Blade materials, testing methods and structural design - Wind Power Generation and Wind Turbine Design - page 432

Gauge
Section

(c)

(b) (d) (e)

(a)

Figure 7: Schematics illustration of test specimens for the determination of
strength data. (a) Specimens for determination of tensile strength
are usually straight-sided for unidirectional composites or dog-bone
shaped, (b) specimens for determination of compressive strength are
short to prevent global buckling, (c) shear strength of laminates can be
determined using the short-beam-shear test or (d) the V-notched beam
(Iosipescu) test. For sandwich structures, rail shear testing is typically
utilized for shear strength measurements (e).

measurements of compressive stress-strain behavior for both monotonic and
fatigue loading of composites.
The shear strength of composite laminates can be measured by the use of a short
beam subjected to three-point bending [18] or by the Iosipescu shear test [19].
Again, the test results must be carefully analyzed and the failure mode documented
since tensile failure can occur prior to shear failure for laminates that possess a
high shear strength. For sandwich structures, shear testing is commonly performed
using a rail shear approach [20].

6.2 Test methods for determination of fracture mechanics properties

Fracture properties are determined from tests of specimens having an artifi cial
crack in the form of a pre-cut notch or a thin slip foil (e.g. by use of Tefl on between
interfaces) introduced during specimen manufacture. However, a machined or arti-
fi cial notch is not as sharp as a real crack. Thus, as discussed later, test methods
that allow the initiation and arrest of cracking are preferred, since they enable
the fracture properties to be determined from a truly sharp crack. An overview
of commonly used specimen geometries for fracture mechanics-based testing of
composite interfaces is given in Fig. 8.
First, methods for determination of the fracture energy of elastically isotropic
materials are reviewed. The Mode I (pure normal opening) fracture properties of
isotropic materials are often determined from the compact tension (CT) specimen.
For thin (isotropic and orthotropic) laminated structures manufactured with the same
material layers, such as composites and sandwich structures, double cantilever beam
(DCB) specimens loaded with wedge forces are commonly employed, both for static
and cyclic crack growth [21]. For both these specimens the energy release rate depends