Failure Analysis of a Hollow Bar TEAM FAIL! Samantha Atmur Timmy Strait Dave Bernick Joe Yoder Alan Korovin.

Failure Analysis of a Hollow BarTEAM FAIL! Samantha AtmurTimmy StraitDave BernickJoe YoderAlan Korovin

Background

• Bar is made of 316L Stainless Steel.• Bar was heat-cleaned at 1200˚C on Silicon

Carbide blocks. (Supposed to be 1200˚F)• Bar is used to rotate (Torque=40 lbf*ft) a material

(Total Force=21.62 lbf) in a vacuum furnace at around 400˚C.

• Bar failed after operating for less than 400 hr. (57,750-60,000 Cycles)

Insert Pic of bar here

Hypothesis

• Heat cleaning at near-melting point (1371 ˚C) Solid state reaction

between SiC block and the bar.

Void creation Changed material

properties

• Failure Cyclic Torsion causing

crack propagation around the void

Decohesive Fracture

Analysis

• Supporting evidence of a solid state reaction

Change in Chemical Composition

Possible change in material properties

Analysis Continued

• Fracture analysis showed stress intensity factor to be much less than the fracture toughness KI << KIC (4 MPa m << 260 MPa m)

• Was not a pure fracture• Evidence of Torsion

• Evidence of a pure shear element (from torsion) The bar fractures at 35 degree angle Crack below fracture at 45 degree

angle  

• Decohesive Fracture Dimple rupture evidence along with

transgranular evidence

Analysis Continued

Analysis Continued• We modeled our void as a notch in

a cylindrical bar

• Estimated number of cycles for crack propagation failure: 9,571 - 16,945 cycles using the max torsional load as an axial load

• Bar lasted between 57,750 -60,000 cycles

• Pure torsion decreases crack growth rate

• "Crystal" void may have been growing due to heating/cooling cycles

Conclusions

• Solid state reaction caused a large void to form and altered material properties

• The cyclic torque applied to the bar lead to crack propagation with the aid of the void acting as a stress concentration

• Once crack reached its critical size the bar experienced a decohesive fracture

Caveats • Stress Concentration Factor approximated with a notched bar• Most of failure surface is covered in tool marks destroying

evidence• Constants for crack propagation from a study using higher

temperatures • Crack propagation analysis was modeled as if void was not

there but using the stress concentration of void• Stress intensity correction factor was taken from aging aircraft

data representing a semicircular crack in a cylindrical tube• Fracture toughness was estimated from 316L annealed steel•  Do not know the effect of heating/cooling cycles and gases

on the "crystal" void

Appendix

Appendix Continued

Appendix Continued

Appendix Continued

Walsh, R.P., Summers, L.T., Miller, J.R. “The 4 K Tensile and Fracture Toughness Properties of a Modified 316LN Conduit Alloy” <http://www.magnet.fsu.edu/library/publications/NHMFL_Publication-4952.pdf>.

Appendix Continued

Chen, C. T., Han, “Stress-Intensity Factor and Fatigue Crack Growth Analyses For Rotorcraft Using AGILE.” The 7th Joint DoD/FAA/NASA Conference on Aging Aircraft. 8-11 September, 2003. <http://airportaircraftsafetyrd.tc.faa.gov/Programs/AgingAircraft/rotorcraft/Publications/SIF%20and%20FCGA%20for%20RC%20Using%20AGILE_CChen.pdf>.

Appendix Continued

“Comparative study on the fatigue crack growth behavior of316L and 316LN stainless steels: e€ect of microstructure ofcyclic plastic strain zone at crack tip.” Journal of Nuclear Materials. Volume 282, Issue 1, November 2000, Pages 32-39. <http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6TXN-41F60C4-2&_user=209810&_coverDate=11%2F30%2F2000&_rdoc=1&_fmt=high&_orig=gateway&_origin=gateway&_sort=d&_docanchor=&view=c&_searchStrId=1728562245&_rerunOrigin=google&_acct=C000014439&_version=1&_urlVersion=0&_userid=209810&md5=0a38baa557ff0725f1f08719ec9b2cde&searchtype=a>.

Appendix Continued

ReferencesVander, Voort George F. Metallography and Microstructures. Materials

Park, OH: ASM International, 2004. Print.

Powell, Gordon W. Failure Analysis and Prevention. [Metals Park,OH]: ASM International, 2007. Print.