OS-C501

Offshore Standard DNV-OS-C501, November 2013
Sec.9 Structural analysis – Page 144
the ply may still carry loads because stresses can be redistributed around the location of failure. The redistribution of
stresses can be of type in-plane (within the same ply) or through thickness (into the neighbouring plies). If
considerable through thickness redistribution occurs, a 3-D progressive failure analysis should be applied, see [2.3].
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2.2.9 The failure analysis is repeated (for the same load level) until no new failure mechanism is detected.
Then, the load level is increased, and a similar failure analysis is performed.
2.2.10 Whenever a violation of the ultimate limit state (ULS) condition is detected the analysis is terminated.
2.3 3-D progressive failure analysis
2.3.1 If at least one of the through thickness stress components cannot be neglected (see [1.3.4]), 3-D effects
shall be taken into account. Then the development of failure is most accurately predicted by 3-D progressive
failure analysis on the ply level.
2.3.2 Initially, non-degraded 3-D ply properties (E 1 , E 2 , G 12 , ν 12, E 3 , G 13 , G 23 , ν 13 and ν 23 ) shall be used in
the progressive non-linear failure analysis.
2.3.3 A crucial effect of the through thickness stresses is the possibility of delamination.
2.3.4 In addition to the failure criteria accounted for in the 2-D failure analysis in [2.2], the delamination failure
criterion at the ply level should now be considered (see Sec.6 [5]).
2.3.5 Otherwise, the 3-D progressive failure algorithm follows the same steps as the 2-D method presented
under [2.2].
2.3.6 3-D ply properties shall be degraded by the same principles as described for 2-D properties in [2.2] and
Sec.4 [9].
2.4 Linear failure analysis with non-degraded properties
2.4.1 In this simplest approximate failure method non-degraded or initial material properties are applied.
2.4.2 The method may be used for both 2-D and 3-D problems, see [1.3.4].
2.4.3 In the results presented for this method, it is assumed that matrix failure occurs prior to fibre breakage,
see [2.1.10]. On the other hand, if fibre failure is not the last failure type to occur in the laminate, the overview
of the connection between analysis methods and failure criteria under [3] is not applicable.
2.4.4 In certain cases, this simplified method, without local degradation of material properties, may offer
considerably incorrect stress/strain distributions, see [2.1.9]. If the error cannot be analysed and included into
the model factor (see [12.3]) a more refined method shall be used.
2.4.5 Stresses and strains should be calculated on the laminate and ply levels.
2.4.6 Before matrix cracking (and other kinds of failure mechanisms) the method predicts correct response
values provided that the underlying analytical or numerical (FE) analysis method is applied within its
assumptions and limitations (see [4] and [5]).
2.4.7 After matrix cracking statically determinate problems result in:
— laminate stiffness – too high
— laminate stresses – correct
— laminate strains – too small
— ply stiffness – E 1 is correct, the other ply properties are generally too large
— ply stresses – σ 1 is too small, the other stress components are generally too large
— ply strains – too small.
2.4.8 After matrix cracking problems with known displacements result in:
— laminate stiffness – too high
— laminate stresses – too large
— laminate strains – correct
— ply stiffness – E 1 is correct, the other ply properties are generally too large
— ply stresses – σ 1 is correct, the other stress components are generally too large
— ply strains – correct.
2.4.9 After matrix cracking statically indeterminate problems result in:
— laminate stiffness – too high
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