OS-C501

Offshore Standard DNV-OS-C501, November 2013
Sec.10 Component testing – Page 166
— based on the individual degradation curves a lifetime analysis can be made
— tests according to [3.3] shall be used to verify the predictions as far as possible.
4.3.6 The lifetime analysis in [4.3.5] may be based on a combination of the measured (or predicted) static
strength of the component and the worst degradation curve of the individual failure mechanisms. This tends to
be a very conservative approach. A better approach is to determine the static failure loads for each failure
mechanism and apply the degradation curves for each failure load.
4.3.7 Obtaining degradation curves and or failure loads for individual failure mechanisms may require
specially designed test pieces that will fail with the required failure mode.
4.3.8 In some cases it may be possible that one failure mode that usually does not occur within practical testing
times can be created by acceleration techniques, e.g. increasing the temperature. In such a case it may be
possible to check two failure modes simultaneously under the accelerated conditions. Evidence for the
acceleration conditions shall be provided.
4.4 Example of multiple failure mechanisms
Guidance note:
Simplified example of multiple failure mechanisms:
The purpose of this example is to explain the concepts used in [4]. The example is a simplified end connector of a
pipe. The pipe is shown in Figure 10-1. It consists of a tube made of a composite laminate and a metal end fitting. The
connection between the two is an adhesive joint.
Laminate
Figure 10-1
Simplified schematic of a connector of a pipe
Steel end fitting
The adhesive joint was designed to be much stronger than the pipe itself. Its predicted failure was at a pressure “A”
as shown in Figure 10-2. Short-term pressure tests confirmed fibre failure in the laminate at level B.
A
Test results
Pressure
MP
C
B
Matrix or adhesive
shear failure
Fibre failure
Max test time
Time
Figure 10-2
Different times to failure for different failure mechanisms
Design life
Long term tests also showed fibre failure. The strength reduction with time was fairly small, as expected for fibre
dominated properties.
However, it is known from other tests that the strength of the adhesive degrades more rapidly with time than the fibre
strength. The predicted reduction of strength is shown in curve A. Even at the end of the design life the pipe should
still fail by fibre failure.
The static strength A of the adhesive joint is, however, not well known and cannot be tested experimentally. If the
strength is totally unknown it can be assumed that the measured static strength (fibre failure at level B) is also the
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