The characterization of vibration-fatigue strength is one of the key parts of mechanical design. It is closely related to structural dynamics, which is generally studied in the frequency domain, particularly when working with vibratory loads. A fatigue-life estimation in the frequency domain can therefore prove advantageous with respect to a time-domain estimation, especially when taking into consideration the significant performance gains it offers, regarding numerical computations. Several frequency-domain methods for a vibration-fatigue-life estimation have been developed based on numerically simulated signals. This research focuses on a comparison of different frequency-domain methods with respect to real experiments that are typical in structural dynamics and the automotive industry. The methods researched are: Wirsching–Light, the ?0.75 method, Gao–Moan, Dirlik, Zhao–Baker, Tovo–Benasciutti and Petrucci–Zuccarello. The experimental comparison researches the resistance to close-modes, to increased background noise, to the influence of spectral width, and multi-vibration-mode influences. Additionally, typical vibration profiles in the automotive industry are also researched. For the experiment an electro-dynamic shaker with a vibration controller was used. The reference-life estimation is the rainflow-counting method with the Palmgren–Miner summation rule. It was found that the Tovo–Benasciutti method gives the best estimate for the majority of experiments, the only exception being the typical automotive spectra, for which the enhanced Zhao–Baker method is best suited. This research shows that besides the Dirlik approach, the Tovo–Benasciutti and Zhao–Baker methods should be considered as the preferred methods for fatigue analysis in the frequency domain.
COBISS.SI-ID: 12402715
The dynamic properties of joints are extremely difficult to model accurately using a purely analytical approach. However, these properties can be extracted from experimental data. In this paper we present a method for establishing a theoretical model of a joint from the substructures and assembly frequency–response function (FRF) data. The identification process considers not only translational, but also rotational degrees of freedom (RDOFs). The validity of the proposed method is demonstrated numerically and experimentally. A combined numerical–experimental approach was used to identify the mass, stiffness and damping effects of a real bolted joint. Using the least-squares method, data from the wide frequency range were used. A substructure synthesis method with the joint effects included was used to check the extracted values.
COBISS.SI-ID: 10470683
Random vibration excitation is a common cause of failure, especially if natural dynamics is excited. The high-cycle vibration-fatigue analysis typically requires the structural dynamics analysis, the response analysis and the fatigue analysis. The material parameters (S-N curve) are obtained at uniaxial stress state. However, in real structures the stress state is rarely uniaxial and the direct application of the S-N curve is difficult. The stress tensor is reduced to a more manageable representation using a multiaxial criterion. In this study, maximum normal stress, maximum shear stress, maximum normal-and-shear stress, C-S criterion, Projection-by- Projection and the Preumont and Piéfort criterion for multiaxial stress state are compared theoretically and experimentally. The crack location and the time-to-failure were compared. The time-to-failure was found relatively accurate with all multiaxial criteria; however, the crack-location estimation was found not to be accurate enough for either of the compared criteria. The study proves the applicability of the vibration-fatigue analysis procedure on real vibrating structures with rich structural dynamics. Random vibration excitation is a common cause of failure, especially if natural dynamics is excited. The high-cycle vibration-fatigue analysis typically requires the structural dynamics analysis, the response analysis and the fatigue analysis. The material parameters (S-N curve) are obtained at uniaxial stress state. However, in real structures the stress state is rarely uniaxial and the direct application of the S-N curve is difficult. The stress tensor is reduced to a more manageable representation using a multiaxial criterion. In this study, maximum normal stress, maximum shear stress, maximum normaland-shear stress, C-S criterion, Projection-by-Projection and the Preumont and Piéfort criterion for multiaxial stress state are compared theoretically and experimentally. The crack location and the time-to-failure were compared. The time-to-failure was found relatively accurate with all multiaxial criteria; however, the crack-location estimation was found not to be accurate enough for either of the compared criteria. The study proves the applicability of the vibration-fatigue analysis procedure on real vibrating structures with rich structural dynamics.
COBISS.SI-ID: 14504475
Instantaneous full-field displacement fields can be measured using cameras. In fact, using high-speed cameras full-field spectral information up to a couple of kHz can be measured. The trouble is that high-speed cameras capable of measuring high-resolution fields-of-view at high frame rates prove to be very expensive (from tens to hundreds of thousands of euro per camera). This paper introduces a measurement set-up capable of measuring high-frequency vibrations using slow cameras such as DSLR, mirrorless and others. The high-frequency displacements are measured by harmonically blinking the lights at specified frequencies. This harmonic blinking of the lights modulates the intensity changes of the filmed scene and the camera-image acquisition makes the integration over time, thereby producing full-field Fourier coefficients of the filmed structure's displacements.
COBISS.SI-ID: 15589915
This research looks at the possibilities for full-field, non-contact, displacement measurements based on high-speed video analyses. A simplified gradient-based optical flow method, optimised for subpixel harmonic displacements, is used to predict the resolution potential. The simplification assumes an image-gradient linearity, producing a linear relation between the light intensity and the displacement in the direction of the intensity gradient. The simplicity of the method enables each pixel or small subset to be viewed as a sensor. The resolution potential and the effect of noise are explored theoretically and tested in a synthetic experiment, which is followed by a real experiment. The identified displacement can be smaller than a thousandth of a pixel and subpixel displacements are recognisable, even with a high image noise. The resolution and the signal-to-noise ratio are influenced by the dynamic range of the camera, the subset size and the sampling length. Real-world experiments were performed to validate and demonstrate the method using a monochrome high-speed camera. One-dimensional mode shapes of a steel beam are recognisable even at the maximum displacement amplitude of 0.0008 pixel (equal to 0.2 [micro]m) and multiple out-of-plane mode shapes are recognisable from the high-speed video of a vibrating cymbal.
COBISS.SI-ID: 15108123
Vibration measurements using optical full-field systems based on high-speed footage are typically heavily burdened by noise, as the displacement amplitudes of the vibrating struc- tures are often very small (in the range of micrometers, depending on the structure). The modal information is troublesome to measure as the structure's response is close to, or below, the noise level of the camera-based measurement system. This paper demonstrates modal parameter identification for such noisy measurements. It is shown that by using the Least-Squares Complex-Frequency method combined with the Least-Squares Frequency- Domain method, identification at high-frequencies is still possible. By additionally incorporating a more precise sensor to identify the eigenvalues, a hybrid accelerometer/high- speed camera mode shape identification is possible even below the noise floor. An accelerometer measurement is used to identify the eigenvalues, while the camera mea- surement is used to produce the full-field mode shapes close to 10 kHz. The identified modal parameters improve the quality of the measured modal data and serve as a reduced model of the structure's dynamics.
COBISS.SI-ID: 15504923
The paper presents a derivation of the consistent tangent operator (CTO) for the cutting-plane algorithm (CPA). For a class of plasticity models that are suitable to be integrated using CPA, an explicit recursive expression is analytically derived and is updated in each iteration of the CPA integration procedure to yield the final value of the CTO when the CPA is converged.
COBISS.SI-ID: 13311515
In this paper, an enhanced numerical method for forming tool design optimisation in three-dimensional (3D) sheet metal forming applications is presented. The applied procedure enables a determination of appropriate forming tool geometry so that the manufacture of a sheet metal product inside specified tolerances would be ensured. In addition to the springback that occurs in the formed part after removal of the forming tools, the impact of the thinning of the sheet metal during the forming process is considered in the method, and both effects are correspondingly compensated for an iterative procedure. Computational efficiency in the E-DA-3D method is achieved mainly because the improved accuracy of the communicated data established corresponding interrelations between the discretised topologies used in the definition of the prescribed product geometry, the current tool geometry, and on this basis actually computed product geometry which is achieved by means of additional point topology mappings. The potential and effectiveness of the method is demonstrated by considering two cases of the forming tool design optimisation that are also experimentally validated.
COBISS.SI-ID: 13403419
The article presents a simple but efficient numerical scheme for the integration of non-linear constitutive equations, in which the principal reason for the inaccuracy of the classical explicit schemes, for example forward-Euler scheme, is effectively eliminated. In the newly developed explicit scheme, where there is no need for iteration, the implementation simplicity of the forward-Euler scheme and accuracy of the approach, which is using the backward-Euler scheme to integrate the constitutive equations, are successfully combined. Computational performance of the proposed next increment corrects error (NICE) integration scheme, particularly regarding the accuracy and the CPU time consumption, is first analysed on a case of complex loading of a material point. When comparing it to the forward-Euler, backward-Euler, trapezoidal and midpoint integration schemes, it turns out that because of its capability of a fast and relatively accurate integration of the constitutive equations, the NICE scheme is very convenient for the integration of constitutive models, where a direct solution technique is used to solve a boundary value problem. Although the deduction of the new integration scheme is general, its implementation for shell applications needs particular care. Namely, in order to satisfy the zero normal stress condition during the whole integration, a through-thickness strain increment has to be adequately chosen in each integration step. The NICE scheme, which was also implemented into ABAQUS/Explicit via User Material Subroutine (VUMAT)interface platform, has been additionally compared with the ABAQUS/Explicit default integration scheme (backward-Euler) and forward-Euler scheme. Two loading case-studies, namely the bending of a square plate and the stretching of a specimen including the onset of necking, are considered with two constitutive models - the von Mises and GTN material model being adopted. Generally, the NICE scheme has demonstrated to be advantageous in cases, where reasonable accuracy and very fast integration of the constitutive model is demanded, which is mostly the case in engineering computations with a direct solution method, for example explicit dynamics and metal forming process simulations.
COBISS.SI-ID: 11160091
We present the results of an experimental investigation on the crystallography of the dimpled patterns obtained through wrinkling of a curved elastic system. Our macroscopic samples comprise a thin hemispherical shell bound to an equally curved compliant substrate. Under compression, a crystalline pattern of dimples self-organizes on the surface of the shell. Stresses are relaxed by both out-of-surface buckling and the emergence of defects in the quasi-hexagonal pattern. Three-dimensional scanning is used to digitize the topography. Regarding the dimples as point-like packing units produces spherical Voronoi tessellations with cells that are polydisperse and distorted, away from their regular shapes. We analyze the structure of crystalline defects, as a function of system size. Disclinations are observed and, above a threshold value, dislocations proliferate rapidly with system size. Our samples exhibit striking similarities with other curved crystals of charged particles and colloids. Differences are also found and attributed to the far-from-equilibrium nature of our patterns due to the random and initially frozen material imperfections which act as nucleation points, the presence of a physical boundary which represents an additional source of stress, and the inability of dimples to rearrange during crystallization. Even if we do not have access to the exact form of the interdimple interaction, our experiments suggest a broader generality of previous results of curved crystallography and their robustness on the details of the interaction potential. Furthermore, our findings open the door to future studies on curved crystals far from equilibrium.
COBISS.SI-ID: 13852187