OS-C501

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Offshore Standard DNV-OS-C501, November 2013 Sec.7 Joints and interfaces – Page 132 2.1.4 The options in Table 7-1 may be used for the different types of joints: Table 7-1 Design approaches for different categories of joints Type of joint Analytical approach Qualification testing Analyses combined with testing (updating) Laminated joint x x x Adhesive joint x x Mechanical joint x x x 2.1.5 The level of all stress (strain) components in all relevant areas of the joint, including stress concentrations, shall be determined according to the same procedures as specified for the rest of the structure. Special emphasis shall be put on possible stress concentrations and local yielding in the joint. It shall be recognised that the stress concentrations in the real structure may be different than determined through the analyses due to e.g. simplifications made, effects of FEM-meshing etc. 2.1.6 An analytical analysis is sufficient, if the stress field can be determined with sufficient accuracy, i.e., all stress concentrations are well characterised and a load model factor γ Sd can be clearly defined. In all other cases experimental testing according to Sec.10 shall be carried out to confirm the analysis. 2.1.7 If the material properties, especially of the interface cannot be determined with sufficient accuracy, experimental testing according to Sec.10 shall be carried out. 2.1.8 Long term performance of a joint may be determined based on long term materials data, if a clear link between the material properties and joint performance can be established. The requirements of [2.1.2] and [2.1.3] also apply for long term performance. 2.1.9 The load cases should be analysed with great care for joints. Relatively small loads in unfavourable directions can do great harm to a jointed connection. Especially loads due to unintended handling, like bending, stepping on a joint etc. should not be forgotten. 2.1.10 Joints may be analysed by testing alone as described in Sec.10 [2]. 2.1.11 The most practical approach is likely to use a combination of analysis and testing. Since a large conservative bias may be necessary in the analysis to account for the many uncertainties in a joint design it is recommended to use the updating procedures of Sec.10 [3.4] to obtain a better utilisation of the joint. The purpose of this approach is to update the predicted resistance of the joint with the results from a limited number of tests in a manner consistent with the reliability approach of the standard. 2.2 Qualification of analysis method for other load conditions or joints 2.2.1 If an analysis method predicts the tested response and strength of a joint based on basic independently determined material properties according to Sec.10 [3], the analysis works well for the tested load conditions. The same analysis method may be used: — for the same joint under different load conditions, if the other load conditions do not introduce new stress concentrations in the analysis — for a joint that is similar to an already qualified joint, if all local stress concentration points are similar to the already qualified joint and all material properties are known independently. 2.2.2 Local stress concentrations are similar if the local geometry of the two joints and the resulting stress fields at these local points can be scaled by the same factor. 2.2.3 An analysis method that predicts the test results properly but not entirely based on independently obtained materials data can only be used for other load conditions or joint geometry if it can be demonstrated that the material values that were not obtained by independent measurements can also be applied for the new conditions. 2.3 Multiple failure modes 2.3.1 Most joint designs can fail by various failure modes. All possible failure modes shall be carefully identified and analysed. See Sec.4. 2.4 Evaluation of in-service experience 2.4.1 In service experience may be used as experimental evidence that a joint functions well. 2.4.2 This evidence shall only be used if the load and environmental conditions of the in-service experience can be clearly defined and if they match or are conservative for the new application. DET NORSKE VERITAS AS
Offshore Standard DNV-OS-C501, November 2013 Sec.7 Joints and interfaces – Page 133 2.4.3 Material properties of the joints to be compared should be similar. The analysis method should be able to address all differences between the joints according to [2.1] and [2.2]. 3 Specific joints 3.1 Laminated joints 3.1.1 Laminated joints rely on the strength of the interface for load transfer. The interface has resin dominated strength properties. Defects in the interface tend to be more critical than defects in the interface of plies of laminate, because the joint interface is the only and critical load path. 3.1.2 The strength of the joint may be different from the through thickness matrix properties of the laminate, because the joint may be a resin rich layer and the joint may be applied to an already cured surface instead of a wet on wet connection. (see manufacturing). The strength of the joint should be documented. 3.1.3 Laminated joints are very sensitive to peel conditions. Peel stresses should be avoided. 3.1.4 For the interface between the joining laminates the matrix design rules given in Sec.5 apply. The resistance of the interface shall be determined with the same level of confidence as specified in Sec.4 [1.6]. It shall be recognised that the resistance of the interface between the laminates may not be the same as the corresponding resistance parameter of the joining laminates. Resin rich layers may even have to be analysed by different failure criteria, e.g., the yield criterion in Sec.6 [6]. 3.1.5 The laminates themselves, including possible over-laminations, shall be analysed like regular laminates. 3.2 Adhesive Joints 3.2.1 All issues related to laminated joints also apply to adhesive joints. 3.2.2 Geometrical details should be clearly specified, especially at points of stress concentrations like the edges of the joints. 3.2.3 The relationship between all elastic constants of both substrates and the adhesive should be carefully considered. Mismatches may introduce stresses or strains that can cause failure of the joint. 3.2.4 Thermal stresses should be considered. 3.2.5 Long term performance of adhesive should be established with great care. The long term performance is not only influenced by properties of the substrate, the adhesive and the interface, but also by the surface preparation and application method. 3.2.6 Relevant long term data shall be established exactly for the combination of materials, geometry, surface preparation and fabrication procedures used in the joint. 3.2.7 An adhesive joint may also introduce local through thickness stresses in the composite laminate that can lead to failure inside the laminate in the joint region. 3.3 Mechanical joints 3.3.1 Mechanical joints are often very sensitive to geometrical tolerances. 3.3.2 Creep of the materials shall be considered. 3.3.3 The pretension of bolted connections shall be chosen by considering possible creep of the material under the bolt. 3.3.4 It is preferred to design the joint in a way that its performance is independent of the matrix. This way matrix cracking or degradation of matrix properties are not important for the performance of the joint. 3.4 Joints in sandwich structures 3.4.1 All aspects related to laminated, adhesive and mechanical joint apply also to sandwich structures. 3.4.2 Sandwich structures have internal joints between core and skin and between cores. These joints are usually evaluated independently, but their properties are treated as an internal part of the sandwich system. Often the core properties are modified to incorporate the joint properties. 3.4.3 When two sandwich structures are joined complicated stress fields may result inside the sandwich structure. Stresses inside the core can be very different near a joint compared to the typical shear stresses in a panel. DET NORSKE VERITAS AS
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OS-C501

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