Influence of key test parameters on SPT results

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1 Indian Journal of Engineering & Materials Sciences Vol. 16, December 2009, pp Influence of key test parameters on SPT results K K Pathak a *, K K Dwivedi b, Manali Shukla a & E Ramadasan c a Advanced Materials and Processes Research Institute (CSIR) Bhopal , India b Department of Mechanical Engineering UIT RGTU Bhopal , India c PIED Group, Bhabha Atomic Research Center, Mumbai , India Received 27 January 2009; accepted 12 October 2009 In order to assess the strength of aged material during the service life, small punch test (SPT) has emerged as a powerful NDT tool of engineers. It is a promising technique for those circumstances where only a small amount of material is available for characterization. SPT involves use of small size samples tested by bending or shearing to fracture in a specially designed die and punch assembly. As the sample and tooling sizes go down, the sensitivity of test parameters on the SPT output becomes an important issue. In order to get realistic material properties, sensitivity of test parameters should be studied. The parameters found to be critical can be given due attention. In this study, influence of several key parameters like fillet radius, ball diameter, sample thickness, yield stress and friction are studied using finite element simulations and their effects are critically examined. The analyses indicate that yield stress is less sensitive parameter as it does not affect peak load and corresponding displacement but fillet radius, ball diameter, plate thickness and friction considerably affect the value of peak load and corresponding displacement. So, these parameters must be precisely and carefully measured during small punch tests. Based on these findings, experimental load stroke data, obtained due to faulty test parameters, may be corrected rather than going for a new test with correct parameters. Keywords: Small punch, stress, strain, finite element, load stroke In service evaluation of mechanical properties of power plant structural materials especially steels used in reactor pressure vessel, fuel cladding and pressure tubes is very important from the remaining life assessment perspective. The major problems associated with this are the non-availability of sufficient material for conventional ASTM test and need of in-service investigation without affecting the functioning. To overcome these limitations, one of the routes followed is testing of miniature specimens. These are non destructive techniques which require very small amount of material as compared to other conventional techniques. These techniques have become quite popular is nuclear industries. Lucas 1 presented a detailed of review of the miniature testing techniques. Among all miniature tests, small punch test (SPT) is most popular. SPT was invented in Japan and soon become very popular worldwide 2. Ha and Fleury 3 and Bulloch 4 reported prediction of fracture toughness using SPT. Kameda and Mao 5 determined ductile brittle transition temperature using SPT. Mao and Kameda 6 reported measurement of material degradation of Cu alloys. Foulds et al. 7 *For correspondence ( kkpathak1@rediffmail.com) reported calculation of fracture toughness using SPT and finite element method (FEM). Yang and Wang 8 applied SPT to determine creep properties of the of the materials. Xu and Xhao 9 talked about miniature testing equipments to characterize materials. Lee et al. 10 applied SPT to predict embrittlement of the cladding material of the inner wall of reactor pressure vessel. Since sample and tooling are of tiny sizes, even a slight deviation in the test parameters may affect the SPT output considerably. In this regard, study of the influence of these test parameters is an important issue. In this study the same has been carried out using finite element analysis. Experimental load stroke data obtained by using faulty test parameters can be realistically corrected using this approach. Small Punch Test Small punch test (SPT) is an advancement of ball punch miniaturized test technique. In SPT, cut samples are metallographically prepared upto 1 µm diamond polish to a thickness of 0.25 mm. The samples are given metallographic finish to avoid deformed layer due to the sample preparation techniques. A punching device is used to punch out

2 386 INDIAN J. ENG. MATER. SCI., DECEMBER 2009 the prepared samples to a disc of 3 mm diameter. Figure 1 shows the schematic diagram of test configuration for testing of disc sample of 3 mm diameter and 0.25 mm thickness. The experimental configuration used in this technique is as per the given specification in Mao and Takahashi 2. It consists of a clamped center-loaded specimen disc of 3 mm diameter and 0.25 mm thickness, an upper and a lower die, four clamping screws, a hardened steel ball punch of 1 mm diameter and a plunger to drive the ball. Figure 2 depicts the fillet radius, ball diameter and the sample thickness to relate the physical dimensions. Small punch tests are carried out using a computer controlled universal testing machine with 500 kg load-cell and a constant crosshead speed of 0.2 mm/min. The tests can be carried out at room temperature and elevated temperatures up to 350 o C. The load and deflection are continuously recorded during testing. The deflection is measured using a finger gauge with a least count of mm. In Fig. 3, a typical load stroke curve is shown. The curve can be divided into 6 zones. Zone I is the elastic response, zone II represent transition between elastic and plastic behaviour. Zone III is related to hardening. In zone IV, geometrical softening and damage occur and zone V is related to failure and crack growth. Zone VI represents the post failure scenario. In order to get material parameters from the load stroke data, inverse engineering approach is employed. Since SPT load stroke data depends on several test parameters, their sensitivity should to be ascertained for realistic material characterization. Test Parameters Following five test parameters and their variations are accounted in the simulation study: (i). Fillet radius = 0.1, 0.2, 0.3 mm (ii). Ball diameter = 0.9,1, 1.1 mm (iii). Sample thickness = 0.24, 0.25, 0.26 mm (iv). Yield stress = 200, 300, 400 MPa (v). Friction (coulomb) = 0.1,0.2,0.3 The standard values of fillet radius, ball diameter and sample thickness are 0.2, 1 and 0.25 mm respectively. Three variations in these parameters are considered to study their influence on SPT output, viz., load stroke curve. FE Analysis of SPT Axisymmetric finite element modeling of the small Fig. 1 Schematic of SPT Setup Fig. 2 Dimensions of SPT setup Fig. 3 Typical Load Stroke Curve

3 PATHAK et al.: INFLUENCE OF KEY TEST PARAMETERS ON SPT RESULTS 387 punch test is carried out using MSC. Superform software 11 for all the cases. Four noded elements are used for this purpose and a typical FE model is shown in Fig. 4. There are 150 elements and 186 nodes in the model. Ball and punch are modeled as rigid and specimen as deformable bodies. Interactions between different bodies are accounted by the in-built contact algorithm of the software. The study material is SS303 steel. Elasto-plastic material modeling is considered for FE simulation. Following values are accounted: Young s modulus (E) = MPa Poisson s ratio (ν) = 0.3 The post yielding behaviour is modeled using the power law equation 12 : σ = kε n where k is the strength coefficient and n is the hardening exponent. k and n values of SS303 are considered as 800 MPa and 0.3 respectively. All the cases are identically deformed to 1.2 mm of punch displacement. Each simulation is attempted in 100 iterations. Typical contours of stress and strain contour in the deform specimen is shown in Figs 5 and 6. It can be observed that maximum stress and strain are around 1430 MPa and which are quite high. Result and Discussion Based on FE simulation results, effects of various test parameters can be studied as follows. Effect of fillet radius Load stroke curves corresponding to various fillet radii, are shown in Fig. 7. It can be inferred that peak load decreases with increase in fillet radius. Corresponding stroke is found to decrease under same circumstances. Thus, depth of penetration in the deformed plate will be more at larger fillet radius. Effect of ball diameter Load stroke curves corresponding to various ball diameter, are shown in Fig. 8. It can be inferred that peak load increases with increase in ball diameter. Corresponding stroke is found to remain same irrespective of ball diameter. It signifies that deformation occurs at higher peak load if ball diameter is increased but depth of penetration of deformed sample remains unchanged for different ball sizes. Fig. 4 FE Model Fig. 5 Effective stress contour (MPa) Fig. 6 Plastic strain contour

4 388 INDIAN J. ENG. MATER. SCI., DECEMBER 2009 Fig. 7 Effect of fillet radius Fig. 9 Effect of sample thickness Fig. 8 Effect of ball diameter Effect of sample thickness Load stroke curves corresponding to various sheet thickness, are shown in Fig. 9. It can be inferred that peak load increases with increase in sample thickness. Corresponding stroke is found to remain same irrespective of thickness. It signifies that deformation occurs at higher peak load if sample thickness is increased but depth of penetration of deformed sample remains unchanged for varying sample thickness. Effect of yield stress Load stroke curves corresponding to various yield stress, are shown in Fig. 10. It can be inferred that peak load and corresponding stroke are independent of the yield stress. Yield stress has prominent role on peak load corresponding to elastic limit in the Zone I. It increases with increase in the yield stress. Effect of friction Load stroke curves corresponding to various friction, are shown in Fig. 11. Peak load and Fig. 10 Effect of yield stress corresponding stroke are found to increase with an increase in friction. Based on these findings, SPT results can be critically analyzed. In case of major deviation, specific parameter may be cross checked. For example, in case of deviation in peak load in Zone I, yield stress will be different indicating aging effect on the material. Effect of these parameters on different zones of load stroke curve can be summarized as: (i) Fillet radius causes deviations in Zones II, III, and IV. (ii) Ball diameter causes deviations in Zones III, IV, and V. (iii) Sample thickness causes deviations in Zones II, III, IV, and V. (iv) Yield stress also causes deviations in Zones II and early part of III apart from the Zone I (v) Friction causes deviations in Zones latter-half of III, IV, and V. Since these implications are inter-related, it is very difficult to sort out individual s share from the

5 PATHAK et al.: INFLUENCE OF KEY TEST PARAMETERS ON SPT RESULTS 389 as it does not affect peak load and corresponding displacement but fillet radius, ball diameter, plate thickness and friction considerably change the value of peak load and corresponding displacement. Hence, these parameters need more attention during the test and simulation. Acknowledgement Financial support for this study, provided by BRNS Mumbai vide sanction number 2006/36/24- BRNS/2806 is gratefully acknowledged. Fig. 11 Effect of friction experimental load stroke curve. However, based on these finding, sensitivities of key test parameters may be studied for giving special attention during the test. In case of any deviation in these parameters from the standard values, load stroke curves may be scaled up/down accordingly. Conclusions In this study, influence of several key test parameters on SPT results are studied using simulation techniques. Their effect on various zones of the load stroke data is examined. It will help in determining the sensitive parameters, to be given special attention, to improve the predictability of test. In the event of deviation, specific parameters rather than the all, need to be crosschecked and load stroke data may be scaled up/down to get the actual one. It is observed that yield stress is less sensitive parameter References 1 Lucas G E, Metall Trans A, 21A, (1990) Xinayun Mao & Hideaki Takahashi, Recommended practice for Small Punch (SP) testing of metallic materials, Japan Atomic Energy Research Institute (1987). 3 Ha J S, Fluery E, Int J Pressure Vessels Piping, 75 (1998) Bulloch J H, Int J Pressure Vessels Piping, 75 (1998) Kameda & Mao, J Mater Sci, 27 (1992) Mao & Kameda, J Mater Sci, 26 (1991) Foulds J R, et al., Fracture toughness by small punch test, ASTM, Yang Z & Wang Z, Int J Pressure Vessels Piping, 80 (2003) Xu Y & Zhao Z, A modified miniature disk test for determining material mechanical properties, ASTM, Lee J, Kim I & Kimura A, J Nuclear Sci, 40 (9) (2003) User s manual, MSC.Superform, MSC Software Corporation, Santa Ana, California USA, (2005). 12 Meyers M A & Chawla K K, Mechanical behaviour of materials, (Prentice Hall), 1999.