OS-C501

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Offshore Standard DNV-OS-C501, November 2013 Sec.6 Failure mechanisms and design criteria – Page 102 Guidance note: The types and directions of loading shown in Figure 6-2 are indicative, and are characteristic of loading associated with the elementary failure mechanisms. However, in real structures, a failure mechanism can occur under various loading conditions. ---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--- 1.3.6 Sandwich structures typically show a sequence of failure mechanisms. These sequences should be considered. If one failure mechanism cannot be well described it may be sufficient to design the component in a way that the preceding failure mechanism will not occur. A typical sequence is: under fatigue loading, crack initiates in the core due to core shearing => crack then propagates in core material => face-core delamination starts when shear crack reaches interface => face-core delamination propagates along the interface until final catastrophic failure. 1.4 Displacements and long term failure mechanisms and failure type 1.4.1 A relationship between failure mechanisms and types according to the principles given in Sec.2 [3.4] is given in Table 6-6 for mechanisms applicable to FRP laminates and sandwich structures. Table 6-6 Relationship between failure mechanisms and failure type Failure Mechanisms Failure Type Unacceptably large displacement Decide individually, see [9.1.3] Stress Rupture Not required for lifetime calculations Fatigue Not required for lifetime calculations 1.5 Link between failure modes and failure mechanisms 1.5.1 The most common failure modes and associated failure mechanisms are listed in Table 6-7. For a new design, an exhaustive list of potential failure modes and failure mechanisms shall be established. A more complete list is given under [7] and App.A. Table 6-7 Minimum list of failure modes and failure mechanisms Minimum list of Failure Mechanisms Comments Failure Modes Fibre Failure Is assumed to cause fracture. Shall always be checked. Matrix Cracking Is assumed to cause fracture in UD laminates. Is assumed to cause fracture in 0/90 laminates loaded in in-plane shear. May reduce compressive fibre strength. May initiate delamination. Otherwise a failure mode that does not influence fracture. Delamination Is assumed to cause fracture if a structure is exposed to through thickness stresses. May be acceptable for in-plane loads. Yielding Shall be checked, unless structure can tolerate large deformations of the material investigated. Buckling May cause fracture. Shall always be checked if compressive and/or significant in-plane shear loads are present. Buckling may be affected by the presence of matrix cracks and delaminations. Fracture Unacceptably large (local/global) displacement It shall be checked that excessive displacements cannot cause fracture. Sandwich core failure Is assumed to cause fracture. Shall always be checked. Sandwich core yield See yielding Sandwich buckling See buckling Stress Rupture Fatigue Effect shall be checked for all failure mechanisms mentioned above. Impact Wear Fire Explosive Decompression Special failure mechanisms that can cause fracture or degradation Chemical decomposition / Galvanic Corrosion DET NORSKE VERITAS AS
Offshore Standard DNV-OS-C501, November 2013 Sec.6 Failure mechanisms and design criteria – Page 103 Table 6-7 Minimum list of failure modes and failure mechanisms (Continued) Minimum list of Failure Mechanisms Comments Failure Modes Fibre Failure Is assumed to cause leakage. Shall always be checked. Matrix Cracking Is assumed to cause leakage unless a liner or other barrier can keep the fluid out of the laminate. Matrix Crack Growth If data exist that show leakage will only occur after a certain crack density has been reached, this failure mechanism may be used instead of simple matrix cracking. No design criterion is given in this document. Delamination May cause leakage, especially if it causes the violation of a displacement requirement. Yielding May cause leakage. Buckling May cause leakage. Unacceptably large May cause leakage. Leakage displacement Sandwich core failure May cause leakage Sandwich core yield May cause leakage Sandwich buckling May cause leakage Stress Rupture Fatigue Impact Wear Fire Explosive Decompression Effects shall be checked for all failure mechanisms mentioned above. Special failure mechanisms that may lead to leakage. Chemical decomposition / galvanic corrosion Buckling Buckling (local or global) Blast/Burst Impact Excessive deformation, Ovalisation, Excessive displacement Wear 2 Design criteria - general approach 2.1 General Sandwich buckling Unacceptably large displacement Impact Unacceptably large displacement Wear Chemical decomposition / galvanic corrosion Buckling is treated as a failure mode and failure mechanism in this standard. Buckling needs special analysis methods and special design criteria. The effect of other failure mechanisms, such as delamination and matrix cracking, on buckling shall be considered carefully. Buckling may lead to violation of displacement requirements. Consider same failure mechanisms as for fracture. Treated as a failure mode and failure mechanism in this standard. 2.1.1 A design criterion shall be assigned to each relevant mechanism of failure. 2.1.2 Design criteria for typical mechanisms of failure and materials are described in the following chapters. 2.1.3 If no design criterion is known for a relevant mechanism of failure the following alternative options may be used: — it may be possible to perform a component test that evaluates the relevant design criteria without the need of a detailed knowledge of the failure mechanisms. This should be documented. See Sec.10 — a design criterion may be proposed. It should be documented by experiments and/or experience that the proposed design criterion is applicable for the component. Details are given under [19]. 2.1.4 A modelling uncertainty shall be assigned to each design criterion and/or the strength parameters used in it. A modelling uncertainty factor is included in the design criteria equations proposed in this standard. DET NORSKE VERITAS AS
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