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On failure modelling in finite element analysis: material imperfections and element erosion
Linköping University, Department of Mechanical Engineering, Solid Mechanics. Linköping University, The Institute of Technology.
2005 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This dissertation concerns failure modelling with material imperfections and element erosion in finite element analyses. The aim has been to improve the element erosion technique, which is simple to use and implement and also computationally inexpensive. The first part of the dissertation serves as an introduction to the topic and as a summary of the methodologies presented in the following part. The second part consists of seven appended papers. In paper A the standard element erosion technique is used for projectile penetration. In papers B and C a methodology that accounts for size effects is developed and applied to crack initiation in armour steel and tungsten carbide. A methodology to better predict the stress state at crack tips with coarse meshes is presented and applied to armour steel in paper D. Papers E and F concern the development of selective mass scaling which allows for larger time steps in explicit methods. Finally, in paper G the previously presented methodologies are used in combination and validated against experimental results on tungsten carbide. The computations show good agreement with the experimental results on failure initiation for both materials, while the computational results on the propagation of cracks show better agreement for the armour steel than for the tungsten carbide.

Place, publisher, year, edition, pages
Institutionen för konstruktions- och produktionsteknik , 2005.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 973
Keyword [en]
finite element method, element erosion, material failure, material imperfections, crack-tip
National Category
Applied Mechanics
Identifiers
URN: urn:nbn:se:liu:diva-4679ISBN: 91-85457-34-5 (print)OAI: oai:DiVA.org:liu-4679DiVA: diva2:20693
Public defence
2005-12-02, C3, C-huset, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
Opponent
Supervisors
Note
On the day of the public defence of the doctoral thesis, the status of articles I, III and IV was Accepted and article VII was Submitted.Available from: 2005-11-09 Created: 2005-11-09 Last updated: 2010-04-06
List of papers
1. Projectile penetration and perforation of high performance concrete: Experimental results and macroscopic modelling
Open this publication in new window or tab >>Projectile penetration and perforation of high performance concrete: Experimental results and macroscopic modelling
2006 (English)In: International Journal of Impact Engineering, ISSN 0734-743X, E-ISSN 1879-3509, Vol. 32, no 7, 1068-1085 p.Article in journal (Refereed) Published
Abstract [en]

Experiments and simulations of penetration and perforation of high performance concrete targets by steel projectiles have been carried out. A Doppler radar monitored the projectile pre-impact velocity history and a high-speed camera captured the projectile residual velocity. A continuum mechanical approach and the finite-element method were used for the simulations. The targets were modelled with the K&C concrete model. For perforation the computational results show acceptable agreement with the experimental results, but not for penetration. The article is concluded with suggestions on how to better model concrete material for the current application.

Place, publisher, year, edition, pages
Pergamon Press, 2006
Keyword
High performance concrete; Projectile; Penetration; Perforation; Experiments; Finite-element analysis
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-13412 (URN)10.1016/j.ijimpeng.2004.11.003 (DOI)
Available from: 2005-11-09 Created: 2005-11-09 Last updated: 2017-12-13
2. Failure modelling in finite element analyses: Random material imperfections
Open this publication in new window or tab >>Failure modelling in finite element analyses: Random material imperfections
2005 (English)In: Mechanics of Materials, ISSN 0167-6636, Vol. 37, no 12, 1175-1179 p.Article in journal (Refereed) Published
Abstract [en]

A phenomenological failure model has been developed where the material is assumed to contain initial material imperfections that influence the material strength only through the largest imperfection. The imperfections are randomly distributed and characterized by material imperfection density functions. In a first approach one type of imperfection is assumed with an exponential imperfection density function. The model parameters are optimized, using results from a deterministic finite element analysis, to match a target function representing the probability of failure in a tensile test. The developed model manages to reproduce this data well.

Keyword
Explicit finite element analysis; Failure initiation; Element erosion; Random imperfections
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-13413 (URN)10.1016/j.mechmat.2005.05.001 (DOI)
Available from: 2005-11-09 Created: 2005-11-09 Last updated: 2009-06-08
3. Weakest link model with imperfection: Application to three point bend of a tungsten carbide
Open this publication in new window or tab >>Weakest link model with imperfection: Application to three point bend of a tungsten carbide
2007 (English)In: International Journal of Refractory Metals and Hard Materials, ISSN 0263-4368, Vol. 25, no 1, 6-10 p.Article in journal (Refereed) Published
Abstract [en]

A general weakest link model adapted to finite element analyses recently proposed by the authors is in this paper applied and optimized against three point bending experiments on two geometrically identical specimens with different sizes. Influence of the choice of description of material imperfections in the model on the result is investigated together with two different types of failure criteria. Due to the choice of failure criteria only one finite element analysis has to be performed. The results show that the model can be fitted well to the experimental results.

Place, publisher, year, edition, pages
Elsevier, 2007
Keyword
Finite element method, Failure initiation, Size effect, Random imperfections
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-13414 (URN)10.1016/j.ijrmhm.2005.10.015 (DOI)000242508800002 ()
Available from: 2005-11-09 Created: 2005-11-09 Last updated: 2012-08-24Bibliographically approved
4. Failure modelling in finite element analyses: Element erosion with crack-tip enhancement
Open this publication in new window or tab >>Failure modelling in finite element analyses: Element erosion with crack-tip enhancement
2006 (English)In: Finite elements in analysis and design (Print), ISSN 0168-874X, E-ISSN 1872-6925, Vol. 42, no 4, 283-297 p.Article in journal (Refereed) Published
Abstract [en]

A method to better handle fracture using element erosion in finite element analyses is proposed. It is assumed that the crack-tip is blunt and that the solution in the vicinity of the crack-tip is separable when described in local polar co-ordinates. The numerical solution is enhanced by scaling strain rates at integration points to better match the state at the crack-tip. This material specific scaling function is empirically determined for modus I steady-state propagation and is applied to two problems. The results show that the method reduces mesh dependency such that the chosen fracture energy can be better matched for different element sizes.

Place, publisher, year, edition, pages
Elsevier, 2006
Keyword
Finite element analysis; Failure; Element erosion; Crack-tip enhancement; Steady-state propagation; Modus I
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-13415 (URN)10.1016/j.finel.2005.07.001 (DOI)
Available from: 2005-11-09 Created: 2005-11-09 Last updated: 2017-12-13
5. Selective mass scaling for thin walled structures modeled with tri-linear solid elements
Open this publication in new window or tab >>Selective mass scaling for thin walled structures modeled with tri-linear solid elements
2004 (English)In: Computational Mechanics, ISSN 1432-0924, Vol. 34, no 2, 134-136 p.Article in journal (Refereed) Published
Abstract [en]

A method for selective mass scaling in explicit finite element analyses of thin walled structures, modeled with solid elements, is introduced. The method aims at increasing the critical time step without significantly altering the dynamical response of the system. The proposed method is based on the exclusion of certain rigid body motions from the applied mass scaling by filtering the local velocity field.

Keyword
Mass scaling, Finite element, Explicit time integration
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-13416 (URN)10.1007/s00466-004-0560-6 (DOI)
Available from: 2005-11-09 Created: 2005-11-09 Last updated: 2009-05-28
6. Selective mass scaling for explicit finite element analyses
Open this publication in new window or tab >>Selective mass scaling for explicit finite element analyses
2005 (English)In: International Journal for Numerical Methods in Engineering, ISSN 0029-5981, E-ISSN 1097-0207, Vol. 63, no 10, 1436-1445 p.Article in journal (Refereed) Published
Abstract [en]

Due to their inherent lack of convergence problems explicit finite element techniques are widely used for analysing non-linear mechanical processes. In many such processes the energy content in the high frequency domain is small. By focusing an artificial mass scaling on this domain, the critical time step may be increased substantially without significantly affecting the low frequency behaviour. This is what we refer to as selective mass scaling. Two methods for selective mass scaling are introduced in this work. The proposed methods are based on non-diagonal mass matrices that scale down the eigenfrequencies of the system. The applicability of the methods is illustrated in two example models where the critical time step is increased by up to 30 times its original size.

Keyword
selective mass scaling, finite element, explicit time integration
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-13417 (URN)10.1002/nme.1293 (DOI)
Available from: 2005-11-09 Created: 2005-11-09 Last updated: 2017-12-13Bibliographically approved
7. Imperfection density function and crack-tip enhancement: Validation against symmetrical bending of circular tungsten carbide plates
Open this publication in new window or tab >>Imperfection density function and crack-tip enhancement: Validation against symmetrical bending of circular tungsten carbide plates
2007 (English)In: Mechanics of materials (Print), ISSN 0167-6636, E-ISSN 1872-7743, Vol. 39, no 8, 753-759 p.Article in journal (Refereed) Published
Abstract [en]

Experiments and finite element analyses of symmetrical bending of circular tungsten carbide plates have been carried out. In the numerical simulations, a material imperfection density that accounts for size effects on the failure strength was considered. In addition, a scaling of the solution near the crack-tips was performed, in order to improve the crack propagation prediction and to ensure a mesh independent fracture energy. The numerical results on failure initiation are shown to be in good agreement with the experiments. However, numerical noise in the solution makes it difficult to handle the crack propagation, which is a problem inherent to all brittle materials.

Place, publisher, year, edition, pages
Elsevier, 2007
Keyword
Explicit finite element analysis, Material imperfections, Element erosion, Tungsten carbide
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-13418 (URN)10.1016/j.mechmat.2006.12.005 (DOI)000246315700003 ()
Available from: 2005-11-09 Created: 2005-11-09 Last updated: 2017-12-13

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