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.

Corrosion as the “Bad Guy”

Corrosion of a can

Image courtesy of sakhorn38 at FreeDigitalPhotos.net

The topic of corrosion makes recurring appearances in the media; it seems that when you hear about one corrosion-related problem, invariably there will be others reported-on at around the same time. There has recently been a spate of articles confirming that corrosion is currently a headache to the oil and gas sector (undersea bolt failures), as well as to the aviation sector (corrosion-induced fatigue of turbine engine blades in the new Dreamliner aircraft). Oftentimes these stories are first published by financial-leaning news outlets (Wall Street Journal, CNN Money, Bloomberg), a result of the high visibility and cost that these incidents bring in terms of replacement and downtime to their respective industries. Enough of these stories circulating over the span of a few news cycles will make any investor wary, and will prompt questions on what is being done from a regulatory standpoint to restore confidence in companies’ operations. This is particularly true when these reports of corrosion failures have impacts (real, or perceived) on public and environmental safety.

Of course, corrosion is not a new phenomenon. We have been observing the process of corrosion for centuries in our manmade structures, and have developed ways to physically mitigate its effects (painting, inspection methods, et cetera). However, it has only been in recent history that we a) have deeper understanding of the electrochemical processes that describe corrosion, and b) have the industrial engineering prowess to design and build ever greater machines and superstructures that help make modern life possible (economically-available energy sources and air travel, being prime examples). The confluence of these two factors drive the need for more development of mechanistic approaches to corrosion mitigation, through the use of computer-assisted modeling and simulation.

To that end, more and more resources are being appropriated for the research of these corrosion mechanisms in many of the materials that are used today. For example, members of the LIFT Consortium (Lightweight Innovations for Tomorrow) have begun work on the development of new models and a material properties database that will allow for more accurate simulations of corrosion in aluminum alloys used in aerospace and other transportation sectors (focusing on aluminum alloys containing copper, lithium, magnesium, manganese, and zinc). The materials database will be characterized to such a degree so that precise information is obtained about the interaction between microstructure and corrosion. The team will begin with the characterization of the industry’s workhorse alloys, and then extend work to evaluate newer alloys crated using various manufacturing techniques. The goal is to mitigate corrosion in a broad spectrum of aluminum alloys through improved simulator capabilities.

However, only half of the equation is being studied by LIFT: the corrosion impact on metals…with no discussion of how that corrosion introduces damage states, from which stress corrosion cracking and other types of corrosion-fatigue can arise. VEXTEC has pioneered development of a software for the U.S. Navy that predicts the statistical distribution of stress corrosion cracking in an alloyed aluminum microstructure that has been exposed to a corrosive environment. This software serves as a basis for all types of materials that are impacted by corrosion: the material modelers can provide the inputs of the corroded damage states into the VEXTEC software, which will in turn simulate the result of in-service loading on the durability of the critical structures of interest.

Until such time as corrosion has been completely removed as a mechanism in a critically-stressed component (and that time is not approaching anytime soon), it is enough to just model the corrosion characteristics…we must also be able to effectively model the subsequent damage growth throughout the component’s service life.

 

2012 TRENDS: DESIGN OF TURBINES FOR COST, AVAILABILITY AND RELIABILITY

Posted by: WindPowerEngineering.com

 By:   

With larger, more powerful turbines coming online, requirements for pitch control are increasing. Experts say the wind-power industry is also beginning to adopt aerospace test technology, the same kind used to vet airplane designs, to simulate loads on wind turbine blades before installation. Read more