![]() This project took Allen two years to complete, and it involved setting up scripts to automate the building of 600 nonlinear models and solutions, covering almost any metal one would ever use. Here, an RS-25 undergoes a hot-fire test. Technology Transfer TASC has been used to determine the fracture toughness of the Space Launch System’s core stage rockets, which will comprise four modified RS-25s-the engines used to launch the Space Shuttle. “I wanted to open up the standard to a whole new group of labs that can run these types of tests for these different industries,” he says. The standard is used not only by NASA but also by airlines for aircraft safety and by oil companies to protect against pipe leaks and explosions, among others. In early 2012, he decided to create a software program that could automate the process, which is based on the American Society for Testing and Materials (ASTM) standard for testing surface cracks, E2899, that he and fellow NASA Marshall engineer Doug Wells had developed and tested over a 10-year period. “Traditionally, you had to have somebody specifically skilled in nonlinear fracture mechanics and material behavior, and he or she needs to run an analysis of that test, which takes a long time and costs a lot of money,” says Allen, noting that it’s typical for such an endeavor to take upwards of eight hours. To come up with the surface crack fracture toughness for plastic-deforming metals, engineers perform laboratory testing combined with what’s called nonlinear finite element analysis, which requires expertise in several niche fields. ![]() Once you exceed their ultimate capability, they just splinter and fail rapidly.” “They’re very stiff and brittle in nature. Then, if you cycle it back and forth enough times, it’ll break.” Contrast that with other materials like glass or some carbon-fiber composite materials that have basically a purely linear response. “You’ve pushed it beyond its linear limit into the plastic regime, into permanent deformation. “If you bend it far enough, it won’t straighten back out,” he explains. Phillip Allen, a materials engineer and structural analyst at Marshall Space Flight Center, says to think of a paperclip. (Even when cracked, depending on the circumstances, metals can continue to be used on flights before needing repair.) This is the amount of permanent deformation they can withstand without failing. To derive fracture toughness for lightweight metals-aluminum and titanium alloys, for example, which are used in airplane and spacecraft hardware-plastic deformation must be taken into account. The software cuts the time needed to perform an analysis from several hours to 20 or 30 minutes. Such a measurement helps engineers assess the point at which a crack will begin to tear and fail structurally. Marshall Space Flight Center materials engineer Phillip Allen developed the free, downloadable program Tool for Analysis of Surface Cracks, or TASC, to simplify determining a metal’s fracture toughness. ![]() To perform such work, engineers first have to determine each material’s fracture toughness-its ability to resist fracture when it contains a crack. By knowing these properties, they can ensure that these spacecraft and structures are safe for flight, which means determining if existing defects, such as cracks, will cause structural failure. When NASA builds a spacecraft, materials engineers at the Agency have the important task of assessing the structural properties of the materials that comprise various hardware, whether it’s the hull of a crew capsule or the external tank of a rocket.
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