Optical technologies for the study of vibrations
Modern camera electronics have improved to such a degree that they are able to measure and monitor very complex and rapidly varying vibrations. Scientists of the TEFFMA (Towards experimental full field modal analysis) project investigated and compared three technologies: electronic speckle pattern interferometry (ESPI); digital image correlation (DIC) and scanning laser doppler velocimetry (SLDV). All three are considered full field technologies because they can measure large areas simultaneously. ESPI, DIC and SLDV are most commonly used to study the structural dynamics of lightweight components, which are particularly sensitive to vibration. TEFFMA began by building a framework to hold instruments using each technology. Software was written to handle data acquisition, visualisation and instrument comparison. Appropriate structural dynamics problems were chosen for investigation. Researchers took measurements for each test case, starting with a vibrating lightweight plate, using all three technologies simultaneously. Broad ranges of frequencies were used, enabling comparative benchmarks to be obtained. TEFFMA built full field experimental models from the frequency response functions, mapping the dynamical behaviour of each structure against the vibrational input. As the same structure was used with each instrument, differences due to boundaries and defects were avoided. Dynamic rotations and surface strains can be calculated from the full field frequency response functions, as maps of the experimental models in the frequency domain. The stress maps can then be used to produce danger maps and fault tolerance assessments, and predict fatigue and damage. Having data available from all three techniques produced much more accurate results, by revealing problems with individual techniques and enabling richer and more tightly constrained numerical models. The project approach to full field coordinated measurement will have applications in mechanical engineering and many other fields, ranging from aerospace to vehicle technologies, electronic component testing in harsh environments, advanced material behaviour analysis, risk assessment, and quality assurance in production.
Keywords
Optical technologies, vibration, TEFFMA, ESPI, DIC, SLDV
