OS-C501

Offshore Standard DNV-OS-C501, November 2013
Sec.6 Failure mechanisms and design criteria – Page 125
Guidance note:
A specified contact pressure is often needed for bolted connections, or if a component is kept in place by friction.
11.3 Initiation of fatigue damage
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11.3.1 Materials may fail due to the cyclic application of loads: this process is called fatigue. A combination
of cyclic loads with an aggressive environment is here also called fatigue. The analysis is generally the same
for both processes, but the material curve describing the reduction of strength with time depends on the
surrounding environment.
11.3.2 All failure mechanisms shall be checked for fatigue, unless a particular failure mechanism may be
acceptable for the structure. The approach is basically the same for all failure mechanisms, but different S-N
curves and residual strength values shall be considered. These are described in Sec.4 and Sec.5.
11.3.3 A fatigue analysis shall contain two steps:
— can the structure survive the expected load sequence
— is the structure strong enough that it can survive all relevant extreme load cases on the last day of its service
life.
11.3.4 A constant amplitude lifetime diagram shall be constructed from the available characteristic S-N curves
charact
(see Sec.4 [3.7] and Sec.5 [3.7]). The characteristic number of cycles to failure n ( γ ⋅ε
) shall be
Sd
applied
extracted for each applied strain condition (amplitude and mean level) from the constant lifetime diagram. The
number of expected cycles to failure shall be found for the applied strains ε applied multiplied by the partial load
model factor γ Sd .
11.3.5 The following design criterion for fatigue shall be used:
with
γ Rd = 1 for a summation over various strain conditions, each of which consists of a combination of a specific
mean strain and a specific strain amplitude, i.e. N>1.
γ Rd = 0.1 if the component is exposed to only one mean strain and one strain amplitude, i.e. N=1.
where n .... n is a function of…
ε applied
n actual
n charact
N
j
t y
γ Sd
γ Rd
γ fat
{ }
γ
fat
γ
{ γ ε }
j
applied
Sd
<
{ ε } 1
j
applied
local response of the structure to the strain condition applied
Rd
t
number of cycles per year at a particular strain condition
characteristic number of cycles to failure under a given strain condition
the total number of strain conditions
index for strain condition
no. of years for the fatigue evaluation (typically equal to the design life)
load-model factor
resistance-model factor
partial fatigue safety factor
N
∑
y
j=
1
n
n
actual
charact
11.3.6 A stress based fatigue analysis may be used as an alternative to [11.3.4] and [11.3.5], using the same
approach, but replacing strains by stresses.
11.3.7 A different γ Rd value may be chosen if it can be documented by experimental evidence. Load sequence
testing for the actual material on representative load sequences shall be used to document the use of a γ Rd in
the range of 1 > γ Rd > 0.1. The minimum is γ Rd = 0.1.
Guidance note:
The factors γ fat γ Rd are designed in such a way that they account for the uncertainty in Miner sum for composites
and provide the desired level of safety. When choosing the default value γ Rd =1 an uncertainty of 10 is assumed for
the Miner sum.
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11.3.8 If the design criterion in [11.3.5] is fulfilled even γ Rd can be multiplied by 20 it may be sufficient to use
typical fatigue data for the laminate without confirming the data by testing. A minimum requirement for using
this clause is that all similarity checks in Sec.4 [8] are fulfilled. A case by case evaluation should be made in
addition, in particular evaluating whether environmental effect could change the fatigue properties of the
laminate relative to the reference data.
γ
Sd
DET NORSKE VERITAS AS