OS-C501

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Offshore Standard DNV-OS-C501, November 2013 Sec.6 Failure mechanisms and design criteria – Page 116 5.3 Delamination growth 5.3.1 Growth of interlaminar cracks can be analysed with a fracture mechanics approach. The crack will propagate when the strain energy release rate G will reach the critical strain energy release rate G critical . The design criterion is then given by: G critical γ F . γ Sd . G < γ . γ G shall be calculated using local interlaminar stresses. For γ Rd see [5.3.2]. 5.3.2 G critical depends often on the crack length: — if the dependence on crack length can be considered γ Rd = 1 — if the dependence on crack length is not known γ Rd = 2 5.3.3 G critical has to be chosen for the appropriate crack opening mode. For mixed mode conditions G critical can be calculated as a weighted average of G I critical and G II critical . 6 Yielding 6.1 General 6.1.1 Yielding design criteria apply to most polymer core materials of sandwich structures. 6.1.2 Yielding design criteria may apply to a matrix material with plastic characteristics if the matrix is located in a region where it is not restrained between fibres, e.g. in the case of resin rich layers. 6.1.3 Yielding applies also to typical liner materials, like thermoplastics or resin rich layers. 6.1.4 The von Mises' yield criterion shall be used to describe materials that yield. The stresses used in this criterion are the principal stresses. where, σ n σ y γ F γ Sd, γ M F characteristic principal stresses, n=1,2,3 characteristic value of the yield stress of the material partial load effect factor load-model factor partial resistance factor γ Rd resistance-model factor, γ Rd = 1.0. 6.1.5 The characteristic yield strength σ y and the corresponding coefficients of variation COV are defined as specified in Sec.4 [1.6] and Sec.5 [1.6]. 6.1.6 When two or several loads are combined, each stress component σ n in direction n can be the result of several combined loads. In that case each stress component σ n j , local response of the structure in direction n due to load j, shall be considered separately as an individual stress component. 6.1.7 The choice of the partial safety factors shall be based on the most conservative partial safety factors obtained when treating each stress component σ n j , local response of the structure in direction n due to load j, as a single load. The material's COV shall always be the COV of the yield strength. 6.1.8 Foam cores under significant hydrostatic pressure or tension shall be checked in addition for ultimate failure of orthotropic homogenous materials [7]. Significant hydrostatic pressure or tension exists if: where σ 1 , σ 2 , σ 3 are the principal stresses with σ 1 > σ 2 > σ 3 . Sd M Rd M Rd 2 2 2 ( σ 1 −σ 2 ) + ( σ 2 − σ 3 ) + ( σ 3 −σ 1 ) σ y γ . γ . γ . γ . < 1 0.1σ 3 ≤ ( σ 1 + σ 2 + σ 3 ) ≤ 0. 1σ 1 3 DET NORSKE VERITAS AS
Offshore Standard DNV-OS-C501, November 2013 Sec.6 Failure mechanisms and design criteria – Page 117 7 Ultimate failure of orthotropic homogenous materials 7.1 General 7.1.1 The criterion can only be used for orthotropic homogenous materials, i.e. materials with three axes of symmetry. Isotropic materials are included, since they are a sub-group of orthotropic materials. This criterion is typically applied to core materials. Strength values shall be determined relative to the axes of material symmetry ([1.4]). The criterion is not valid for fibre reinforced laminates, because laminates are not homogeneous. 7.1.2 The criterion can be applied to brittle core materials or to polymeric materials after yield. 7.1.3 The following design criterion should be used when the stress in one direction is dominating compared to the stresses in the other directions. The stress in one direction is said to be dominating when the criterion in [7.1.4] is not satisfied. The criterion shall be applied for tensile and compressive stresses. where, n σ nk ∧ direction of the dominating stress characteristic value of the local load effect of the structure (stress) in direction n σ nk characteristic value of the strength (stress to failure for component n) γ F partial load effect factor γ Sd partial load-model factor γ M partial resistance factor γ Rd partial resistance-model factor, γ Rd = 1.0. Local response and strength must be given in the same co-ordinate system. Guidance note: A typical example for core materials in a sandwich would be to check that the through thickness shear stress does not exceed the shear strength. In many cases the shear stress is the dominating stress component, however, this shall be checked with the criterion in [7.1.4]. ---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--- 7.1.4 The interaction between the stress components in several directions shall be taken into consideration when the following criterion is satisfied. In that case, there is no dominating stress and the interaction can not be disregarded. max i where i and n refer to the directions: 11, 22, 33, 12, 13, 23. For isotropic materials the directions shall be either the principal normal stresses or the principal shear stresses. For orthotropic materials the directions shall be the material axes. The same co-ordinate systems shall be used in [7.1.3] and [7.1.4]. 7.1.5 When the interaction between the stress components in several directions shall be taken into consideration, the design criterion below shall be applied. The criterion shall be applied for tensile and compressive stresses. where, n refers to the directions 11, 22, 33, 12, 13, 23 characteristic value of the local load effect of the structure (stress) in the direction n σ nk ∧ σ ∧ σ ik ik σ / ∑ nk ≤ 10 n ≠ i ∧ σ nk γ . γ γ . γ . σ σ nk characteristic value of the strength (stress to failure for component n) γ F partial load effect factor γ Sd, partial load-model factor γ M partial resistance factor γ Rd partial resistance-model factor, γ Rd = 1.25. F Sd . γ F M . γ Sd Rd . nk ∧ σ nk < γ . γ ∑ n M ⎛ ⎜ σ ⎜ ∧ ⎝ σ Rd nk nk ⎞ ⎟ ⎟ ⎠ 2 < 1 DET NORSKE VERITAS AS
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