Influence of Anisotropy on Edge Fracture of Advanced High-strength Steels

Zinan Li

Influence of Anisotropy on Edge Fracture of Advanced High-strength Steels

Julkaisun otsikon käännös: Anisotropian vaikutus edistyneiden erittäin lujaan teräksen reunamurtumaan

Tutkimustuotos: Doctoral Thesis › Collection of Articles

Abstrakti

Edistyneet erittäin lujat teräkset (AHSS) on kehitetty erinomaisilla ominaisuuksilla laajoihin sovelluksiin autoteollisuudessa. Näistä kaksoisfaasiteräkset (DP-teräkset) ja karkaisun ja jakautumisen teräkset (QP-teräkset), jotka edustavat kehityksen ensimmäistä ja kolmatta sukupolvea, herättävät suurta kiinnostusta. Vaikka näillä materiaaleilla on parannetun lujuuden etuja, niiden muovattavuus muodostusprosesseissa, erityisesti voimakkaasti deformaation kokeneilla paikallisalueilla, muuttuu arvaamattomaksi. AHSS on alttiimpi reunamurtumille kuin perinteiset pehmeät teräkset, mikä johtuu paitsi vähentyneestä venyvyydestä myös erityisestä paikallisesta murtumakäyttäytymisestä. AHSS:n reunamurtumakäyttäytymisen syvällisempi ymmärtäminen edellyttää karakterisointimenetelmän löytämistä, joka tarkasti kuvaa murtumavauriota. Tutkimuksen tavoitteena on ennustaa reunamurtumia tarkasti huomioimalla usein laiminlyötyjä tekijöitä, kuten anisotropian kehitys, paikallinen plastinen lokalisaatio ja jännitystilariippuvainen murtumakäyttäytyminen. Tavoitteen saavuttamiseksi käytetään yhdistelmää kokeellisia ja numeerisia lähestymistapoja. Ensinnäkin suoritettiin joukko vetokokeita, joissa käytettiin erilaisia geometrioita ja kuormitussuuntia, DP- ja QP-terästen anisotrooppisten murtumaominaisuuksien karakterisoimiseksi. Seuraavaksi suoritettiin reiän laajennuskokeita (HET) reunamuovattavuuden arvioimiseksi sekä sileillä että leikatuilla reunoilla. Klassisen murtumakäyttäytymisen jännitystilariippuvuuden lisäksi tarkastelun keskiössä on anisotrooppinen käyttäytyminen sekä plastisuuden että murtuman osalta. Muuttuva ei-assosioitu Hill48-malli (enHill48) sovellettiin kuvaamaan anisotrooppista plastisuutta, kun taas anisotrooppiset murtumamallit kehitettiin kuvaamaan murtumakäyttäytymistä. Tutkimuksen uutuusarvo piilee siinä, että tutkituissa DP-teräksissä reunamurtumat johtuvat pääasiassa anisotropian aiheuttamasta muodonmuutoksen paikallistumisesta. Sen sijaan tutkituissa QP-teräksissä reunamurtumat ovat vähemmän herkkiä anisotrooppiselle plastisuudelle, mutta ne riippuvat vahvasti anisotrooppisesta murtumakäyttäytymisestä ja jännitystriaxialiteetista, joita hallitsevat murtumarajat. Osittain anisotrooppinen murtumamalli osoittautui tehokkaaksi DP-teräkselle anisotrooppisen plastisuuden ja lokalisaation kuvaamisessa, mutta sen kyky kuvata tarkasti murtumadominoitua paikallista muovattavuutta QP-teräksessä on rajallinen. Näin ollen QP-teräkselle ehdotetaan täysin anisotrooppista mallia, joka perustuu plastisten muodonmuutosten lineaariseen transformaatioon vaurion aloituskriteerissä. Tämä malli validoidaan sekä anisotrooppisen murtumakäyttäytymisen että reunamuovattavuuden osalta, ja sen ennustetarkkuus on parantunut. Havainnot korostavat anisotropian keskeistä roolia reunamurtumien arvioinnissa, mikä ei ainoastaan paranna mallinnuksen tarkkuutta vaan myös esittelee suunnittelukonseptin materiaalien reunamuovattavuuden parantamiseksi.

Julkaisun otsikon käännös	Anisotropian vaikutus edistyneiden erittäin lujaan teräksen reunamurtumaan
Alkuperäiskieli	Englanti
Pätevyys	Tohtorintutkinto
Myöntävä instituutio	Aalto-yliopisto
Valvoja/neuvonantaja	Lian, Junhe, Vastuuprofessori Salmi, Mika, Ohjaaja
Kustantaja	Aalto University
Painoksen ISBN	978-952-64-2205-3
Sähköinen ISBN	978-952-64-2206-0
Tila	Julkaistu - 2024
OKM-julkaisutyyppi	G5 Artikkeliväitöskirja

Tutkimusalat

reunamurtuma
anisotropia
paikallinen muovattavuus
sitkeä murtuma
edistyneet erittäin lujat teräkset
karkaisun ja jakautumisen teräs
kaksoisfaasiteräs

Pääsy asiakirjaan

http://urn.fi/URN:ISBN:978-952-64-2206-0

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@phdthesis{cfc03c97c65b4ba59fcde70b3bf5941d,

title = "Influence of Anisotropy on Edge Fracture of Advanced High-strength Steels",

abstract = "Advanced high-strength steels (AHSS) have been developed with superior properties for wide applications in the automotive industry. Among them, dual-phase (DP) steels and Quenching and Partitioning (QP) steels, as representatives from the first and third generations of development, are attracting great interest. Despite the benefits of the enhanced high strength, the formability of these materials becomes unpredictable in the forming processes, especially in the largely deformed local area. AHSS is more susceptible to edge fracture compared to traditional mild steels, due not only to reduced ductility but also to distinct local fracture behavior. To deepen the knowledge of the edge fracture behavior of AHSS, it is necessary to find a characterization method that accurately describes fracture failure. The study aims to accurately predict edge fracture by addressing often-overlooked factors such as anisotropy evolution, localization, and stress-state-dependent fracture behavior. To achieve this goal, a combination of experimental and numerical approaches is employed. First, a category of tensile tests with various geometries and loading directions was conducted to characterize the anisotropic fracture properties of DP and QP steels. Next, hole expansion tests (HET) were carried out to assess edge formability under both smooth and blanked edge conditions. In addition to the classic fracture dependency on stress states, the spotlight is on the anisotropic behavior in terms of both plasticity and fracture. The evolving non-associated Hill48 (enHill48) model is applied to describe anisotropic plasticity, while the anisotropic fracture models are formulated to represent fracture behavior. The novelty of this study lies in uncovering that the edge fractures of examined DP steel are primarily driven by anisotropy-induced strain localization, while the edge fractures of the investigated QP steel, though less sensitive to anisotropic plasticity, are strongly dependent on anisotropic fracture and stress triaxiality, governed by fracture limits. A partially anisotropic fracture model is found effective for DP steel in capturing anisotropic plasticity and localization but remains limited in accurately describing fracture-dominated local formability for QP steel. Consequently, a fully anisotropic model is proposed for QP steel, based on the linear transformation of plastic strain within the damage initiation criterion. This model is validated for both anisotropic fracture behavior and edge formability with improved prediction accuracy. The findings emphasize the crucial role of anisotropy in edge fracture assessment, which not only enhances modeling precision but also introduces a design concept to elevate material edge formability.",

keywords = "edge fracture, anisotropy, local formability, ductile fracture, advanced high-strength steels, quenching and partitioning steel, dual-phase steel, reunamurtuma, anisotropia, paikallinen muovattavuus, sitke{\"a} murtuma, edistyneet eritt{\"a}in lujat ter{\"a}kset, karkaisun ja jakautumisen ter{\"a}s, kaksoisfaasiter{\"a}s, edge fracture, anisotropy, local formability, ductile fracture, advanced high-strength steels, quenching and partitioning steel, dual-phase steel",

author = "Zinan Li",

year = "2024",

language = "English",

isbn = "978-952-64-2205-3",

series = "Aalto University publication series DOCTORAL THESES",

publisher = "Aalto University",

number = "281",

address = "Finland",

school = "Aalto University",

}

TY - BOOK

T1 - Influence of Anisotropy on Edge Fracture of Advanced High-strength Steels

AU - Li, Zinan

PY - 2024

Y1 - 2024

N2 - Advanced high-strength steels (AHSS) have been developed with superior properties for wide applications in the automotive industry. Among them, dual-phase (DP) steels and Quenching and Partitioning (QP) steels, as representatives from the first and third generations of development, are attracting great interest. Despite the benefits of the enhanced high strength, the formability of these materials becomes unpredictable in the forming processes, especially in the largely deformed local area. AHSS is more susceptible to edge fracture compared to traditional mild steels, due not only to reduced ductility but also to distinct local fracture behavior. To deepen the knowledge of the edge fracture behavior of AHSS, it is necessary to find a characterization method that accurately describes fracture failure. The study aims to accurately predict edge fracture by addressing often-overlooked factors such as anisotropy evolution, localization, and stress-state-dependent fracture behavior. To achieve this goal, a combination of experimental and numerical approaches is employed. First, a category of tensile tests with various geometries and loading directions was conducted to characterize the anisotropic fracture properties of DP and QP steels. Next, hole expansion tests (HET) were carried out to assess edge formability under both smooth and blanked edge conditions. In addition to the classic fracture dependency on stress states, the spotlight is on the anisotropic behavior in terms of both plasticity and fracture. The evolving non-associated Hill48 (enHill48) model is applied to describe anisotropic plasticity, while the anisotropic fracture models are formulated to represent fracture behavior. The novelty of this study lies in uncovering that the edge fractures of examined DP steel are primarily driven by anisotropy-induced strain localization, while the edge fractures of the investigated QP steel, though less sensitive to anisotropic plasticity, are strongly dependent on anisotropic fracture and stress triaxiality, governed by fracture limits. A partially anisotropic fracture model is found effective for DP steel in capturing anisotropic plasticity and localization but remains limited in accurately describing fracture-dominated local formability for QP steel. Consequently, a fully anisotropic model is proposed for QP steel, based on the linear transformation of plastic strain within the damage initiation criterion. This model is validated for both anisotropic fracture behavior and edge formability with improved prediction accuracy. The findings emphasize the crucial role of anisotropy in edge fracture assessment, which not only enhances modeling precision but also introduces a design concept to elevate material edge formability.

AB - Advanced high-strength steels (AHSS) have been developed with superior properties for wide applications in the automotive industry. Among them, dual-phase (DP) steels and Quenching and Partitioning (QP) steels, as representatives from the first and third generations of development, are attracting great interest. Despite the benefits of the enhanced high strength, the formability of these materials becomes unpredictable in the forming processes, especially in the largely deformed local area. AHSS is more susceptible to edge fracture compared to traditional mild steels, due not only to reduced ductility but also to distinct local fracture behavior. To deepen the knowledge of the edge fracture behavior of AHSS, it is necessary to find a characterization method that accurately describes fracture failure. The study aims to accurately predict edge fracture by addressing often-overlooked factors such as anisotropy evolution, localization, and stress-state-dependent fracture behavior. To achieve this goal, a combination of experimental and numerical approaches is employed. First, a category of tensile tests with various geometries and loading directions was conducted to characterize the anisotropic fracture properties of DP and QP steels. Next, hole expansion tests (HET) were carried out to assess edge formability under both smooth and blanked edge conditions. In addition to the classic fracture dependency on stress states, the spotlight is on the anisotropic behavior in terms of both plasticity and fracture. The evolving non-associated Hill48 (enHill48) model is applied to describe anisotropic plasticity, while the anisotropic fracture models are formulated to represent fracture behavior. The novelty of this study lies in uncovering that the edge fractures of examined DP steel are primarily driven by anisotropy-induced strain localization, while the edge fractures of the investigated QP steel, though less sensitive to anisotropic plasticity, are strongly dependent on anisotropic fracture and stress triaxiality, governed by fracture limits. A partially anisotropic fracture model is found effective for DP steel in capturing anisotropic plasticity and localization but remains limited in accurately describing fracture-dominated local formability for QP steel. Consequently, a fully anisotropic model is proposed for QP steel, based on the linear transformation of plastic strain within the damage initiation criterion. This model is validated for both anisotropic fracture behavior and edge formability with improved prediction accuracy. The findings emphasize the crucial role of anisotropy in edge fracture assessment, which not only enhances modeling precision but also introduces a design concept to elevate material edge formability.

KW - edge fracture

KW - anisotropy

KW - local formability

KW - ductile fracture

KW - advanced high-strength steels

KW - quenching and partitioning steel

KW - dual-phase steel

KW - reunamurtuma

KW - anisotropia

KW - paikallinen muovattavuus

KW - sitkeä murtuma

KW - edistyneet erittäin lujat teräkset

KW - karkaisun ja jakautumisen teräs

KW - kaksoisfaasiteräs

KW - edge fracture

KW - anisotropy

KW - local formability

KW - ductile fracture

KW - advanced high-strength steels

KW - quenching and partitioning steel

KW - dual-phase steel

M3 - Doctoral Thesis

SN - 978-952-64-2205-3

T3 - Aalto University publication series DOCTORAL THESES

PB - Aalto University

ER -

Influence of Anisotropy on Edge Fracture of Advanced High-strength Steels

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