PSI - Issue 7

Kentaro Wada et al. / Procedia Structural Integrity 7 (2017) 391–398 K. Wada et al./ Structural Integrity Procedia 00 (2017) 000–000

397

7

3.3. Discussion As shown in Fig. 2, the fatigue limit of SAE52100 steel was markedly increased at the R of −10. The fatigue limit of the specimen was at first determined at the crack initiation limits of R = − 5 , −3 and − 1, but was subsequently determined at the crack non-propagation limit of R = −10. The mechanism for enabling the presence of a non propagating crack at R = − 10 is discussed in this section. Crack initiation can be facilitated by compressive yielding ahead of the notch root. A greater amount of compressive yielding occurs ahead of the notch root, particularly at lower stress ratios, resulting in the potential generation of a tensile residual stress field. Pippan (1987) suggested that, in a compression-compression fatigue test, the tensile residual stress field ahead of the notch root can assist FCG within the distance, ω , as explained by the following equation:

2

π 8

K

(6)

=

ω

2

σ

y

where, K is the stress intensity factor calculated for the notch and σ y is the yield stress. According to Pippan, once the crack tip exceeds the plastic zone via the tensile residual stress field, the crack loses its driving force and can no longer propagate under cyclic compression. Table 1 outlines a comparison between the length of non-propagating cracks measured from the notch root, a − a init , and the yielding zone size, ω , as translated by Eq. (6), where σ y = 2161 MPa was used. The value of K was estimated by the following equation:

0.65 σ = ⋅

π

K

area

(7)

min

where, σ min is the minimum stress. The value of ω was 56 μm at R = −10 , some eight times larger than the value obtained at R = −5. It should be noted that, during this study, the loading cycle was not compression-compression, but included a tension element which may also have acted as a driving force. As demonstrated in Figs. 5 and 6, during the FCG process, crack closure is developed, which can act as a resistance force against crack growth. It was presumed that the crack closure was mainly caused by roughness or plastic deformation, whereas further investigations is needed to elucidate the mechanism. In addition, a compressive residual stress field beyond the tensile stress field may also contribute to crack-arrest. Accordingly, the propagation or non propagation of the crack was determined as a result of competition between the driving and resistance forces. Namely, at higher stress ratios ( e.g. R = −5, −3 and −1), the resistance force could not develop stro ngly enough to arrest crack growth and, in consequence, propagation of the crack could not be stopped, once initiated. On the other hand, at lower stress ratios ( e.g. R = −10), crack closure was facilitated, overcoming the driving force, thus enabling the non propagation of cracks.

Table 1: Comparison of the compressive yielding zone size, ω , as given by Eq. (6), with the crack length measured from the notch root, a – a init . R

2 a init ( μ m)

a − a init ( μ m)

σ min (MPa)

ω ( μ m)

− 5

300 300

− 585

7

0

− 10

− 1636

56

118