Wind turbine designers are working to provide blade designs that allow a turbine connected to the blades or rotor to effectively convert wind into electricity. The blades must also be designed properly to withstand inertial forces, aerodynamic forces, and structural forces to provide a relatively long service life and safe operation. Like all rotating machines, wind turbines are generators of fatigue, and every revolution of their components, including the turbine blades, produces a load or fatigue cycle, with each of these cycles causing a small, finite amount of damage that eventually may lead to fatigue cracks or other failures.
Modeling may be used in some cases to determine service life of a turbine blade during normal operations. Modeling has its limitations, including variations in the as-built/manufacture blade design and the difficulty in accurately modeling operational conditions with varying and sometimes random loading. As a result, wind turbine blades are typically laboratory tested to determine that their fatigue strength or characteristics are adequate for a desired service life. Wind turbine or rotor blade testing is used to verify that laminations in the blade are safe, e.g., the layers used to fabricate a blade do not separate or delaminate, and to verify that the blade will not break under repeated stress.
Several types of test systems are used to apply loads to wind turbine blades. One type uses a linear hydraulic actuator to apply the desired loads to the blade. The base or root of the blade is mounted to a rigid and very large test stand, and the linear hydraulic actuator is mounted to the blade some distance from the root or base and from the test stand. This type of apparatus is advantageous in that it can be used to apply loads in any desired direction by simply mounting the hydraulic actuators at the desired positions on the blade, and by orienting the actuators in the appropriate directions, e.g., for sequential flapwise and edgewise testing. However, these systems often use a large actuator, and a relatively complex hydraulic system with pumps and hoses to operate the actuator to oscillate the blade or test article. The size of the test stand, with its large concrete blocks, and the complexity and size of the hydraulic actuator, makes these testing systems difficult to move and time-consuming and expensive to build and set up, which limits the number of systems, and forces blade manufacturers to ship blades to the testing facilities for fatigue testing.
The Base Excitation Test System (BETS) is a multi-axis, degree-of-freedom blade testing system that effectively utilizes base excitation (e.g., shaking or oscillating a base or root) to provide more efficient fatigue testing of wind turbine blades. During operation, the system provides simultaneous displacements of a test article, such as a blade, in multiple degrees of freedom (e.g., translations and/or rotations) by concurrently moving or shaking a blade support structure in two or more directions.
One advantage of this system is that it removes the need for specialized hydraulic equipment such as pumps, hoses, and actuators. This is done using a motor to create oscillatory motion and resonate a blade in either the flapwise or edgewise direction. The motor and flywheel system rotate a link that is attached to a frame with a prismatic joint. The frame’s vertical deflection causes the system frame to rotate about a revolute joint mounted to the ground. The blade is mounted to the frame and oscillates with the frame. In contrast to traditional test stands, the frame is mobile because it is self-supporting and requires little anchoring to the ground. A variation of the concept would be to cantilever a weight from the blade stand to reduce the loads at the follower carriage.
The blade support also may include a hub pivotally mounted to the testing platform that receives the base of the test article. The excitation input assembly may include a leverage arm or frame that extends from the hub, and an actuator may apply forces to the leverage arm (e.g., at or toward a tip of the arm) in a direction that is substantially parallel to a longitudinal axis of the test article, which causes the test article to oscillate in the vertical direction. The hub may also be mounted to the platform for translational or sliding movement in a plane parallel to the testing platform or the like, and the second actuator may be a linear actuator that applies forces to the blade support proximate to the pivot axis of the hub (e.g., along an axis of a shaft/pin extending out from the hub and pivotally supported by a frame that is, in turn, attached to the testing platform).