Constraint-Correction of Cleavage Fracture Data From Miniature Specimens and Improved Estimates of Lower Bound Fracture Toughness

2009 ◽  
Author(s):  
Colin J. Madew ◽  
David W. Beardsmore ◽  
Richard O. Howells
Keyword(s):  
Fracture Toughness ◽  
Lower Bound ◽  
Cleavage Fracture ◽  
Crack Tip ◽  
Small Scale ◽  
Small Scale Yielding ◽  
Fracture Data ◽  
Scale Yielding ◽  
Miniature Specimens ◽  
Constraint Correction

Current assessments of pressurised components use fracture data collected on conventional size, 25 mm and 10 mm thick fracture specimens. It would be advantageous to be able to measure fracture toughness on what has commonly been termed miniature specimens (i.e. smaller than 10mm) as this would allow a more economical use of available plant material. However, tests on miniature specimens generally produce values of fracture toughness which over-estimate the fracture toughness of the material (as evaluated from the 25 mm or 10 mm specimens). In particular, the measured scatter in the data exhibits lower-bound values that are higher than the values obtained with conventional size specimens. A study has thus been undertaken in order to examine a methodology to derive fracture toughness from miniature specimens and allow a better determination of the lower-bound values. When cleavage fracture toughness tests are carried out using miniature specimens, the values of critical J obtained do not directly determine the cleavage fracture toughness of the material. This is because a loss of crack-tip constraint will generally occur before fracture. In such cases, it is necessary to apply an appropriate constraint correction to map the measured values to their equivalent small-scale yielding values. This paper uses a method for carrying out constraint corrections in order to assess data obtained from a recent UK miniature fracture toughness specimen testing programme. The method is based on the notion of matching areas enclosed by a same-stress contour of maximum principal stress around the crack tip in the specimen and small-scale yielding geometries. In applying the method, two-dimensional, plane strain finite element models of the specimen geometries have been developed together with a boundary layer model of the reference small-scale yielding condition to determine the appropriate areas.

Plastic Stress Intensity Factors for Small Scale Yielding in Antiplane Shear by Caustics

1982 ◽  
Vol 49 (4) ◽  
pp. 754-760 ◽  
Author(s):  
P. S. Theocaris ◽  
C. I. Razem
Keyword(s):  
Stress Intensity Factor ◽  
Stress Intensity ◽  
Intensity Factor ◽  
Crack Tip ◽  
Antiplane Shear ◽  
Elastic Behavior ◽  
Small Scale ◽  
Small Scale Yielding ◽  
Scale Yielding ◽  
Plastic Stress

The KIII-stress intensity factor in an edge-cracked plate submitted to antiplane shear may be evaluated by the reflected caustic created around the crack tip, provided that a purely elastic behavior exists at the crack tip [1]. For a work-hardening, elastic-plastic material, when stresses at the vicinity of the crack tip exceed the yield limit of the material, the new shape of caustic differs substantially from the corresponding shape of the elastic solution. In this paper the shape and size of the caustics created at the tip of the crack, when small-scale yielding is established in the vicinity of the crack tip, were studied, based on a closed-form solution introduced by Rice [2]. The plastic stress intensity factor may be evaluated from the dimensions of the plastic caustic. Experimental evidence with cracked plates made of opaque materials, like steel, corroborated the results of the theory.

Estimation of K in Ductile CT and SENB Specimens under LEFM and SSY Regimes by J Integral Method: A Review

2013 ◽  
Vol 275-277 ◽  
pp. 242-246
Author(s):  
Bhimsen Karadin ◽  
Nilesh Satonkar ◽  
Sunil Bhat
Keyword(s):  
Integral Method ◽  
Crack Tip ◽  
Small Scale ◽  
J Integral ◽  
Small Scale Yielding ◽  
Critical Limit ◽  
Ductile Materials ◽  
Ductile Mode ◽  
Scale Yielding ◽  
Material Fracture

Stress intensity factor (K) is the measure of severity of stress at the crack tip. When K exceeds the critical limit (i.e., the material fracture toughness), the crack grows. K is valid in brittle materials (LEFM) and to some extent in ductile materials also provided there is small scale yielding (SSY) at the crack tip. The paper reviews the numerical methodology to obtain KI of ductile, Mode I cracked, CT and SENB test specimens in LEFM and SSY regimes with the help of J integral method. The numerical values are successfully compared with the theoretical values.

Small Scale Yielding at a Crack Normal to the Interface Between an Elastic and a Yielding Material

1991 ◽  
Vol 239 ◽  
Author(s):  
Ming Y. He ◽  
R. M. McMeeking ◽  
Ning T. Zhang
Keyword(s):  
Finite Element Analysis ◽  
Finite Element ◽  
Brittle Material ◽  
Crack Tip ◽  
Asymptotic Solutions ◽  
Ductile Material ◽  
Small Scale ◽  
Small Scale Yielding ◽  
Element Analysis ◽  
Scale Yielding

ABSTRACTBy using the elastic singular field as a prescribed loading condition, small scale yielding solutions are obtained for a crack normal to the interface between a brittle and a ductile material. Results for both a crack in the brittle material and one in the ductile material are obtained by finite element analysis. The crack tip fields obtained by the finite element analysis are compared with the asymptotic solutions. It is found that near the tip the stress fields approach the asymptotic solutions. If the crack is in the brittle material, the high triaxial stresses are developed near the interface ahead of the crack tip.

Prediction of Initiation Toughness Under Small Scale Yielding (SSY) and J0.2BL vs. Q Fracture Locus Using Local Approach Methodologies in 304SS

2012 ◽  
Author(s):  
A. Wasylyk ◽  
A. H. Sherry ◽  
J. K. Sharples
Keyword(s):  
Fracture Toughness ◽  
Small Scale ◽  
Initiation Toughness ◽  
Single Edge ◽  
Small Scale Yielding ◽  
Local Approach ◽  
Three Point Bending ◽  
Fracture Locus ◽  
Scale Yielding ◽  
Cracked Specimens

Structural integrity assessments of structures containing defects require valid fracture toughness properties as defined in national and international test standards. However, for some materials and component geometries, the development of valid toughness values — particularly for ductile fracture — is difficult since sufficiently large specimens cannot be machined. As a consequence, the validity of fracture toughness properties is limited by the development of plasticity ahead of the crack tip and the deviation of crack tip conditions at failure from small scale yielding. This paper described the use of local approach models, calibrated against invalid test data, to define initiation toughness in 304 stainless steel pipe material. Three fracture toughness geometries were tested, shallow cracked single edge cracked specimens tested under three point bending, deep cracked single edge cracked specimens tested under three point bending, and deep cracked single edge cracked specimen tested under tension. Initiation toughness and J-Resistance curves were defined for each specimen using the multi-specimen technique. All initiation toughness values measured were above the specimen validity limits. The fracture conditions at initiation were analysed using three local approach models: the Generalised Rice & Tracey, High Constraint Rice & Tracey and the Work of Fracture. The adequacy of local approaches to define the fracture conditions under large strains in 304 stainless steels was demonstrated. A modified boundary layer analysis combined with the local approach models was used to predict the “valid” initiation toughness under small scale yielding condition in this material by defining a J-Q fracture locus. The analytically derived fracture locus was compared to the J-Q values obtained experimentally and shown to be consistent.

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