OS-C501

Offshore Standard DNV-OS-C501, November 2013
Sec.5 Materials – sandwich structures – Page 87
Guidance note:
Static strength values estimated from an S-N curve are typically conservative.
When subjected to very high cyclic load over short period of time, the reduced static strength may be much lower than
an estimated value from S-N curve - and here wrongly assumed being a conservative approach.
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3.8.3 Static strains to failure shall be obtained from the reduced static strength and the cycle dependent
stiffness value.
3.8.4 If the S-N curve is used and is not linear in a log presentation, the static strength cannot be calculated by
the above equation, but shall be taken directly from the S-N curve.
3.8.5 Higher static mechanical properties values may be used on the ground of experimental evidence. The
procedure in Sec.4 [3.4] may be used.
Guidance note:
The static strength for a given stress amplitude may be higher than the value on the S/N curve, if the specimen is
exposed to less cycles than given on the S/N curve. For some PVC foams, 90% of the static strength is preserved up
to almost the end of fatigue life. At the end of the cyclic life, static strength decreases very significantly.
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3.9 Effect of high loading rates - shock loads - impact
3.9.1 The application of a high loading rates may cause the core or the adhesive material to behave differently.
3.9.2 Strain rate effects are material-dependant but also vary with temperature.
3.9.3 Typical apparent effects of high loading rates are:
— increase in strength
— increase in modulus
— decrease of strain to failure
— change of failure mode from ductile or plastic to brittle.
Guidance note:
The effect of high strain rate is more pronounced for polymeric materials than for wooden or metallic materials.
For polymeric materials, such as polymeric foam, strain rate effects are significant for values superior to 5% per
second at room temperature
For balsa wood core materials, strain rate effect is negligible for temperature range belonging to –20 +40°C and for
strain due to slamming of waves in marine applications.
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Table 5-7 Typical increase in strength due to strain rate
increase by 4 decades, i.e. 0.001/s to 10/s
Core Material
Typical increase in strength
Aluminium honeycomb 12%
Nomex honeycomb 20%
End-grain balsa 30%
linear PVC foam 80%
cross-linked PVC foam 28%
3.9.4 Decrease of strain to failure under high strain rate regime may be critical at stress concentration areas,
for example, area of load introduction, joints, inserts.
3.9.5 When strain rates effects are unknown, strength and elasticity modulus for quasi-static conditions should
be used together with strain to failure at high strain rate - as a conservative approach.
3.10 Characteristic values
3.10.1 Characteristic values shall be used for all stress rupture and S-N curves in this standard. The procedure
to obtain these values is the same as given in Sec.4 [3.11].
DET NORSKE VERITAS AS