Fatigue Testing of the Airframe

Before a new aircraft is allowed to transport passengers, it has to demonstrate its strength in a dynamic test. In this life cycle fatigue test, all important pans of the assembled test aircraft are loaded in a test ng with the load spectra which the aircraft has to expect in real airline operation. This includes the very rare maximum loads which arc only felt by very few aircraft. To provide a safety margin, the fatigue test must simulate twice the cycle number (and load events) before final certification. Aircraft in airline operation must never reach more than half the number of cycles which were tested in the dynamic lest rig.

For SCTs flying faster than Mach 1.8. this includes thermal fatigue testing and thermal cycling (like for Concorde). For SCTs flying at most Mach 1.8. the highest temperatures (about 80 *C> arc reached on ground, when the aircraft slays in the sun without any wind. So these tem­peratures are not different to existing aircraft and well within the limits of known materials Bui at Mach 2 mean temperatures at the aircraft skin arc about 105 ‘C. the hottest points at 125 ‘C. and at higher Mach numbers even higher.

Thermal fatigue testing has to simulate the stresses related to temperature gradients Temperature distribution and its gradients must therefore be simulated. Establishment of tem­perature gradients is a transient process which cannot be accelerated without changing (and mostly increasing» the gradients and stresses. Therefore cycling can only he accelerated for the equilibrium temperature limes: long ground stops and cruise after heating up all fuel.

New SCTs will he designed for a service of 60 000 flight hours and 20 000 cycles. When assuming that the simulation time saved by equilibrium flight roughly is as long as the ground simulation time for cooling down, this means fatigue testing of 120 000 h or 13.7 years! Today’s certification rules require certification to be completed within 12 years, although this may be altered for an SCT. Such long and complicated tests are very expensive. And faults detected during those tests may ground the whole fleet, at least the high cycle aircraft.

To save time and money it is proposed to develop a new technology for thermal fatigue certification: use partial simulation of representative subassembly pans with shoncr heating and cooling cycles. These partial simulations will he transposed into a total aircraft simulation required for certification which is based on a certified numerical simulation method.

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