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381 aerospace applications of acoustic emission

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381 Aerospace Applications of Acoustic Emission Testing
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The above discussion of acoustic emission testing during hydrostatic testing points out the need to verify the structural integrity of filament wound composite rocket motor cases at proof pressures lower than 70 to 80 percent of expected burst pressure. A related goal is the prediction of burst pressures on the basis of hydrostatic proof test data. The remainder of this work discusses the attainment of these two objectives with a set of 12 fiberglass epoxy bottles with diameters of 146 mm (5.75 in.). 25 Since its inception, the solid rocket motor industry has used hydrostatic proof testing as a means of verifying the structural integrity of rocket motor cases before propellant loading. This procedure consists of filling each case with water and pressurizing it once to a proof load in excess of the maximum expected operating pressure. Although hydrostatic proof testing has proven successful in verifying the structural integrity of metal cases, it has not always been as successful for composite materials. In subscale composite pressure vessels, there have been instances where chambers have successfully passed the one-time hydrostatic proof test and then have failed below maximum expected operating pressure on the very next pressurization cycle. 25 The reason for this is hydrostatic proof test damage to the composite structure. Whenever a composite pressure vessel is loaded, three fundamental types of damage can occur: matrix cracking, delamination and fiber breaks. Using acoustic emission test instrumentation, all three of these mechanisms are routinely observed during hydrostatic proof testing. 16,25-27 In terms of rocket motor performance, matrix cracking is the least damaging of the three failure mechanisms. The filament wound case is lined with a rubber insulator, which not only keeps the case from burning through during firing but also prevents pressure leaks. From the acoustic emission test data, it can be seen that matrix cracking occurs almost from the onset of pressurization and continues throughout the hydrostatic proof test cycle. Typically, it will occur to some extent over the entire case, with localized buildups at stress concentration points. Delaminations generally occur at higher pressures and in regions of high flexure, where the interlaminar shear stresses are the greatest. Although delaminations are critical in regions where two or more parts are joined together and can adversely affect buckling strength, their effect on the pressure vessel strength seems to be positive rather than negative; that is, they tend to relieve stress in such a way as to increase the burst pressure strength rather than decrease it. 25 The fibers in filament wound composite rocket motor cases are the primary load bearing component during the pressurization of firing. Hence, fiber breaks are the most damaging of the three failure mechanisms. Although some localized fiber breaks are occasionally sensed very early in the pressurization
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  • Fall '19
  • Fighter aircraft, Nondestructive testing, Acoustic Emission, Acoustic Emission Testing

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