PSI - Issue 17

Pavel Konopik et al. / Procedia Structural Integrity 17 (2019) 479–486 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction

Determination of mechanical properties with the use of sub-sized specimens is a topic of high interest nowadays. The application of the sub-sized samples is quite wide in all cases where only limited amount of the experimental material is available such as evaluation of additively manufactured products properties, residual life of in-service components, properties determination of developed nanostructured materials, assessment of dilatometric samples used for thermal and thermo-mechanical treatment development, local properties of components, weld joints, etc. The residual lifetime can be evaluated by the standard mechanical test techniques, such as the tensile test, uniaxial creep test, the Charpy or the fracture toughness test. In particular, fracture mechanics has achieved high significance in determining the ultimate load limit values and in assessing the integrity of a large number of engineering structures of different types. However, standard mechanical tests used to determine the fracture toughness involve extraction of large blocks of material and are not applicable to in-service components. The development and in-service application of essentially non-destructive, miniature material sample removal systems (e. g. the surface sampling systems, such as Electric Discharge Sampling Equipment - EDSE) provided a practical incentive for development of small specimen test methods to evaluate material properties, e. g. fracture toughness. The EDSE can cut out small slices (‘boat sampling’) about 3 mm thick and approximately 20 mm wide x 25 mm long from thick-section components. The remaining cavities have round edges and do not usually require subsequent repair. There has been developed several non-destructive or semi-destructive methods such as Small Punch Test (SPT) (Konopik, 2012) or Automated Ball Indentation (ABI) (Haggag, 1993) used for considered purposes. Drawback of these methods is they are using for the properties quantification some correlations with limited validity and higher tolerance bounds stemming from the measurement and evaluation uncertainties and also accounting for different loading modes between these methods and standard testing methods (e.g. SPT x Charpy impact notch toughness, ABI x fracture toughness,...). Therefore development of small size specimen techniques using miniaturized standard size samples is important, because these tests maintain very important advantage – the same loading mode as standard test samples (Brynk, 2012). Using advantage of application of the newest measurement equipment and techniques, miniaturized samples can be successfully used providing much more reliable data than presently used methods using correlation approach. Furthermore, such a data can be used as an input data influence on FEM simulation (Dzugan, 2014), for calibration of fracture locus (Spaniel, 2014), identification of ductile damage parameters (Dzugan 2018) or characterization of material after thixoforming (Jirkova, 2018). 2. Experimental material As the experimental material, two structural steels was chosen – namely 34CrNiMo6 and 27NiCrMoV 15-6. Both are currently used for production of several parts in energy industry like steam turbine parts and others highly stressed machine parts. Such above mentioned components in service can be degraded by stresses and temperature; therefore the prediction of their remained life is critical. The chemical composition is summarized in Table 1.

Table 1. Chemical composition of investigated materials Material C Mn Si Cr Ni

Mo

P

S

V

27NiCrMoV 34CrNiMo6

0,27 0,36

0,28 0,75

0,14 0,22

1,5

3,7

0,35 0,25

0,01 0,01

0,01 0,01

0,1

1,55

1,67

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