nisms to fatigue improvement described above.
The influence of microstructure on the fatigue crack propagation behavior of gas metal arc welds and base metals of 316L and AL6XN austenitic stainless steel has been investigated using conventional fa- tigue testing and constant DK testing pro- cedures. The following conclusions can be drawn from this research:
1) Large grain sizes in both the weld metal and base metal produce a rough fracture surface that leads to improved fa- tigue resistance.
2) The observed improvement in fa- tigue resistance occurs at low stress inten- sity ranges when the plastic zone size is ap- proximately equal to or less than the grain size.
3) The improved fatigue resistance with increasing grain size can be attributed to three main factors: 1) a tortuous crack path that requires formation of a larger surface area for a given length of crack propagation, 2) crack growth out of the Mode I plane, which reduces the stress in- tensity range available for crack growth, and 3) roughness induced closure that shields the crack from part of the applied load.
4) Quantitative estimates of the DK level below which grain size effects are ex- pected to occur are in reasonable agree- ment with the observed experimental re- sults.
funding for this research. The authors would also like to acknowledge Mike Rex, John Gregoris, and Gene Kozma at Lehigh University for assistance with fa- tigue crack propagation sample prepara- tion and testing and Arlan Benscoter for assistance with metallography. The au- thors also gratefully acknowledge Ravi Menon of Stoody Company for prepara- tion of the welds.
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The authors thank the United States Office of Naval Research for providing
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