OS-C501

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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 DET NORSKE VERITAS AS
Offshore Standard DNV-OS-C501, November 2013 Sec.10 Component testing – Page 167 strength of the adhesive joint. Applying the degradation curve to this strength gives curve B. This is the approach in 306, giving very conservative long-term pressures. Alternatively, if testing up to a certain time has always shown that the tube failed by fibre failure, then it is possible to apply the degradation curve of the adhesive joint to the longest test time. This is done in curve C, based on the approach in [4.3.5]. The resulting maximum pressure MP, is relatively good value. The simplifications of this example are mainly that more than two failure mechanisms are involved in such a joint. The laminate may show matrix cracking and delamination due to through thickness stresses. The adhesive joint may fail in the adhesive or one of the interfaces. Each of these may have different degradation curves. ---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--- 5 Updating material parameters in the analysis based on component testing 5.1 5.1.1 If test results do not agree with the analysis a change of few material parameters in the analysis may create agreement between test results and analysis. Such changes should be avoided and shall be made with great caution. 5.1.2 Material parameters may be changed if the results can be confirmed by independent tests on the material level. 5.1.3 If such tests cannot be made the analysis is closely linked to the particular test geometry. Such an analysis shall not be used for other geometry or loading conditions unless it can be shown that the adjusted material parameters have a physical meaning. Guidance note: Changes of material parameters due to damage, like matrix cracking, as described in Sec.4 can be made. These values are based on independent material tests and are needed for a non-linear analysis. ---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--- DET NORSKE VERITAS AS
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OS-C501

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