The mechanical properties of the material, such as yield strength, ultimate tensile strength, elongation and reduction in cross-sectional area of the specimen are basic mechanical characterisation values in engineering design.
The evaluation of the local mechanical properties of in-service components or as input data for Finite Element Method (FEM) simulation requires an accurate data obtained from the measurement of small volume of tested material.
Sub-sized specimens testing techniques
The sub-sized specimen testing techniques can be successfully applied when the insufficient volume of the material is available (i.e. testing of bulk nanomaterials), for the evaluation of the current mechanical properties of in-service components in remaining life assessment (i.e. power generation equipment), as well as in development of new alloys to ensure the lowest possible cost [1-4].
The mechanical testing of miniaturized specimens can play a significant role in investigating local mechanical properties in metal sheets and parts made of them. Sheet metal forming has found an application in almost all the industries. Small specimens can be extracted from the parts before or after deformation in order to evaluate remaining plasticity.
Figure 1 shows the miniaturized testing specimen with the weldment in the middle section, extracted from a washing machine drum. Tensile tests of these small samples have permitted to evaluate the local properties of the mentioned component.
Fig. 1: Miniaturized tensile tests specimen with the weldment in the middle section.
Miniaturized tensile tests (MTT) are advantageously used for standard mechanical characteristics determination, including anisotropy. Furthermore, Miniaturized tensile tests allows the mechanical testing in a wide temperature range and at high velocity of deformation for the strain rate sensitivity investigation (from quasi-static conditions to dynamic mechanical testing) [5].
Specimens preparation process for miniaturized testing and set up
Specimens prepared for mechanical testing can be cut from in-service components by conventional machining. However, this method results in the detachment of large fragments of metal parts, overheating, concentration of stresses and poor surface qualities. These shortcomings are undesirable for in-service components and has to be avoided.
For this reason, the sub-sized specimens can be extracted using a portable electric discharge equipment (EDSE). This device applies the consumable electrodes to remove material using electric spark erosion with a cooling medium. EDSE cuts out small parts of the investigated material without leaving sharp corners. It allows to reduce the stress concentration in the region of extraction [6]. Obtained boat-shaped samples can be used for small sample machining for the tensile, impact, fatigue and fracture toughness testing.
The miniaturized testing results can be sensitive to the conditions of surface finishing, such as roughness or dimensional accuracy. The literature data show that the smallest dispersion of tensile strength values can be obtained thanks to the surface polishing using abrasive paper and alumina suspension [7]. To ensure the data consistency, the thickness of the testing specimen should be at least 10 times higher than the material grain size [8].
On the other hand, the results of some miniaturized testing methods as the Small Punch Test (SPT), have to be converted into conventional parameters (tensile and fracture toughness properties, transition temperature, etc.) and the conversion has to be validated for each material to assure the accuracy of the methodology.
The Miniaturized tensile tests have minimal material requirements, the same loading mode as in standard tensile tests and the resulting values can be used directly without any additional correlation. The Miniaturized tensile tests specimen geometry was developed based on the disc-shaped SPT specimen with a diameter of 8 mm. The specimen development is shown in Figure 2. The finite element method verification of the stress distribution in the testing specimen demonstrated higher stress concentration in the transition from the gauge section to the specimen shoulders [1]. Nevertheless, the results of the further experimental investigation with variation of the specimen geometry did not show the problem connected with incorrect location of the crack initiation [4].
Fig. 2. a) SPT specimen geometry, b) MTT specimen geometry developed based on SPT specimen, c) currently used MTT specimen geometry.
The effect of the scale factor on the values of yield strength, ultimate tensile strength and uniform elongation is negligible but the total elongation values are strongly sensitive on the specimen geometry. The thickness-to-width ratio can affect the neck geometry and the angle between the neck and load axis. The specimen shoulder curvature influences the creation of zones with lower strain level. The length-to-width ratio can influence the stress concentration near the specimen shoulders, and consequently cause the change of obtained yield strength value [4].
During the test, the deformation is measured optically using a contactless measuring system based on the Digital Image Correlation (DIC) method. A stochastic pattern is applied to the specimens for the optical monitoring of displacement that can be recorded by one (2D mode) or more (3D mode) cameras. During the test, the pixels configuration corresponding to the position of the pattern dots is changing along the material deformation. The DIC system is tracing and comparing the pattern of each image with the reference one. Thanks to this method, it is possible to measure the deformation and displacement of the whole specimen, as well as locally in selected regions. Figure 3 presents the testing setup for the miniaturized tensile test with gripping tools for sub-sized specimen mechanical testing.
The DIC system is tracing and comparing the pattern of each image with the reference one. Thanks to this method, it is possible to measure the deformation and displacement of the whole specimen, as well as locally in selected regions. Figure 3 presents the testing setup for the miniaturized tensile test with gripping tools for sub-sized specimen mechanical testing.
Fig. 3. Testing setup for the miniaturized tensile test.
Results and applications
The results obtained from the Miniaturized tensile tests (MTT) can be also used for remaining plasticity determination, the evaluation of Lankford coefficients, and strain-hardening determination, as well as for the determination of fracture locus.
The data obtained by this method can be applied as input data in FEM simulations. For calibration of the required advanced materials model, many different specimens’ configurations are used. All of these types of miniaturized mechanical tests are under development and can be applied in sheet metal testing.
Sub-seized specimen testing techniques can also be widely applied to perform tensile tests, high-cycle and low-cycle fatigue tests, creep tests, impact Charpy-tests with FATT determination and fracture toughness tests.
Results can be evaluated directly or have to be recalculated with respect to the different geometries i.e. in mini-Charpy results it is necessary to calculate the coefficient expressing a shift of FATT related to the specimen dimensions [5].
Mini-specimens allow to determine several mechanical properties of the material and can be widely applied to other tests
The mechanical properties possible to determine from the mini-specimens are tensile characteristics (Rm, Re, Reh, Rel, A, Z) including flow curve evaluation, strain rate sensitivity determination, Lankford coefficients (r-value) and strain-hardening coefficients (C, n) calculation, ductile–to-brittle transition temperature investigation and fatigue/fracture toughness properties.
References
[1] Dzugan J, Konopik P, Rund M and Prochazka R 2015 Determination of local tensile and fatigue properties with the use of sub-sized specimens PVP-2015 1A ed Xu SX, Hojo K, Cipolla RC (New York: Amer Soc Mechanical Engineers)
[2] Kumar K, Pooleery A et al. 2014 Use of Miniature Tensile Specimen for Measurement of Mechanical Properties ICONS-2014 (Kalpakkam) 86 ed Jayakumar T, Sandhya R, Rao BPC, Bhaduri AK (Amsterdam, Elsevier Science BV) pp. 899-909
[3] Dzugan J, Spaniel M et al. 2018 Identification of ductile damage parameters for pressure vessel steel NUCL ENG DES 328, pp. 372-80
[4] Gussev MN, Busby JT et al. 2015 Role of Scale Factor During Tensile Testing of Small Specimens ASTM STP 1576 ed Sokolov MA, Lucon E (West Conshohocken/PA USA: ASTM International) pp. 31-49
[5] ChvostováE, Horváth J et al. 2019 Optimization of test specimen dimensions for thermal power station exposure device, IOP Conf. Ser.: Mater. Sci. Eng. 723, PING 2019
[6] Okamoto K, Kitagawa H et al. 2009 Development of electric discharge equipment for small specimen sampling, International Journal of Pressure Vessels and Piping 86(9), pp. 633-636
[7] Suzuki S, Sato S et al. 2015 Influence of Surface Roughness on Tensile Strength of Reduced-Activation Ferritic/Martensitic Steels Using Small Specimens, STP1576 ed Sokolov M and Lucon E (West Conshohocken, PA: ASTM International, 2015), pp. 3-11 [8]Konopik P et al. 2019 Applicalibity of miniature tensile test in the automotive sector IOP Conf. Ser.: Mater. Sci. Eng. 461
