OS-C501

Offshore Standard DNV-OS-C501, November 2013
Sec.4 Materials - laminates – Page 54
— the matrix is not the main load carrying material. The component can carry the loads with a fully cracked
matrix according to Sec.9 [2.2], i.e., all matrix dominated ply properties are set close to 0.
— the total number of cycles does not exceed 1500.
3.8.6 If the structure is exposed to through thickness cyclic loads the fatigue performance shall be
demonstrated by testing on the actual laminate or component.
Guidance note:
Matrix cracks develop very easily during fatigue. A design should be avoided where the structural integrity or any
critical performance requirement relies on matrix cracking not occurring under fatigue conditions.
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3.9 Static strength reduction due to cyclic loads
3.9.1 Fibre dominated static strength is not changed under cyclic fatigue for most continuous glass and carbon
fibres. The same is true for Aramid fibres loaded in tension only. The long term static strength according to
[3.4] may be used as the strength at the end of cyclic loading. The mean fatigue load should be used as the
permanent static load under fatigue.
3.9.2 In all other cases: If a laminate is exposed to a cyclic stress of any magnitude for a number of cycles N,
the static strength (or strain to failure) influenced by that stress shall be estimated from the pertinent S-N curve:
log σ = log σ 0fatige - α log N
or
log ε = log ε 0 fatigue - α log N
where N is the number of cycles expected during the lifetime of the structure.
The characteristic strength shall be determined according to Sec.4 [3.11]. The coefficient of variation COV of
the strength after a certain time should be the same as the COV for short term data, unless a COV of remaining
strength has been measured directly.
3.9.3 If the S-N curve is not linear in a log-log presentation, the static strength cannot be calculated by the
above equation, but shall be taken directly from the S-N curve.
3.9.4 Higher static strength values may be used with experimental evidence.
Guidance note:
A possible way to document that the residual strength is higher than given by the S-N curve is:
a) Expose the test sample to a fatigue load for 90% of the cycles to failure expected according to the S-N curve.
b) Measure the remaining strength after this exposure time.
c) Repeat step a and b for at least one more stress level.
d) If the remaining strength of the tests is the same, it can be assumed that the remaining strength is also the same
up to 90% of the lifetime for lower load levels, provided no changes in failure modes are expected. The possible
change of failure modes should be analysed
e) Measurements could be made for other test periods than 90% of the lifetime.
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3.9.5 The reduction of strength of matrix dominated properties may be ignored if the conditions of [3.8.5] are
met.
3.9.6 If the cycle dependent strength is known, static strains to failure shall be obtained from the reduced static
strength and the cycle dependent stiffness value. If the cycle dependent strain to failure is known, static
strengths shall be obtained from the reduced static strains to failure and the cycle dependent stiffness value.
3.10 Effect of high loading rates - shock loads - impact
3.10.1 The effect of high loading rates is a slight increase of stiffness, slight increase of strength and possibly
a reduction of strain to failure, especially for ductile materials.
3.10.2 It is conservative to assume the same strength values as for static properties. Higher strength values
shall be documented.
3.11 Characteristic values
3.11.1 Characteristic values shall be used for all stress rupture and S-N curves in this standard.
3.11.2 Characteristic values shall be established with a 97.5% tolerance (probability of not being exceeded)
and 95% confidence.
DET NORSKE VERITAS AS