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  • 1
    Electronic Resource
    Electronic Resource
    Crystallization of the catalytic subunit of Saccharomyces cerevisiae acetohydroxyacid synthase (2001)
    Copenhagen : International Union of Crystallography (IUCr)
    Acta crystallographica 57 (2001), S. 1321-1323 
    Details
    ISSN: 1399-0047
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Acetohydroxyacid synthase (AHAS; E.C. 4.1.3.18) is the first enzyme in the biosynthetic pathway of the branched-chain amino acids isoleucine, leucine and valine. It is a thiamin diphosphate-dependent enzyme which catalyses the decarboxylation of pyruvate and its condensation with either 2-ketobutyrate or a second molecule of pyruvate to give 2-aceto-2-hydroxybutyrate or 2-acetolactate, respectively. The enzyme is the target of sulfonylurea and imidazolinone herbicides, which act as potent and specific inhibitors. Here, the crystallization and preliminary X-ray diffraction analysis of the catalytic subunit of Saccharomyces cerevisiae AHAS is reported. Data to 2.7 Å resolution have been collected using synchrotron radiation (Advanced Photon Source, Chicago). Crystals have unit-cell parameters a = 95.8, b = 110.0, c = 178.9 Å and belong to the space group P212121. Preliminary analysis indicates there is one dimer located in each asymmetric unit.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0907444901011635
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  • 2
    Electronic Resource
    Electronic Resource
    Momentum and energy processes during jackhammer operation (1989)
    Springer
    Rock mechanics and rock engineering 22 (1989), S. 205-229 
    Details
    ISSN: 1434-453X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Architecture, Civil Engineering, Surveying , Geosciences
    Notes: Summary An analytical model has been developed that describes the motion of the bit of a pneumatic jackhammer and the forces exerted at its tip during penetration of a target. This model assumes that the energy is transmitted in the form of a one-dimensional longitudinal wave without inclusion of the effect of lateral inertia. The piston and the bit can be of arbitrary shape, but are approximated here by uniform stepped sections. A computer program was developed to provide numerical results for the operation of the jackhammer/target system. The analysis includes the pressures that act on both the top and bottom of the piston and additional thrusts that are applied by the operator, an external thrust device, due to the weight of the jackhammer, or as the result of a combination of these effects. The program includes (1) the incident wave for an arbitrarily shaped piston/bit system, (2) the force history at any position of this unit, (3) the force profile along the bit at any instant, (4) the indentation history of the jackhammer bit when the force-indentation relation for the bit/target combination is knowna priori, (5) the rebound velocity of the piston, and (6) the energy transfer from the piston to the target. Numerical examples indicating the results of the calculations are presented and compared with data from corresponding experiments that involved the penetration of an Ingersoll-Rand jackhammer with five different tip shapes into Sierra granite. Both the force histories and indentation histories at the bit tip were measured; correspondence with predicted values was very good. The effects of variations in air pressure and applied thrust as well as the character of the force-indentation relation on the energy transferred to the target are also discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01470987
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  • 3
    Electronic Resource
    Electronic Resource
    A force-indentation model for brittle rocks (1989)
    Springer
    Rock mechanics and rock engineering 22 (1989), S. 127-148 
    Details
    ISSN: 1434-453X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Architecture, Civil Engineering, Surveying , Geosciences
    Notes: Summary A force-deformation model is developed for localized loading of brittle rocks that depicts the action of a percussive drill bit. This model consists of a series of successive cycles each of which is composed of a crushing and a chipping phase. Crushing involves the fragmentation or comminution of individual grains or grain clusters and is manifested by an increase in the applied force, while chipping is associated with the ejection of crushed material accompanied by a reduction in the load. This work represents the foundation for the analysis of the response of a brittle rock to the action of a jackhammer. Based on the hypotheses of a constant resistive pressure exerted by the target and a constant friction coefficient between the bit and target, the reactive target force during crushing is shown to vary linearly for a wedge and quadratically for a conical bit tip as a function of indentation. During chipping, a linear force-identation relation with a negative slope is utilized and the shear force acting on the chip, but not the normal force, is reduced to zero. Thus, the force at the end of chipping is finite. Previous investigators have shown that, when using the Coulomb failure criterion, the curve connecting the points denoting the initiation of chipping in the force-indentation plane is also linear for a wedge indenter and quadratic for a conical bit tip. However, the corresponding force levels are known to have been overestimated. In the present study, the failure strength of brittle rock will be modified both by a Weibull parameter to account for a size effect and a tip shape factor. The Weibull parameters are also employed to define the initiation of chipping. Good correlation has been obtained between the predictions of the model and corresponding experimental results.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01583958
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  • 4
    Electronic Resource
    Electronic Resource
    Investigation of crack formation during loading of brittle rock (1990)
    Springer
    Rock mechanics and rock engineering 23 (1990), S. 53-63 
    Details
    ISSN: 1434-453X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Architecture, Civil Engineering, Surveying , Geosciences
    Notes: Conclusions The response of elastic/brittle targets to loading by conical-, wedge-and hemispherical-indenter shapes representing jackhammer bit tips was analyzed. Theoretical predictions of crack extent and direction are based on values of the maximum principal strains and the associated extension strain criterion in the target assuming a semi-infinite, homogeneous and isotropic solid. Corresponding static experiments using an MTS machine and dynamic percussive tests involving an encased circular target disk of Sierra granite have been conducted. The damage pattern consisting of a crater, a crushed zone, a region of multiple minor cracks as well a some longer fissures has been delineated. Comparison with theoretical predictions indicate that cracks are produced by maximum tensile strain, with their directions predicted well by the present theory. The upper bound predictions of the crack extent are also in reasonable accord. Deviations are due to the presence of inhomogeneities and anisotropies in the tests specimens.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01020422
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  • 5
    Electronic Resource
    Electronic Resource
    A model of a pneumatic jackhammer system (1992)
    Springer
    Rock mechanics and rock engineering 25 (1992), S. 49-61 
    Details
    ISSN: 1434-453X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Architecture, Civil Engineering, Surveying , Geosciences
    Notes: Summary A complete model of a pneumatic jackhammer system has been developed. Application of this model requires that two preliminary experiments be performed. The first experiment produces the relationship between piston impact velocity and the pressures acting on the top and bottom surfaces of the piston, while the second determines the force-indentation behavior for the bit/target system. The model analysis leads to the prediction of the jackhammer efficiency and target response, including target penetration and crack propagation. This model is not dependent on the jackhammer type or size, and may be applied to any such system. A jackhammer system, consisting of an Ingersoll Digger, a 30° conical bit tip, and a Sierra granite target, was used to validate the model. The efficiency, target penetration, and crack extension were measured for this system and found to be in good agreement with predicted results.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01041875
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  • 6
    Electronic Resource
    Electronic Resource
    Performance of a pickax (1986)
    Springer
    Rock mechanics and rock engineering 19 (1986), S. 27-52 
    Details
    ISSN: 1434-453X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Architecture, Civil Engineering, Surveying , Geosciences
    Notes: Summary Single and multiple impact tests were conducted with a commerically available 3.58 kg pickax with a 60° conical tip and a 30° wedge tip in the input energy range from 1.01 to 59.4 J. Five target materials were employed: (1) 2024-0 aluminum, (2) diorite, (3) sandstone, (4) cement-sand, and (5) green shale. Force and penetration data were collected using strain gages and a non-contact displacement probe. It was found that when the impact process involves cracking and chipping, the maximum penetration was linearly proportional to the input energy and the forcepenetration relation exhibited non-linear characteristics. When the impact process does not involve cracking or chipping, the penetration was related to the square root of the input energy and the force-penetration relation exhibited a nearly linear relation. Thus, empirical models involving a linear spring resistive force or a constant resistive force were found to reasonably describe the penetration and input energy data. In addition, an analytical model involving a pickax mass inertia term and a piecewise linearized force-penetration function was constructed that portrayed the force and penetration histories. Good results were obtained for both conicaland wedge-tip cases. From the multiple impact tests it was found that the efficiency of the pickax operation decreases by about 50 to 60 percent after ten impacts, suggesting that the impact point be moved in order to obtain an improved performance.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01043445
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  • 7
    Electronic Resource
    Electronic Resource
    Response of dry- and liquid-filled porous rocks to static and dynamic loading by variously-shaped projectiles (1986)
    Springer
    Rock mechanics and rock engineering 19 (1986), S. 235-260 
    Details
    ISSN: 1434-453X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Architecture, Civil Engineering, Surveying , Geosciences
    Notes: Summary The present investigation was undertaken to study the response of both dry and liquid-filled porous rocks to static and dynamic loading of penetrators with various tip shapes. Eighteen static and forty-eight dynamic tests were conducted involving 6.35 mm diameter indenters featuring flat, hemispherical and 60° conical tip shapes on three different rocks in the energy range from 1.57 to 5.63 J. Force-penetration data were collected by means of strain gages attached to the penetrators. For all three materials, the magnitude of the resistive forces for a given input energy decreased in the following order: (i) dynamic penetration into a dry sample, (ii) dynamic penetration into a fluid-filled specimen, (iii) static loading of the fluid-filled rock, and (iv) static, loading of the dry rock. An unusual observation for the dynamic force-penetration curves was the temporal disparity between the peak force and maximum deformation, which must be attributed to inertial factors. Based on the experimental data, an analytical rigid-body model was constructed that contained both a static and a dynamic component of the resistive force with the objective of quantifying the effect of dynamic loading and the presence of fluid on the response of the rocks. The dynamic component was portrayed as a viscous resistance proportional to the velocity of the penetrator and the contact surface area which is a function of the indentation. It was found that the model provides a good predictive capability for the dynamic force-indentation relations for prescribed materials and tip geometries upon use of but a single constant for the viscosity.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01039997
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  • 8
    Electronic Resource
    Electronic Resource
    Development of texture and texture gradient in Al-Cu-Li (2195) thick plate (2000)
    Springer
    Journal of materials science 35 (2000), S. 3189-3195 
    Details
    ISSN: 1573-4803
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract Texture resulting from micromechanical processing plays an important role in the anisotropy of materials. The determination of texture components is a useful way to present texture data. The types of components present in the texture of a material can be related to other material aspects including predictions of yield loci using polycrystal methods. Al-Cu-Li 2195 thick plate was cold rolled to produce various reductions in thickness. Texture analysis was performed on the various rolled materials at different positions through the thickness of the plates. The texture components are consistent among the various rolled specimens at equivalent thickness positions. A texture gradient is observed to exist through the plate thickness that may indicate other microstructural information about the alloy. This texture gradient evidences the effects of increasing amounts of deformation on texture intensity, texture components observed, and changing modes of deformation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1004838330687
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  • 9
    Electronic Resource
    Electronic Resource
    Energy absorption in polymer crazing (1985)
    New York : Wiley-Blackwell
    Journal of Polymer Science: Polymer Physics Edition 23 (1985), S. 683-693 
    Details
    ISSN: 0098-1273
    Keywords: Physics ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The total energy absorbed by a craze during its development in creep is analyzed and calculated on the basis of a time-dependent theory of crazing. Experimental measurements of the craze length have been utilized in the energy calculations. For polystyrene the initial energy absorption in the craze region is found to be several hundred times that in the uncrazed medium. This ratio decreases sharply in a short period of time to about 50 to 1 and less and remains low afterward. For polycarbonate, somewhat similar behavior has been found. The initial strain energy absorption by crazing is about 200 times that in the uncrazed region. The energy ratio reduces rapidly to about 55 to 1 and tends to level off thereafter. However, in general, the amount of strain energy absorbed does increase as a function of time, as it should.
    Additional Material: 9 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/pol.1985.180230406
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  • 10
    Electronic Resource
    Electronic Resource
    Strength model of adhesive bonded composite pipe joints under tension (1991)
    Stamford, Conn. [u.a.] : Wiley-Blackwell
    Polymer Engineering and Science 31 (1991), S. 533-538 
    Details
    ISSN: 0032-3888
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: A theoretical model is developed to predict the strain of the pipe, coupling, and adhesive under tensile loading of an adhesive bonded joint. The model is found to be within 10 percent of the experimental pipe and coupling strain. Based on the model, several failure modes and their locations are defined and related to the measured data. In this investigation, delamination is the dominating mode of failure. The delamination stress for each test sample is within 7 percent of the average theoretical delamination stress. In addition, the effect of the coupling length, coupling Young's modulus, adhesive shear modulus, and adhesive thickness on the delamination failure are investigated. The model shows that decreasing the modulus of the coupling improves the delamination failure load; however, the coupling strain at the middle of the joint is increased by this variation. Increasing the shear modulus of the adhesive provides the most significant improvement of the joint delamination failure load. Two geometric factors, the joint length and the adhesive thickness also affect the joint failure load. The joint delamination failure load can only be significantly improved by increasing the bonding length up to a certain limit. Increasing the adhesive thickness increases the delamination failure load, however, a large gap between the pipe and coupling may contribute to misalignment during installation which may result in imposed moments under tensile loading. This study can supply the manufacturers with the appropriate design parameters to improve the joint performance significantly under tensile loading.
    Additional Material: 10 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/pen.760310710
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