OS-C501

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Offshore Standard DNV-OS-C501, November 2013 Sec.6 Failure mechanisms and design criteria – Page 114 F F F F f + nn 1 = , if σ < 0 fibre II A + n = nt 1c with, A t = A c = 1.6 c 1 = R p R σ z ⊥ ( + ) ⊥Ψ A ⊥Ψ p − R ( + ) ⊥Ψ A ⊥Ψ , F , F − nn = − n = R ( −) ⊥Ψ A ⊥Ψ = 1 1 , F A nl = R R⊥ || ⊥⊥ p p R ( −) ⊥Ψ A ⊥Ψ τ τ nl nt = tan ψ ( + ) ( + ) ( + ) p ⊥Ψ p p ⊥ = ⊥⊥ || cos 2 ψ + sin 2 ψ R A A ⊥Ψ R ⊥⊥ R ⊥|| ( −) p 2 ⊥|| cos ψ + sin R ( −) ( −) p ⊥Ψ p ⊥⊥ 2 = A A R ⊥Ψ R ⊥⊥ ⊥|| R A ⊥⊥ d R⊥ = 2(1 + p (−) ⊥⊥ where the shape parameters ) ψ ( + ) ( −) ( + ) ( −) p ⊥|| , p ⊥|| ,p ⊥⊥ ,p ⊥⊥ should be determined experimentally. If they are not available the following default values shall be used: ( + ) ( −) ( + ) p = 0.30, p = 0.30, p = 0.15, p z R ⊥ = σ ∧ matrix , R d = matrix , shear ⊥ σ ∧ ⊥|| = . 4.3.4 The characteristic strength for each of the stress components matrix , matrix , shear 2t 2c and the corresponding coefficients of variation COV n are defined as specified in Sec.4 [1.6]. The combined COV comb is defined as: COV comb = max n (COV n ) 4.3.5 When two or more loads are combined, each stress component σ nk in direction n can be the result of several combined loads. In that case each stress component σ nk j , which is the local load effect of the structure in direction n due to load j, shall be considered separately as an individual stress component to determine the COV. or 2t σ ∧ 2c ( −) ⊥|| ⊥|| ⊥⊥ ⊥⊥ = σ ∧ 12 σ ∧ R 12 σ ∧ COV comb ⎛ j ⎞ ⎛ = ⎜∑σ . COVn ⎟ / ⎜ nk ∑ ⎝ n ⎠ ⎝ n COV comb. = max n (COV n ) Guidance note: This approach is conservative compared to the approach of Tukstra’s rule as used for the fibre design criteria. This approach has been chosen for simplification. In the case of fibre failure, only the strains parallel to the fibre directions have to be considered, whereas for matrix cracking all stress directions may interact. ---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--- 4.3.6 The choice of the partial safety factors shall be based on the most conservative partial safety factors obtained when treating each stress component σ nk j , which is the local load effect of the structure in direction n due to load j, as a single load. j σ nk ⎟ ⎠ ⎞ 0.27 DET NORSKE VERITAS AS
Offshore Standard DNV-OS-C501, November 2013 Sec.6 Failure mechanisms and design criteria – Page 115 4.3.7 The partial safety factors γ F and γ M shall be chosen as described in Sec.8 with a COV equal to COV comb , for both the characteristic strengths and the local load effects (see [4.3.4]to [4.3.6]). 4.3.8 The resistance model factor γ Rd shall be chosen to be 1.1. The model factor shall ensure a conservative result with respect to the simplifications made regarding the treatment of combined loads. 4.3.9 Matrix failure cannot be checked on a laminate level, it shall always be checked on a ply level. 4.4 Obtaining orientation of the failure surface 4.4.1 The orientation of the fibre failure surface is critical if a structure is loaded in compression. Matrix crack failure surfaces with an orientation of 30 o to 60 o relative to the plane of the laminate can reduce compressive fibre strength and reduce the resistance to delamination. 4.4.2 The orientation of the failure surface should be determined with the Puck design criterion by finding the angle q at which the matrix design criterion in [4.3.2] reaches its maximum. 4.4.3 If the laminate may have matrix cracks with an orientation of 30 o to 60 o relative to the plane of the laminate the compressive fibre strength shall be measured on laminates with the presence of such cracks and this value shall be used in the fibre design criterion (see this section under [3]). In this case the tested laminate should be equal to the one used in the component. Guidance note: Matrix cracks with an orientation of 30 o to 60 o occur mainly when the ply is exposed to high in-plane shear stresses or compressive stresses normal to the fibre direction. 4.5 Matrix cracking caused only by shear ---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--- 4.5.1 Some laminates may fail (rupture) due to shear in the plies without fibre failure. This condition was described in [3.1.5]. In this case matrix cracking due to stresses transverse to the fibres is acceptable. To check for this condition the matrix failure design criteria described in [4.1]-[4.3] may be used by applying them just for shear stresses. 4.5.2 For simple 2-D in-plane conditions the matrix cracking design criterion in [4.2] reduces to: where, σ 12 ∧ matrix 12 σ γ F γ Sd γ M characteristic value of the local load effect of the structure (stress) in the in-plane shear direction 12 characteristic value of the stress components to matrix cracking in the in-plane shear direction 12 partial load effect factor partial load-model factor partial resistance factor γ Rd partial resistance-model factor, γ Rd = 1.0 The co-ordinate system is the ply co-ordinate system. 4.6 Matrix failure checked by component testing 4.6.1 Refer to section on component testing (Sec.10). 5 Delamination 5.1 General 5.1.1 Delamination is a separation of plies. Delaminations are debonded areas that can grow gradually, once they are initiated. 5.1.2 Delaminations can also be debonding between core materials and skins. 5.2 Onset of delamination γ . γ . σ F Sd 12 ∧ matrix 12 σ < γ . γ 5.2.1 The onset of delamination due to in-plane stresses or strains is difficult to predict. It is known that delaminations will not initiate before matrix cracks have formed. It is, therefore, a conservative choice to model the onset of delamination with the matrix cracking criteria from [4]. M Rd DET NORSKE VERITAS AS
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