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In this study, instrumented indentation testing was conducted at room temperature for the investigation of the effect of strain rate on the hardness and yield strength in the weld zone of a commonly used structural steel, SM520. A number of indentation tests were undertaken at a number of strain rate values from 0.02 s −1 to 0.2 s −1 in the weld metal (WM), heat-affected zone (HAZ), and base metal (BM) regions of the weld zone.
Journal of Science and Technology in Civil Engineering NUCE 2019 13 (3): 104–112 INDENTATION FOR INVESTIGATION OF STRAIN RATE EFFECT ON MECHANICAL PROPERTIES IN STRUCTURAL STEEL WELD ZONE Pham Thai Hoana,∗, Nguyen Ngoc Vinhb , Nguyen Thi Thanh Tungc a Faculty of Building and Industrial Construction, National University of Civil Engineering, 55 Giai Phong road, Hai Ba Trung district, Hanoi, Vienam b Department of Civil and Environmental Engineering, Sejong University, 98 Gunja-dong, Gwangjin-gu, Seoul, South Korea c Faculty of Civil Engineering, Vinh University, 182 Le Duan street, Truong Thi district, Vinh city, Nghe An province, Vietnam Article history: Received 22/07/2019, Revised 21/08/2019, Accepted 22/08/2019 Abstract In this study, instrumented indentation testing was conducted at room temperature for the investigation of the effect of strain rate on the hardness and yield strength in the weld zone of a commonly used structural steel, SM520 A number of indentation tests were undertaken at a number of strain rate values from 0.02 s−1 to 0.2 s−1 in the weld metal (WM), heat-affected zone (HAZ), and base metal (BM) regions of the weld zone The mechanical properties including yield strength (σy ) and hardness (H) in WM, HAZ, and BM were then computed from the applied load-penetration depth curves using a proposed method As the result, the effects of strain rate indentation on yield strength and hardness in all regions of the weld zone were evaluated The results displayed that hardness and yield strength in the weld zone’s components are influenced on the strain rate, where both hardness and yield strength decrease with the decreasing strain rate Keywords: indentation; mechanical properties; strain rate effect; structural steel; weld zone https://doi.org/10.31814/stce.nuce2019-13(3)-10 c 2019 National University of Civil Engineering Introduction The excellent weldability and machinability of structural steel, which caused by it’s high strength, stiffness, toughness, and ductility, have led to the common usage of this material in many construction fields including buildings, bridges, tunnels and in the manufaction of machinery parts and equipments [1–3] Welding is considered as the efficient method to form the strong joints between the steel parts, where the structural steel is used However, the welding joints are also considered as the weakest parts of structures [4] The heating or cooling stages influence the microstructures in the weld zone, including the weld metal (WM) region, the heat-affected zone (HAZ), and base metal (BM) region near the weld due to the transformation of solid-state phases, leading to the change of material properties in the weld zone [5–8] Thus, the properties in the local regions of weld joints need to be evaluated The high ductility and energy dissipation capacity have also been the important reason for the wide utilization of structural steel in both static and seismic applications It has been pointed out that the mechanical properties of structural steel are governed by the metallurgical aspects and strongly ∗ Corresponding author E-mail address: hoanpt@nuce.edu.vn (Hoan, P T.) 104 Hoan, P T., et al / Journal of Science and Technology in Civil Engineering dependent on the conditions of strain rate [9–11] For example, Luecke et al [11] carried out the dynamic loading tensile tests for several low-carbon steel types and concluded that their tensile and yield strength decrease with the decreasing strain rate However, the influences of strain rate on the hardness and yield strength in the weld zone of structural steel have not been well reported Since these effects are the important factor for the engineering analyses as well as the steel structure designs in the both static and dynamic problems, it is essential to have a comprehensive investigation of the strain rate influences on the hardness and yield strength in the weld zone of structural steel Instrumented indentation testing (IIT) has been known as an efficient method in extracting material properties at both the macro- and nano-scales [12] For characterization of the mechanical properties under different strain rate levels, it has also proved to be reliable and efficient since this approach can not only provide accurate results [13–16] but also overcome the uneconomical and time-consuming dynamic tensile loading tests This work aims to evaluate the strain rate influences on the hardness and yield strength in the weld zone of structural steel using instrumented indentation tests A number of indentations were undertaken at a number of strain rate values from 0.02 s−1 to 0.2 s−1 in the weld zone (WM), heat-affected zone (HAZ), and base metal (BM) regions The mechanical properties including yield strength (σy ) and hardness (H) in WM, HAZ, and BM were then computed from applied load-penetration depth curves of indentation using a proposed method As the result, the effects of strain rate indentation on yield strength and hardness in all regions of the weld zone were investigated Methods Fig illustrates a typical load-depth (P-h) curve of an elastic-plastic material to a three-sided Berkovich indentation [17] From this curve, several indentation characteristic parameters, such as the maximum penetration depth hm , the maximum applied load Pm , the residual depth after unloading hr , the initial unloading slope S , the loading curvature C, the projected of contact Ac , the residual plastic work W p , and the recovered elastic work We , can be extracted As can be seen in Fig 1, hm , Pm , hr , W p , and We , can be directly obtained from the P-h curve, while S , C, and Ac can only be extracted based on the description of loading and unloading curves For sharp indenters, the initial unloading slope S , the loading curvature C, and the projected of contact Ac can be expressed, respectively, as follows [17]: dP = mB (hm − hr )m−1 dh h=hm Pm C= hm Ac = 24.5h2c S = (1) (2) (3) where hc = hm − ε∗ Pm /S and ε∗ = 0.75 for sharp indenters [17] The mechanical properties of the indented material, such as elastic modulus E, yield strength σy , strain hardening exponent n, and hardness H, can thus be evaluated from these above indentation parameters Numerous analytical approaches that allow the determining of the mechanical properties from the indentation load-penetration depth curve have been proposed in recent years [17–21] Among which Oliver and Pharr’s method [17] has been considered as one of the most popular methods to extract elastic modulus and hardness of indented material, while the algorithm proposed by Pham et al [21] could be considered as a representative approach for determination of yield strength and 105 C= Pm hm2 (2) Ac = 24.5hc2 (3) = hm −P. T.,Pmet al S and =of0.75 where hc Hoan, for sharp indenters in[17] / Journal Science and Technology Civil Engineering * * Figure indentation Figure1.1.Typical Typical load-depth load-depth (P-h) curve of indentation The mechanical properties of the indented material, such as elastic modulus E, yield strength y, of strain hardening exponent n, and H, can thus evaluated strain hardening exponent structural steel In Oliver andhardness Pharr’s method, thebe value of E and H from these indentation parameters be extracted from theabove following relations [17]: Numerous analytical approaches that allow the determining of the mechanical √ properties from the indentation load-penetration π S Er =in recent depth curve have been proposed √ years [17-21] Among which Oliver and 2β A Pharr’s method [17] has been considered cas one of the most popular methods to −1 − v2i material, while the algorithm − vof2 indented extract elastic modulus and hardness E = + proposed by Pham et al [21] rcould beE considered Ei as a representative approach for determination of yield strength andPm strain hardening exponent of structural steel In H value = of E and H can be extracted from the following Oliver and Pharr’s method, the Ac can (4) (5) (6) where Er is the reduced modulus owing to the effects of elastic deformation of the indenter E, υ, Ei and υi are the elastic modulus and Poisson’s ratio of the sample and indenter, respectively For determination of the yield strength, the algorithm proposed by Pham et al [21] that allows extracting yield strength σy of structural steel was used In this method, the yield strength σy in the weld zone can be determined with respect to α using the following polynomial equations [21]: Er∗ = σy 4 jk n j−1 αk−1 i=1 j=1 k=1 S = ∗ E r hm 4 Er∗ C i−1 bi jk n j−1 αk−1 ln i=1 j=1 k=1 (7) Er∗ σy i−1 (8) where α is defined as the ratio of the strain at starting point of strain hardening and the yield strain and jk and bi jk are coefficients [21] The α value of the weld zones can be obtained with the aid of FE analysis of indentation by correlating the experimental with the simulated load- depth curves The details of the procedure for determination of the yield strength in the weld zones from the proposed method and FE analysis can be found in the previous work [22] Experiments One of the most common used structural steel (SM520) with the chemical composition listed in Table was chosen to be investigated The weld with suitable weld material in the form of double V 106 Hoan, P T., et al / Journal of Science and Technology in Civil Engineering groove butt with no root gap was employed to connect two 12 mm-thickness steel plates by welding using metal arc method under an 100 A of current and 22 V of voltage Table Compositions of weld and steel material (in wt.%) Material C Si Mn P S Al Cu Ni Steel Weld 0.17 0.12 0.41 0.35 1.39 1.44 0.017 0.010 0.001 0.003 0.04 0.04 0.02 - 0.13 From the welded plate, a slice across the weld was cut out by water jet cutting method Such cutting method did not affect the transformation of properties in the weld zone The slice was then used for the preparation of indentation specimen according to ASTM E3-01 standards [23] The smooth and flat of specimen surface were ensured to meet the requirement of the indentation standard after being polished in seven stages by silicon carbide papers, poly diamond particles, and colloidal silica with the fineness of the last stage about 40 nm Such a smooth and flat surface of specimen for indentation tests is considered as main criterion to eliminate the surface roughness in the similarity analysis and to minimize the occurance of error in the tests [24] Indentation testing was carried out using a Nano Hardness Tester at room temperature comforming to ASTM E2546-07 standard [25] The diamond Berkovich indenter with elastic modulus of Ei = 1141 GPa and Poission ratio of υ = 0.07, was Journal of of Science Science and and Technology Technology in in Civil Civil Engineering Engineering NUCE 2019 employed Indentation tests were undertaken in threeJournal regions, WM, HAZ, and BM ofNUCE the2019 weld zone by using the displacement control mode without a holding time at a number of strain rate values from 0.02 s−1 to 0.2 s−1 A 50 µm spacing - grid of × indenting points were performed at each strain rate value of 0.02 s−1 , 0.04 s−1 , 0.1 s−1 , and 0.2 s−1 for each region of the weld, leading to the total of 100 and 300 indenting points in each region and in all regions of the weld, respectively All indentations were also carried with a maximum applied load of 2000 nm in order to obtain the composite behavior instead of a microstructural phase response from the tests Fig shows the cut-out location of the specimen from the welded plate and the polishing specimen surface, on which the indenting positions Journal of Science and Technology in Civil - NUCE 2019 sample in the indentation test machine are also illustrated, and theEngineering installation of the (a) (a) (a) (a) (b) (b) (b) (c) (c) (c) Figure (a) Cut-out Cut-out from from the the welded welded plate plate by by water water jet, jet, (b) (b) Indenting Indenting positions positionsin ineach each region, and and (c) (c) Sample Sample installed installed in in the the indentation indentation test test machine machine Figure (a) Cut-out from the welded plate by water jet, (b) Indenting positions in each region and (c) Sample installed in the indentation test machine Results and discussion discussion 4.1 Indentation response response The representative representative indentation indentation responses responses of of the the material material in in BM, BM, HAZ, HAZ, and and WM, WM, at different strain rate rate levels, levels, together together with with the the averaged averaged load-depth load-depth curves curves in in these these regions at a certain strain strain rate rate level level of of 0.2 0.2 ss-1-1 are are displayed displayed in in Fig Fig 3 ItIt isis seen seen from from Figs 3a-c that strain rate 107 rate during during indentation indentation tests tests influences influences both both the the shape shape and and the the magnitude of the load-depth load-depth curves curves of of the the tests tests For For all all regions regions in in the the weld, weld, the the loading curvature tends tends to to decrease decrease with with the the decreasing decreasing strain strain rate rate during during the the tests, tests, leading to the higher maximum maximum applied applied load load with with the the higher higher strain strain rate rate level level due dueto tothe the 66 Hoan, P T., et al / Journal of Science and Technology in Civil Engineering Results and discussion Journal of Science Science and and Technology Technology in in Civil Civil Engineering Engineering NUCE NUCE 2019 2019 Journal of 4.1 IndentationJournal response of Journal ofScience Scienceand andTechnology Technology in in Civil Civil Engineering Engineering NUCE NUCE 2019 2019 The representative indentation responses of the material in BM, HAZ, and WM, at different strain rate levels, together with the averaged load-depth curves in these regions at a certain strain rate level applied constant maximum displacement in all the tests Regarding to the correlation applied constant maximum displacement in all the Regarding to the applied maximum displacement in tests Regarding to the correlation applied constant maximum displacement in all all the the tests tests Regarding toindentation the correlation correlation of 0.2 s−1constant are displayed in Fig It is seen from Figs 3(a)–3(c) that strain rate during tests between the indentation response in different regions of the weld, the distinguishable influences both the shape and the magnitude of the load-depth curves of the tests For all regions in between the indentation response in different regions of the weld, the distinguishable between between the the indentation indentation response response in in different different regions regions of of the the weld, weld, the the distinguishable distinguishable the weld, the loading curvature tends to decrease with the decreasing strain rate during thebetests, leadvariation of the indentation curves obtained from BM, HAZ, and WM can observed variation of the indentation curves obtained from variation BM, HAZ, and WM can be observed variationof ofthe theindentation indentation curves curves obtained obtained from from BM, BM, HAZ, HAZ, and and WM WM can can be beobserved observed ing to the higher maximum applied load with the higher strain rate level due to the applied constant in Fig 3d can be seen from this figure that both loading curvature and applied load that both loading curvature and load in ItIt this figure loading curvature and applied load in Fig Fig 3d 3d It can can be be seen seen from this figure that that both loading curvature and applied applied load maximum displacement in all from the tests Regarding to theboth correlation between the indentation response of the indentation lowest and these parameters are highest in the curves in the BM are lowest and these parameters are highest in in regions ofcurves the weld, variation of the indentation curves obtained from of the in the BM and parameters are the ofdifferent the indentation indentation curves inthe thedistinguishable BM are are lowest lowest and these these parameters are highest highest in inthe the BM, HAZ, and three WM can be observed in observation Fig 3(d) It can be seen from this figure that both loading WM among regions This observation consists with the available results of consists with the available results WM three regions This consists with the available results of WM among among three load regions This observation observation consists with the available resultsareof of curvature and applied of the indentation curves in the BM are lowest and these parameters indentation of other structural steels [3, 6, 22] and responses for the weld zone of other structural steels [3, 6, 22] and indentation responses for the of structural steels and indentation responses for regions the weld weld zone of other other structural steels [3, [3, 6, 6,of22] 22] and highest in the WM among three Thiszone observation consists with the available results indencorresponds to the lowest hardness and yield strength in BM among three regions, yield strength in among tation responses thelowest weld zone of other and structural steels [3, 6, 22] and to theregions, lowest corresponds to hardness strength in BM among three regions, corresponds toforthe the lowest hardness and yield yield strength in BM BMcorresponds among three three regions, hardness and yield strength among three regions, respectively, which will be discussed below respectively, below which willinbeBM discussed below respectively, respectively,which which will will be be discussed discussed below below (a) (a) (a) (a) (c) (b) (b) (b) (b) (b) (d) (c) (d) (d) (c) (d) (c) (d) −1 Figure Indentation responses (P-h curves) in (a) BM, (b) HAZ, (c) WM and (d) All regions(c) at εWM, = 0.2 sand Indentation responses (P-h curves) in (a) BM, (b) HAZ, Figure curves) in (a) BM, (b) HAZ, (c) WM, and Figure3 3.Indentation Indentation responses responses (P-h (P-h curves) curves) in in (a) (a) BM, BM, (b) (b) HAZ, HAZ, (c) (c) WM, WM, and and Figure -1 (d) All regions at == 0.2 ss-1 regions at 0.2 -1 -1 (d) All All regions regions at == 0.2 0.2 ss (d) 108 at rate effect on mechanical properties in the weld zone 4.2 Strain properties in the weld zone 4.2.Strain Strainrate rateeffect effect on on mechanical mechanical properties properties in in the the weld weld zone zone 4.2 Hoan, P T., et al / Journal of Science and Technology in Civil Engineering 4.2 Strain rate effect on mechanical properties in the weld zone Journal of Science and Technology in Civil Engineering - NUCE 2019 H (MPa) From the applied load-depth curve of indenta4000 tion test, the contact area Ac and maximum load Pm can be easily measured and then the hardness 3500 was computed using Eq (6) The calculated results are presented in Fig in such the way to 3000 show the change of hardness with respect to different strain rate levels in all regions of the weld as 2500 well as the distinguishable hardness values in the BM HAZ WM 2000 three regions In this figure, each presented hard0.00 0.05 0.10 0.15 0.20 ness value is the mean of 25 values obtained from ( ) an indentation test series in individual region, toFigure Strain rate effect hardness the weld zone Figure Strain rateoneffect onofhardness gether with the corresponding error bar of ±1 stanWhile the hardness can beofdirectly extracted from indentation curve P-h, the the weld zone dard deviation, which are listed in Table The yield strength in the regions of the weld zone was determined using Eqs (7) and (8), best fit curve of the changing trend of hardnessin in which an unknown parameter corresponding to the yielding part of the structural each region is also accompanied steel’s - curve () needs to be pre-estimated for each region The value of each region is estimated using the results from the analysis of indentation FE simulation, Table Hardness in each region which of weld at in different are zone detailed previousstrain works rate [3, 6,levels 22] In present work, by applying such BM Strain rate 0.02 0.04 0.10 0.20 ∗ H (MPa) STDEV∗ (MPa) 2950 2996 3130 3216 70 57 62 138 approach, the values at each strain rate values for BM, HAZ, and WM regions were estimated and the yield strength in each region of the weld zone was extracted for HAZ WM certain strain rate level With the same illustrated way for hardness in Fig 4, the strength(MPa) in each region of the weld zone at different H extracted (MPa) yield STDEV H (MPa) STDEV (MPa)values of strain rate are presented in Fig The corresponding values in this figure are also listed in 3110 148 3380 85 Table for more clarity 3159 76 3441 74 Table Yield strength in each region of weld zone at different strain rate levels 3312 44 BM 3572 HAZ 82 WM 3375 240 3680 y 153 Strain rate STDEV* STDEV STDEV y y (MPa) Standard deviation 0.02 420.08 (MPa) 14.19 (MPa) 455.61 (MPa) 20.42 (MPa) 436.18 (MPa) 23.96 From Fig 4, the effecta of strain rate on the hardness region of465.69 weld zone7.80 are clearly 0.04 in individual 432.03 16.87 447.10 13.79 observed The same trend is that higher strain rate0.10 level leads to the 14.79 higher hardness all regions 448.28 480.42 for 15.04 464.13 8.44 WM, HAZ and BM of the weld zone This trend is0.20 recognized that the hardness value 15.66 quite rapidly 461.18 11.32 495.55 474.73 18.42 increases as strain rate level increases from 0.02 s−1*Standard to 0.04deviation s−1 and the increment of hardness reduces when strain rate levels change from 0.1 s−1 to 0.2 s−1Fig It5,istheinteresting to observe that strength the change From strain rate influences on the yield in individual region observed The same is that higherto strain level leads of hardness in individual region with respectaretoclearly different strain ratetrend levels seem be rate obeyed an to the higher yield strength for all regions BM, HAZ and WM of the weld zone This trend is also exponential function, as can be seen in Fig It is also noted that the hardness in BM is lower than the corresponding one in HAZ at a certain strain rate level, while the corresponding hardness value in WM is highest in the weld zone These results match well with the trends reported in previous works for the weld zone of other structural steels [3, 22] While the hardness can be directly extracted from indentation curve P-h, the yield strength in the regions of the weld zone was determined using Eqs (7) and (8), in which an unknown parameter corresponding to the yielding part of the structural steel’s σ-ε curve (α) needs to be pre-estimated for each region The α value of each region is estimated using the results from the analysis of indentation FE simulation, which are detailed in previous works [3, 6, 22] In present work, by applying such approach, the α values at each strain rate values for BM, HAZ, and WM regions were estimated and the yield strength in each region of the weld zone was extracted for certain strain rate level With the same illustrated way for hardness in Fig 4, the extracted yield strength in each region of the weld 109 Hoan, P T., et al / Journal of Science and Technology in Civil Engineering Journal Science and Technology in Civil Engineering - NUCE 2019 zone at different values of strain rate are presented in Fig 5.ofThe corresponding values in this figure are also listed in Table for more clarity recognized that the increment of yield strength values when strain rate value changes -1 different strain rate levels Table Yield strength in each region of weld from 0.02 s-1 tozone 0.04 sat is greater than the increment of yield strength when strain rate BM Strain rate 0.02 0.04 0.10 0.20 ∗ σy (MPa) STDEV∗ (MPa) 420.08 432.03 448.28 461.18 14.19 16.87 14.79 11.32 Standard deviation value changes from 0.1 s-1 to 0.2 s-1 Similar to the hardness, the change of yield strength in individual HAZ region with respect to different WMstrain rate levels seem to be obeyed an exponential function, as can be seen in Fig Considering the correlation σbetween STDEV (MPa) y (MPa) y (MPa) the yield strength(MPa) values in σ each region, theSTDEV yield strength in BM is lowest in the weld zone, while the yield strength in HAZ at a certain strain rate level is higher 455.61 20.42 436.18 23.96 than the corresponding one in WM This result indicates that the chosen weld material 465.69 7.80 447.10 13.79 in this case eventhougth satify the requirements for the weld, it is still should be 480.42 15.04 464.13 chosen better in order to avoid the failure of the weld 8.44 due to the weld material 495.55 these obtained 15.66results are consistent 474.73 with the reported 18.42 trends for the same However, structural steel weld zone in the previous works [3] From Fig 5, the strain rate influences on the 550 yield strength in individual region are clearly observed The same trend is that higher strain rate 500 level leads to the higher yield strength for all re450 gions BM, HAZ and WM of the weld zone This trend is also recognized that the increment of yield 400 strength values when strain rate value changes −1 −1 from 0.02 s to 0.04 s is greater than the inBM WM HAZ 350 crement of yield strength when strain rate value 0.00 0.05 0.10 0.15 0.20 0.25 ( ) changes from 0.1 s−1 to 0.2 s−1 Similar to the hardness, the change of yield strength in individFigure Strain rate effect on yield strength in the weld zone Figure Strain rate effect on yield strength ual region with respect to different strain rate levFor the validation of reliability and weld accuracy of the present results, the obtained in the zone els seem to be obeyed an exponential function, as and yield strength at a certain strain rate of 0.02 s-1 were compared with the hardness corresponding values in the same weld zone, which are available in the literature [3], can be seen in Fig Considering the correlation as listed Table It is obvious that hardness andinyield values at strain rate between the yield strength values in each region, theinyield strength in BM is lowest thestrength weld zone, of 0.02 s-1 in this work match well with corresponding reported ones [3] The relative while the yield strength in HAZ at a certain strain rate level is higher than the corresponding one in error in case of hardness is within ± 3.72%, while it is even smaller in case of yield WM This result indicates that the chosen weld material in this case eventhougth satify the requirestrength with the error within ± 3.25% The observation and comparison indicates that ments for the weld, it is still should be chosen order to avoid failure the weld due to the better obtainedin results in this work arethe accurate and of reliable the weld material However, these obtained results are consistent with the reported trends for the same Table Comparison of mechanical properties at strain rate of 0.02 s-1 between structural steel weld zone in the previous works [3] present and previous works [3] For the validation of reliability and accuracy of the presentYield results, the obtained hardness Hardness and yield strength (MPa) (MPa) strength at a certain strain rate of 0.02 s−1 were compared with the corresponding values in the same weld zone, which are available in the literature [3], as listed in Table It is obvious that hardness and 10 Table Comparison of mechanical properties at strain rate of 0.02 s−1 between present and previous works [3] BM HAZ WM Yield strength (MPa) Hardness (MPa) Present work Previous work [3] Error % Present work Previous work [3] Error % 420.08 455.61 436.18 426.41 470.92 434.57 −1.48 −3.25 0.37 110 2950 3110 3380 3064 3185 3475 −3.72 −2.35 −2.72 Hoan, P T., et al / Journal of Science and Technology in Civil Engineering yield strength values at strain rate of 0.02 s−1 in this work match well with corresponding reported ones [3] The relative error in case of hardness is within ± 3.72%, while it is even smaller in case of yield strength with the error within ± 3.25% The observation and comparison indicates that the obtained results in this work are accurate and reliable Conclusions In this study, the influences of strain rate on the hardness and yield strength of a typical structural steel (SM520) weld zone was investigated using indentation The following conclusions can be withdrawn: - Strain rate during influences on both the shape and magnitude of the indentation applied loaddepth curves For all the regions in the weld, the loading curvature increase as the strain rate during indentation increases, leading to the higher maximum applied load with the higher strain rate level due to the applied constant maximum displacement in all the tests - Strain rate level has strong effect on the hardness for all regions in the weld zone The trend is that the hardness values quite rapidly increase as strain rate value increases from 0.02 s−1 to 0.04 s−1 and the increment of hardness reduces when strain rate value change from of 0.1 s−1 to 0.2 s−1 The trend seems to be obeyed an exponential function - Strain rate level has strong effect on the yield strength for all regions in the weld zone The trend is that the increment of yield strength values when strain rate value changes from 0.02 s−1 to 0.04 s−1 is greater than the increment of yield strength when strain rate value changes from 0.1 s−1 to 0.2 s−1 Similar to the hardness, the change of yield strength in individual region with respect to different strain rate levels seem to be obeyed an exponential function In conclusion, the mechanical properties in the investigated structural steel weld zone are strongly influenced by indentation strain rate and the relationships between hardness and yield strength with strain rate obtained in present study provide an 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indentation data obtained from finite element simulations and experimental results International Journal of Mechanical Sciences, 62(1):34–46 [21] Pham, T.-H., Kim, J J., Kim, S.-E (2015) Estimating constitutive equation of structural steel using indentation International Journal of Mechanical Sciences, 90:151–161 [22] Pham, T.-H., Kim, S.-E (2015) Determination of mechanical properties in SM490 steel weld zone using nanoindentation and FE analysis Journal of Constructional Steel Research, 114:314–324 [23] ASTM E3-01 (2007) Standard guide for preparation of metallographic specimens ASTM International, West Conshohocken, PA [24] Miller, M., Bobko, C., Vandamme, M., Ulm, F.-J (2008) Surface roughness criteria for cement paste nanoindentation Cement and Concrete Research, 38(4):467–476 [25] ASTM E2546-07 (2007) Standard practice for instrumented indentation testing ASTM International, West Conshohocken, PA 112 ... regions regions at == 0.2 0.2 ss (d) 108 at rate effect on mechanical properties in the weld zone 4.2 Strain properties in the weld zone 4.2 .Strain Strainrate rateeffect effect on on mechanical. .. compositions in the weld zone of structural steel using nanoindentation Journal of Constructional Steel Research, 99:121–128 [2] Pham, T.-H., Kim, S.-E (2015) Nanoindentation for investigation of microstructural... obeyed an exponential function In conclusion, the mechanical properties in the investigated structural steel weld zone are strongly influenced by indentation strain rate and the relationships between