شبيه سازي رفتار ميكرو ساختار فولاد دوفازي تحت كشش

چكيده انگليسي :

Microstructural Modeling of Dual Phase Steel During Tensile Test Shakib Katani Email s katani@me iut ac ir Date of submission February 28 2012 Department of Mechanical Engineering Isfahan University of Technology Isfahan 84156 83111 IranDegree M Sc Language Farsi Supervisor Saeed Ziaei Rad Email szrad@cc iut ac irAbstractNowadays in material engineering the use of lightweight components is a basic matter For developingmaterial properties and also due to the economical and ecological aims the mass of a structure should bereduced and at the same time its structure quality should be strengthened Up to now multiphase steels suchas DP dual phase steels offer high strength and ductility and have widely used in automotive industry Theincreased strength of multiphase steels is the consequence of grain refinement and precipitation hardening bycoexisting of softer and harder phases and various grain sizes The steel used in this study is AISI 5115 thatoffer impressive mechanical properties such as continuous yielding behavior high work hardening rate andsuperior strength ductility combination which is made it suitable for machine elements In this study thedual phase steel was fabricated by a special heat treatment procedure A series of experiments were thencarried out on the produced steel to obtain its mechanical characteristics and to make it ready forphotographing in micro dimensions Next using scanning electron microscope SEM pictures were takenfrom some regions of the manufactured steel Two sets of finite element models were then created Thegeometry of the first set was created from the SEM images of the material This method was developed toseparate grain boundary from ferrite and martensite phases with a relatively high accuracy Since theboundaries have significant role in material behavior it is also need to be recognized between the materialphases At the end of this stage the pictures were mapped into a matrix involves arrays which show the typeof the phases and boundaries Another method based on DEM is used to create random models Thistechnique is used a Voronoi type algorithm to construct geometries which are statistically similar to the SEMimages As it is known ferrite is a ductile phase and fractures with dimpled features due to the voidformation and coalescence On the other hand martensite is a brittle phase and fractures in cleavage mode Considering the fracture surface of material the Gurson model is an appropriate model for simulating thedamage inside the ductile material Therefore it is a good idea to represent the ferrite and boundary using aGTN model and martensite with elastoplastic behavior In the next step parameters of GTN are determined The representative material volume RMV is modeled by two approaches 1 a unit cell containing adiscrete spherical void at its center and 2 a unit cell having the same void volume fraction which obeys theGurson Tvergaard constitutive relation The macroscopic stress strain response and the void growth andcoalescence behavior of the voided cell are obtained from detailed finite element analyses and the resultsshow strong dependencies on stress triaxiality and the initial void volume fraction The micromechanicsparameters of the GT model q1 and q2 are calibrated to minimize the differences between the predicted voidgrowth rate and macroscopic stress strain relation by the GT model and the corresponding finite elementresults of the voided RMV Results of simulations obtained from SEM images shows that the simulation dataare in good agreement with experimental results and also analysis of simulation clearly show that thedeformation of the material is accompanied by the nucleation of voids that were originally not present in thematerial It is showed that voids are initiated from the boundary which is between two crystals commonlywith different phases ferrite and martensite The obtained results from the finite element model show thatthe failure always happen in boundaries and then growth ov

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شبيه سازي رفتار ميكرو ساختار فولاد دوفازي تحت كشش

آناليز تصاوير , مدل آسيب گرسون -تورگارد-نيدلمن , مدل سازي ميكروساختارها , مكانيزم شكست