OS-C501

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Offshore Standard DNV-OS-C501, November 2013 Sec.4 Materials - laminates – Page 62 6.2 Change of production method 6.2.1 A change in production method, e.g., going from hand lay-up to filament winding, is usually accompanied by many other changes. Different resins are used, different fibres or sizings may be used, and the fibre fraction and void content may change significantly. 6.2.2 A re-qualification of the materials data shall be done if the production process is not similar to the original process - see [8.3]. A production process is similar as long as changes introduced by the new process can be quantified as described in the following parts of [6] and the process group is not changed. 6.2.3 Examples of process groups are: — hand-lay-up — spray-lay-up — pre-preg lay-up — autoclave curing (in combination with one of the other processes — resin injection processes — resin transfer moulding — centrifugal moulding — filament winding — tape winding. 6.3 Change of processing temperature and pressure 6.3.1 For simple processes of thermoset materials with only one cure temperature: — curing temperature of the reference data was higher than in reality. data shall be re-qualified — curing temperature of the reference data was lower than in reality: data are valid, as long as the temperature did not exceed the resin manufacturer's maximum curing temperature. 6.3.2 For simple processes of thermoset materials with only one cure pressure: — curing pressure of the reference data was higher than in reality: data shall be re-qualified — curing pressure of the reference data was lower than in reality: data are valid, as long as the pressure did not exceed the resin manufacturer's maximum curing pressure. 6.3.3 For thermoplastic composites where the plies are welded or fused together, a processing window of pressure and temperature should be established in which test data are applicable. 6.3.4 For complex curing cycles with varying temperature and pressure, any change to the process shall require requalification of the data. 6.4 Change of post cure procedure 6.4.1 Post-curing has no influence on fibre dominated tensile properties. 6.4.2 Post curing may improve fibre dominated compressive properties and all matrix dominated properties. 6.4.3 For some matrix systems post curing accelerates some slow crosslinking reactions that would also take place at room temperature. This is the case for many types of polyester. For these systems data obtained from post cured specimens are also valid for materials that were not post cured. 6.4.4 For some matrix systems post curing creates a better crosslinked network that will never be achieved if the laminate remains at room temperature. This is the case for many vinyl esters and epoxies. For these systems data obtained from post cured specimens are not valid for materials that were not post cured. 6.5 Change of void content 6.5.1 If the void content of the reference data was higher than in reality, data are valid. 6.5.2 If the void content of the reference data was more than 10% (relative) lower than in reality, data shall be re-qualified. 6.6 Correction for change in fibre volume fraction 6.6.1 For UD-plies: DET NORSKE VERITAS AS
Offshore Standard DNV-OS-C501, November 2013 Sec.4 Materials - laminates – Page 63 — Young’s modulus in fibre direction — Young’s modulus normal to fibre direction — In-plane shear modulus — Tensile strength in fibre direction — Compression strength in fibre direction X V * *0 f 1 = X1 ⋅ 0 0 V if V f < V f f * *0 X 1 = X 0 1 if V > f V f — Tensile strength normal to fibre — Compression strength normal to fibre — In plane shear strength The superscript 0 , identifies originally measured data. 6.6.2 These formulas are valid if: — the void content does not change by more than 10% (relative change) — the change of fibre volume fraction is not more than 10% (absolute change). 6.6.3 Measured fatigue properties may also be scaled by the formulas given above and within the limits given in [6.6.2]. In addition, fibre strength data shall only be corrected for fibre volume fractions up to 35%. If the fibre volume fractions are higher than 35%, considerable reductions in fatigue performance have been observed in the past, and fatigue data shall be re-established. 6.6.4 If the fibre fractions change by more than the validity range given in [6.6.2], the properties shall be reestablished. 6.6.5 Laminates with dry fibres (due to too high volume fractions or bad processing) have poor properties and shall not be accepted. 6.7 Control of fibre orientation: 6.7.1 If the variation of fibre angles is reduced compared to that of the reference data all data are valid. 6.7.2 If the variation of fibre angles is increased compared to that of the reference data fibre dominated strength and stiffness values shall be re-qualified. 6.7.3 If the variation of fibre angle of the reference data and the actual data is known the effect on stiffness and tensile strength may be calculated with laminate theory. Strength and stiffness shall be modified according to these calculations. These calculations shall not be used if the variation in fibre angles has increased by more than 5 o (absolute change). 6.7.4 The stiffness used in design shall be the mean of the effect of the variable fibre orientation while the strength shall be the strength of the least favourable fibre orientation, i.e. the minimum strength. 6.8 Control of fibre tension: V 0 E1 = E1 ⋅ V 6.8.1 For laminates with constant fibre tension: if the fibre tension is reduced compared to the reference data and fibres do not crimp or change orientation, all data are valid. If the fibre tension increases, fibre dominated values shall be re-qualified. f 0 f 0 1−V 0 f E2 = E2 ⋅ 1−V G 12 = G 1 0 12 X = X f 1−V ⋅ 1−V 0 1 V ⋅ V 0 Y1 ≈ Y 1 Y XY * *0 1 ≈ Y1 ≈ f 0 f 0 12 XY 12 0 f f DET NORSKE VERITAS AS
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OS-C501

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