OS-C501

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Offshore Standard DNV-OS-C501, November 2013 Sec.6 Failure mechanisms and design criteria – Page 98 SAFETY ISSUES FAILURE TERMINOLOGY Safety class Each sub-structure may belong to a different safety class (high, normal, low) Structure or product Entity being designed. May be sub-divided into substructure, components, parts and details. Example: Pipeline sub-divided into pipe sections Functional requirements Requirement that the structure or part of the structure shall fulfil. Example: Pipeline A) pressure containment B) tightness Limit states Each failure mode shall be related to ULS or SLS Failure modes Fashion in which the structure fails Example: Pipeline A) burst (ULS) B) leakage (ULS) Failure types Degree of pre-warning intrinsic to a given failure mechanism. Each failure mechanism shall be related to one of the failure types (brittle, plastic, ductile) Failure mechanisms Underlying phenomenon at the material level that determines the failure mode. Depending on its level of severity a failure mechanism can lead to several failure modes. Example: Pipeline A) fibre failure (brittle) B) matrix cracking & delamination (ductile) Failure criteria Example: Pipeline A) maximum strain in the fibre direction B) maximum stress transverse to the fibres combined with shear stress Figure 6-1 Failure terminology DET NORSKE VERITAS AS
Offshore Standard DNV-OS-C501, November 2013 Sec.6 Failure mechanisms and design criteria – Page 99 1.1.3 For the material in consideration, all the relevant mechanisms of failure shall be listed. A minimum list of failure mechanisms is given in Table 6-1. The failure mechanisms are linked to typical material types. Table 6-1 Failure mechanisms for different materials Failure Mechanisms Fibre Failure Matrix Cracking Delamination Yielding Ultimate failure of isotropic or anisotropic homogenous materials Elastic buckling Unacceptably large displacements Stress Rupture Fatigue Wear Fire* Explosive decompression* Impact* Chemical decomposition Material Type Laminates and Sandwich Skins Laminates Laminates and Sandwich Core/Skin Interface Core materials, liners, resin rich areas Core materials All materials All materials All materials, all failure mechanisms. All materials, all failure mechanisms. All materials All materials All materials All materials All materials * these items are load conditions, but are treated here as failure mechanisms to simplify the approach in the standard. Guidance note: The mechanisms of failure of composites can be discussed at different material levels. Failure can be considered to happen in the matrix or in the fibre. On a larger scale, it can happen to the individual ply (or core). Eventually, one can consider the whole thickness of the structure as one quantity, i.e. the laminate or the sandwich structure. ---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--- 1.1.4 In some cases, a critical sequence of mechanisms of failure may be required for a failure mode to occur. That sequence should be specified (considering the “domino effect”), if relevant. Guidance note: Different sequences may lead to the same failure mode. In this case, the structure shall only be considered as failed, if the whole sequence of mechanisms of failure modes has happened. ---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--- 1.1.5 The type of failure corresponding to each failure mechanism shall be determined. See Sec.2 [3.4] for the definition of the three types of failure. Failure types for typical failure mechanisms are indicated in the following chapters. Guidance note: The type of failure determines the partial safety factors (see Sec.2). The determination of the type of failure is a critical step in the design process and can lead to significant differences in the magnitude of the partial safety factors. ---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--- 1.1.6 For each location, a link between possible mechanisms of failure and failure modes shall be established. Possible links between failure modes and the failure mechanisms of FRP laminates and sandwich structures are described in [1.5]. This is not exhaustive and it should be used for guidance only. 1.1.7 Special considerations for interfaces between laminates and steel or laminates and laminates are covered in Sec.7. DET NORSKE VERITAS AS
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