OS-C501

Recommendations

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Offshore Standard DNV-OS-C501, November 2013 Sec.5 Materials – sandwich structures – Page 90 5.2.5 In the case of recently-manufactured foam core materials used in sandwich structures, the effect of outgassing under elevated temperature may induce subsequent delamination at the core-facing interface. 5.3 Effect of water 5.3.1 The effect of water on adhesive and core materials can be insignificant when the time of exposure is short. 5.3.2 If the laminate of the sandwich protect the entire core and the adhesive from the exposure, the resistance to water may be less critical. However, water diffuses through laminates and may degrade core properties. 5.3.3 Some core materials are specially treated or sealed to reduce any effects of water. The advantageous effect of the treatment shall be documented. If the treated core material is used in a component a quality procedure shall ensure that no untreated core material is used. 5.3.4 The effect of water on cellular core materials is typically severe for open-cell micro-structure and much less severe for closed-cell materials. 5.3.5 Moisture content has an important influence on the mechanical properties of wooden core materials. 5.3.6 Balsa and other wood core materials are very susceptible to water penetration and consequent swelling, debonding and rot, unless properly sealed and laminated. Guidance note: Flat-grain balsa (in which the grain lies parallel to the panel surface) has greater susceptibility to water permeation and is generally less satisfactory as a core material than 'End-grain balsa'. ---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--- 5.3.7 Crosslinked PVC cores and specially sealed and treated end grain balsa cores have demonstrated no degradation in a marine environment when embedded in typical laminates. Some documentation with respect to the resistance to a sea water environment shall be provided. 5.3.8 Resistance to water of core materials at high pressures in deep water is unknown and shall be demonstrated if relevant. 5.3.9 The effect of seawater is generally less severe than the effect of fresh water. 5.3.10 The effect of distilled water is more severe than the effect of fresh water. 5.3.11 The combination of water and high temperature may be worse than the individual effects of temperature and water. 5.3.12 In case of a core material permanently exposed to water, long term properties should be documented. 5.4 Effect of chemicals 5.4.1 The compatibility of cores and adhesives to the exposure to chemicals shall be demonstrated. 5.4.2 In a qualitative way chemicals tend to have the same effects as water on cores and adhesives. The degradation rates shall be obtained for the actual materials in question. 5.4.3 If the laminate of the sandwich protect the core and the adhesive from the exposure, the resistance to chemicals may be less critical. However, most chemicals can diffuse through laminates and may attack the core. 5.5 Effect of UV radiation 5.5.1 Core materials are typically embedded in a laminate and are not exposed to direct UV radiation. 5.5.2 If core materials are exposed to UV radiation their resistance shall be documented and quantified if necessary. 5.6 Electrolytic corrosion 5.6.1 Possible electrolytic corrosion should be considered when metallic components and carbon fibres are used in a sandwich structure. 5.7 Combination of environmental effects 5.7.1 The combination of environmental effects on materials, like combined humidity and heat, may be worse than the individual effects. DET NORSKE VERITAS AS
Offshore Standard DNV-OS-C501, November 2013 Sec.5 Materials – sandwich structures – Page 91 5.7.2 Test data should be obtained of the combined effect of environments on the material properties if relevant. The worst relevant combination of environments should be used for testing. 6 Influence of process parameters and core density 6.1 Core production 6.1.1 Core materials are generally manufactured in a well-controlled process. This standard does not address manufacturing methods of core materials. It is assumed that the measured and reported core properties can be reproduced and guaranteed by the core manufacturer. 6.1.2 In some cases the core properties may be influenced by the joining methods used to join the skins or core sections. This aspect is described in [6.2]. 6.2 Sandwich production 6.2.1 The skins are usually laminated onto the core or glued onto the core. Sections of the core may also be joined with adhesives or some fillers. 6.2.2 The joints may influence the properties of the core either by modifying the core material itself or by becoming part of the core system. Core properties shall be measured and evaluated with the presence of such joints. 6.2.3 Changes in process parameters effect joints in a similar way as described for the matrix in a laminate in [6]. 6.3 Influence of core density 6.3.1 For core materials that are available in different densities, all mechanical properties shall be assumed to depend on the density unless evidence exists that suggests otherwise. 6.3.2 All relationships between properties and density shall be confirmed by experimental evidence for the particular core type. Data may be interpolated, but should not be extrapolated to densities outside the measured range. 6.3.3 If a relationship can be established between core density and certain core properties qualification of material properties may be simplified as described in section [8.6]. 7 Properties under fire 7.1 Introduction 7.1.1 The performance of composites in a fire is a complex process, because the various constituent materials respond differently to a fire. 7.1.2 The requirements under fire conditions can usually be found in the fire codes for a particular application. 7.1.3 Fire codes may implicitly assume that the structure is built of steel or metal. The relevance of a fire code to composite materials shall be checked carefully. 7.1.4 Since most composites are flammable and temperature sensitive most applications use protective measures to reduce the impact of fire. In this case the fire performance of the complete system, composite structure with fire protection shall be evaluated. 7.1.5 Some aspects of fire performance can be modelled, but some experimental testing shall always be done to demonstrate fire performance. Guidance note: Sandwich panels with no internal cavities, such as wood and foam core materials, will not allow any stack effect or neither help the transportation of combustion products. Conversely, sandwich structures with internal cavities will enhance these phenomenon. 7.2 Fire reaction ---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--- 7.2.1 Fire reaction describes the response of a composite to fire in terms of flammability, flame spread, smoke development and emission of toxic gases. All these aspects shall be documented if relevant. 7.2.2 Special additives or fillers are often added to composites to improve fire reaction. DET NORSKE VERITAS AS
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OS-C501

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