Abstract:
The use of composite materials is rapidly growing in the construction of aerospace
structures. Most of the aircraft parts made from composite materials, like wing skins,
spoilers, fairings, and flight controls, are being used due to their reduced weight as
compared to aluminum parts. New generation large commercial aircraft are being designed
with all composite fuselage and wing structure. The main advantages of composites over
conventional metallic materials are their high strength to weight ratio, corrosion resistance
and tailored stiffness. Uni-directional composites have predominant mechanical properties
in one direction and vary with the direction relative to the axis of structure. The aero-elastic
properties, such as stiffness and dynamic stability, also depend on the stacking sequence
of the plies.
Fatigue and Damage Tolerance (F&DT) assessment of these advanced composite
materials is an emerging field of research. In aircraft structural integrity analysis, the
damage tolerance and fatigue life is investigated against a cyclic loading spectrum. The
particular spectrum includes the stress/loading levels counted during a flight of certain
duration. The occurrences of z-axis load factors ‘Nz’ may include higher gravitational
acceleration ‘g’ levels. While maintaining a certain g level occurrence at higher angle of
attack, wing structure vibrates with the amplitudes of its natural frequencies. The cyclic
stress amplitudes of vibration depend upon the natural frequencies of vibrating structure,
i.e. lower frequency gives higher amplitudes and vice versa. These fluctuating load
amplitudes are superimposed on the higher ‘g’ level mean loads during fatigue analysis.
These additional cycles are very critical in DTA studies.
