The steady, periodic, random loads lead to static and dynamic deformations of the blades; these deformations also consist of steady, periodic, and random components. All these distributed spanwise loads lead to oscillating stresses in the blades, which vary spanwise and over each cross section. Because of the strong influence of aerodynamics and structural dynamics on the internal loads, this problem is very much interdisciplinary. One cannot choose a material for these machines "out of context" (i.e., without considering the aeroelastic aspects of the problem). For additional discussion of the load characterization, see de Vries (1979).

BLADE FAILURE EXPERIENCE

At the blade root, the oscillating stresses are passed into the rotor hub and certain harmonics are passed through the shaft into the tower. Many failures involve the connecting structure between the blade and the hub. Others involve failures of the steel connectors between the main part of the blade and the bearing, which allows for pitching of the outboard section in stall-controlled machines. Still others involve subassembly joints in the main blade component and chordwise cracking of the blade shell.

In order to put blade failure experience in perspective, it should be noted that the preponderance of such failures have occurred at joints and other discontinuities in the rotor structure. There is a category of blade designs and type that has experienced a high failure rate principally due to defects in manufacture and design. Other manufacturers of blades, however, have not experienced these failures. At present the oldest blades have been in service for about 8 years. Because of a lack of knowledge of fatigue properties and performance at large numbers of cycles, it is expected that these blades will be replaced after about 10 years of service--far short of their design life of 30 years.

As a side note, relative advantages and disadvantages of pitch-controlled versus stall-controlled wind turbine designs are not well understood. While this is not a materials issue, per se, such configuration parameters do influence the structural design, which is, in turn, closely coupled to the material selection. For example, U.S. Windpower blades are of the pitch-controlled variety and have exhibited relatively high reliability based on in-field operations. While all the factors influencing reliability are not clear, configuration will certainly have an impact. A similar statement can be made concerning HAWT versus VAWT configurations. VAWT blades have some advantages concerning loads since they are not subject to once-per-revolution gravitational stress reversals. They do, however, experience a once-per-revolution aerodynamic loading reversal. Considering all the aspects of design, however, one cannot rule out any of the well-known configurations at this time.

High-cycle fatigue is not as serious a problem for most applications in the aerospace industry as it is for helicopters. Some of the same problems in helicopters are also present in wind turbines. Part of the fatigue problem with wind turbine rotors is manifested as leading and trailing edge cracks that form as the blades undergo continual use. Research has shown that, because early design efforts focused almost entirely on blade flapping moments and gave inadequate attention to inplane (chordwise or lead-lag) moments, these cracks were not anticipated (Conover and Young, 1989). Although poor quality control is a factor in the development of these cracks, it would appear that design procedures that do not take all necessary aspects into account are major contributors. Indeed, extant response codes (such as FLAP [Wright et al., 1988]) provide neither in-plane bending nor torsional contributions to the stress prediction. With composite blades, which typically have three-dimensional stress fields, this is far from adequate.

LESSONS FROM HELICOPTER EXPERIENCE

The helicopter industry has taken a rather conservative approach to changing the structural and configuration design of the rotor/hub (by this we



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