OS-C501

Offshore Standard DNV-OS-C501, November 2013
Sec.10 Component testing – Page 163
from a log(stress)-log(lifetime) diagram for the anticipated lifetime. If more tests are made the requirements
are given in DNV-OS-C501 Sec.4 [8.8.6].
3.3.5 Stress rupture testing for normal safety class: at least one survival test shall be carried out. The specimen
should not fail during the survival test and it should not show unexpected damage. The requirements to the test
results are:
— tests should be carried out up to three times the maximum design life with realistic mean loads that the
component will experience. If constant load testing is carried out tests should be carried out up to 30 times
the design life to compensate for uncertainty in sequence effects.
— if the anticipated lifetime exceeds 1000 hours testing up to 1000 hours may be sufficient. The load levels
should be chosen such that testing is completed after 10 3 hours. The logarithms of the two test results shall
fall within µ-2σ of the logarithm of the anticipated lifetime, where µ is the mean of the logarithm of the
predicted lifetime and σ is one standard deviation of the logarithm of the predicted lifetime, both interpreted
from a log(stress)-log(lifetime) diagram for the anticipated lifetime. If more tests are made the requirements
are given in DNV-OS-C501 Sec.4 [8.8.6].
3.3.6 For low safety class long term testing is not required.
3.3.7 The sequence of the failure modes in the test shall be the same as predicted in the design. If the sequence
is different or if other failure modes are observed, the design shall be carefully re-evaluated.
3.3.8 The average of the measured number of cycles or time until occurrence of each critical failure shall never
be less than the predicted characteristic lifetime or numbers of cycles. Critical failure modes are failure modes
that are linked to a limit state.
3.3.9 Tests should be carried out with a typical load sequence or with constant load amplitude. If a clearly
defined load sequence exists, load sequence testing should be preferred.
3.3.10 Whether reduced test times compared to the component's life are acceptable should be evaluated based
on the anticipated failure modes and whether extrapolation of the data to longer lifetimes is possible. This will
mainly depend on the confidence and previous knowledge one has about the failure modes that are tested.
3.3.11 In some cases high amplitude fatigue testing may introduce unrealistic failure modes in the structure.
In other cases, the required number of test cycles may lead to unreasonable long test times. In these cases an
individual evaluation of the test conditions should be made that fulfils the requirements of [3.3.2] or [3.3.3] as
closely as possible.
3.3.12 The static strength of the structure after long term exposure shall be taken as the extrapolation of the
long term test data of the fatigue or stress rupture tests.
3.3.13 Higher static strength values after long term exposure may be used if experimental or theoretical
evidence can be provided. The same arguments as given in Sec.4 [3] may be used for matrix and fibre
dominated properties. A procedure to obtain strength data after long term exposure is suggested in Sec.4 [3.4]
and [3.9].
3.3.14 Additional tests may be required if resistance to a failure mode cannot be shown by analysis with
sufficient confidence and if this failure mode is not tested by the tests described above.
3.4 Procedure for updating the predicted resistance of a component
3.4.1 The resistance of the component is R and is assumed to be normally distributed:
R∈N(µ R ,σ R 2 )
where,
µ R = mean value of the resistance of the component (generally unknown).
σ R = standard deviation of the resistance of the component, representing the natural variability in the material
properties and the manufacturing/production process, and here assumed known.
3.4.2 The characteristic value of the resistance is specified as a specific quantile in the distribution of the
resistance, here defined as:
x C =µ R -2σ R
However, because µ R is unknown, the true characteristic value x C is also unknown.
3.4.3 Estimates of µ R and x C prior to testing are sought. One way of obtaining such prior estimates is to carry
out an analysis of the component by means of available analysis models.
The estimate µ RA of µ R is obtained from a single analysis using mean values for the material properties. The
uncertainty in the estimate µ RA should also be assessed, expressed in terms of a standard deviation σ m ’, and
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