This research is focused on a comparison of classic and strain experimental modal analysis (EMA). The modal parameters (the natural frequencies, the displacement mode shapes (DMSs) and the damping) of real structures are usually identified with classic EMA, where the responses are measured with motion sensors (e.g. accelerometers). Strain EMA is a special approach in the field of EMA, where the responses are measured with strain sensors. Classic EMA is the preferred method, but strain EMA offers advantages that are important for particular applications: for example, the direct identification of strain mode shapes (SMSs), which is important in the vibration-fatigue and damage-identification models. The next advantage is that strain EMA can sometimes be used, for experimental/geometrical reasons, where classic EMA cannot. There are also drawbacks: for example with strain EMA only, the mass-normalization of the DMSs and SMSs cannot be performed. This study researches the theoretical similarities and differences of both EMA approaches. Furthermore, the accuracy of both approaches for the case of a free%free supported beam and a free%free supported plate is investigated. Classic and strain EMA were performed with a piezoelectric accelerometer and the piezoelectric strain gauges, respectively. The results show that the accuracy of strain EMA results (the natural frequencies, DMSs and the damping) is comparable to the accuracy of classic EMA.
COBISS.SI-ID: 13425947
When dealing with small and light structures, difficulties occur when measuring the modal parameters. The resonant frequencies are usually relatively high and therefore a wide frequency range is needed for the measurement. Furthermore, the mass that is added to the structure by the sensors causes structural modifications. To overcome these difficulties, an improved method using an operational modal analysis instead of an experimental modal analysis is proposed in this study. It is derived from the sensitivity-based operational mode-shape normalisation with a consideration of the mode-shape variation. The measurement of the excitation force is not needed, because the operational modal analysis is used and only two simultaneous response measurements at an unknown excitation are required. The proposed method includes the cancellation of the added mass, resulting in mode shapes and resonant frequencies of the unmodified structure. The numerical and experimental results on small and light structures are compared with the results of the experimental modal analysis. The comparison shows that the proposed approach allows measurements over a wide frequency range and increases the accuracy of the results compared to the sensitivity-based operational mode-shape normalisation and also compared to the particular experimental modal analysis method that was used in this study. The advantages of the proposed method can be seen whenever the mass that is added to the structure by the accelerometer is not negligible and therefore a variation of the mode shapes occurs.
COBISS.SI-ID: 13110043
Car components are exposed to the random/harmonic/impact excitation which can result in component failure due to vibration fatigue. The stress and strain loads do depend on local stress concentration effects and also on the global structural dynamics properties. Standardized fatigue testing is long-lasting, while the dynamic fatigue testing can be much faster; however, the dynamical changes due to fatigue are usually not taken into account and therefore the identified fatigue and structural parameters can be biased. In detail: damage accumulation results in structural changes (stiffness, damping) which are hard to measure in real time; further, structural changes change the dynamics of the loaded system and without taking this changes into account the fatigue load in the stress concentration zone can change significantly (even if the excitation remains the same).
COBISS.SI-ID: 13555483