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Impact Damage Characterization and Damage Tolerance of Composite Sandwich Airframe Structures-Phase II

The impact responses and the damage states in flat composite sandwich panels with thin
facesheets were investigated in Phase I and were found to be dependent on the diameter of the
spherical steel impactor. The residual strength of impact-damaged sandwich panels under static
in-plane compressive loads was also found to be dependent on the nature of the damage state.
The impact damage states due to blunt impactors, depending on the size of the damage, are
difficult to detect in service and can degrade the residual strength more than a punctured skin. In
Phase II, the detectability of impact damage states using different field inspection techniques was
experimentally investigated for different facesheet and core combinations. The impact damage
in honeycomb core sandwich panels was better detected using instruments that exploited the
local vibrational characteristics of the sandwich structure, while the damage in foam core panels
were more amenable to acoustic-based techniques. The effect of facesheet stiffness on the
sensitivity of the field inspection techniques was investigated. The behavior of the sandwich
panel with impact damage and subjected to in—plane compressive loads was investigated in
detail. The impact damage behaved in a characteristic sequence of events leading to contrasting
final failure modes. The sequence of events was dependent on the facesheet stiffness and the
transverse compressive properties of the core material.

The effects of panel curvature on the impact damage resistance of sandwich panels were
experimentally investigated for limited sandwich configurations. The effects of the radius of
cylindrical panels, boundary conditions, facesheet type, and core type were also examined. The
decreasing radius of curvature increased the global bending stiffness, but decreased the local
contact stiffness due to the radial compressive properties of the honeycomb core. The decrease
in radius increased susceptibility to damage for sharp impactors but decreased damage for blunt
impactors.

The fatigue behavior of honeycomb core and foam core sandwich panels with impact damage
due to a 3" diameter impactor was investigated, including the effect of load ratio and load level
on the fatigue life. No significant trend could be observed with load ratio. The load/life was
very flat and exhibited a well known composites sudden death behavior. At higher fatigue loads,
the early failures were attributed to impingement on the static strength distribution. The
specimens surviving the predefined infinite life of 150,000 cycles were further tested for
degradation in residual strength.