High Cycle Fatigue Strength of Spring Steel with Small Scratches

Fatigue strength of spring steel with small scratches

“…Here, σ w0 is in MPa and HV is in kgf/mm 2 . By substituting HV = 533 into Equation , σ w0 = 1.6 × 533 = 853 MPa is obtained, which is substantially the same value of σ w0 = 850 MPa determined by the previous experiment (Figure ). The relationship of Equation holds when fatigue limit is determined by the threshold condition for the propagation of a crack nucleated from slip bands on the material surface, and Equation is usually applicable for HV < 400.…”

“…Thereafter, these cracks survived as a mode I nonpropagating crack at the root of scratch for the subsequent vast number of cycles until N = 10 8 cycles. This result indicates that torsional fatigue limit is governed by the threshold condition for the propagation of a mode I crack, which is the same condition as observed for uniaxial fatigue limit of the same material with a circumferential notch …”

“…A previous study showed that uniaxial fatigue limit of a smooth specimen, σ w0 , of SWOSC‐V can be predicted by the following equation: …”

“…Figure shows the variation of the stress amplitude, τ a , as a function of the size of 0° scratches represented by or t , in which the prediction by Equations and is shown by a solid line together with the data of uniaxial fatigue limit of specimens with circumferential notches exposed to rotating bending . It is noted that the actual notch depths measured at the deepest point of scratches were used for t to plot the experimental data.…”

“…The residual stress measured at the surface in the axial direction using X‐ray diffraction was within ± 20 MPa. An average value of the micro‐Vickers hardness, HV , measured at the cross and the longitudinal sections of a specimen was 533 (kgf/mm 2 ), and anisotropy of hardness was not observed . To quantitatively examine the effects of a long, shallow defect present on the surface of spring wire, by using a gadget shown in Figure , a small scratch was introduced into the surface of specimens in different directions: either in the longitudinal direction (0°) or in the oblique direction (45°), as shown in Figure .…”