The mean value and standard deviation of the safety margin M are thus:
M350 200 150 MPa
352240 53.15 MPa
M
whereby the reliability index may be calculated as:
150
2.84
53.15
Finally the failure probability is determined as:
( 2.84) 2.4 10^3
PF
,
Example 6.2 – Error Accumulation Law
As an example of the use of the error propagation law consider a right angle triangle ABC,
where B is the right angle. The lengths of the opposite side b and adjacent side a are
measured. Due to measurement uncertainty the length of the sides a and b are modelled as
independent Normal distributed random variables with expected values a = 12.2, b = 5.1
and standard deviations a = 0.4 and b = 0.3, respectively. It is assumed that a critical
condition will occur if the hypotenuse c is larger than 13.5 and the probability that this
condition should happen is to be assessed.
Based on the probabilistic model of a and b the statistical characteristics of the hypotenuse
c given by:
cab^22
may be assessed through the error propagation model given by Equations (6.13)-(6.14),
yielding:
0
22
2
22
1 22 22
()
i
ab
n
Xa
i i
Ec
Var c fa
x ab ab
2
b
b
!
"
"#$% xx
x^
which by inserting for and a b their expected values yields:
22
22
22 22
12.2 5.1 13.22
12.2 5.1
0.182
12.2 5.1 12.2 5.1
ab
Ec
Var c
As seen from the above the variance of the hypotenuse c depends on the chosen linearization
point. If instead of the mean value point a value corresponding to the mean value plus two
standard deviations was chosen the variance of would have been: c
222222
(^13) 0.4 5.7 0.3 0.149
13 5.7 13 5.7
Var c