OS-C501

Offshore Standard DNV-OS-C501, November 2013
Sec.4 Materials - laminates – Page 55
3.11.3 This section is applicable for estimating the characteristic (and subsequently the design) time to failure
under a specified load for a laminate exposed to static or cyclic load, provided the plot of log stress vs. log time
is linear.
3.11.4 If the linear relationship cannot be documented, an equivalent approach shall be used, taking the nonlinearity
into account.
3.11.5 Values shall be based on data that are fairly evenly distributed over the plot of log time to failure vs. log
load, or log number of cycles vs. log load. Load is usually expressed as stress or strain. At least 15 data points
should be used.
3.11.6 To obtain the characteristic curve the mean S-N curve of the form:
log σ = log σ 0 fatigue - α log N
or the mean stress rupture curve of the form:
log σ = log σ 0 stress rupture - β log t
shall be converted to the form:
log(X) mean = log(X 0 ) – k⋅logσ
where X represents the time (or number of cycles) to failure under a sustained stress σ (or stress range σ). X is
a function of σ and exhibits a natural variability from point to point within the material.
logσ
0
log X =
0
β
stress rupture
k =
1
β
or
or
log
log =
X
0
1
k =
α
σ 0 fatigue
α
Guidance note:
Usually, estimates of k and logσ 0stressrupture (or logσ 0fatigue ) can be obtained from linear regression analysis of log X
on log σ.
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3.11.7 When the standard deviation σ ε of the variations in log(X) about the mean is constant, i.e. when σ ε does
not depend on the sustained load or stress range σ, then the characteristic value of log(X) c can be taken as:
log(X) c = log(X 0 ) – k⋅logσ - x⋅σ ε
in which σ ε is estimated from available tests, and x is taken from Table 4-7 depending on the number n of
available data pairs (logs, log X) from tests.
Table 4-7 Values of coefficient x
x
n (# of tests) Case 1 Case 2
10
15
20
50
100
Infinite
3.9
3.4
3.1
2.6
2.4
2.0
4.7
4.0
3.7
3.0
2.6
2.0
3.11.8 The coefficient values marked as Case 1 are valid and can be used for sustained loads or cyclic stresses
within the range of σ-values covered by available tests, i.e. whenever the available tests cover a wide enough
range of σ-values. These coefficient values will be non-conservative if applied for sustained loads or stresses
σ outside the range of log σ-values covered by available tests. When values for x are needed for σ-values
outside this range, coefficient values marked as Case 2 can be used for extrapolation within a concentric range
of logσ twice the length of the range covered by tests.
3.11.9 The mean curve can be transformed back into the standard formulation of an S-N curve, or stress rupture
or fatigue curve using the same equations as given above.
log
— Stress rupture curve: σ
X
1
0
log 0stress
rupture = and β = with X 0 as time.
β
k
log
X
1
0
— Fatigue curve: log σ 0 fatigue = and α = with X 0 as number of cycles.
α
k
3.11.10 The characteristic mean curve can be transformed back into the standard formulation of an S-N curve
or stress rupture curve using the same equations as given above.
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