Journal of Accident Investigation
Journal of Accident Investigation
Journal of Accident Investigation
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MATTHEW R. FOX, CARL R. SCHULTHEISZ, JAMES R. REEDER, AND BRIAN J. JENSEN<br />
the surface for matrix rollers (pieces <strong>of</strong> fractured matrix<br />
material rolled into cylindrical shapes by the relative motion <strong>of</strong><br />
the fracture surface during cyclic loading), which would have<br />
indicated fatigue. Fracture surfaces <strong>of</strong> the remaining samples<br />
were cleaned ultrasonically in water before being coated with a<br />
conductive layer <strong>of</strong> gold and palladium. 6 Typically, delamination<br />
samples about 2 inches square were taken from widely spaced<br />
areas on the exposed fracture surfaces in an effort to identify<br />
overall trends. (See table 1.) Samples were also taken from<br />
areas where the delamination surface morphology changed<br />
(mostly at the ends <strong>of</strong> plies in the lay-up) to explore for local<br />
differences in stress state or crack propagation direction.<br />
Investigators took more than 300 SEM photographs <strong>of</strong><br />
translaminar fractures in the main attachment areas <strong>of</strong> the<br />
vertical stabilizer and examined more than 1 0 square inches<br />
<strong>of</strong> the delamination surfaces at high magnification. For<br />
translaminar fractures intersecting the lug attachment hole, they<br />
examined the entire fracture surface at high magnification, and<br />
for translaminar fractures above the lugholes, they examined<br />
several inches <strong>of</strong> the total extent <strong>of</strong> the fracture.<br />
One challenge facing investigators during the fractographic<br />
analysis was the relatively small amount <strong>of</strong> reference material<br />
dealing specifically with fractographic examination <strong>of</strong><br />
fabric-reinforced composites. Most <strong>of</strong> the literature describing<br />
fractography <strong>of</strong> composites focuses on unidirectional tape<br />
lay-ups. However, fabrics have unique characteristics, such as<br />
variation in resin content on delamination surfaces and less<br />
fiber pullout in translaminar fractures relative to tape-reinforced<br />
materials, as investigators found in the accident airplane. For<br />
example, in the unidirectional lay-ups reported in the literature,<br />
river marks were typically only observed in Mode I (opening<br />
displacement between fracture faces) loading. However, in the<br />
fabric construction <strong>of</strong> the accident airplane where evidence <strong>of</strong><br />
Mode II (sliding displacement between fracture faces) loading<br />
was observed, river marks were also found in the matrix-rich<br />
areas near the bundle crossings, and in the base <strong>of</strong> hackles 7 where<br />
a bundle at one orientation transitioned to a perpendicular<br />
crossing bundle. River marks in the bundle crossings were used<br />
to identify a general direction <strong>of</strong> fracture propagation upward<br />
and aftward for both <strong>of</strong> the large delaminations (at the forward<br />
left and aft left attachments). (Investigators also explored the<br />
river marks at the base <strong>of</strong> the hackles during their examination<br />
6 A. Sjögren, L.E. Asp, and E.S. Greenhalgh, Interlaminar Crack Propagation<br />
in CFRP: Effects <strong>of</strong> Temperature and Loading Conditions on Fracture<br />
Morphology and Toughness, in Composite Materials: Testing and Design, and<br />
Acceptance Criteria ASTM STP 1416, Nettles and Zureick, eds., 2002.<br />
7 “Hackles are matrix fracture features that indicate a significant<br />
component <strong>of</strong> shear across the fracture surface. Hackles are formed<br />
when matrix microcracks that are spaced fairly regularly along planes <strong>of</strong><br />
maximum tension join together.” National Transportation Safety Board,<br />
In-Flight Separation <strong>of</strong> Vertical Stabilizer, Aircraft <strong>Accident</strong> Report NTSB/<br />
AAR-04/04, NTSB Public Docket (Washington, DC: NTSB, 2004).<br />
<strong>of</strong> the delaminations at the forward right lug as described later<br />
in this paper.) Because manufacturers are increasing their use<br />
<strong>of</strong> composites with fabric reinforcements in airplane structures,<br />
more research is needed to characterize fracture surfaces<br />
generated under controlled laboratory conditions to help failure<br />
analysts in interpreting fractographic details.<br />
Fracture Surface Observations and Discussion<br />
During the visual examination, investigators found that the<br />
vertical stabilizer was largely intact with no significant areas <strong>of</strong><br />
skin buckling. An overall view <strong>of</strong> the vertical stabilizer as it<br />
was being recovered from the water <strong>of</strong> Jamaica Bay is shown in<br />
figure 4. At the lower end, each <strong>of</strong> the six attachment locations<br />
had separated from the fuselage either by fractures that<br />
intersected the lug attachment hole or by fractures through the<br />
structure above the hole. A schematic <strong>of</strong> the lower end <strong>of</strong> the<br />
vertical stabilizer is shown in figure , which shows a general<br />
fracture location for each lug, pointing to overall views <strong>of</strong> each<br />
lug fracture. Portions <strong>of</strong> rib 1, the rib 1 rib-to-skin attach angle,<br />
and the lower end <strong>of</strong> the forward spar also were fractured. In<br />
addition, the trailing edge panels were damaged in several<br />
locations. 8<br />
Description <strong>of</strong> Main Lug Fractures<br />
Translaminar fractures on the right aft, right forward, and left<br />
forward main lugs intersected the attachment hole. For the<br />
remaining three main lugs, translaminar fractures intersected<br />
the structure above the lug. Each <strong>of</strong> the lugs had delaminations<br />
in the lug area and/or in the structure above the lug. Safety<br />
Board Materials Laboratory factual reports contain details <strong>of</strong><br />
the fractographic examination. 9 Some <strong>of</strong> the delaminations<br />
extended into the main portion <strong>of</strong> the vertical stabilizer,<br />
and the extent <strong>of</strong> these delaminations was determined using<br />
nondestructive inspection (NDI), including ultrasonic<br />
inspection and x-ray-computed tomography scanning and<br />
imaging. Safety Board Materials Laboratory factual reports<br />
contain the results <strong>of</strong> the NDI. 10<br />
MACROSCOPIC FRACTURE FEATURES<br />
On the right side <strong>of</strong> the vertical stabilizer, the roughness <strong>of</strong> the<br />
main lug translaminar fractures was in general consistent with<br />
overstress fracture in primarily tensile loading. Delaminations<br />
8 National Transportation Safety Board, Materials Laboratory Factual Report<br />
02-083, NTSB Public Docket, 2002.<br />
9 (a) NTSB, Materials Laboratory Factual Report 02-083, NTSB Public<br />
Docket, 2002; (b) NTSB, Materials Laboratory Factual Report 03-018,<br />
NTSB Public Docket, 2003.<br />
10 (a) NTSB, Materials Laboratory Factual Report 02-078, NTSB Public<br />
Docket, 2002; (b) NTSB, Materials Laboratory Factual Report 03-033,<br />
NTSB Public Docket, 2003.<br />
14 NTSB JOURNAL OF ACCIDENT INVESTIGATION, SPRING 2006; VOLUME 2, ISSUE 1