OS-C501

Recommendations

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Offshore Standard DNV-OS-C501, November 2013 Sec.6 Failure mechanisms and design criteria – Page 128 14 High / low temperature / fire 14.1 General 14.1.1 The effects of fire and high/low temperature shall be considered by using the appropriate material properties as described in Sec.4 and Sec.5 within the design criteria described in this section. 14.1.2 High temperatures and fire may introduce changes in the material, even if no other design criteria are violated. The following shall be considered as a minimum: — melting — burning — removal of material — phase transitions 14.1.3 Specific requirements to fire performance are not given in this standard. These requirements shall be obtained from other codes or regulations covering the application. 14.1.4 Composite structures usually need special fire protection to fulfil fire performance requirements. Special care shall be taken to ensure that the insulation works properly, including joints and attachments. The joints should remain tight in a fire and insulation should not detach. 15 Resistance to explosive decompression 15.1 Materials 15.1.1 Materials that are exposed to fluids under high pressure tend to absorb some fluid. If the pressure is removed rapidly (rapid decompression) the fluid inside the material wants to expand and wants to diffuse out of the material. If the material's structure does not allow the fluid to move out rapidly, or if the molecular strength of the material is not strong enough to contain the expanding fluid, severe microscopic damage may happen to the material. 15.1.2 The resistance of a material to the effects of rapid decompression shall be tested experimentally, if relevant. Tests shall be carried out at two pressure levels: The maximum expected pressure and the low typical service pressure. Guidance note: The reason to test at two pressures is based on the two effects interacting during a rapid decompression scenario: Diffusion and Molecular strength. 15.2 Interfaces ---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--- 15.2.1 Similar to the effect observed in materials, fluids can accumulate between interfaces of materials with different diffusion constants. A rapid reduction of pressure may destroy the interface, because the fluid wants to expand. In the case of liners or thin materials, the liner may deform substantially, buckle, or even crack. 15.2.2 The designer should think about venting arrangements in the structure to avoid the build-up of fluids in interfaces. 15.2.3 The resistance of an interface to the effects of rapid decompression shall be tested at the maximum expected pressure, unless it can be shown that venting arrangements prevent the build-up of fluids. 16 Special aspects related to sandwich structures 16.1 General 16.1.1 Sandwich structures are built of a light weight core embedded between two faces (or skins). Design criteria are discussed for skins, cores and the core skin interface. 16.2 Failure of sandwich faces 16.2.1 The same general design criteria as discussed above apply. 16.2.2 Both faces shall be checked for failure, since they may be exposed to different stress states. 16.3 Failure of the sandwich core 16.3.1 Many core materials show plastic behaviour. If yielding cannot be accepted in the sandwich structure the yield criterion under [6] shall be applied. DET NORSKE VERITAS AS
Offshore Standard DNV-OS-C501, November 2013 Sec.6 Failure mechanisms and design criteria – Page 129 16.3.2 Ultimate failure of brittle and plastic core materials shall be checked with the design criterion for orthotropic homogenous materials under [7]. 16.3.3 In many cases the dominant stress in a core is shear, causing shear yield or failure or tensile failure in 45 o to the through thickness direction. This case is also covered by the criteria given under [6] and [7]. 16.3.4 Core indentation due to local compressive stress shall be checked. The core will indent if the compressive strength of the core is exceeded (see [16.3.1] and [16.3.2]). 16.4 Failure of the sandwich skin-core interface 16.4.1 Interface failure between the skin and core of a sandwich can be treated the same way as a delamination ([5]). 16.4.2 Care shall be taken to use proper material properties when applying the design criterion. Interface properties may differ from the core or laminate properties. 16.4.3 Sandwich structures with PVC core show typically no failure directly in the interface, but they fail inside the PVC core close to the interface. PVC core properties may be used in the design criteria. 16.4.4 Sandwich structures with Balsa core fail typically in the interface. 16.5 Buckling of sandwich structures 16.5.1 The same buckling criteria as for general structures apply, see under [8]. 16.5.2 Sandwich structures show some special buckling modes. The following modes shall be considered: 16.5.3 Face wrinkling will occur when the compressive direct stress in the face will reach the local instability (or wrinkling) stress, i.e. σ ≥ ˆ 16.5.4 Global buckling will occur when the axial load will reach the critical buckling load, i.e. 16.5.5 Shear crimping will occur when the direct stress will reach the crimping stress, i.e. 16.5.6 Face dimpling or intercellular buckling occurs only in sandwich structures with honeycomb and corrugated cores. Face dimpling will occur when the compressive stress in the cell wall reaches the buckling stress of a honeycomb cell or a corrugated core, i.e. 17 Chemical decomposition / galvanic corrosion 17.1 General σ σ face σ wrinkling P ≥ P cr. face σ crimping 17.1.1 A material may degrade due to chemical decomposition. This effect is covered in principle in this standard by the time dependence of strength and stiffness values. 17.1.2 The extrapolation of long term data under environmental exposure describes usually the gradual effect of environment or load on the properties. Chemical decomposition may act suddenly and rapidly and may not be detected by mechanical long term tests. 17.1.3 If a material is exposed to chemicals, possible chemical decomposition should be considered. ≥ 17.1.4 Possible galvanic corrosion should be considered when carbon fibre composites are in contact with metal. Usually the metal degrades first, but in some cases also damage to the matrix and the fibres can happen. Carbon composites should be electrically isolated from metal components. ˆ ≥ ˆ core σ dinpling DET NORSKE VERITAS AS
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OS-C501

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