OS-C501

Offshore Standard DNV-OS-C501, November 2013
Sec.9 Structural analysis – Page 146
2.6 Two-step non-linear failure analysis method
2.6.1 The method may be applied for both 2-D and 3-D problems, see [1.3.4].
2.6.2 In the results presented for this method it is assumed that matrix failure occurs prior to fibre breakage,
see [2.4.3].
2.6.3 In the first step response calculations are performed with non-degraded material properties.
2.6.4 In regions (e.g. finite elements) where the strains (or stresses) exceed the level for matrix cracking (or
other failure mechanisms), the in-plane material properties are degraded according to the method presented in
[2.5.5].
2.6.5 The final step consists of response calculations with the locally degraded material properties.
2.6.6 If the final calculations break down, e.g. due to ill-conditioned structural matrices, one should repeat the
final step with non-degraded through thickness parameters.
2.6.7 For problems related to the local degradation of material properties, see [2.2.7].
2.6.8 For difficulties arising in numerical calculations when using locally degraded values equal to 0, and the
possibilities to apply larger values for the degraded parameters, refer to [2.2.4] and [2.2.5].
2.6.9 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 under [4] and [5]).
2.6.10 After local occurrences of matrix cracking statically determined problems result in:
— laminate stiffness – generally correct, locally too low
— laminate stresses – correct
— laminate strains – generally correct, locally too large
— ply stiffness – generally correct, locally E 1 is correct and the other ply properties are mostly too small
— ply stresses – generally correct, locally σ 1 is too large and the other stress components are mostly too small
(zero)
— ply strains – generally correct, locally too large.
2.6.11 After local occurrences of matrix cracking problems with known displacements result in:
— laminate stiffness – generally correct, locally too low
— laminate stresses – generally correct, locally too small
— laminate strains – correct
— ply stiffness – generally correct, locally E 1 is correct and the other ply properties are mostly too small
— ply stresses – generally correct, locally σ 1 is correct and the other stress components are mostly too small
(zero)
— ply strains – correct.
2.6.12 After local occurrences of matrix cracking statically indeterminate problems result in:
— laminate stiffness – generally correct, locally too low
— laminate stresses – generally correct, locally between too small and correct
— laminate strains – generally correct, locally too large
— ply stiffness – generally correct, locally E 1 is correct and the other ply properties are mostly too small
— ply stresses – generally correct, locally σ 1 is too large and the other stress components are mostly too small
(zero)
— ply strains - generally correct, locally too large.
2.6.13 The applicability of this method in conjunction with various failure mechanisms is discussed under [3].
2.7 Through thickness 2-D analysis
2.7.1 [2.2] deals with an in-plane 2-D analysis method that is applicable if through thickness stresses can be
neglected, see [1.3.4]. The in-plane 2-D approach is frequently used in conjunction with global analysis of
relatively large composite structures.
2.7.2 On the other hand, certain structural details, in which plane strain conditions prevail, may be analysed
by a through thickness (cross section) 2-D approach.
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