The research presented in this article focuses on a 9-m CX-100 wind turbine blade, designed by a team led by Sandia National Laboratories and manufactured by TPI Composites Inc. The key difference between the 9-m blade and baseline CX-100 blades is that this blade contains fabric wave defects of controlled geometry inserted at specified locations along the blade length. The defect blade was tested at the National Wind Technology Center at the National Renewable Energy Laboratory using a schedule of cycles at increasing load level until failure was detected. Researchers used digital image correlation, shearography, acoustic emission, fiber-optic strain sensing, thermal imaging, and piezoelectric sensing as structural health monitoring techniques. This article provides a comparison of the sensing results of these different structural health monitoring approaches to detect the defects and track the resultant damage from the initial fatigue cycle to final failure.

A fatigue test of a wind turbine blade was conducted a t the National Renewable Energy Laboratory in the fall of 1994. Acoustic emission monitoring of the test was performed, starting with the second loading level. The acoustic emission data indicated that this load exceeded the strength of the blade. From the first cycle a t the new load, an oil can type of deformation occurred in two areas of the upper skin of the blade. One of these was near the blade root and the other was about the middle of the tested portion of the blade. The emission monitoring indicated that no damage was taking place in the area near the root, but in the deforming area near the middle of the blade, damage occurred from the first cycles a t the higher Ioad. The test was stopped after approximately one day and the blade was declared destroyed, although no gross damage had occurred. Several weeks later the test was resumed, to be continued until gross damage occurred. The upper skin tofe approximately one half hour after the cycling was restarted.

Data on durability, petrography and microstructure are presented for 27 specimens of Waikato Coal Measures mudrocks: fine-grained, carbonaceous rocks, which are dominated by kaolinite clays, and have low durabilities. The poor durability is not caused by swelling clays. Rather, slaking is believed to result from the release of residual stresses within the rock following weakening by water adsorption on to clay surfaces and a consequent loss in cohesiveness. This mechanism favours fragmentation into small, water-stable fragments, rather than dispersion of clays into the water. The amount of day-sized material in the rocks provides the most direct control on the durability, but mierostructural features are also recognized as important: specimens with a discontinuous matrix have a slightly lower durability than those with a continuous matrix.

The technology of acoustic emission (AE) testing has been advanced and used at Sandia for the past 40 years. AE has been used on structures including pressure vessels, fire bottles, wind turbines, gas wells, nuclear weapons, and solar collectors. This monograph begins with background topics in acoustics and instrumentation and then focuses on current acoustic emission technology. It covers the overall design and system setups for a test, with a wind turbine blade as the object. Test analysis is discussed with an emphasis on source location. Three test examples are presented, two on experimental wind turbine blades and one on aircraft fire extinguisher bottles. Finally, the code for a FORTRAN source location program is given as an example of a working analysis program. Throughout the document, the stress is on actual testing of real structures, not on laboratory experiments.

Prepared by Sandia National Laboratories Albuquerque, New Mexlco 87185 and Livermore, California 94550 for the United States Department of Energy under Contract DE-AC04-94AL85000 Approved for public release; distribution is unlimited.