Short Bar Research Articles

Stress concentration factors for axial and shear loading applied to short flat bars with projections

The finite element method has been used to obtain stress concentration factor data for flat ‘T’ shaped components, subjected to axial and shear loading, with both remote and local restraints being applied. The ratio of length to depth (or slenderness ratio) of the shank region is generally less than 3:1. For these short components under shear (or bending) loads, primary bending stresses are no longer dominant and transverse shear is an important secondary effect. Comparisons are made between the predictions with remote and local restraints in order to quantify the effect of localizing the restraints (or load).

Preform design in extrusion by the FEM and its experimental confirmation

An undesirable convex configuration at the front of an axisymmetric product occurs in the extrusion of a short bar, that is required to be removed by machining in order that the front of the product be flat. However, a flat front to a product can be obtained without machining by the use of an appropriate design of preform. Here the preform shape to produce a flat front after extrusion is designed using the rigid-plastic finite-element method and its backward-tracing scheme. Experiments into the extrusion process with the same conditions as those of the numerical simulations have been carried out to confirm the results relating to the numerical preform design, the results showing that the numerically designed preform is satisfactory in achieving the design purpose of a flat front the axisymmetric extrusion processes. The confirmation of the backward-tracing scheme of the finite-element method is valuable and necessary for wide application of the design scheme to preform design in practical forming processes.

Study of fracture and erosive wear of plasma sprayed coatings

Double cantilever beam (DCB) and short bar (SB) specimens were used to determine the critical strain energy release rate (GIC) of plasma-sprayed coatings ofZrO2-base ceramic and WC-Co coatings. Erosion rates were measured for various erosive conditions. By comparing theGIc data and erosion rate (Ev) data with X- ray diffraction and fractographic analysis, fracture and erosion mechanisms of plasma-sprayed coatings were proposed. Based on the erosion models for brittle materials, a proportional relationship between the erosion rate,Ev, andGIC was derived as follows: $$E_v \propto \left( {\frac{1}{H}} \right)^{{1 \mathord{\left/ {\vphantom {1 4}} \right. \kern-\nulldelimiterspace} 4}} \times \left( {G_{IC} } \right)^{ - {2 \mathord{\left/ {\vphantom {2 3}} \right. \kern-\nulldelimiterspace} 3}} $$ (1) whereH is hardness. An increase inGIC leads to a decrease inEv. The exponent value of -2/3 ofGIC was confirmed by the experimental data.

Acetylcholine receptor bars and transmitter release in frog neuromuscular junctions

After labelling with rhodaminated α-bungarotoxin, acetylcholine receptors in cutaneous pectoris muscles of normal adult frogs ( Rana temporaria) appear as brightly fluorescent straight bars, usually extending over the whole gutter. Here we investigated first whether receptor bars can undergo changes and secondly whether they would provide a structural correlate for the strength of a junction. Bars of low fluorescence intensity, as well as short or discontinuous receptor bars consisting of two or three segments, suggest plasticity at the receptor/active zone level. In order to elucidate this notion, receptor bars were studied at different seasons which have previously been shown to be associated with structural changes. In two groups of frogs kept under laboratory conditions simulating wintertime and summertime, respectively, the length and number of receptor bars and the amount of discontinuous bars were investigated. Synaptic contact length, which is the summed length of labelled synaptic branches, and the number and total length of receptor bars did not differ significantly. A clear difference between Group I (“winter” frogs) and Group II (“summer” frogs) was found in the number of discontinuous bars, which was almost twice as high in Group I compared with Group II (6.4 ± 3.3% S.D. vs 3.4 ± 1.3% S.D., n = 8 and 7 muscles, respectively, P < 0.05). In addition, the average length of individual bars was slightly longer in Group I frogs (2.16 ± 0.7 gmm S.D. vs 2.07 ± 0.12 /gmm S.D.,0.1 < P < 0.05). Transmitter release has been shown to be different in these two groups—as determined from endplate potential measurements in tubocurarine-containing bathing solutions—although it was equal when measured in low Ca 2+/high Mg 2+ [Dorlöchter M. et al. (1991) Pflügers Arch. 418, Suppl. 1, R31]. We also investigated whether receptor bars would be a reasonable structural correlate of synaptic function by comparing different measures of transmitter release with different structural parameters in 19 identified junctions. The mean quantal content) m) of a junction was positively correlated with the number and total length of receptor bars, but not with synaptic contact area or length. Amplitudes of the first, maximum, and plateau endplate potentials (corrected for a common resting potential and apparent input resistance) at tetanic nerve stimulation (40 Hz for 2 s) in tubocurarine block were strongly correlated with both synaptic contact length and total receptor bar length ( r = 0.90 for maximum endplate potential); correlations between m and any structural measure were significantly worse. In junctions with a small first endplate potential, facilitation, expressed as the ratio of the maximum/first endplate potential, was higher and the maximum endplate potential was reached later in the train than in those with a large first endplate potential. In contrast, depression was less pronounced or absent in junctions with a small first endplate potential; the ratio of plateau to first endplate potential was larger, and the decline of endplate potential amplitude to below the value of the first endplate potential occurred later in the train or did not occur within 80 impulses. Thus, plateau endplate potentials were more similar among junctions than the first endplate potential; the coefficient of variation was 0.48 for plateau and 0.58 for first endplate potentials. We concluded that fluorescent acetylcholine receptor bars undergo redistribution in the course of environmental changes. Furthermore, their close correlation, especially with the maximum endplate potential in a train, makes them excellent parameters for comparisons of structure and function. The considerable variability in transmitter release among junctions is counterbalanced to some degree by the processes of facilitation and depression.

A materials failure relation of accelerating creep as empirical description of damage accumulation

A general materials failure relation, $$\ddot \Omega = {\rm A}\dot \Omega ^\alpha$$ , describes accelerating creep of materials with rate coefficients α andA, by relating rates of deformation, $$\dot \Omega$$ , to changes in deformation rate, $$\ddot \Omega$$ (Voight, 1988). Time of failure can be extrapolated from inverse rate versus time data, and α andA may be derived to permit one to calculate the failure time. The method is of value for quantitative hazard assessments. Mechanisms leading to damage accumulation during accelerating creep include creep fracture by stress corrosion and power law lattice deformation. These mechanisms are examined here as phenomenologically related to the materials failure relation. Apparently, both mechanisms favour α≅, where α is the parameter of the materials failure relation controlling the sensitivity to accelerating activity. For pure shear governed by power law creep of powerp, under constant load, α=2.0 andA=p. Stress corrosion is widely described by Charles' equation, relating crack velocity to stress intensity during subcritical crack growth by the stress corrosion indexn. The relationship betweenn and α is given by α=(2n−2)/n. The quantitative, predictive qualities of the general materials failure relationship are illustrated with examples from Mount Toc, Italy, and Mount St. Helens, Washington. Six chevron-notched short bar experiments under constant load serve as examples for accelerating creep fracture in the laboratory. The experiments were conducted on silstones of the Ithaca formation, which show a mean stress corrosion index ofn=78±24. Analysis with regard to the materials failure relation results in a mean α=2.0±0.3, which complies with the established relationship betweenn and α.

Measurement of the critical strain energy release rate of plasma-sprayed coatings

Based on the compliance calibration method of linear elastic fracture mechanics, three kinds of specimens, i.e. single-edge-notched single-edge-notched bend specimen, double-cantilever-beam (DCB) specimen and short-bar (SB) specimen, have been used to determine the critical strain energy release rate ( G IC) of plasma-sprayed coatings of ceramic ZrO 2 and cermet WC-Co series. By comparing the results and procedures, it was found that DCB and SB methods or a combination of the two, can properly evaluate the value of fracture toughness, optimize the spraying technique and direct the design in plasma-sprayed coatings.

Structure–property relationships of SiC reinforced advanced Al–Zn–Mg–Cu alloy

he mechanical properties of extruded particulate composites of an enhanced 7000 series alloy have been examined and have been related to a macromodel of composite behaviour. The best tensile properties were found in the T6 condition: the 10 vol.-%SiC composite was found to have 624 MN m−2 proof stress, 690 MN m−2 ultimate tensile strength, 10% ductility (elongation), and 87 GN m−2 Young's modulus. As would be expected, the Young's modulus was found to increase with volume fraction of reinforcement and, at a level of 25 vol.-%SiC, was 112 GN m−2. The tensile modulus of the unreinforced alloy was found to be 73 GN m−2. An explanation for the variation of tensile property behaviour with heat treatment is proposed. The fracture toughness of the composites was determined via the short bar method; toughness values were high, ranging from 34 MN m−3/2 for 10 vol.-%SiC in the underaged temper to 19 MN m−3/2 for 20 vol.-%SiC in the overaged temper. The relationships between properties and microstructure are discussed.MST/1505

Delamination fracture: Effect of fiber orientation on fracture of a continuous fiber composite laminate

Delamination fracture toughness (DFT) of continuous fiber reinforced resin matrix composites is of significant interest because failure by delamination is considered to be the weak link in the fracture process. In this investigation, the effect of fiber orientation on delamination fracture toughness was examined. A modified version of the monolithic short bar test specimen (the hybrid short bar, HSB) was used to determine DFT in relationship to the orientation of the reinforcing fibers. The fracture toughness was determined for a graphite/epoxy composite laminate in which the crack direction-fiber orientation (CD/FO) angle varied from 0 to 90°. The fracture toughness increased as the CD/FO angle increased. The observed increase in fracture toughness as a function of fiber misorientation was attributed to deviant crack tip behavior. Misorientation of the fibers promoted crack tip bifurcation, deflection, and twisting. Such deviant behavior increased mixed mode fracture character locally at the crack tip; and as a consequence, the near field stress intensity at the crack tip was reduced. Since the local crack tip driving force is reduced, the applied load must be increased to advance fracture in the laminate, hence, an apparent increase in the DFT is observed. Results showed that the fracture toughness increased by nearly a factor of 2 as the CD/FO angle increased from 0 to 90°.

Response characteristics of the cells of cortical area 21a of the cat with special reference to orientation specificity.

1. Extracellular recording using tungsten-in-glass microelectrodes was conducted on 115 neurons in area 21a of fifteen anaesthetized cats. Quantitative analysis using computer-controlled display and collecting routines were used to investigate the excitatory and inhibitory regions of the receptive field and to see if interaction, within and between these regions, contributed to the response properties of the cells. 2. The responses of the cells in the sample appeared to arise from a single, homogeneous class. All cells had single discharge regions which responded with composite ON/OFF firing to a stationary flashing bar. The same region also responded to moving light and dark bars and edges. There was little evidence of inhibition as measured by the suppression of spontaneous or induced firing. Most cells had relatively small receptive fields (primary width: mean = 2.1 +/- 0.9 deg (S.D.); n = 108), all were binocular and were located within 15.0 deg of the visual axes. 3. All cells responded well to slowly moving stimuli but generally failed to respond to stimuli moving faster than 10.0 deg s-1. All responses were bi-directional and, although many showed evidence of length summation, there was no sign of linear summation. 4. Despite the absence of significant sideband inhibition many cells were acutely tuned for orientation (half-width at half-height: mean = 15.6 +/- 5.3 deg; n = 48). To investigate this property further, cells were analysed to assess the effect of changing the length of a moving bar stimulus on the acuteness of the orientation tuning curve. Short bars, of similar length to the width of the receptive field, had orientation tuning curves of equivalent sharpness to those obtained with longer bars. 5. The equivalence of orientation tuning for long and short bars stands in contrast to the results obtained for both simple (S) and complex (C) cells of the striate cortex where tuning for the longer bar is sharper than that for the shorter. The result from area 21a cells is consistent with the absence of sideband inhibition and can be related to an input from the striate cortex that passes through a threshold barrier. 6. The orientation tuning of cells of area 21a can be explained if it is assumed that they receive their major input from C or complex cells of the striate cortex in which firing must reach a threshold frequency to activate the recipient cell.

Unidirectional solidification of Al-Si eutectic with the accelerated crucible rotation technique

The accelerated crucible rotation technique (ACRT) is applied to Al-Si eutectic to reveal the effect of forced convection on the unidirectional solidification structure. Experimental results show that if the crucible rotation is started from the beginning of the unidirectional solidification, irregular eutectic with short bars and chunks of silicon will be formed. But if a normal Bridgman process is experienced before the crucible rotation is applied, a more satisfactory directional needle-like silicon structure where the silicon is straight, long, and more regular will be formed during the ACRT process. In order to get a satisfactory unidirectional needle-like silicon structure, ACRT must be used with suitable solidification parameters (temperature gradientG and growth rateR). The rotation method of the crucible is also important.

An engineering tool for fracture toughness testing

A chevron notch four-point bend geometry fracture specimen is presented which complements the chevron notched short rod and short bar geometry used in ASTM E1304, the newest standard for fracture toughness testing. Although the four-point bend samples require slightly more material, sample preparation is less involved. Therefore total testing costs are lower using this four-point bend geometry when testing materials with low to moderate toughness to yield strength ratios. To test the new four-point bend samples, a stand alone toughness tool has been developed which allows a fracture toughness measurement to be made in about 30 seconds. It is shown that the fracture toughness values, measured using this toughness tool, are identical to the values obtained when loading the samples in a servo hydraulic testing system using a four-point bend fixture in displacement control. The fracture toughness values obtained using either loading method agree with fracture toughness values obtained according to ASTM E399.

Formula omitted] Determination using a chevron notched compact tension specimen

The stress intensity factor of a standard compact tension specimen modified with a 37° chevron notch similar to those employed in short rod and short bar specimens has been evaluated using the boundary integral equation method as well as an experimental compliance test method. The minimum normalized stress intensity factor obtained was found to be in agreement within 10%. The plane strain fracture toughnesses, K IC , of three tool steel materials, viz. Assab XW-41, Assab DF2 and Rochling RCC Extra steel, have been determined at various tempering temperatures from as-quenched to 600°C using the modified compact tension specimens and also by the ASTM-E399 standard testing method. At tempering temperatures below 500°C close agreement, generally within less than 10%, has been observed in each case. As the ductility of the materials increased at higher tempering temperature, however, overestimation was observed.

Fracture toughness measurements on SiC/Al2O3 composite

Three test techniques are addressed for the measurements of plane strain critical stress intensity factors,KIC, on monolithic Al2O3 and SiC-whisker/Al2O3 composite: a four-point test on chevron-notched bend bars; a four-point test on single edge-notched bend bars; and a fractometric test on chevron-notched short bars. The tests were performed on 99.80% Al2O3 and 30 vol % SiC whisker-reinforced Al2O3. Bend bar test techniques yielded more realistic stress intensity factors,KIC, on the SiC-whisker/Al2O3 composite than the short-bar test results. Chevron-notched bend-bar tests yielded relatively higher critical stress intensity factors, on both Al2O3 and SiC/Al2O3, possibly due toR-curve effects, suggesting the use of stress intensity factor as a function of crack length instead of using the minimum value. Ambiguous results,KIC, obtained from short-bar tests on SiC/Al2O3 composite, strongly suggests the need to run compliance calibration tests on ceramic composites to determine an appropriateK-factor.

Control of vergence: Gating among disparity inputs by voluntary target selection

The ability of human beings to shift their vergence between multiple disparity stimuli was investigated. The stimulus was a stereogram consisting of a single bar (1 x 0.3 degree) projected in the center of a larger circular pattern (28 degrees dia) of dots (0.3 degree dia). In the initial condition, the subjects perceived a single bar in the center of a cluster of dots, all lying in a single depth plane. The subjects were instructed to fixate either the bar or one of the dots close to the bar. Stepwise changes of target-vergence of either the bar, or of all dots, or of both configurations in opposite directions, were imposed in a random sequence. Under these conditions, ocular vergence was controlled by the target-vergence of the selected target at all times, even if this implied a loss of binocular fusion for the non-selected target. The effect of target selection per se, without concomitant changes in retinal position of the selected target, was studied in two experienced subjects with stabilized image techniques. The subjects viewed a configuration containing short vertical bars placed at either side of a long vertical bar. The short bars on different sides of the long bar had opposite (crossed or uncrossed) disparities. After stabilization of the configuration while the subject fixated the long bar, subjects attempted to fixate the short bars alternatingly. Due to the stabilization the ensuing eye movements did not affect the retinal positions, and thus the disparity, of the bars. Attempted fixation of a parafoveally viewed, short bar induced vergence responses in the appropriate direction.(ABSTRACT TRUNCATED AT 250 WORDS)

A synaptic antigen (B16) is localized in retinal synaptic ribbons

This morphological and biochemical study examines the cytoplasmic synaptic determinant recognized by a monoclonal antibody (B16). This antibody was generated by using an immunosuppression protocol that generates antibodies to relatively rare antigens. The B16 antibody labels structures in the brain that are dot-shaped and in the retina that resemble synaptic ribbons in their location, size, developmental emergence, and biochemical composition. The antigen is apparently conserved across species as it is found in retinas from lizards, frogs, fish, birds, mice, rats, rabbits, cats, and monkeys. This paper focuses on observations in the murine retina. Labeling in the outer plexiform layer of the retina is confined to the margin between the outer plexiform layer (OPL) and the outer nuclear layer. The labeled structure resembles a semiellipse or an arc with the open end facing the OPL and the top facing the outer nuclear layer. Overall, the arc is approximately 1 micron in length and less than 0.5 micron thick. Approximately 10% of the labeled arcs occur in a proximal stratum of the OPL and form a planar cluster that resembles a flat plaque parallel to the OPL. Five to ten arcs are found in each plaque. The arcs found within the plaques are approximately 50% smaller than the larger isolated arcs. Counterstaining with peanut agglutinin (PNA), a lectin that recognizes cone photoreceptors and their associated processes, demonstrates that the plaques are associated with the cone pedicles. Animals that have a higher ratio of cones/rods than mice demonstrate a much higher ratio of plaques/isolated arcs in the OPL. The structure labeled in the inner plexiform layer resembles a short bar (0.8 micron long by less than 0.5 micron wide) that is confined to the inner half of the inner plexiform layer in mice. The relative mobility (Mr) of the B16 antigen obtained from mouse retinal and brain tissue is 88 kD, as determined by SDS-PAGE followed by Western blotting. The mouse 88 kD protein is relatively soluble (precipitates at 70% ammonium sulphate) and elutes at a pH of 7.3 from an isoelectric focusing column. It appears that the determinant recognized by the B16 antibody is a previously undescribed synaptic protein that is associated with the synaptic ribbons in photoreceptor and bipolar terminals of most vertebrate retinas.

Characterization of the interface in the plasma-sprayed HA coating/Ti-6Al-4V implant system.

The successful use of plasma-sprayed hydroxyapatite (HA) coatings on Ti-alloy implants for implant-to-bone fixation requires strong adherence of the ceramic coating to the underlying metal substrate. In this study, the metal-ceramic interface was evaluated using mechanical, chemical, and structural characterization methods. Evaluations of an HA-coated Ti-6Al-4V implant system using a modified short bar technique for interfacial fracture toughness determination revealed relatively low fracture toughness values. Additionally, conventional tensile bond strength testing indicated much lower values than previously reported. Using high resolution electron spectroscopic imaging, evidence of chemical bonding was revealed at the plasma-sprayed HA/Ti-6Al-4V interface, though bonding was primarily due to mechanical interlock at the interface. This study illustrates the benefits of, and the need for, a multilevel approach to evaluate and improve these plasma-sprayed ceramic-metal substrate interfaces.

Stimulus-Dependent Assembly Formation of Oscillatory Responses: I. Synchronization.

Current concepts in neurobiology of vision assume that local object features are represented by distributed neuronal populations in the brain. Such representations can lead to ambiguities if several distinct objects are simultaneously present in the visual field. Temporal characteristics of the neuronal activity have been proposed as a possible solution to this problem and have been found in various cortical areas. In this paper we introduce a delayed nonlinear oscillator to investigate temporal coding in neuronal networks. We show synchronization within two-dimensional layers consisting of oscillatory elements coupled by excitatory delay connections. The observed correlation length is large compared to coupling length. Following the experimental situation, we then demonstrate the response of such layers to two short stimulus bars of varying gap distance. Coherency of stimuli is reflected by the temporal correlation of the responses, which closely resembles the experimental observations.

Orientation discrimination in the cat: Its cortical locus. I. Areas 17 and 18

An elementary unit of visual pattern and form perception is thought to be the orientation of edges; this element has been studied extensively by neurophysiologists using oriented line segments or bars. These same stimuli have been used in the present study to measure threshold discriminations in cats before and after cortical lesions of areas 17 and/or 18. Control experiments showed that the discriminations were made by using a single cue, orientation, and that other stimulus parameters, width, length and contrast of the bar, were optimized. The extent of the lesions was evaluated anatomically from cell and fiber stained sections through cortex and thalamus, matched to retinotopic maps of Tusa et al. (Cortical Sensory Organization, Vol. 2, Humana Press, pp. 1-31, '81) and Sanderson (Journal of Comparative Neurology 143:101-118, '71), and physiologically from visual field position of receptive fields of cells recorded in areas neighboring the lesions. Lesions involving area 17 and large parts of area 18 produced a marked deficit in orientation discrimination which included a loss in retention, and after retraining a substantial increase in thresholds for up to 3 years when tested with long bars. There was no recovery of discrimination when the animals were tested with short bars. Lesions which involved area 17 plus small parts of 18, or lesions of areas 18 and 19, produced no retention deficit and resulted in an increase in thresholds only at low contrast and narrow width. These experiments revealed an excellent correlation between lesion locus and size and behavioral deficit. They indicate that the cortical representation of bar orientation used for discrimination is distributed within and across areas 17 and 18. The spread of the distribution depends on other stimulus parameters such as bar width and length. Furthermore the experiments show that neither the most narrowly tuned cells nor the X-cell system is required for fine orientation discrimination of a long bar.

Axial responses in visual cortical cells: Spatio-temporal mechanisms quantified by Fourier components of cortical tuning curves

The responses of 81 cells from area 17 in paralysed and anaesthetized cats were studied with moving spots and moving bars of different lengths. Tuning curves were measured and plotted as polar-plots. The strongest response of visual cortical cells to a moving bar occurs when the stimulus trajectory crosses the long axis of the receptive field (Hubel and Wiesel 1962). The optimal orientation for a moving and a flashing bar are identical, so that this response-type has been called the orientational component. For a moving spot, however, in most cases the strongest response occurs for motion along the receptive field long axis (axial component). Thus, the axial and orientational components are orthogonal (Wörgötter and Eysel 1989). It is shown that orientational and axial components can display direction selectivity and for short bar stimuli a superposition of the two orthogonal components is demonstrated. Such a superposition in general, resulted in a polar-plot with four peaks 90 degrees apart from each other (four-symmetrical polar-plot). Polar-plots with three or two response peaks were also found; the actual number of response peaks depending on the direction selectivity of the components. In many cells pure axial responses could be elicited with a light spot which stimulates only motion dependent mechanisms. Thus, it was concluded that temporal facilitation is strongly involved in the generation of axial responses. Fourier analysis of polar-plots (SDO-analysis, Wörgötter and Eysel 1987; Wörgötter et al. 1990) was applied to determine the tuning strengths of the different components.(ABSTRACT TRUNCATED AT 250 WORDS)

YANAGIHARA Short Bar Method for Measuring Impact Force.

This paper presents a method for measuring the correct impact force acting on the top of a short bar. In this method, the impact force is calculated from the stress detected by the strain gauge at one point on the short bar. The theory of this measuring method based on the theory of propagation of longitudinal elastic stress wave is very simple. It is confirmed by the impact test that this measuring method is suitable in practical use. This method can be applied to the measurement of force in the range from impact to quasi-static force.