Material Performance Centre University of Manchester UNTF 2010 Andrew Wasylyk UNTF 2010 Assessment...

Material Performance Centre

University of Manchester

UNTF 2010

Andrew Wasylyk UNTF 2010

Assessment of Ductile Tearing and Plastic collapse in 304 SS

Andrew Wasylyk

Andrew Wasylyk UNTF 2010UNTF 2010

Aim

• Assessment of defect behavior against crack initiation toughness can be highly pessimistic representations of failure load by unstable tearing or plastic collapse for low yield high toughness materials

• We aim to quantify the dynamically evolving relationship between “failure” by crack initiation, crack instability and plastic collapse.

• Use a combination of experimental and modeling approaches to quantify the relationship between crack initiation, ductile crack growth and the development of local and global plasticity in standard and novel specimens

What? Why? How? Conclusions!


plastic zone

Fracture process areaPlastic zone

Fracture Modes: Competition


*Anderson, Fracture Mechanics: Fundamentals and Applications, 2005


Problems and issues• Materials with low yield and high

tearing modulus can exhibit increase in toughness with crack propagation

• Represented by a J-Resistance curve

• Structural components can experience loss of constraint and plastic collapse – this process competes with tearing

• In order to take advantage of the increase in toughness through crack growth, a better understanding of the interaction of these mechanisms is required



The J-Resistance curve is highly dependant on the test specimen constraint

High constraint test specimens provides conservative (lower bound) values of fracture toughness when analysing low constraint structures

J-Resistance curve: Constraint effect



J vs Load can be superimposed on the J-Resistance curve

The intersections between the two curve represent the onset of stable then unstable crack growth

Constraint has a high influence on the prediction of the extent of stable crack growth

Onset of Stable crack growth

Onset of unstable crack growth

High constraint Specimens can lead to conservative predictions of structural instability.

Adequate constraint correction can lead to more accurate predictions enabling known conservatism to be quantified

J-Resistance curve: Ductile Tearing

Prediction



Approach to study

Aim:• Development of a generic approach to defect tolerance assessment of components showing

high ductility characteristics that take account of the inter-relationship of Δa, P & J

• Analysis of the dynamic relationship between crack initiation, propagation and development of local and global plasticity.

• Experimental studies including Image Correlation and conventional and micro mechanistic FE modelling

Material used: 304 (L) Stainless Steel• Has low initiation toughness and high tearing modulus.• Used in nuclear cooling system piping• Fully Austenitic • Highly ductile• Low carbon content



Fracture Mechanics Experiment:

Fracture toughness testing of scaled down Compact Tension specimens of thicknesses B=25, 15, designed according to British Standard 7448-4.

Experimental set-up:

Specimens are first fatigue-cracked to crack length (a) to specimen width (w) ratio of 0.55.

Fracture toughness testing under displacement control promoting stable crack extension

Experiment will be monitored using: Image Correlation Load Line Opening Displacement


Experiments


Material Properties



Image Correlation

• Optical tracking of local displacement of features on the surface of the specimen

• Surface preparation:– 25mm CT: White paint coating with

random black speckles

– 15mmCT: Oxalic Acid electro-etching, I=6V,t=12min

• Displacement mapping obtained using Digital Image Software (DaVis).

• Equivalent plastic strain calculated using strain components obtained from DIC



Unloading Compliance



J-Resistance Curves



Finite Element: Plastic collapse



Conclusions

• Specimen size had little effect on J initiation values corrected for crack tip blunting

• Yielding of the remaining ligament (Limit Load) occurs before crack initiation.

• Extensive plasticity occurs (>2% strain) occurs before significant tearing is observed.

• Specimen size had little influence on the relationship between initiation and Yielding of the remaining ligament

• Specimen size influenced the crack propagation instability



Questions???

Material Performance Centre University of Manchester UNTF 2010 Andrew Wasylyk UNTF 2010 Assessment...

Documents

Transcript of Material Performance Centre University of Manchester UNTF 2010 Andrew Wasylyk UNTF 2010 Assessment...