OS-C501

Offshore Standard DNV-OS-C501, November 2013
Sec.9 Structural analysis – Page 145
— laminate stresses – between correct and too large
— laminate strains – generally 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 – generally too small.
2.4.10 The applicability of this method in conjunction with various failure mechanisms and the need for
modifying certain failure criteria are discussed under [3]. The fibre failure criterion shall only be used with the
factor γ a defined in [3.2]. It shall also be ensured that the ply strain and stress in fibre direction are multiplied
by γ a in all other failure criteria or analysis where strain or stress values exceed the level of matrix cracking.
2.5 Linear failure analysis with degraded properties
2.5.1 In this approximate failure method globally degraded material properties are applied. This implies
effectively that matrix cracking has occurred in the laminate, and that the laminate is not checked for matrix
cracking, see [3.1.3].
2.5.2 The method may be applied for both 2-D and 3-D problems, see [1.3.4] and [2.5.5].
2.5.3 In the results presented for this method it is assumed that fibre breakage is the last failure mechanism to
occur, see [2.4.3].
2.5.4 This method should be mainly used for statically determined problems. Otherwise this simplified
method, with global 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.
2.5.5 The material properties are degraded in the entire domain by changing certain ply elasticity parameters.
For in-plane 2-D analysis the stiffness in the fibre direction, E 1 , of each ply is kept unaltered, while the rest of
the 2-D ply properties should be changed according to Sec.4 [9], assuming the matrix has cracked due to high
ply stresses τ 12 and σ 2 . This is equivalent to using the degraded stiffness (E nonlin see Figure 9-1) in the laminate
stress-strain relation for loads in the fibre directions. If 3-D analysis is required, the in-plane parameters are
dealt with as in the 2-D analysis. Through thickness parameters should only be changed according to Sec.4 [9],
if through thickness stresses cause matrix cracking.
2.5.6 For difficulties arising in numerical calculations when using degraded values equal to 0, and the
possibilities to apply larger values for the degraded parameters, refer to [2.2.5] and Sec.4 [9].
2.5.7 Stresses and strains should be calculated at the laminate and ply levels.
2.5.8 For in-plane 2-D analysis statically determined problems result in:
— laminate stiffness – too low
— laminate stresses – correct
— laminate strains – too large
— ply stiffness – E 1 is correct, the other ply properties are generally too small
— ply stresses – σ 1 is too large, the other stress components are generally too small(zero)
— ply strains – too large.
2.5.9 For in-plane 2-D analysis problems with known displacements result in:
— laminate stiffness – too low
— laminate stresses – too small
— laminate strains – correct
— ply stiffness – E 1 is correct, the other ply properties are generally too small
— ply stresses – σ 1 is correct, the other stress components are generally too small (zero)
— ply strains – correct.
2.5.10 For in-plane 2-D analysis statically indeterminate problems result in:
— laminate stiffness – too low
— laminate stresses – between too small and correct
— laminate strains – generally too large
— ply stiffness – E 1 is correct, the other ply properties are generally too small
— ply stresses – σ 1 is too large, the other stress components are generally too small(zero)
— ply strains – generally too large.
2.5.11 The applicability of this method in conjunction with various failure mechanisms is discussed under [3].
DET NORSKE VERITAS AS