150 J.H. Jensen 640Hz X:818.5 Freq:818.75 Hz Freq:767 Hz A: D1 Freq Resp B: D1 Freq Resp kg/Ib kg/Ib 1 4 100 1 4 100 mg/Ib mg/Ib LogMag LogMag decades decades Y:115.705 g/Ib Damp:1.6091 Damp:1.0154 c-3 c-3 Hz 840Hz AVG: 16 840Hz 640Hz X:767.25 Hz Y:24.573 g/Ib AVG: 16 Fig. 14.5 The frequency response measurement of the cowling used to extract the frequency and damping of the modes after the crack was installed Fig. 14.6 Damping values for the four cowling modes verses excitation time increase in damping value after the fault was installed. The predicted damping increase after crack initiation is clearly visible as a step increase in damping. There is a small step increase in the three higher order modal frequencies after the damage is done and a decrease in the modal frequency of the first bending mode Fig. 14.7. Shaking the first bending mode through the crack initiation point would reveal the transitional changes in both frequency and damping of the higher order modes. This transitional data would be the key to detecting crack initiation. 14.7 Honeycomb Laminate Control Surface The honeycomb laminate composite control surface section was not expected to behave in the same manner as the all metal cowling section. The C-141 trailing edge control surface was a rectangular shape with a triangular cross section. The structure consisted of an aluminum skin bonded to a honeycomb core. The modal frequencies were widely spread with heaver damping so it was only practical to monitor two modes in one frequency span. Two sets of damping values were extracted
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