Application of small punch test to estimate mechanical behaviour of SUS304 austenitic stainless steel

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The small punch test with an application of a relatively small specimen has recently become a reliable material mechanics testing method. In this study, the small punch test is set up based on the conventional mechanical testing machine for SUS304 stainless steel to evaluate the mechanical properties of SUS304 steel at different displacement rates of the punch in quasi-static loading condition in the case of with and without heat treatment.

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Nội dung Text: Application of small punch test to estimate mechanical behaviour of SUS304 austenitic stainless steel

Vietnam Journal of Mechanics, Vol. 46, No. 2 (2024), pp. 138 – 151 DOI: https:/ /doi.org/10.15625/0866-7136/20801 APPLICATION OF SMALL PUNCH TEST TO ESTIMATE MECHANICAL BEHAVIOUR OF SUS304 AUSTENITIC STAINLESS STEEL The Yen Doan , Hang Thi Pham∗ Faculty of Mechanical Engineering, Thuyloi University, 175 Tay Son, Dong Da, Hanoi, Vietnam ∗ E-mail: pthang@tlu.edu.vn Received: 16 May 2024 / Revised: 29 May 2024 / Accepted: 31 May 2024 Published online: 11 June 2024 Abstract. The small punch test with an application of a relatively small specimen has recently become a reliable material mechanics testing method. In this study, the small punch test is set up based on the conventional mechanical testing machine for SUS304 stainless steel to evaluate the mechanical properties of SUS304 steel at different displace- ment rates of the punch in quasi-static loading condition in the case of with and without heat treatment. Although heat treatment has an insigniﬁcant effect on the microstructure and hardness of the material, the mechanical properties of the material in the small punch test are greatly reduced after heat treatment. Both cases with and without heat treatment have a similar tendency for the rate - sensitivity of the applied force - displacement curve. A higher value of force is applied to obtain the same value of displacement at a low dis- placement rate in the stable plastic deformation zone. Meanwhile, the maximum value of applied force is higher at a higher displacement rate in the stage that initiation of crack might appear In the examined range of displacement rate, a positive rate - sensitivity of displacement at the maximum force. Therefore, a correlation between equivalent fracture strain and fracture toughness of the material can be achieved. Keywords: small punch test, SUS304 steel, strength of material, fracture toughness. 1. INTRODUCTION The mechanical properties of materials such as strength and ductility are usually evaluated by traditional testing techniques, for example, tension, compression or three- point bending test performed on a conventional mechanical testing machine. In 1982, a new experimental methodology called the small punch test has been designed by Mana- han et al. [1] for assessing the mechanical properties of irradiated materials by using very
Application of small punch test to estimate mechanical behaviour of SUS304 austenitic stainless steel 139 small specimen. Then, this technique has been developed to investigate the mechanical properties and fracture toughness of both ductile and brittle materials [2, 3] during last decades. This testing method has many advantages through the application of a rela- tively small specimen with a disc shape having a diameter of 3–10 mm and a thickness of 0.1–07 mm [4, 5]. Moreover, the small punch test in quasi-static condition can be per- formed on a conventional tensile testing machine with the design of a new ﬁxture [6, 7]. Therefore, this experimental method is cost-effective and has become a reliable material mechanics testing method that has attracted the attention of the scientiﬁc community. On the other hand, SUS304 austenitic stainless steel is widely used not only in the chemical and food industries but also for mechanical structures due to its good corrosion resistance and high mechanical properties [8]. This steel is considered as a representa- tive of the steels with phase transformation by plastic deformation [9, 10]. The unstable austenite phase of SUS304 steel can transform into martensitic phase during plastic defor- mation of the steel. Due to the formation of martensitic phase, the mechanical properties of the steel are considerably improved. Although some past studies examined the inﬂu- ence of strain-induced martensitic transformation in SUS304 steel in various deformation modes, the inﬂuence of phase transformation of this steel due to plastic deformation dur- ing the small punch test is still unclear. Furthermore, according to previous study [11], the stability of austenitic phase of this steel is strongly affected by heat treatment process. Therefore, an investigation on the effect of heat treatment on the mechanical properties of SUS304 stainless steel in the small punch test is indispensable. Recently, several studies have been carried out on the mechanical properties of austenitic stainless steel in the small punch test. Sunjaya et al. [12] performed ﬁnite ele- ment simulation for material behaviors of 304H stainless steel after different heat treat- ment conditions during the small punch test It is reported that the ductility of the investi- gated material is strongly affected by heat treatment. Mahmudi et al. [13] established the correlation between results from the small punch test and tensile yield as well as ultimate stress for type-304 austenitic stainless steel. Yang et al. [14] investigated the effect of the geometrical factors and some process parameters on plastic damage of SUS304 steel in the small punch test. Then, ductile fracture of 306L stainless steel in the small punch test was examined by Kub´k et al. [15]. Rate - sensitivity of fracture characteristics of SUS304 ı was investigated by Pham et al. [9] during the small punch test in quasi-static and impact loading condition. More recently, the small punch test was applied for an investigation of the inﬂuence of the inhomogeneous microstructure of 316L stainless steel multi-pass weld joint of a nuclear power plant done by Fan et al. [16]. In the previous study [6], the authors established the small punch test for an investigation of rate - sensitivity of mechanical properties for aluminum alloy A1050-H14. According to this study, the value of maximum force and the punch displacement at the maximum force obtained from the
140 The Yen Doan, Hang Thi Pham small punch test can be used for an evaluation of strength and fracture toughness of ma- terial, respectively. On the other hand, the investigation of Pham et al. [11] presented that the α’-martensitic phase can be observed in the microstructure of type-304 austenitic stainless steel after an appropriate heat treatment during grinding and polishing because of strain-induced martensitic transformation mechanism. In this study, the small punch test using relatively small specimen is set up based on the conventional mechanical testing machine for SUS304 stainless steel. The experimental works are performed at different displacement rate of the punch in quasi-static condition. The results on the curve of applied force and punch displacement are used to discuss the mechanical behavior of the material. The inﬂuence of heat treatment process on the results of force - displacement curve is examined. Then, the rate - sensitivity in the range of low displacement rate of mechanical properties of the steel obtained from the small punch test is discussed in the case of with and without heat treatment. 2. METHODOLOGY 2.1. Materials The material used in present study is SUS304 austenitic stainless steel. The inves- tigated material was analyzed for chemical composition with the results shown in Ta- ble 1. Compared to Japanese Industrial Standards (JIS), the chemical composition of the main elements such as carbon, chromium, nickel, ... is all within the standard range for SUS304 stainless steel. The specimen is heated by the electrical resistance furnace device LHT08/16. According to previous investigation [11], the heating temperature of heat treatment process should be over than 1000 ◦ C to increase the probability for martensitic transformation by plastic deformation. Therefore, in present study, the heating temper- ature is set up at 1050 ◦ C and the holding time is 20 minutes. After that, the specimen is quenched in water. The hardness of material is measured by a Rockwell hardness tester AR-20. The microstructure observation is performed on a metallurgical microscope Olympus GX41. Table 1. Chemical compositions of SUS304 steel Chemical element C Si Mn Cr Ni Co N V Wt. (%) 0.053 0.31 1.17 18.8 8.03 0.22 0.113 0.103 Chemical element P S Mo Al Cu Ti W Fe Wt. (%) 0.023 0.004 0.007 0.001 0.02 0.001 0.007 Blan.
3 3 Wt. (%) 0.053 0.31 1.17 18.8 8.03 0.22 0.113 0.103 Application P small punch0.053 estimate mechanical behaviour of SUS304 austenitic 0.22 Chemical element Wt.of(%) S test to Mo 0.31 1.17 Al Cu 18.8 8.03 Ti 0.113 stainless steel Fe W 0.103 141 Chemical element P S Mo Al Cu Ti W Fe Wt. (%) 0.023 0.004 0.007 0.001 0.02 0.001 0.007 Blan. 2.2. Small punch test Wt. (%) 0.023 0.004 0.007 0.001 0.02 0.001 0.007 Blan. Fig. 1 shows the schematic view and a photograph of the jig for the small punch test. 2.2. Small punch test The experimental model basically consists of an upper die, a lower die and a punch. A 2.2. Small punch test steel rod shows Fig. schematicthe schematichead of 1.2 photograph foristhe jig for the small punch test. The Fig. 1 punch with a hemisphericala photograph mm radius used. A punch test. The the view and of the jig small small disc-shaped experimental model basically consists of an upper die, a lower die and aof the A steel rod punch with 1 shows view and a punch. aspecimen isexperimental model basically consists of andisc-shaped specimen and a punch. the centerdie are hemispherical placed 1.2 the center of theA small die. Then,lower lower die and upper of head of in mm radius is used. lower upper die, a the die is placed in A steel rod punch with clamped by hemispherical head specimens with clamped by of screws. thickness and the center athe screws. The of 1.2 mm radius is used. A small disc-shaped specimen is placed in 10 mm of the lower die. Then, the lower die and upper die aredimensions the 0.5 mmThe specimens with the lower die. Then, the lower die and upper die are clamped by the screws. The specimens with dimensions of dimensions of 0.5 mm thickness and 10mm arediameter are this study. this study. is appliedis the in diameter0.5 mm thickness and 10mm in diameter in applied in appliedverticalload The load applied are applied in this study. The load is applied via in The direction to via vertical direction to the center of the center of theleading the plasticpunch, leading the plastic deformation center of the specimen through the punch, specimen through leading the plastic deformation via vertical direction to the specimen through the punch, the deformation of material. of material. of material. (a) a) b) (b) a) b) Fig. 1. a) Schematic presentation and b) A photograph of the jig for the small punch test Fig. 1. (a) Schematic2presentation and (b) A photograph of the jig for the small the small punchtest on a Fig. 1. a) Schematic presentation and b) A photograph of the jig for thepunch test Fig. shows a photograph of the experimental arrangement for small punch based Fig. 2 conventional testing machine experimental arrangement forRauenstein 5KN, Typ Z0M5/91. A new shows a photograph of the VEB Thuringer Industriewerk the small punch based on a conventional testing machine VEB Thuringer Industriewerk Rauensteinon a rigid plate.the small punch Fig. 2 showsas above described for the small punch test is placed 5KN, Typ Z0M5/91. A new is placed fixture a photograph of the experimental arrangement for A force sensor fixture as above described loadtesting machine VEBto obtain the applied force force sensor is placed KN, At based on a conventional the small punch test is placed on a rigid plate. A value Rauenstein 5 between the for block and the testing jig Thuringer Industriewerk during the experiment. between the load block time,the testing jig to obtain is used to record the displacement of experiment. Atplate. It is the same and the displacement sensor the applied force value during the the supporting the same time, necessary to ensure that is used todescribed displacementplate and the punch placed on a Typ Z0M5/91. displacement sensor the movement ofthe for the small the supporting plate. It via vertical the A new ﬁxture as above record the supporting of punch test is head is rigid plate. direction arethe movement ofresults of applied force and movement of the via vertical processes A force equivalent. The between the load block punch head punch are necessary to ensure that sensor is placedthe supporting plate and the and the testing jig to ob- direction are equivalent. data processorof applied force TMR-2111. the same time, results processes from the tain the applied force value during applied force - time At The and the punchthe displacement through a The results Multi Recorder and movement experimental are obtained the experiment. curve of punch through aisdatasensors record the displacement of the supporting plate. Itdisplacement - the en- sensor used to processor Multi Recorder TMR-2111. The experimental results obtained from time curve, will be plotted for respectively. Then, the curve of the relationship between applied force andis necessary to punch displacement of the sensors will be plotted for applied force - time curve and punch displacement - time curve, sure that the movement of the relationship between applied force and displacement of thedisplacement rate for the small punchthe supporting plate experiments are conductedvia vertical direction respectively. Then, the curve of test can be achieved. The and the punch head at different punch of the punch in quasi-static condition as 0.82; 1.09; 1.63; and 2.17 mm/s. for the small punch test can be achieved. The experiments are conducted atof the punch are processes are equivalent. The results of applied force and movement different displacement rate of the punch in quasi-static condition as Recorder 1.63; and 2.17 The experimental results obtained through a data processor Multi 0.82; 1.09; TMR-2111. mm/s. from the sensors will be plotted for applied force - time curve and punch displacement - time curve, respectively. Then, the curve of the relationship between applied force and displacement of the punch for the small punch test can be achieved. The experiments are conducted at different displacement rate of the punch in quasi-static condition as 0.82, 1.09, 1.63, and 2.17 mm/s.
4 4 142 The Yen Doan, Hang Thi Pham Fig. 2. Experimental arrangement of the small punch test in conventional tensile te Fig. 3 describes a photograph of the specimen using in the small punch test. T wire-cut from sheets made of SUS304 steel. Then, the surfaces of specimen are Fig. 2. Experimental arrangement of the smallto 2000test inin order to reduce the friction between the s silicon carbide papers up punch grit conventional tensile testing machine Fig. 2. Experimental arrangement of the small punch test ininfluence of surface geometry factors on the mech punch head as well as eliminate the conventional tensile testing machine Fig. 3 describes a photograph of the during plastic deformation. of the material specimen using in the small punch test. The specimens are wire-cut from sheets made a photographsteel. Then, the surfaces of specimen are ground by using Fig. 3 describes of SUS304 of the spec- silicon carbide papers up to small punch order to reduce the friction between the specimen and the imen using in the 2000 grit in test. The punch head as well are eliminate the influencemade of geometry factors on the mechanical properties specimens as wire-cut from sheets of surface of the material during plastic the surfaces of specimen SUS304 steel. Then, deformation. are ground by using silicon carbide papers up to 2000 grit in order to reduce the friction be- tween the specimen and the punch head as well as eliminate the inﬂuence of surface ge- ometry factors on the mechanical properties of the material during plastic deformation. The results obtained from the small punch test are discussed based on the relationship between applied force - punch displacement. Fig. 3. Specimen using in the small punch test Fig. 3. Specimen using in the small Fig. 4 shows a typical force results obtained from the small punch test are discussed based on the rela The - displacement punch test applied force - punch displacement. Fig. 4 shows a typical force - displacement cur the small punch test. From this figure, the shape of the obtained curve of the small p different from that obtained from traditional experiments such as tensile or three-po According to Abendroth in the small punchcurve can be divided into 5 zones corr Fig. 3. Specimen using et al. [17], this test The results obtained from the small punch test are discussed based on the relationship between
Application of small punch test to estimate mechanical behaviour of SUS304 austenitic stainless steel 143 curve obtained from the small punch test. From this ﬁgure, the shape of the obtained curve of the small punch test is very different from that obtained from traditional exper- iments such as tensile or three-point bending test. According to Abendroth et al. [17], 5 this curve can be divided into 5 zones corresponding to the deformation stages of the deformation stagesZone I ismaterial. Zone I isdeformation region of the material. Next, zone II material. material. of the the elastic bending the elastic bending deformation region of the Next, zone II shows a transition between deformation and plastic deformation. The third stage third stage shows a transition between elastic elastic deformation and plastic deformation. The of zone III is called is called the deformation zone. zone. Zone IV correspondsato a less stable de- of zone III the plastic plastic deformation Zone IV corresponds to less stable deformation and the initialformation and the initial crack might appear infinal stage. In theapplied force will decrease after crack might appear in this stage. In the this stage, the ﬁnal stage, the applied force will decrease after reaching its maximum value. reaching its maximum value. Fig. 4. Typical curve of applied force - punch displacement in thethe small punch test Fig. 4. Typical curve of applied force - punch displacement in small punch test 3. RESULTS AND DISCUSSION 3. RESULTS AND DISCUSSION Fig. 5 shows microstructure observation of SUS304 steel before and after heat treatment. From this figure, the difference microstructure observationsteel before andbefore heat after heat treat- Fig. 5 shows in the microstructure of of SUS304 steel after and treatment cannot be seen ment. From this ﬁgure, the difference in the microstructure of steel before and after the typical clearly. The microstructure mostly shows a homogeneous austenite phase similar to microstructuretreatment cannot be seen clearly. The microstructure mostly shows a homogeneous because of heat of SUS304 stainless steel. The appearance of shear bands can be observed plastic deformation of similar to the typical microstructure of SUS304 stainless steel. and ap- austenite phase the surface layer of the specimen during grinding The polishing for metallurgical observation. bands can be observed because of[11], although phase transformation during pearance of shear According to previous studies plastic deformation of the surface quenching cannotthe specimen during grinding and polishing for might become less stable Ac- this phase layer of be obtained, the austenite phase of the steel metallurgical observation. and might transform to previous studies [11], although phase transformation during quenching can- cording into martensitic phase during plastic deformation. Moreover, the hardness of not be obtained, the austenite phase of the steel might become less stable and this phase investigated material is 55HRA before heat treatment and 52HRA after heat treatment, respectively. It might transform into martensitic phase during plastic deformation. Moreover, the hard- can be considered that an improvement of hardness of the material as well as a considerable change of ness of investigated material is 55HRA before heat treatment and 52HRA after heat treat- the microstructure due to heat treatment cannot be observed. However, as above discussion, the case ment, respectively. It can be considered that an improvement of hardness of the material of heat treatment with more unstable austenitic phase might increase probability for martensitic as well as a considerable change of the microstructure due to heat treatment cannot be transformation by plastic deformation during the small punch test. Therefore, it is expected that the mechanical properties of the steel will be improved by heat treatment in the small punch test.
this figure, the difference in the microstructure of steel before and after heat treatment cannot be seen this figure, the difference in the microstructure of steel before and after heat treatment cannot be seen clearly. The microstructure mostly shows a ahomogeneous austenite phase similar to the typical clearly. The microstructure mostly shows homogeneous austenite phase similar to the typical microstructure of SUS304 stainless steel. The appearance of shear bands can be observed because of microstructure of SUS304 stainless steel. The appearance of shear bands can be observed because of plastic deformation of the surface layer of the specimen during grinding and polishing for plastic deformation of the surface layer of the specimen during grinding and polishing for metallurgical observation. According to previous studies [11], although phase transformation during metallurgical observation. According to previous studies [11], although phase transformation during quenching cannot be obtained, the austenite phase of the steel might become less stable and this phase quenching cannot be obtained, the austenite phase of the steel might become less stable and this phase might transform into martensitic phase Yen Doan, Hang Thi Pham might 144 transform into martensitic phase during plastic deformation. Moreover, the hardness of The during plastic deformation. Moreover, the hardness of investigated material isis55HRA before heat treatment and 52HRA after heat treatment, respectively. ItIt investigated material 55HRA before heat treatment and 52HRA after heat treatment, respectively. can be observed. However,improvement of hardness of theof heat treatmentas a considerable change of can beconsidered that an improvement of hardness of the material as well as a considerable change of considered that an as above discussion, the case material as well with more unsta- themicrostructure due to heat treatment cannot be observed. However, as above discussion, the case the microstructure due to heat treatment cannot be observed. However, as above discussion, the case ble austenitic phase might increase probability for martensitic transformation by plastic of heat treatment with more unstable austenitic phase might increase probability for martensitic of heat treatment with more unstable austenitic phase might increase probability for martensitic deformation during the small punch test. Therefore, it is expected that the mechanical transformation by plastic deformation during the small punch test. Therefore, ititisisexpected that the transformation by plastic deformation during the small punch test. Therefore, expected that the properties of the steel will be improved by heat treatment in the small punch test. mechanical properties of the steel will be improved by heat treatment in the small punch test. mechanical properties of the steel will be improved by heat treatment in the small punch test. 6 Fig. 6 presents a photograph of specimen after the small punch test. A local defo be seen from this figure. During the experiment, when the force is applied via vertical d (a)a) Before heat treatment with the punch (b) b) is After treatment a concave shape with a hat-sha part in contact Before heatheat treatment treatment headAfter deformed into b)is heat heat between a) Before Meanwhile, the part of specimen that After heat treatment the upper and lower dies m treatment clamped Fig. 5. MicrostructureIn of SUS304 steeldeformation, after heat one side of the specimen initiates a Fig. 5. Microstructure the final stage of before and after heat treatment deformed. of SUS304 steel before and a crack on treatment Fig. 5. Microstructure of SUS304 steel before and The obtained result of morphology of the deformed leading the fracture of the specimen. after heat treatment very similar to the typical deformation of the specimen in the small punch test. Fig. 6 presents a photograph of specimen after the small punch test. A local deformation can be seen from this ﬁgure. During the exper- iment, when the force is applied via vertical di- rection, the part in contact with the punch head is deformed into a concave shape with a hat- shaped proﬁle. Meanwhile, the part of spec- imen that is clamped between the upper and lower dies might not be deformed. In the ﬁ- nal stage of deformation, a crack on one side of the specimen initiates and expands, leading the fracture of the specimen. The obtained re- Fig. 6. Photograph of specimen after exper- Fig. 6. Photograph of specimen after experiment at punch displacement rate of 2.17 sult of morphology of the deformed specimen iment at punch displacement rate of is very similar to the typical deformation the the To ensure of reliability of the experimental results, the experiments at each displ 2.17 mm/s were repeated at least three times. Then, the obtained results of applied force - punch d specimen in the small punch test. relationship at different rate of punch displacement are shown in Fig. 7 for the unheat To ensure the reliability of the experimental results the experiments at displacement curves are quite s specimens. It can be seen that the applied force - punch each displace- typical applied force - punch displacement curve obtained from the small punch test as s ment rate were repeated at least three times. deformation stages of the material are quite corresponding to tho past studies [9]. The Then, the obtained results of applied force punch displacement relationship at different rate can punch that the small punch was successfully set up and Fig. 4. From these results, it of be seen displacement are shown in Fig. 7 for the unheated treatment specimens. It can arrangement might be reliable.force punch results from this experimental be seen that the applied
Fig. 6. Photograph of specimen after experiment at punch displacement rate of 2.17mm/s Fig. 6. Photograph of specimen after experiment at punch displacement rate of 2.17mm/s Application of small punch test to estimate mechanical behaviour of SUS304 austenitic stainless steel 145 To To ensure the reliability thethe experimental results, the experiments each displacement rate ensure the reliability of of experimental results, the experiments at at each displacement rate were repeated at least three times. Then, thethe obtained results of applied forcepunch displacement obtained results of applied force - - punch displacement were repeated at least threequite similar to the typical applied force punch displacement displacement curves are times. Then, relationship at different rate of of punch displacement are shown in Fig.forfor the unheated treatment relationship at different rate punch displacement are shown in Fig. 7 7 the unheated treatment specimens. It cancanfrom thethat the punch test- as- showndisplacement studiesare quite similar to the curve obtained seen that small applied force punch in the past curves [9]. The defor- specimens. It be be seen the applied force punch displacement curves are quite similar to the typical applied force -the punch displacement curve obtained fromthose shown testFig.as shown in the typical applied force - material are quite corresponding to the small punch test shown in the mation stages of punch displacement curve obtained from the small punch in as 4. From past studies [9]. The deformation stages small punch was successfully set up and the those shown in past studies [9]. can be seen that the of of the material are quite corresponding to obtained in these results, it The deformation stages the material are quite corresponding to those shown Fig. 4. From these results, it can be be seen that themight punch was successfully set up and the obtained Fig. 4. From these experimentalseen that the small punch reliable. results from this results, it can arrangement small be was successfully set up and the obtained results from this experimental arrangement might be be reliable. results from this experimental arrangement might reliable. 7 (a) Displacement rate of 1.09 mm/s b) Displacement raterate 1.09 mm/s (b) Displacement rate of 1.63 mm/s b) Displacement of of 1.09 mm/s b) Displacement rate of 1.63 mm/s b) Displacement rate of 1.63 mm/s (c) Displacement rate of 2.17 mm/s c) Displacement rate of 2.17 mm/s Fig.7. Relationship of applied force - punch displacement at various punch displacement rate for Fig. 7. Relationship of applied force - punch displacement at various punch displacement rate unheated-treatment specimens for unheated-treatment specimens Next, force-displacement curve for SUS304 steel under the small punch test at different displacement rate is described in Fig. 8. The influence of the punch displacement rate can be seen clearly. According to Cao et al. [18], the total consumption energy small punch test at differ- be Next, force-displacement curve for SUS304 steel under the in the small punch test can calculated from the area under the applied Fig. 8.- The inﬂuence of the curve until the maximum value ent displacement rate is described in force punch displacement punch displacement rate of applied seen clearly. According8, it can be observed total consumptionconsumption issmall at can be force. Thus, from Fig. to Cao et al. [18], the that total energy energy in the higher higher displacement rate in the investigated range of displacementforce -of the punch from 0.82 to punch test can be calculated from the area under the applied rate punch displacement 1.63curve untilobtain same value of displacement in the plastic deformation stage (zone observedcases mm/s. To the maximum value of applied force. Thus, from Fig. 8, it can be III), two of lower displacement rate indicate higher at higher displacement rate to the investigated that total energy consumption is higher levels of force compared in the cases of higher displacement rate. Meanwhile, the values of maximum force as well as displacement at the maximum force increase with an increase of displacement rate from 0.82 mm/s to 1.63 mm/s. However, the value of maximum force at 2.17 mm/s is slightly smaller than that at 1.63 mm/s.
Fig.7. Relationship of applied force - punch displacement at various punch displacement rate for unheated-treatment specimens Next, force-displacement curve for SUS304 steel under the small punch test at different displacement rate is described in Fig. 8. The influence of the punch displacement rate can be seen clearly. According to Cao et al. [18], the total consumption energy in the small punch test can be calculated from the area under the applied force - punch displacement curve until the maximum value 146 The Yen Doan, Hang Thi Pham of applied force. Thus, from Fig. 8, it can be observed that total energy consumption is higher at higher displacement rate in the investigated range 0.82displacement rateobtain same value of 0.82 to range of displacement rate of the punch from of to 1.63 mm/s. To of the punch from 1.63 mm/s. To obtain same value of displacement in the plastictwo cases of lower displacement displacement in the plastic deformation stage (zone III), deformation stage (zone III), two cases of lowerrate indicate higher levels of force compared to of force of higher displacement rate. higher displacement rate indicate higher levels the cases compared to the cases of displacement rate. Meanwhile, of maximum force as wellforce as well as displacement at the maximum Meanwhile, the values the values of maximum as displacement at the maximum force force increase with an an increase of displacement rate from 0.82mm/s to 1.63 mm/s. However, the value increase with increase of displacement rate from 0.82 mm/s to 1.63 mm/s. However, of maximumvalue of maximum force at 2.17 smaller than thatsmaller than that at 1.63 mm/s. the force at 2.17 mm/s is slightly mm/s is slightly at 1.63 mm/s. Fig. 8. Force - displacement curve at various punch displacement rate before heat treatment Fig.8. Force - displacement curve at various punch displacement rate before heat treatment Fig. 9 describes the effect of heat treatment on the force - displacement curve ob- Fig. 9 describes small punch test fortreatment on at different displacement rate in quasi- tained from the the effect of heat SUS304 steel the force - displacement curve obtained from the small punch test for SUS304 steel at different displacement rateseen.quasi-static condition. A static condition. A considerable inﬂuence of heat treatment can be in At low displace- considerable influence ofmm/s,treatment canmaximum applied force and displacement 0.82mm/s, the ment rate of 0.82 heat the values of be seen. At low displacement rate of at the values of maximum applied force and displacement higher displacement force decrease after heat maximum force decrease after heat treatment. At at the maximum rate in the investi- treatment. At higher except for the elasticin the investigated a decrease in the value of force can gated range, displacement rate deformation stage, range, except for the elastic deformation be seen in the case of heated treatment in the plastic deformation stage. As above men- tion, the value of maximum force obtained in the small punch test can be used to calculate the ultimate strength of material. As a result, it can be considered that the heat treatment has a considerable effect on the ultimate strength of material in the investigated range of displacement rate. An improvement of the ultimate strength cannot be achieved by heat treatment. Furthermore, in the case of 0.83 mm/s, a reduction in the displacement at the maximum force due to heat treatment might induce a decrease in the fracture toughness of the material.
8 8 stage, a a decrease in the value of force can be seen in the case of heated treatment in the plastic stage, decrease in the value of force can be seen in the case of heated treatment in the plastic deformation stage. As in above mention, thecan be seen in the force of heated treatment small punch test deformation stage.in above value of force value seen in the case force obtained in the the the plastic stage, a decrease As the mention, the value of maximum of heated treatment small punch test stage, a decrease the value of force can be of maximum case obtained in the in in plastic candeformationto calculate the ultimate strength ofof of maximum result,obtainedconsidered that the heat candeformation stage. As above mention, the of material. As a force obtainedbebe considered punch test bebe used calculate the ultimate strength value maximum result, it can in thethe small that the heat used to stage. As above mention, the value material. As a force it can in small punch test treatmentbe used considerable ultimateonon the of of material. Asresult,material in the investigatedheat of of treatment has a considerable ultimate strength material. As a of of it cancan considered that therange can be used a to calculate the effectstrength ultimate strength result, it be be considered that the heat can has to calculate the effect the ultimate strength a material in the investigated range displacementhas rate. considerable effect on the ultimatestrength cannot bebethethe investigatedtreatment. displacementhas considerable effect on of the ultimatestrength ofcannot inachieved byby heatrange of An improvement the ultimate strength material in investigated range of treatment rate. aAn improvement of the ultimate strength of material treatment a achieved heat treatment. Furthermore, in in rate. case improvement aestimate mechanical behaviour ofcannot be at the by by steelforce due to to Furthermore,ApplicationAnsmall punch test of of the ultimatethe displacement achievedstainless heat force due displacement rate. case improvement to thereduction strengthdisplacement achieved heat treatment. 147 displacement the of ofof 0.83mm/s, reduction in instrength SUS304 austeniticmaximum treatment. the An 0.83mm/s, a ultimate the cannot be at the maximum heat treatment might theinduce0.83mm/s,in in the fracture toughness of the material. heat treatmentin in induce aof 0.83mm/s,thereduction toughness of the material. maximum force due to Furthermore, mightcase of decrease a reduction in in the displacement thethe Furthermore, the case a decrease a fracture the displacement at at maximum force due to heat treatment might induce a decrease in thethe fracture toughness thethe material. heat treatment might induce a decrease in fracture toughness of of material. a)(a) Displacementrate ofof 0.82 mm/s Displacement rate of of 0.82 mm/s Displacement rate of 0.82 mm/s a) a) Displacement rate 0.82mm/s a) Displacement rate 0.82 mm/s (b)b)Displacementrate of of mm/s /s b)b)Displacement rate of 1. 09 mm Displacement of of 1. 1. mm/s Displacement rate 1.09 b) Displacement raterate1.09 0909 mm/s (c) Displacement rate of 1.63 mm/s Displacement rate of 1.63 mm/s c) c) Displacement rate of 1.63 mm/s d) Displacement raterate2.17 2.17mm/s (d) Displacement rateof 2.17 mm/s d) Displacement of of mm/s c) c)Fig.9. Effect of heat treatment on force - displacement curved) Displacement rate of 2.17 mm/s Displacement rate of of 1.63 mm/s Displacement rate 1.63 mm/s d) Displacement rate of 2.17 mm/s Fig.9. Effect of heat treatment on force - displacement curvedifferent punch displacement raterate at at different punch displacement Fig. 9. Effect Effect oftreatment on force - displacement curve at different punch displacement rate ofEffect of heat treatment on forcedisplacement curve at different punch displacementrate Fig.9. Fig.9. heat heat treatment on force - - displacement curve at different punch displacement rate The relationship of of applied forcepunch displacement at various punch displacement rate in the the The relationship applied force - - punch displacement at various punch displacement rate in case of of heat treatmentof applied in in-Fig. 10.displacement at at variousprocess strongly affects the thethe case heat treatmentof applied force punch Although, the heat treatment process strongly affects in The relationship is presented force -10. Although, the heat treatment punch displacement rate the The relationship is presented Fig. punch displacement various punch displacement rate in case ofof displacement is curve applied force -thethe tendency of treatmentvarious punch displace- case Thetreatment curveof as shown in Fig. 9, tendency of heat treatment process strongly affects forceheat displacement is presented in Fig. 10. Although, themechanical properties of material in the thethe - - relationship as shown in Fig. 9, forceheat treatment presented in Fig. 10. Although, the heatmechanical process strongly affects the punch displacement at Also, a properties of material in case ofdisplacement curvequite similar to that of of tendency of mechanical properties higher valuein the case of heated treatment isshownsimilar to that thethe unheated treatment propertieshighermaterial the heated treatment is asquite unheated treatment case. Also, a of value of of case. force - displacement curve as shown in Fig. 9, the force - ment rate in the case in heat treatmentthe presented in Fig. 10. Although, of material in in Fig. 9, tendency of mechanical total consumption energy of the small punch test can be obtained at higher rate of punch the heat treat- total heated treatment quite similar punch is the unheated treatment rate of punch higher value displacement case ofofrange from 0.82 mm/squite similar to that ofcan be obtained at higher case. Also, higher value of of case heated treatment is is in the smallto that of the unheated treatment case. Also, a a displacement consumption energy test in in the range from 0.82 mm/s to 1.63 mm/s. - displacement curve as shown in Fig. 9, the ten- ment process energy in into 1.63 the punch test can be obtained at higher rate of punch displacement the affects mm/s. strongly the small punch test can be obtained at higher rate of punch displacement total consumption energy the small total consumption force in in the rangemechanical properties of material inin Figs. 8 andheated rate - sensitivity of applied the range fromthe 0.82 mm/s to-1.63 mm/s. curve the case ofand 10, the rate - sensitivity ofsimilar dency of from results ofto 1.63 mm/s. From the 0.82 mm/s forcedisplacement curve in Figs. 8 10, the treatment is quite applied From results of force - displacement force in in the low range oftreatment case. Also, a higherpresented Fig. 11.11. detail, Fig. 11(a) toforce of the unheated displacement rate achieved as as valuein in Fig.consumption energy that the low range of displacement rate is is achieved presented of total In In detail, Fig. 11(a) presents athe negativeofof- force displacement curveforce Figs. and displacement- of sensitivity of applied From negative rateforce - - displacement curve in From the results results rate sensitivity of of applied in Figs. 8 8 and 10, the ratesensitivity of applied 10, the rate - - sensitivity applied force value at displacement of 1mm. However, a a inpresents a range test can be obtained achievedvalue at forcepositive low punchofof displacement forceis higher rate ofrange of displacementdetail, Fig. 11(a) 1mm. However, force in small sensitivity displacement force isatthe investigated punch displacementIn inin Fig. 11(a) in the rate - range of maximum rate in in the investigated range in Fig. 11. detail, in range the presented in Fig. 11. In rate the quasi- positive the low sensitivity of maximum rate achieved as as presented of displacement rate quasi- rate - presents testaisnegative to 1.63 sensitivity of means that thevalue might displacement excellentHowever, a a presents test mm/srate - sensitivity This applied the steel mightat possess more of 1mm. However, from 0.82 indicated in Fig. mm/s. static a negative rate -Fig. 11(b). of means that forcesteel at possess more excellent mechanical static is indicated in 11(b). This applied force value displacement of 1mm. mechanical positive rate at sensitivity ofof maximumin both in the with and without heatdisplacement rateplasticquasi- positive rate at sensitivity maximum force cases with and without range of displacement rate plastic properties - - lower displacement rate both in cases investigated heatof treatment during the in lower displacement rate in force the investigated range treatment during the in quasi- properties the results of force - displacement curve in Figs. 8 and 10, the rate - sensitiv- From staticdeformation stagein in small11(b). This OnOnthat the hand, a positivepossess - sensitivity maximum static test is indicatedthe Fig. punch test. meansthe other hand, might rate -more excellent mechanical deformation stage of ofFig. small punch means otherthe steel a positive rate more excellent mechanical test is indicated 11(b). This test. the that steel might possess sensitivity of of maximum ity of applied forcethe the low range of displacement rate is achieved as presented in in properties at at lower displacement rate in both cases with and without heat treatment during the plastic properties lower displacement rate in both cases with and without heat treatment during the plastic deformation Instage of the small punch test. On the other rate a positive rate ofsensitivity forcemaximum deformation detail, Fig. 11(a) presents athe other hand, - a positive rate -applied of of value Fig. 11. stage of the small punch test. On negative hand, sensitivity - sensitivity maximum at displacement of 1 mm. However, a positive rate - sensitivity of maximum force in the investigated range of displacement rate in quasi-static test is indicated in Fig. 11(b). This means that the steel might possess more excellent mechanical properties at lower
148 The Yen Doan, Hang Thi Pham 9 displacement rate in both cases with and without heat treatment during the plastic de- force is shownstage of11(b), leading an improvement other hand, a positive ratehigher deformation formation in Fig. the small punch test. On the of the ultimate strength at - sensitivity of maximum force is shown in Fig. 11(b), leading an improvement of the ultimate strength rate. at higher deformation rate. 9 is shown in Fig. 11(b), leading an improvement of the ultimate strength at higher deformation Fig.10. Force - displacement curve at various displacement rate after heat treatment Fig. 10. Force - displacement curve at various displacement rate after heat treatment 10 Fig.10. Force - displacement curve at various displacement rate after heat treatment (a) a) Rate-- sensitivity applied force atforce at displace- Rate sensitivity of of applied displacement of 1 mm (b) Rate - sensitivity of of maximum force b) Rate – sensitivity maximum force ment of 1 mm Fig.11. Rate - sensitivity of applied force in the cases of with and without heat treatment a) Rate - sensitivity of applied force at displacement of 1 mm Fig. 11. Rate - sensitivity of applied forceshows rate - sensitivity of displacement at the maximum applied force in the ca Fig. 12 in the cases of with and without heat treatment with and without heat treatment. Both cases with and without heat treatment indicate a positive sensitivity of displacement of the punch at the maximum applied force. According to previous Fig. 12 shows rate - sensitivity of displacement at the maximumcorrelation with thein the fracture stra [19], fracture toughness of material might indicate a applied force equivalent cases of with and without heat treatment.from results of the displacement at the maximum force. From results of Fig. can be evaluated Both cases with and without heat treatment might be said that the material shows higher value of equivalent fracture strain at higher displac indicate a positive rate - sensitivity the displacement of the punch at the maximum applied with the results rate in of investigated range of displacement rate. This tendency is consistent force. According to previous integral for determine fracture toughness of reported in Pham et al. [20] for the same material study [19], fracture toughness as material might indicate a result, a correlation between equivalent fracture strain in the small punch test and fracture toughn correlation with the equivalent material canstrain that cansmallevaluated from results of the condition. the fracture be obtained in the be range of displacement rate in quasi-static
b) Rate – sensitivity of maximum force Fig.11. Rate - sensitivity of applied force in the cases of with and without heat treatment Fig. 12Application of small punch test to estimate mechanical behaviour of SUS304 austenitic stainless steelin the cases of shows rate - sensitivity of displacement at the maximum applied force 149 with and without heat treatment. Both cases with and without heat treatment indicate a positive rate - sensitivity of displacement of the punch at the maximum applied force.itAccording to previousthe displacement at the maximum force. From results of Fig. 12, might be said that study [19], fracture shows higher value of equivalent fracture strain at higher displacement rate in material toughness of material might indicate a correlation with the equivalent fracture strain that can the evaluated from results of the displacement at the tendency is consistent results of results it be investigated range of displacement rate. This maximum force. From with the Fig. 12, might be said that for material shows highertoughness as reported in Pham at higher displacement on J-integral the determine fracture value of equivalent fracture strain et al. [20] for the ratesame material. As range of displacement rate. This tendency is consistentstrain the the small J- in the investigated a result, a correlation between equivalent fracture with in results on integral for determine fracture toughness as reported in Pham et al. [20] for the same material. As a punch test and fracture toughness of the material can be obtained in the small range of result, a correlation between equivalent fracture strain in the small punch test and fracture toughness of the displacement rate in quasi-static condition. material can be obtained in the small range of displacement rate in quasi-static condition. Fig. 12. Rate - sensitivity of displacement at the maximum applied force in the cases of with Fig.12. Rate - sensitivity of displacement at the maximum applied force in the cases of with and without and without heat treatment heat treatment 4. CONCLUDING REMARKS CONCLUDING REMARKS The small punch tests are performed to evaluate the mechanical properties of SUS304 steel at different displacement rates of the punch in quasi-static loading condition in the case of with and without heat treatment. The obtained results show that although heat treatment has an insigniﬁcant effect on the microstructure and the hardness of the ma- terial, the ultimate strength of the material determined in the small punch test is greatly reduced after heat treatment. However, the tendency of the rate - sensitivity of the ap- plied force - displacement curve in the cases with and without heat treatment is quite similar. In the stable plastic deformation zone, the value of applied force value needs to be higher to obtain the same displacement value at low displacement rate However, in the stage of unstable plastic deformation then leading to fracture, the applied force in- creases at higher displacement rate In the examined displacement rate range, a positive - rate sensitivity of displacement at the maximum force. Therefore, a correlation between equivalent fracture strain and fracture toughness of the material can be achieved.
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