PSI - Issue 14

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

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7

The main concept of the method is a Failure Assessment Diagram (FAD) constructed in coordinates “relative load L r vs. relative SIF K r ”. The relative load is Y , , , ) / ( ,     a l S L F t b r . (14) Specific cases mentioned in BS 7910: a) semi-elliptic surface crack,

2

2

/ 3 σ b

σ / 9 σ 



al 

b

t

r L



ς

,

,

(15)



0.42

( 2 ) S

S l

) ς (1 

σ

Y

b) embedded elliptic crack,

2

2

Y (16) A commonly used function determining the boundary between the safe and unsafe areas as per R6 procedure is K r = K I /( K c ) J = f R6 ( L r ) = (1 – 0.14 L r 2 )(0.3 + 0.7exp(–0.65 L r 6 )). (17) In the method suggested in the present study, calculation is made in terms of J-integral, and after the special consideration the function (17) was replaced by a similar function f 2  1/ f R6 2 (18) that is used as a link between J-integral and its elastic part J el , see Ilyin and Filin (2013),   2.7 2 1 13.3 0.5    r L f for 0.5  r L , (18)   E f f K J J / 1 2 2 el 2 2 I      . (19) The use of this assessment procedure is necessary when the conservative (“absolute”) condition of resistance to cleavage fracture cannot be met, the same was formulated by Ilyin at al. (2009) as J n S E / 2 c 3 Y   . (20) The reserve factor n 3 amounts from 2.1 to 3.2 for base metal and from 1.5 to 2.2 for welded joints depending on loads, flaw size and residual stress. The reasons of non-fulfillment of (20) are:  High strength of materials and/or lowered fracture toughness of welded joints at the temperature Td,  High scatter of experimental Jс values typical for ductile-to brittle transition temperature interval and/or poor production technology,  Large thickness of structural elements,  An assessed structure is made of materials not certified as per modern requirements,  An initial purpose or design load of the structure is changed. The economic reasons to apply the calculation procedure are obvious: substantiation of applicability for cheaper material, less repair of welded joints, service life extension and so on. The volume of this paper makes impossible to give the full description of procedures used in calculation assessment, so an example of their application is suggested. It relates to a hypothetic pressure vessel with the following parameters:  Wall thickness S = 200 mm,  Yield strength  Y (20°C) = 500 MPa,  Design case of normal operation, the minimum design temperature is T d =-10°C,  Service stress  t = 350 MPa,  b = 80 MPa,  Welded joint appearance is shown in Fig. 3. It also shows chosen assessment points. 2 σ ] ς ) 4 ς [(1 σ ] ς ) 4 ς / 3) [(1 ςσ σ / 3 ( ςσ σ 2            t t b t b r L , a S / 0.8    . 3.2 Example of calculation procedure