Fatigue and creep wearout failures in electronics—a historical retrospective
Fatigue and creep wearout failures in electronics—a historical retrospective
Fatigue—lessons (maybe) learned
Fatigue, and the lack of proper
understanding of the fatigue mechanism
and the resulting damage, has historically
caused spectacular catastrophes with the
concomitant losses of human lives and
economic setbacks. ‘Liberty’ ships broke in
half and bridges failed. One of the more
infamous examples is the first jet airliner,
the de Havilland ‘Comet’ shown in Figure
3.
The Comet was built by de Havilland
in Great Britain and was the first jet
aircraft for commercial passenger service. It
was put in service on May 2, 1952, serving
on BOAC’s London-to-Johannesburg
route, and the first disaster diagnosed as
fatigue-related occurred on January 10,
1954. A Comet broke up in mid-air at Figure 2. Manson-Coffin Plot for an electrodeposited copper.
8,000 m (26,000 ft) over deep water in the
Mediterranean Sea off the island of Elba,
because the localized plastic deformation followed the then-accepted practices, the
of Napoleon’s exile fame, with the loss of
accommodated some of the strain, and state of the art was simply inadequate to
35 passengers and crew
2,3
.
the remainder of the strain resulted provide the insights for this extension
An investigation was begun, and all
primarily in fatigue loading in the elastic in engineering application, and the
Comets were grounded. A search for the
regime. Thus, the test section of the cabin unfortunate testing sequence seemingly,
plane wreck was unsuccessful because
survived 16,000 simulated flight fatigue but incorrectly, reassured de Havilland of
of the water depth at the crash site. On
cycles—equivalent to ~40,000 hours of the soundness of the design.
the very day, April 8, 1954, on which the
service—giving de Havilland confidence, The Comet disaster, which took
search off Elba was to be abandoned,
albeit false, that the aircraft cabin had the lives of some 500 people, set the
another Comet aircraft disintegrated in
a fatigue life well beyond that required European passenger aircraft industry back
mid-air, this time between Naples and
for the planned design life of 10 years or by almost 30 years, allowing American
Sicily.
10,000 flights
5
. manufacturers Boeing, Douglas and
Because the maximum flight altitude
Unfortunately, the airframes of the Lockheed, who in the 1950’s were behind
of the Comet was about 13,700 m (45,000
planes put in service did not have the in their development efforts, to dominate
ft), whereas previous commercial propeller-
benefit of a prior overstress test, and the market. With some notable exceptions,
driven planes had typical cruising altitudes
they crashed after only 1,290 and 900 such as in 1955 the ‘Caravelle’ from
of about 6,100 m (20,000 ft), comfortable
pressurized flights, respectively. The SNCA du Sud-Est in France, the European
breathing of the passengers required
aircraft whose wreckage was discovered off aircraft manufacturers did not fully
pressurization to 56.9 kPa (8.25 lb/in
2
)
Naples had begun to crack at the corners recover until—after the heavily subsidized
relative to the outside environment
4
. It
of the aerial cut-outs and the sharp corners joint British-French development of the
was recognized that this much more severe
of the window and doors (note the square supersonic ‘Concord’ airplane, put in
compression-decompression cycle for each
windows in Figure 3). Tests subsequent to service in 1976—the A320 Airbus became a
flight would place the Comet fuselage
the crashes showed that the fuselage made commercial success in 1987.
under significantly higher cyclic loads
with a postcard-thin (0.7 mm/0.028 in) On April 28, 1988, the Boeing 737
than had been experienced by previous
skin had cut-outs, such as windows and of Aloha Flight 243 lost a 5.5 x 6.5 m (18
airplanes. Further, the early Comet
hatches, with corner radii too small to x 21 ft) section of the forward cabin roof
versions were designed as short-to-medium
prevent critical stress concentrations and shortly after reaching a cruising altitude
range aircraft; thus, the compression-
the airframes disintegrated as the result of of 7,300 m (24,000 ft)
6,7
. Fortunately, the
decompression cycle might be repeated
fatigue cracks originating at the window airframe of the Boeing 737 is significantly
five to six times a day. Consequently, a
and hatch corners
4,5
. more robust than was the Comet airframe,
fuselage test section was constructed for
While the de Havilland designers and the plane was able to turn around and
testing. Prior to the pressure cycling tests,
most likely to save money and time, the
same fuselage was first overpressure tested
with multiple exposures to pressures as
high as 114 kPa (16.5 lb/in
2
)
5
. During the
overpressure tests, the weakest links in the
fuselage construction were unknowingly
subjected to loads high enough to cause
substantial local yielding and plastic
deformation. Subsequent fatigue testing no
longer had the same effect on the airframe
structure it would have had otherwise,
Figure 3. Photograph of de Havilland Comet jetliner.
www.globalsmt.net Global SMT & Packaging – September 2009 – 39
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