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  • STRUCTURAL MECHANICS  (3)
  • Mechanical impedance  (2)
  • Radial stress  (2)
  • 1985-1989  (7)
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  • 1
    ISSN: 1432-0789
    Keywords: Biopores ; Root growth ; Transpiration ; Simulations ; Mechanical impedance ; Triticum aestivum
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Geosciences , Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: Summary The use of vertical biopores by wheat (Triticum aestivum) seminal roots for easy access to the subsoil and the consequences for plant water supply and yield has been investigated by computer simulation. Parameters included were: biopore density and diameter, depth of cultivation and strength of the subsoil — all under a wide range of seasonal weather conditions. The model predicts that biopores add significantly to root penetration at depth, even at a density of 0.1% v/v of small, vertical pores, while 1.5% to 2.0% v/v can ensure maximum root penetration. When the growing season is shorter a larger number of biopores is needed to ensure timely root penetration to depth. With shallow tillage, biopores occur closer to the soil surface, and their importance is increased. Deeper root penetration invariably gives greater water uptake and transpiration, but may have a negative effect on grain yield, especially under the driest climatic conditions. An increase in early water use may result in less soil water being available during the grain-filling period. The effect of biopores on plant transpiration varies from year to year, depending on the amount of rain and its distribution in time, and on the amount of soil water stored at time of sowing.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Springer
    Plant and soil 94 (1986), S. 59-85 
    ISSN: 1573-5036
    Keywords: Cohesion ; Gossypium hirsutum ; Helianthus annuus ; Penetrometer resistance ; Pisum sativum ; Plastic failure ; Radial stress ; Root diameter ; Root growth pressure ; Soil aggregates ; Tangential stress ; Tensile stress
    Source: Springer Online Journal Archives 1860-2000
    Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: Summary The axial force required for penetration of soil aggregates by roots of pea (Pisum sativum cv. Greenfeast), cotton (Gossypium hirsutum cv. Sicot 3) and sunflower (Helianthus annuus cv. Hysun) seedlings was measured. Effects of aggregate size and strength on root penetration behaviour were investigated. Maximum axial root growth pressure (P x ) was estimated from the maximum axial root growth force (F max) and mean root diameter. F max, time (T max) to attainF max, andP x all increased with increase in size and strength of aggregates. A significant interactive effect of size and strength of aggregate on root diameter was observed.F max,T max and root diameter were significantly different for different plant species. Maximum penetrometer pressure (P′) was compared with the axial pressures generated during root penetration. The penetrometer probe was found to overestimate the root growth pressure by a factor of 1.8 to 3.8.P x /P′ decreased with increase in size and strength of aggregates. A theory was developed to estimate radial and tangential stresses adjacent to the soil-root interface assuming cylindrical deformation by the root in aggregates of finite size. The stresses were calculated using shear cohesion values, estimated from tensile strength measurements, and with an assumed value of soil internal friction. Radial and tangential stresses adjacent to the root axis increased with increase in dimensionless aggregate radius and aggregate strength. Tensile stress adjacent to the root axis is predicted to result in plastic failure of finite sized aggregates during root penetration.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    Springer
    Plant and soil 94 (1986), S. 43-58 
    ISSN: 1573-5036
    Keywords: Aggregate size ; Blunt probe ; Penetrometer pressure ; Plastic failure ; Plastic front ; Probe penetration ; Radial stress ; Root penetration ; Tangential stress
    Source: Springer Online Journal Archives 1860-2000
    Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: Summary Maximum penetrometer pressure was measured on artificial soil aggregates of finite size (2–29 mm) using blunt probes (total cone angle 60°) driven at 3 mm min−1. Maximum penetrometer pressure increased asymptotically with increase in dimensionless aggregate radius,b/a, wherea andb are the probe and aggregate radii, respectively. A theory was developed for penetration of blunt probes into soil aggregates of finite size. The theory assumed that plastic failure occurs out to a radius,R, and that beyond this only elastic straining occurs. This theory can be applied to estimate the radial and tangential stresses adjacent to a blunt probe. The estimated radial and tangential stresses increased with increase in dimensionless aggregate radius,b/a. The radius of the plastic front,R, around the probe is predicted to increase with increased aggregate size. The results also demonstrate the effect of soil shear cohesion and internal friction angle onR. The results are discussed with reference to root penetration.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    Springer
    Plant and soil 99 (1987), S. 211-218 
    ISSN: 1573-5036
    Keywords: Aeration ; Barley ; 3,5-Diiodo-4-hydroxybenzoic acid ; Extensibility ; Mechanical impedance ; Osmotic potential ; Pea ; Root elongation ; Wheat ; Young's modulus
    Source: Springer Online Journal Archives 1860-2000
    Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: Summary This paper reports the results of two series of experiments. In the first the effects of DIHB on the rate of root elongation were compared on unstressed roots and on roots stressed by mechanical impedance and by inadequate levels of aeration. Barley plants were grown in beds of small glass spheres through which nutrient solution was circulated. Mechanical impedance of 25 kPa was applied by subjecting the beds to a confining pressure. Inadequate aeration was obtained by reducing the oxygen concentration in the nutrient solution to 5%. The second series examined possible effects of DIHB on the elastic modulus of root tips of wheat and pea. Elastic modulus gives an indication of the behaviour of roots in structured soil where penetration of peds can be limited by the buckling of root tips. The elastic modulus was measured in experiments of the static cantilever type on roots previously immersed in solutions of polyethylene glycol of different osmotic potential. Elastic modulus measurements can also detect any changes in turgor pressure and wilting characteristics of roots and can therefore help to identify the mechanisms of action of DIHB. DIHB caused increases in root elongation relative to controls in all cases: 26±5.7% in unstressed roots, 14±6.4% in mechanically impeded roots and 54±9.8% in roots growing in 5% oxygen. DIHB had no effect on the elastic modulus, osmotic or turgor pressure of the roots. It is concluded that DIHB acts by modifying the cell wall extensibility factor.
    Type of Medium: Electronic Resource
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  • 5
    Publication Date: 2019-06-28
    Description: The mechanics of fatigue crack growth under constant-amplitudes and variable-amplitude loading were examined. Critical loading histories involving relatively simple overload and overload/underload cycles were studied to provide a basic understanding of the underlying physical processes controlling crack growth. The material used for this study was 7091-T7E69, a powder metallurgy aluminum alloy. Local crack-tip parameters were measured at various times before, during, and after the overloads, these include crack-tip opening loads and displacements, and crack-tip strain fields. The latter were useed, in combination with the materials cyclic and monotonic stress-strain properties, to compute crack-tip residual stresses. The experimental results are also compared with analytical predictions obtained using the FAST-2 computer code. The sensitivity of the analytical model to constant-amplitude fatigue crack growth rate properties and to through-thickness constrain are studied.
    Keywords: STRUCTURAL MECHANICS
    Type: NASA-CR-172597 , NAS 1.26:172597
    Format: application/pdf
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  • 6
    Publication Date: 2019-07-27
    Description: This paper compares experiments with an analytical model of fatigue crack growth under variable amplitude. The stereoimaging technique was used to measure displacements near the tips of fatigue cracks undergoing simple variations in load amplitude-single overloads and overload/underload combinations. Measured displacements were used to compute strains, and stresses were determined from the strains. Local values of crack driving force (Delta-K effective) were determined using both locally measured opening loads and crack tip opening displacements. Experimental results were compared with simulations made for the same load variation conditions using Newman's FAST-2 model. Residual stresses caused by overloads, crack opening loads, and growth retardation periods were compared.
    Keywords: STRUCTURAL MECHANICS
    Type: Effects of load and thermal histories on mechanical behavior of materials; Feb. 25, 26, 1987; Denver, CO; United States
    Format: text
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  • 7
    Publication Date: 2019-07-12
    Description: Ignoring crack growth retardation following overloads can result in overly conservative life predictions in structures subjected to variable amplitude fatigue loading. Crack closure is believed to contribute to the crack growth retardation, although the specific closure mechanism is dabatable. The delay period and corresponding crack growth rate transients following overload and overload/underload cycles were systematically measured as a function of load ratio and overload magnitude. These responses are correlated in terms of the local 'driving force' for crack growth, i.e. the effective stress intensity factor range. Experimental results are compared with the predictions of a Dugdale-type (1960) crack closure model, and improvements in the model are suggested.
    Keywords: STRUCTURAL MECHANICS
    Type: Engineering Fracture Mechanics (ISSN 0013-7944); 33; 6 19
    Format: text
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