Page 429 - Wind Energy Handbook

BLADES 403

on Figure 7.15, and is compared with that for a similar stall-regulated machine
having the same section modulus. The cross section is designed to resist the extreme
bending moment for the stall-regulated machine of 130 kNm. In both cases the S–N
curve index is taken as 10, and the IEC Class A turbulence intensity assumed. It is
apparent that the pitch-regulated machine fatigue damage is concentrated around
rated speed, and that the total damage is an order of magnitude greater than the total
for the stall-regulated machine. As the 12 m radius section modulus to resist the
extreme flapwise moment is likely to be less for the pitch-regulated machine, fatigue
loading is likely to be more critical than indicated by the comparison in the figure.

Factors affecting fatigue criticality

The relative criticality of fatigue and extreme loading is determined by the material
properties and safety factors adopted, as well as by the loadings themselves. As an
aid to comparison, the fatigue loading can be described in terms of the notional one
cycle equivalent load, ó eq(n¼1) , which is defined as the stress range of the single
reverse loading cycle that would cause the same total fatigue damage as the actual
fatigue loading on the basis of the design S– N curve. Then fatigue is critical if
ó eq(n¼1) ó ext
. ª L (7:10)
2ó 0d ó cd
where ó 0d is the stress amplitude given by the reverse loading fatigue design curve
at N ¼ 1, ó ext is the stress resulting from the extreme loading case, ª L is the load
factor and ó cd is the design compression stress (which is assumed not to be
governed by buckling considerations). The condition may be rewritten in terms of
characteristic stress values as follows:

ó eq(n¼1) ª mu ó 0k
. 2ª L (7:11)
ó ext ª mf ó ck
or as
ó eq(n¼1) ª mu ó 0k
. 2:7 with ª L set to 1:35:
ó ext ª mf ó ck
As is implicit from the survey of GFRP and wood laminate properties in Sections
7.1.6 and 7.1.7, the value of ó 0k =ó ck can vary between about 1.0 and 1.4. If the
derivations of the characteristic ultimate and fatigue strengths are statistically
similar, the IEC rules indicate that the respective partial materials safety factors
should be taken as equal, whereas, as noted in Section 7.1.6, the GL rules for GFRP
imply a value of 1.5 for the ª mu =ª mf ratio. Thus in principal the parameter
ª mu ó 0k
2ª L
ª mf ó ck
governing fatigue criticality can take a wide range of values of between 2.7 and
about 6.