PSI - Issue 13

Shigeru Hamada et al. / Procedia Structural Integrity 13 (2018) 1026–1031 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

1029

4

 xy 㻲㼍 㼠㼕㼓 㼡㼑㻌㼏㼞㼍㼏㼗

㻹㼕㼏㼞㼛㼙 㼑㼏㼔㼍㼚 㼕㼟㼙

 xy

(grain) Boundary

㻹㼛㼐㼑㻌㻵 㻵 㼘㼛㼍㼐 㼕㼚 㼓

 xy

 xy

The weakest crystal orientation area for the crack initiation

Fig. 3. Fatigue crack propagation and its micromechanism for Mode II loading.

2.3. Difference between phenomena and their application The greatest difference between CSFCG and DAFCP lies in whether the same phenomenon occurs for each cyclic loading. That is, in CSFCG, one fatigue crack growth occurs with one loading cycle. However, in DAFCP, the crack propagates once owing to damage accumulation by multiple loading cycles. Therefore, in DAFCP, we cannot use the concept of d a /d N , which has been used earlier to represent fatigue crack propagation. Table 1 presents the results of various considerations for CSFCG and DAFCP.

Table 1. Comparison between CSFCG and DAFCP. “?” means unconfident. CSFCG

DAFCP

d a /d N concept

Valid

Invalid Intermittent propagation by damage accumulation Invalid (?) Damage accumulation is related to Δ K (?)

Growth in every cycle

Δ K concept

Valid Dislocation emission is related to Δ K

PICC concept Ductile striation

Valid

Invalid

Formed

Not formed

RICC (RISS) concept

Valid

Invalid

(hypothesis)

(hypothesis)

Work hardening effect

Positive

Negative Some “ d amage” is accumulated

Hardness improvement

Mean compressive stress effect

Positive

Neutral

Δ K eff decreases

Crack initiation is not affected Strongly induces propagation (damage localization?)

Texture

Not affected strongly

3. Phenomena under Mode II loading in literature

As mentioned above, even if Mode II loading is applied, the same phenomenon does not necessarily occur. Figure 4 shows the fatigue crack morphology reported in literature (Otsuka et al. 1975, Doquet and Bertolino 2008, Gates and Fatemi 2016) and mechanisms that occurred when applying Mode II loading. Application of cyclic shear loading, i.e., Mode II loading, to the crack, is equivalent to applying tension compression depending on the viewing direction by stress conversion as shown in Fig. 4. Here, as a crack growth mechanism, dislocation emission from the crack tip and fatigue crack growth occur. If the fatigue crack continues to grow in one direction, it forms a branch crack. However, it may be possible to repeat flexion. If this flexion interval is short, in the macro viewpoint, the crack seems to grow linearly. Figure 5 shows a previously reported fatigue crack morphology different from that in Fig. 4 and a presumed mechanism, which occurred when Mode II loading was applied (Hamada et al. 2018).