Ductile fracture assessment using parameters from small specimens

Dissertation

Heli Talja

Research output: ThesisDissertationMonograph

Abstract

The topic of this thesis is computational aspects in the assessment of ductile failure in metals. The first part briefly describes the micromechanics of ductile crack growth and methods for assessing it. The "classic" approach to describe material behaviour using fracture mechanics is summarised. The limitations of the one parameter approach based on the stress intensity factor K or the J-integral are described. Attempts to extend the application field of fracture mechanics parameters by introducing triaxility or constraint parameters are also presented. Different local approach methodologies are summarised. Special attention is paid to the modified Gurson model, which is based on micro-mechanical studies of void initiation, growth and coalescence. The main part of the work consists of numerical analyses with the modified Gurson model. The parameters of the model are first determined by matching tensile test results by finite element analysis, and then applied to J-R curve prediction. This methodology is applied to several reactor pressure vessel materials: A533B, 20 MnMoNi 5 5 and austenitic VVER-440 cladding. As a result, the applicability of different specimen types for the parameter determination of the modified Gurson model has been evaluated. Because a combination of experimental and numerical work is needed, it proved to be most feasible to use specimens which can be simulated with two-dimensional or axisymmetric finite element models. Further, a practical way to treat anisotropic material behaviour using the modified Gurson model by using separate parameter sets for different orientations has been proposed and verified. The correspondence between the observed scatters in tensile and fracture mechanical test results has been examined. Best agreement was obtained fitting the scatter of tensile tests by varying the values of initial parameters. Reasons for apparently higher ductility measured from sub-sized than standard size tensile specimens were studied by detailed comparison of stress and strain states. The observed differences could partly be clarified. In the case of austenitic VVER-440 reactor pressure vessel cladding material, the transferability of the modified Gurson model parameters proved to be quite limited, which is interpreted to be partly due to different microcrack initiation and competing failure mechanisms in this kind of material. However, lack of some essential experimental data limits the possibility to draw precise conclusions on this. Finally, suggestions for future work are presented. One important application of the modified Gurson model would be to study the specimen size and geometry dependence of J-R curves.
Original languageEnglish
QualificationDoctor Degree
Awarding Institution
  • Helsinki University of Technology
Place of PublicationEspoo
Publisher
Print ISBNs951-38-5243-1
Electronic ISBNs951-38-5244-X
Publication statusPublished - 1998
MoE publication typeG4 Doctoral dissertation (monograph)

Fingerprint

fracture mechanics
vessel
micromechanics
methodology
parameter
type specimen
microcrack
ductility
failure mechanism
coalescence
void
crack
material
geometry
metal
prediction
test
reactor
comparison
method

Keywords

  • stainless steels
  • fracturing
  • cracking (fracturing)
  • fracture mechanics
  • crack propagation
  • nuclear reactors

Cite this

Talja, H. (1998). Ductile fracture assessment using parameters from small specimens: Dissertation. Espoo: VTT Technical Research Centre of Finland.
Talja, Heli. / Ductile fracture assessment using parameters from small specimens : Dissertation. Espoo : VTT Technical Research Centre of Finland, 1998. 140 p.
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title = "Ductile fracture assessment using parameters from small specimens: Dissertation",
abstract = "The topic of this thesis is computational aspects in the assessment of ductile failure in metals. The first part briefly describes the micromechanics of ductile crack growth and methods for assessing it. The {"}classic{"} approach to describe material behaviour using fracture mechanics is summarised. The limitations of the one parameter approach based on the stress intensity factor K or the J-integral are described. Attempts to extend the application field of fracture mechanics parameters by introducing triaxility or constraint parameters are also presented. Different local approach methodologies are summarised. Special attention is paid to the modified Gurson model, which is based on micro-mechanical studies of void initiation, growth and coalescence. The main part of the work consists of numerical analyses with the modified Gurson model. The parameters of the model are first determined by matching tensile test results by finite element analysis, and then applied to J-R curve prediction. This methodology is applied to several reactor pressure vessel materials: A533B, 20 MnMoNi 5 5 and austenitic VVER-440 cladding. As a result, the applicability of different specimen types for the parameter determination of the modified Gurson model has been evaluated. Because a combination of experimental and numerical work is needed, it proved to be most feasible to use specimens which can be simulated with two-dimensional or axisymmetric finite element models. Further, a practical way to treat anisotropic material behaviour using the modified Gurson model by using separate parameter sets for different orientations has been proposed and verified. The correspondence between the observed scatters in tensile and fracture mechanical test results has been examined. Best agreement was obtained fitting the scatter of tensile tests by varying the values of initial parameters. Reasons for apparently higher ductility measured from sub-sized than standard size tensile specimens were studied by detailed comparison of stress and strain states. The observed differences could partly be clarified. In the case of austenitic VVER-440 reactor pressure vessel cladding material, the transferability of the modified Gurson model parameters proved to be quite limited, which is interpreted to be partly due to different microcrack initiation and competing failure mechanisms in this kind of material. However, lack of some essential experimental data limits the possibility to draw precise conclusions on this. Finally, suggestions for future work are presented. One important application of the modified Gurson model would be to study the specimen size and geometry dependence of J-R curves.",
keywords = "stainless steels, fracturing, cracking (fracturing), fracture mechanics, crack propagation, nuclear reactors",
author = "Heli Talja",
note = "Project code: V7SU00091",
year = "1998",
language = "English",
isbn = "951-38-5243-1",
series = "VTT Publications",
publisher = "VTT Technical Research Centre of Finland",
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}

Talja, H 1998, 'Ductile fracture assessment using parameters from small specimens: Dissertation', Doctor Degree, Helsinki University of Technology, Espoo.

Ductile fracture assessment using parameters from small specimens : Dissertation. / Talja, Heli.

Espoo : VTT Technical Research Centre of Finland, 1998. 140 p.

Research output: ThesisDissertationMonograph

TY - THES

T1 - Ductile fracture assessment using parameters from small specimens

T2 - Dissertation

AU - Talja, Heli

N1 - Project code: V7SU00091

PY - 1998

Y1 - 1998

N2 - The topic of this thesis is computational aspects in the assessment of ductile failure in metals. The first part briefly describes the micromechanics of ductile crack growth and methods for assessing it. The "classic" approach to describe material behaviour using fracture mechanics is summarised. The limitations of the one parameter approach based on the stress intensity factor K or the J-integral are described. Attempts to extend the application field of fracture mechanics parameters by introducing triaxility or constraint parameters are also presented. Different local approach methodologies are summarised. Special attention is paid to the modified Gurson model, which is based on micro-mechanical studies of void initiation, growth and coalescence. The main part of the work consists of numerical analyses with the modified Gurson model. The parameters of the model are first determined by matching tensile test results by finite element analysis, and then applied to J-R curve prediction. This methodology is applied to several reactor pressure vessel materials: A533B, 20 MnMoNi 5 5 and austenitic VVER-440 cladding. As a result, the applicability of different specimen types for the parameter determination of the modified Gurson model has been evaluated. Because a combination of experimental and numerical work is needed, it proved to be most feasible to use specimens which can be simulated with two-dimensional or axisymmetric finite element models. Further, a practical way to treat anisotropic material behaviour using the modified Gurson model by using separate parameter sets for different orientations has been proposed and verified. The correspondence between the observed scatters in tensile and fracture mechanical test results has been examined. Best agreement was obtained fitting the scatter of tensile tests by varying the values of initial parameters. Reasons for apparently higher ductility measured from sub-sized than standard size tensile specimens were studied by detailed comparison of stress and strain states. The observed differences could partly be clarified. In the case of austenitic VVER-440 reactor pressure vessel cladding material, the transferability of the modified Gurson model parameters proved to be quite limited, which is interpreted to be partly due to different microcrack initiation and competing failure mechanisms in this kind of material. However, lack of some essential experimental data limits the possibility to draw precise conclusions on this. Finally, suggestions for future work are presented. One important application of the modified Gurson model would be to study the specimen size and geometry dependence of J-R curves.

AB - The topic of this thesis is computational aspects in the assessment of ductile failure in metals. The first part briefly describes the micromechanics of ductile crack growth and methods for assessing it. The "classic" approach to describe material behaviour using fracture mechanics is summarised. The limitations of the one parameter approach based on the stress intensity factor K or the J-integral are described. Attempts to extend the application field of fracture mechanics parameters by introducing triaxility or constraint parameters are also presented. Different local approach methodologies are summarised. Special attention is paid to the modified Gurson model, which is based on micro-mechanical studies of void initiation, growth and coalescence. The main part of the work consists of numerical analyses with the modified Gurson model. The parameters of the model are first determined by matching tensile test results by finite element analysis, and then applied to J-R curve prediction. This methodology is applied to several reactor pressure vessel materials: A533B, 20 MnMoNi 5 5 and austenitic VVER-440 cladding. As a result, the applicability of different specimen types for the parameter determination of the modified Gurson model has been evaluated. Because a combination of experimental and numerical work is needed, it proved to be most feasible to use specimens which can be simulated with two-dimensional or axisymmetric finite element models. Further, a practical way to treat anisotropic material behaviour using the modified Gurson model by using separate parameter sets for different orientations has been proposed and verified. The correspondence between the observed scatters in tensile and fracture mechanical test results has been examined. Best agreement was obtained fitting the scatter of tensile tests by varying the values of initial parameters. Reasons for apparently higher ductility measured from sub-sized than standard size tensile specimens were studied by detailed comparison of stress and strain states. The observed differences could partly be clarified. In the case of austenitic VVER-440 reactor pressure vessel cladding material, the transferability of the modified Gurson model parameters proved to be quite limited, which is interpreted to be partly due to different microcrack initiation and competing failure mechanisms in this kind of material. However, lack of some essential experimental data limits the possibility to draw precise conclusions on this. Finally, suggestions for future work are presented. One important application of the modified Gurson model would be to study the specimen size and geometry dependence of J-R curves.

KW - stainless steels

KW - fracturing

KW - cracking (fracturing)

KW - fracture mechanics

KW - crack propagation

KW - nuclear reactors

M3 - Dissertation

SN - 951-38-5243-1

T3 - VTT Publications

PB - VTT Technical Research Centre of Finland

CY - Espoo

ER -

Talja H. Ductile fracture assessment using parameters from small specimens: Dissertation. Espoo: VTT Technical Research Centre of Finland, 1998. 140 p.