{"id":5038,"date":"2019-10-08T10:56:31","date_gmt":"2019-10-08T17:56:31","guid":{"rendered":"https:\/\/nau.edu\/mechanical-engineering\/?p=5038"},"modified":"2020-04-30T08:58:52","modified_gmt":"2020-04-30T15:58:52","slug":"major-step-forward-nau-engineer-developing-a-more-accurate-failure-prediction-model-for-ductile-metals","status":"publish","type":"post","link":"https:\/\/in.nau.edu\/mechanical-engineering\/major-step-forward-nau-engineer-developing-a-more-accurate-failure-prediction-model-for-ductile-metals\/","title":{"rendered":"\u2018Major step forward\u2019: NAU engineer developing a more accurate failure prediction model for ductile metals"},"content":{"rendered":"

Every day, Americans rely on engineered systems such as bridges, buildings and offshore structures that are subject to extreme weather, earthquakes and a range of other conditions. Knowing how materials used in these systems respond to stressors is crucial to creating components that successfully and efficiently do the job.<\/p>\n

Engineers rely on mathematical models to predict how the materials they use will react to stressors. However, some \u201cstate-of-the-art\u201d models for ductile metals that have been in place for years have done an inadequate job of predicting when and how these materials will eventually fail when exposed to repeated multidirectional permanent deformation, a condition known as multiaxial ratcheting.<\/p>\n

Heidi Feigenbaum<\/strong>, professor in NAU\u2019s Department of Mechanical Engineering<\/a>, recently received a $544,758 grant from the U.S. Department of the Army to develop a mathematical model that will more accurately predict how deformation will accumulate and materials will ultimately fail under cycles of twisting, pulling and pushing. Such a mathematical model has the potential to reduce structure weight, minimize cost, maximize material efficiency\u2014and ultimately ensure that the Army will have safer and more reliable systems.<\/p>\n

Feigenbaum will work with NAU professor Constanin <\/strong>Ciocanel<\/strong> and graduate and undergraduate students on the project. She will also collaborate with engineers at the University of California, Davis and the Institute of Thermomechanics, Academy of Sciences in the Czech Republic, who are funded through different sponsors.<\/p>\n

Predictive model can be leveraged to improve safety, reliability and material efficiency<\/strong><\/p>\n

\u201cIn this work,\u201d said Feigenbaum, \u201cwe will develop a means to predict and analyze ratcheting failure in ductile metals such as steel and aluminum, which can be leveraged to improve safety, reliability and material efficiency in a wide variety of structures and systems. This fundamental research will be a major step forward in the field, as despite years of research, predictions made with state-of-the-art models differ dramatically from experimental findings.\u201d<\/p>\n

Developing a predictive model is difficult because many variables are involved, such as the load itself, how the material was loaded in the past and whether and how outside influences might affect it.<\/p>\n

\u201cWhen you load a metal but don\u2019t load it too much and then remove the load, it goes back to its original shape. But if you load it too much, it permanently deforms, and that\u2019s called plastic deformation,\u201d Feigenbaum said.<\/p>\n

Ratcheting occurs when metals are subject to cyclic plastic deformation.<\/p>\n

\u201cPredicting ratcheting is actually very challenging to model mathematically because the deformation depends on how much you load it, how it was previously loaded and what other stresses it experiences in the process,\u201d Feigenbaum said. \u201cPipes are a good example. If you load one with internal pressure and then twist it and pull on it, it behaves very differently than if you twist it, pull it and then load it with internal pressure. The order and history matter when it comes to permanent deformation of metals.\u201d<\/p>\n

Ratcheting can lead to failure in metal systems exposed to extreme weather, earthquakes or repetitive mechanical or thermal conditions. For example, bridges are stressed by everyday heating and cooling. Airplanes are stressed by changing pressure in altitude as they move up and down. Feigenbaum suspects that current predictive models fall short precisely because of the cyclical nature behind ratcheting.<\/p>\n

\u201cWe think that these models might be just a little off in the initial prediction, but because the loading and stress repeat many times, the prediction\u2019s error increases,\u201d she said. \u201cI don\u2019t mean hundreds or thousands of cycles\u2014I mean dozens. Let\u2019s return to the pipe example. It fills and empties time and time again. Imagine it\u2019s full and an earthquake hits. That earthquake causes the pipe to twist and pull and perhaps elongate over and over, and then it subsides. If our prediction of how the material will react is slightly off but repeats with every cycle, that error grows.\u201d<\/p>\n

According to Feigenbaum, because the current models haven\u2019t accurately predicted ratcheting, engineers have overdesigned systems to avoid any risk. That leads to heavier, more expensive and potentially less efficient equipment and structures.<\/p>\n

\u201cImagine I\u2019m a pipe designer. I know ratcheting might occur\u2014an earthquake or some unexpected cyclic loading\u2014so I\u2019ll make my pipe three times thicker than I\u2019m sure it needs to be,\u201d she said. \u201cThat\u2019s often the solution: Let\u2019s overdesign to avoid any permanent deformation. We overuse materials because we cannot otherwise ensure safety. If we could predict the safety without that overdesign, we could be much more efficient with our use of materials.\u201d<\/p>\n

Feigenbaum\u2019s team plans to use continuum mechanics and thermodynamics principles to investigate ratcheting and come up with a rigorous yet simple model that can be applied to a variety of metals and loading conditions. To do this, the team will address two questions:<\/p>\n