| Stress and strain measurements are often desired for testing and monitoring of structures. One application where strain is often used to define certification requirements and determine when maintenance is required is on a wind turbine blade. However, current methods of using strain gages present certain challenges, as strain is only captured at discrete points, and therefore defects or failures may be undetected if their locations are unknown. A process was developed for obtaining full-field dynamic strain on a wind turbine blade. This approach utilizes displacement measurements taken at a limited number of points. Analytical reduction and expansion theories are used to expand the measurements to displacement vectors for each node of a finite element model of the structure, which is then used to predict dynamic stress and strain on all surfaces. This technique does not require the identification of loads or boundary conditions of the structure of interest.;Various tests, which were comprised of static, sinusoidal, pluck, and multiple-sine excitations, were performed to validate the proposed approach. Measurements were taken using strain gages and optical measurement systems that utilize the principles of digital image correlation and digital photogrammetry. Predicted strain results compared very well with measured strain on both exposed and hidden surfaces. The proposed technique demonstrates significant promise for monitoring wind turbine blades, as well as a wide range of applications for which structural health monitoring is an important tool. |
