- J . A . J O H N W A N G A N D K E N C . L I U

( O A K R I D G E N A T I O N A L L A B O R A T O R Y )

An Innovative Technique for Evaluating Fracture Toughness of

Graphite Materials

PRESENTED BY : PRACHI AGARWAL (SC09B071), VI semester

OBJECTIVES

To understand the paper and look into the new experimental techniques coming up in the field of Fracture Mechanics

To evaluate the technique proposed and compare its results with the conventional techniques

AIM OF THE PAPER

To introduce a new technique to effectively overcome the deficiencies of the traditional testing techniques

The paper focuses on graphite (brittle material) and A302B steel (ductile material) testing

The main application of the test results for graphite will be in Nuclear Pressure Vessels and Piping (PVP) systems

Using the results to modify current regulations on safety assessment

BACKGROUND

ASTM standard test methods use compact tension (CT) and compact disk tension (CDT) specimen of specified dimensions

Plane strain condition has to be ensured

Charpy V-notch (CVN) impact test used for nuclear reactor pressure vessels

Elements of uncertainties in these techniques

Large safety factors considered hence material wastage

LIMITATIONS OF CONVENTIONAL TECHNIQUES

Direction of crack propagation is unpredictable

Lack of methods for uniform distribution of applied load

Stresses at and near the free ends are anamolous resulting in shear lip formation

Size effect is not considered

Large data scatter in results

Fracture behaviour under mixed mode loading not well known

NEW TECHNIQUE PROPOSED

Spiral Notch Torsion Fracture Toughness Test (SNTT)

Measures intrinsic fracture toughness ( KIC)

Pure torsion applied to uniform cylindrical specimen with a notch line that spirals around at a 45 degree pith

Suitable for testing a wide variety of materials used in pressure vessels and piping structural components

ADVANTAGES OF SNTT

Ensures Mode I crack for 45⁰ pitch

Defined direction of crack propagation

Ensures uniform distribution of applied loads

Size miniaturization

Mixed mode loading conditions can be tested

Applicable to both ductile and brittle materials

KIC values for functionally gradient (non homogeneous) materials and metal-matrix composites can be determined

SNTT SYSTEM

Applies pure torsion to cylindrical specimen having a notch line that spirals at a 45 deg pitch angle

Pure torsion creates uniform equibiaxial tension/ compression stress field on each of concentric cylinders

Grooved line effectively becomes a Mode I crack mouth opening

Plane strain achieved on every plane normal to spiral groove

TEST SET UP

Torsion tests performed on a closed loop controlled, electro-hydraulic, biaxial testing system

Shear strain measured by biaxial extensometer

Rossette strain gauge for cross calibration

Pure torsion achieved with zero axial force in control

Pre-cracking of metallic specimen accomplished by cyclic torsion using Haver sine wave form

DEFORMATION MECHANISM

When grooved specimen is sectioned into segments perpendicular to the groove line, each of the segment can be viewed as a CT specimen with a notch

Since all CT specimen are bonded the compatibility condition is satisfied

In absence of V groove the state of stress of a round bar under pure torsion can be depicted as tension (normal to 45⁰ pitch) and compression (tangential to 45⁰ pitch) of equal magnitude

When a notch is introduced, a tri-axial tensile stress field will evolve around notch root area

Hence the grooved line effectively becomes a Mode I cracking

SNTT KIC EVALUATION

Due to 3-D non-coplanar crack front of SNTT configuration and lack of closed form solutions KIC values are obtained with the aid of a three dimensional finite element computer code TOR3D-KIC

Fig (a) used for brittle SNTT specimen with shallow crack front

Fig (b) used for ductile specimen

FEM model contains 8000, 20-node quadratic brick elements

SNTT SPECIMEN SIZE REDUCTION

CT specimen test are size specific SNTT specimen is a manifestation

of CT specimen having width equivalent to the total length of spiral notch

Key information needed for determining KIC value is manifested in within a small region near crack tip, therefore rod specimen can be miniaturized

Brittle materials need only shallow surface notch which reduces the size further as compared to conventional techniques which use deep notch

A302B steel specimen : uniform gauge section of 20.3 mm diameter and 76.2 mm gauge length

Spiral V-groove depth of 1.9 mm

Fractured at 519.7 Nm

Specimen miniaturization yielded same result

MIXED MODE LOADING

CT test specimen require a complex test set up

Can be done in SNTT by varying the pitch angle

Test results indicated reduction to 50% in toughness and 30% in tearing modulus in ductile material

Synergistic impact due to combination of normal stress (Mode I) and torsion (Mode III)

In brittle material, Mode I dominates failure mode

GRAPHITE TESTING

Graphite specimen:

Gauge length- 25.4 mm

Gauge diameter- 15 mm and 25 mm

Shallow spiral groove

No pre-cracking

COMPARISON OF SNTT RESULTS WITH CONVENTIONAL METHOD TEST RESULTS

MATERIALS SNTT (KIC IN MPa√m) CONVENTIONAL METHOD –CT (KIC in MPa√m)

A302B steel 55.8 55.0

7475-T7351 Al 51.3 51.0

Mullite ceramic 2.21 2.20

Graphite 1.0 1.0

FURTHER SCOPE

The technique has been proposed as a technique for evaluating wind turbine blade composites

Refer: An Innovative Technique for Evaluating the Integrity and Durability of Wind Turbine Blade Composites (FY2010 Report) by Jy-An John Wang and Fei Ren

Also proposed as to develop fracture toughness testing protocol for concrete materials

Refer: Developing an Innovative Field Expedient Fracture Toughness Testing Protocol for Concrete Materials by Jy-An John Wang and Fei Ren

Can be used in bi-material interface toughness research

Refer: An Innovative Technique for Bi-Material Interface Toughness Research by John Jy-An Wang, Ian G. Wright, Ken C. Liu, Michael J. Lance

CONCLUSIONS

SNTT overcomes many limitations inherent in traditional techniques and introduces new possibilities in fracture toughness testing

Conforms to the classical theory of fracture toughness

Can test multiple modes of stress

Controls crack propagation

Consistent results

Not limited by size of the sample

Potential for use to determine KIC values of interface of non homogeneous materials

Agreement between SNTT data and reported data in literature is remarkable, in view of possible material variation, inhomogenity, and anisotropy

These factors confirm that proposed technique is a reliable way

Can be used as an effective tool for nuclear pressure vessel and piping (PVP) system surveillance

THANK YOU