OS-C501

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Offshore Standard DNV-OS-C501, November 2013 Sec.4 Materials - laminates – Page 48 3 Properties under long term static and cyclic and high rate loads 3.1 Introduction 3.1.1 For all mechanical data three types of properties are relevant. These are static properties, properties under constant permanent static loads or deformations, and properties under cyclic loads or deformations. 3.1.2 Long term properties, like all properties are effected by exposure conditions. Long term data should be obtained for the environment and exposure conditions the material is used in. Some aspects related to changes due to exposure conditions are given in Sec.4 [5]. 3.1.3 Permanent static loads may have the following effects: — creep: a visco-elastic or plastic deformation with time. This effect is accompanied by a reduction of the elastic modulus — stress rupture: the material may loose strength leading to failure after some time — static strength reduction: the static short-term strength (often called residual strength) may become reduced. 3.1.4 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. 3.1.5 Cyclic loads may have the following effects: — reduction of elastic properties: usually due to the formation of matrix cracks. — fatigue failure: the material may loose strength leading to failure after a certain time. — static strength reduction: the static short-term strength (often called residual strength) may be reduced. 3.1.6 Fibre and matrix dominated properties show different characteristics with respect to long-term loads or deformations. 3.1.7 The long term properties of all static properties listed in Sec.4 [2] should be documented if relevant to the application. Measurements should be made on laminates that represent the actual lay-up as closely as possible. 3.1.8 Long term properties should be based on effects due to representative loads and environments. The loads described in Sec.3 [9.5] should be used. 3.1.9 Simplified approaches may be used if it can be documented that the results describe a worst case scenario. 3.1.10 For extrapolation of test data beyond the measured time see this section [3.11]. 3.1.11 Three aspects shall be considered when evaluating effects of long-term loads (Sec.6): — the effect of change of elastic parameters shall be checked by analysing the structure with initial and changed stiffness values — a lifetime analysis shall be carried out to establish that the structure will not fail within its design life — it shall be shown that the structure still tolerates possible extreme loads at the last day of its design life. This check has to investigate the change of the static properties with time. 3.2 Creep 3.2.1 The application of a permanent load may lead to increasing deformation of the material denoted as creep. This plastic deformation may permanently change the shape of the component. 3.2.2 Under the constant load, the increase of deformation results in an apparent reduction of the modulus of elasticity, and the reduction is denoted as the creep modulus. However, creep is plastic deformation process. The response to short term loads is not influenced by the long term loads and is governed by the original elastic constants. 3.2.3 Creep is a phenomenon mainly observed in the matrix. However, fibres may show some creep behaviour as well. 3.2.4 The creep of the composite laminate is a combined effect of the creep of the matrix and the fibres. 3.2.5 Ideally creep shall be measured on the actual laminate for the relevant loading condition. 3.2.6 For fibre dominated elastic parameters creep data of the same fibre type may be used to estimate creep. DET NORSKE VERITAS AS
Offshore Standard DNV-OS-C501, November 2013 Sec.4 Materials - laminates – Page 49 3.2.7 For short fibre composites all elastic constants shall be considered to be matrix dominated with respect to creep. Creep shall be measured for the combination of matrix and fibres. 3.2.8 For matrix dominated elastic constants creep data of the matrix alone shall not be used to estimate creep. 3.2.9 Tensile creep data of matrix dominated properties may be used to estimate creep in compression. Tensile creep data for fibre dominated properties shall not be used to estimate creep in compression because in compression viscoelastic effects of the matrix may reduce fibre support and give higher creep than measured under tension. 3.2.10 Compressive creep data shall not be used to estimate creep in tension. 3.2.11 The change in strain with time can be predicted using the following visco elastic equation (Findley’s theory). The first part describes the time independent elastic response and the second part describes the time dependent visco elastic response. ε = ε + elastic ε plastic or σ σ ε = + Eelastic E plastic with Eelastic E plastic the elastic modulus obtained from the quasi static data, typically for the duration of about 1 minute the time dependent plastic modulus obtained from creep data. 3.2.12 The time dependent plastic modulus is given by: where: t = time after loading ρ = constant for the visco elastic equation (in MPa) n = constant for the visco elastic equation (dimensionless) 3.2.13 The equation in [3.2.12] can also be expressed for a time dependent creep modulus E creep with a time independent and a time dependent part: 1 1 1 = + 3.3 Stress rupture E creep 3.3.1 The time to failure under a permanent static stress is described by a stress rupture curve. E 1 plastic E 3.3.2 The stress rupture curve should be represented as: log σ = log σ 0stress rupture - β log t or σ = σ 0stress rupture - β log t where t is the time to failure under a permanent stress σ. Other formats of the equation may be used if supported by experimental evidence. 3.3.3 The material parameters σ 0stress rupture and β shall be determined experimentally or be based on typical data as described in [8]. 3.3.4 Ideally stress rupture shall be measured on the actual laminate for the relevant loading condition and environment. 3.3.5 For fibre dominated strength values stress rupture data of the same fibre type may be used to estimate stress rupture. 3.3.6 For short fibre composites stress rupture of the matrix due to shear in the matrix shall be considered in addition to stress rupture of the fibres. 3.3.7 For matrix dominated strengths, stress rupture data of the matrix alone shall not be used to estimate stress rupture. Stress rupture shall be measured for the combination of matrix and fibres. = elastic ρ t n E plastic DET NORSKE VERITAS AS
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OS-C501

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