THE EFFECT OF STIFFNESS OF THE LOADING SYSTEM ON DETERMINATION OF TENSILE CHARACTERISTICS FROM THE RESULTS OF SMALL PUNCH TESTS

The assessment of the residual lifetime of critical components of industrial plants requires the knowledge of mechanical properties prior to operati on, respecting all technological operations realized throughout the manufacture of the componen t, a d the knowledge of mechanical properties after actual time of operation (actual m echanical properties). The conventional mechanical tests require relatively large volume of testing material and extracting it from an operating component can impair its integrity. The n eed for evaluating the actual mechanical properties by direct testing method has led to deve lopment of innovative techniques based on miniaturized specimens. Among these, a technique ca ll d the Small Punch (SP) test has emerged as a promising candidate. It enables measurement o f the realistic material properties at the critical locations in the component both prior and after long-term operation. Yield stress and tensile strength at ambient temperature are determi n d from empirical correlations between SP and standardized test results. In the present paper the empirical correlations obtained in two laboratories for determination of yield stress and tensile strength of low alloy steel of type 14MoV6-3 in as received state from the results of S P tests are compared and the differences obtained are discussed. The results obtained demons trate the significance of loading system stiffness on the results of SP tests at laboratory temperature.


Introduction
The residual lifetime assessment and potential for possible failure of in-service components is a critical issue in the safety and reliability analysis of industrial plants, in particular for operating power stations and petrochemical plants which are approaching the end of their design lives [1].The conventional mechanical tests require relatively large volume of testing material and extracting it from an operating component can impair its integrity.In such situations, mechanical tests based on small specimens test techniques are efficient for characterizing the mechanical properties of components.A promising candidate based on very limited amount of testing DOI 10.12776 [2] and Chinese Code of Practice [3,4] give guidance on the procedure to be followed when carrying out Small Punch tests aimed at evaluation of tensile and fracture characteristics of the metallic materials.On the basis of common experimental programme, realized in both laboratories on low alloy steel of 14MoV6-3 type in as-received state and after long-term operation at 540°C, the database of results of standardized tensile tests, impact tests, fracture toughness tests and the SP tests results in temperature range -193°C -+20°C was obtained.
In the present paper the empirical correlations obtained in both laboratories for determination of yield stress and tensile strength at ambient temperature from the results of SP tests are compared and the differences obtained are discussed.

Experimental material and methods
A pipe ø 457 x 28 mm in the as-received state made of low alloy steel of 14MoV6-3 type was used as the testing material.Chemical composition of the testing materials is shown in Table 1.SP tests at both laboratories were carried out under crosshead control at crosshead speed of 1.5 mm/min.The objective of the test is to produce a load-displacement (punch displacement, crosshead displacement) record (see Fig. 2) and/or load-specimen deflection record, which contains information about the elastic-plastic deformation and strength properties of the material.The load-displacement curves obtained can be utilised to derive empirical correlations between SP and standardised test results [1,[7][8][9][10][11] or they can be analysed in terms of elastic-plastic finite element methods [12][13][14].Fig. 3 shows the comparison of empirical correlations for determination of yield stress at ambient temperature for 14MoV6-3 steel in as-received state.Empirical correlations were expressed as the dependences of yield stress on parameter F e /(h 0 ) 2 [15][16][17], where F e is the load characterizing the transition from linearity to the stage associated with the spread of the yield zone through the specimen thickness (see Fig. 1), because it was proved that the parameter F e /(h 0 ) 2 eliminated any difference in disc specimen thicknesses on load F e [1,18].expressed as the dependences of tensile strength on parameter F m /(u m .h 0 ) where F m is the maximum load recorded during SP test and u m is the displacement corresponding to the maximum load F m .In this case the parameter F m /(u m .h 0 ) is used to eliminate any differences in disc specimen thicknesses on F m and u m [1,18].

Discussion
It is evident that mainly the correlations for a determination of ultimate tensile strength are significantly different in both laboratories.To explain the significant differences in empirical correlations for determination of tensile strength from the results of SP tests the values of maximum load during SP test F m and displacement corresponding to maximum load u m obtained in both laboratories are summarized in Table 4. Values listed in the table are the average of three SP tests.• The comparison of the results obtained in both laboratories have proved that the displacement corresponding to maximum load u m is affected mainly by the stiffness of the testing machine while the maximum load recorded during SP test F m is affected by the testing rig probably mainly by the shape of the chamfer edge of the bore diameter of the receiving die.
• The results obtained suggest that empirical correlations obtained in different laboratories cannot automatically be used for determination of yield stress and tensile strength at room temperature.This fact should be taken into account when revising the document CWA 15627 and during ASTM standards preparation.

Fig. 2
Fig. 2 Load-displacement curve recorded during a small punch test of a ductile material

Fig. 3 Fig. 4
Fig. 3 Comparison of empirical correlations for yield stress obtained at MMR and SME for 14MoV6-3 steel in the as-received state

Fig. 5
Fig.5shows the differences in stiffness of test fixtures used for penetration tests carried out in laboratory of SME on servo-mechanical testing machine CSS 44 000.

Fig. 5 Fig. 6 Fig. 6
Fig. 5 Differences in stiffness of test rigs used for penetration tests carried out on the testing machine CSS 44 000 [2]s.v20i4.425p-ISSN1335-1532e-ISSN 1338-1156 material to be sampled is the Small Punch Test Technique.It is mechanical testing method used at the present time to obtain tensile, fracture and creep characteristics with the specimens of disc shape 0.25-0.5 mm thick and 8-10 mm diameter.In 2007 CWA 15627 "Small Punch Test method for Metallic Materials"[2]was issued by CEN (European Committee for Standardization).In 2012 the solution of bilateral project, focused on the comparison of Codes of Practice for determination of tensile and fracture characteristics by SP tests between EU and China, was initiated in the frame of Czech-Chinese Scientific and Technological Cooperation.The participants of the project were Material & Metallurgical Research Ltd. (MMR), Ostrava, Czech Republic and School of Mechanical Engineering, East China University of Science and Technology, Shanghai, China.Both European Code of Practice

Table 1
Control chemical analysis of the tube in the as-received state In order to compare the effect of testing conditions and testing rigs, standardized round bar tensile test specimens 8 mm in diameter, oriented in longitudinal direction and semi-products of SP disc specimens of 8 mm and 10 mm in diameter oriented in R-L direction were manufactured in mechanical workshop of MMR for both laboratories.Tensile tests at ambient temperature were carried out in the laboratory MMR on servo-hydraulic testing machine MTS 100 kN under stroke control at the rate 1.5 mm/min.The same tests were carried out at the School of Mechanical Engineering (SME) in Shanghai on servo-hydraulic testing machine Instron 8800 at the rate 1.0 mm/min.Results obtained in both laboratories are summarized in Segments of the size 405 x 70 mm, cut from the pipe, were heat treated by 7 different regimes HT1 to HT7 (see Table2) to obtain significantly different yield stresses and tensile strengths.

Table 3 .
Values listed in the table are the average values of three measurements.The results obtained show a good agreement between the average values of yield stress, tensile strength, elongation and reduction of area determined in both laboratories.

Table 3
Comparison of tensile test results at ambient temperature carried out in both laboratories MMR) and steel ball with the hardness greater than 60 HRC (at SME) were used for SP tests in the temperature range -193°C to ambient temperature.DOI 10.12776/ams.v20i4.425p-ISSN 1335-1532 e-ISSN 1338-1156

Table 4
Comparison of F m and u m obtained in both laboratories during SP tests at ambient