6. Consider the “D-spar" of a 360-inch rotor blade illustrate below. The "D-spar" has a uniform "E"-glass composite cross section consisting of a thin component of +45° crossply fiberglass (x-ply) bonded together with a thicker component of 0° unidirectional fiberglass (UNI); assume the same geometry between blade stations 63 and 73. The x-ply is bonded to the UNI fibers to increased torsional stability of the spar. Assume that the blade is pined at station 29.5 (+y into the paper), material properties and flight test load data are: a. Find the maximum shear force, multicell shear flows and the torsional rigidity at station 65 checkpoints 4 and 9. During manufacture of the blade voids commonly occur between lamina of composite laminates. These voids are located by ultrasonic inspection and filled to acceptable size tolerances with resin. Number forty(#40) sized drill bits are used to access the void cavities. b. (24-point extra credit) assume damage due to # 40 drill holes (0.098" diameter x 0.375" depth)at 4 inch spacing in a straight line from stations 65 to 68; a) what is the stress concentration due to shearing forces mid-section of the drill holes? b) what is the reduction in torsional rigidity?and c) what is the as-damage maximum allowable torque assuming a n=1.25 margin of safety? c. (24-point extra credit) assume that +45°crossply doublers are know to reinforce damaged fiberglass structure due to drill holes; a) what is the stiffness of 0.009" x 6.0" oval (a=3.0"b=1.5") doubler? b) how many layers of a doubler are required to restore torsional rigidity to as-manufactured; overlap doubler layers by 1.0 inch? And c) what is the restored margin of safety?

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