The Comet’s Resonance

aircraft window design has improved fatigue resistance

A Modern Aircraft Window (Image courtesy of podpad at FreeDigitalPhotos.net)

A couple of months ago, there was an anniversary that might not be very well-known: July 27, 1949. It is a date as momentous for air travel as it is for the advancement of the field of fatigue and fracture mechanics. On this date, the de Havilland Comet, the world’s first jet airliner designed and built for commercial passengers, underwent its first test flight in Hertfordshire, England. The prototype performed admirably, and paved the way for the Comet’s entry into service by the British Overseas Airways Corporation in 1952. The designs of the Comet 1 and 1A aircraft were revolutionary, with two de Havilland Ghost turbojet engines built into each wing, a pressurized cabin for the comfort of 44 passengers, and large square windows yielding a generous visual perspective that was rarely seen by civilians before that time. Unfortunately, it was the convergence of the last two features (pressurization and square-shaped windows) that led to a series of fatal crashes in the first two years of the Comet’s service. The entire fleet was grounded in 1954 while investigations took place, the results of which concluded that repeated pressurization/re-pressurization caused cracks to initiate and grow at the corners of the planes’ square windows. During each pressurization cycle, the fuselage’s metal was being further “fatigued” with cracks originating from locations of high “stress concentration” at these window corners. The terms “fatigue” and “stress concentration” were relatively new at the time, as materials science (as we now know it) was still a new field of study. The Comet was redesigned in subsequent years, with oval windows and other safety improvements, but by then (the late 1950s) the market had been overtaken by Boeing’s larger and longer-range 707 model. Boeing went on to dominate the commercial airliner industry for decades to come.

The Comet’s legacy is not completely negative however; these early failures helped develop the backbone of fatigue and fracture mechanics that would be used, refined and evolved over the next 70 years. Indeed, it was only 20 years after that first test flight of the Comet that NASA’s engineering team supported a successful moon landing! Industries beyond aviation and space exploration have benefited from this science as well: heavy machinery, transportation, naval, energy, medical devices…all have been fundamentally changed by the furtherance of materials science principles.

VEXTEC continues this evolutionary effort, by incorporating these “physics of failure” principles into our probabilistic Virtual Life Management® technology. We differentiate ourselves from other computational fatigue methods, by combining a component’s inherent microstructural variability with physics-based damage mechanisms and realistic loading histories to accurately predict fatigue life. As structures become increasingly more complex, with continual demands for lighter-weight materials (for both manufacturing and operational cost savings) and better performance, the need for a comprehensive reliability simulation technology becomes clear. No one wants to be the next disastrous chapter in this Comet’s Tale.

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