OS-C501

Offshore Standard DNV-OS-C501, November 2013
Sec.14 Calculation example: two pressure vessels – Page 193
4.5 Matrix dominated ply strength and strain to failure
4.5.1 The characteristic strengths and strains to failure transverse to the fibre direction and in shear are taken
from the representative data from App.F. Data were corrected according to Sec.4 [8.12]. The strain to failure
is given by: ε = σ / E.
4.5.2 The change of the matrix dominated properties under fatigue is uncertain. Matrix cracks can develop
even if ply stresses are below the level for initiation of matrix cracks under quasi-static loads. However, the
same value as the original strength is used for the water tank, since stresses are low and the load could be carried
by a vessel full of matrix cracks (Sec.4 [3.8.5]). The prerequisites for not changing the strength of the matrix
of the water vessel under fatigue are fulfilled as shown in the analysis in [6.8]. For the gas vessel matrix cracks
are acceptable and fracture of the matrix need not be considered.
4.5.3 The change of the matrix dominated properties under permanent loads is treated similar to the fatigue
case. The same value as the original strength is used for the water tank, since stresses are low and the load could
be carried by a vessel full of matrix cracks (Sec.4 [3.4.4]). The prerequisites for not changing the strength of
the matrix of the water vessel under permanent load are fulfilled as shown in the analysis in [6.7]. For the gas
vessel matrix cracks are acceptable and fracture of the matrix need not be considered.
4.5.4 When matrix cracks have developed the strength parameters are not relevant anymore, since the cracks
are assumed to be present already.
4.6 Time to failure for fibre dominated properties
4.6.1 For cyclic loads the characteristic S-N curve from App.F [F.4.2.2] can be used to establish that the fibres
have sufficient lifetime. The curve for R = 0.1 can be used since the pressure vessel is cycled between 0 and
maximum load. The characteristic fatigue curve is given as:
4.6.2 The characteristic stress rupture curve from [4.4.5] can be used to establish that the fibres have sufficient
lifetime. The characteristic stress curve is given as:
4.7 Time to failure for matrix dominated properties
4.7.1 The design of the gas vessel is based on the philosophy that matrix cracks are acceptable and the time to
initiation of matrix cracks does not have to be checked.
4.7.2 The design of the water vessel is based on the philosophy that stresses are so low and cycle numbers are
low enough, that the time to initiate matrix cracks is less than the lifetime of the component. The method
requires that the conditions in Sec.4 [3.3.10] and Sec.4 [3.8.5] are fulfilled. These conditions are checked in
[6.6].
4.8 Test requirements
[ ε( N )] = 0.
− 0.101log( N )
log 063
log
[ σ ( t)
] = log[ 348] − 0.0423log( t)
abs
4.8.1 Test requirements should be seen in combination with the analysis results in the following sections.
Different properties are critical for the gas vessel with liner and the water vessels without liner. Test
requirements are also explained in the following analysis sections, but are summarised here.
4.8.2 Since representative properties are used for both vessels, it should be verified that these representative
properties are applicable for the actual laminates used.
4.8.3 For the gas vessel with liner only the fibre dominated properties are critical. According to Sec.4 [8.6] the
tensile and compressive strength of the laminates in fibre direction should be confirmed. The Young's modulus
should also be measure during theses tests. Since stress rupture is the critical failure mode for this application
nine stress rupture tests up to 10 000 hours according to Sec.4 [8.8] should be carried out. (Note that the
requirement is only three survival tests up to 1000 hours for normal safety class Sec.4 [8.9].)
4.8.4 For the water vessel without liner the fibre dominated and matrix dominated properties are critical.
According to Sec.4 [8.6] the tensile and compressive strength of the laminates in fibre direction and transverse
to the fibres should be confirmed. The Young's modulus should also be measure during theses tests. Long term
properties do not need confirmation, since stress levels are very low.
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