Analyses of instrumented hardness test and small punch test are done using experimental tests and finite element simulation. These methods can be used when some material properties are unknown and there is no way to cut larger pieces from the structure to determine them using standard size specimens.С использованием экспериментальных данных и конечноэлементного моделирования анализируются экспериментальные методы инструментальной оценки твердости и индентирования малых образцов. Данные методы можно использовать при определении некоторых

неизвестных механических свойств материала, когда нет возможности вырезать образцы

стандартных размеров из исследуемой конструкции.Із використанням експериментальних даних і скінченноелементного моделювання аналізуються експериментальні методи інструментальної оцінки твердості й індентування малих зразків. Дані методи можна використовувати при

визначенні деяких невідомих механічних властивостей матеріалу, коли немає

можливості вирізати зразки стандартних розмірів із досліджуваної конструкції

Beleznai, R.

Jonas, Sz.

Koves, T.

Lenkey, G.B.

Rozsahcgyi, P.

Szavai, Sz.

Наукова електронна бібліотека періодичних видань НАН України (Vernadsky National Library of Ukraine)

UDC 539.4Determination of Mechanical Properties of Aged NPP ComponentsUsing Instrumented Hardness Testing and Other Miniature SpecimenTesting TechniquesG. B. Lenkey, Sz. Szàvai, P. Rîzsahegyi, T. Kîves, Sz. Jînàs, and R. BeleznaiBay Zoltan Nonprofit Ltd., Institute for Logistics and Production Systems, Miskolc,HungaryÓÄÊ 539.4Îïðåäåëåíèå ìåõàíè÷åñêèõ ñâîéñòâ ýëåìåíòîâ ÀÝÑ ïîñëå ñòàðåíèÿñ ïîìîùüþ èíñòðóìåíòàëüíîé îöåíêè òâåðäîñòè è äðóãèõ ìåòîäîâèñïûòàíèÿ ìàëûõ îáðàçöîâÃ. Á. Ëåíêåé, Ø. Øàâàè, Ï. Ðîæàõåãèé, Ò. Êîâåñ, Ø. Éîíàñ, Ð. ÁåëåçíàèÁàé Çîëòàí Ëòä., Èíñòèòóò ëîãèñòèêè è ïðîèçâîäñòâåííûõ ñèñòåì, Ìèøêîëüö,ÂåíãðèÿÑ èñïîëüçîâàíèåì ýêñïåðèìåíòàëüíûõ äàííûõ è êîíå÷íîýëåìåíòíîãî ìîäåëèðîâàíèÿ àíàëè-çèðóþòñÿ ýêñïåðèìåíòàëüíûå ìåòîäû èíñòðóìåíòàëüíîé îöåíêè òâåðäîñòè è èíäåíòèðî-âàíèÿ ìàëûõ îáðàçöîâ. Äàííûå ìåòîäû ìîæíî èñïîëüçîâàòü ïðè îïðåäåëåíèè íåêîòîðûõíåèçâåñòíûõ ìåõàíè÷åñêèõ ñâîéñòâ ìàòåðèàëà, êîãäà íåò âîçìîæíîñòè âûðåçàòü îáðàçöûñòàíäàðòíûõ ðàçìåðîâ èç èññëåäóåìîé êîíñòðóêöèè.Êëþ÷åâûå ñëîâà: èíñòðóìåíòàëüíàÿ îöåíêà òâåðäîñòè, èíäåíòèðîâàíèå ìàëûõîáðàçöîâ, êîíå÷íîýëåìåíòíîå ìîäåëèðîâàíèå.Introduction. During the operation of nuclear power plants (NPP) mechanicalproperties of the structural materials can change due to several factors such asirradiation, temperature, load level and fatigue loading, corrosion, etc. Such achange in material properties influences the lifetime of the equipment (ageing). Itis essential for the fitness for service assessment to have reliable information onactual mechanical properties of an operating structure. In most cases, it is notallowed to cut larger pieces from an operating component to manufacture specimensand perform destructive laboratory tests, because it may have significant impact onthe component’s integrity. The determination of the necessary data is possible withnon-destructive or semi-destructive techniques.1. Instrumented Hardness Test. In several cases, the hardness measurement isthe only accepted non-destructive testing method whose results can be used toassess the actual mechanical properties such as yield stress or tensile strength.Since the scatter of the properties calculated directly from the pure hardness valueis usually too large and the number of properties to be assessed is limited, methodshave been developed for the evaluation of the mechanical properties from theload–indentation curve using instrumented hardness testing. The most reliable© G. B. LENKEY, Sz. SZÀVAI, P. RÎZSAHEGYI, T. KÎVES, Sz. JÎNÀS, R. BELEZNAI, 201364 ISSN 0556-171X. Ïðîáëåìû ïðî÷íîñòè, 2013, ¹ 4method is the so-called automated ball indentation (ABI) test [1]. This methodrequires quite a complex measurement device (Fig. 1) and application on a realstructure can be problematic. For handling the difficulties in the case of on-sitemeasurements, a simplified measurement procedure has been implemented formanually operated instrumented hardness tester neglecting the download cycles.The method has been successfully tested on different materials of real componentsat the Mechanical Testing Laboratory of BAY-LOGI. True stress–true strain curveswere derived from the indentation curves and the results were validated withtensile tests and with finite element modeling as well.1.1. Test Parameters and Evaluation Method. The Mat-Tec Unihard S3 typeportable instrumented indentation test machine was used, for which an extraaluminum frame was designed and manufactured, in order to clamp the machineproperly on the tested equipment surface (Fig. 1). In the case of carbon steel 4magnet blocks and in the case of austenitic steel two fixing belts could be used.The maximum applied indentation load was 500 N. The indenter was tungstencarbide cone with 1 mm tip radius. The load and indentation depth curves wererecorded and evaluated. The ABI method was used [1] to determine the truestress–true plastic strain curve, the yield stress and the ultimate tensile strength ofthe tested materials. These parameters were also determined with tensile tests carriedout at room temperature and were compared with the indentation test results.Using the true stress–true strain curve determined with tensile tests and withthe indentation tests, the load–indentation depth curve also was determined usingfinite element methods in order to validate the results obtained from the instrumentedhardness testing. Three different material types were tested: austenitic steel, carbonsteels with and without Lüders strain. Some kind of statistical analysis was alsocarried out to determine the uncertainty of the instrumented hardness measurementwith conservative approximation.1.2. Results of the Hardness Test. The results of the finite element method(FEM) validation show that the indentation curves and the finite element resultsalong the loading part of the curve are in good agreement (Fig. 2.)Fig. 1. Instrumented hardness tester on a component.ISSN 0556-171X. Ïðîáëåìû ïðî÷íîñòè, 2013, ¹ 4 65Determination of Mechanical Properties of Aged NPP Components ...The true stress–true strain curves determined from indentation curves are ingood agreement with those determined from tensile tests (Figs. 3–5.). The yieldstress and the ultimate tensile strength calculated with ABI method fitted well withthe tensile test results. The differences were not more than 3.7% in the case of yieldstress, and 4.5% in the case of ultimate tensile strength for each tested materialtype. The calculated measurement uncertainties were 5.85% for yield stress and7.46% for the ultimate tensile strength. The measurement uncertainty calculationwas based on a conservative method. To determine this with more accuracy, thework be will continued in the future and more extended analysis of the testparameters will be needed.Despite of the fact that the curves, the yield stress and the ultimate tensilestrength calculated with ABI method fitted well with the tensile test results, theresults have some scatter and not always provide accurate data for furtherFig. 2. FEM validation of the evaluation method.Fig. 3. True stress–true strain curve of austenitic steel.66 ISSN 0556-171X. Ïðîáëåìû ïðî÷íîñòè, 2013, ¹ 4G. B. Lenkey, Sz. Szàvai, P. Rîzsahegyi, et al.assessment. Nevertheless, the ABI method does not provide fracture mechanicsproperties, so other alternative way should be applied. The selected method is thesmall punch test (SPT), which is one of the miniature specimens’ testing techniquesand belongs to the semi-destructive testing methods.2. Small Punch Test. In the framework of a research programme, the smallspecimen test techniques, like SPT, were studied to understand the size effect andto find an up-to-date measurement technique to assess the ageing problems ofstructural materials of NPP. The aim of our work was to evaluate the mechanicalproperties of 15H2MFA, 22K, X6CrNiTi 18-10 (08H18N10T) and their welds andheat-affected zone at room temperature, to compare these data with the results fromconventional testing techniques and the above mentioned ABI test. The SPT is auseful technique in residual lifetime assessment owing to the lower tests evaluationcost compared to destructive methods and has higher lifetime evaluation accuracythan the nondestructive test techniques. But the SPT has a disadvantage: theloading originates biaxial stress state in the specimen during the test which causescomplication in the determination of the correct material properties.Fig. 4. True stress–true strain curve of carbon steel without Lüders strain.Fig. 5. True stress–true strain curve of carbon steel with Lüders strain.ISSN 0556-171X. Ïðîáëåìû ïðî÷íîñòè, 2013, ¹ 4 67Determination of Mechanical Properties of Aged NPP Components ...The tests have been carried out with the typical specimen size of the SPT. Inmost literature sources, the SPT specimen has 8 mm diameter and 0.5 mmthickness [2] (Fig. 6). The force–displacement curves are registered in each test.The diameter of the ball (2.5 mm) and also the geometry of the testing clampingdevice were chosen in accordance with most of the test equipment found in theliterature. The SPT was performed according to the guidelines of the EuropeanCode of practice [3].The aim of the tests was to determine the yield strength of the steels by theso-called two-tangent method from the registered force–displacement curves. Theresult of the SPT test was validated by finite element method using MSC.Marc&Mentat code [4].The typical SP force–displacement curves had four distinct regions (Fig. 7).The first part is the elastic bending, the second is the plastic bending, the third partis the membrane stretching and the last one is the region of plastic instability. Theforce Py corresponds to the limit of the elastic region and the beginning of theplastic deformation [2]. The tests were made by an Instron 8874 servo-hydraulicbiaxial tensile tester machine (Fig. 8) at room temperature. The displacement ismeasured with an extensometer and a deflection meter at the same time.The geometry of the smallpunch test specimenThe half of the 3D modelof the testing deviceFig. 7. Typical SP force–displacement curve.68 ISSN 0556-171X. Ïðîáëåìû ïðî÷íîñòè, 2013, ¹ 4G. B. Lenkey, Sz. Szàvai, P. Rîzsahegyi, et al.Fig. 6. SPT specimen and device.2.1. SPT FEM Model. The SPT can be modeled as a 2D axis-symmetricmodel, because the specimen, the clamping device and the load have cylindricalsymmetricity. The model is shown in Fig 9. The conjecture is that the materialswith high strength can cause deformation in the testing tool itself, so it is modeledby three parts. The upper die has two parts, because there is a large deviation in theresults in the case of high-strength steels. The upper die (Fig. 9, upper die 2/1) canmove up 0.1 mm during the test by the leverage of the specimen. This movementcan compensate the deviation in the elastic-bending zone between the measurementand the FEM results. The three parts of the device are linked with a spring to eachother in the model to consider the elasticity of the clamping device. The mesh ofthe lower die is finer than the upper ones, because higher deformation can beexpected in the device. The same consideration was taken in the case of the balland the specimen, too. The ball and the disk were inserted into displacement in thehorizontal direction. The puncher is modeled as a rigid contact body and it isrepresented by a curve which is on the upper part of the ball. Its position ischanging during the test. The device, the ball and the SP specimen are modeled asa deformable contact body. The total number of the finite elements in the model is1213, the specimen has 650 four-node type quadrilateral elements.Fig. 8. The testing machine and the sections of the specimens after the test.Small punch test Sections of the specimensFig. 9. FEM of the SPT.ISSN 0556-171X. Ïðîáëåìû ïðî÷íîñòè, 2013, ¹ 4 69Determination of Mechanical Properties of Aged NPP Components ...The material properties are defined using true stress–true plastic strain curves,which are obtained by tensile tests. In favour of the more precise results, the curvesare extended with a fitting curve in the higher strain value region. The next twodiagrams show the hardening curves of the 22K and X6CrNiTi 10-18 (Figs. 10 and11). The experimental test of the 15H2MFA material is in process yet and the FEMresult is not published here. Figure 12 shows the distribution of the total equivalentplastic strain field in the SP specimen after the test. Making comparison of theforce–displacement curves from the test with the FEM analyses, very goodagreement can be obtained (Figs. 13 and 14).Fig. 10. True stress–true plastic strain curve (22K).Fig. 11. True stress–true plastic strain curve (X6CrNiTi 10-18).Fig. 12. The total equivalent plastic strain distribution in the specimen after the test.Fig. 10 Fig. 11Fig. 13. FEM results comparison with measurement (22K).70 ISSN 0556-171X. Ïðîáëåìû ïðî÷íîñòè, 2013, ¹ 4G. B. Lenkey, Sz. Szàvai, P. Rîzsahegyi, et al.2.2. Analysis of the Effects of the Friction, the Gap in the Clamping Deviceand the Tolerance of the Specimen Thickness. Performing finite element analysisfor the clamping device without the 0.1 mm gap between the upper parts, theresults of the test and calculation show a large difference in the first, elastic-bendingregion. Consideration of the gap in the model provides good agreement betweenthe test and calculation. The expectation of the clamping device deformationproved to be correct. In case of the 22K material, the gap is not needed.Consequently, the difference in the strength of the materials causes inaccuracy inthe measurement, so the steels with high strength need compensation (Fig. 15). Thedifferent friction coefficients are causes difference in the results of the maximumforces as it can be seen in Fig. 16. There is no effect in the elastic-bending region,however, there is large difference in the plastic instability zone. Work [2] providesan equation to determine the tensile strength based on the maximum force from theforce–displacement curve, but that expression is valid for larger ball diameter. Thatequation is derived from 10 types of steels used for power plant components. Thetolerance of the specimen thickness 0.005 mm is assumed in [5]. The question isthat how bigger tolerance affects the results? The results show high dependence onthe tolerance of the thickness, so the thickness of the specimens is one of the mostimportant parameter in this test (Figs. 17 and 18). The allowed tolerance causes nodeviation, but higher difference in the thickness changes the results, particularlythe value of the maximum force.Fig. 14. FEM results comparison with measurement (X6CrNiTi 10-18).Fig. 15. Effect of the gap in the loading frame.ISSN 0556-171X. Ïðîáëåìû ïðî÷íîñòè, 2013, ¹ 4 71Determination of Mechanical Properties of Aged NPP Components ...2.3. Determination of the Yield Strength from the SP Curves. There is a newway to evaluate the yield strength of materials [6]. For this the Py value isnecessary, which is the maximum load at the upper surface of the specimen whenthe material properties begin to change, the elastic bending region turns into theplastic bending region (Fig. 7) 0 2 2.,Pty(1)Fig. 16. FEM results of the higher difference than tolerance from the literature.Fig. 17. FEM results for the different friction coefficients.Fig. 18. FEM results for the tolerance of the specimen thickness.72 ISSN 0556-171X. Ïðîáëåìû ïðî÷íîñòè, 2013, ¹ 4G. B. Lenkey, Sz. Szàvai, P. Rîzsahegyi, et al.where  is the material constant (for X6CrNiTi 10-18,  0.477; for 22K, 0.36), Py is the yield load, and t is the thickness of the specimen (0.5 mm).The comparison of the yield strengths from the Eq. (1) and from tensile tests arepresented in Table 1.Two tangent lines should be drawn on the force–displacement curve (one forthe elastic-bending zone and the other is for the plastic bending region). Then theintersection point should be projected to the force axis of the diagram. Theobtained value is the Py (Fig. 19).Conclusions. Analyses of the automated ball indentation test and the smallpunch test are performed using experimental tests and finite element simulation.Both test techniques have been applied for determination of the material propertiesof NPP structural materials. The ABI test shows some scatter in the resultshowever it is practically applicable when only non-destructive test technique ispermitted. The validation of the SPT was also completed and the results from finiteelement analyses corroborated the measurement results. The yield strengthdetermination shows that the calculation results are good agreement with theresults of the tensile tests. These methods can be applied when some materialproperties are unknown and there is no way to cut larger pieces from the structureto determine them with standard size specimens.Ð å ç þ ì å²ç âèêîðèñòàííÿì åêñïåðèìåíòàëüíèõ äàíèõ ³ ñê³í÷åííîåëåìåíòíîãî ìîäåëþ-âàííÿ àíàë³çóþòüñÿ åêñïåðèìåíòàëüí³ ìåòîäè ³íñòðóìåíòàëüíî¿ îö³íêè òâåð-äîñò³ é ³íäåíòóâàííÿ ìàëèõ çðàçê³â. Äàí³ ìåòîäè ìîæíà âèêîðèñòîâóâàòè ïðèT a b l e 1Comparison of the Measured Yield StrengthsSteel Yield strengthfrom SP (MPa)Yield strengthfrom FEM (MPa)Yield strengthfrom tensile test (MPa)22K 244 245 250X6CrNiTi 10-18 270 276 305Fig. 19. Determination of the Py yield load from SP curves with two-tangent method.ISSN 0556-171X. Ïðîáëåìû ïðî÷íîñòè, 2013, ¹ 4 73Determination of Mechanical Properties of Aged NPP Components ...âèçíà÷åíí³ äåÿêèõ íåâ³äîìèõ ìåõàí³÷íèõ âëàñòèâîñòåé ìàòåð³àëó, êîëè íåìàºìîæëèâîñò³ âèð³çàòè çðàçêè ñòàíäàðòíèõ ðîçì³ð³â ³ç äîñë³äæóâàíî¿ êîíñò-ðóêö³¿.1. F. M. Haggag, “In-situ measurements of mechanical properties using novelautomated ball indentation system,” in: W. R. Corvin, F. M. Haggag, andW. L. Server (Eds.), Small Specimen Test Techniques Applied to NuclearReactor Vessel Thermal Annealing and Plant Life Extension, ASTM STP1204 (1993), pp. 27–44.2. I. Klevtsov and A. Dedov, “Experience in tensile properties determination bysmall punch test,” in: Proc. 1st Int. Conf. on Determinination of MechanicalProperties of Materials by Small Punch and Other Miniature Testing Techniques(Aug. 31–Sept. 2, 2010, Ostrava, Czech Republic), Metallurg. J., LXIII,Special Issue, 128–132 (2010).3. R. Hurst and K. Matocha, “The European Code of practice for small punchtesting – where do we go from here?,” in: Proc. 1st Int. Conf. on Determininationof Mechanical Properties of Materials by Small Punch and Other MiniatureTesting Techniques (Aug. 31–Sept. 2, 2010, Ostrava, Czech Republic),Metallurg. J., LXIII, Special Issue, 5–11 (2010).4. MSC.Marc&Mentat User’s Guide.5. I. Nonaka, A. Kanaya, S. Komazaki, and K. Kobayashi, “Standardization oftest method for small punch creep testing in Japan,” in: Proc. 1st Int. Conf. onDeterminination of Mechanical Properties of Materials by Small Punch andOther Miniature Testing Techniques (Aug. 31–Sept. 2, 2010, Ostrava, CzechRepublic), Metallurg. J., LXIII, Special Issue, 12–18 (2010).6. P. Du, X. Ling, Z. Zhou, and T. Xu, “Study on influence factors of smallpunch test to estimate the yield strenght by energy model,” in: Proc. 1st Int.Conf. on Determinination of Mechanical Properties of Materials by Small Punchand Other Miniature Testing Techniques (Aug. 31–Sept. 2, 2010, Ostrava,Czech Republic), Metallurg. J., LXIII, Special Issue, 133–137 (2010).Received 03. 10. 201274 ISSN 0556-171X. Ïðîáëåìû ïðî÷íîñòè, 2013, ¹ 4G. B. Lenkey, Sz. Szàvai, P. Rîzsahegyi, et al.

Determination of Mechanical Properties of Aged NPP Components Using Instrumented Hardness Testing and Other Miniature Specimen Testing Techniques

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Determination of Mechanical Properties of Aged NPP Components Using Instrumented Hardness Testing and Other Miniature Specimen Testing Techniques

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Наукова електронна бібліотека періодичних видань НАН України (Vernadsky National Library of Ukraine)