Aerospace Engineering

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Ultrasonic nondestructive evaluation techniques and the effects of fiber architecture on mechanical performance in multi-directionally reinforced textile composites

1995 , Hale, Richard , Daniel O. Adams , David K. Hsu , Aerospace Engineering

Constant load stress rupture tests were performed on alloy 908. The test matrix used varied oxygen concentration, applied load, temperature, and percent cold work. The effect of modifying surface residual stresses on intergranular oxidation and cracking was examined using constant strain C-ring tests;The mechanism for high temperature intergranular fracture in alloy 908 is stress assisted intergranular oxidation cracking. A direct correlation between percent intergranular fracture and oxygen concentration was observed. This result was comparable to the oxidation assisted, intergranular fracture behavior of alloy 718. Internal oxidation, for temperatures at or near 650∘ C and under an applied tensile stress, manifests itself as intergranular oxidation in alloy 908. Intergranular oxidation penetrates to the same depth as predicted for internal oxidation. The concentration of Cr incorporated into the intergranular oxide increases with increasing test duration. This result is consistent with internal oxidation theory. The depth of intergranular oxidation is controlled by both the oxygen partial pressure and the Cr concentration of the alloy. These factors influence the growth of intergranular Cr2O3 and NiCr2O4 precipitates that serve to block the diffusion path for oxygen. Analogous to internal (matrix) oxidation, a transition from intergranular to external oxidation in alloy 908 occurs when the partial pressure of oxygen at the surface is below 7.6x10-4 Torr;The activation energy for intergranular oxidation in air was determined to be 202 kJ/mole for this alloy. At low (<195 ppm) oxygen concentrations, the activation energy for intergranular oxidation increased to 500 kJ/mole. This higher value reflects the oxygen concentration dependence observed for intergranular oxidation. The dependence on alloy Cr concentration is similar to that observed for intergranular stress corrosion cracking in aqueous environments;An oxygen concentration threshold based on zero percent intergranular fracture is a better indicator of the potential for intergranular fracture during heat treatment than one based on time to rupture. An oxygen partial pressure below 1x10-4 Torr is recommended for heat treatment of alloy 908 under residual or applied stresses. Shot peening appears to be relatively simple solution to the stress accelerated grain boundary oxidation problem. (Copies available exclusively from MIT Libraries, Rm. 14-0551, Cambridge, MA 02139-4307. Ph. 617-253-5668; Fax 617-253-1690.)

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Flexible composite propeller design using constrained optimization techniques

1997 , Khan, Abdul , Daniel O. Adams , Vinay Dayal , Aerospace Engineering

An investigation of a conventional propeller, made from composite materials, was conducted in which its characteristics were studied under quasi-static aerodynamic loading. Also, optimized designing of a composite propeller was performed for various constrained and unconstrained design objectives. Only symmetric ply stacking sequences were considered to eliminate the effect of centrifugal force on the propeller. Results show that the ply stacking sequence has an effect on the propeller characteristics of a conventional propeller. Proper stacking sequence of the composite propeller improves its performance as compared to its metallic counterpart. An improvement of about 47% in the propeller thrust coefficient was observed for one of the cases. Similarly, improvement was observed in other propeller characteristics as well. The classical blade element theory of propellers is used to calculate propeller characteristics and the aerodynamic force distribution on the propeller blades. The finite element method is used to calculate the resulting deformation of the propeller blades. In the present study, the propeller is modeled as a variable thickness plate, discretized into a number of quadrilateral shear-deformable finite elements. Propeller characteristics are calculated for ply orientation angles ranging from -90 degrees to +90 degrees to study these parameters as a function of orientation angle. These analyses were performed for six different values of advance ratios and at two different initial blade setting angles. Improvement in propeller design, by only changing the stacking sequence, was also considered using techniques of numerical optimization. It is shown that different design objectives can be achieved by changing the stacking sequence. For carrying out design optimization of propeller, Fortran subroutine "CONMIN" was used.

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The effect of stiffener parameters on the farfield sound radiation from composite beams: Experiment and theory

1997 , Slaughter, Julie , J. Adin Mann, III , Daniel O. Adams , Aerospace Engineering

The effects of spatial and material variations on farfield sound radiation from carbon/epoxy composite beams were investigated. Specifically, the geometry of a single stiffener of carbon/epoxy embedded in a beam, periodically spaced stiffeners along the length of a beam, and single stiffeners of different materials embedded in beams were all investigated. Because of a lack of methods to sufficiently model the details of spatially varying material properties, experimental data were used to identify important parameters that needed to be included in a model. Vibration testing was performed for three frequency ranges: 500 Hz to 1500 Hz, 1500 Hz to 2500 Hz, and 2500 Hz to 3500 Hz. The geometry of a single stiffener of carbon/epoxy affected the region of the stiffener that radiated sound. Periodically spaced stiffeners tended to have a global stiffening effect at lower frequencies and to act like individual stiffeners for frequencies where the wavelength is shorter than the stiffener. A stiffener of viscoelastic material reduced the sound power radiated from the region of the stiffener, and stiffeners of other materials radiated sound from the ends of the stiffener. Vibration experiments were performed on fiber-reinforced composite beams to verify a cubic spline based solution of a simple Euler-Bernoulli beam model. The model seemed to accurately predict the shape and location of farfield sound radiation from stiffeners. However, limitations on the available boundary conditions prevented accurate prediction of the magnitude of sound radiation. The model was then used to predict sound radiation from beams with varying stiffener bond lengths and these results were experimentally verified. In general a longer stiffener bond length causes less sound radiation. However, there seems to be a critical length at which the sound radiation reaches a maximum.