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At least 829 records · Page 46

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NASA NTRS1978

Mechanics of failure of composite materials

Composite materials are both inhomogeneous and anisotropic. Both of these characteristics affect the internal stress distributions since inhomogeneity involves variations in both strength and stiffness. The fracture mechanics of nonuniform materials are considered, taking into account the effect of nonuniformity on stress distributions near the crack tip, predicted yield zones in nonuniform and uniform materials, and the fracture of a center-notched unidirectional specimen. The mechanics of failure of laminated materials is discussed. It is found that the development of damage in a laminate with increasing load and, possibly, increasing numbers of cycles of loading is peculiar to the laminate in question, i.e., the material system, the stacking sequence, and the geometry. Approaches for monitoring damage development are also described.

Reifsnider, K. L.↗

NASA NTRS1978

Titanium/beryllium laminates - Fabrication, mechanical properties, and potential aerospace applications

The paper describes an investigation to assess the fabricability, mechanical properties, and possible aerospace applications of adhesively-bonded titanium/beryllium Tiber laminates. The results of the investigation indicate that structural laminates can be made which have: a modulus of elasticity comparable to steel, fracture strength comparable to the yield strength of titanium, density comparable to aluminum, impact resistance comparable to titanium, and little or no notch sensitivity. These laminates can have stiffness and weight advantages over other materials, including advanced fiber composites, in some aerospace applications where buckling resistance, vibration frequencies, and weight considerations control the design.

Chamis, C. C.↗

NASA NTRS1978

Metallurgical characterization of the fracture of aluminum alloys

A systematic investigation was conducted to examine the fracture behavior of the structural aluminum alloys 2024, 6061, 7075, and 7178 (in selected heat treatments) tested under several controlled conditions. The investigation included both time independent (tensile, shear, and precracked notch-bend) fractures and time dependent (fatigue and stress corrosion) fractures. Specimens were obtained from both sheet and plate material and tested in longitudinal and transverse orientations. Strain rate effects on fracture morphology were examined in tension and shear tests. Fatigue fracture studies included an examination of the influence of minimum-to-maximum-load ratio on fracture morphology. Second-phase particles observed on fracture surfaces and metallographically prepared sections and corrosion products associated with stress corrosion fractures were analyzed chemically using scanning electron microscopy and energy-dispersive X-ray analysis. Fracture morphology was related to the microstructural features, the testing conditions, and the form of commercial product.

Bhandarkar, M. D.↗

NASA NTRS1979

The fracture behavior of graphite/epoxy laminates

The results of uniaxial tensile tests conducted on a variety of graphite/epoxy laminates containing narrow rectangular slits and square or circular holes with various aspect ratios are discussed. The techniques used to study stable-crack or damage-zone growth - namely, birefringence coatings, COD gages, and microscopic observations are discussed. Initial and final-fracture modes are discussed as well as the effect of notch size and shape and laminate type on the fracture process. Characteristic lengths are calculated using the point, average and inherent flaw theories and comparisons with observations are discussed. Further, the effect of flaw geometry on stress states and deformations is assessed.

Yeow, Y. T.↗

NASA NTRS1979

A correlative study between analysis and experiment on the fracture behavior of graphite/epoxy composites

The paper compares the fracture behavior of a composite material by using the analytical models of Waddoups et al. (1971), Whitney and Nuismer (1974, 1975), and Snyder and Cruse (1975) with experimental results from tests performed on center-notched tensile strips. Laminate configurations of (0 deg)8s, (0 deg/90 deg)4s, (+ and -45 deg)4s, and (0 deg/+ and -45 deg/0 deg)2s from T300/934 graphite/epoxy are tested. These particular configurations are used so that the effect of various degrees of anisotropy can be studied. The procedure adopted uses the results from one test for crack size aspect ratio to predict the results of tests of other aspect ratios. For those methods that use a characteristic dimension, predictions are made by assuming the magnitude of this dimension to be constant. The validity of this assumption for a laminate is assessed by comparing predicted and experimental results. Analytical models using a characteristic dimension are compared to the model developed by Cruse (1973).

Yeow, Y. T.↗

NASA NTRS1979

Flexible stator control on the Galileo spacecraft

Galileo is a dual-spin spacecraft designed to deliver a probe to Jupiter and then orbit the planet. The stator, or despun section, contains four flexible modes below 10 Hz and the despun actuator is separated from the inertial sensors by this flexibility. Control loop separation by bandwidth proved unacceptable due to performance requirements. To obtain the desired performance, a control scheme was devised which consists of three parts. First, flexibility damping and control notch filtering are accomplished by phase locked loop techniques. Second, slewing maneuvers are produced by torque profiles which are nonexcitatory to the structure. Finally, a low bandwidth perturbation controller is supplied to remove spacecraft disturbances.

Kopf, E. H.↗

NASA NTRS1980

Predicting lifetime of cast parts

Life expectancy of cast aluminum machine parts can be predicted accurately from fatigue tests at 78 K on notched specimens of aluminum alloy. Method was developed for rocket engine turbopump parts made of high strength, heat treatable alloy with high silicon content; however, technique is applicable to other aluminum casting alloys.

Cooper, R. A.↗

NASA NTRS1979

Characterization of an oxide dispersion strengthened superalloy, MA-6000E, for turbine blade applications

Alloy MA 6000E was developed by the mechanical alloying process for turbine blade applications. The nominal composition of the experimental alloy is Ni-15CR-2Mo-4W-4.5Al- 2.5Ti-2Ta- .15Zr-.05C-.01B-1.1Y2O3. The 1000 hour rupture strength in the longitudinal direction is about 145 MPa at 1093 C and about 483 MPa at 760 C. The alloy displays normal three-stage creep behavior. Typically the creep elongation is 3.5% at 760 C and 2% at 1093 C. The alloy is notch ductile (K sub 1 = 3.5). The rupture properties of the alloy are not significantly degraded by thermal cycling or prior stress isothermal exposure. The alloy also has excellent longitudinal high and low cycle fatigue resistance. Limited testing indicates that MA 6000E posesses good off-axis mechanical properties. The transverse tensile elongation at 760 C is about 3%. The 100 hour transverse rupture strength is 331 MPa at 760 C and about 55 MPa at 1093 C.

Kim, Y. G.↗

NASA NTRS1979

Theoretical study of aerodynamic characteristics of wings having vortex flow

The aerodynamic characteristics of slender wings having separation induced vortex flows are investigated by employing three different computer codes--free vortex sheet, quasi vortex lattice, and suction analogy methods. Their capabilities and limitations are examined, and modifications are discussed. Flat wings of different configurations: arrow, delta, and diamond shapes, as well as cambered delta wings, are studied. The effect of notch ratio on the load distributions and the longitudinal characteristics of a family of arrow and diamond wings is explored. The sectional lift coefficients and the accumulated span loadings are determined for an arrow wing and are seen to be unusual in comparison with the attached flow results. The theoretically predicted results are compared with the existing experimental values.

Reddy, C. S.↗

NASA NTRS1979

Analysis of a unidirectional composite containing broken fibers and matrix damage

An analytical solution is developed for the determination of the stresses and displacements in a unidirectional fiber-reinforced composite containing an arbitrary number of broken fibers as well as longitudinal yielding and splitting of the matrix. The solution is developed using a materials-modeling approach which is based on a shear-lag stress transfer mechanism. The equilibrium equation in the axial direction gives a pair of integral equations which are solved numerically. Excellent agreement is shown to exist between the solution and experimental results for notched unidirectional boron/aluminum laminates without splitting. For brittle matrix composites (i.e. epoxy) equally good results are indicated for both matrix yielding and splitting. For yielding without splitting the fracture strength depends on crack length while for large splitting it is crack length independent.

George, J. G.↗

NASA NTRS1980

SPS pilot signal design and power transponder analysis, volume 2, phase 3

The problem of pilot signal parameter optimization and the related problem of power transponder performance analysis for the Solar Power Satellite reference phase control system are addressed. Signal and interference models were established to enable specifications of the front end filters including both the notch filter and the antenna frequency response. A simulation program package was developed to be included in SOLARSIM to perform tradeoffs of system parameters based on minimizing the phase error for the pilot phase extraction. An analytical model that characterizes the overall power transponder operation was developed. From this model, the effects of different phase noise disturbance sources that contribute to phase variations at the output of the power transponders were studied and quantified. Results indicate that it is feasible to hold the antenna array phase error to less than one degree per power module for the type of disturbances modeled.

Lindsey, W. C.↗

NASA NTRS1980

Development of a laminate fatigue analysis

A fatigue analysis technique developed to predict damage growth in notched laminates is described. Features of the analysis include: criteria to relate matrix failure to cyclic stresses in and between plies; correlation of delamination growth with strain-release rate; and in-plane shear modulus change is related to cyclic shear stresses. A simplified finite element model is used to determine stresses in laminates that contain matrix damage. Failure criteria are integrated with the finite element model to form the fatigue analysis.

Roderick, G. L.↗

NASA NTRS1980

Satellite power system (SPS) magnetron tube assessment study

Taks performed to extend the data base and to define a technology development program for the magnetron directional amplifier for the SPS are reviewed. These include: (1) demonstrating the tracking of phase and amplitude of the microwave output to phase and amplitude references; (2) expanding the range of power over which the directional amplifier will operate; (3)recognizing the importance of amplitude control in overall system design and in simplifying power conditioning; (4) developing a preliminary design for the overall architecture of the power module; (5) demonstrating magnetron starting using the amplitude control system; (6) mathematically modelling and performing a computerized study of the pyrolytic graphite radiating fin; (7) defining the mass of the magnetic circuit for the SPS tube; (8) noise measurement; (9) achieving harmonic suppression by notch reflection filters; (10) estimating the mass of the transmitting antenna; (11) developing a magnetron package with power generation, phase control, and power condition functions; and (12) projecting magnetron package characteristics.

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NASA NTRS1980

Low-temperature solder for joining large cryogenic structures

Three joining methods were considered for use in fabricating cooling coils for the National Transonic Facility. After analysis and preliminary testing, soldering was chosen as the cooling coil joining technique over mechanical force fit and brazing techniques. Charpy V-Notch tests, cyclic thermal tests (ambient to 77.8 K) and tensile tests at cryogenic temperatures were performed on solder joints to evaluate their structural integrity. It was determined that low temperature solder can be used to ensure good fin-to-tube contact for cooling-coil applications.

Buckley, J. D.↗

NASA NTRS1979

Frequency effect on the fatigue life of a laminated composite

Effects of load frequency on the fatigue behavior of notched (+ or - 45 deg)2s laminated graphite/epoxy composites are investigated. Experimental results are obtained using four frequencies at three load levels. Temperature rises near the center hole are also measured. It is found that when temperature increase during the fatigue test is small, fatigue life increases as the load frequency increases. However, an increase in the near hole temperature results in a reduction in fatigue life. A theoretical model based upon crack propagation in viscoelastic media is presented. The model is modified to account for the temperature effect.

Sun, C. T.↗

NASA NTRS1980

Analysis of a unidirectional composite containing broken fibers and matrix damage

An analytical solution is developed for the determination of the stresses and displacements in a unidirectional fiber-reinforced composite containing an arbitrary number of broken fibers as well as longitudinal yielding and splitting of the matrix. The solution is developed using a 'materials-modeling' approach which is based on a shear-lag stress transfer mechanism. The equilibrium equation in the axial direction gives a pair of integral equations which are solved numerically. Excellent agreement is shown to exist between the solution and experimental results for notched unidirectional boron/aluminum laminates without splitting. For brittle matrix composites (i.e., epoxy) equally good results are indicated for both matrix yielding and splitting. For yielding without splitting the fracture strength is found to depend on crack length while for large splitting it is crack length independent.

Goree, J. G.↗

NASA NTRS1980

Beat by beat stroke volume assessment by PDE in upright and supine exercise

A 3.0 MHz pulse Doppler echocardiograph was used to estimate instantaneous stroke volume and cardiac output in 8 men during steady state supine and upright exercise at 300 kpm/min which were compared with other studies utilizing invasive procedures. The mean transients in heart rate and stroke volume and cardiac output for the first 20 sec of exercise in each posture were then determined. Centerline blood velocities were obtained in the ascending aorta with the transducer positioned manually in the suprasternal notch. Mean supine values for stroke volume and cardiac output at rest and exercise were 111 (6.4) and 112 ml (9.7 L/min), respectively, for supine. The corresponding results for upright were 76 (5.6) and 92 ml (8.4 L/min). These values compare favorably with prior studies. The transient response of cardiac output following the onset of upright was about twice as fast as in S because of the rapid and almost immediate upsurge in stroke volume. In supine, only heart rate served to augment cardiac output as stroke volume initially fell. The faster initial aortic flow in upright must represent the rapid mobilization of pooled venous blood from the leg veins which more than accounts for the additional volume (184 ml) of blood passing through the aorta during upright compared with supine in the first 20 sec.

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NASA NTRS1980

Changes in cardiac output and tibial artery flow during and after progressive LBNP

A 3.0 MHz Pulsed Doppler velocity meter (PD) was used to determine blood velocities in the ascending aorta from the suprasternal notch before, during and after progressive 5 min stages of lower body negative pressure (LBNP) in 7 subjects. Changes in stroke volume were calculated from the systolic velocity integrals. A unique 20 MHz PD was used to estimate bloodflow in the posterior tibial artery. With -20 torr mean stroke volume fell 11% and then continued to decline by 48% before LBNP was terminated. Mean tibial flow fell progressively with LBNP stress, due to an increase in reverse flow component and a reduction in peak forward flow and diameter. Stroke volume increased and heart rate fell dramatically during the first 15 sec of recovery. The LBNP was terminated early in 2 subjects because of vasovagal symptons (V). During V the stroke volume rose 86% which more than compensated for the drop in heart rate. This implies that V is accompanied by a paradoxical increase in venous return and that the reduction in HR is the primary cardiovascular event. During the first 15 sec of recovery these 2 subjects had a distinctive marked rise to heart rate reminiscent of the Bainbridge reflex.

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