Peter Avitabile

Optical Non-contacting Vibration Measurement of Rotating Turbine Blades II

Identifying the structural dynamics of rotating components can be difficult. Often times, structural dynamic measurements are obtained while the structure is in a static configuration. There are differences that exist in the structural behavior when comparing these statically performed tests and the dynamic characteristics when in operation. In order to evaluate the actual system while in operation, slip-rings are used during testing with measurements made at only a very few selected points. But this slip-ring configuration can be problematic, suffer from measurement noise and the attached sensors can obscure the true dynamic response due to mass loading and aerodynamic effects. 3D digital image correlation (DIC) has been used to capture the out-of-plane motion on the surface of a small scale rotating fan blade. This work extends prior efforts, by quantifying the performance of the optical measurement on a 46 in (1.17m) diameter, rotating wind turbine. The optical measurements are made using DIC (10,000+ measurement points) and dynamic photogrammetry (providing dozens of effective measurement locations). The motion of the turbine as measured using DIC, photogrammetry and accelerometers is compared at several discrete points. The proposed measuring approaches via DIC and dynamic photogrammetry enable full-field dynamic measurement and monitoring of rotating structures in operation.

Prediction of full field dynamic strain from limited sets of measured data

Dynamic response is an important consideration for design of structures due to operating or occasional loadings. The resulting dynamic stress strain is also of concern for fatigue and structural health monitoring. Typically, the actual loading and structural condition (boundary conditions, environmental condition, geometry, mechanical properties, etc.) are not necessarily known. Much effort is expended in attempting to identify the loads and appropriate model for prediction of these types of events. At best, the forces and actual boundary conditions are approximate and have an effect on the overall predicted response and resulting stress-strain that is identified for subsequent evaluation. Experimental data can only be obtained from limited sets of points, such as those typically collected with accelerometers. These are normally used in the evaluation the state of a structure in service condition. More recently, Digital Image Correlation (DIC) and Dynamic Photogrammetry (DP) have become very important techniques to measure the surface response. These are non-contact and full-field techniques, which allow that much more simultaneous data to be measure. The sets of limited surface data that are collected can be used in conjunction with an expansion algorithm to obtain full field information. The finite element model mass and stiffness matrices are used to obtain the normal constitutive relations as well as the modal characteristics. This information is used to develop the expansion algorithm and for the stress recovery during the back substitution process typically employed.

Dynamic Stress–Strain on Turbine Blade Using Digital Image Correlation Techniques Part 1: Static Load and Calibration

Often times, wind turbine blades are subjected to static and dynamic testing to identify the performance levels that can be achieved for a particular configuration. These tests are a necessary part of the validation process. Typically, a variety of different static and dynamic measurements are made using a variety of different transducers. Typically, only a handful of strain gages are deployed to capture strain information. Recent advances in digital image correlation (DIC) and dynamic photogrammetry (DP) have allowed new opportunities for blade inspection, structural health monitoring, and full-field vibration testing. The primary benefit to using DIC is that the measurement approach is not limited to identifying the displacement or strain at only a few discrete measurement locations, but instead makes full-field surface measurements possible. These techniques are currently being explored on several wind turbine blade applications and can provide a wealth of additional information that was previously unobtainable. This paper, which is the first part of a two part paper, presents the static strain measurements and calibration of the system overall. The strain distribution along the length of the structure is compared to the finite element model. The data analysis is used to assure that the model is calibrated for the dynamic testing results; dynamic testing results are presented in the second part of this paper.

Dynamic Stress–Strain on Turbine Blades Using Digital Image Correlation Techniques Part 2: Dynamic Measurements

Often times, wind turbine blades are subjected to static and dynamic testing to identify the performance levels that can be achieved for a particular configuration. Many times only a handful of strain gages are deployed to capture that information.

Dynamic characteristics of a wind turbine blade using 3D digital image correlation

Digital image correlation (DIC) has been becoming increasingly popular as a means to perform structural health monitoring because of its full-field, non-contacting measurement ability. In this paper, 3D DIC techniques are used to identify the mode shapes of a wind turbine blade. The blade containing a handful of optical targets is excited at different frequencies using a shaker as well as a pluck test. The response is recorded using two PHOTRON™ high speed cameras. Time domain data is transferred to the frequency domain to extract mode shapes and natural frequencies using an Operational Modal Approach. A finite element model of the blade is also used to compare the mode shapes. Furthermore, a modal hammer impact test is performed using a more conventional approach with an accelerometer. A comparison of mode shapes from the photogrammetric, finite element, and impact test approaches are presented to show the accuracy of the DIC measurement approach.

Inspection and monitoring of wind turbine blade-embedded wave defects during fatigue testing

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.

Using High-Speed Stereophotogrammetry Techniques to Extract Shape Information from Wind Turbine/Rotor Operating Data

Stereophotogrammetry techniques used in concert with 3D point tracking (dynamic photogrammetry) software are advantageous for the collection of operating data on large wind turbines (or helicopter rotors) over conventional accelerometer-data acquisition systems (DAQ) for several reasons. First, this is a non-contacting technique that doesn’t require the use of mounted accelerometers and electrically noisy slip rings. Second, the optical targets (measurement points) that are mounted to the blade surfaces can remain in place for long periods of time and be used for subsequent measurements without extended/overly complicated setup time. Third, deflection data can be collected on many more points on a turbine/rotor surface beyond what is capable of a conventional multi-channel data acquisition system and accelerometer setup. Operating data has previously been collected on a 1.17 m Southwest Windpower Air BreezeTM wind turbine [1] using Stereophotogrammetry and this data has been used to extract operating deflection shapes from the structure. The purpose of this work is to improve upon the experimental methods used on the 1.17 turbine by Warren [2] and apply these improved methods to a larger, 2.56 m diameter turbine/rotor analog, and collect operating data on the structure. This data was collected outdoors and shape information was extracted from this operating data and compared to that taken with a standard, impact test.

Damage detection and full surface characterization of a wind turbine blade using three-dimensional digital image correlation

The increasing demand for wind power has led to a significant increase in the number and size of wind turbine blades manufactured globally. As the number and physical size of turbines deployed grow, the probability of manufacturing defects being present in composite turbine blades also increases. As capital blade costs and operational and maintenance expenses increase in ever larger turbine systems, the need for inspection of the structural health of large-scale turbine blades during operation critically increases. One method for locating and quantifying manufacturing defects, while also allowing for the in situ measurement of the structural health of blades, is monitoring the full-field deformation and strain of a blade. In a demonstration of this methodology, static tests were performed on a Sandia National Laboratories CX-100 9-m composite turbine blade to extract full-field displacement and strain measurements. Three-dimensional digital image correlation was used. Measurements were taken at previously identified damaged areas near the blade root, along the high- and low-pressure surfaces. The results indicate that the measurement approach can clearly identify failure locations and discontinuities in the blade curvature under load. Postprocessing of the data, using a stitching technique of digital image correlation snapshots taken along the length of the blade, allows observation of the shape and curvature of the entire blade. The experiment demonstrates the feasibility of the approach and reveals that the technique can be readily scaled to accommodate utility-scale blades. As long as a trackable pattern is applied to the surface of the blade, measurements can be made in situ when a blade is on a manufacturing floor, installed in a test fixture, or installed on a rotating turbine. The results demonstrate the potential of the optical measurement technique for use in the wind industry.

Comparison of Some Wind Turbine Blade Tests in Various Configurations

As part of the SEM Dynamic Substructuring Subgroup, several different dynamic modeling scenarios are to be studied in an attempt to identify an overall substructuring modeling strategy that can be used. A wind turbine system was chosen as a test bed to deploy some of those techniques. Two separate wind turbines with a total of six blades are available for this study.

Comparison of Modal Parameters Extracted Using MIMO, SIMO, and Impact Hammer Tests on a Three-Bladed Wind Turbine

As part of a project to predict full-field dynamic strain of rotating structures (e.g. wind turbines or helicopter rotors), a validated numerical model of a structure is required. In this case, a small wind turbine was used. To understand the dynamic characteristics and validate a finite element model of a three-bladed wind turbine, several experimental modal analysis tests were conducted on the turbine attached to a 500-lb steel block. The test structure consisted of three 2.3-m blades mounted to a hub that was attached to the block using a shaft and a lathe chuck. In three separate tests, the structure was excited using a single shaker, multiple shakers, and an impact hammer; the responses of the structure to the excitations were measured using 12 triaxial accelerometers. The results reveal several very closely spaced modes present within the turbine in the test configuration. The natural frequencies and mode shapes obtained by using three different methods were compared to demonstrate the differences (e.g. strengths and weaknesses) between each excitation technique. The paper reports the results obtained and lessons learned during the experimental modal tests of the wind turbine.