| Summary: | 碩士 === 淡江大學 === 航空太空工程學系 === 89 === Considerable fracture work has been done on the ductile materials for tensile loading since this mode was thought to be the most dangerous and was also the easiest to test in the laboratory. However, most structures do not experience just one type of loading but combinations of tension and shear. This situation is commonly known as mixed mode loading. The rotating machines such as compressor turbine blades, cooling fans, propellers, and helicopter rotor blades… etc., are easy to sustain such mixed loading and the fracture occur.
In this research, the mixed mode fracture of a pair of highly rotating metallic blades have been investigated at room temperature using single edge notched specimens. A set of 2-bladed rotor is driven by a 220 volt AC motor and the rotating speed is fixed as 850 rpm. The notch is located in various blade positions from blade root to tip. The correlation of notches at blade leading edge and trailing edge are also studied. A simple theoretical model of a flapping Bernoulli-Euler-Beam like rotor blade model is established for analytic study.
The experimental results show that the crack growth for the notched blade at leading edge is faster than the trailing edge notched blade. The near-root notched blade crack occurs earlier than the blade that the notch located near the tip. The cracks grow fast in the case of the rotor blade with higher angle-of-attack. In the analytic study of blade flapping motion, it is found that the aerodynamic force plays an important role in rotating machines. The aerodynamic force lifts the highly rotating blades. Due to the fact from the experimental data, it is shown that the blade flapping bending moment causes the crack occurring at the lower part of the rotating blade with angle-of-attack. It is noted that the aerodynamic force should be included in doing the rotating fracture problems.
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