Creaking and Cracking With Fatigue, the Structures of Our World Need Therapy
Blacksmiths called it Krupp Krankheit , or “Krupp’s disease.” Early metallurgists blamed it on the disintegration of a metal’s “intergranular cement.”
It has torn apart planes, made bridges collapse and cracked nuclear reactor components. After it recently turned an Aloha Airlines jetliner into a flying convertible, the peculiar phenomenon of metal fatigue has become a headline topic. What is this metal disease and how endangered are we by it?
Fatigue has been around since the Bronze Age, when artisans first observed that metals (and, it turns out, essentially all materials) subjected to cyclic stresses will eventually fail--no matter how low the stress levels. While a metal can be stressed indefinitely at relatively high loads applied in only one direction, the simple process of repeatedly reversing the direction of the stress will lead to cracking and rupture. And the greater and more frequently cycled the load, the faster the failure.
Airplanes, bridges, automobiles, elevators and dozens of other technological contraptions in which we entrust our lives daily are constantly fatiguing: Microscopic cracks form and grow in materials undercyclic stress. Eventually they link up. That can lead to disaster. The same process that makes it possible to break a coat hanger or a paper clip by bending it rapidly back and forth is causing fatigue in the most sophisticated of structures and devices.
There is no cure for Krupp’s disease, but its cause is fairly well understood. Atomic-size faults known as dislocations apparently arrange themselves under cyclic conditions to form microcracks. If these faults can be immobilized, then the cracking process can be slowed. Over the past few decades technologists have developed several alloys and materials treatment techniques that can accomplish this--at least for a time. This means that not every plane or bridge is a disaster waiting to happen. But, just like the human beings they serve, our machines and structures are aging and fatigue is a prime cause.
The first commercial jetliner went into service in this country 30 years ago; some of the aircraft still in operation date from that period. The Aloha plane--built in 1969--was one of the oldest Boeing 737s in operation. Most planes in the sky, however, were built with advanced, fatigue-resistant materials that have generally held up well.
Virtually the opposite is true for another--and much larger--set of transportation structures: the nation’s bridges. There reportedly is one bridge failure every two days somewhere in this country. Many of our 230,000 bridges are corroding and fatiguing dangerously; one-fourth of them may be unsafe. Since we are upgrading our bridges at a rate of about 3,000 a year, it will take decades to reverse the aging--and potential dangers--in all of the country’s spans. In nuclear reactors, another group of rapidly aging technological devices, fatigue brought on by cyclic thermal stressing has resulted in cracking in major components, including fuel elements.
What must be done to minimize the threat posed by aging materials is clear: More inspections using advanced detection techniques, more repairs, more replacements and stepped-up monitoring of fatigue-prone structures are called for. This means designing more effective and expanded programs in the Federal Aviation Administration, Federal Highway Administration, Nuclear Regulatory Commission, local and regional public works departments and other government agencies charged with protecting public safety. In addition, research and development in both the private and public sectors aimed at controlling fatigue and other materials debilitations must be accelerated.
This means spending more money, including more taxpayer money, which is not a universally appealing thought in today’s deficit-ridden economy. But these expenditures must be looked upon as preventive medicine: It will cost far less in dollars, injuries and lives to prevent fatigue-related disasters than to suffer from them.
Metal fatigue is another reminder that our technological sophistication has its limits. We must not forget that even streamlined, high-tech devices made of complex aluminum and steel alloys, just like the flesh, can get old and weak. For someone who spent the better part of six years in a university laboratory breaking things by fatigue, this is not easy to forget: The memory of long nights spent listening to a loud, oscillating fatigue tester--disturbingly similar to the up-and-down droning sound of an airplane as it plies the night sky--waiting for a test specimen to crack (it always did, sometimes much sooner than expected) is a powerful reminder of the importance of properly maintaining our aging technological infrastructure.
