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Harris' Shock and Vibration Handbook Part 11

Tham khảo tài liệu 'harris' shock and vibration handbook part 11', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | PYROSHOCK TESTING 26.29 FIGURE 26.17 Typical shock response spectrum and acceleration timehistory from a tuned or tunable resonant fixture test. The shock response spectrum is calculated from the inset acceleration time-history using a 5 percent damping ratio. required by the tuned resonant fixture methods described below. Pendulum hammers of the general type shown in Fig. 26.8 have been used as well as pneumatically driven pistons or air guns. The method which is selected must provide repeatability and control of the impact force both in magnitude and duration. The magnitude of the impact force controls the overall test amplitude and the impact duration must be appropriate to excite the desired mode of the tuned resonant fixture. In general the impact duration should be about one-half the period of the desired mode. The magnitude of the impact force is usually controlled by the impact speed and the duration is controlled by placing various materials e.g. felt cardboard rubber etc. on the impact surfaces. Resonant Plate Bending Response . The resonant plate test method23-24 is illustrated in Fig. 26.18 which shows a plate usually a square or rectangular aluminum plate freely suspended by some means such as bungee cords or ropes. A test item is attached near the center of one face of the plate which is excited into resonance by a mechanical impact directed perpendicular to the center of the opposite face. The resonant plate is designed so that its first bending mode corresponds to the knee frequency of the test requirement. The first bending mode is approximately the same as for a uniform beam with the same cross-section and length. Appendix 1.1 provides a convenient design tool for selecting the size of the resonant plate. The plate must be large enough so that the test item does not extend beyond the middle third of the plate. This assures that no part of the test item is attached at a nodal line of the first bending mode. Usually the resonant fixture with an .