PSI - Issue 13

Vitor Scarabeli Barbosa et al. / Procedia Structural Integrity 13 (2018) 367–372 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

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measurements of fracture toughness data, current RPV material surveillance programs make extensive use of small fracture specimens to assess changes in fracture toughness properties over its operational life. Specifically, three point bend testing of precracked Charpy V-notch (PCVN) specimens becomes necessary when severe limitations exist on material availability as this specimen configuration is predominant within surveillance capsule programs. A recently developed procedure to characterize fracture toughness data over the DBT region, often known as the Master Curve approach (Wallin, 1991; Wallin, 2002; McCabe et al., 2005) and standardized in the form of ASTM E1921-18 (ASTM E1921, 2018), makes extensive use in practice of PCVN specimens to determine a reference temperature, T 0 , and the associated median fracture toughness, K Jc(med) , applicable to a wide range of structural ferritic steels. However, as the specimen size is reduced (relative to the standard 1T specimen), the evolving crack tip plastic zones developing from the free surfaces with increased loading affect strongly the crack-front size over which high levels of near-tip stress triaxiality (constraint) are maintained. These changes in the crack-tip stress fields over a relatively small thickness in connection with a smaller sampling volume for cleavage fracture influence the measured toughness values, including their statistical scatter and mean value. Such features clearly affect the fracture toughness dependence on temperature and, consequently, produce potential differences in T 0 -values measured using small size specimens and larger fracture specimens. This work addresses an experimental investigation on the cleavage fracture behavior of an ASTM A572 high strength, low alloy structural steel using nonstandard PCVN configurations having varying specimen span over width ( S/W ) ratio. The experimental results show a rather marked effect of specimen span on J c -values which can help to mitigate the effects of constraint loss often observed in smaller fracture specimens. An exploratory application to determine the reference temperature, T 0 , derived from the Master Curve methodology (which defines the dependence of fracture toughness with temperature for the tested material) also provides additional support for using non-standard PCVN specimens in routine fracture applications.

Nomenclature W

specimen width

b 0

initial remaining ligament nominal crack length

a

F ( J c )

cumulative failure probability based on median Rank probability estimate

J

J -integral

J c J 0

J -integral at cleavage instability

characteristic toughness of the Weibull distribution for J c -values threshold toughness of the 3-P Weibull distribution maximum J -value corresponding to a given deformation limit

J min J lim

B specimen thickness K Jc-med median toughness of the Weibull distribution for K Jc -values K min lower bound (threshold) fracture toughness K 0

fracture toughness corresponding to 63.2% cumulative failure probability

M

nondimensional deformation limit

S T

specimen span - distance between specimen supports

test temperature

reference temperature corresponding to a K Jc (median) of 100 MPa√m Weibull modulus (shape parameter) of toughness distribution

T 0

α

σ ys σ uts

material yield stress material ultimate stress

Ramberg – Osgood strain hardening exponent specimen thickness corresponding to 1 inch (25.4 mm)

n

1T

CVN Charpy-V Notch specimens CMOD Crack mouth opening displacement