低周波応力期間を変更した間欠重畳応力条件下の場合

Fig.5.8（a）及びFig.5.12（a）に示した元の応力履歴において，低周波応力期間を増加

させた種々の間欠重畳応力条件を作り，これらの条件で疲労亀裂伝播シミュレーション を実施した．その間欠重畳応力履歴をFig.5.18とFig.5.19に示す．

1 2 3

0.002 0.004 0.006 0.008 0.01 0.012 0.014

Original

Only low frequency stress Only superimposed stress

Enveloped low frequency stress and original low frequency stress

Only enveloped low frequency stress

Number of cycles (low frequency component): N_L (x 10⁵)

Alternating plastic zone size [mm]

169 (a) Original

(b) Lv1 increment of low frequency stress

(c) Lv2 increment of low frequency stress

0 1 2 3 4 5 6 7 8 9

Number of cycles (low frequency component): N

_L

(x 10

⁵

)

S: Superimposed stress L :Low frequency stress

S1 S2 S3

L1 L2 L3 L4 S4 L5 S5 L6 S6

0 1 2 3 4 5 6 7 8 9

Number of cycles (low frequency component): N

_L

(x 10

⁵

)

S: Superimposed stress L :Low frequency stress

S1 S3

L1 L2 L3 L4 S4 L5 S5 L6 S6

0 1 2 3 4 5 6 7 8 9

Number of cycles (low frequency component): N

_L

(x 10

⁵

)

S: Superimposed stress L :Low frequency stress

S1

L1 L2 L3 L4 S4 L5 S5 L6 S6

170

(d) Lv3 increment of low frequency stress

(e) Only low frequency stress

Fig.5.18 Applied loading conditions (ID: S3-3_25_2_1)

(a) Original

0 1 2 3 4 5 6 7 8 9

Number of cycles (low frequency component): N

_L

(x 10

⁵

)

S: Superimposed stress L :Low frequency stress

S1

L1 L2 L3 L4 L5 S5 L6 S6

0 1 2 3 4 5 6 7 8 9

Number of cycles (low frequency component): N

_L

(x 10

⁵

)

S:Superimposed stress L:Low frequency stress

L1 L2 L3 L4 L5 L6

0 1 2 3 4 5

Number of cycles (low frequency component): N

_L

(x 10

⁵

)

S: Superimposed stress L: Low frequency stress

S1 S2 S3

L1 L2 L3

171

(b) Lv1 increment of low frequency stress

(c) Lv2 increment of low frequency stress

(d) Only low frequency stress

Fig.5.19 Applied loading conditions (ID: S3-3_50_2_1)

0 1 2 3 4 5

Number of cycles (low frequency component): N

_L

(x 10

⁵

)

S:Superimposed stress L:Low frequency stress

S1 S3

L1 L2 L3

0 1 2 3 4 5

Number of cycles (low frequency component): N

_L

(x 10

⁵

)

S:Superimposed stress L:Low frequency stress

S1

L1 L2 L3

0 1 2 3 4 5

Number of cycles (low frequency component): N

_L

(x 10

⁵

)

L: Low frequency stress

L1 L2 L3

172

Fig.5.18に示した間欠重畳応力条件下において解析結果をFig.5.20に示す．Fig.5.20で

も低周波成分のサイクル数に対する伝播履歴を示している．想定初期亀裂寸法が短くか つ総サイクル数もさほど長期間でない解析のため，伝播挙動に大きな差異は確認できな いものの亀裂伝播曲線を詳細に眺めると，低周波応力期間の増加に伴い，疲労亀裂伝播 が速くなることが分かる．

(a) Overall view

0 3 6 9

1.5 2.0 2.5

Number of cycles (low frequency component): N_L (x 10⁵)

Half crack length: a [mm]

: Original

: Only low frequency stress : Lv2 of low frequency stress : Lv3 of low frequency stress : Lv1 of low frequency stress

Loading sequence: S3−3_25_2_1

173 (b) Enlarged view

Fig.5.20 Comparison of fatigue crack growth curves in the case of increasing low frequency stress period

次にFig.5.19に示した間欠重畳応力条件下における解析結果をFig.5.21に示す．これ

についても伝播挙動に大きな違いは見られないものの亀裂伝播曲線を詳細にみると，低 周波成分のサイクル数が1.0 10 ~ 1.5 10 ⁵  ⁵回付近（Fig.5.19（a）において重畳応力期間S1 に相当）で， 元の応力履歴（a）の方が低周波応力成分のみ（d）の場合よりも疲労亀裂 伝播速度が速くなっている．また，低周波応力のサイクル数が3.5 10 ~ 4.4 10 ⁵  ⁵回付近 において，伝播速度の大小関係と間欠重畳の程度が不規則に変化しており，Fig.5.20の ように低周波応力期間を増加させる度に疲労亀裂伝播速度が速くなるという挙動は示 していない．したがって， 応力履歴次第では，必ずしも低周波応力期間を増加させる 度に疲労亀裂伝播速度が速くなるとは限らないと考えられる．

6 7 8 9

2.0

Number of cycles (low frequency component): N_L (x 10⁵)

Half crack length: a [mm]

: Original

: Only low frequency stress : Lv2 of low frequency stress : Lv3 of low frequency stress : Lv1 of low frequency stress

Loading sequence: S3−3_25_2_1

174 (a) Overall view

(b) Enlarge view 1

0 1 2 3 4 5

1.5 1.7 1.9

Number of cycles (low frequency component): N_L (x 10⁵)

Half crack length: a [mm]

: Original

: Only low frequency stress : Lv1 of low frequency stress : Lv2 of low frequency stress

Loading sequence: S3-3_50_1

0 1 2 3

1.5 1.7

Number of cycles (low frequency component): N_L (x 10⁵)

Half crack length: a [mm]

: Original

: Only low frequency stress : Lv1 of low frequency stress : Lv2 of low frequency stress

Loading sequence: S3-3_50_1

175 (c) Enlarge view 2

Fig.5.21 Comparison of fatigue crack growth curves in the case of increasing low frequency stress period

ドキュメント内 九州大学学術情報リポジトリ (ページ 176-183)