Journal of Accident Investigation

Fractographic Examination Procedures and Challenges
For most common airplane structural metals, visual
inspection or low-power magnification is often sufficient to
determine fracture mechanism and direction. For metals, the
fracture plane, surface roughness, radial marks, chevrons, shear
lips, and general deformation all provide macroscopic clues to
the fracture mechanisms, direction of fracture propagation,
and relative motion of mating surfaces. Preexisting cracks in
metals often show staining or changes in color associated with
corrosion. 4 Using these clues, experienced investigators can
examine large areas of damaged structure relatively quickly to
identify fracture origins and areas requiring closer inspection.
However, composites by their nature present their own
set of challenges. Visual clues to preexisting fractures, such
as flat fracture surfaces with curving boundaries or staining
from corrosion, which are easy to see in structural metals, are
in general not as visible in composites. Furthermore, visual
cues to fracture propagation direction that are sometimes
apparent in composite structures, such as crack branching in
translaminar fractures (fractures that break fibers) or banding
in delaminations (fractures between layers), were not apparent
in many of the fractures of interest on the accident airplane.
Because visual cues were not present in many of the fractures,
3 Fractography is the examination of fracture surfaces and adjacent areas
to determine conditions that caused the fracture. See ASM Handbook,
Volume 11: Failure Analysis and Prevention, eds. W.T. Becker and R.J.
Shipley, ASM International, 2002.
4 K. Mills and others, eds., Fractography, ASM Handbook Vol. 12 (ASM
International: Metals Park, Ohio, 1987).
MATERIALS EXAMINATION OF THE VERTICAL STABILIZER FROM AMERICAN AIRLINES FLIGHT 87
two 45-degree fabric layers
two zero-degree tape layers
stringer outer flange layers
(zero-degree tape)
Figure 3. Microstructure of sample RS1. The cross-section shown is in a plane that is oriented parallel to the
plus or minus 45-degree fiber direction. Fiber and void content were determined using computer optical image
analysis of polished micrographs. The layup in each sample was determined from optical micrographs of the
specimens that were assembled into mosaics like the one shown here.
the composite fractures in the accident airplane required an
especially time-consuming examination because the area to
be examined using high magnification was substantially larger
than what is typically required for overstress fractures of similar
metal structures.
Investigators first conducted a visual inspection of the
translaminar fractures and delaminations. This examination
included mapping the fractures to help determine fracture
propagation directions from crack branching patterns, recording
features indicating translaminar fracture under tension or
compression, and, in the delaminations, identifying any visual
cues to changes in fracture mechanism or mode.
Using results of the visual examination as a guide, investigators
used scanning electron microscopy (SEM) to determine the
fracture mechanism and fracture propagation direction on the
translaminar fractures and on the delamination surfaces, and
to identify the layers involved, fracture mechanisms, modes of
fracture, and propagation directions. The SEM examination also
enabled investigators to distinguish between fatigue fractures
and preexisting cracks, which may appear similar during a visual
examination. Results of the SEM examination were used to
check construction of the vertical stabilizer and rudder against
the manufacturing drawings and to determine how the fractures
related to the loading of the overall structure.
Two samples, one from each of the two large delaminations,
were not cleaned, and were examined first in order to explore
R.J. Kar, “Atlas of Fractographs,” in Composite Failure Analysis Handbook
Volume 2: Technical Handbook (Northrop Corporation, Aircraft Division,
1992).
NTSB JOURNAL OF ACCIDENT INVESTIGATION, SPRING 2006; VOLUME 2, ISSUE 1 13