A demonstration of the capability of the inhomogeneous wave theory to simulate backward displacement of ultrasonic-bounded beams [M. A. Breazeale and M. Torbett, Appl. Phys. Lett. 29, 456 (1976)] has been demonstrated recently [N. F. Declercq, J. Degrieck, R. Briers, and O. Leroy, Appl. Phys. Lett. 82, 2533 (2003)]. The current report applies the theory of the diffraction of inhomogeneous waves and shows how this theory is capable of simulating, explaining, and understanding the experiments mentioned above. The theory reveals the existence of leaky Scholte-Stoneley waves, a phenomenon suggested theoretically [N. F. Declercq, J. Degrieck, R. Briers, and O. Leroy, J. Acoust. Soc. Am. 112, 2414 (2002)] and observed experimentally [A. A. Teklu, M. A. Breazeale, J. Acoust. Soc. Am. 113, 2283 (2003)]. Moreover, the present paper shows that the classical Fourier decomposition of bounded beams is unable to simulate the backward beam displacement. This work also elucidates the nature of Wood anomalies in Diffraction spectra.
Although the effect of high and low temperatures on the mechanical properties of Ti-6Al-4 V has already been extensively studied, extreme temperatures are usually considered in these investigations. Instead, the present work focuses on a range of temperatures not far from room conditions. This particular range of temperatures is important not only because it is commonly found in service, but also because it corresponds with the temperatures reached by plastic work heating when the material is plastically deformed. Isothermal tensile tests show that relatively low temperatures (of less than 100 °C) already have a non-negligible effect on the plastic flow of the material. Numerical simulations of the heat generation in the test samples and the heat transfer to the surrounding environment show that there is a significant temperature increment in samples loaded at low strain rates. As is confirmed by experiments at these speeds, thermal softening, together with the strain rate effect, is playing an important role, even at strain rates at which isothermal conditions are often assumed. Finally, the suitability of the Johnson-Cook plasticity model to describe the observedbehaviouris discussed.
The design and testing of composite point absorbers under slamming load is the central theme in this paper. The research is situated within the SEEWEC project which was launched at the end of 2005 in the Sixth Framework of the European Union. The SEEWEC stands for the sustainable economically efficient wave energy converter. First, the energy situation in Europe is discussed to explain the objective of these point absorbers within this research. Next, a small filament winding machine has been designed to produce these point absorbers on the laboratory scale. Also, a set-up for laboratory scale slamming tests was designed and built. Several test samples were produced on the winding machine and tested on the slamming set-up. First, a cone was tested as a reference case. Validation of this case followed suit. Next, a deformable and a non-deformable cylinder were compared. Finally, a possible point absorber was tested. Finite element analysis (FEA) calculations were conducted and computational fluid dynamics (CFD) simulations are initiated.
An important requirement for tissue engineering scaffolds is matching of the functional mechanical properties to their natural tissue counterpart.Specifically for arteries this comprises the elastic response of the vessel wall to blood pressure.Human aorta has a low elastic modulus when compared to some FDA-approved synthetic polymer materials frequently used in tissue engineering.The current research endeavours to expand the existing production technology of 3D plotting to winding of micro-extruded filaments in order to obtain flexible polymer tubes with continuous fibre.Tube scaffolds are manufactured by conventional 3D plotting and by winding.Their structure and quasi-static mechanical properties are evaluated and compared to human aorta.Winded tubes are found to be far more suitable for application as a blood vessel scaffold than their 3D plotted counterparts.
The deformation behaviour of ductile metals is determined by phenomena at different physical length scales. In most ductile metals subjected to high strain-rates, the development of adiabatic shear bands (ASB) is one element of that process. In ASBs, the highly localized shear deformation occurs in a narrow band which exhibits very large strains relative to the neighbouring material. Since the deformation time is very small, the process happens adiabatically. The resulting temperature increase by plastic work causes thermal softening and further localization of plastic deformation. ASBs are observed in many applications such as machine chips, forging, ballistic impact loading and fracture. In the past, a dozen of experimental techniques have been developed to characterize the process of ASB formation. One often used technique is based on the dynamic deformation of hat-shaped specimens in a split Hopkinson pressure bar (SHPB) setup [1]. Figure 1 shows a sketch of the specimen with an indication of characteristic lengths. Due to the specific geometry of this specimen, shear strains are concentrated in a narrow region. This technique is especially popular among metallurgists because even materials that do not localize spontaneously in shear can be forced up to shearing failure. On the other hand, the determination of material properties from these experiments is not straightforward because of the complex stress state in the shearing region. Figure 1: Hat-shaped specimen More insight in the stress distribution in the specimen is needed in order to better understand the experimental data obtained by this technique. Furthermore, as the nucleation and propagation of ASBs depends on the stress condition [2], knowledge of the stress distribution is crucial. Several researchers used different specimen dimensions in their experiments. Moreover, the outcome of these experiments is affected not only by the testing material but also by the specimen geometry. In [1] the stress distribution of one particular specimen geometry was investigated using FE simulations. The goal of this contribution is to relate the specimen dimensions with the stress distribution in the specimen. This has previously been done for other specimen geometries: e.g. truncatedconic specimens [2], dogbone-shaped tensile specimens [3] and double shear specimens [4]. In addition, the existence of an optimal specimen geometry to achieve an as pure as possible shearing stress state, is studied. The main tool in this study is the FE method. A 3D axis-symmetric model has been defined, using ABAQUS/explicit. The load is applied by a uniform velocity of the boundaries of the specimen. Strain-rate dependency of the material behaviour is modelled by the Johnson-Cook phenomenological model. Heat generated due to the plastic work is included, while heat conduction is not implemented. To overcome difficulties from extensive element distortion, ALE adaptive meshing is used. Figure 2 shows the distribution of the shear stress in the shear region of a sample. Figure 2: Simulation of the shear stress Attention is paid on the stress tri-axiality as well as on the ratio of the hydrostatic pressure (related to r2-r1, Figure 1) to the shear stress in the shear region. ACKNOWLEDGEMENTS The authors would like to acknowledge The Interuniversity Attraction Poles Programme (IUAP) of the Federal Science Policy of Belgium and the partners of IUAP VI (www.m3phys.be) REFERENCES [1] Bronkhorst C.A., Cerreta E.K., Xue Q., Maudlin P.J., Mason T.A. and Gray G.T., An experimental and numerical study of the localization behaviour of tantalum and stainless steel. Int. J. Of Plasticity Vol 22, pp. 1304-1335, 2006. [2] Li J.R., Yu J.L. and Wei Z.G., Influence of specimen geometry on adiabatic shear instability of tungsten heavy alloys. Int. J. Impact Engineering Vol. 28, pp. 303-314, 2003. [3] Verleysen P. and Degrieck J., Experimental and numerical study of the response of steel sheet Hopkinson specimens. J. Phys. IV France Vol. 134, pp. 541-546, 2006. [4] Klepaczko J.R., Stress concentrators and rate effects in formation of adiabatic shear bands. Final technical report, European research office of the US-army, N68171-95-C-9071, 1996
High performance composites, such as carbon-fibre reinforced plastics (CFRP), are increasingly being used in high engineering industries. Their wide acceptance introduces an issue regarding the bonding of these materials and their mechanical behaviour. As this reinforced thermoplastic is not easily bonded with adhesives due to the chemical inertness, the fusion bonding process could be used to make a structural bond. In this paper, the interlaminar fracture behaviour of infrared welded bonds was investigated based on experimental analysis. The material used, is a 5-harness satin-weave (5HS) carbon fabric-reinforced polyphenylene sulphide (PPS). Laminates were welded using infrared light and a delamination was introduced by a Kapton film. Welding parameters were first optimized using lapshear tests, then mode I and mode II tests were conducted to determine the fracture toughness behaviour of the welded bonds. Tests under mode I loading were carried out using double cantilever beam (DCB) specimens whereas for mode II loading, three-point end notched flexure (ENF) specimens were considered. Crack growth under mode I and mode II loading conditions was observed to be unstable resulting in a sawtooth like load-displacement response, but nevertheless, values for the fracture toughness were derived.
The radiation theory has proved to produce results in agreement with experiments when the conversion is investigated of a Scholte - Stoneley wave at the extremity of a fluid loaded plate. The drawback of the radiation mode theory is that it is not universally applicable and is also very cumbersome. The advantage is that it is an exact method. There is a trend in acoustics to develop finite element models to describe the interaction of sound waves with materials. This investigation compares a newly developed finite element model to simulate the considered effect. Results are compared with the exact results obtained by the radiation mode theory [J. Acoust. Soc. Am. 101(3), 1347-1357, 1997] and experimental observations [J. Acoust. Soc. Am. 95(1), 13-20, 1994]. The study shows correspondence between the finite element analysis and earlier obtained experimental and theoretical results as a function of a relationship between mesh properties and the evanescence (rather than the wave length) of the considered Stoneley waves.
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