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