| Summary: | 碩士 === 國立中興大學 === 土木工程學系所 === 94 === In this research, a new testing method for fatigue was proposed that the traditional hydraulic press machine was replaced by a pair of vibratory motors (centrifugal force provided by rotary blades) with identical loading amplitudes (m0eωp2) on the basis of force control. Besides, the fatigue behavior of lightweight concrete (LWAC) beams subjected to cyclically dynamic loading was investigated and compared with that of corresponding normal weight concrete (NWC) beams. Dynamic loading tests were implemented with 4 levels of loading magnitude (Pmax/Py = 0.71, 0.62, 0.54, and 0.45). For each level, the loading range (R=Pmin/Pmax) was set to be constant values (R=0.57). The oscillating frequencies (ωP) used were 30 Hz and 12 Hz respectively with the corresponding range of frequency ratios (ωP/ωn) of 1.23~1.57 and 0.49~0.63 scattered on both sides of resonant region (ωP/ωn=1). The dynamic responses (displacement and acceleration) and magnification factors (Rd and Ra) of NWC and LWAC beams were evaluated for the feasibility of new testing method under the two above-mentioned cases (ωP/ωn>1 and ωP/ωn<1). Further, if the specimens subjected to two million cycles of dynamic loading without entire failure, the static loading tests were anew performed to access the residual strength of beams.
The experiment results showed that, the most critical factor affecting the amplitude of dynamic responses (displacement and acceleration) of beams was the ratio of the applied oscillating frequency (ωP) to the systematically natural frequencies (ωn) of beams. As the oscillating frequency was 30 Hz, the amplitude of dynamic responses (displacement and acceleration) would decrease with the magnitude of dynamic loading (amplitude and median) increasing. However, in contract, there would be an opposite tendency obtained from the results of specimens as the oscillating frequency was 12 Hz. The deflections of all specimens (both ωP=30 Hz and 12 Hz included) depend primarily on the magnitude of dynamic loading (amplitude and median). The larger dynamic loading (amplitude and median) would cause higher values of the deflections of specimens. In addition, the residual strength and stiffness of specimens were mainly influenced by the amplitude of dynamic responses of the beams themselves. Except for the specimens accompanying resonance (such as the case of ωP = 12 Hz with Pmax/Py = 0.54), the residual strength and stiffness of others would decrease with the increase of the amplitude of dynamic responses. Eventually, according to the results of verification for the applied oscillating frequencies (ωP), systematically natural frequency (ωn), amplitude of dynamic loading (m0eωP2), amplitude of displacement response and components (X-, Y-, and Z directions) of dynamic responses (displacement and acceleration), the new testing method proposed in this paper for fatigue was practicable with ωP/ωn>1 (ωP/ωn≒1.41 suggested). Under such condition, the time it would take for the period of fatigue testing could be reduced substantially.
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