Model leta helikoptera

U ovom radu izrađen je model leta helikoptera. Za dinamiku leta primijenjen je 6DOF model. Za model rotora koristi se analitički matemetički model koji je primjenjen na glavni rotor i, uz neke preinake, na repni rotor. Modelirani su i trup letjelice te vertikalni i horizontalni stabilizator. Cijeli model rađen je unutar MATLAB/Simulink okružja te je napravljeno povezivanje s FlightGear vizualizacijskom aplikacijom za simulaciju u stvarnom vremenu. Najvažnije komponente uspoređene su s podacima iz literature odnosno s drugim, verificiranim, modelima. Primjenjeni 6DOF model je verificiran s postojećim modelom. Model glavnog rotora je uspoređen s rezultatima modela po teoriji elementarnog kraka (BET) u lebdenju i horizontalnom letu. Model repnog rotora uspoređen je u lebdenju. Rezultati analize i usporedbe pokazuju dobra poklapanja. Manja odstupanja su objašnjena i svojstvena su korištenju analitičkog modela rotora

An inverse method for local stiffness identification based on scanning laser measurements

The basic principle of inverse methods for the identification of material model parameters is to compare measured observations on a test specimen in a given test setup with virtual observations computed with a numerical model of the test specimen. The unknown parameters in the numerical model are updated until the computed observations match the measurements. Many inverse methods have already been proposed for the identification of uniform material properties in beamlike or plate like specimens based on a limited amount of observations. However, if the material properties in the specimens vary from point to point, more measured observation information is necessary. This paper presents an inverse method that can identify the local bending stiffness distribution in test beams based on the observation of the curvatures of vibration mode shapes. The test beams are freely suspended and the mode shapes are activated by acoustical excitation. The curvatures of the mode shapes are measured with a scanning laser and compared with curvatures computed with a finite element model of the test beams. The presentation will discuss the principle of the used inverse method, the experimental test set-up, the numerical model and give identified stiffness distributions results on composite material test beams. The presentation will end with some conclusions about the results and suggestions for future research

Simulacija leta helikoptera u stvarnom vremenu modelom prve razine

U ovom radu prezentiran je simulacijski model leta helikoptera. Za dinamiku leta primjenjen je 6DOF model, dok je rotor opisan modelom prve razine, analitičkim modelom rotora koji je primjenjen na glavni rotor, a uz određene preinake i na repni. Trup letjelice, horizontalni i vertikalni stabilizator su također modelirani pri čemu nije razmatrano međudjelovanje glavnog rotora na repni kao ni na preostale elemente konfiguracije helikoptera. Simulacija u stvarnom vremenu realizirana je u MATLAB/Simulink okružju te je napravljeno povezivanje sa FlightGear aplikacijom za vizualizaciju leta. Rezultati modela leta helikoptera su po komponentama uspoređeni s podatcima dostupnim iz literature. Rezultati analize i usporedbe pokazuju dobro poklapanje, a uočena odstupanja su objašnjena i svojstvena su primjeni modela rotora prve razine

Nondestructive characterization of the elastic properties of orthotropic composites with ultrasound

Due to the increasing use of composite materials in critical applications demanding light weight to high strength ratio, reliable evaluation tools are necessary. The field of ultrasound is a well known non-destructive method, in which the classical C-scan has already proven its usefulness in visualizing defects, delaminations, ... Though, because of the limitations for quantitatively characterizing a material using a C-scan, the quest for more sophisticated methods is a big challenge in current research. A polar scan, which makes use of oblique incidence of ultrasound, can serve as a prominent successor of more classical methods. By gathering the reflected (or transmitted) amplitude of the ultrasonic beam, one is able to built a polar plot which covers a certain solid angle. Typically, dark rings are observed in a polar plot which can be linked to the generation of critical bulk waves (pulsed regime) or to the generation of surface waves (harmonic regime). Both types of waves are vigorous entangled with the elastic constants of the material. This implicates that a polar scan is a great candidate tool to investigate anisotropic materials non-destructively, where the mapped amplitudes are a local fingerprint of the investigated material

Comparison of shearography to scanning laser vibrometry as methods for local stiffness identification of beams

Local stiffness of Euler–Bernoulli beams can be identified by dividing the bending moment of a deformed beam by the local curvature. Curvature and moment distributions can be derived from the modal shape of a beam vibrating at resonance. In this article, the modal shape of test beams is measured by both scanning laser vibrometry (SLV) and shearography. Shearography is an interferometric optical method that produces full-field displacement gradients of the inspected surface. Curvature can be obtained by two steps of derivation of the modal amplitude (in the case of SLV) or one step of derivation of the modal shape slope (in the case of shearography). Three specially prepared aluminium beams with a known stiffness distribution are used for the validation of both techniques. The uncertainty of the identified stiffness distributions with both techniques is compared and related to their signal-to-noise ratios. A strength and weakness overview at the end of the article reveals that the shearography is the technique that shows the most advantages

Experimental and numerical polar scans of several anisotropic materials using pulsed and harmonic ultrasonic beams

Ultrasonic non-destructive testing is a well known technique in present days, in which the C-scan is the most wide spread. Though, because of the inherent limitations of most methods to quantitatively characterize (damaged) composite materials, the quest for more sophisticated methods is put forward. This study reports experimentally registered polar scans using an in house developed ultrasonic test setup for some typical composite materials. Both pulsed and harmonic ultrasonic beams are considered, which impinge onto the immersed anisotropic layer under investigation. Numerical computations are shown and compared with the experimental results. The experimental polar scan of a carbon fabric/PPS laminate with an artificially added delamination shows a drastic change in the observed patterns, compared to the one of the undamaged carbon fabric/PPS laminate. Combined with the sensitivity to the local stiffness tensor, a polar scan can become a great tool to quantitatively evaluate (damaged) composite materials

Ultrasonic characterization of subsurface 2D corrugation

The ultrasonic backscattering technique is employed for the characterization of a 2D surface corrugation which is superposed on or hidden on the backside of a polycarbonate sample. In contrast to previous studies where the incident angle at well-defined and a-priori known symmetry orientations of the surface structure is varied in order to extract the characteristic periodicities, the backscatter polar scan method incorporates an additional variation of the orientation of the vertical insonification plane within the experimental measurement protocol. As such, the characteristic periodicities as well as the surface symmetries can be extracted without any prior knowledge of the surface structure. As a benefit compared to optical methods, we have also validated this extended methodology for the investigation of a 2D subsurface corrugation. Although the diffraction conditions do not change in comparison with a visible 2D surface corrugation, we remark that additional attention is required in the sense that the elastic properties of the substrate material put further restrictions to the range of applicable ultrasonic frequencies. The characterized periodicities and symmetries are in excellent agreement with the design parameters of the (hidden) 2D surface grating

Local bending stiffness identification of beams using simultaneous Fourier-series fitting and shearography

In this paper, we present a novelmethod for the identification of the local bending stiffness of beams. We use shearography to capture measurements of vibrating beams, so the input data for the identification is the modal slope - the differential of the modal shape. The modal slope is fitted by two Fourier-series functions, one of which is derived froma thin-beam model. The local bending stiffness is identified as the one correspondingwith the best match between the measured and the two fitted modal slopes. This identification method, which we call simultaneous Fourier-series fitting, is demonstrated on numerically-generated inputs, as well as on experimental measurements. We use a flat, concave and convex beam, as well as beams with locally varying bending stiffness mimicking local damage to verify the method. It is shown that the method gives accurate results and is robust to noise. Additionally, it has advantageous properties that make it useful and practical: using this method, it is possible to perform the identification from only a sub-region of a beam and evenwithout specifying the boundary conditions. (C) 2018 Elsevier Ltd. All rights reserved

A novel ultrasonic strain gauge for single-sided measurement of a local 3D strain field

A novel method is introduced for the measurement of a 3D strain field by exploiting the interaction between ultrasound waves and geometrical characteristics of the insonified specimen. First, the response of obliquely incident harmonic waves to a deterministic surface roughness is utilized. Analysis of backscattered amplitudes in Bragg diffraction geometry then yields a measure for the in-plane strain field by mapping any shift in angular dependency. Secondly, the analysis of the reflection characteristics of normal incident pulsed waves in frequency domain provides a measure of the out-of-plane normal strain field component, simply by tracking any change in the stimulation condition for a thickness resonance. As such, the developed ultrasonic strain gauge yields an absolute, contactless and single-sided mapping of a local 3D strain field, in which both sample preparation and alignment procedure are needless. Results are presented for cold-rolled DC06 steel samples onto which skin passing of the work rolls is applied. The samples have been mechanically loaded, introducing plastic strain levels ranging from 2% up to 35%. The ultrasonically measured strains have been validated with various other strain measurement techniques, including manual micrometer, longitudinal and transverse mechanical extensometer and optical mono- and stereovision digital image correlation. Good agreement has been obtained between the ultrasonically determined strain values and the results of the conventional methods. As the ultrasonic strain gauge provides all three normal strain field components, it has been employed for the extraction of Lankford ratios at different applied longitudinal plastic strain levels, revealing a strain dependent plastic anisotropy of the investigated DC06 steel sheet

Errors in shearography measurements due to the creep of the PZT shearing actuator

Shearography is a modern optical interferometric measurement technique. It uses interferometric properties of coherent laser light to measure deformation gradients on the μm/m level. In the most common shearography setups, the ones employing Michelson interferometer, the deformation gradients in both x- and y-direction can be identified by setting angles on the shearing mirror. One of the mechanisms for setting the desired shearing angles in the Michelson interferometer is using the PZT actuators. This paper will reveal that the time-dependent creep behaviour of the PZT actuators is a major source of measurement errors. Measurements at long time spans suffer severely from this creep behaviour. Even for short time spans, which are typical for shearographic experiments, the creep behaviour of the PZT shear actuator induces considerable deviation in the measured response. In this paper the mechanism and the effect of the PZT creep is explored and demonstrated with measurements. For long time-span measurements in shearography, noise is a limiting factor. Thus, the time-dependent evolution of noise is considered in this paper, with particular interest in the influence of the external vibrations. Measurements with and without the external vibration isolation are conducted and the difference between the two setups is analyzed. At the end of the paper some recommendations are given for minimizing and correcting the here studied time-dependent effects