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Control systems.-Static tests of the control systems to their limit loads should be conducted. Also, operating tests of the control systems loaded to 80 percent of their limit load values specified in Chap. 1 should be conducted to show that the control systems will operate properly when subjected to loads which simulate flight conditions.
Launching and towing release mechanisms.—Static tests of the launching and towing release mechanisms to their limit loads should be conducted. Also, operation tests of release mechanisms should be conducted to show they will function properly when loaded throughout the range from zero to 100 percent of the limit force values specified in Chap. 1 for release mechanisms. . These operational tests should include the back load trip release mechanism, if installed. Hardware used in towlines should be of sufficient rigidity to avoid distortion and possible malfunctioning or breakage after repeated use. The above recommendations concerning operation tests do not apply to "open-hook” launching fittings used solely for shockcord type launching apparatus. The strength of the control system actuating the release mechanism is covered in Chap. 1.
TEST PROCEDURE These operation tests should be conducted with the fuselage supported and restrained in a manner which will avoid any excessive "springing" of the fuselage nose sections when the test load is suddenly released. Inasmuch as the purpose of the test is to demonstrate the releasing characteristics of the mechanism under load, the fuselage resisting loads may be applied at any convenient points on the fuselage near the release mechanism location.
TEST LOADS, APPARATUS AND METHODS The determination of test loads, the apparatus used and the methods of conducting the tests should be brought to the attention of the regional office concerned prior to conduct of the tests.
ULTIMATE LOAD AND DESTRUCTION TESTS An ultimate load test is used to determine the ability of the structure to withstand its ultimate design load. For conventional type structures, the stress analysis determines this satisfactorily but if the structure is of an unconventional type, an analysis cannot be relied on. In other cases, the manufacturer may not wish to submit a complete stress analysis. Ultimate load tests are, therefore, recommended in such cases as an available method for determining whether the structure will fail before it reaches the required ultimate load.
When a static load test is carried to the point where the maximum carrying capacity of the structure is reached, the test is usually referred to as a destruction test. An ultimate load test is not necessarily a destruction test.
In cases wherein the strengths of structures are demonstrated by ultimate load tests in lieu of stress analyses, the proposed test program should be submitted for verification before the tests are conducted. Particular items which should be given special consideration in an ultimate load test are (1) the testing of all members, or portions of the structure requiring excess factors of safety and (2) taking into account the question of material variations from standard specifications and possible discrepancies between the test specimen and drawings.
MATERIAL TESTS Standard properties.—Drawings which are to be used as a basis for a type certificate will specify certain minimum guaranteed material properties, usually by reference to existing standard specifications. It is not necessary that the glider applicant substantiate the strength characteristics of standard materials if the purchasing invoices show they are in accordance with standard specifications. In cases where new or unconventional materials are used, special strength tests should be conducted to establish guaranteed minimum properties suitable for design.
Stress-strain diagrams. In general, the most useful data are obtained from a stress-strain diagram obtained in a tension test and such diagrams should be obtained in all cases where new materials are used. This diagram permits the determination of the following important characteristics:
1. Ultimate tensile stress
3. Modulus of elasticity (E) Further information on strength properties of materials, stressstrain diagrams, et cetera, can be obtained from ANC-5, materials textbooks, et cetera.
Special materials tests.-In many cases, special tests should be conducted to account for factors difficult to evaluate in a stress analysis. The effects of welding after heat treatment are difficult to predict in some cases. Stress-concentration caused by poor detail design will often reduce the allowable stresses considerably below standard values. Most metals show marked reductions in allowable stresses after being subjected to repetitive loadings even in cases wherein the stresses are relatively low.
METHODS OF CORRECTING TO STANDARD It is necessary to know the type of failure which is critical in order that proper correction factors may be used to reduce the results to standard. For a "built-up” structure, failure can occur in many different ways and at different places. In general, it is only necessary to derive correction factors for the particular portion and for the particular mode of failure which occurred. When the total load sustained is 15 percent greater than the “ultimate” load required, no additional material corrections are necessary. When the strengths of fittings are demonstrated by ultimate load tests, material correction factors are not considered necessary since the fitting design factor of 1.15 is believed sufficient to cover it.
The material correction factor should be made by multiplying the test load sustained at failure by the ratio of standard strength of the material to the strength of a specimen taken from the structure.
The particular strength property involved will depend largely on the mode of failure. In general, it is desirable to obtain a stress-strain diagram for the material specimen. A chemical analysis might be advisable if there is doubt as to the actual material used in the test structure.
TEST PROCEDURES Jigs.—Tests of tail surfaces, wings and similar components are usually conducted by mounting the surface to be tested onto a specially built jig or framework, using the regular attachment fittings of the unit being tested. The jig should conform to the glider structure as far as possible. In cases where the attachment of a component to the fuselage involves the distribution of concentrated loads into a thin-walled structure, it is highly desirable to test the surfaces while attached to the actual structure or to the portion affected; otherwise the strength and rigidity of the jig will be imparted to the test thereby leading to erroneous conclusions of excess strength. Special care should be taken to obtain net deflections of the surface tested; that is, the deflections of the jig should be deducted from the total deflection.
Loading schedule.-A loading schedule should be prepared. The schedule shows the load distribution to be used and the values of the loads to be applied at each stage of the loading process. When the load is to be applied by means of bags of shot or by weights, it is usually expedient to weigh each increment of loading in advance and to assign it to a marked space on the floor, so that no confusion will result. The loads can be divided into suitable increments of about one-sixth (16.7 percent) of the ultimate load. In the usual case, such increments will be one-fourth (25 percent) of the required “limit” load, so that the limit value of test load will have been reached at the fourth increment. The ultimate value of test load will then be reached at the sixth increment. After reaching the ultimate load, the size of the increments should be reduced so that the second additional increment will produce 115 percent of the “ultimate” load. However, if the structure should show signs of failing at any time, the loading increments should be accordingly reduced so that the test loads will exceed the failing load by as small a margin as possible.
Supports.—It is advisable to support the unit being tested by means of jacks during the load application. A safety framework or blocking should be provided in all cases so that the structure will not deflect too much after failure. This not only protects workmen and observers but also permits an accurate determination of the point of initial failure and may permit continuation of the test after local reinforcement if such is desirable. Deflection sticks should be attached at various points of the test specimen and a level should be provided for reading the scales, which should preferably be graduated in tenths of an inch. See figs. 2-1 and 2-II.
Special procedure in limit load tests.—At the start of the test, it is advisable to apply at least a part of one increment and remove it before measuring the initial positions of the deflection stations. After each load increment is applied, the jacks should be lowered for a period of at least 1 minute before deflection readings are taken. When the total "limit" load has been loaded onto the structure and readings have been obtained, the entire load should be removed, preferably one increment at a time. The deflection readings at zero load should then be obtained.
Special procedure in ultimate load tests.—The procedure outlined for the limit load test described in the preceding paragraph should be followed until the limit load test is completed. It will be noted that the ultimate load test, if conducted first, would not permit the determination of the permanent set caused by the "limit" load. After the limit load test, the loading should be continued beyond the limit load value in accordance with the loading schedule. As the ultimate load is approached, the structure should be carefully observed and any unusual behavior noted. The increments should be reduced if any signs of failure are observed. If the structure should fail locally before reaching ultimate, it is permissible to reinforce the failed portion. When that is practicable, the test should then be resumed. The details of reinforcement should be carefully noted. If the material correction factors will be small, it is not necessary to proceed to the 115 percent overload. If the test specimen cannot be used after test, it is desirable to continue the test to destruction, that is; through to complete failure. When failure begins during the lowering of the jacks, it is advisable to remove some of the load before completely removing the support, in order that the minimum load causing failure can be determined as closely as possible.
Special procedures when ultimate load tests are conducted in lieu of stress analysis.-In such cases the procedure is the same as for ulti
mate load tests except that it will often be necessary to prove that specified extra loads (higher factors of safety) can be carried by certain portions of the structure. Additional design conditions may also have to be investigated. Whenever possible, adequate photographs should be taken and samples of the material should be obtained and tested.
Check of test structure.-In most cases, a conformity check is made of the test specimen against the drawings and other design data used as a basis for certification.
In the case of ultimate load tests in lieu of stress analysis, a particularly thorough conformity check is made. After the test, the portions that failed usually are further checked for dimensions and strength properties.
Figure 2-1. Chordwise extension piece to magnify wing torsional deflection readings.
TEST REPORT In all cases the manufacturer making a test is required to submit a complete report covering details of tests. The report should include photographs or drawings of the test setup and the test specimen; photographs of failed parts or sections; records of deflections and readings taken; date of test; identification number of report; serial and model number of glider and signature of responsible witnesses and/or test personnel. In addition, the following points should be covered when applicable:
• Substantiation by references or computations of the selection
of critical test conditions and loadings.