ambient temperature: (a) AS800; (b) SN282. A backside crack is outlined as a dotted line in (b) SN282.

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Figure 9

Comparison in post-impact strength between hardened steel ball projectiles (this work) and silicon nitride ball projectiles [30]. The size of projectiles was 1.59 mm diameter for both cases.

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Figure 10

A schematic of elastic foundation considered in the analysis. A target fully supported on elastic foundation is subject to impact force F by an impacting projectile.

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Figure 11

Predicted backside cracking velocity (Vbc ) as a function of target thickness (t) for AS800 and SN282 silicon nitrides impacted by 1.59 mm-diameter hardened steel ball projectiles at ambient temperature. Experimental data for AS800 and SN282 are included.

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