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Assessment of Ductile Tearing and Plastic collapse in 304 SSAndrew WasylykMaterial Performance CentreUniversity of ManchesterUNTF 2010

Andrew WasylykUNTF 2010Andrew WasylykUNTF 20101AimAssessment 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!UNTF 2010Andrew WasylykUNTF 2010plastic zone

Fracture process area

Plastic zone

Fracture Modes: CompetitionWhat?Why?How?Conclusions!*Anderson, Fracture Mechanics: Fundamentals and Applications, 2005UNTF 2010Andrew WasylykUNTF 20103Plastic zone: In a loaded component containing a defect and made of a materials that experience high amounts of work hardening, a zone of plastic deformation exists ahead of the defect. For those materials, as the load increases, two scenarios can occur:

Plastic collapse:If the plastic zone size increases up to the point of meeting the edge of he material without crack initiation, plastic collapse occurs and constraint effects are lost in the component. This happens when your material has a low yield strength but high initiation toughness or when the ligament ahead of the defect is small. Ductile tearing: in ductile tearing occurs when after the crack initiates. A plastic zone will surround the fracture process area in which ductile tearing occurs. Ductile crack growth occurs in the following three stages:

Crack tip blunting and onset of crack growthStable crack growthRapid tearing

If you look at the damage mechanisms ahead of the crack tip, the following mechanism can be seen:Void formation at inclusions or second phase particles (carbides) : decoherance of the matrice or fracture of the particle. Void growth: increased plastic deformation of the material will lead to local void growth ahead of the defect tipVoid nucleation: as void size a concentration ahead of the crack tip reaches a critical value, the ligaments between the voids fail and crack growth occurs.

Problems and issuesMaterials 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

What?Why?How?Conclusions!UNTF 2010Andrew WasylykUNTF 20104

The J-Resistance curve is highly dependant on the test specimen constraintHigh constraint test specimens provides conservative (lower bound) values of fracture toughness when analysing low constraint structures

J-Resistance curve: Constraint effectWhat?Why?How?Conclusions!UNTF 2010Andrew WasylykUNTF 20105J-resistance curves shows the J required for an amount of crack extension.The J-Resistance curve is highly dependant on the test specimen constrainThe bigger specimens will follow the high constraint curvesThe smaller specimens will follow the low constraint curves

Components in structures are often in low constrain conditions

of the experiment is to look at the J-resistance curve, which shows the evolution of a local energy parameter, J, with crack growth. We can see on the curve that

J vs Load can be superimposed on the J-Resistance curveThe intersections between the two curve represent the onset of stable then unstable crack growthConstraint has a high influence on the prediction of the extent of stable crack growthOnset of Stable crack growthOnset 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 quantifiedJ-Resistance curve: Ductile Tearing Prediction What?Why?How?Conclusions!UNTF 2010Andrew WasylykUNTF 20106Approach to studyAim:Development of a generic approach to defect tolerance assessment of components showing high ductility characteristics that take account of the inter-relationship of a, 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 SteelHas low initiation toughness and high tearing modulus.Used in nuclear cooling system pipingFully Austenitic Highly ductileLow carbon content

What?Why?How?Conclusions!UNTF 2010Andrew WasylykUNTF 20107

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 extensionExperiment will be monitored using:Image CorrelationLoad Line Opening Displacement

What?Why?How?Conclusions!ExperimentsUNTF 2010Andrew WasylykUNTF 20108

Material PropertiesWhat?Why?How?Conclusions!UNTF 2010Andrew WasylykUNTF 20109Material PropertiesWhat?Why?How?Conclusions!

UNTF 2010Andrew WasylykUNTF 201010Image Correlation Optical tracking of local displacement of features on the surface of the specimenSurface preparation:25mm CT: White paint coating with random black speckles15mmCT: Oxalic Acid electro-etching, I=6V,t=12minDisplacement mapping obtained using Digital Image Software (DaVis).Equivalent plastic strain calculated using strain components obtained from DIC

What?Why?How?Conclusions!UNTF 2010Andrew WasylykUNTF 2010Unloading ComplianceWhat?Why?How?Conclusions!

UNTF 2010Andrew WasylykUNTF 201012Series of experiments designed to analyse the Tearing characteristics of the material.

This is done by doing unloading compliance testing to obtain J-Resistance Curves using the unloading gradients to calculate crack propagation and the area under the curve to obtain the J according to BS7448

The quality of the results is highly dependant on the quality of the experimental set-up and due to the difficulty of reproduction of results a batch of results is highly recommended

For austenitic stainless steels which are low yield, high toughness materials, recommended the use of side grooving (Austenitic:20% side grooving at a 90degree angle).

Done a series of initial unloading compliance tests.Two of the tests were non-sidegrooved SpecimensOne of the test was a sidegrooved Specimen

Similar testing done by (find who!!!) yielded slightly different resultsUnloading ComplianceWhat?Why?How?Conclusions!

UNTF 2010Andrew WasylykUNTF 201013Series of experiments designed to analyse the Tearing characteristics of the material.

This is done by doing unloading compliance testing to obtain J-Resistance Curves using the unloading gradients to calculate crack propagation and the area under the curve to obtain the J according to BS7448

The quality of the results is highly dependant on the quality of the experimental set-up and due to the difficulty of reproduction of results a batch of results is highly recommended

For austenitic stainless steels which are low yield, high toughness materials, recommended the use of side grooving (Austenitic:20% side grooving at a 90degree angle).

Done a series of initial unloading compliance tests.Two of the tests were non-sidegrooved SpecimensOne of the test was a sidegrooved Specimen

Similar testing done by (find who!!!) yielded slightly different resultsJ-Resistance CurvesWhat?Why?How?Conclusions!

UNTF 2010Andrew WasylykUNTF 201014J-resistance curves were calculated from the unloading curves and the area under the curve from the unloading compliance testing

J(0.2), 0.5 and 1 for each case and compare

J-Resistance curves are geometry dependant due to constrain influence over crack growth. Compact tension specimens which are highly constrained specimens are used for the hardness testing in laboratory and then implemented on less constrained components by the industry in order to obtain conservative estimates of material crack propagation. Figure 2 d) shows that the side grooved specimens requires less energy imputed for crack growth after 0.5mm crack growth. This is due to triaxiality maintained throughout the crack growth due to the side groove.This is due to the fact sidegrooves introducing a stress concentrator on the face of the specimen along the crack propagation plane. This results in more consistent results due to the fact there is no constraint reduction as the material is plasticly deformed ahead of the crack tip. This in turns leads to more conservative estimates of crack growth. J-Resistance CurvesWhat?Why?How?Conclusions!

UNTF 2010Andrew WasylykUNTF 201015J-resistance curves were calculated from the unloading curves and the area under the curve from the unloading compliance testing

J(0.2), 0.5 and 1 for each case and compare

J-Resistance curves are geometry dependant due to constrain influence over crack growth. Compact tension specimens which are highly constrained specimens are used for the hardness testing in laboratory and then implemented on less constrained components by the industry in order to obtain conservative estimates of material crack propagation. Figure 2 d) shows that the side grooved specimens requires less energy imputed for crack growth after 0.5mm crack growth. This is due to triaxiality maintained throughout the crack growth due to the side groove.This is due to the fact sidegrooves introducing a stress concentrator on the face of the specimen along the crack propagation plane. This results in more consistent results due to the fact there is no constraint reduction as the material is plasticly deformed ahead of the crack tip. This in turns leads to more conservative estimates of crack growth. Finite Element: Plastic collapse

What?Why?How?Conclusions!UNTF 2010Andrew WasylykUNTF 201016ConclusionsSpecimen size had little effect on J initiation values corrected for crack tip bluntingYielding 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 ligamentSpecimen size influenced the crack propagation instabilityWhat?Why?How?Conclusions!UNTF 2010Andrew WasylykUNTF 2010Questions???

UNTF 2010Andrew WasylykUNTF 2010