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

2. Background Bar is made of 316L Stainless Steel.
Insert Pic of bar here
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)

3. 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

4. Analysis Supporting evidence of a solid state reaction
Change in Chemical Composition
Possible change in material properties

5. Analysis Continued Evidence of Torsion
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

6. Analysis Continued 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

7. Analysis Continued We modeled our void as a notch in a cylindrical bar
Estimated number of cycles for crack propagation failure: 9, ,945 cycles using the max torsional load as an axial load
Bar lasted between 57, ,000 cycles
Pure torsion decreases crack growth rate
"Crystal" void may have been growing due to heating/cooling cycles

8. 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

9. 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

10. Appendix

11. Appendix  Continued

12. Appendix  Continued

13. 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” <

14. 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 September, <

15. Appendix  Continued
“Comparative study on the fatigue crack growth behavior of
316L and 316LN stainless steels: e€ect of microstructure of
cyclic plastic strain zone at crack tip.” Journal of Nuclear Materials. Volume 282, Issue 1, November 2000, Pages <

16. Appendix Continued References
Vander, Voort George F. Metallography and Microstructures. Materials
Park, OH: ASM International, Print.
Powell, Gordon W. Failure Analysis and Prevention. [Metals Park,
OH]: ASM International, Print.