1、U NASA AN INVESTIGATION OF THE EFFECTS OF SURROUNDING STRUCTURE ON SONIC FATIGUE Prepnred by LOCKHEED-GEORGIA COMPANY yarietta, Ga. for LatIgZey Research Center f - NATIONAL AERONAUTICS AND SPACE ADMINISTRAT ION WASH NASA cz 4 R -1 “ Provided by IHSNot for ResaleNo reproduction or networking permitt
2、ed without license from IHS-,-,-7-18 J NASA CR-1536 TECH LIBRARY KAFB, NM J INVESTIGATION OF THE EFFECTS OF SURROUNDING STRUCTURE ON SONIC FATIGUE 1/ By Thomas F. Nelson d Prepared under Contract No. NAS 1-8120 by 8 -QI LOCKHEED-GEORGLA COMPANY Marietta, Ga. for Langley Research Center NATIONAL AERO
3、NAUTICS AND SPACE ADMINISTRATION Far sale by the Clearinghouse far Federal Scientific and Technical information Springfield, Virginia 22151 - CFSTl price $3.00 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-. .I AN INVESTIGATION OF THE EF!FECTS OF S
4、URROUTDING STRUCTURE OM SONIC FATIGUE By Thomas F. Nelson Locklzeed-Georgia Company SuNjNzaRy Four different sizes of a basic grid-like skin-stringer structure were tested in sonic fatigue in order to evaluate the effect of specimen complexity on sonic fatigue test data. Specimen size was increased
5、systematically by progressive addition of panel bays, beginning with a single skin bay and adding up to forty-eight similar skin bays around the centrally located reference panel. Random rms strain was the control quanity in conducting the tests and the primary results consisted of tVrenty-one data
6、points from tests of the four configurations which are plotted in the forrn of S-N curves. Data points from all four configurations correlate well with a single computed S-N curve for a = 4 within a 20$ stress deviation band. Strain and acceleration data are included on specimen frequency and amplit
7、ude response to both discrete frequency and random acoustical loading, INTRODUCTI016 One of the basic objectives of structural scnic fatigce testing is to be able to correlate more closely the laboratory test results with actual in-semice performnce of aircraft. Usually for reasons of economy, the l
8、aboratory tests are run on small portions of the aircraft structure. There is seldom any scaling down or modeling effect, but often the complexity of the actxal. aircraft structure is drastically reduced. From a standpoint of obtaining basic sonic fatigue data that could be used by the designer for
9、specific applications, sane of the test variables which could affect these data are: surround structure O temperature O manner of SPL loading (discrete or random) O of edge atta.cbments or clamps s Good simlation of all of these variables is the edge attachment of a skin bay. The generally high degr
10、ee of correlation demonstrated by these.tests was due in part to uniformity of failure modes where skin cracks occurred around the edge attachments at mid span of the long edge of a skin bay. Symmetryabout a central skin bay was maintained in these tests and a verification of these results for assym
11、etrical panel construction should not be assumed on the basis of these tests. Program monitor for this investigation was Mr. Carl E. Rucker, NASA-Langley. Laboratory testing was done by Mr. J. 0. Rasor, assisted by Mr. Ken Smith. Mr. J, 2. Carroll was responsible for the analytical design considerat
12、ions of Appendix A, The latter are Lockheed-Georgia personnel. SYMBOLS A a b B C d db ax E e f, fo distance from fastener to “heel“ of stringer, inches long side of plate, inches short side of plate, inches scan filter effective band width, zlz distance from neutral axis, inches fastener diameter, i
13、nches decibels, with a reference of 0.0002 dynes/cm finite area strip of Rayleigh probability density plot Youngs modulus of elasticity, psi statistical error frequency, Hz 2 8 2 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-. . c sound pressure, p
14、si gravitational unit, 3a6 in/sec panel dimension coefficient frequency, cycles per second plate thickness, inches flange thickness, inches radius of gyration, inches 2 stress concentration factor, stress at concentration nominal stress beam or stringer length, inches bending moment, inch-pounds num
15、ber of stress cycles to failure (subscripts added in Appendix B) nmber of degrees of freedom pounds per square inch Rayleigh probability density for ratio of peak-to-rms stress power spectral density, (measured quantity) /az panel dimension coefficient resistance end capacitance time constant rcot m
16、ean square stress vs cycles to failure sound pressure level, db stress, -psi fa8 tener spacing, inches data sample length, seconds load density, lb/in maximum panel deflection mode shape rms stress, psi, or standard deviation microinches, inches x 10 2 -6 dsrr pinq rat io, actual damping critical da
17、mping sound pressure spectral density, (psi) /BZ 2 3 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-. TEST SPECIMENS Basic Design Considerations To accomplish the purpose of this test program, two extremes in specimen size and complexity were select
18、ed. The largest size was a specimen with an area of about 6 ft by 10 ft containing forty-nine bays of the basic test section. The simplest specimen possible, of the same type structure, consisted of one hsy of skin riveted to a simple, rigid support fixture. Various other specimen con- figurations b
19、etween these two extremes were selected for evaluation. A typical aircraft skin-stringer type structure was selected. The connect- ing bays of structure were structurally the same and the same stringer and ring cross section was used in all specimens. The intent was to keep all specimen parameters c
20、onstant except the extent of specimen structure surrounding, or supporting, a central test section. Four different configurations of test specimens were selected, each differ- ing only in the extent of similar structure surrounding the central test section. The basic test seotion was considered to b
21、e a flat rectangular skin panel sup- ported by stringers along the two longitudinal edges and heavier rings along the transverse edges, Countersunh rivets attached the skin to the stringers and rings. The particular dimensions, materials and construction details were in- tended to be typical of airc
22、raft skin-stringer structure subject to sonic fatigue. Each of the four specimen configurations differed in the number of similar skin bays. A particular specimen is identified according to the number of skin bays. The basic skin panel consisted of a rectangular sheet of 7075-T6 clad aluminum, O.G5
23、inches thick and measuring 8 inches by 18 inches between attaching rivet lines. The rivets were 5/32 inch diameter, countersunk into the outside surface of the skin, The stringer and ring members were the same for all configurations, except or length which depended on the size of each configuration,
24、 Fabrication of the intersection of the stringers and rings at the four corners of each skin panel, or bay, was the same for all supported skin panels in each configuration, These specimens were designed using the considerations explained in Appendix A. This Appendix contains a typical method for an
25、alyzing structure for Sonia fatigue. A detail description of each specimen configuration follows. Sinsr;le Bay Configuration - Considering that the aircraft structure under consideration is a. continuous pattern of skin panels supported by a grid of support structure, the most elementary specimen wo
26、uld be a single bay skin panel attached by the same type of attachments to a rigid test fixture. This config- uration retained the aircraft skin panel dimensions and edge attachment stress concentrations, but did not incorporate the edge rigidity of the aircraft support 4 Provided by IHSNot for Resa
27、leNo reproduction or networking permitted without license from IHS-,-,-structure. For this purpose, a panel of skin was riveted to a 0.75 inch thick steel plate with a cutout for the skin panel. The same type of rivets were used as for the other configurations. A sketch of the specimen and support f
28、ixture is shown in Figure 1, Mine Bar Configuration - Nine skin bays in a three-by-three arrangement are included, supported by a grid of stringers and rings. This resulted in one basic test section, or one skin bay, isolated from any effects of fixture support by one other skin bay in all direction
29、s. This configuration is shown in Figure 2. As for all configurations, the test bay was 8 x 18 inches between attachment lines. The surrounding bays were smaller in order that the response frequency of the surrounding bays be different from that of the center bay. Failure was desired in the center b
30、ay, not in the outer edges of the specimen. In addition, the attachments through the skin in the outer bays were not countersunk, to decrease the chance of a skin crack outside the test section. Substructure details are shown in Figure 3. Twenty-five Eiay Configuration - This design contained twenty
31、-five skin bays with the central bay separated from the support fixture by two bays in all directions. The outer bays which tied into the support fixture were smaller than the others, and did not contain countersunk rivets. See Figure 4 for a sketch of this configuration. Forty-nine Bag Configuratio
32、n - This was the largest specimen tested measuring about five feet by ten feet and containing forty-nine skin bays. This design had three other bays separating the center bay from the support fixture. A doubler sheet was installed over the outside skin bays to reinforce the skin at the support fixtu
33、re tie-in. A sketch of this configuration is included as Figure 5. Facility and Equipment A progressive wave type test chamber was used for these tests. The chamber dimensions at the test section were six feet in height, ten feet in length and fourteen inches in width. The flat specimen formed one s
34、ide of the chamber with plywoorl panels used to fill in the area around the specimens. The opposite side of the chamber was formed by steel doors. Sound was introduced at one end of the chamber through two rods of six exponential horns from twelve electro-pneumatic noise transducers. At the other en
35、d of the wave chamber was a single horn which expanded into a muffler tunnel. A photograph of this test chamber is included as Pigure 6. Heavy, movable beams spanning the specimen side of the chamber horizontally and vertically were used to support the smaller specimen support fixtures. Specimens we
36、re positioned such that the center was located between about two to five feet downstream from the upstream edge of the test section. A single layer of cloth tape was used to separate all specimens from the surface of the fixtures. This was done to avoid a failure due to friction or corrosion bet- we
37、en the aluminum specimen and steel support. A forty-nine bay specimen is shown installed in the test chamber in Figure 7+ 5 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-The usable frequency range of the wave chamber facility for fatigue testing is
38、 from 60 Hz to 1000 Hz. Maximum SPL capacity is 164 db in the range of 100 Hz to 500 Hz. Total acoustical power available is 48,000 watts. Description of Test Setups Single Bag Confimration - The support fixture and specimen for this config- uration are almost integral and the mounting of the simple
39、 sheet specimen has been previously described. The steel plate fixture had two cutouts so that two speci- mens could be tested at the same time. A sketch of this fixture ia shown in Figwre 1. The support fixture was bolted at top and bottom to the side of the wave chamber. Microphones were suspended
40、 by bungee cord in front of each specimen and located midway between the specimen and the opposing chamber wall. Between one and four specimens were tested together during this program, therefore, the pattern of microphone locations was changeable. These locations are shown in Figure 0. The micropho
41、nes were the condenser type with a usable SPL range of from 05 db to 185 ab and a frequency range of 10 Hz to 4000 Hz. Overa31 SPL readout accuracy within 21.5 db. The microphones were non-directional and were positioned verti- aally in the wave chamber. Figure 9 is a photograph of the microphones i
42、n place for a test of a twenty-five bay specimen. Strain gage locations were coordinated for all configurations. Since strain was the criteria for setting the sound pressure level (SPL) for each test, gage locations had to be comparable between specimens, if results were to be comparabb. The first s
43、pecimen carried eight gages and the following twelve specimens, at least four each. The particular gages for each specimen are called out in the plots and tables in the test results section of this report. Figures 10 and 11 show the gage locations applicable to any specimen. For this configuration,
44、gage locations 2, 2A and 6 were above the edge of the cutout in the steel plate and locations 3 and 4 were in the center of the bay. The strain gages were the Constantan wire grid type which were bonded with Epoxy adhesive to the specimen. Nine Bag Confipuration - Each specimen panel was fastened to
45、 a steel “piotzlre frame“ fixture by a double row of screws through the skin and angle clips Rf the ends of each stringer and ring. 411 screws were the protruding head type 1.1, order to reduce the chance of failure around the outside edge of the specimen. A sketch of the specimen mounted in the sup
46、port fixture is included as Figure 12. The support fixture was bolted to the support beams in the side of the wave chamber with the long side of the specimen horizontal. Microphone locations are shown in Figure 8, and strain gage looations in Figure 10. The gage locations selected for each specimen
47、are listed in the results section of this report. Accelerometer locations are shown in Figure 13. * The accelerometers were the piezo-electric type, weighing 2.8 grama and with a usable frequency range of 2 to 5000 Hz. 6 Provided by IHSNot for ResaleNo reproduction or networking permitted without li
48、cense from IHS-,-,-. Twenty-five Bay Gonfiquration - The inst,alletion of this specimen in the test chamber was similar to that of the nine bay specimen. A sketch of the su;lport fixture is shos;n fn Figure 14. KicropkLone locations were the same as for the nirie tay conflguratint:; one microphone i
49、n front of each three central skin bays. At least eight of the stan- dard strain gage locations were selected for each sgecimer., including the primary gage locations of Figure 11, Accelerometers were mounted in the first four stan- dard locations of Figure 13 for a low level random survey. Forty-nine Bay Confi-ration - This confi,aration was considered too large to be supported by a o
