Size Effect on the Fracture Properties of Nuclear Graphite Gyanender Singh, Haiyan Li and Alex Fok...

Size Effect on the Fracture Properties of Nuclear Graphite

Gyanender Singh, Haiyan Li and Alex Fok

University of Minnesota

Weibull Model

• Based on the Weakest link theory. • Quantifies size effect on strength:

m is the Weibull modulus, σfi is the failure stress for the Component i with effective volume Vi, i=1,2.

Viability of Weibull model

George Quinn, J. Am. Ceram. Soc., 73(8), 1990

• Characteristic strength of bend bar as a function of effective volume

• RBSN: Reaction-bonded silicon nitride

Viability of Weibull modelGeorge Quinn, J. Am. Ceram. Soc. 74(9), 1991

• Studies on three different grades of silicon nitride reviewed.

• Strength decreases with increase in the effective volume.

Deviations from the Weibull Model - Graphite• Experimental study conducted by Brocklehurst

(1977)

• Specimens of IM1-24 graphite subjected to tension and four-point bending

• Flexural strength increases with increase in specimen volume for smaller volumes

• Tensile strength increases with increase in specimen volume and the trend levels off for specimens with greater volumes.

Deviations from the Weibull Model - Graphite

Mitchell et al., Journal of Nuclear Materials (2003)

Deviations from the Weibull Model – IG-11 Graphite

Li et al., Carbon (2013)

Summary for Graphite

• Strength can increase with increase in volume/strain gradient.

• Weibull modulus can also increase with increase in volume/strain gradient.

Previous Work ConductedLi and Fok , Journal of Nuclear Materials (2009)

Deterministic model with material strain-softening (post-peak stress retention) was employed to predict the failure process of quasi-brittle materials subjected to different strain gradients.

Considered rectangular beams under bending and L-shaped beams under tension.

Previous Work Conducted

before material damage

during material damage but prior to cracking

during crack propagation

Stress/strain distributions under pure bending

Li and Fok , Journal of Nuclear Materials (2009)


Without strain-softening With strain-softening



Results from Monte Carlo analysis with 3000 specimens per test. Properties were assumed to follow a 2-parameter Weibull distribution.



Nemeth et al. , Carbon (2013)



Mitchell et al., Journal of Nuclear Materials (2003)


Conclusions:•Model correctly predicts the failure behavior of specimens with different strain gradients.•Strain softening (post-peak stress retention) leads to higher flexural strength than tensile strength.•Increase in Weibull modulus with increasing strain gradient correctly predicted.

Limitations of the study:•Less successful for L-shaped specimens.•All specimens are assumed homogeneous, even though different from each other.•Size effect not considered.



• Conducted 3-point bend test on notched NBG-18 graphite specimens of three different sizes.

• Evaluated critical stress intensity factor (fracture toughness) and the corresponding Weibull modulus.

• Employed Digital Image correlation to measure effective crack length.

• Bazant’s size-effect model was used to study the effect of size on fracture toughness.

• The width of the specimen is represented by d and fracture toughness by KIC.

• KIf and cf are the size independent parameters of the Bazant Law. These parameters were evaluated.


Results:

• With a decrease in specimen size the fracture toughness as well as Weibull modulus decreased.

• Size effect is due to different effective crack lengths: effect is smaller in bigger specimens.


Current Work• Extends Li and Fok (2009)• Addresses size effect with more sophisticated FE models• Perform Monte Carlo analyses for

• Tensile test for 3 different sizes of specimens• Flexural test for 4 different sizes of specimens• Notched-beam flexural test for 2 sizes of specimens

• Corresponding experimental studies: Brocklehurst (1977) and Li et al. (2013)

Current Work

σ

δ

GIC

• Addresses heterogeneity in the material.

Current Work: Simulation

Heterogeneous interface

Current Work: Approach

mσ σc mG GIC

Tensile (rectangular beams) C C P P

Bending Notched P P C C

Un-notched P P P P

Tensile (L-shaped specimen) P P P P

C

C

P

Determine mσ and σc through midsize specimens

Determine mG and GIC through midsize specimens

Obtain through simulation (30 specimens) using the calibrated values

Current Work: Results

Current Work: ResultsTension

Bending (un-notched)

Brocklehurst (1977) obtained Weibull modulus as 16 for 4-point bending specimens with volumes greater than 1 cm3. Volumes of the specimens in current study were: 250, 31, 2, 0.03 cm3.

Price (1976) performed tensile tests and obtained Weibull modulus of 8.6 and 9.7 for small and large tensile specimens with volumes 724 and 9847 mm3. Volumes of the specimens in current study were 250, 2 and 0.03 cm3

Current Work: Results

Weibull moduli comparison

Small Size Medium Size Large Size

Current study 12.1 18.3 20.3

Experiment (Li et al.) 18.1 35.5 41.1

Why does Strength Depend on Strain Gradient?

Stress

Strain

Why Does Weibull Modulus Increase With Strain Gradient?

Single die

Two dices

. . .

Many dices

Why Does Weibull Modulus Increase With Size?

Probability of a large specimen containing all-sized defects is high. Fracture is therefore controlled by large defects only, which gives a higher Weibull modulus.

Probability of small specimens containing all-sized defects is low. Therefore, fracture distribution depends on all the defects, which gives a lower Weibull modulus.

Acknowledgement

Funded by the DOE Office of Nuclear Energy’s Nuclear Energy University

Programs (NEUP)

Size Effect on the Fracture Properties of Nuclear Graphite Gyanender Singh, Haiyan Li and Alex Fok...

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