Environmental Engineering Reference
In-Depth Information
whose size is equal to or greater than the critical size χ
cr
in the operating
mode of the product:
c
1
bef
S
∫
N
=
Nd
() .
cc
res.cr
res
m
÷
cr
When
N
res.cr
< 1 the safety
S
of the product (reliability criterion based on
fracture resistance) is estimated by the expression:
S
S
= −
1
.
res.cr
If
N
res.cr
>
1 the produce does not have any safety margin and can not
be allowed to operate.
Residual defectiveness
N
res.d.s
in the range defects which are important
for reliability is determined as the number of defects that are larger than
the maximum size of defects [χ]
d.s
in service
c
1
bef
∫
S
S
N
=
NdN
( )
at
cc
1.
<
res.d.s
res
res.d.s.
m
÷
d.s.
The reliability of the product
R
is defined by the equation
R
S
= −
If
N
res.d.s
>
1, the product does not have any safety margin. The third
range describes accurately the quality of manufacture.
In constructing the histograms, the horizontal axis χ should include the
critical defect size, even if all identified defects did not reach critical sizes
in inspection.
In inspection of several similar products all the results are added up and
presented in the form of a histogram. The greater the number of inspected
items the higher is the reliability of the final result.
In the
second variant
a test sample is produced to determine the
dependencies
P
dd
,
N
in
=
f
(χ) and
N
res
= φ(χ) (the residual defects) are made
the test sample.
Taking into account the actual operating loads and service conditions
for the product (e.g. pipeline in Fig. 5.42) the fracture mechanics methods
(with safety factors taken into account) are used to determine the set of
defects (discontinuities) of the critical size allowed in service as well as
the allowable size of discontinuities in manufacture (Fig. 5.43).
The test sample is made in the form the product and on the scale of
approximately 1:1 to the product or its most critical part. The most critical
part of the product is the part of the product in which defects are most
likely to form (welds, areas of maximum service effects, etc.) or destruction
of which poses a risk. The test sample is produced from the same material
1
.
res.d.s
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