OS-C501

Offshore Standard DNV-OS-C501, November 2013
Sec.6 Failure mechanisms and design criteria – Page 124
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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10.4.9 If the design criterion in [10.4.8] is fulfilled even if 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 cases 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.
10.4.10 The selection of the partial safety factors is based on the following assumptions: The long term static
load is defined as a conservative mean value, i.e. no uncertainty needs to be considered for that variable. The
partial load effect factor γ F is set equal to 1.0.
10.4.11 Under permanent loads, composite materials show a reduction of strength with time for various failure
mechanisms under permanent loads. All relevant failure mechanisms shall be checked for the reduced strength
values under extreme load conditions. More details about strength reduction can be found in Sec.4 and Sec.5.
Possible reduction of Young's moduli should also be considered, depending on the analysis methods used (see
Sec.9).
10.4.12 If the criterion is used in combination with a linear non-degraded analysis according to Sec.9 [2.4], all
strains and stresses in fibre direction above the level to initiate matrix cracking shall be multiplied by the
analysis factor γ a from [3.2].
10.4.13 The partial fatigue safety factor γ fat is defined in Sec.8 [5] for different safety classes. The same safety
class as determined for the static failure mechanism shall be used.
11 Long term cyclic loads
11.1 General
11.1.1 The fatigue load conditions, as defined in Sec.3 [9.6], shall be used as the applied load when checking
fatigue. An approach similar to the one given in Sec.3 [9.6.5] should be used, indicating the mean and
amplitude of applied stress or strain as a function of the number of cycles. The load conditions shall be based
on a conservative estimate.
11.1.2 Fatigue analysis may be performed on a ply (lamina) level or on a laminate level. However, fatigue data
shall always be measured on laminates with representative lay-ups to ensure that the data represent the
interactions between plies.
11.1.3 If a component is exposed to static and cyclic long term loads the combined effect shall be taken into
account. As a conservative choice the effects may be taken to be additive. Other combinations may be used if
experimental evidence can be provided. If fatigue is analysed or tested with a mean load that corresponds to
the permanent static load, effects of static and cyclic fatigue may be considered separately.
11.1.4 Matrix cracks may develop under cyclic loads even if the maximum stress is below the level to initiate
matrix cracking in a static test.
11.2 Change of elastic properties
11.2.1 Fatigue loads may change the elastic properties of a material. The change is of permanent nature in most
cases. How the modulus changes with the number of cycles is described in Sec.4 and Sec.5. Usually
experimental confirmation of the change of elastic properties is required.
11.2.2 The result of a change of the elastic properties can be a redistribution of stresses in a larger structure or
exceeding a maximum displacement requirement. If the redistribution of stresses is of concern a stress analysis
with the changed elastic constants shall be performed. If displacement requirements shall be observed the
displacement criterion (see Sec.6 [9]) shall be fulfilled for the relevant elastic properties in the analysis.
Guidance note:
The change of elastic constants is usually a result of an accumulation of matrix cracks under cyclic fatigue.
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11.2.3 In some cases a certain contact pressure may be needed for a component. It shall be documented that
the change of elastic constants due to cyclic fatigue will not reduce the contact pressure to an unacceptable
level.
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