OS-C501

Recommendations

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Offshore Standard DNV-OS-C501, November 2013 Sec.5 Materials – sandwich structures – Page 76 Table 5-2 Adhesive specifications, process parameters and conditioning parameters Constituent adhesive material(s): Generic adhesive type (e.g. epoxy, polyester) Specific adhesive type trade name Specific adhesive type batch number Catalyst (trade name and batch number) Accelerator (trade name and batch number) Fillers (trade name and batch number) Additives (trade name and batch number) 1.2.3 It shall be specified whether internal gases need to be removed from the core material before laminates can be applied. Typically the gases diffuse out by themselves some time after the core has been produced. A minimum storage temperature between production and use should be specified in that case. 1.2.4 Any pretreatment applied to the core before the laminates are applied should be specified. An example would be sealing of the outer pores with resin. 1.3 Lay-up specification 1.3.1 A sandwich structure is made of a sequence of layers. All materials, both core(s) and laminate(s), and their stacking sequence shall be clearly identified. 1.3.2 The orientation of non-homogenous or anisotropic materials shall be clearly specified on the materials level and the structural level. 1.3.3 Laminates and core(s) shall be specified such that they can be described by a sequence of stacked orthotropic plies, see also Sec.4. 1.4 Isotropic/orthotropic core layers 1.4.1 A core layer is defined as a volume element with three axis of symmetry with respect to mechanical properties. 1.4.2 All layer sequences shall be described by a combination of the three-co-ordinate systems. Ply angles shall be specified between the fibre, laminate co-ordinate system and the main core direction (x-direction). 1.4.3 Typically, there are two possible microstructure alignments: — 0/90 cell alignment found in orthotropic cores. Cells run parallel to each other within the same plane. The 3 main directions to which material properties are refereed are; width (W), length (L) and transverse (T) or x-, y- and z-direction. Typical cores are honeycomb, balsa wood and other corrugated core — random cell alignment in quasi-isotropic core. Cells are randomly oriented without any preferred direction. A typical reinforcement type of this class is cellular foam core. 1.4.4 The following is assumed in this standard: — for cellular cores, i.e. wood and foam, material behaviour and mechanical properties are considered at macroscopic scale, i.e. material properties are taken as an average over a volume of about cm 3 — the measured material properties should be based on a scale that is compatible with the scale of the structural analysis. 1.4.5 In regions of high local stress concentrations, material properties on a microscopic scale may be needed. 1.4.6 These simplifications are generally valid. However, their applicability shall always be checked. Other modelling methods may be preferred for certain material combinations. 1.5 Mechanical and physical properties 1.5.1 All properties are dependent on the constituent materials, the processing and conditioning environment. It is natural to first separate the properties into laminate(s) and core(s), and interfaces. Interfaces are the core skin connection and possibly other adhesive joints between sections of cores. 1.5.2 For the sandwich facings, see Sec.4. If the faces are made of metallic materials, relevant codes for these materials shall be used. 1.5.3 The mechanical and physical properties of the core are very much dependent upon the nature of the material used for the core whether it is foam, honeycomb, wood or corrugated. For example, honeycomb cells can be made of paper and polyester resin, or aluminium, or Aramid and epoxy. DET NORSKE VERITAS AS
Offshore Standard DNV-OS-C501, November 2013 Sec.5 Materials – sandwich structures – Page 77 1.5.4 It is possible that a structure is loaded in such a way that some material properties are not relevant. In that case the non-relevant properties do not have to be known, but it shall be documented that the properties are not relevant for the application. 1.5.5 If cores of the same type are used in different layers in the component, one test series is sufficient to determine their properties. 1.5.6 Properties can be established as new, see App.D, or checked against typical values. The procedure is given in Sec.4 [8.6]. 1.5.7 Mechanical properties depend on the load conditions and the environment. Parameters related to the topics should be accounted for, see this section headings [3], [4] and [5]. 1.5.8 For test data, the load conditions and the environment parameters should be reported. 1.6 Characteristic values of mechanical properties 1.6.1 See Sec.4 [1.6]. 2 Static properties 2.1 General 2.1.1 All material properties shall be given with full traceability of materials and conditions. Test results are only valid if the information given in Table 5-1 and Table 5-2 is be available. Tests shall be reported as mean, standard deviation, and number of tests. 2.1.2 For many applications the static properties after exposure to long term loads and environments are more important than the static properties of a new material. This fact should be kept in mind when selecting materials and developing a test programme. Long term properties are described in the following sections. 2.2 Static properties 2.2.1 The complete list of orthotropic data for core and adhesive materials is shown in the following tables. Recommendations for test methods to obtain the properties are given in App.D. 2.2.2 Laminate faces elastic constants, strains, strengths and other mechanical properties are described in Sec.4. 2.2.3 When different adhesives are used to bond faces and core together, or core layers together, a distinction shall be made between adhesive(s) and matrix properties. In some cases, matrix and adhesive materials properties are significantly dissimilar. 2.2.4 Static properties are assumed to be identical to quasi-static properties. Accordingly, strain rate should not exceed a value of about 1% per minute. 2.2.5 The yield point for ductile core materials is defined as the 2% offset point. 2.2.6 The orthotropic static data for core materials are the following (note that other co-ordinate systems may be chosen to describe the orthotropic behaviour, e.g. cylindrical co-ordinates): Table 5-3 Mechanical static properties for core materials Mechanical parameter Unit Reference in Appendix D for measurement method In-plane orthotropic elastic constants E xt core linear Tensile modulus of elasticity of core in x-direction in [GPa] [D.2.1] the linear range E xc core linear Compressive modulus of elasticity of core in x- [GPa] [D.2.2] direction in the linear range E yt core linear Tensile modulus of elasticity of core transverse to y- direction in the linear range [GPa] [D.2.1] E yc core linear Compressive modulus of elasticity of core transverse [GPa] [D.2.2] to y-direction in the linear range G xy core linear In plane shear modulus of core in the linear range [GPa] [D.2.3] E xt core non-linear Tensile modulus of elasticity of core in x-direction in [GPa] [D.2.1] the non-linear range E xc core non-linear Compressive modulus of elasticity of core in x- [GPa] [D.2.2] direction in the non-linear range DET NORSKE VERITAS AS
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