OS-C501

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Offshore Standard DNV-OS-C501, November 2013 Sec.4 Materials - laminates – Page 50 3.3.8 Tensile stress rupture data of matrix dominated properties may be used to estimate stress rupture in compression. Tensile stress rupture data of fibre dominated properties shall not be used to estimate stress rupture in compression because in compression viscoelastic effects of the matrix may reduce fibre support and give lower stress rupture values than measured under tension. 3.3.9 Compressive stress rupture data shall not be used to estimate stress rupture in tension. 3.3.10 If the component cannot tolerate matrix cracking, effects of long term static loads can only be ignored if both of the conditions below are fulfilled: — the stresses in the matrix are below the level of initiation of matrix cracking according to Sec.6 [3] — The matrix is not the main load carrying material. The component can carry the loads with a fully cracked matrix according to Sec.9 [2.2], i.e., all matrix dominated ply properties are set close to 0. 3.4 Static strength reduction due to permanent static loads 3.4.1 If a laminate is exposed to a permanent stress of any magnitude for a time t the static strength influenced by that stress, often called residual strength, should be estimated from the stress rupture curve: log σ = log σ 0stress rupture - β log t or σ = σ 0stress rupture - β log t The characteristic strength shall be determined according to Sec.4 [3.11]. The coefficient of variation COV of the strength after a certain time should be the same as the COV for short term data, unless a COV of remaining strength has been measured directly. Other formats of the equation may be used if supported by experimental evidence 3.4.2 Higher static strength values may be used with experimental evidence. Guidance note: A possible way to document that the residual strength is higher than given by the stress rupture curve is: a) Expose the test sample to a permanent load for 90% of the failure time expected according to the stress rupture curve. b) Measure the remaining strength after this exposure time. c) Repeat step a and b for at least one more stress level. d) If the remaining strength of the tests is the same, it can be assumed that the remaining strength is also the same up to 90% of the lifetime for lower load levels, provided no changes in failure modes are expected. The possible change of failure modes should be analysed. e) Measurements could be made for other test periods than 90% of the lifetime. ---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--- 3.4.3 The long term strains to failure may have an elastic and a plastic component. Strains shall be calculated based on [3.2]. 3.4.4 Static strength reduction of matrix dominated strength properties can be ignored if the conditions of [3.3.10] are fulfilled. 3.5 Stress relaxation 3.5.1 Permanent static deformations may have the following effects: — stress relaxation: a visco elastic or plastic process reducing the stresses in the material. This effect is accompanied by a reduction of the elastic modulus — residual strength reduction: the static short-term strength may be reduced. 3.5.2 The application of a permanent deformation may lead to stress relaxation. This is described as a reduction of the Modulus of elasticity. The result of the reduction of the Modulus of elasticity is a reduction of stress in the structure under the constant deformation. 3.5.3 Stress relaxation is a phenomenon mainly observed in the matrix. However, fibres may show some stress relaxation behaviour as well. 3.5.4 Ideally stress relaxation should be measured on the actual laminate for the relevant loading condition. 3.5.5 For fibre dominated elastic constants stress relaxation data of the same fibre type may be used to estimate the change of the modulus. Stress relaxation shall be measured for the combination of matrix and fibres. 3.5.6 For short fibre composites all elastic constants shall be considered to be matrix dominated with respect to stress relaxation. DET NORSKE VERITAS AS
Offshore Standard DNV-OS-C501, November 2013 Sec.4 Materials - laminates – Page 51 3.5.7 For matrix dominated elastic constants stress relaxation data of the matrix alone shall not be used to estimate the change of the modulus. 3.5.8 Tensile stress relaxation data of matrix dominated properties may be used to estimate stress relaxation in compression. Tensile stress relaxation data of fibre dominated properties shall not be used to estimate stress relaxation in compression because in compression viscoelastic effects of the matrix may reduce fibre support and give lower stress relaxation values than measured under tension. 3.5.9 Compressive stress relaxation data shall not be used to estimate stress relaxation in tension. 3.5.10 Creep modulus measurements may be used to estimate modulus changes under permanent deformation. 3.6 Change of modulus of elasticity under cyclic fatigue 3.6.1 The Modulus of elasticity of a composite laminate tends to reduce under the effect of cyclic fatigue. The main reason for the modulus change is the formation and accumulation of matrix cracks during tensile fatigue loads. The matrix cracks reduce the matrix dominated axial stiffness values. 3.6.2 The in-plane elastic ply constants of plies with thermoset matrix may be estimated to change to the values given in Table 4-6 after extensive cyclic fatigue exposure (about 10 6 cycles): Table 4-6 Change of modulus of elasticity under cyclic fatigue E 1 fibre UD-ply Modulus of elasticity in main fibre 10% reduction for glass and carbon fibres. Drops direction significantly for Aramid fibres loaded in compression E 2 matrix UD-ply Modulus of elasticity transverse to drops to 0 in tension main fibre direction no change in compression E 1 linear Modulus of elasticity in 0 fibre Drops to 0.9 E 1 non-linear from static measurements in cross-ply direction in the liner range tension. No change in compression E 2 linear cross-ply E 1 non-linear cross-ply E 2 non-linear cross-ply Modulus of elasticity normal to the 0 fibre direction in the liner range Modulus of elasticity in 0 fibre direction at the failure point Modulus of elasticity normal to the 0 fibre direction at the failure point G 12 linear In plane shear modulus in the linear range G 12 non-linear In plane shear modulus at the failure slight drop (unknown) point ν 12 Ply major Poisson’s ratio slight drop (unknown) ν 21 Ply minor Poisson’s ratio ν 21 = ν 12 E 2 / E 1 3.6.3 Experimental results may be used to demonstrate different changes of the elastic parameters during cyclic fatigue for specific laminates or loading conditions. 3.6.4 The structure shall be analysed for the values of the elastic parameters before fatigue damage has taken place and for the values of the elastic parameters after fatigue damage has taken place. 3.6.5 If the structure is exposed to through-thickness cyclic loads a degradation of the through-thickness properties shall be considered. Experimental evidence shall be provided. 3.6.6 The in-plane matrix dominated modulus does not change if the conditions in [3.8.5] are fulfilled. 3.7 Cycles to failure under cyclic fatigue loads Drops to 0.9 E 2 non-linear from static measurements in tension. No change in compression Is a combined effect of changes to fibre properties and matrix properties. Roughly a weighted average of the effects to E 1 fibre and E 2 matrix . Is a combined effect of changes to fibre properties and matrix properties. Roughly a weighted average of the effects to E 1 fibre and E 2 matrix . slight drop (unknown) 3.7.1 The number of cycles N to failure under a cyclic stress is described by an S-N curve for a specified R- ratio. 3.7.2 The R-ratio is defined as the minimum stress divided by the maximum stress. 3.7.3 For calculation of the R-ratio, note that tensile stresses are defined as positive, while compressive stresses are defined as negative. DET NORSKE VERITAS AS
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OS-C501

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