文档介绍：第八章疲劳裂纹扩展
疲劳裂纹扩展速率
疲劳裂纹扩展寿命预测
影响疲劳裂纹扩展的若干因素
疲劳裂纹扩展速率试验
返回主目录
1
前节回顾:
The plot of log da/dN versus log
DK is a sigmoidal (S形) curve.
This curve may be divided into
three regions.
At low stress intensities, cracking
behavior is associated with threshold effects.
In the mid-region, the curve is essentially linear.
Finally, at high DK values, crack growth rates are extremely high and little fatigue life is involved.
lg da
/
dN
1 2 3
10
-5 -6
10
-9
lg
( K)
D
~
c
K
=(1-
R)
K=(1-R)Kmax
th
D
K
2
Most of the current application of LEFM concepts to describe crack growth behavior are associated with region 2. In this region the log da/dN versus log DK curve is approximately linear and lies roughly between 10-7 and 10-4 mm/cycle. Many curve fits to this region have been suggested. The Paris equation, which was proposed in the early 1960’s, is the most widely accepted.
大多数用线弹性断裂力学描述裂纹扩展的应用是与区域2相关的。在这一区域,logda/dN - log DK曲线近似线性且在10-7-10-4 mm/c间。已有许多拟合曲线提出,60年代初的Paris公式是应用最广的。
3
影响疲劳裂纹扩展的若干因素
K是控制da/dN的最主要因素。
平均应力、加载频率、环境等的影响较次要,但有时也不可忽略。
同一材料, 由不同形状、尺寸的试件所得到的da/dN-K曲线相同。
da/dN-K曲线可以描述疲劳裂纹扩展性能。
K
4 10 20 40
lgda/dN (m/c)
-9
-8
-7
-6
碳钢
R=
K
4
1. 平均应力或应力比的影响
注意到
a=(1-R)max/2,
m=(1+R)max/2;
有:
故a 给定时, R ,m 。
讨论应力比的影响,
就是讨论平均应力的影响。
a
m
R
R
s
s
)
1
(
)
1
(
-
+
=
R>0、R<0影响趋势不同。
th
K
R= 0 -1
lgda/dN
D
lg( K)
D
实验结果
5
th
K
R= 0 -1
lgda/dN
D
lg( K)
D
R>0的情况
R>0时,min>0。
a 给定,R , min , max 。
三个速率区域内,da/dN均增大。da/dN-K 曲线整体向左移动。
Forman公式:
K
K
R
K
C
dN
da
C
m
D
-
-
D
=
)
1
(
)
(
K=(1-R)Kmax KmaxKc,
分母0,da/dN。
KKth,da/dN0。
若考虑Kth的影响,有:
da
dN
C
K
K
R
K
K
m
th
m
c
=
-
-
-
[
(
)
(
)
]
(
)
D
D
D
1
6
th
K
R= 0 -1
lgda/dN
D
lg( K)
D
低速率区,R,Kth。
R<0的情况:负应力存在,
对da/dN三区域的影响不同。
情况比R>0时复杂得多。
0 .2 .4 .6 .8
87654321
低碳钢
低合金钢
不锈钢
A517-F
9301
A508C
A533B
不同钢材的R-Kt