Computational Evolution of Strain Localization through Grain Boundaries in Irradiated Polycrystals

Lame Jouybari, Amirhossein (author), Cizelj, Leon (mentor), El Shawish, Samir (comentor)

In this thesis, the mechanisms of plastic strain localization in neutron-irradiated polycrystalline materials are extensively examined with their connection to Irradiation-Assisted Stress Corrosion Cracking (IASCC). New theoretical and numerical advances are introduced within the framework of Strain Gradient Crystal Plasticity (SGCP) that predict the formation and evolution of slip and kink bands, commonly termed “clear channels”. A novel enhanced theory (Enhanced SGCP) is proposed, in which the amplitude of the gradient term in the free energy expression is specified as a function of the local strain, thereby implicitly capturing microstructural evolution. This formulation introduces a variable length scale that enables control over the shape and width of the bands. With an appropriately chosen dependence, the bands become not only regularized but also overcoming limitations of classical SGCP models, in which band spreading and loss of plastic strain localization are observed at higher loads. Within the Enhanced SGCP framework, two models are introduced that differ in the chosen gradient contribution to the free energy. The Enhanced MicroSlip-SGCP model regularizes both types of bands, whereas the Enhanced MicroCurl-SGCP model regularizes only kink bands. In both models, the effects of grain size, the length scale, the boundary conditions at grain boundaries on macroscopic hardening, the microscopic evolution of dislocations, and stress concentrations at grain boundaries are examined in detail. Using an in-house computational code, an efficient framework based on the Fast Fourier Transform is established, enabling rapid solution of the highly nonlinear equations of the Enhanced SGCP theory at high resolutions. The results are validated against analytical solutions and a commercial finite-element code. Simulations indicate that, under MicroHard conditions, mechanical stresses at grain boundaries increase markedly, supporting the hypothesis of intergranular cracking consistent with IASCC mechanisms. Qualitatively, the results agree with experimental observations in irradiated stainless steels and zirconium alloys.

• Language: English
• Keywords: Strain Gradient Crystal Plasticity, Irradiated Polycrystalline Materials, Grain Boundary, FFT-based method, Plastic Strain Localization, Slip Band, Kink Band, Irradiation-Assisted Stress Corrosion Cracking Cracking.
• Work type: Doctoral dissertation
• Typology: 2.08 - Doctoral Dissertation
• Organization: FMF - Faculty of Mathematics and Physics
• Year: 2026
• PID: 20.500.12556/RUL-178802
• COBISS.SI-ID: 267309315
• Publication date in RUL: 30.01.2026

Secondary language

• Language: Slovenian
• Title: Računski razvoj lokalizirane deformacije preko kristalnih mej v obsevanih polikristalih

Abstract

V disertaciji obširno obravnavamo mehanizme lokalizacije plastične deformacije v obsevanih polikristaliničnih materialih ter njihovo povezavo z nastankom napetostne korozije pod vplivom nevtronskega sevanja (angl. IASCC). Uvedemo nove teoretične in numerične izboljšave, s katerimi na osnovi teorije kristalne plastičnosti z gradientom deformacije (angl. SGCP) napovemo nastanek in razvoj drsnih ter pregibnih lokaliziranih deformacijskih pasov, ki so znani kot »čisti kanali«. Predlagamo novo razširjeno teorijo SGCP, v kateri amplitudo gradientnega člena v izrazu za prosto energijo definiramo kot funkcijo lokalne deformacije, s čimer posredno zajamemo razvoj mikrostrukture. Tako uvedemo spremenljivo dolžinsko skalo, ki omogoča nadzor nad obliko in širino simuliranih pasov. S primerno izbrano odvisnostjo postanejo pasovi ne le regularizirani, temveč tudi konstantne širine, s čimer presežemo omejitve klasičnih modelov SGCP, kjer pri večjih obremenitvah opazimo širjenje pasov oz. izgubo lokalizacije plastične deformacije. V okviru razširjene teorije SGCP predstavimo dva modela, ki se razlikujeta po izbranem gradientnem popravku proste energije. Model MicroSlip-SGCP regularizira oba tipa pasov, medtem ko model MicroCurl-SGCP regularizira le pregibne pasove. V obeh modelih podrobno preučimo, kako velikost kristalnih zrn, dolžinska skala in robni pogoji na mejah kristalnih zrn vplivajo na makroskopsko utrjevanje, mikroskopski razvoj dislokacij ter koncentracijo napetosti na kristalnih mejah. Na podlagi lastne računske kode vzpostavimo učinkovit računski okvir, ki temelji na metodi hitre Fouriereve transformacije in omogoča hitro reševanje močno nelinearnih enačb znotraj razširjene teorije SGCP pri visokih ločljivostih računske mreže. Rezultate validiramo z analitičnimi rešitvami ter s komercialnim programom na osnovi metode končnih elementov. Simulacije pokažejo, da se pri pogojih MicroHard mehanske napetosti na kristalnih mejah močno povečajo, kar podpira hipotezo o nastanku medkristalnih razpok v okviru IASCC. Kvalitativno se rezultati ujemajo z eksperimentalnimi opažanji v obsevanih nerjavnih jeklih in cirkonijevih zlitinah.

• Keywords: kristalna plastičnost z gradientom deformacije, obsevani polikristalinični materiali, kristalna meja, metoda na osnovi hitre Fouriereve transformacije, lokalizacija plastične deformacije, drsni pas, pregibni pas, napetostna korozija pod vplivom nevtr