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  • 1
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    Measurement and analysis of critical crack tip processes during fatigue crack growth (1985)
    In:  CASI
    Details
    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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  • 2
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    Fatigue crack growth with single overload - Measurement and modeling (1987)
    In:  Other Sources
    Details
    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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  • 3
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    Buckling of conical shell with local imperfections (1974)
    In:  CASI
    Details
    Publication Date: 2019-06-27
    Description: Small geometric imperfections in thin-walled shell structures can cause large reductions in buckling strength. Most imperfections found in structures are neither axisymmetric nor have the shape of buckling modes but rather occur locally. This report presents the results of a study of the effect of local imperfections on the critical buckling load of a specific axially compressed thin-walled conical shell. The buckling calculations were performed by using a two-dimensional shell analysis program referred to as the STAGS (Structural Analysis of General Shells) computer code, which has no axisymmetry restrictions. Results show that the buckling load found from a bifurcation buckling analysis is highly dependent on the circumferential arc length of the imperfection type studied. As the circumferential arc length of the imperfection is increased, a reduction of up to 50 percent of the critical load of the perfect shell can occur. The buckling load of the cone with an axisymmetric imperfections is nearly equal to the buckling load of imperfections which extended 60 deg or more around the circumference, but would give a highly conservative estimate of the buckling load of a shell with an imperfection of a more local nature.
    Keywords: STRUCTURAL MECHANICS
    Type: NASA-TM-X-2991 , L-9331
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  • 4
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    Automated preliminary design of simplified wing structures to satisfy strength and flutter requirements (1972)
    In:  CASI
    Details
    Publication Date: 2019-06-27
    Description: A simple structural model of an aircraft wing is used to show the effects of strength (stress) and flutter requirements on the design of minimum-weight aircraft-wing structures. The wing is idealized as an isotropic sandwich plate with a variable cover thickness distribution and a variable depth between covers. Plate theory is used for the structural analysis, and piston theory is used for the unsteady aerodynamics in the flutter analysis. Mathematical programming techniques are used to find the minimum-weight cover thickness distribution which satisfies flutter, strength, and minimum-gage constraints. The method of solution, some sample results, and the computer program used to obtain these results are presented. The results indicate that the cover thickness distribution obtained when designing for the strength requirement alone may be quite different from the cover thickness distribution obtained when designing for either the flutter requirement alone or for both the strength and flutter requirements concurrently. This conclusion emphasizes the need for designing for both flutter and strength from the outset.
    Keywords: STRUCTURAL MECHANICS
    Type: NASA-TN-D-6534 , L-7876
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  • 5
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    Analytical investigation of an inflatable landing system having omnidirectional and multiple impact capabilities (1969)
    In:  CASI
    Details
    Publication Date: 2019-06-27
    Description: Inflatable landing system investigated for impact normal to smooth surface
    Keywords: STRUCTURAL MECHANICS
    Type: NASA-TN-D-5236
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  • 6
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    Modelling and measurement of crack closure and crack growth following overloads and underloads (1989)
    In:  Other Sources
    Details
    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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  • 7
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    Reinforcement of metals with advanced filamentary composites (1974)
    In:  Other Sources
    Details
    Publication Date: 2019-07-13
    Description: This paper reviews some recent applications of the concept of reinforcing metal structures with advanced filamentary composites, and presents some results of an experimental investigation of the tensile behavior of aluminum and titanium reinforced with unidirectional boron/epoxy. Results are given for tubular and flat specimens, bonded at either room temperature or elevated temperature. The composite reinforced metals showed increased stiffness over the all-metal counterpart, as predicted by the rule of mixtures, and the results were independent of specimen geometry. The tensile strength of the born/epoxy reinforced metals is shown to be a function of the geometry of the test specimen and the method of bonding the composite to the metal.
    Keywords: STRUCTURAL MECHANICS
    Type: Conference on Composite materials: Testing and design; Mar 21, 1973 - Mar 22, 1973; Williamsburg, VA
    Format: text
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