A History of Research and a Review of Recent Developments

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The effects of explosive loading
residual strength and life after damage—all part of the study of the impact
damage tolerance of aircraft structures.
It was realized that existing design guidelines and specifications did not
fully address the projectile damage threat, and to improve design methods
Avery, Porter and Lauzze reviewed structural integrity requirements [8.25]
and resistance to battle damage. The damage in metal sheet, stiffener and
plate structures typical of modern aircraft takes the form of cracks, spallation,
petals, holes, dents or gouges, and the authors pointed out that for a given
target material the type of damage depends on sheet thickness, projectile velocity
and impact angle. A ‘damage regime’ diagram for 0.30 armour piercing
projectiles in plating made from the aluminium alloy 7075–T6, taken from
ref. [8.25] was shown in Figure 7.23. The most useful way of quantifying
projectile damage is by the ‘lateral damage’ notion, which is defined as the
diameter of an imaginary circle that just encloses the limits of fracture or
material removal. A typical variation of damage size with projectile velocity
was shown in Figure 7.24.
In skin and stiffener structures there are several types of damage
configurations, depending on whether the lateral damage is confined to the
skin between stiffeners, spreads across a stiffener with the stiffener remaining
intact, or spreads across a stiffener with the stiffener failing. The latter type is
normally the critical case for vulnerability analysis. If they remain intact the
stiffeners frequently provide a crack-arresting capability which can significantly
improve the residual strength of a battle-damaged structure.
The response of aircraft structures to impact damage was also examined,
for example, by Massmann [8.26], who considered the residual strength of
damaged structures by using finite element analytical methods and fracture
mechanics techniques. He developed a structural strength model, which included
an idealization of the wing of the US F84 aircraft. F84 wings had been shot at
during field tests, giving typical damage patterns of holes and cracks. Wing
plating in which the damage resulted in cracks only was analysed by using the
fracture toughness theory of Griffith, Westerg and associates to determine
residual strength. When the crack ran into a circular hole, the residual strength
was found to be a function of hole radius and crack length.
Test results and analysis showed that hits close to the front spar of the wing
considerably reduced the load capacity, and that wings constructed of stiffened
panels and three spars had a residual strength of over 2.5 times the residual
strength of milled integral panels having two spars (both geometries having
been designed to have similar initial load capacities). The stiffened panel design
was greater in weight than the integrally milled design, but in terms of the
entire weight of the aircraft, the increase in weight was less than 1% and the
decrease in manoeuvrability negligible.
In addition to the possible loss of strength from impact damage, the stiffness
of the aircraft structure may be altered, and stiffness degradation can lead to
aero-elastic problems associated with flutter, control and load redistribution.