OS-C501

Offshore Standard DNV-OS-C501, November 2013
Sec.6 Failure mechanisms and design criteria – Page 109
3.5 Fibre failure checked by component testing
3.5.1 Refer to section on component testing (Sec.10)
3.6 Fracture mechanics approach
3.6.1 The fibre design criteria described above can always be used. However, in the presence of stress
concentrations that reach infinity a fracture mechanics approach may be applied.
3.6.2 Stress concentration can be caused by the following factors:
— cut-outs
— discontinuous linear and smooth geometry (including rough edges)
— joints which include bolted joints, bonded joints, and other mechanical joints
— mismatch of elastic properties between two adjacent components or materials
— voids and damage due to material fabrication.
3.6.3 Unidirectional laminates should never be used in the presence of infinite stress concentrations, because
matrix cracks and delaminations can propagate from that point through the structure with nearly no resistance.
3.6.4 In the presence of infinite stress concentrations matrix cracking and delamination will occur. If that is
not acceptable on a local level, the design shall be changed to remove the stress concentration.
3.6.5 The suggested design criterion is the point stress criterion: Failure occurs when the stress or strain at a
distance d 0 away from the tip of the stress concentration point is equal to or greater than the strength of the unnotched
material, see Figure 6-3. This means the design criteria described above shall be applied at a distance
d 0 away from the stress concentration point.
x
σ
( 0 , y) < ˆ σ
y = R + d
x 0
σ 0
σ x
d 0
( )
R
y
Figure 6-3
Point Stress Criterion.
3.6.6 The distance d 0 has to be determined experimentally for the laminate in question.
3.6.7 The stress field ahead of the stress concentration point may be calculated by analytical or FE methods.
Guidance note:
For an infinite orthotropic plate, with a circular hole, subjected to a uniform stress, σ x ∞ , applied parallel to the x-axis
at infinity, the normal stress, σ x , along the y-axis ahead of the hole, see Figure 6-4 below, can be expressed as:
where
σ
x
∞ 2
4
6
σ ⎪
⎧ ⎛ ⎞ ⎛ ⎞
⎡ ⎛ ⎞ ⎛ ⎞
8 ⎤ ⎪
⎫
x R R
∞ R R
= ⎨ 2 + ⎜ ⎟ + 3 ⎜ ⎟ − K
T
− 3 ⎢ 5 ⎜ ⎟ − 7 ⎜ ⎟ ⎥ ⎬
2 ⎪⎩ ⎝ y ⎠ ⎝ y ⎠
⎢⎣
⎝ y ⎠ ⎝ y ⎠ ⎥⎦
⎪⎭
K
∞
T
⎧
⎪ ⎡
= 1 + ⎨ 2 ⎢
⎪⎩
⎢⎣
E
E
x
y
⎤
⎥
⎥⎦
0 .5
− ν
xy
+ 0 .5
E
G
x
xy
⎫
⎪
⎬
⎪⎭
0 .5
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