OS-C501

Offshore Standard DNV-OS-C501, November 2013
Sec.9 Structural analysis – Page 153
9 Swelling effects
9.1 General
9.1.1 Changes in gas or fluid absorption from the environment resulting in dimensional changes of the body
shall be taken in account. The general swelling strains, e i , can be expressed as
where β i is the swelling expansion coefficients and C is swelling agent concentration inside the laminate.
9.1.2 Accordingly, the stress-strain relations shall be modified to account for the stress free environmentally
induced expansion strains as follows:
10 Analysis of sandwich structures
10.1 General
10.1.1 A typical load carrying sandwich structure has the following characteristics; it is build up of three
elements: two faces, usually stiff and strong; a core, weaker and lighter; a joint, continuous along each of the
two interfaces. Additionally, see the definition in Sec.14.
10.1.2 All the sandwich structures that do not fall into the above definition are denoted special sandwich
structures. A distinction is made between typical and special sandwich panels. Simple formulas are provided
for design of typical sandwich panels whereas special ones shall be designed on the basis of more rigorous
analyses and possibly testing.
10.1.3 A decision to use 2-D or 3-D analysis shall be made depending on the level of significance of the
through thickness stresses/through width strains (see [1.3.4]). If all through thickness stress components may
be neglected, in-plane 2-D analysis may be applied, and if plane strain conditions prevail, a through thickness
2-D approach may be adopted. Otherwise, 3-D analysis should be performed.
10.1.4 In the context of FEA of sandwich structures (one of) the following element types or combinations
should be applied:
— a single layer of layered shell elements through the thickness of the entire sandwich material (for in-plane
2-D analysis, see [1.3.4])
— (layered) shell elements for the faces and solid elements for the core (for 3-D and through thickness 2-D
analysis, see [1.3.4]). In this case a compensation may be desirable for the change in stiffness, or
alternatively, in order to avoid overlapping areas, shell elements can be positioned adequately without the
need for modifying the material properties by using the eccentricity property of the element. Depending on
the commercial package used this option is not always available
— solid elements for both faces and core (for detailed 3-D and through thickness 2-D analysis, see [1.3.4]).
10.1.5 For the analysis of sandwich structures, special considerations shall be taken into account, such as:
— elements including core shear deformation shall be selected
— for honeycomb cores one shall account for material orthotropy, since honeycomb has different shear
moduli in different directions
— local load introductions, corners and joints, shall be checked
— curved panels with small radii of curvature shall be analysed in 2-D (through thickness direction) or 3-D to
account for the transverse normal stresses not included in shell elements.
10.1.6 The load combinations and associated load factors and the surrounding environmental conditions
established in Sec.3 shall be applied to the loads to calculate stresses and strains in the structure.
10.1.7 Each point in the structure shall be checked for all times against the specified functional requirement
and corresponding failure modes.
10.1.8 Failure criteria for each mechanisms of failure are described in Sec.6.
10.2 Elastic constants
e = β C i
i
{ ε}
= [ S ]{ σ } + { e}
10.2.1 Each laminate shall be described with the suitable set of elastic constants as mentioned in section 4
dealing with monolithic structures.
10.2.2 Core materials are generally orthotropic and are described by more than two elastic constants (see
Sec.5). However, most FE codes can only describe isotropic core materials. If the elements applied in the FEA
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