A modified split hopkinson torsional bar in studying shear localization

A modified split Hopkinson torsional bar (SHTB) is introduced to eliminate the effect of the loading reverberation of the standard SHTB on the study of evolution of shear localization. The effect, the cause and the method by which to eliminate loading wave reverberation are carefully analysed and discussed. By means of the modified apparatus, the post-mortem observation of tested specimens can provide data on actual evolution of micro-structure and micro-damage during shear localization. Some test results of shear banding conducted with this apparatus support the use of the modified design. Moreover, the modification makes possible the correlation of evolving micro-structures to the transient shear stress-strain recording.

Elimination of loading reverberation in the split hopkinson torsional bar

The loading reverberation is a multiple wave effect on the specimen in the split Hopkinson torsional bar (SHTB). Its existence intensively destroys the microstructure pattern in the tested material and therefore, interferes with the study correlating the deformed microstructure to the macroscopic stress-strain response. This paper discusses the problem of the loading reverberation and its effects on the post-mortem observations in the SHTB experiment. The cause of the loading reverberation is illustrated by a stress wave analysis. The modification of the standard SHTB is introduced, which involves attaching two unloading bars at the two ends of the original main bar system and adopting a new loading head and a couple of specially designed clutches. The clutches are placed between the main bar system and the unloading bars in order to lead the secondary loading wave out of the main bar system and to cut off the connection in a timely manner. The loading head of the standard torsional bar was redesigned by using a tube-type loading device associated with a ratchet system to ensure the exclusion of the reflected wave. Thus, the secondary loading waves were wholly trapped in the two unloading bars. The wave recording results and the contrasting experiments for examining the post-mortem microstructure during shear banding both before and after the modification highly support the effectiveness of the modified version. The modified SHTB realizes a single wave pulse loading process and will become a useful tool for investigating the relation between the deformed microstructure and the macroscopic stress-strain response. It will play an important role especially in the study of the evolution of the microstructure during the shear banding process. (C) 1995 American Institute of Physics.

Avaliação da estabilidade da cirurgia de avanço mandibular através da superposição de modelos tridimensionais

Embora a cirurgia de avanço mandibular seja considerada um procedimento altamente estável, existem algumas preocupações clínicas em relação a mudanças nos côndilos e nos segmentos proximais, que podem levar a recidiva sagital e abertura de mordida. A avaliação dos resultados da cirurgia através de ferramentas de geração e superposição de modelos virtuais tridimensionais (3D) permite a identificação e quantificação dos deslocamentos e remodelação óssea que podem ajudar a explicar as interações entre os componentes dentários, esqueléticos e de tecido mole que estão relacionados a resposta ao tratamento. Este estudo observacional prospectivo avaliou, através de tomografia computadorizada de feixe cônico (CBCT), mudanças na posição/remodelação 3D dos ramos mandibulares, côndilos e mento. Assim, exames CBCT de 27 pacientes foram adquiridos antes da cirurgia (T1), imediatamente após a cirurgia(T2), e 1 ano após a cirurgia(T3). Uma técnica automática de superposição na base do crânio foi utilizada para permitir a avaliação das mudanças ocorridas nas regiões anatômicas de interesse (RAI). Os deslocamentos foram visualizados e quantificados em mapas coloridos 3D através da ferramenta de linha de contorno (ISOLINE). Pelo teste t pareado compararam-se as mudanças entre T1-T2 e T2-T3. O coeficiente de correlação de Pearson verificou se os deslocamentos ocorridos nas RAI foram correlacionados entre si e entre os tempos de avaliação. O nível de significância foi determinado em 0,05. O avanço mandibular médio foi de 6,813,2mm em T2 e 6,363,41mm em T3 (p=0,13). Entre T2 e T3, a posição do mento variou positivamente (≥2mm) em 5 pacientes negativamente em 7. 12% dos pacientes sofreram recidivas ≥4mm. Para todas as outras RAI avaliadas, apenas a porção inferior dos ramos (lado direito - 2,342,35mm e lado esquerdo 2,972,71mm) sofreram deslocamentos médios >2mm com a cirurgia. No acompanhamento em longo prazo, esse deslocamento lateral da porção inferior dos ramos foi mantido (lado direito - 2,102,15mm, p=0,26; e lado esquerdo -2,762,80, p=0,46), bem como todos os outros deslocamentos observados (p>0,05). As mudanças na posição do mento foram correlacionadas a adaptações pós-cirúrgicas nos bordos posteriores dos ramos (esquerdo r=-0,73 e direito r=-0,68) e côndilos (esquerdo r=-0,53 e direito r=-0,46). Os deslocamentos médios sofridos pelas estruturas do lado esquerdo foram suavemente maiores do que no direito. Correlações dos deslocamentos ocorridos entre T1-T2 e T2-T3 mostraram que: os deslocamentos dos côndilos esquerdos com a cirurgia foram negativamente correlacionados às adaptações pós-cirúrgicas destes (r=-0,51); e que o deslocamento da porção superior do ramo esquerdo com a cirurgia foi correlacionado à adaptação pós-cirúrgica ocorrida nos bordos posteriores (r=0,39) e côndilos do mesmo lado (r=0,39). Pode-se concluir que: (1) os deslocamentos causados pela cirurgia foram de modo geral estáveis no acompanhamento de 1 ano, mas identificou-se uma considerável variação individual; (2) as mudanças pós-cirúrgicas na posição do mento foram correlacionadas a adaptações sofridas pelos côndilos e bordos posteriores dos ramos; e que (3) deslocamentos suavemente maiores causados pela cirurgia nas estruturas do lado esquerdo levaram a maiores adaptações pós-cirúrgicas no segmento proximal deste lado.

Split-aperture laser pulse compressor design tolerant to alignment and line-density differences

We introduce a four-pass laser pulse compressor design based on two grating apertures with two gratings per aperture that is tolerant to some alignment errors and, importantly, to grating-to-grating period variations. Each half-beam samples each grating in a diamond-shaped compressor that is symmetric about a central bisecting plane. For any given grating, the two half-beams impinge on opposite sides of its surface normal. It is shown that the two split beams have no pointing difference from paired gratings with different periods. Furthermore, no phase shift between half-beams is incurred as long as the planes containing a grating line and the surface normal for each grating of the pair are parallel. For grating pairs satisfying this condition, gratings surfaces need not be on the same plane, as changes in the gap between the two can compensate to bring the beams back in phase. © 2008 Optical Society of America.

A chemically inert Rashba split interface electronic structure of C-60, FeOEP and PTCDA on BiAg2/Ag(111) substrates

The fields of organic electronics and spintronics have the potential to revolutionize the electronics industry. Finding the right materials that can retain their electrical and spin properties when combined is a technological and fundamental challenge. We carry out the study of three archetypal organic molecules in intimate contact with the BiAg2 surface alloy. We show that the BiAg2 alloy is an especially suited substrate due to its inertness as support for molecular films, exhibiting an almost complete absence of substrate-molecular interactions. This is inferred from the persistence of a completely unaltered giant spin-orbit split surface state of the BiAg2 substrate, and from the absence of significant metallic screening of charged molecular levels in the organic layer. Spin-orbit split states in BiAg2 turn out to be far more robust to organic overlayers than previously thought.

Retail prices of split, smoked, dried bream and dried Lake Tanganyika sardines at Lusaka markets, 1968-1970

This paper reports the results of a retail price survey at Lusaka markets on split, smoked, dried bream (Tilapia spp.) in 1968-69 and dried Lake Tanganyika sardines (Limnothrissa miodon and Stolothrissa tanangicae) in 1969-70. During the survey periods the average prices of these products maintained a level 70% to 130% higher than those fixed by the Government. Price fluctuations in relation to season, size and quality of the products were discussed. The prices were affected by season and size of the products but not by quality in terms of appearance and smell of the non-cooked products.