Papers by Keyword: Laser Vibrometer

Abstract: Precision of the machining centers is influenced by many factors among which are mechanical vibrations that occur in the machine-tool-part-device system. Evaluation of vibration level for five axes machining centers is a very important issue considering structural and functional complexity of these equipments. The paper presents researches performed at University of Oradea to evaluate vibration of TMA-AL 550 five axes machining center, equipped with a two rotating axes table, during the machining process. There have been used two methods: evaluation of the dynamic behavior of the spindle using a laser vibrometer and evaluation of mechanical vibrations of the five axes machining center using triaxial accelerometers. Measurement data had been processed using spectral analysis and time-frequency parameters, revealing frequency domains which can cause high amplitudes of vibration. Results also show that the measured vibration levels are not exceeding the accepted limits.

Abstract: Identification of modal parameters is crucial especially in aerospace applications whereby the interactions of airflow with aircraft structures can result in undesirable structural deformations. This structural deformation can be predicted with knowledge of the modal parameters. This can be achieved through conventional modal testing that requires a known excitation force in order to extract these dynamic properties. This technique can be experimentally complex because of the need for artificial excitation and it also does not represent actual operational condition. The current work presents part of research work that address the practical implementation of operational modal analysis (OMA) applied to a cantilevered hybrid composite plate exposed to low speed airflow in a wind tunnel. A single contactless sensing system via a laser vibrometer is employed to measure the response. OMA technique applied in a wind-on condition succeeded in extracting the modal parameters of the hybrid composite plate which correlate well with modal testing using impact hammer excitation.

Abstract: The investigation was focused on Carbon Fibre Reinforced Polymers (CFRP). In the first part of research the aim was the characterization of CFRP surfaces. These surfaces were influenced by release agent, hydraulic fluid, moisture and overheating. In the second part of research adhesive bond quality was investigated. Three different cases of possible weak bonds were focused on. Weak bond caused by release agent contamination, moisture contamination and poor curing of adhesive. The characterization was conducted using laser vibrometry used as NDT tools. An active element in the form of piezoelectric transducer was used to excite the samples made out of CFRP material. Laser vibroemter was used to register the surface response. Combining the piezoelectric excitation with laser sensing a tool was obtained to measure precisely the propagating elastic waves. The excited waves were measured in defined points by the vibrometer obtaining the wavefield. In order to characterize the surface and bonding quality an indicator was proposed based on propagating wave parameters. The guided elastic wave velocity depends material properties (Young modulus, density, Poisson ratio) and thickness of the sample. It was assumed that comparison of the velocities can provide an information about the bond condition. All the investigated scenarios showed deviation from the reference case.

Abstract: The reported research concerns experimental investigation toward the monitoring of an aircraft panel. Guided wave propagation phenomena were used to obtain information about the state of the monitored structure. A curved aluminium panel with rivets was investigated. Piezoelectric transducer was used to excite guided waves in chosen structural element. The generated signal was amplified before applying it to the transducer in order to ensure measurable amplitude of excited guided waves. Measurement of the wave field was realized using laser scanning vibrometer that registered the velocity responses at a points belonging to a defined mesh. This contactless measurement technique allowed to investigate phenomena related to wave propagation in the aircraft panel. In the first stage, due to high complexity of the element, baseline measurements were taken. Next, a discontinuity (additional mass) was introduced on the panel surface and the measurements were repeated. Signal processing methods for features extraction from signals were proposed. These features were applied in order to detect and localize the presence anomalies in the investigated panel. The signal processing was conducted in MATLAB with the procedures developed by the authors. The used measurement technology (vibrometer) allowed to register whole wavefield of the propagating guided waves. This allowed to visualize the interaction of the waves with rivets. After introducing the discontinuity on the panel surface wave interaction with it was investigated. Two positions of the additional mass were considered. One just before the riveted stiffener and second after the stiffener. Because of this the influence of the stiffener on the damage detection abilities could be investigated. It can be concluded that the guided wave can be used for monitoring of such complex structures. The vibrometer measurements allowed learn about the guided wave propagation phenomena and perform successful damage localization.

Abstract: Rotor unbalance and shaft misalignment are common major concerns in rotating system. In this study, vibration analysis for unbalanced rotor and misaligned shaft components using non-contact laser Doppler vibrometer (LDV) were examined. The vibration acceleration magnitude (m/s²) and FFT spectrum from the both fault detections were recorded by the portable laser Doppler vibrometer and analysed by the vibration analysis software. The result shows that combination of unbalance and parallel misalignment fault conditions can produce higher vibration thus reduce the life span of the rotor system. In this case, the unbalance fault condition combined with parallel misaligned rotor gave highest vibration magnitude followed by simply unbalanced rotor and parallel misalignment fault only. This experiment shows that the non-contact method has successfully detected the unbalance and parallel misalignment fault in the rotor system.

Abstract: A method to predict the modal density and radiation efficiency of orthogonally stiffened cylindrical shell is presented in this paper. The modal density is derived using energy method from the undamped natural frequencies of stiffened shell by treating the stiffeners as discrete elements. The band averaged radiation efficiency is estimated by categorizing the structural modes into acoustically fast and acoustically slow modes. The predicted parameters are compared with experimental results using laser vibrometer measurements. Phase-roll off has been observed in laser vibrometer measurements for which a correction procedure is applied. This prediction method can be easily extended to shells with stiffeners having different size, orientation etc.