Small size testing methods in the field of mechanical testing have been used for many years, however recently activities in this field are dramatically increasing. The main reasons for increasing demand are new applications and also measuring capabilities. New applications are for example local properties measurement for complex material models or characterization of additively manufactured parts. This increasing demand for applications leads recently for trials to standardize these methods under ASTM and ISO. Recent developments in testing methods and their standardization state are being presented here.
The aim of this paper is to show the applicability of miniature tensile test (M-TT) for local properties determination on steel sheets for drawing and forming which are mostly use in automotive industry. Knowledge of local mechanical properties is a key factor for parts quality assurance. To assess the M-TT reliability, the tensile results from standard sized and miniature sized geometry will be compare each other. Various mechanical properties, such as the ultimate tensile and yield strengths the elongation, and reductions in area will be obtained. Moreover, thanks to advanced measurement techniques like digital image correlation with high resolution, strain hardening parameters and plastic strain ratio can be evaluated as well. Finally, obtained flow stress curves will be calibrated by inverse analysis in Abaqus FEM Base software.
Application of the small punch test in technical practice is critically dependent on the existence of reliable methods for conversion of data measured by means of such a biaxial test to equivalent quantities of uniaxial tests. In the paper, various methods for recalculation of force in small punch test to stress in conventional creep test are investigated. The methods include: (i) classical “CEN Work Shop Agreement” model, (ii) empirical “force to stress conversion” method, (iii) modified Chakrabarty method and (iv) “constant deflection rate” method. The methods are examined using the data obtained by testing Fe-Al based alloy with additions of chromium and cerium. The tested alloy enables verification of conversion methods for different crystallographic lattices, namely D03 at lower temperatures and B2 at elevated temperatures.
The small punch (SP) testing technique was applied to five heats of Gr.91 steel, which had been actually used for boiler pipings in different USC power plants for long periods of time, to investigate the applicability of this testing technique to the assessment of heat-to-heat variation of creep property. The SP creep test was carried out at the temperature of 650°C and under the loads of 190, 230, 300 and 400 N using a small disk-type specimen. The experimental results revealed that the SP creep rupture strength (rupture life) and the deformation rate were different depending on the heat. These differences were qualitatively in good agreement with those observed in the uniaxial creep test. The obtained results indicated that the SP creep testing technique could be a strong tool for the assessment of heat-to-heat variation of in-service boiler pipings. It was also revealed that the SP creep rupture test data were relatively well converted to the uniaxial creep ones using the load/stress conversion coefficient obtained from the SP tests.