Page 156 - Biaxial Multiaxial Fatigue and Fracture

The Influence of Static Mean Stresses Applied Normal to the Maximum Shear Planes in ... 141

CONCLUSIONS

For a given fatigue life, data from this test program exhibit an inverse linear relationship between
the alternating shear stress and the static mean stress normal to the maximum shear stress
amplitude planes for static mean stresses smaller than 500 MPa and 76 MPa for the hard and soft
steels, respectively. Increasing the tensile mean stress beyond these values does not result in a
further decrease in the alternating shear stress for a given fatigue life.
When static mean stresses normal to the plane of maximum alternating shear stress are high,
fracture surface asperities are unmarked. Asperity heights increase with increasing tensile mean
stress until the stress at which the crack faces no longer touch is achieved.
The modified Findley parameter condensed the majority of the experimental results within a
2x band on a parameter versus fatigue life curve. For tensile static mean stresses larger then
Sint, the crack faces were found to be fully separated, a condition previously identified as crack
face interference free growth. This cracking mechanism is incorporated into the modified
Findley parameter by placing a limiting value Sint above which the mean stress value used in
the parameter is kept constant.

ACKNOWLEDGEMENTS

The authors would like to thank NSERC for the funding to complete this program of study and
the Canadian Armed Forces. The authors would also like to thank Dr. Bonnen, Dr. Conle, Mr.
Barber, Mr. Boldt and Mr. Alberts for their contributions to this research.

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