Page 148 - Biaxial Multiaxial Fatigue and Fracture

The Influence of Static Mean Stresses Applied Noma1 to the Maximum Shear Planes in _.. 133

For the SAE 1045 steel, BHN 203, as the length of the fatigue crack increased in depth and
surface length, the applied hoop pressure sine wave became pointed. The number of cycles until
failure was taken as the number of cycles until this change in the applied hoop pressure sine
wave was observed.

ANALYSIS OF RESULTS
Fatigue life curves were obtained for various magnitudes of static mean stress applied normal to
the maximum shear planes for the hard and soft SAE 1045 steel. Figures 6 and 7 show that the
fatigue strength decreases with increasing static mean stress until the crack. surfaces are
interference free for the hard (BHN 456) and soft (BHN 203) SAE 1045 steel, respectively. The
fatigue strength remains constant for applied tensile static mean stresses larger than these mean
stress values.

1.00E+02 1.00E+03 1.00E+04 1.00E+05 1.00E+06 1.00E+07
Life (Nf)

A 500 MPa to 740 MPa Mean Stress 0 350 MPa to 375 MPa Mean Stress
0 6 MPa to -35 MPa Mean Stress -120 MPa to -140 MPa Mean Stress
m -290 MPa to -320 MPa Mean Stress
Fig. 6. Fatigue life data for various static mean stresses levels applied normal to the planes of
maximum alternating shear stress for SAE 1045 steel, BHN 456.

Figures 8 and 9 were obtained by constructing best fit curves to the test results in Figs 6 and 7
and taking the value of the alternating shear stress for given fatigue lives from these curves. In
Figs 8 and 9, there is a linear relationship between the maximum alternating shear stress and the
static mean stress on the critical planes for static mean stresses less then Sint. When the tensile
static mean stress exceeds the interference free stress, (Sint) , the alternating shear stress stops
decreasing and remains constant.