Blade materials, testing methods and structural design - Wind Power Generation and Wind Turbine Design - page 431

(a) (b)
Figure 6: Schematic illustration of (a) stress-strain behavior, which describes
the continuum response up to the peak stress, s (onset of localization)
and (b) behavior after localization which is described in terms of a
traction-separation relationship called a cohesive law.
6 Materials testing methods
Because the cost of full-scale blade testing and certifi cation is signifi cant, various
laboratory tests are being developed to better access the structural reliability of com-
posite laminates and adhesive joints under conditions that simulate expected stress or
strain states in the turbine blade. In this section, test methods that can be used for the
measurement of relevant material properties are reviewed. The material properties
that infl uence the damage development in a wind turbine blade can be divided into
the elastic properties, the strength properties and fracture mechanics properties.
6.1 Test methods for strength determination
Different test methods are used for tensile and compressive strength determination.
An overview of common test methods for strength measurements is given in Fig. 7.
Tensile strength is determined by uniaxial tensile tests [12, 13]. Tensile test speci-
mens are usually long, straight-sided or have a narrow gauge section (dog-bone shape)
to ensure that failure develops in the gauge section region where the stress fi eld is uni-
form. Bending tests can sometimes be used to estimate tensile strength [14, 15]; but
bending failure can also occur by shear or compression failure and the failure mode
and test results must be carefully analyzed. Moreover, because of the non-linear stress
state developed in bending, the initiation of damage on one side of a specimen will
result in a shift of the neutral axis of the specimen which further complicates interpre-
tation of test results. Therefore, tensile tests which provide a uniform volume of stressed
material are preferred in order to avoid invalid determination of tensile strength.
Specimens for compressive failure are short in order to avoid buckling of the spec-
imen [ 16 ]. Unfortunately, the stress fi eld is not uniform in the specimens and
speci mens frequently fail away from the gauge section. In an effort to improve the
accuracy of compressive testing, various approaches have been developed to
reduce bending and transverse loading. In a test compressive test fi xture designed by
Bech et al . [17], hemispherical bearings are used to reduce transverse loading bend-
ing strains during compressive loading. The test fi xture provides more accurate