Advances in Experimental Mechanics V

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Authors: Carl T.F. Ross, Marcus Engelhardt, Andrew P.F. Little
Abstract: This paper describes an experimental and an analytical and numerical investigation into the buckling behaviour of cylindrical composite tubes under external hydrostatic pressure. The investigations concentrated on fibre reinforced plastic tube specimens made from a mixture of three carbon and two E-glass fibre layers. The lay-up was 0°/90°/0°/90°/0; the carbon fibres were laid lengthwise (0°) and the E-glass fibres circumferentially (90°). The theoretical investigations were carried out using a simple solution for isotropic materials, namely a well-known formula by “von Mises” and also by finite element analyses using ANSYS. The experimental investigations showed that the composite specimens behaved similarly to isotropic materials tested by various other researchers. The specimens failed by the common modes associated with this study, namely due to elastic buckling, inelastic buckling and axisymmetric yield failure. Furthermore it was discovered that the specimens failed at changes of the composite lay-up due to the manufacturing process of these specimens. These changes seem to be the weak points of the specimens. For the theoretical investigations two different types of material properties were used to analyse the composite. These were calculated properties derived from the properties of the single layers given by the manufacturer and experimentally obtained properties. Two different approaches were chosen for the investigation of the theoretical buckling pressure, a program called “MisesNP”, based on a well-known formula by von Mises for single layer isotropic materials, and two finite element analyses using the famous computer package called “ANSYS”. This latter analyses simulated the composite with a single layer orthotropic element (Shell93) and also with a multi layer element (Shell99). It was found out that the results obtained with ANSYS predicted questionable buckling pressures that could not be reproduced logically. Nevertheless this report provides Design Charts for all approaches and material types. These Design Charts allow the possibility of obtaining a ‘plastic knockdown factor’. The theoretical buckling pressures obtained using MisesNP or ANSYS can then be divided by the plastic knockdown factor, to give predicted buckling pressures. This method can be used for the design of full-scale vessels.
Authors: Ch.R. Koenig, D.H. Mueller, O. Focke, Mircea Calomfirescu
Abstract: Fibre reinforced polymers (FRP) offer a high potential to reduce kinetic energy. As a consequence of this, fibre reinforced polymers often have a higher risk of being exposed to impact loads. The knowledge of the mechanisms and of the material loading during and shortly after an impact load is essential for an ‘impact-load-monitoring-system’ to predict possible structural failures. Especially a prognosis of structural failures caused by – often unrecognized - barely visible impacts is an important factor. Primary impact damages often leads to a sudden structural failure. These unheralded failures seems to be one of the most important problems in product development for the aircraft industry. By measuring the structural response at several discrete measurement points an impact can be detected, reconstructed and also rated.
Authors: Ana M. Amaro, Paulo N.B. Reis, A.G. Magalhães, Marcelo F.S.F. de Moura
Abstract: The aim of present work is to study the influence of the impactor diameter and boundary conditions on low velocity impact on carbon-fibre-reinforced epoxy laminates. Experimental tests were performed on [04,904]s laminates, using a drop weight-testing machine. Circular plates were tested under low velocity impacts for two diameters of the hemispherical impactor, 12.7 mm and 20 mm, and considering similar impact energies, 2.6 J for the first impactor and 3 J for the second one. Rectangular and square plates were analysed under low velocity impacts with different boundary conditions. The impacted plates were inspected by X-radiography. Numerical simulations were also performed considering interface finite elements compatible with three-dimensional solid elements including a cohesive mixed-mode damage model, which allows to model delamination between layers. The impact tests showed that both the impactor’s diameter and boundary conditions have influence on the delaminated area. Good agreement between experimental and numerical analysis for shape, orientation and size of damage was obtained.
Authors: S.K. Srivastava, I.P. Singh
Abstract: Hybrid laminates of glass fibre reinforced plastics (GFRP) are being increasingly used for marine structures under multidirectional loadings, due to their anisotropic behavior, corrosion resistance, high specific strength and stiffness. Therefore appropriate laminate configuration for marine environment applications is an important field of study. Five types of fibre epoxy laminates configurations, resulting from different combinations of three layers of chopped strand mats (CSM) and woven roving (WR) were fabricated using the vacuum resin transfer moulding (VRTM) technique. These were investigated for the effect of seawater on its impact properties. The results showed a significant reduction in the impact strength in all types of wet specimens. This behavior may be attributed to penetration of water molecules in the composites. The impact properties of hybrid laminates using a mixture of CSM and WR were found to be better than combination of laminates comprising only CSM and WR under both dry and wet conditions for marine structure.
Authors: Robert A.W. Mines
Abstract: Structural foams are used as cores in sandwich construction. In the application of foreign object impact loading of sandwich structures, the core will suffer dynamic multi axial deformation and crush. This means that experimental study is required for the crush behaviour of structural foams at various strain rates, and numerical simulation foam models need to be calibrated with dynamic data. A number of foams are considered, namely Divinycell PVC foam, Rohacell PMI foam and Alporas aluminium foam. Also, new generation metallic micro lattice structures are discussed
Authors: M.A. Garcia-Romeu-Martinez, Michael A. Sek, Vincent Rouillard, V.A. Cloquell-Ballester
Abstract: During distribution, consignments undergo numerous handling processes both mechanized and manual. These operations are known to produce drops and impacts of varying severity which have a potential to cause damage to the product. These shocks are the main parameters required for the optimum design of protective packaging systems. The severity of the shocks is often described in terms of their effective (free-fall equivalent) drop height (EDH) and impact orientation, in order to facilitate the laboratory testing conducted on a free-fall apparatus. The preferred approach is to survey the shocks with self-contained tri-axial shock recorders and process the data in such a way that statistical distributions of expected drop heights and orientations are obtained. On the other hand the Real Drop Height (RDH) method, based on the measurement of free fall time, is also used, mainly to discriminate between free-fall events and more commonly occurring complex causes of shocks, primarily for the quality control of distribution environment. The focus of the paper is on the EDH method and on the use of characteristic parameters of the tri-axial acceleration shock pulse to determine the EDH. An accurate estimate of the coefficient of restitution between the instrumented test package and the impact surface must be known and this poses a problem as it cannot always be established for every event during distribution. Consequently, the adopted approach is to calibrate an instrumented test package and obtain an estimate of the coefficient of restitution between the package and a test impact surface which is generally assumed to be hard relative to the cushioned package. The paper addresses the pitfalls and investigates various algorithms of determining the EDH from recorded shock data. It presents an analysis of the influence and errors associated with various methods used to estimate velocity change from characteristic parameters of a shock pulse such as the pulse width, the peak acceleration and its temporal location. The effects of analyzing the orthogonal acceleration vectors separately, as opposed to the resultant vector, are discussed. The results of a number of free-fall experiments, undertaken in controlled conditions, are used to validate and calibrate the proposed method for determining the EDH for free-fall drops on hard surfaces.
Authors: Takashi Yokoyama, Kenji Nakai
Abstract: High strain-rate compressive responses of AA7075-T651 and its welds as produced by the friction stir welding (or FSW) process are investigated using the conventional split Hopkinson pressure bar. Cylindrical specimens machined along the thickness direction of the base material (AA7075-T651) and the friction stir (FS) welds are used in the static and impact compression tests. The micro-hardness tests are conducted across the centerline of a FS welded AA707-T651 joint in order to examine the microstructural change. It is shown that FSW reduces the compressive flow stress of the FS weld (weld nugget) to below that of the base material, and both the base material and the FS weld exhibit almost no strain rate effects up to nearly € ε˙ =103/s.
Authors: Stefan Andreas Ritt, Alastair F. Johnson
Abstract: This paper is concerned with methods to determine the resultant impact load pulse on test structures during gas gun tests. The muzzle of a gas gun carries an instrumented target support designed to measure force-time-pulses of the impact event on structures mounted on the device. The target during impact is supported on two long slender metallic bars with axial strain gauges. By measuring the impact strain pulses in the support bars and assuming longitudinal wave propagation, it is possible to determine the impact load pulse on the target. The paper describes the strain measuring device and methods for determining the impact loads during impact. Gas gun test results with different projectiles are presented and the load pulses compared with a direct load measurement from a target load cell. The paper discusses possibilities and limitations of the device for quantitative force pulse measurements.
Authors: Thorsten Siebert, Thomas Becker, Karsten Spiltthof, Isabell Neumann, Rene Krupka
Abstract: The reliability for each measurement technique depends on the knowledge of it’s uncertainty and the sources of errors of the results. Among the different techniques for optical measurement techniques for full field analysis of displacements and strains, digital image correlation (DIC) has been proven to be very flexible, robust and easy to use, covering a wide range of different applications. Nevertheless the measurement results are influenced by statistical and systematical errors. We discuss a 3D digital image correlation system which provides online error information and the propagation of errors through the calculation chain to the resulting contours, displacement and strains. Performance tests for studying the impact of calibration errors on the resulting data are shown for static and dynamic applications.

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