Applied Mechanics and Materials Vols. 24-25

Abstract: Simulation result of a structural dynamics problem is dependent on the techniques used in the finite element model and the major task in model updating is determination of the changes to be made to the numerical model so that dynamic properties are comparable to the experimental result. In this paper, the dynamic analysis of a thin wall structure ( approx. 1.5±0.1 mm thick) was realized using the Lanczos tool to extract the modes between 0 and 200 Hz, but the interest was to achieve a good aggreement between the first ten natural frequencies. A shell element with mid size nodes was used to improve the finite element result and the model was tunned using the damping constant, material properties and discretization. The correlation of the results from the impact excitation response test and the finite element was significantly improved. A correlation coefficient of 0.99 was achieved after tunning the model.

Abstract: Stick-Slip motion is the basis for the description of a great variety of phenomena characterized by the presence of sliding friction between bodies with elastic features. In this article a simple experimental equipment for the analysis of this kind of dynamics is described. A wide set of possible experimental observations and measures are presented. This equipment has been tested at the university of Napoli Federico II in courses for undergraduate students and in the teacher training school for secondary education.

Abstract: Compressive stress-strain loops of selected polymers at strain rates up to nearly 800/s are determined in a strain range of nearly 8% on the standard split Hopkinson pressure bar. Four different commercially available extruded polymers are tested at room temperature. The compressive stress-strain loops at low and intermediate strain rates are measured on an Instron testing machine. The effects of strain rate on the Young's modulus, flow stress and dissipation energy are discussed. It is shown that the area included within the stress-strain loop increases with increasing strain rate as well as a given strain, that is, all four extruded polymers tested exhibit intrinsic strain-rate dependent viscoelastic behavior and a high elastic aftereffect following complete unloading.

Abstract: This work concerns the shape identification of curvilinear objects, for example bent beams or wires in mechanics. The beam’s digital picture is analyzed with the introduced Virtual Image Correlation method. This one consists in finding the optimal correlation between the beam’s image and a virtual beam, whose curvature field is described by a truncated series. The gray level and amplitude of the virtual beam does not need to reproduce exactly the ones of the physical beam image. The analytical form of the optimal shape allows one to derive mechanical properties: the identification of the Young’s modulus of a bar is given as an example. We will also show the robustness of the method with regards to the quality of the image.

Abstract: The achievement of high levels of confidence in finite element models involves their validation using measured responses such as static strains or vibration mode shapes. A huge amount of data with a high level of information redundancy is usually obtained in both the detailed finite element prediction and the full-field measurements so that achieving a meaningful validation becomes a challenging problem. In order to extract useful shape features from such data, image processing and pattern recognition techniques may be used. One of the most commonly adopted shape feature extraction procedures is the Fourier transform in which the original data may be expressed as a set of coefficients (coordinates) of the decomposition kernels (bases) in the feature space. Localised effects can be detected by the wavelet transform. The acquired shape features are succinct and therefore simplify the model validation, based on the full-field data, allowing it to be achieved in a more effective and efficient way. In this paper, full-field finite element strain patterns of a plate with a centred circular hole are considered. A special set of orthonormal shape decomposition kernels based on the circular Zernike polynomials are constructed by the Gram-Schmidt orthonormalization process. It is found that the strain patterns can suitably be represented by only a very small number of shape features from the derived kernels.

Abstract: This report deals with an innovative method (Own-Weight Cantilever Method) to measure Young’s modulus of flexible thin materials. A newly developed method is based on the large deformation theory considering large deformation behaviors due to own-weight in flexible thin materials. Analytical solutions are derived by using Bessel Functions. By means of measuring the horizontal displacement or the vertical displacement at a free end of a cantilever, Young’s modulus can be easily obtained for various flexible thin and long materials. Measurements were carried out on a piano wire. The results confirm that the new method is suitable for flexible thin wires.

Abstract: Tensile tests were performed in order to identify the stiffness components of superconducting windings in the shape of rings (also called ‘double pancakes’). The stereo image correlation technique was used for full-field displacement measurements. The strain components were then obtained from the measured displacement fields by numerical differentiation. Because differentiation is very sensitive to spatial noise, the displacement maps were fitted by polynomials before differentiation using a linear least-square method. Then, in the orthotropy basis, the four in-plane stiffnesses of the double pancake were determined using the Virtual Fields Method.

Abstract: The paper describes a novel method based on Digital Image Correlation (DIC) for measuring the static modulus of active PGA graphite specimens during conventional four-point bend strength tests. DIC has been used in combination with finite element and numerical modelling to monitor the displacement fields developing in the specimen during testing, and calculate representative modulus values. Details of the model and results are presented for two specimen geometries and a range of materials with different levels of exposure and density, tested in the perpendicular and parallel orientations. The calculated static modulus values from the DIC measurements confirm the trend between modulus, flexural strength and density. Comments and observations on the uncertainty in the measurement are also presented.

Abstract: In this paper the effect of delamination position on the critical buckling load and buckling mode of hybrid composite beams is investigated. Experimental and numerical studies are carried out to determine the buckling load of delaminated composite beams. The laminated composite beams with various laminate designs of [G90]6, [C90]8, [C0/G0]4 and [C90/G90]4 were manufactured and tested to find the critical buckling load. Three different defect positions were placed through the thickness to find three main buckling modes. It was found that delamination position and lay-up can affect the buckling mode and also the critical buckling load. By approaching the delamination position to the outer surface of the specimen the buckling load decreases. The buckling process of hybrid and non-hybrid composite beams was also simulated by finite element software ANSYS and the critical buckling loads were verified with the relevant experimental results.

Abstract: Three Pultruded Fibre Reinforced Polymer (PFRP) 152 x 152 x 6.4mm Wide Flange (WF) beams were fabricated with a central two-plate splice joint. The 6.4mm thick PFRP splice plates were 210, 410 and 610mm long. Each beam was tested in symmetric four-point bending about its major and minor-axis and deflections, rotations and surface strains were recorded. Beam transverse stiffnesses, support rotations and splice rotational stiffnesses were quantified and compared with theoretical predictions. Predicted deflections were 3.5% to 18.5% larger and support rotations were 10% smaller to 14.2% larger than the experimental values. Splice end rotations were generally poorly predicted.