PSI - Issue 42

Gauri Mahalle et al. / Procedia Structural Integrity 42 (2022) 570–577 Mahalle et al./ Structural Integrity Procedia 00 (2019) 000 – 000

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Nomenclature Yield strength Ultimate tensile strength Maximum punch load Yield load Punch displacement Specimen thickness R Absolute temperature ( K ) Q Activation (kJ/mol), ̇ strain rate σ flow stress (MPa) A, α , n, 0 , 1 & 2 Material constants ̇ Shear strain rates τ Shear stress T

Universal gas constant (8.314 J/mol K)

Miniature testing techniques over conventional tests are more popular for obtaining mechanical properties and mechanical performance assessment of structural materials for valuable, scarce, and difficult-to-process materials (Lucas 1990; Rabenberg et al. 2014). One of the Miniature testing techniques, Small Punch Test (SPT), developed by Manahan et al. 1986; Manahan, Argon, and Harling 1981, is a useful technique for evaluating the location-dependent mechanical properties using miniature specimens. SPT attracts much attention due to its simplicity in the experimental setup, usage of a small material specimen, and better correlation of attained results with conventional tensile testing techniques (Arunkumar 2020, Manahan et al. 1986). In SPT, the upper surface of a miniature disc specimen is subjected to compressive force transferred from a small hemispherical punch indenter. The loading of the indenter can be under a constant displacement rate (a tensile deformation) or a constant force (a time-dependent creep response) (Jeffs et al. 2015; Manahan, Argon, and Harling 1981). The stress state witnessed in the SPT contrasts with the stress state witnessed in uniform tensile elongation. Sufficient literature was reported to correlate the generated properties from SPT and conventional tests (Altstadt et al. 2016a; Bruchhausen et al. 2016; Cuesta, Alegre, and Lorenzo 2014; Fernández et al. 2017). Fernández et al. 2017 investigated and compared the mechanical properties of three different sintered materials by tensile test and SPT. Also, the effect of porosity on the mechanical properties for different thickness locations was analyzed (Fernández et al. 2017). Further, Moreno 2018 stated four different methodologies to calculate the yield strength from the characteristic load for disc-shaped specimens and round cylindrical bars for different grades of aluminum alloys and structural steel (Moreno 2018). Due to the increasing use of SPT in worldwide test centres, a European Code of Practice (EUCoP) and ASTM equivalent standards have been developed for tensile, creep, fatigue, and fracture testing (Anon 2020; Lancaster et al. 2022). In the present work, a set of SPT is carried out to understand the strain rate sensitivity of T91 Ferritic-martensitic steel at two different temperatures. The deformation characteristics of T91 steel at different temperatures and strain rates are modelled by introducing the power-law and hyperbolic sine equation. Finally, the affect of material parameters on deformation behaviour is analyzed using the load-displacement curve. 2. Materials and Experimental Methods In this study, a commercially available T91 Ferritic-martensitic steel was used. The microstructure and chemical composition of the parent T91 steel are given in Fig.1(a) and Table 1 respectively. The 0.5 mm thick discs of 8 mm diameter were prepared from a T91 steel cylinder. Fig 1(b) shows the schematic for the SPT test setup which includes a shear punch fixture with a 2 mm diameter flat cylindrical punch and a 3.2 mm diameter receiving hole. For high temperature testing, after locating the specimen in the fixture, the assembly of the specimen and fixture were accommodated by the split furnace. Prior to the test, the SPT fixture was kept in a furnace for 20 minutes to achieve thermal equilibrium in the testing setup. During the testing, the applied load, P, was measured using the load cell as a