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Dental Wire Failure Analysis

Dental Wire Failure Analysis. Brad James, Ph.D., P.E. Background. Three-coil dental wire had been in service for approximately two months Orthodontist stated the cables were made of nitinol Cable fractured while eight year old patient was eating Japanese food

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Dental Wire Failure Analysis

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  1. Dental Wire Failure Analysis Brad James, Ph.D., P.E.

  2. Background • Three-coil dental wire had been in service for approximately two months • Orthodontist stated the cables were made of nitinol • Cable fractured while eight year old patient was eating Japanese food • Fortunately, no injury occurred due to fracture

  3. Background

  4. Background

  5. Analysis SEM of as-received wires:

  6. Analysis SEM following cleaning:

  7. Analysis Wire Number 1:

  8. Analysis Wire Number 1:

  9. Analysis Wire Number 2:

  10. Analysis Wire Number 2:

  11. Analysis Wire Number 3:

  12. Nitinol? Don’t think so…..

  13. Use Fracture Mechanics to Estimate Wire Loads • Critical crack length approach: • Given KIC, and critical crack length, use K solution to determine the maximum load • Make sure LEFM applies! (crack length, plastic zone size, etc.) • Striation spacing approach: • Determine da/dN and ΔK from striation spacing near final fracture (need da/dN vs. ΔK for material and environment) • Modify ΔK for any R-ratio effects (e.g. Forman equation) • Determine peak load using appropriate K-solution (need crack length at striation measurement location)

  14. Examples of Striation Spacing as a Function of da/dN From: D. L. Davidson and J. Lankford, “Fatigue Crack Growth in Metals and Alloys: Mechanisms and Micromechanics”, International Materials Reviews, 1002, Vol. 37, No. 2, pp. 45-76

  15. Conclusions • High cycle fatigue failure • Non-zero stress ratio, reversed bending stress condition • Wire was austenitic stainless steel (probably 304), not nitinol! • Haven’t gotten around to determining maximum load on wire yet….

  16. New Wires, All Better!

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