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At least 523 records · Page 29
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

Fatigue response of notched graphite/epoxy laminates

Tests were performed to determine the damage states in quasi-isotropic graphite/epoxy laminates with center holes caused by cyclic tensile loading. The influence of the stacking sequence on the initiation and interaction of damage modes and the relationship between damage, strength, stiffness, and life of the laminates were also studied. X-ray radiography, moire interferometry, and stiffness change were used to monitor damage. Fatigue damage in both laminates began with matrix cracks around the holes leading to delaminations. In laminates cycled at the same percent of notched tensile strength the stacking sequence influenced the density of the matrix cracks and the modes and distribution of the damage. Ply cracking was also caused by the stacking sequence. The damage states in the two laminates produced stiffness changes of 15 to 20 percent, different rates of change in residual strength, and a factor of two to four difference in fatigue life. It was determined that continued cyclic loading produced matrix cracks which led to fatigue of the laminates.

Kress, G. R.↗
NASA NTRS1986

An analytical investigation of a conceptual design for the station transverse boom rotary joint structure

A study was conducted to define an annular ring, discrete roller assembly concept for the space station transverse boom rotary joint. The concept was analyzed using closed-form and finite element techniques, to size structural members for a range of joint diameters and to determine necessary equivalent stiffnesses for the roller assemblies. Also, a mass study of the system was conducted to determine its practicality, and maximum loads in the joint were identified. To obtain the optimum balance between high stiffness and low structural mass in the design of the rotary joint, it is necessary to maximize the diameter of the annular ring within operational constraints (i.e., shuttle cargo bay size). Further, a rotary joint designed with the largest possible ring diameter will result in minimum operational loads in both the roller assemblies and the transition truss members while also allowing minimum design stiffnesses for the roller assemblies.

Lake, M. S.↗
NASA NTRS1986

Development of computer program NAS3D using Vector processing for geometric nonlinear analysis of structures

An algorithm for vectorized computation of stiffness matrices of an 8 noded isoparametric hexahedron element for geometric nonlinear analysis was developed. This was used in conjunction with the earlier 2-D program GAMNAS to develop the new program NAS3D for geometric nonlinear analysis. A conventional, modified Newton-Raphson process is used for the nonlinear analysis. New schemes for the computation of stiffness and strain energy release rates is presented. The organization the program is explained and some results on four sample problems are given. The study of CPU times showed that savings by a factor of 11 to 13 were achieved when vectorized computation was used for the stiffness instead of the conventional scalar one. Finally, the scheme of inputting data is explained.

Mangalgiri, P. D.↗
NASA NTRS1985

Nonlinear Dynamic Analysis of Deploying Flexible Space Booms

A fundamental investigation of the planar deployment and lock-up of two flexible boom type appendages which have attached tip masses and are connected to a central rigid body through a rotational spring is presented. Nondimensional parameters are identified and it is shown that, in general, the solution depends only on two mass ratios and one nondimensional stiffness parameter. Results are presented for boom tip deflections, deployment time and root moments at lock-up. A threshold value of the nondimensional stiffness parameter is identified beyond which boom deflections become large. Also, a thorough examination of the effect of nonlinear terms in the equations governing the deployment phase is performed. Nonlinear terms in the deployment equations due to kinematics and structural deformation are required to predict more accurately boom deflections, but retention of an inconsistent set of nonlinear terms leads to erroneous results. In particular, retaining nonlinear kinematic terms while neglecting nonlinear structural terms can produce inaccurate results even below the threshold stiffness value.

Mcgowan, P. E.↗
NASA NTRS1986

Evaluation of a hybrid hydrostatic bearing for cryogenic turbopump application

A hybrid hydrostatic bearing was designed to operate in liquid hydrogen at speeds to 80,000 rpm and radial loads to 440 n (100 lbf). The bearing assembly consisted of a pair of 20-mm angular-contact ball bearings encased in a journal, which was in turn supported by a fluid film of liquid hydrogen. The size and operating conditions of the bearing were selected to be compatible with the operating requirements of an advanced technology turbopump. Several test parameters were varied to characterize the bearing's steady-state operation. The rotation of the tester shaft was varied between 0 and 80,000 rpm. Bearing inlet fluid pressure was varied between 2.07 and 4.48 MPa (300 and 650 psia), while the fluid sump pressure was independently varied between 0.34 and 2.07 MPa (50 and 300 psia). The maximum radial load applied to the bearing was 440 N (110 lbf). Measured hybrid-hydrostatic-bearing stiffness was 1.5 times greater than predicted, while the fluid flow rate through the bearing was 35 to 65 percent less than predicted. Under two-phase fluid conditions, the stiffness was even greater and the flow rate was less. The optimal pressure ratio for the bearing should be between 0.2 and 0.55 depending on the balance desired between bearing efficiency and stiffness. Startup and shutdown cyclic tests were conducted to demonstrate the ability of the hybrid-hydrostatic-bearing assembly to survive at least a 300-firing-duty cycle. For a typical cycle, the shaft was accelerated to 50,000 rpm in 1.8 sec. The bearing operated for 337 start-stop cycles without failure.

Spica, P. W.↗
NASA NTRS1986

A comparison of experimental and theoretical results for leakage, pressure gradients, and rotordynamic coefficients for tapered annular gas seal

A brief review of current annular seal theory and a discussion of the predicted effect on stiffness of tapering the seal stator are presented. An outline of Nelson's analytical-computational method for determining rotordynamic coefficients for annular compressible-flow seals is included. Modifications to increase the maximum rotor speed of an existing air-seal test apparatus at Texas A&M University are described. Experimental results, including leakage, entrance-loss coefficients, pressure distributions, and normalized rotordynamic coefficients, are presented for four convergent-tapered, smooth-rotor, smooth-stator seals. A comparison of the test results shows that an inlet-to-exit clearance ratio of 1.5 to 2.0 provides the maximum direct stiffness, a clearance ratio of 2.5 provides the greatest stability, and a clearance ratio of 1.0 provides the least stability. The experimental results are compared to theoretical results from Nelson's analysis with good agreement. Test results for cross-coupled stiffness show less sensitivity of fluid prerotation than predicted.

Elrod, D. A.↗
NASA NTRS1986

Fatigue behavior of continuous-fiber silicon carbide/aluminum composites

Four lay-ups of continuous fiber silicon carbide (SCS2) fiber/aluminum matrix composites were tested to assess fatigue mechanisms including stiffness loss when cycled below their respective fatigue limits. The lay-ups were 0 (sub 8), 0(sub 2)/ + or - 45 (sub 2s), 0/90 (sub 2s), and 0/ + or - 45/90 (subs). The data were compared with predictions from the author's previously published shakedown model which predicts fatigue-induced stiffness loss in metal matrix composites. A fifth lay-up, + or - 45 (sub 2s), was tested to compare shakedown and fatigue limits. The particular batch of silicon-carbide fibers tested in this program had a somewhat lower modulus (340 GPa) than expected and displayed poor bonding to the aluminum matrix. Good agreement was obtained between the stiffness loss model and the test data. The fatigue damage below the fatigue limit was primarily in the form of matrix cracking. The fatigue limit corresponded to the laminate shakedown for the + or - 45 (sub 2s) laminate.

Johnson, W. S.↗
NASA NTRS1986

Fatigue response of notched laminates subjected to tension-compression cyclic loads

The fatigue response of a ((0/45/90/-45)(sub s))(sub 4) T300-5208 graphite-epoxy laminate with a drilled center-hole subjected to various components of tensile and compressive cyclic loads was investigated. Damage evaluation techniques such as stiffness monitoring, penetrant-enhanced X-ray radiography, C-scan, laminate deply and residual strength measurement were used to establish the mechanisms of damage development as well as the effect of such damage on the laminate strength, stiffness and life. Damage modes consisted of transverse matrix cracks, initiating at the hole, in all plies, followed by delamination between plies of different orientation. A characteristic stiffness repsonse during cyclic loading at two load levels was identified and utilized a more reliable indicator of material and residual properties than accumulated cycles. For the load ratios of tension-compression loading, residual tensile strength increased significantly above the virgin strength early in the fatigue life and remained approximately constant to near the end of life. A technique developed for predicting delamination initiation sites along the hole boundary correlated well with experimental evidence.

Bakis, C. E.↗
NASA NTRS1987

Theoretical approach to obtaining dynamic characteristics of noncontacting spiral-grooved seals

The dynamic characteristics of spiral-grooved seals are theoretically obtained by using the Navier-Stokes equation. First, with the inertia term of the fluid considered, the flow and pressure in the steady state are obtained for the directions parallel to and perpendicular to the groove. Next, the dynamic character is obtained by analyzing the steady state and by analyzing the labyrinth seal. As a result, the following conclusions were drawn: (1) As the land width becomes shorter or the helix angle decreases, the cross-coupling stiffness, direct and cross-coupling damping, and add mass coefficients decrease; (2) As the axial Reynolds number increases, the stiffness and damping coefficients increase. But the add mass coefficient is not influenced by the axial Reynolds number; (3) The rotational Reynolds number influences greatly the direct and cross-coupling stiffness and direct damping coefficients; and (4) As the journal rotating frequency increases, the leakage flow decreases. Therefore zero net leakage flow is possible at a particular rotating frequency.

Iwatsubo, Takuzo↗
NASA NTRS1987

SSME seal test program: Test results for smooth, hole-pattern and helically-grooved stators

All of the listed seals were tested in a liquid Halon test facility at high Reynolds numbers. In addition, a helically-grooved-stator seal was tested in an air seal facility. An analysis of the test results with comparisons to theoretical predictions supports the following conclusions: (1) For small seals, the Hirs' friction-factor model is more restricted than had been thought; (2) For smooth seals, predictions of stiffness and damping improve markedly as the radical clearance is reduced; (3) Friction-factor data for hole-pattern-seal stators frequently deviates from the Hirs model; (4) Predictions of stiffness and damping coefficients for hole-pattern-stator seals is generally reasonable; (5) Tests for the hole-pattern stators at reduced clearances show no clear optimum for hole-pattern seals with respect to either hole-area ratio or hole depth to minimum clearance ratios; (6) Tests of these hole-pattern stators show no significant advantage in net damping over smooth seals; (7) Tests of helically-grooved seal stators in Halon show reasonable agreement between theory and prediction for leakage and direct stiffness but poor agreement for the net damping coefficient.

Childs, Dara W.↗
NASA NTRS1987

On dynamic loads in parallel shaft transmissions. 2: Parameter study

Solutions to the governing equations of a spur gear transmission model, developed in NASA TM-100180 (AVSCOM TM-87-C-2), are presented. Factors affecting the dynamic load are identified. It is found that the dynamic load increases with operating speed up to a system natural frequency. At operating speeds beyond the natural frequency the dynamic load decreases dramatically. Also. it is found that the applied load and shaft inertia have little effect on the dynamic load. Damping and friction decrease the dynamic load. Finally, tooth stiffness has a significant effect on dynamic loading; the higher the stiffness, the lower the dynamic loading. Also, the higher the stiffness the higher the rotating speed required for dynamic response.

Lin, Edward Hsiang-Hsi↗
NASA NTRS1988

Numerical modeling of multidimensional flow in seals and bearings used in rotating machinery

The rotordynamic behavior of turbomachinery is critically dependent on fluid dynamic rotor forces developed by various types of seals and bearings. The occurrence of self-excited vibrations often depends on the rotor speed and load. Misalignment and rotor wobbling motion associated with differential clearance were often attributed to stability problems. In general, the rotative character of the flowfield is a complex three dimensional system with secondary flow patterns that significantly alter the average fluid circumferential velocity. A multidimensional, nonorthogonal, body-fitted-grid fluid flow model is presented that describes the fluid dynamic forces and the secondary flow pattern development in seals and bearings. Several numerical experiments were carried out to demonstrate the characteristics of this complex flowfield. Analyses were performed by solving a conservation form of the three dimensional Navier-Stokes equations transformed to those for a rotating observer and using the general-purpose computer code PHOENICS with the assumptions that the rotor orbit is circular and that static eccentricity is zero. These assumptions have enabled a precise steady-state analysis to be used. Fluid injection from ports near the seal or bearing center increased fluid-film direct dynamic stiffness and, in some cases, significantly increased quadrature dynamic stiffness. Injection angle and velocity could be used for active rotordynamic control; for example, injection, when compared with no injection, increased direct dynamic stiffness, which is an important factor for hydrostatic bearings.

Hendricks, R. C.↗
NASA NTRS1986

An experiment for determining the Euler load by direct computation

A direct algorithm is presented for computing the Euler load of a column from experimental data. The method is based on exact inextensional theory for imperfect columns, which predicts two distinct deflected shapes at loads near the Euler load. The bending stiffness of the column appears in the expression for the Euler load along with the column length, therefore the experimental data allows a direct computation of bending stiffness. Experiments on graphite-epoxy columns of rectangular cross-section are reported in the paper. The bending stiffness of each composite column computed from experiment is compared with predictions from laminated plate theory.

Thurston, Gaylen A.↗
NASA NTRS1988

Tension fatigue analysis and life prediction for composite laminates

A tension fatigue life prediction methodology for composite laminates is presented. Tension fatigue tests were conducted on quasi-isotropic and orthotropic glass epoxy, graphite epoxy, and glass/graphite epoxy hybrid laminates. Edge delamination onset data were used to generate plots of strain energy release rate as a function of cycles to delamination onset. These plots were then used along with strain energy release rate analyses of delaminations initiating at matrix cracks to predict local delamination onset. Stiffness loss was measured experimentally to account for the accumulation of matrix cracks and for delamination growth. Fatigue failure was predicted by comparing the increase in global strain resulting from stiffness loss to the decrease in laminate failure strain resulting from delaminations forming at matrix cracks through the laminate thickness. Good agreement between measured and predicted lives indicated that the through-thickness damage accumulation model can accurately describe fatigue failure for laminates where the delamination onset behavior in fatigue is well characterized, and stiffness loss can be monitored in real time to account for damage growth.

Obrien, T. K.↗
NASA NTRS1988

Extension and validation of a method for locating damaged members in large space trusses

The damage location approach employs the control system capabilities for the structure to test the structure and measure the dynamic response. The measurements are then used in a system identification algorithm to produce a model of the damaged structure. The model is compared to one for the undamaged structure to find regions of reduced stiffness which indicate the location of damage. Kabe's 3,4 stiffness matrix adjustment method was the central identification algorithm. The strength of his method is that, with minimal data, it preserves the representation of the physical connectivity of the structure in the resulting model of the damaged truss. However, extensive storage and computational effort were required as a result. Extension of the damage location method to overcome these problems is the first part of the current work. The central system identification algorithm is replaced with the MSMT method of stiffness matrix adjustment which was previously derived by generalizing an optimal-update secant method form quasi-Newton approaches for nonlinear optimization. Validation of the extended damage location method is the second goal.

Smith, Suzanne Weaver↗
NASA NTRS1989

Role of circumferential flow in the stability of fluid-handling machine rotors

The recent studies of the dynamic stiffness properties of fluid lubricated bearing and seals by the authors have yielded most of the generalized characteristics discussed and used in this paper. They include bearing and seal nonlinear fluid film properties associated with stiffness, damping, and fluid average circumferential velocity ratio. Analytical relationships yield the rotor system's dynamic stiffness characteristics. This paper shows the combination of these data to provide the fluid-induced rotor stability equations.

Bently, D. E.↗
NASA NTRS1989

A three-node C deg element for analysis of laminated composite sandwich shells

A three-node flat shell element with C deg rotation fields has been developed for analysis of arbitrary composite shells. The element may consist of any number of orthotropic layers, each layer having different material properties and angular orientation. The formulation includes coupling between bending and extension, which is essential for analysis of unsymmetric laminates. Shearing deflections are included, since laminated and sandwich construction frequently results in shear stiffness much smaller than bending stiffness. Formulation of the element is straightforward, and calculation of its stiffness matrix is simple and fast. Convergence of solutions with mesh refinement is uniform for both thin and thick shells and is insensitive to element shape, although not as rapid as some other elements that lack one or more capabilities of the newly developed element. An experimental verification of the shall element is reported in the appendix.

Martin, C. Wayne↗
NASA NTRS1988

Analysis and test of a space truss foldable hinge

The Mini-Mast is a 20-meter long deployable, three-longeron, truss-beam being used to develop analytical and experimental methods for predicting the physical behavior of large space structures. With 57 corner-body hinges and 54 mid-body hinges, the Mini-Mast is highly joint-dominated, necessitating inclusion of the compliance of the joints in analytical models. This study demonstrates an approach for calculating the stiffness properties of a complicated hinge called the mid-body hinge. The process includes detailed modeling with solid-body modeling software and the use of finite element analysis. Load-deflection tests were conducted to determine the axial stiffness of the mid-body hinge. This is compared to the axial stiffness value determined from a finite element analysis.

Nimmo, N. A.↗