{"id":55780,"date":"2019-05-28T15:03:55","date_gmt":"2019-05-28T22:03:55","guid":{"rendered":"http:\/\/in.nau.edu\/news\/?p=55780"},"modified":"2019-05-28T16:14:12","modified_gmt":"2019-05-28T23:14:12","slug":"feigenbaum-ductile-metals","status":"publish","type":"post","link":"https:\/\/in.nau.edu\/news\/feigenbaum-ductile-metals\/","title":{"rendered":"\u2018Major step forward\u2019: NAU engineer developing a more accurate failure prediction model for ductile metals"},"content":{"rendered":"\n

May 28, 2019<\/p>\n\n\n\n

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

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

Heidi Feigenbaum<\/strong>, professor in Northern Arizona University\u2019s Department\nof Mechanical Engineering<\/a>,\nrecently received a $544,758 grant from the U.S. Department of the Army to\ndevelop a mathematical model that will more accurately predict how deformation\nwill accumulate and materials will ultimately fail under cycles of twisting,\npulling and pushing. Such a mathematical model has the potential to reduce structure\nweight, minimize cost, maximize material efficiency and ultimately ensure the\nArmy will have safer and more reliable systems. <\/p>\n\n\n\n

Feigenbaum will\nwork with fellow mechanical engineering professor Constanin <\/strong>Ciocanel<\/strong> and graduate and undergraduate students on\nthe project. She also will collaborate with engineers at the University of\nCalifornia, Davis and the Institute of Thermomechanics,\nAcademy of Sciences in the Czech Republic, who are funded through different\nsponsors.<\/p>\n\n\n\n

Improving\nsafety, reliability and material efficiency<\/strong><\/h4>\n\n\n\n

\u201cWe will develop a means to predict and\nanalyze ratcheting failure in ductile metals such as steel and aluminum, which\ncan be leveraged to improve safety, reliability and material efficiency in a\nwide variety of structures and systems,\u201d Feigenbaum\nsaid.<\/strong> \u201cThis fundamental research will be a\nmajor step forward in the field, as despite years of research, predictions made\nwith state-of-the-art models differ dramatically from experimental findings.\u201d<\/p>\n\n\n\n

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

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

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

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

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

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

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

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

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