OS-C501

Offshore Standard DNV-OS-C501, November 2013
Sec.4 Materials - laminates – Page 52
3.7.4 The material curve of fibre dominated properties for the lifetime strength analysis should be described as:
log σ = log σ 0 fatigue - α log N
or
log ε = log ε 0 fatigue - α log N
3.7.5 The strain representation is simpler, because it is applicable to a wider group of materials and fatigue
data are less effected by volume fraction changes. The strain representation can be obtained from the stress
representation by using the relationship σ = E ε.
3.7.6 The double logarithmic representation of fatigue data shall be chosen.
3.7.7 All fatigue curves shall be obtained from load controlled tests, unless the structure is clearly only exposed
to deformation controlled fatigue.
3.7.8 S-N curves should be preferably obtained for R ratios relevant for the application. Minimum
requirements are given in [3.7.9]-[3.7.11].
3.7.9 If the structure is exposed to tensile and compressive fatigue, at least data for R= -1 shall be available.
3.7.10 If the structure is only exposed to tensile fatigue, data for R with 1 < R ≤ 0 may be used.
3.7.11 If the structure is only exposed to compressive fatigue, data between R= -1 or R=10 may be used.
3.7.12 Care shall be taken to identify whether fatigue data are given as stress amplitude or stress range.
3.7.13 A constant amplitude lifetime diagram shall be constructed from the fatigue curves if the structure is
exposed to fatigue stresses of other R ratios than the measured ones or to various R-ratios. The diagram can be
used to extrapolate expected number of cycles to failure for different combinations of mean and amplitude.
Guidance note:
Constant amplitude lifetime diagrams CAL are commonly used to obtain fatigue lifetimes for a given stress amplitude
and mean. Fatigue data are often only available for three R-ratios, R=10, -1, and 0.1. These data represent three lines
in the CAL diagram, other values have to be extrapolated. Linear extrapolations may be used, giving the CAL diagram
typically triangular shape.
Figure 4-3 gives an example of a CAL diagram.
- The diagram was based on characteristic fatigue curves measured at the R-ratios R = 10, -1, and 0.1. In addition
the characteristic static tensile and compressive strains at failure were needed.
- The CAL diagram can be divided into four sectors in this case. The sectors are shown in Figure 3. Within each
sector constant life lines were drawn for lifetimes of 10, 100, 1000, ... cycles. These lines are assumed to be straight.
- For sectors 1 and 4 all lines were connected to the static tensile and compressive strains at failure.
- If fatigue data at other R-ratios exist an equivalent approach with more (or less) sectors can be used.
The expected lifetime N exp for a given strain amplitude α and mean η can be found by the following procedure (see
also Figure 4-3):
1) Draw the point P in the constant amplitude life diagram representing the given strain amplitude α and mean η.
2) Draw a line a from the origin of the constant amplitude life diagram (0 mean, 0 amplitude) through and beyond
the point P.
3) Identify the two closest constant life lines nearest to P, n 1 and n 2 , where n 2 is the line with the higher number of
cycles to failure.
4) Measure the length a 1 on line a between the two constant life lines n 1 and n 2 nearest to P.
5) Measure the length a 2 on line a between point P and the constant life line n 2 with the higher number of cycles
nearest to P.
6) Find the line b nearest to P representing fatigue life of a measured R-ratio, e.g. R=10, or R=-1, or R=0.1.
7) Measure the length b 1 on b between n 1 and n 2 .
8) Calculate b 2 = b 1 a 2 / a 1
9) Find the strain amplitude e CAL corresponding to point Q that lies on b at a distance b 2 away from the intersection
of b and n 2 .
10) Obtain the characteristic value of the expected number of cycles N exp for ε Cal using the measured characteristic
S-N curve.
DET NORSKE VERITAS AS