Thursday, August 28, 2014

Orientation Dependence of the Fracture Behavior of Single-crystal Tungsten ☆


Polycrystalline tungsten at room temperature shows a brittle fracture behavior, which is also strongly influenced by the grain structure and texture as well as sample dimensions. To gain insight into the mechanical response of individual grains, an experimental program has been set up to test small scale samples under microbending starting with a notched tungsten single crystal oriented with the {110}<> crack system along the loading direction. Related to this experimental program a finite element study has been performed to analyze the crack propagation in such single-crystal tungsten micro cantilevers. The aim of the present numerical work is to investigate the influence of the single-crystal orientation on the fracture process.
A finite element (FE) model of the notched microbeam was created taking plastic deformation at the crack tip into account. Plastic deformation is implemented using a crystal plasticity approach formulated by Asaro (1983) and written by Huang (1991). Furthermore, the fracture process with crack propagation is described by a cohesive zone model. The simulations of microbending allow for evaluating the details of the fracture process more accurately. The results reveal details of the developing plastic zone as well as the current crack propagation and the J-integral in dependence of the crystal orientation and notch geometry.

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Thursday, August 14, 2014

Piezoelectric single crystal ultrasonic transducers for biomedical applications

Piezoelectric single crystals, which have excellent piezoelectric properties, have extensively been employed for various sensors and actuators applications. In this paper, the state-of-art in piezoelectric single crystals for ultrasonic transducer applications is reviewed. Firstly, the basic principles and design considerations of piezoelectric ultrasonic transducers will be addressed. Then, the popular piezoelectric single crystals used for ultrasonic transducer applications, including LiNbO3 (LN), PMN–PT and PIN–PMN–PT, will be introduced. After describing the preparation and performance of the single crystals, the recent development of both the single-element and array transducers fabricated using the single crystals will be presented. Finally, various biomedical applications including eye imaging, intravascular imaging, blood flow measurement, photoacoustic imaging and microbeam applications of the single crystal transducers will be discussed.

Keywords:Ultrasonic transducer; Single crystal; Biomedical applications.

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