Finite amplitude flexural vibrations at ultrasonic frequencies in metallic bars

9 páginas, 15 figurasThis paper deals with a theoretical and experimental study about the nonlinear behavior of metals subjected to intense ultrasonic, flexural vi brations. In the theoretical analysis a one-dimensional, second-order wave equation is established and solved by means of the successive approximation method. The solution for standing, flexural waves is obtained by applying the superposition principle. Spatial distributions of force, momentum, and particle velocity are derived as well as the waveforms. The experimental study is carried out with resonant, prismatic bars driven at their central sections at frequencies in the range of 20-30 kHz. The samples are driven by means of a piezoelectric transducer. The vibration amplitudes and waveforms are monitored by using a laser vibrometer. Good agreement is found comparing the experimental and theoretical results.This research was supported by the Plan Nacional de Tecnologías Avanzadas de la Producción (research project TAP 93-230). The authors thank Dr. F. Montoya-Vitini and Mr. P.T . Sánz-Sánchez for the designm and construction of the electronic excitation system and Dr. T. Hoffmann for revision.Peer reviewe

Energy-saving compression valve of the rock drill

The relevance of the research is due to the necessity to create pneumatic rock drills with low air consumption. The article analyzes the reasons for low efficiency of percussive machines. The authors state that applying a single distribution body in the percussive mechanism does not allow carrying out a low-energy operating cycle of the mechanism. Using the studied device as an example, it is substantiated that applying a compression valve with two distribution bodies separately operating the working chambers makes it possible to significantly reduce the airflow. The authors describe the construction of a core drill percussive mechanism and the operation of a compression valve. It is shown that in the new percussive mechanism working chambers are cut off the circuit by the time when exhaust windows are opened by the piston and air is not supplied into the cylinder up to 20% of the cycle time. The air flow rate of the new mechanism was 3.8 m3/min. In comparison with the drill PK-75, the overall noise level of the new machine is lower by 8-10 dB, while the percussive mechanism efficiency is 2.3 times higher

Педагогические технологии как дидактический инструментарий подготовки специалиста в инженерном вузе

Рассмотрено понятие педагогической технологии, раскрыта сущность контекстной и задачной технологий, обоснован выбор технологий, адекватных профилю инженерного образования

Parity-violating elastic electron scattering and nuclear structure

We discuss parity-violating elastic electron scattering as a complementary tool in the race for more precise determinations of neutron densities in nuclei. Isovector and isoscalar densities and form factors in N > Z and N = Z stable nuclei are discussed taking Pb-208 and Si-28 as examples. Distorted wave calculations of parity-violating asymmetries are shown and are compared to the plane wave impulse approximation. The extraction of the ratio between neutron and proton monopole form factors is discussed. The isospin mixing produced by Coulomb interaction in the ground state of N = Z nuclei with Skyrme selfconsistent mean fields is also discussed.This work was supported by Ministerio de Ciencia e Innovación (Spain) under Contract FIS2008-01301. J.M.U. acknowledges support from INTAS Open Call grant No 05- 1000008-8272, and Ministerio de Ciencia e Innovación (Spain) under grants FPA-2007- 62616 and FPA-2006-07393, and UCM and Comunidad de Madrid under grant Grupo de Física Nuclear (910059).Peer reviewe