OS-C501

Offshore Standard DNV-OS-C501, November 2013
Sec.6 Failure mechanisms and design criteria – Page 105
If combination (1) is the worse, then the design criterion to be checked is given by:
If combination (2) is the worse, then the design criterion to be checked writes:
A conservative value of the load combination factor Ψ = 0.7 can be used for Ψ snow and Ψ wind
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2.4 Time dependency and influence of the environment
2.4.1 Design criteria are given as static design criteria in this section. It is assumed that time or influence of
the environment does not change the design criteria themselves. However, the properties used in the design
criteria shall be the appropriate properties for the point of time where the analysis is carried out. The change of
properties with time is described in Sec.4 and Sec.5.
2.4.2 Time dependency is seen here in the widest possible sense. Time dependency considers influence on
properties due to permanent static and cyclic loads, due to environmental effects, and all possible combinations.
3 Fibre failure
3.1 General
γ
γ
wave wave snow snow snow
F
. Sk
+ γ
F
. Sk
. Ψ <
S
wave wave wave snow snow
F
.
k
. Ψ + γ
F
.
k
<
3.1.1 Fibre failure is defined here as the failure of a ply by fracture of fibres. The fibre strength or strain to
failure is based on test results from plies or laminates as described in Sec.4. Ply failures are measured as rupture
of the ply in fibre direction.
3.1.2 The maximum strain criterion should be used to check fibre failures.
3.1.3 Other design criteria may be used if it can be shown that they are equal or conservative compared to the
maximum strain criterion given here. See for example [3.3].
3.1.4 Fibre failure should be checked at the ply level, not at the laminate level.
3.1.5 Laminates are made of plies with different fibre orientations. If the minimum angle between fibres at any
position in the laminate is larger 45 o , matrix cracking or deformation due to in plane ply shear stresses may
cause rupture of the laminate. In this case matrix cracking due to ply shear should also be checked to avoid
fracture, burst or leakage (see also [4.5] and [3.3]), unless it can be shown that matrix cracks or deformations
can be tolerated by the laminate under the relevant loading conditions.
Guidance note:
A pipe made of ±55 laminate with a liner can tolerate matrix cracks and shear deformations, as long as the pipe sees
only internal pressure. If the pipe must carry axial loads or bending moments in addition to the pressure, fibres would
want to reorient themselves to a different angle, a complicated condition. This is only avoided as long as the shear
properties of the pipe are intact.
A pipe made of a 0/90 laminate can tolerate matrix cracks and shear deformations under internal pressure and axial
loads. This pipe would have problems with axial torsion, since the stresses due to torsion have to be carried by the
matrix.
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3.1.6 Regardless of the analysis method used, these laminates should always be analysed with non-degraded
in-plane shear moduli G 12 .
3.1.7 Laminates are made of plies with different fibre orientations. If the minimum angle between fibers at any
position in the laminate is larger than 70 o , matrix cracking or deformation due to in plane ply shear stresses or
stresses transverse to the fibres may cause rupture of the laminate. In this case matrix cracking due to all
possible stress components should also be checked to avoid fracture, burst or leakage (see also [4.1] to [4.3]),
unless it can be shown that matrix cracks or deformations can be tolerated in by the laminate under the relevant
loading conditions.
Guidance note:
This condition is typical for UD laminates where all fibres run parallel in one direction throughout the thickness of
the laminate. Great care should be taken when using such laminates due to their low properties in all other directions
than the fibre direction.
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S
γ
γ
R
k
wave
M
.
R
γ
k
snow
M
.
γ
Rd
Rd
DET NORSKE VERITAS AS