Problem 3 (20 points) The de Havilland Comet was the first jet-powered commercial airliner. It was built before fracture

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Problem 3 20 Points The De Havilland Comet Was The First Jet Powered Commercial Airliner It Was Built Before Fracture 1 (56.07 KiB) Viewed 103 times

Problem 3 (20 points) The de Havilland Comet was the first jet-powered commercial airliner. It was built before fracture mechanics methodologies had been implemented in the aerospace industry. In 1954, two Comets suffered catastrophic failures originating at windows or hatches as shown in Figure 6 (next page). Flight histories for each aircraft included only about 2000 flights! The fuselage skin is subjected to zero-to-tension loading as a result of cyclic pressurization, one cycle per flight (FYI, the amount of the pressure cycle is about 8.3 psi at 35,000 feet of elevation). Repeated pressurization caused cracks to form and grow at stress concentrations (like windows), and when cracks reached a critical size the plane suffered catastrophic failure. The gross hoop stress in the region of the failures has been found to be about 28 ksi. The fuselage skin was made from an aluminum alloy with S, = 60 ksi, S, = 65 ksi, K. = 45 ksi-vin, and crack growth rate which can be described by the Paris Law if C=2.0x10° and m-4.0 (inch-ksi units). a) Model a crack emanating from a window corner as an edge crack in an infinitely wide plate and assume an initial crack size of 0.02 inch. Predict the number of flights it would take for this initial crack to grow to a critical crack size. (A closed form solution should be developed.) b) During development of the Comet, the plane was fatigue tested. In fact, a full-size final prototype cabin was tested under the action of cyclic pressurization. During testing, extra high "proof" pressures were occasionally applied in an effort to provide a margin of safety. No failure occurred in these tests after 18,000 cycles of pressurization. Describe what, if anything, led to the discrepancy between this initial testing and actual aircraft performance.

LA JONY. RIVETS YOU COSE OF PLATE FUSELAGE DATIN LAR JOWT Hoop STRESS coil 'BUCALE SECONDARY FAILURE T FRANE IN OF TEARS DUT Tran COLE OF FRAME ET LES F DOW REMAINED PLACE FRANE SPRUNG OUTWARDS AT BOTTON CORNERS. FRANE HR FIG. 6 THE PROGRESS OF THE FAILURE OF THE FORWARD ESCAPE HATCH ON THE PORT SIDE-COMET G-ALYU. HANEH PUCALE PUCALES CAUSED RELEASE OF PRESTLAE ·DRIC OF FAILURE Bildiag TENSION MAITE AT HAN [urea N MINETS ON AFT LINE OF FRANC STEARING ALONE FIND for NVE'S WH TENSION ✓ ✓ NEY, DIRECTION OF SHIN SEPARATION CHOWN BY ARROWS SHIN NOVING (HOARD SHOWN. SHIN MOVING OUTWARD SHOWN DIRECTION OF TENSION LOADS TO CAUSE FAILURE CONSIDER JUST ONE CRACK