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(CATEGORY) RESPONSE-TO-NOISE STUDIES OF SOME AIRCRArm AND SPACECRAFT STRUCTURES - - By Philip M. Edge, Jr., and Carl E. Rucker NASA Langley Research Center Langley Station, Hanrpton, Va.
Presented at the Second International Conference on Acoustical Fatigue GPO PRICE $ OTS PRICE(S) $ Hard copy (HC)
Microfiche (M F) &-
Dayton, Ohio
April 29 - Fay 1 , 1964
RESPONSE-TO-NOISE STUDIES OF SOME AIRCRAFT AND SPACECRAFT STRUCTURES .
\ By P h i l i p M. Edge, Jr.,* and Carl E. h c k e r * NASA Langley Research Center INTRODUCTION ,- Thelresponse of a i r c r a f t o r spacecraft structures)to complex L o i s e inputs involves such important variables as btrucbural nlaterials, fapric-9tion tech- environmentalAandit-ions. A t t h e Langley Research Center niaues, and r e l a t e d w e have a research e f f o r t underway which ranges i n scope from t h e study o f b o i s e responses of simple panels t o acoustic environmental studies of l a r g e and corn- plex space p a y l o a d d The purpose of t h i s paper i s t o present a s t a t u s report on some of these current research studies.
The types of studies t o be covered a r e indicated by the sketches i n t h e The first p a r t of t h e paper w i l l deal with &ne1 studies, and first figure.
then b r i e f discussions w i l l be given of studies involving s h e l l s t r u c t u r e s and f u l l - s c a l e payloads. The sketch i n t h e upper l e f t i l l u s t r a t e s simple aluminum panels which a r e being studied t o determine some e f f e c t s of varying t h e panel curvature on panel dynamic response. The sketch at t h e center represents a b i s c o - e l a s t i c paneq on which fatigue studies are being made at elevated tempera- t u r e s . O n t h e r i g h t i s i l l u s t r a t e d a corrugation-stiffened panel incorporating a thermal expansion j o i n t . The s h e l l structure i l l u s t r a t e d represents a 1/5-scale Saturn I ttlox" tank. The studies t o be described a r e p a r t of a corn- prehensive program t o evaluate t h e use of models i n support of s t r u c t u r a l l y scaled dynamic s t u d i e s of t h e complete Saturn vehicle. Finally, a description w i l l be given of a f u l l - s c a l e environmental t e s t on a complete Agena instrument package as represented by t h e sketch i n t h e lower r i g h t of t h e figure. For such complex s t r u c t u r e s as these, f u l l - s c a l e proof t e s t i n g i n a r e a l i s t i c environment i s t h e only s a t i s f a c t o r y approach currently available.
PANEL FATIGUE STUDIES Studies of response of panels t o noise have been conducted both i n house and under contract i n order t o increase our knowledge of t h e dynamic behavior of some of t h e configurations being considered f o r application t o high-performance f l i g h t vehicles.
Simple Curved Panels Studies involving t h e e f f e c t s of curvature were made f o r simple aluminum- a l l o y panels, as i l l u s t r a t e d i n figure 2. The panels were 20 inches by
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Aerospace Engineer.
L-3937 20 inches and of 0.020-inch-thick, 2024-T3 clad aluminum. They were attached t o steel t e s t f i x t u r e s with 3/16-inch-diameter a i r c r a f t b o l t s w i t h washers, and were torqued t o 25 inch-pounds. Distance from t h e center l i n e of t h e b o l t hole With t h i s t o t h e panel edge was 5/8 inch, and b o l t spacing w a s 1- inches.
method of attachment, t h e free o r e f f e c t i v e s i z e of t h e panel w a s approximately 17.5 inches by 17.5 inches. Panels were instrumented with s t r a i n gages and, f o r random noise tests, were oriented r e l a t i v e t o t h e a i r j e t noise source as indicated i n t h e figure. Tests have been made with radii of curvature of 4 f e e t , 8 feet, and i n f i n i t y ( f l a t panel). Response c h a r a c t e r i s t i c s of t h e panels at these curvatures were determined by means of a d i s c r e t e frequency noise source which could be operated i n such a manner as t o sweep through a range of frequencies.
Shown i n f i g u r e 3 are t h e r e s u l t i n g root-mean-square bending s t r a i n responses p l o t t e d as a function of frequency f o r each of t h e t h r e e panel curva- tures. The driving noise (normal impingement) w a s held at a constant 125 dB l e v e l while sweeping through t h e range of frequency. The c h a r a c t e r i s t i c f l a t panel response i s i l l u s t r a t e d i n t h e bottom p a r t of t h e figure. For t h e input noise l e v e l of t h e experiment, it can be seen t h a t nonlinear e f f e c t s are evi- dent. A s t i f f e n i n g o r hardening i s seen t o occur i n t h e form of a r i s e i n t h e response frequency as s t r a i n amplitude increases.
I n t h e case of t h e 4-foot-radius curved panel a markedly d i f f e r e n t response p a t t e r n is obtained, as shown i n t h e t o p of t h e figure. The s t r a i n l e v e l s a r e lower, there i s r e l a t i v e l y more response a t t h e higher frequencies, and there i s a trend toward a softening; t h a t is, t h e response frequency tends t o decrease with increasing s t r a i n amplitude. The 8-foot-radius panel e x h i b i t s response c h a r a c t e r i s t i c s which are intermediate between those of t h e others and, a t t h i s p a r t i c u l a r input l e v e l , behaves e s s e n t i a l l y as a l i n e a r system.
Random noise t e s t s i n which a number of these panels were t e s t e d t o failure were made i n Langley's air j e t f a c i l i t y , which i s shown i n the photograph of
f i g u r e 4. The noise i s t h e mixing noise of t h e air jet, and t h e view of t h e
figure i s looking upstream i n t o t h e j e t e x i t . Ahead of t h e e x i t are four 90' pipe bends which a r e useful f o r increasing t h e noise l e v e l and f o r s h i f t i n g t h e noise energy i n t h e low-frequency d i r e c t i o n . Panel specimens a r e located par- a l l e l t o t h e j e t boundary i n each of f o u r positions around t h e j e t exhaust.
Overall sound pressure l e v e l s of up t o 160 dB are obtained i n t h i s manner. Some of t h e r e s u l t s obtained with t h i s shown with t h e aid of t h e b a r graph setup are of figure 5 , where t h e range of times t o failure f o r a number of panels a t each It i s seen t h a t an increase i n t h e t i m e t o failure i s curvature a r e indicated.
associated with increased curvature. The f a i l u r e s obtained were skin cracks located along t h e edge of t h e washers (see f i g . 2 ) , and s t r a i n measurements were obtained near these failure locations. The curved panel s t r a i n data show a marked reduction i n l e v e l compared with those f o r the flat panels. The f a c t t h a t t h e curved panels d i d not e x h i b i t much longer times t o f a i l u r e suggests t h a t s i g n i f i c a n t l y d i f f e r e n t stress concentratkon f a c t o r s may e x i s t .
Figure 6 shows mean-square spectra of sound input and s t r a i n response f o r f l a t and 4-foot-radius curved panels.
Sound pressure w a s measured at the reference point i l l u s t r a t e d i n the figure. Mean-square sound pressures per cps (estimated from measurements using a 25-cps bandwidth f i l t e r ) are p l o t t e d versus frequency i n t h e upper graph of t h e figure. The above analysis shows energy of t h e input t o be concentrated l a r g e l y at 100 cps. A spectrum o f s t r a i n responses (estimated from measurements using a 25-cps bandwidth f i l t e r ) i s shown i n the lower graph i n t h e figure. Mean-square s t r a i n i n microinches p e r inch per cps i s p l o t t e d versus frequency f o r a f l a t panel and curved panel. The curves represent t h e sum of t h e edge bending and membrane s t r a i n s a t t h e gage location shown, which i s t h e same as f o r t h e comparable d i s c r e t e frequency data of f i g - ure 3. I n both cases panel response w a s sharply tuned near 100 cps where t h e input spectrum peaked; however, t h e panel with g r e a t e r curvature has a g r e a t e r response at t h e higher frequencies. The f l a t panel ( s o l i d l i n e ) bending s t r a i n peaked a t 100 cps, and t h e membrane s t r a i n peaked at twice the bending f r e - quency. For t h e 4-foot radius of curvature panel the membrane response w a s low and lacked prominent peaks; thus t h e response spectrum of f i g u r e 6 i s essen- t i a l l y the bending s t r a i n responses. A prominent peak i s s t i l l shown between 100 and I25 cps.
Visco-Elastic Panels Studies t o determine some e f f e c t s of combined elevated-temperature-intense- noise environments on fatigue of commercially available visco-elastic panels are underway under a contract with North American Aviation of Columbus, Ohio. Con- f i g u r a t i o n s f o r which some data have been obtained are i l l u s t r a t e d schematically i n figure 7. The panels a r e 24 inches square with two hat-shaped s t r i n g e r s riveted across t h e back side. These stringers were located 5 inches from t h e The center l i n e , thereby forming a 10-inch by 24-inch panel.bay i n t h e center.
v i s c o - e l a s t i c panels were made up of two aluminum facing sheets of 0.016-inch
thickness with a 0.017-inch thickness o f visco-elastic material ( t r a d e name -
A control panel of Dyna-Damp) bonded between t h e sheets as i l l u s t r a t e d .
0.051-inch-thick aluminum was constructed and t e s t e d f o r use as a reference f o r comparing t h e r e s u l t s obtained with t h e visco-elastic panels. All aluminum used i n these tests w a s 2024-T3 clad. The edges of t h e t e s t panels were riveted t o 1--inch channel frames which i n t u r n were attached t o t h e mounting frame of t h e acoustic t e s t chamber. The tests were made i n t h e grazing incidence test cham- ber of North American's d i s c r e t e frequency s i r e n f a c i l i t y . I n t h e w a l l of t h i s chamber t h e panels were mounted w i t h the s t i f f e n e r s running v e r t i c a l l y on t h e panel surface away from t h e noise ( f i g . 7). I n t h e o3pcsite w a l l of t h e t e s t chamber w a s a series of quartz tube heat lamps f o r heating t h e panels. The t e s t i n g technique used was t o sweep through a frequency range i n t h e operation of t h e s i r e n at t h e sound pressure l e v e l of t h e t e s t . From t h i s frequency sweep t h e fundamental resonant frequency of t h e panel center bay w a s determined from Three con- strain-gage measurements, and t h e tests were run a t t h i s frequency.
t r o l panel specimens and t h r e e visco-elastic panel specimens were t e s t e d a t sound pressure l e v e l s of 148, 154, and 160 dB, both a t room temperature and at 200° F.
Some results from these t e s t s are shown i n figure 8. Shown i n t h i s figure are t h e cycles t o f a i l u r e as a function of noise l e v e l i n decibels. Failure w a s judged to have occurred when skin cracks were first visible.
These cracks gen- erally were detected near the rivet attachments of the center bay of the tes% specimens. The plot on the left shows results obtained at room temperature, and the plot on the right shows the results obtained at 200° F. The data points shown are the average of the data obtained on the three panels tested under each condition. These data show that the visco-elastic panels generally endure a larger number of cycles before failure is detected. This margin of improvement f o r the 2 0 0 ' F conditions.
is greater for the room-temperature conditions than This result suggests that the visco-elastic material may be losing some of its effectiveness at the elevated temperature condition.
Corrugation-Stiffened Panels The last panel study to be described is concerned with a structure designed for application as a hot structure for lifting reentry vehicles. The acoustic study was part of a Langley project to design and test such a structure under a variety of environmental conditions associated with reentry loads and elevated temperatures (see ref. 1 ) . This acoustic study was made in the random noise environment of the Langley air jet facility.
The structure studied is illustrated in figure 9 . A s shown, the test panel structure is 12 inches wide by 48 inches long and forms the top surface of an 8-inch-deep box.
The part of the structure which was studied in detail was an expansion joint which is illustrated by the inserted sketch. The material used was light-gage Inconel X, fabricated by a welding-riveting process. The top skin was bonded and stiffened by a corrugated under skin, welded to form a com- posite panel. Of special interest in these acoustic tests are the panel skin terminations at the expansion joint. The outer skin was terminated by a row of rivets at the expansion joint attachment. The inner skin was terminated by a Z-member which was spot welded to each of the'corrugation flats. These spot welds were found to fail during acoustic fatigue tests, and outer skin surface cracks developed along the attachment rivet line. The effects of noise level and edge attachment detail on the growth of the outer skin crack are indicated in figure 10.
Shown in this figure are plots of crack length as a function of time in minutes. Data on the left are for sound pressure levels of 163 dB and 156 dB, and for the condition of an initial failure of the undersurface welds. It can be seen that the crack growth is markedly faster for the higher noise level condition. The experiment was repeated for the condition of a riveted instead of a welded Z-attachment of the internal structure in the region of initial failure. A s a result of using rivets in this internal structure, the time to failure of the outer skin and the associated rate of crack growth were markedly reduced, as can be seen in the right-hand figure. A point to be made from these data is that the structural integrity of interior attachment points of this type of built-up panel construction can be very important in the time to failure of skin surfaces and also in the crack growth problem.
It should also be mentioned that, although skin surfaces may be readily inspected, the inaccessible under- structures may be an important inspection problem relating to the fatigue of the skin surfaces.
TANK VIBRATIONS A comprehensive series of studies is underway at Langley to determine the feasibility of using dynamic models of launch vehicles to predict the vibration characteristics of full-scale vehicles. A s part of this general study, a 1/5-scale model of one of the Saturn I "lox" tanks is being used for some spe- cial noise and vibration studies. One of the setups in use is illustrated in figure 11. Shown is an echo-free room equipped with an air jet noise generator and instrumentation for making accurate noise surveys under the controlled lab- oratory conditions. The fuel tank, which is 11 feet long by 14 inches in diam- eter, was supported at the base by a cable suspension system attached to the ceiling structure of the room. The tank is an aluminum shell having a wall thickness of approximately 0.020 inch, and with its attachment structure at the two ends is structurally scaled to accurately represent the full-scale vehicle.
The capability exists for the tank to be mechanically excited by an elec- tric vibrator and then acoustically excited by the random noise of an air jet.
Accelerometers were installed along the length of the tank, and a microphone array was used for noise measurements. This setup of figure 11 permits a study of the tank response to both mechanical and acoustical excitation and, in addi- tion, provides a measure of its acoustic radiation under controlled conditions.
As an indication of the vibration response of structures of this type, two spectra obtained by William M. Thompson, Jr., of LRC are presented in figure 12.
Acceleration responses for a location on the tank surface midway between its ends are shown as a function of frequency for both the empty condition and the full-of-fluid condition. For each condition the sinusoidal driving force of 1 pound was applied at one end and was varied slowly throughout the test range.
The roll-off at high frequency is due to limitation of instrumentation. It can be seen that the tank exhibits a very complex vibration response, and a large For this particular model, the number of vibration modes can be identified.
dominant modes happened to occur in the frequency range of conversational speech, and a voice input to the mechanical shaker was quite faithfully repro- duced into audible signals by the tank. As a matter of information, the vibra- tion response of the tank was not markedly different when filled with fluid, as can be seen in the right-hand graph. The peak acceleration amplitudes are approximately equal to those of the empty tank, and the most readily observed difference in tank response is the appearance of some clearly defined, low- tank is filled with fluid. Further frequency modes for the case where the experiments are planned in order to evaluate the structural and radiation a broad-band acoustic damping and the manner in which the tank responds to input.
ENVIRONMENTAL STUDIES OF A N INSTRUMENT PACKAGE Acoustic environmental tests of an Agena instrument package have recently been made in the noise field of the Langley 9- by 6-~oot Thermal Structures Tunnel. The tests were made at the request of the Air Force and in association with Lockheed Aircraft of Sunnyvale, California. Figure 13 shows a photograph . .
of the Agena forward equipment rack in testing position at the tunnel. The view is looking toward a 12-foot-diameterdiffuser exit of the tunnel.
The Agena In this pos- model is cradled in a shock-mounting suspension onboard a trailer.
ition, the test specimen is located at the center-line elevation of the tunnel exhaust and only a few feet from the exhaust stream boundary. The Agena package was about 5 feet in diameter and about 5 feet long. The aft end was sealed off with a nonflight structure, and the fore end was equipped with a dummy nose cone. The package was of skin stringer construction with skin surfaces being beryllium. Tests were made in a random noise environment, and accelerometer measurements were taken on structural members throughout the model.
14. At Representative results from these studies are included in figure the top of the figure is shown the acoustic input in one-third octave levels.
At the bottom of the figure are shown the resulting accelerations in one-third octave bands measured at two locations in the model. The acoustic input is noted to have a flat spectrum within about 5 dB from 1OO'cpsto 10,000 cps. The acceleration responses, on the other hand, are noted to increase with increasing frequency up to about 2,000 cps which is approaching the useful upper limit of The accelerations of the shell struc- frequency of the accelerometer equipment.
ture were noted to peak at about 3Og, whereas the peak accelerations measured on the more massive gyro guidance equipment are markedly lower, as would be expected, and in this case did not exceed about 6g. Such experiments as these are frequently required for complex structures designed for specific accelera- tion tolerances since at present there are no acceptable analytical means for accomplishing this task. It is hoped that basic information such as is pre- sented here will be useful in the formulation of acceptable prediction schemes for future vehicles.
CONCLUDING REMARKS A brief status report has been given on current studies of the dynamic responses of panel shells and complete equipment packages to acoustic inputs.
The objectives of these various studies are to provide information for improving existing fabrication techniques of flight structures exposed to noise loads and the development of criteria for the establishment of vibration and acoustic specifications for complex structures of flight vehicles.
REFERENCE 1 . Pride, Richard A . , Royster, Dick M . , and Helms, Bobbie F. : Design, Tests, and Analysis of a Hot Structure for Lifting Reentry Vehicles.
NASA 'I" D-2186, 1964.
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