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Experimental Techniques - 相似文献
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Hamid Nayeb-Hashemi 《Experimental Mechanics》1987,27(1):51-56
Turbine-generator shafts are often subjected to a complex transient torsional loading. Such a torsional loading may initiate
yielding at the outer radius of the shaft or in the fillets. The methods for predicting turbine-shaft fatigue life due to
transient loading depent upon the mode of crack growth from an undetected crack. The most common location for the existence
of a crack is the fillets or shoulders of the shaft.
Specimens were designed from AISI 4340 steel with two diametrically opposed flat surfaces. Initial defect orientations of
0 deg, 45 deg and 90 deg with respect to the sepcimen axis on the fillet were studied. The specimens were subjected to cyclic
torsion with zero mean torque and with a torque amplitude necessary to cause yielding at the outer radius of the specimen.
When initial defects were aligned with a plane of maximum shear stress (0 deg and 90 deg), the cracks propagated along that
plane. For 45-deg defects (aligned to a plane of maximum tensile stress) the crack still propagated along the plane of maximum
shear. However, the number of cycles to initiate and to propagate the crack to failure for 45-deg defects were (two to three
times) larger than those for 0-deg and 90-deg defects.
Mode II and Mode III crack-growth rates were measured from specimens containing 0-deg and 90-deg defects. It was found that
the crack-growth rate in Mode II was higher than in Mode III. However, all the specimens failed due to reduction of the net
cross section, mostly attributed to Mode III crack growth. Similar results were obtained from specimens of turbine-shaft material
(A469 steel), and 2024 aluminum with different rolling directions.
Fatigue-crack-growth rates in Mode III were measured from circumferentially notched bar. They were found to be a unique function
of ΔK
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alternating stress intensity in Mode III. It was found that the mechanism of crack growth is produced by the formation and
linkage of elongated cavities at the crack tip. 相似文献
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The use of a microhardness indentation as a controlled flaw in a flexural specimen for measuring fracture toughness in advanced ceramics is critically evaluated. From a review of fracture-toughness testing methodologies for ceramics it is shown that currently there are not any standardized or widely agreed uponK
Ic test procedures. The problems associated with the controlled flaw test are discussed and analyzed. An experimental program is used to investigate the effects of residual stress on the measurement of fracture toughness in a wide variety of sintered and hot-pressed advanced ceramics including TiB2, Al2O3, SiC, ZrO2, Si3N4. The validity of the test procedure for evaluatingK
Ic of advanced ceramics in general is put forth. 相似文献
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Amin Ajdari Babak Haghpanah Jahromi Jim Papadopoulos Hamid Nayeb-Hashemi Ashkan Vaziri 《International Journal of Solids and Structures》2012,49(11-12):1413-1419
We investigated the mechanical behavior of two-dimensional hierarchical honeycomb structures using analytical, numerical and experimental methods. Hierarchical honeycombs were constructed by replacing every three-edge vertex of a regular hexagonal lattice with a smaller hexagon. Repeating this process builds a fractal-appearing structure. The resulting isotropic in-plane elastic properties (effective elastic modulus and Poisson’s ratio) of this structure are controlled by the dimension ratios for different hierarchical orders. Hierarchical honeycombs of first and second order can be up to 2.0 and 3.5 times stiffer than regular honeycomb at the same mass (i.e., same overall average density). The Poisson’s ratio varies from nearly 1.0 (when planar ‘bulk’ modulus is considerably greater than Young’s modulus, so the structure acts ‘incompressible’ for most loadings) to 0.28, depending on the dimension ratios. The work provides insight into the role of structural organization and hierarchy in regulating the mechanical behavior of materials, and new opportunities for developing low-weight cellular structures with tailorable properties. 相似文献
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Amin Ajdari Hamid Nayeb-Hashemi Ashkan Vaziri 《International Journal of Solids and Structures》2011,48(3-4):506-516
The in-plane dynamic crushing of two dimensional honeycombs with both regular hexagonal and irregular arrangements was investigated using detailed finite element models. The energy absorption of honeycombs made of a linear elastic-perfectly plastic material with constant and functionally graded density were estimated up to large crushing strains. Our numerical simulations showed three distinct crushing modes for honeycombs with a constant relative density: quasi-static, transition and dynamic. Moreover, irregular cellular structures showed to have energy absorption similar to their counterpart regular honeycombs of same relative density and mass. To study the dynamic crushing of functionally graded cellular structures, a density gradient in the direction of crushing was introduced in the computational models by a gradual change of the cell wall thickness. Decreasing the relative density in the direction of crushing was shown to enhance the energy absorption of honeycombs at early stages of crushing. The study provides new insight into the behavior of engineered and biological cellular materials, and could be used to develop novel energy absorbent structures. 相似文献
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