PSI - Issue 14

Taslim D. Shikalgar et al. / Procedia Structural Integrity 14 (2019) 529–536 T.D.Shikalgar et al./ Structural Integrity Procedia 00 (2018) 000–000

530

2

1. Introduction In 1980s, the small punch test (SPT) was developed to determine mechanical properties of nuclear materials in case of limited availability of materials insufficient for conducting conventional standard tests. This test was mainly focused to assess the change in tensile and fracture properties of nuclear structural materials. The SPT consists of a small square or circular shaped clamped specimen in a die, is punched until fracture using the rigid spherical ball to obtain load v/s displacement data. From these test data, yield load, maximum load, displacement at failure and area under the curve can be extracted. SPT has received an increasing amount of attention over the last decades with the aim to obtain elastic plastic material properties. A European Code of Practice (EUCoP) for Small Punch tensile, fracture testing was developed and launched by CEN in 2006, further revised in 2007 (CWA 15627). Over the years, there have been several attempts to determine fracture properties of a material using SPT. This is done by using empirical equations developed for the estimation of fracture initiation parameter based on the minimum thickness of the specimen at the point of fracture. In such methodology, determination of fracture properties in the absence of a specimen without pre-crack makes the results doubtful. Recently, test on miniature pre-cracked specimens has emerged as an alternative to determine fracture properties of nuclear materials. J. M. Alegre et al. (2014, 2016) performed small punch test on pre-cracked specimens to estimate fracture toughness parameters of PH 15.5 H1025 material using experimental load-displacement curve for different initial pre-crack lengths, crack tip opening displacement (CTOD) and numerically determined J-integral. To validate the proposed methodology, fracture properties obtained from p-SPT were compared with those results obtained from conventional Compact Tension (CT) specimen tests following the ASTM E1820 standard (2001). T.E. Garcia et al. (2015) performed interrupted tests on pre-notched specimens to analyze the evolution of notch mouth opening displacement (δ SPT ) of CrMoV grade steels. E. Cardenas et al.(2011)also performed tests on pre-notched specimens of X-70TT material and calculated the instantaneous slope from the experimental load v/s displacement data. J-initiation was calculated at the point where slope started decreasing. Garcia et al. and Cardenas et al. compared numerical elastic plastic analysis data with the experimental data. They commented that large number of different materials needs to be tested to validate p-SPT technique. In this paper, p-SPT specimens are used to determine the fracture toughness (J-initiation) of nuclear structural steel 20MnMoNi55 and T91. The p-SPT fixtures are fabricated as per guidelines given in CEN Code (2007). The tests are conducted to get load v/s punch displacement data. Elastic-plastic FE analysis is carried out and numerically obtained load v/s displacement data are compared with the experimental results. This analysis also helped to compute J-integral near the crack tip as a function of load. FE analysis is then repeated using the micromechanical Gurson-Tvergaard-Needleman model to know the load at which crack is initiated. The modified q 2 near crack tip scheme proposed by Dutta et al. (2008) is employed to improve predictions of the crack-initiation load. Computed J initiation has good matching with the value quoted in the literature, which shows the viability of the method. The methodology described in this paper has the potential to determine fracture initiation toughness of aged nuclear materials using pre-cracked small punch tests. 2. Testing of p-SPT specimens 2.1. Materials The materials investigated in the present work are 20MnMoNi55 and T91. These steels are widely used for nuclear reactor pressure vessels because of a good combination of mechanical and fracture properties. Chemical composition and mechanical properties are shown in Table 1 and 2 respectively. The average true stress-strain curves of both materials are shown in Fig.1.An exponential law (Eq. 1) is used to represent true stress (σ) v/s plastic strain ( ε p ) data.

Table 1. Chemical composition (wt %) Material C

Si

Mn

S

P

Ni

Cr

Mo

Al

N

Fe

20MnMoNi55

0.19 0.099

0.23 0.32

1.33 0.43

0.001 0.007

0.52 0.062 0.53

0.056

0.009 Bal

T91

-

0.02

0.24

8.80 0.96

-

-

Bal