OS-C501

Offshore Standard DNV-OS-C501, November 2013
Sec.5 Materials – sandwich structures – Page 86
Guidance note:
For polymeric foam - Elasticity modulus of polymeric foam cores typically varies less than 10% until short before
failure for high cycle fatigue and maximum stress levels pertaining to the linear range. For low cycle fatigue and
maximum stress levels close to ultimate values, elasticity modulus varies as much as 100% through the entire life.
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3.6.7 The elastic constants can be estimated to change to the values in Table 5-6 for crosslinked PVC and balsa
cores after extensive cyclic fatigue exposure:
Table 5-6 Effects of cyclic fatigue on elastic constants
Core Mechanical
Properties
Mechanical parameter
Effects of cyclic fatigue exposure
E t core Tensile modulus of elasticity of core 10% reduction for high-cycle fatigue.
E c core Compressive modulus of elasticity of core 10% reduction for high-cycle fatigue.
G xy core In-plane shear modulus of core 10% reduction for high-cycle fatigue.
E zt core Out-of-plane core tensile elasticity modulus 10% reduction for high-cycle fatigue
E zc core Out-of-plane core compressive elasticity modulus 10% reduction for high-cycle fatigue.
G xz core Out-of-plane core shear modulus 10% reduction for high-cycle fatigue.
G yz core Out-of-plane core shear modulus 10% reduction for high-cycle fatigue.
ν xy core Major Poisson’s ratio of core (unknown)
ν yx core Minor Poisson’s ratio of core (unknown)
ν xz core Core Poisson’s ratio normal to the core plane (unknown)
3.7 Cycles to failure under fatigue loads
3.7.1 The number of cycles N to failure under a cyclic stress is described by an S-N curve for a specified R-
ratio.
3.7.2 The R-ratio is defined as the minimum stress divided by the maximum stress.
3.7.3 The core material curve for the lifetime strength analysis should be described as:
log σˆ = log σ 0 fatigue - α log N
3.7.4 The strain representation can be obtained as described earlier in this section.
3.7.5 All fatigue curves shall be obtained from load controlled tests, unless the structure is clearly only exposed
to deformation controlled fatigue.
3.7.6 S-N curves should be preferably obtained for R ratios relevant for the application. Minimum
requirements are given in [3.7.7]-[3.7.10].
3.7.7 If the structure is exposed to tensile and compressive fatigue, at least data for R= - 1 shall be available.
3.7.8 If the structure is only exposed to tensile fatigue, data between R= - 1 or R= 0.1 may be used.
3.7.9 If the structure is only exposed to compressive fatigue, data between R= - 1 or R=10 may be used.
3.7.10 Care shall be taken to identify whether fatigue data are given as stress amplitude or stress range.
3.7.11 A Hall diagram shall be constructed from the fatigue curves if the structure is exposed to fatigue stresses
of other R ratios than the measured ones or to various R-ratios.
3.7.12 Ideally, fatigue should be measured on the actual sandwich structure for the relevant loading condition
and environment.
3.7.13 S-N curves may also be measured for specific load sequences if relevant. This may be beneficial,
because Miner sum calculations would not be needed for that load sequence. The validity of the data for other
load sequences would have to be demonstrated.
3.8 Static strength reduction due to cyclic loading
3.8.1 If a core is exposed to a cyclic stresses of any magnitude for a number of cycles N, the static strength
influenced by that stress shall be determined.
3.8.2 If static strength reduction values are not available, the S-N curve may be used as a conservative estimate,
as long as loads never exceed the static yield strength of the core.
DET NORSKE VERITAS AS