Environmental Engineering Reference
In-Depth Information
In this case, the permissible stress is taken as the expected stress in a
dangerous cross section at a given load.
We take the coefficient of variation of the load A q = 0.100, ie.,
approximately the same as for the material strength σ T . Then the safety
characteristics and probability of failure for both cases can be defined by
the formulas:
1.90
1.0
[ ]
2
at
s =
1400 kgf/cm
g=
=
4;
V
0.000032;
=
0.106
2
1.90
2
+
0.1
2
1.67
1.0
[ ]
at
s =
1400 kgf/cm
2
g=
=
3.3;
V
0.00045.
=
2
2
2
0.106
1.67
+
0.1
This implies that an increase in permissible stress to 200 kgf/cm 2
increases in this case the probability of failure by approximately 14 times.
However, this probability still remains sufficiently small.
3.2 Probability of failure under cyclic loading causing
fatigue of constructional materials
Under cyclic loading the basic process of aging is the cumulation of fatigue
damage. Fatigue of structural steels is determined by the characteristics of
fatigue which can be represented in the form of the fatigue curve (Fig. 3.3).
If the conditions of cyclic loading are stationary (i.e., constant loading
amplitude), then the probability of failure can be determined using the
approach described above in section 3.1.
Let the endurance limit of a component be a random quantity distributed
normally with parameters
s
,s
s is the endurance limit of
(
lg
lg
s−
lg
s s−
components,
is the standard deviation, the ratio
υ
=s
/
s is
lg
s−
lg
s−
lg
lg
the variation coefficient).
The distribution function of stress amplitudes is characterised by the
values:
￿ ￿ ￿ ￿ ￿
s - the average amplitude of the stress cycle; s s
- standard
aF ], MPa
3.3 Fatigue curve of type 0Cr18Ni10Ti steel.
Search WWH ::




Custom Search