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- 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