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Karl-Evert Fällström

Bio: Karl-Evert Fällström is an academic researcher from Luleå University of Technology. The author has contributed to research in topics: Bending & Wave propagation. The author has an hindex of 7, co-authored 13 publications receiving 193 citations.

Papers
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Journal ArticleDOI
TL;DR: In this paper, the impact of a ballistic pendulum on an anisotropic plate is modeled as a Dirac pulse in space and time, and a method for determining material parameters, and the mean contact time, from the interferograms is hence developed.
Abstract: Propagating bending waves are studied in plates made of aluminum and wood. The waves are generated by the impact of a ballistic pendulum. Hologram interferometry, with a double pulsed ruby laser as the light source, is used to record the out of plane motion of the waves. Elliptic-like fringes visualize differences in wave speed for different directions in the anisotropic plate and circular ones are obtained for the isotropic plate. The experimental data for the isotropic plate compare favorably with analytical results derived from the Kirchhoff-plate equation with a point impact of finite duration. A similarity variable is found when starting conditions are modeled as a Dirac pulse in space and time, that brings new understanding to the importance of specific parameters for wave propagation in plates. A formal solution is obtained for a point force with an arbitrary time dependence. For times much larger than the contact time, the plate deflection is shown to be identical to that from a Dirac pulse applied at the mean contact time. A method for determining material parameters, and the mean contact time, from the interferograms is hence developed.

53 citations

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TL;DR: In this paper, an electronic speckle pattern interferometer (ESPI) is used to determine modes of vibration in rectangular, orthotropic, free-free plates; that is using a noncontact, nondestructive, optical method.
Abstract: An electronic speckle pattern interferometer (ESPI) is used to determine modes of vibration in rectangular, orthotropic, free-free plates; that is using a noncontact, nondestructive, optical method. It is shown, using the finite element method (FEM), that each of the first three modes of vibration in rectangular orthotropic plates has a strong dependence upon only one of the main material parameters, namely the in-plane shear modulus and the two Young's moduli, respectively. With this one-variable dependence it is a simple task to determine the effective material parameters. This method has several obvious advantages compared to the use of test bars and it can be extended to give a measure of the damping parameters and probably also be used for production control. Preliminary results are presented and discussed.

40 citations

Journal ArticleDOI
TL;DR: In this article, material parameters of anisotropic plates are determined by changing the quotient between the length of the sides of the plates, so that the second and third modes of vibration degenerate into the well-known cross-shaped and ring-shaped modes, respectively.
Abstract: Material parameters of anisotropic plates are determined. Rectangular anisotropic plates are tuned by changing the quotient between the length of the sides, so that the second and third modes of vibration, for free-free boundary conditions, degenerate into the well-known cross-shaped and ring-shaped modes, respectively. The first three modes of vibrations for these plates are determined by optical methods. The propagation of bending waves, generated by the impact of a ballistic pendulum on the plates, has also been optically studied. It is found that bending waves generated in the middle of the tuned plates will reach the boundaries of the plates simultaneously. This gives a relationship between main material parameters. Using this relation and Rayleigh's method for the first three modes of vibrations, the main material parameters for the plates, that is, the effective two Young's moduli, the shear modulus, and the Poisson's ratio, are determined.

30 citations

Journal ArticleDOI
TL;DR: In this article, material parameters in an anisotropic plate are determined using two non-destructive measuring techniques: real-time phase-stepped electronic speckle pattern interferometry and dual-reference-beam pulsed holographic interFERometry.

22 citations

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TL;DR: In this paper, the influence of local changes in thickness and delaminations upon the fringe pattern is presented, together with a method to interpret the fringe patterns when several defects are present, where the size of a defect has to be of the same size or larger than the wavelength of the bending wave to be visible.
Abstract: Double exposure holographic interferometry, with a ruby laser as the light source, is used to study the influence of local changes in thickness, of delaminations and variations in damping upon propagating transient bending waves in plates and shells. The bending waves are created by the impact of a pendulum. Defects cause changes in the wave impedance of the plate, which will give rise to changes relative to the undisturbed fringe pattern, by reflected and transmitted waves. The size of a defect has to be of the same size or larger than the wavelength of the bending wave to be visible. Since bending waves are dispersive, high frequency components travelling faster than low frequency ones, a small defect is most likely to be observed shortly after impact. The influence of local changes in thickness and delaminations upon the fringe pattern is presented, together with a method to interpret the fringe pattern when several defects are present.

10 citations


Cited by
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Journal ArticleDOI
TL;DR: In this paper, a variety of techniques used to obtain the mechanical properties of materials at high rates of strain (⩾10 s−1) are summarised, including dropweight machines, split Hopkinson pressure bars, Taylor impact and shock loading by plate impact.

683 citations

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TL;DR: In this article, the authors derived a criterion for small-mass (wave controlled) impact response of orthotropic plates and showed that the response type is governed by the impactor-plate mass ratio and not by impact velocity.
Abstract: Impact duration strongly influences the impact response of plates. Long impacts cause a quasi-static response influenced by the plate size and boundary conditions. Short impacts cause a response governed by wave propagation unaffected of plate size and boundary conditions. This paper shows that the response type is governed by the impactor–plate mass ratio and not by impact velocity and derives a criterion for small-mass (wave controlled) impact response of orthotropic plates. Published criteria for large-mass (quasi-static) impact are discussed. Small-mass impacts on composite laminates are shown to be more critical than large-mass impacts of the same energy. Use of mass criteria for selecting analytical response models and considerations for sandwich plates are also discussed.

297 citations

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TL;DR: The main numerical techniques that have been developed to carry out fully automated analysis of fringe patterns resulting from solid mechanics experiments are reviewed, and a unified treatment for both speckle and smooth-wavefront interferograms is presented.
Abstract: The paper reviews the main numerical techniques that have been developed to carry out fully automated analysis of fringe patterns resulting from solid mechanics experiments. These include temporal and spatial phase shifting interferometry, temporal and spatial phase unwrapping, and calculation of strain fields from the phase maps. Systematic and random errors associated with the various procedures are also analysed. A unified treatment for both speckle and smooth-wavefront interferograms is presented, and the common features underlying many of the algorithms are emphasized. The paper is illustrated with applications that include ball impact (moire photography), bending waves in orthotropic plates (double-pulsed dual-reference wave holography) and finite strains in propellant grains (fine grid technique).

124 citations

Journal ArticleDOI
TL;DR: In this paper, an approximate analytical solution for the dynamic response of an infinite specially orthotropic plate impacted by an impactor with a semispherical tip is presented, which is valid for low mass impacts.
Abstract: This paper presents an approximate analytical solution for the dynamic response of an infinite specially orthotropic plate impacted by an impactor with a semispherical tip. Thus, the solution is valid for low mass impacts. The analysis is an extension and rederivation of a solution for isotropic plates proposed by Zener. The analysis assumes a Hertzian contact law and is based on Kirchhoff's plate equation. The plate response is expressed in terms of contact force, contact pressure, central displacement, central curvature, and size of the impact affected area. The response is computed from a dimensionless differential equation in time, which is only dependent on the inelasticity parameter lambda. Lambda is a function of the impact velocity and variables describing the impactor and the plate. For a given lambda, the response can be interpolated from the solution plots for a number of representativ e values of lambda. Results computed from the model are compared with published numerical analyses and a number of experiments, and a close agreement is noted. Finally, the analysis shows the time-dependent velocity of a flexural wave propagating from the impact center.

110 citations

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TL;DR: In this paper, a method of combining finite element analysis and optimum design is adopted to inversely determine the elastic constants in order to simplify the modeling processes and to reduce complicated derivation in the numerical method.

99 citations