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Offshore Standard DNV-OS-C501, November 2013 Sec.6 Failure mechanisms and design criteria – Page 100 1.2 FRP laminates - failure mechanisms and failure type 1.2.1 A relationship between failure mechanisms and types according to the principles given in Sec.2 [3.4] is given for FRP laminates in Table 6-2. Table 6-2 Relationship between failure mechanism and failure type for FRP laminates Failure Mechanisms Failure Type Fibre Failure Brittle Matrix Cracking Brittle, if cracks are bridged by fibres: Plastic if only used as a criterion for leakage: Ductile Delamination Brittle Elastic buckling Brittle, plastic or ductile according to type of structure and loading. 1.2.2 In some cases a failure mechanisms is linked in a conservative way to a failure mode. If the failure mechanisms is only linked to that failure mode in a conservative way, a different failure type than stated in [1.2.1] may be used based on the criteria in Sec.2 [3.4]. Guidance note: For example: onset of matrix cracking may be linked to leakage, even though it is known that a fairly large number of matrix cracks must be present to cause leakage in most laminates. If matrix cracking is not linked to any other failure modes than leakage, the failure type “ductile” may be chosen. ---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--- Guidance note: For elastic buckling the failure type is determined by the post-buckling behaviour. For elastic buckling of most simple, symmetrical columns and struts the failure type may be considered plastic. For plates supported on all edges the behaviour is often ductile. For some shell and optimised stiffened plate structures the behaviour may be brittle. Note that deformations associated with elastic buckling may trigger other failure mechanisms such as fibre failure, with consequent change of failure type. ---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--- 1.2.3 Failure mechanisms are often described in more detail for FRP laminates than the mechanisms given in [1.2.1]. The Table 6-3 links the detailed failure mechanisms to the ones used in this standard. Table 6-3 Mechanisms of failure for FRP laminates Fibre Matrix Ply Inter-laminar Laminate Fibre failure (tensile or Matrix cracking Fibre/matrix Delamination Rupture (1) compressive) debonding (2) Fibre buckling (1) Matrix yielding Interlaminar tensile Global buckling failure (3) Interlaminar shear failure (3) Local buckling Laminate creep Laminate fatigue described in this standard by (1): fibre failure, (2): matrix cracking, (3): matrix cracking and or delamination. 1.2.4 Laminates 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. Typical sequences are: — matrix cracking => delamination => fibre failure — debonding and matrix cracking => fibre buckling => fibre failure — delamination => crack propagation due to fatigue => global buckling. Unusual but possible sequence: — wedge shaped matrix cracks => component failure in compression. 1.3 Sandwich structures - failure mechanisms and failure type 1.3.1 Sandwich structures are built of a light weight core embedded between two faces (or skins). Design criteria are given for skins, cores and the core-skin interface. DET NORSKE VERITAS AS
Offshore Standard DNV-OS-C501, November 2013 Sec.6 Failure mechanisms and design criteria – Page 101 1.3.2 Failure mechanisms and types for the faces are the same as for FRP laminates described in [1.2]. 1.3.3 A relationship between failure mechanisms and types according to the principles given in Sec.2 [3.4] is given in Table 6-4 for sandwich structures. Table 6-4 Relationship between failure mechanism and failure type for sandwich structures Failure Mechanisms Failure Type Crack Growth in core-skin interface Typical plastic Debonding of core skin interface Brittle Yielding of core Depends on core material, see [1.3.4] Ultimate failure of anisotropic homogenous core Brittle material Local elastic buckling of skin laminates Assume brittle unless plastic or ductile type can be documented Global elastic buckling of panel See [1.2.1] 1.3.4 Table 6-5 indicates typical failure types for yielding of common core materials at normal laboratory loading rates (strain rate about 1% per minute). Failure types can change with loading rates. The failure types indicated are typical cases and shall be documented for the specific material, based on the definition given in Sec.2 [3.4]. Table 6-5 Typical failure types for various core types Yielding of Core Material Typical Failure Type Paper honeycomb Ductile Aluminium honeycomb Ductile Glass/Phenolic honeycomb Does not yield Nomex honeycomb Does not yield Balsa wood Does not yield Polyurethane foam Does not yield Polystyrene foam Does not yield Polyvinyl chloride foam (linear) Ductile or plastic Polyvinyl chloride foam (cross-linked) Ductile or plastic Poly-methacryl-imide foam Does not yield Corrugated core Material-dependent 1.3.5 Failure mechanisms are often described in more detail for sandwich structures than the mechanisms given in [1.3.2] and [1.3.3]. Figure 6-2 relates some typically illustrated or discussed failure mechanisms to the ones used in this standard. a) face/core yielding/fracture b) core shear c) buckling - face wrinkling d) delamination e) general buckling f) buckling - shear crimping g) buckling - face dimpling h) (h) core indentation - core yield. Figure 6-2 The Failure Mechanisms in a Sandwich Beam DET NORSKE VERITAS AS
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OS-C501

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