PSI - Issue 14

Filin V.Yu. et al. / Procedia Structural Integrity 14 (2019) 758–773 Filin V.Yu, Ilyin A.V. / Structural Integrity Procedia 00 (2018) 000–000

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One more parameter suggested by Alan Wells in 1961 is CTOD (  ). It is a “deformation” parameter of fracture. In SSY conditions and plane stress,

2 4 (1 K

2

)

 

I

.

(9)

 

  E

Y

This formula is not applicable for high-toughness materials, another models have been developed by Donald S. Dugdale and Grigory Barenblatt. Generally, CTOD is related to J-integral as    Y J m , (10) where m = 2 in plane strain conditions and m = 1 for plane stress state. A first attempt to use EPFM parameters to assess defects in structures in a standard document was BS-PD 6493 «Guidance on some methods for the derivation of acceptable levels for defects in fusion welded joint» where interpolation formulas were given allowing to find  for a defect in a structure: (12) Now FEM methods are widely applied to calculate fracture mechanics parameters for a defect in a structure. However a lot of issues still exist in assessment procedures, e.g. a proper account of residual stress in welded structures. 3. Calculation procedure providing for cleavage fracture resistance 3.1 General provisions EPFM strength condition related to cleavage fracture resistance is needed when for an analyzed structure the problem of strength assessment is out of LEFM conditions. In terms of J-integral, c n J J   1 . (13) Referring to the problems named in item 1, one of the main designer’s tasks is a choice of material suitable for particular structure (critical level of fracture toughness J c ). So the procedure includes the load parameter calculation in terms of J , choice of a reference flaw size (conservatively assumed to be a crack) in view of applicable methods and amount of nondestructive inspection, and setting the reserve factor related as to the flaw size as to the reliability of knowledge about material properties ( J c ) in the conditions corresponding to all the design cases. The second main task is an evaluation of admissible flaw sizes for welded joints. In Russia a standard procedure in terms of J-integral has been developed with the authors’ participation for welded joints of subsea pipelines and introduced into the Rules for the Classification and Construction of Subsea Pipelines, Russian Maritime Register of Shipping, 2017. Other tasks are: evaluation of material fracture toughness at certification for general purpose, assessment of loads that could be borne by a particular material in particular thickness and loading conditions, assessment of real flaws found in existing structures. On contrary to the above list, in the world practice the last task is considered to be the most important. Several documents have been issued for safety assessment of flaw containing structures: CEGB Report R/H/R6 “Assessment of the Integrity of Structures Containing Defects”, SSM Rapport 2008-01 “A combined deterministic and probabilistic procedure for safety assessment of components with cracks”, DNVGL-ST-F101 “Submarine Pipeline Systems”, BS 7910:2013+A1:2015 “Guide to methods for assessing the acceptability of flaws in metallic structures”. If a reverse task needs to be solved following the listed documents, it has to be solved in iterations, with no clear understanding of the reference flaw size assignment. / e e 0.5 for 2 Y 2    Y Y e A        e e , (11) / e e 0.5 for 0.25 2 Y Y Y      e e e A .