A Damage-Mechanics Based Approach to Structural Design

of ITER Components

US-ITER TBM Meeting

UCLA

Nov. 3-5, 2003

S. Sharafat1, N. Ghoniem1, R. Odette2, and E. Diegele3

1UCLA, 2UCSB, 3FZK

OUTLINE

1. Integrated Dynamic Fusion Material Property Digital Data-base

2. Structural Design Approach for Novel Components

3. VISTA: Virtual International StructuralTest Assembly

Integrated Fusion Material Digital Data-Base

DATA CORRELATIONSTHEORY &

SIMULATION &EXPERIMENTS

ACCELERATED DESIGN OF MATERIALS

WEB-DATABASE for Accelerated Materials Design

I. MATERIAL PROPERTIES

a. material data,

b. process information,

c. statistical

parameters.

II. MODELS:

a. Parameterization of attributes,

b. Piece-wise polynomial fits, and

c. Physical description.

III. DESIGN

a. Constitutive Eq.

b. Damage Function

c. FEM

WEB-Digital Database

Interactive Web Data Base

Al-Foam: Example of Accelerated Materials Design

DATA CORRELATIONSTHEORY &

SIM ULATION &EXPERIM ENTS

ACCELERATED DESIGN OF MATERIALS

KNOWN:• Thermal k• Bubble Density• Size Distribution• Process (blowing)

KNOWN:• Rule of mixture

k(P)

FEM to Determine Effects:• Bubble density• Size distribution• Statistical process

variations

Identify Process Parametersto optimize

• Thermal k, vs.• Young’s Modulus

Al-Foam solid model:- Bubble size r = 1 – 5 um- Gaussian Distribution- 5% Porosity

Modify Bubble Blowing Process

Interactive Web Data Base

Material Data Base for Accelerated Materials Design

APPROACH

Use Relational Database Management Systems (RDBMS) combined with the Internet to develop a database backed Web site.

Develop a DYNAMIC Web site to eliminate the cost of updating and archiving STATIC pages.

Provide features for Content and Collaboration Management (CCM) by the Fusion Materials Community:

Community members upload data onto the web site instead of a centralized web meister

The Dynamic RDBMS backed Web site is functional (testing phase)

Fusion Materials Digital Database Web-Site:

1. Choose Material

2. SelectProperty

4. Retrieve PDF File

3. Mouse Rollover ShowsData Point Value

5. View Author’s Comments

Structural Design Approach for Novel Components

Structural Design Objectives

• The objective of every structural design is to develop a structure that is able to perform its functions while meeting the constraints of safety and cost.

• An important element in this process is the specification of maximum permissible failure modes that are relevant to the structure.

• Structure reliability (complement of the maximum permissible failure) depends foremost on the consequence of the type of failure.

Level of Consequence

Inconvenience Interference with

Operation

Suspension of Operation

Loss of Mission, Compromise

Structural Integrity

Total Loss of Structure

MINOR

MODERATE

SERIOUS

CRITICAL

CATASTROPHIC

Structure Failure Consequences

Structure Reliability

• Reliability-based design criteria are usually “calibrated” to existing structures (of the same class) having a history of successful service.

• However, novel ITER structures clearly exceed the scope of existing design standards “calibration exercise” is not feasible.

• Structure reliabilities need to be derived from more fundamental considerations.

Derivation of Target Reliability

for ITER Structure

Define System Define performance requirements Identify failure modes

Identify system failureconsequences

Identify structural categories Define limit states

Select componentreliability

Identify intermediatefailure consequences

Analytical/FEM Existing

structure codes(IISDC*)

Scientificconsiderations

Engineeringjudgment

Derive sub-systemand componentreliabilities in eachrelevant limit state

*ITER Interim Structural Design Criteria

Develop / Include:“Damage Functions”

after Bhattacharya B., et al.

VISTA: Virtual International Structural Test Assembly

VISTA: Virtual International Structural Test Assembly

• VISTA is an International Project currently including EU and the US:– UCLA, UCSB, FZK, ANL, …

• Goal:– Provide a methodology and physical basis to evaluate failure paths to establish

structural integrity assessment methods.• Include 3-D geometric realization• Include interaction of different sources of loading and damage.

• Approach:– Combine wide range of models including:

• Constitutive and damage laws, • Finite Element models, • Geometrical configurations and • Loading conditions.

• Perform “virtual experiments” over a wide range of conditions, to evaluate a range of potential interactions and failure paths.

VISTA: Virtual International Structural Test Assembly

• VISTA is based on a general Damage Mechanics methodology for identifying primary limit states of structures.

• VISTA accounts for large scale 3-D geometric effects by FE-modeling of the entire 3-D radial-toroidal Reactor Sector

• VISTA aims at identifying, both tangible and intangible damage pathways

VISTA: Hierarchical Approach

Full Geometry 3-D Models Boundary

Conditions for 2-D Models

2-D Fracture and Plastic Flow

Localization

1. Hierarchical FEM:

3-D Concept:

2D Detailed Failure Analysis:Fracture and Plastic Flow

3-D FW/B Detail

VISTA: Hierarchical Approach

– Constitutive laws of plasticity, visco-plasticity, creep, swelling or other subcategories are based on Multi-Scale integration of materials theory, models, simulations and experiment.

2. Hierarchical Material Models:

Model:creepswellingdeformationsintering, etc.

Particle Mechanics:overall properties= f(,t)k = f(t), etc.

FEM:3-D Geometric

EffectsMULT-ISCALE MODELING

– VISTA plans to represent the hierarchical multi-physics involving the interactions of thermo-hydraulics, thermal, electro-magnetic, thermo-mechanical, and structural aspects of the function of a component structure.

3. Hierarchical FEM :

Importance of Damage Mechanics

Between 1985 and 1995 a total of 32 Containment pressure boundary degradations were reported (Naus et al., 1996)

• Corrosion resulted in 50% loss of local thickness in 18 NPPs*.

• Other effects leading to structural deterioration included:– Fatigue, including crack initiation and propagation to fracture– Elevated temperature creep– Irradiation effects.

• Physics of these damage processes are reasonably understood.

*NPP: Nuclear Power Plant

Continuum Damage Mechanics

• Continuum damage mechanics (CDM)* relates the effects of microstructural defects (voids, discontinuities, inhomogeneities) to measured macroscopic quantities (stiffness, Poisson's ratio, etc.)

• Essential CDM assumption:• Damage growth is a volume-wide degradation of microstructure

• CDM is useful in modeling accumulation of damage in a material PRIOR to formation of a detectable defect

• With the formation of a macroscopic defect, the essential CDM assumption breaks down. DAMAGE MECHANICS

*Lemaitre J., “How to use damage mechanics,” Nucl. Engr. & Des. 80(1984)233-245

VISTA: Damage Mechanics

*NPP: Nuclear Power Plant

Typical Thermo-Mechanical

Effects:• Ductile

deformation• Creep damage• Fatigue damage

Irradiation Effects: Synergistic Effects

of Microstructure Changes and Loads

DAMAGE MECHANICS:

• Use Mechanistic Models (“Damage Functions”) of structural deterioration for time-dependent structural analysis of:

– Ductile deformation damage as a function of plastic strain (uniaxial)– Creep damage as a function of time– Fatigue damage as a function of number of cycles– Irradiation damage as a function of time

• Take into account loading and strength uncertainties

FEM – Based Structural Aging

Model:

VISTA Damage-Mechanics Approach

I. Damage Mechanics:• Develop “damage-functions” for

(i) ductile deformation, (ii) creep, (iii) fatigue,

(iv) irradiation effects, based on damage mechanics• Develop a FEM- based Structural Aging Model using these

“damage functions.”

II. Large-Scale 3-D Geometric Features:• Develop entire FULL 3-Dimensional FW/Blanket segment• Include support structure, ducts, and sub-component interfaces

Example of a Needed Analysis of a HCPBB

A comprehensive thermo-mechanical FEM analysis of a Helium-Cooled Pebble Bed Blanket at steady state operation would include:

– Solid modeling of the entire 3-D radial–toroidal TBM box including the TBM-housing, ITER support structure, etc.

– Thermal stresses caused by the temperature gradients in the TBM and TBM support structure (both radial and toroidal T-variations)

– Mechanical loads due to coolant pressure in normal operating conditions

– Mechanical loads due to TBM component gravitational loads

– Mechanical loads due to accidental over-pressurization of the TBM box

– Stresses caused by a central plasma disruption (I=20 MA0 in 20 ms)

– Radiation-induced dimensional changes of breeder and multiplier (He and neutron damage (damage-functions).

VISTA: Example of Damage Model for Pebble-Bed/Structure Interaction

Developing Damage-Model for Modeling Interaction between Breeder Pebble Bed and Structure:

– The interface shear tests of soil-structure shows that the failure of a rough soil–structure interface is accompanied with strong strain-softening and normal dilatancy during shearing*.

– A damage model needs to be developed to characterize such behavior of a rough interface with nine parameters.

– The DAMAGE-MODEL (relation matrix between stress and strain increments in the interface) has to be derived and the damage model has to be incorporated into a FEM program.

– Such a damage model should capture the interface behavior, such as hardening, softening, and shear dilatancy.

*Liming Hu and Jia Liu Pu, “Application of damage model for soil–structure interface,” Computers and Geotechnics 30 (2003) 165–183

SUMMARY

• First Phase of an Internet-Based Dynamic Fusion Material Digital Database is nearing completion.

• Structural Design Reliability of Novel Components need to be derived from fundamental considerations based on Damage-Mechanics.

• VISTA: a Damage-Mechanics based full-scale 3-D geometric FEM approach is under development to identify component failure pathways.

References:

Bhattacharya B., et al., “Developing target reliability for novel structures: the case of the Mobile Offshore Base,” Marine Structures 14(2001)

Allen DE., “Criteria for design safety factors and quality assurance expenditure.” Proceedings of the Third International Conference on Structural Safety and Reliability, Trondheim, Norway, 1981. p. 667-78.

Lemaitre J., “How to use damage mechanics,” Nucl. Engr. & Des. 80(1984)233-245

Naus, D. J., et al., “Aging of the containment pressure boundary in light water reactor plants,” Proc. Water Reactor Safety Information Meeting, Rockville, MD, October 1996.

Liming Hu and Jia Liu Pu, “Application of damage model for soil–structure interface,” Computers and Geotechnics 30 (2003) 165–183