Sailors and pilots a generation or two ago learned the hard way what is obvious to today's engineers: even the strongest materials have their breaking point.
During World War II, several huge cargo ships ripped apart and sank, sometimes before they had been launched, because the metals they were made with broke apart along continuous welds. And in the 1950s a whole line of British aircraft was taken out of service because of structural problems.
Since then industry has become more aware of the problem of fracture in high-strength metals, but testing the limits of these materials has remained an expensive, cumbersome process. Now two University of Rochester engineers have come up with a tool they believe reduces the time and cost needed to test the materials.
Graduate student Eric Stromswold and his adviser, David Quesnel, have developed a "toughness tool" that quickly and reliably measures how well a material resists the growth of a fault. the toughness tool costs only a few hundred dollars and can be used to perform dozens of test each hour.
In a series of tests on such materials as soldered joints, high-strength steel alloys and high-strength aluminum, results from the toughness tool matched up closely with results obtained through testing procedures laid down by the American Society for Testing and Materials (ASTM).
"Most methods for testing are long and complicated, and they usually require a great deal of engineering judgment," says Quesnel, professor of mechanical engineering. "This simplifies the process. You put a sample in the toughness tool, and you snap it off. This tells you the maximum load it can bear, which for many materials is a good indicator of its toughness."
The tool is designed to measure how well a material resists a crack. "You need to know how long a crack can be before the material fails, and how fast the crack will grow," says Stromswold. "The answer will determine when a material needs replacing or how often inspections are necessary."
The test is useful for a variety of materials which often fail through fracture, such as high-strength steel, aluminum, and titanium. Such materials are commonly used in airplanes, spacecraft, and even trucks -- wherever low weight and high strength are a priority. They're also used by makers of machine and cutting tools used in manufacturing.
The tool requires samples to be prepared with a "chevron notch," a small notch which predisposes a sample to crack sharply in a particular manner when placed under stress. The notch eliminates the need for pre-cracking the sample, which is normally done by repeatedly loading it over the course of several hours. Toughness tool samples are about two inches long and a half-inch wide, which is slightly smaller than those used in conventional (ASTM E399) tests.
"We wanted an instrument which could be used by the average person to break most materials," says Stromswold, who estimates that about 62 lbs. of force is enough to break samples of even the toughest materials using the toughness tool.
A sample is placed between the two identical wrench heads of the toughness tool, which is made of hardened steel and resembles a specially designed torque wrench. The lower wrench head is held fixed in a vise, while the operator exerts a steadily increasing force to the upper wrench head by pushing down on the handle. A plastic marker indicates the amount of force that was needed to break the sample.
Stromswold and Quesnel acknowledge that the toughness tool may not be ideal for all testing situations. In particular, a more elaborate procedure might be called for if an engineer wants full details on the fracture mechanics of a certain material. The tool's greatest use may be for quality control, where technicians must monitor large numbers of parts quickly. "This would allow even small companies to test their incoming materials for fracture toughness before manufacturing," says Quesnel.
A paper on the toughness tool by Stromswold and Quesnel, "An Engineering Tool for Fracture Toughness Testing," has been accepted for publication in the journal Engineering Fracture Mechanics.
tr