OS-C501

Recommendations

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Offshore Standard DNV-OS-C501, November 2013 Sec.9 Structural analysis – Page 148 σ σˆ E lin E nonlin Figure 9-1 Typical stress-strain relation for a laminate containing 0, 45 and 90 layers ε corr Matrix cracking in 90 layers Matrix cracking in 45 layers εˆ Fibre failure in 0 layer. Laminate failure ε 3.3 Creep, stress relaxation and stress rupture-stress relaxation 3.3.1 The presence of creep, stress relaxation and stress rupture-stress relaxation in composite structures depends on the level of stresses and or strains and the condition of the constituent materials (intact, presence of cracks or other failures). 3.3.2 Only analysis methods that provide conservative estimates of stresses/strains and material conditions should be applied in predictions of phenomena like creep. 3.4 Fatigue 3.4.1 Failure due to fatigue may develop under long term cyclic loading conditions. 3.4.2 Development of fatigue failure depends on the maximum strains during each cycle, as well as the total number of cycles with strains exceeding prescribed limits. 3.4.3 Thus, analysis methods providing conservative strain estimates may be applied in conjunction with fatigue. 4 Analytical methods 4.1 General 4.1.1 Analytical methods can be divided into two classes: Analytical solutions of (differential) equations or use of handbook formulae. 4.2 Assumptions and limitations 4.2.1 Analytical methods shall not be used outside their assumptions and limitations. Guidance note: The main disadvantage of available analytical solutions is that simplifications often put too many restrictions on geometry, laminate build-up etc. and hence, are insufficient in the design of more complex composite structures. Handbook formulae are usually too simple to cover all the design issues and are also in general not sufficient. Simplified isotropic calculation methods should not be used, unless it can be demonstrated that these methods give valid results. ---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--- 4.2.2 For analytical analysis of sandwich structures, special care shall be taken with assumptions in approximate solutions that may be found in textbooks. 4.3 Link to numerical methods 4.3.1 Analytical solutions or handbook formulae used within their assumptions and limitations may be used to validate finite element analysis results. DET NORSKE VERITAS AS
Offshore Standard DNV-OS-C501, November 2013 Sec.9 Structural analysis – Page 149 5 Finite element analysis 5.1 General 5.1.1 Only recognised FE programs should be used. Other programs shall be verified by comparison with analytical solutions of relevant problems, recognised FE codes and/or experimental testing. 5.2 Modelling of structures – general 5.2.1 Element types shall be chosen on the basis of the physics of the problem. 5.2.2 The choice of the mesh should be based on a systematic iterative process, which includes mesh refinements in areas with large stress/strain gradients. 5.2.3 Problems of moderate or large complexity shall be analysed in a stepwise way, starting with a simplified model. 5.2.4 Model behaviour shall be checked against behaviour of the structure. The following modelling aspects shall be treated carefully: — loads — boundary conditions — important and unimportant actions — static, quasi-static or dynamic problem — damping — possibility of buckling — isotropic or anisotropic material — temperature or strain rate dependent material properties — plastic flow — non-linearity (due to geometrical and material properties) — membrane effects. 5.2.5 Stresses and strains may be evaluated in nodal points or Gauss points. Gauss point evaluation is generally most accurate, in particular for layered composites, in which the distribution of stresses is discontinuous, and should therefore be applied when possible. Guidance note: The analyst shall beware that Gauss point results are calculated in local (element or ply based) co-ordinates and must be transformed (which is automatically performed in most FE codes) in order to represent global results. Thus, Gauss point evaluation is more time-consuming than nodal point calculations. ---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--- 5.2.6 Support conditions shall be treated with care. Apparently minor changes in support can substantially affect results. In FE models, supports are typically idealised as completely rigid, or as ideally hinged, whereas actual supports often lie somewhere in between. In-plane restraints shall also be carefully treated. 5.2.7 Joints shall be modelled carefully. Joints may have less stiffness than inherited in a simple model, which may lead to incorrect predictions of global model stiffness. Individual modelling of joints is usually not appropriate unless the joint itself is the object of the study. See also requirements for the analysis of joints in Sec.7. 5.2.8 Element shapes shall be kept compact and regular to perform optimally. Different element types have different sensitivities to shape distortion. Element compatibility shall be kept satisfactory to avoid locally poor results, such as artificial discontinuities. Mesh should be graded rather than piecewise uniform, thereby avoiding great discrepancy in size between adjacent elements. 5.2.9 Models shall be checked (ideally independently) before results are computed. 5.2.10 The following points shall be satisfied in order to avoid ill-conditioning, locking and instability: — a stiff element shall not be supported by a flexible element, but rigid-body constraints shall be imposed on the stiff element — for plane strain and solid problems, the analyst shall not let the Poisson’s ratio approach 0.5, unless a special formulation is used — 3-D elements, Mindlin plate or shell elements shall not be allowed to be extremely thin — the analyst shall not use reduced integration rule without being aware of possible mechanism (e.g. hourglass nodes). DET NORSKE VERITAS AS
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OS-C501

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