Tong W. Wind Power Generation and Wind Turbine Design

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Blade Materials, Testing Methods and Structural Design 455

strength for two delaminations occurred when the delaminations split the laminate

into sub-laminates of similar thickness.

7.3.4 Modeling of a full main spar to determine the webs inﬂ uence

on ultimate strength

The deformation and strength of a box girder used as a spar in a 34 m wind turbine

blade was examined by Branner et al . [ 76 ]. The study also examined the effect

of sandwich core properties on ultimate strength of the box girder. In Fig. 27 , the

transverse (vertical or 90°) strains for both faces of the sandwich web are com-

pared with both linear and non-linear FE analysis. The strains presented in Fig. 27

are measured on the upper part of the web towards the leading edge at the position

where the failure was observed (also indicated in the ﬁ gure). The longitudinal

normal strains (in the z -direction) caused by bending of the box girder lead to

associated strains in the transverse direction (the y -direction) due to the Poisson’s

Figure 26: Comparison of experimental and FE predicted failure loads for

specimens containing a delamination in the C position and one of

the positions from A to G [ 75 ].

456 Wind Power Generation and Wind Turbine Design

effect. The transverse strains are positive (tension) in the shear webs of the upper

half part of the box girder where the bending cause compression and the transverse

strains are negative (compression) in the lower half part.

Initially, the transverse strain increased linearly with load, see Fig. 27 . However,

as the load increases, the strains become non-linear with respect to applied load

and it is noted that the graphs deﬂ ect towards compressive strains.

90 deg Strain Gauges on shear web towards leading edge

100

200

300

400

500

600

700

800

900

0% 20% 40% 60% 80% 100%

% of ultimate load of the blade including skin

Microstrain

FEM NL-Outer surface

FEM NL-Inner surface

Exp.-Outer surface

Exp.-Inner surface

FEM Lin.-Outer surface

FEM Lin.-Inner surface

Figure 27: Comparison of FE results and measurements for back to back strain

gauges on upper web part. The positions for the strain gauges are indi-

cated in the sketch (top) [ 76 ].

Blade Materials, Testing Methods and Structural Design 457

The difference between the linear and the non-linear results is, at least in part,

caused by the Brazier effect. The crushing pressure (that is the transverse stresses

due to curvature described in Section 7.1.2) is ﬂ attening the cross section and

introduces compressive strains into the shear webs. This crushing pressure varies

with the square of the applied load as found by Brazier [ 49 ], resulting in the noted

deviation from linear responses. The ﬂ attening of the cross section will probably

initiate buckling which accelerates the failure evolution. Other non-linear phe-

nomena, such as changes in geometry and loading conﬁ guration (which follows

the geometry in the non-linear analysis), will also contribute to the observed non-

linearities. For further discussion of this see Jensen et al . [ 77 ].

In the study in Branner et al . [ 76 ], it was found that the corner stiffness greatly

inﬂ uence the overall non-linear behavior of the box girder. The webs take a larger

part of the overall cross section deformation when the corners are stiff. It is seen

from the stiff corner models, that the core density have some inﬂ uence on the ulti-

mate strength. In contrast, the soft corner models show that the core density has

almost no inﬂ uence on the ultimate strength. The ultimate strength of a box girder

was also studied experimentally and numerically [ 78 ]. It was found that for the

shear webs, failure was initiated by shear failure of the core (see Fig. 27 ).

7.3.5 Testing of long sections of a main spar to determine buckling behavior

Kühlmeier [ 79 ] worked with buckling of wind turbine blades in his PhD thesis. In

Kühlmeier et al . [ 80 ], a 9 m long airfoil blade section, designed to fail in buckling,

was build and tested destructively in a four-point bending conﬁ guration. Based on

numerical analysis, it was found that a linear buckling analysis will over-predict the

ultimate strength of the blade. How much the strength is over-predicted depends

on the size of the imperfections present in the blade and the sensitivity of the

structure with respect to the imperfections. It was suggested that a bifurcation

buckling analysis with a knockdown factor applied on the buckling load will give

a good estimate on the ultimate strength of the blade. The value of the knockdown

factor will depend on the size of the imperfections as well as the imperfection

sensitivity. For the 9 m blade section a factor of 1.25 was found to assess the

ultimate strength of the blade.

7.4 Full wind turbine blade models with damage

Overgaard and Lund [ 81 ] analyzed the full-scale blade collapse described in

Section 4 [ 2 ]. A geometrically non-linear and linear pre-buckling analysis was per-

formed for predicting the failure of the blade due to local buckling on the suction

side of the airfoil. The geometrical imperfection sensitivity of the blade was evalu-

ated by imposing the strain gauge measurement for the full-scale experiment as an

imperfection pattern. Figure 28 displays the response of the obtained imperfection

amplitude where the 23% amplitude model ﬁ ts the best of the evaluated models.

It is seen that the imposed imperfection pattern is directly proportional with the

longitudinal strain measured in the strain gauges [ 82 ]. An important discovery is

that the buckling shape is unaffected by the presence of geometrical imperfections,

458 Wind Power Generation and Wind Turbine Design

but the local strain at the maximum geometric imperfection amplitude is linearly

depending on the imperfection amplitude. The epic centre of the buckling shape

mode is at the core and ﬂ ange material transition, but the structural collapse of the

structure is at the geometric imperfection amplitude.

The advantages of applying a sandwich construction, as opposed to traditional

a single skin laminate, in the ﬂ anges of a load-carrying box girder, was studied by

Berggreen et al . [ 83 ] for a future, very large, 180 m wind turbine rotor. The study

showed that buckling by far is the governing criterion for the single skin design.

A signiﬁ cant weight reduction by more than 20% and an increased buckling

capacity was obtained with the sandwich construction in comparison with the

Imperfection Amplitudes and Flapwise Test Results

Geometric Imperfection Sensitivity

Normalized Rotor Radius

Normalized Imperfection Pattern

Normalized Longitudinal Strain

Figure 28: Response of imperfection amplitude compared with test results [ 81 ].

Blade Materials, Testing Methods and Structural Design 459

traditional single skin composites. However, introducing a sandwich construction

in the ﬂ ange was found to result in a globally more ﬂ exible structure, making

tower clearance the critical criterion.

Overgaard and Lund [ 84 , 85 ] simulated the full-scale blade test till failure [ 2 ] by

FE analysis using a cohesive zone modeling. A mixed mode bilinear cohesive law

was used. The combination of a relative coarse mesh and the use of constitutive

laws with softening leads to severe solution difﬁ culties, and therefore an efﬁ cient

and robust solution strategy for dealing with large three-dimensional structures

was implemented.

This work shows that it is possible to predict structural behavior of a wind turbine

blade based on non-linear fracture mechanics in a geometrically non-linear frame-

work, i.e. account for buckling and delamination interaction (see Fig. 29 ). The

numerical damage predictions were compared with a ﬂ apwise static test result. The

damage simulation displayed strong geometric and material instability interaction

which indeed caused a progressive collapse of the wind turbine blade as seen in the

full-scale experiment [ 2 , 82 ]. According to the model, the critical buckling load of

the blade triggered a delamination at the corner and in middle of the ﬂ ange at the

point of inﬂ ection of the buckling pattern. This started a progressive chain of events

that lead to a structural collapse of the complete wind turbine blade.

8 Perspectives and concluding remarks

This chapter has presented an overview of blade materials, blade design and test-

ing methods. With more knowledge evolving, the blade design will be made more

accurately and safety factors may be reduced. However, safety factors should only

Figure 29: Simulation of failure of a box girder (progressive delamination growth

in combination with buckling) [ 85 ].

460 Wind Power Generation and Wind Turbine Design

be reduced if quality control and damage assessment methods are improved. The

reduction of safety factors could lead to an increasing amount of damage evolution

in wind turbine blades in service. The safely level can nevertheless be maintained

or increased if structural health monitoring (SHM) techniques are used. The per-

spective is that the turbine blades will be equipped with a sufﬁ cient amount of

sensors that enables the detection of damages with high ﬁ delity. The extra costs of

the SHM sensors and systems may be less than the cost savings by skipping the

regular service inspection, at least for large offshore wind turbines, where access is

difﬁ cult and costly. The potentially earlier detection of damage by SHM will give

the owner of the wind turbine the possibility to inspect, repair or replace the blades

before the damages become serious. The cost savings come mainly from the fact that

the vast majority of the blades are not expected to develop signiﬁ cant damages; with

the use of SHM, these blades would not require any manual inspection.

From a knowledge point of view, it is envisioned that the future developments is

towards a better understanding of the damage evolution at a smaller length scales,

leading to a better, more complete and more fundamental understanding of the dam-

age evolution in composite structures. This opens the possibility for the development

of larger and better, more damage tolerant materials.

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