OS-C501

Offshore Standard DNV-OS-C501, November 2013
Sec.14 Calculation example: two pressure vessels – Page 192
4.3.2 The Poisson’s ratio is 0.29 according to the representative data from App.F. It is not changed for other
laminate types.
4.3.3 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. An accumulation
of matrix cracks would reduce the matrix dominated stiffness values. The stiffness may drop to 0 (Sec.4 [3.6.1]
and [3.6.2]). A value close to 0 (value for a cracked matrix) is used in the analysis of the gas tank. The same
value as the original modulus 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 [3.6.6] and [3.8.5]). The prerequisites for not changing the modulus of the
water vessel under fatigue are fulfilled as shown in the analysis in [6.6], where it is shown that matrix cracks
will not develop within the lifetime of the vessel.
4.3.4 Elastic parameters under permanent load do not change. The permanent load may cause plastic
deformation (Sec.4 [3.2.2]).
4.3.5 A 10% reduction for the exposure to water has been used for long-term properties.
4.3.6 When matrix cracks have developed the parameters are set to 1% of the original value according to
(Sec.9 [2.2.8]). Values are not reduced further to account for the influence of water.
4.4 Fibre dominated ply strength and strain to failure
4.4.1 The characteristic strength to failure in fibre direction is 534 MPa according to the representative data
from App.F. Data are for stitch-bonded materials while the vessel is made by filament winding. Data should be
corrected according to Sec.4 [8.12]:
— 534 MPa × 0.6 / 0.8 = 401 MPa.
Guidance note:
It is recommended to use data measured from a laminate made by filament winding to avoid using the corrections
made above. Good filament wound laminates can have as good properties as flat panels (see also Sec.4 [8.12.6]).
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
4.4.2 The characteristic strain to failure is given by: ε = σ / E = 401 MPa / 23.7 GPa = 1.69%.
4.4.3 The long term fibre strength is not effected by fatigue, but by long term static loads (Sec.4 [3.9]). This is
evaluated in [4.4.5].
4.4.4 Exposure to water gives typically a reduction of 10% for strength and modulus according to App.F
[F.5.2.1].
The strength is reduced by permanent loads as described in the stress rupture equation in Sec.4 [3.4.1]. The
representative values give a 55.6% reduction for the characteristic (long-term) mean
∧ strength in 25 years relative to
the (corrected, see [4.4.1] and Sec.4 [8.12]) mean short-term strength ( σ ) according to the values given in
1t
App.F [F.4.2.3]:
log[σ(t)] = log[σ(l)] – β log(t), with σ(l) = 0.888 and b = 0,0423.
This formula is expressed in normative strength values (absolute characteristic strength per mean short-term
strength) and time in minutes. The stress rupture curve above may be expressed with respect to the absolute
strength:
where
∧
∧
σ 1t
mean
is the corrected mean (short-term) strength
( = 614 MPa × 0.6 / 0.8 = 461 MPa).
σ 1t
mean
[ σ ( t)
] = log 0.888 − 0.0423log( t)
Inserting this value into the formula above gives
log[ σ ( t)
] = log[ 409] − 0.0423log( t)
log
abs
This results in a long-term strength after 25 years of 461 MPa × (1-0.556) = 205 MPa.
abs
4.4.5 The long-term elastic strain to failure is given by: ε = σ / E = 205 MPa / 23.7 GPa = 0.87%.
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⎢
⎣
4.4.6 A 10% reduction of properties has been used to account for the exposure to water under long-term loads.
4.4.7 The fibre dominated strength properties are not influenced by matrix cracks and the same values are used
for the laminate with and without matrix cracks (Sec.9 [2.2]).
∧
σ
mean
1t
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⎥
⎦
DET NORSKE VERITAS AS