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Russian Physics Journal, Vol. 45, No. 3, 2002

1064-8887/02/4503-0261$27.002002 Plenum Publishing Corporation 261

STRESS-STRAIN CURVES, FRACTURE MECHANISMS, AND SIZE

EFFECT FOR LOW-CARBON LOW-ALLOYED STEELS WITH

A QUASI-COMPOSITE STRUCTURE

É. V. Kozlov, A. V. Plevkov, A. B. Yur’ev, and V. E. Gromov UDC 681.142 : 535

The present paper is devoted to a detailed study of low-carbon low-alloyed reinforcing steel bars of different

diameters. The mechanical characteristics of a reinforcing bar as a whole and its quasi-composite parts are

determined and their dependences on the diameters of samples are established. The composition and defect

structure of reinforcing steel are investigated. Fracture of steel after uniaxial tension is examined. Particular

attention is given to the quasi-composite structure of reinforcing steel formed during the manufacture of the bar.

The stress-strain dependence and the size effect are studied in detail.

INTRODUCTION

In the last decades, low-alloyed reinforcing steel bars of different diameters, hardened by the method of interruptedthrough quenching in fast rolling mills of the Open Joint-Stock Company Western Siberian Integrated Metallurgical Plant(OJSC WSIMP), have been widely used for the production of building and reinforcing concrete structures. This method of thermal hardening allows reduction of ferromanganese in the production of reinforcing steel bars and elimination of additional technological operations [1]. In this case, structural steel possesses the structure of a natural composite [2].

The process of production of reinforcing steel bars developed in the 70s is described in detail in the literature (for example, see [3]). The process of thermomechanical hardening has several stages. The scheme of thermomechanicalhardening is shown in Fig. 1. Starting from the temperature of rolling termination (1100°C), the reinforcing bar is cooled byan air-water flow down to 950°C in a pre-quencher (PQ). In the first stage, it is cooled fast, and the surface layer isquenched to a certain depth. In this case, according to the scheme of interrupted quenching, metal in the central region of the bar is in the austenite state. At the same time, the near-surface layer has undergone the martensitic transformation. The

arising temperature gradient allows the near-surface layer to be heated and self-tempered in the second stage. Then thereinforcing bar is put on the roller table of a quencher in which the structure of the central zone of the metal bar is formed(in the third stage). The final microstructure is formed during the cooling period when the temperature in the bar crosssection decreases down to ambient temperature [3].

The investigation of the structure and mechanical properties of low-carbon low-alloyed structural steel is a veryimportant problem.

It is well known from the literature that low-alloyed steel subjected to thermomechanical through hardening (TMH)in a rolling mill is inhomogeneous over the bar cross section. The structure of individual layers and their mechanical

properties have not yet been adequately studied [4]. Meanwhile, the evolution of deformation of a quasi-composite sampleunder loading is complex in character. It cannot be fully understood without studying the properties of the composite as awhole and its individual layers. In the present paper, an attempt is made to remedy this situation.

Diameters of commercially available reinforcing steel bars vary significantly. It is natural that bars with different

diameters have different mechanical properties after thermomechanical hardening in a rolling mill by the same procedure.Investigations of the dependences of the mechanical properties of TMH reinforcing steel bars on the bar diameter were notconducted previously; the present paper fills this gap.

Tomsk State University of Architecture and Building; Siberian State Industrial University. Translated fromIzvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 49–60, March, 2002.



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Several parameters are commonly used to characterize the mechanical properties of reinforcing steel bars, namely,the yield stress σy, ultimate tensile stress σu.t, strain δ, and reduction of area at fracture Ψ. Of course, these characteristicscannot describe in detail the physics of deformation of low-carbon low-alloyed steels. The present paper analyzes the maincritical characteristics of the dependence σ = f (ε) and their correlation with the structure of the material and with the

parameters of the fracture surface. It is aimed at studying the laws of plastic deformation and fracture of reinforcing steel bars of different diameters and also their defect structure and composition.

1. EXAMINED MATERIAL

We investigated TMH reinforcing steel bars satisfying specifications 14-1-5254-94 with crescent cross sections 12,14, 16, 18, 22, 25, 32, and 40 mm in diameters produced at the OJSC WSIMP (Novokuznetsk). The elemental compositionof examined reinforcing steel is given in Table 1.

2. INVESTIGATION TECHNIQUES

Several techniques were used to perform investigations at different scale levels.

2.1. Mechanical tests

Reinforcing steel bars were chosen according to the requirements of the All-Union State Standard 12004-81. Beforetesting, scribe marks spaced at distances multiple of 5 or 10 mm (depending on the bar diameter) were marked on theworking section of the examined bar with a ruling machine, snap gauge, or center-punch. Tension tests of one-piecereinforcing bars 12, 14, 16, 18, 22, and 25 mm in diameters (Fig. 2a) were performed on an MR-500 testing machine under tensile forces as great as 60 ton. Reinforcing steel bars 32 and 40 mm in diameters were tested on Amsler and R-100machines under tensile forces as great as 150 ton. The samples with a working section of 1 × 1 × 10 mm (Fig. 2) were cut

Fig. 1. Scheme of thermal hardening shown in the thermalkinetic diagram of the type described in [3].

TABLE 1

Chemical elementStatistical parameters

Carbon Silicon Manganese Nickel Chromium Copper

Average value 0.17 0.07 0.49 0.36 0.19 0.50Standard deviation 0.019 0.015 0.034 0.017 0.013 0.017



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from different regions of the reinforcing bar and tested in uniaxial tension. The shapes of these samples were described in[5]. They were tested on an Instron machine. The samples were cut continuously by the spark cutting method starting fromthe surface considering the structure of the layers detected by the methods of optical and electron microscopy and the

boundaries between them. Testing in uniaxial tension was conducted at room temperature with a rate of 5⋅10 –4 s –1; P – L

diagrams were automatically recorded, where P is the load and L is the elongation of the examined bar. These diagramswere then recalculated into σ – ε (stress-strain) diagrams. Figure 3 shows their typical form. The microhardness of reinforcing steel bars was also determined along the bar diameters starting from the surface toward the center.

2.2. Metallographic analysis (optical microscopy)

The samples chosen for metallographic analysis represented discs with thicknesses of 1.5–2 mm cut perpendicularly to the longitudinal axis of the reinforcing steel bar on a spark machine. Then these samples were groundand polished using grinding discs and a PTF-2 machine. To reveal the microstructure of the samples, chemical andelectrolytic etching by special chemical reagents (50% brine, acetic + nitric acids, and saturated solution of chromiumanhydride in orthophosphoric acid) were used. The etching time and the sequence of etching by chemical reagents and

cooling of electrolytes by liquid nitrogen were preliminary adjusted. The above-indicated methods allowed us to obtainmore complete information on the microstructure of the examined samples. Microsections of the examined reinforcing steel

bars were observed with a MIM-10 metallograph microscope at magnifications of 100–1000.

2.3. Method of transmission electron microscopy (TEM)

TEM investigations were performed using ÉM-125K and ÉM-125 transmission electron microscopes. The foilswere prepared by the electrolytic method in reference reagents. Based on the results of metallographic analysis, the structureand phase content of each detected zone of the bars with the above-listed diameters and of the boundaries between thesezones were analyzed. The qualitative and quantitative parameters of the structure were determined including the sizes of martensite packets and laths; dislocation density ρ; density of intraphase and interphase boundaries; location, volume

fraction P , and degree of dispersion (sizes and shapes) of the carbide phase; elastic-plastic stresses (the excess dislocationdensity ρ±). The structural parameters were determined by the methods of stereometry. Electron diffraction patterns wereindexed using the methods described in [6]. To analyze the phase content of steel and the phase composition and finestructure of the matrix in more detail, video and TV images were used with subsequent data processing on personalcomputers [7].

a

b

Fig. 2. Samples of the initial reinforcing steel bar at different scale levels: a) reinforcing bar as a whole(12 mm in diameter) and b) the sample cut from one of the structural zones of the bar 40 mm in diameter.



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2.4. Method of scanning electron microscopy (SEM)

This method was used to analyze qualitatively the fracture surface based on the morphology of macroscopicstructures. Quantitative parameters of the fracture surface such as sizes of dimples in regions of ductile fracture and contentof different types of fracture morphologies were also determined. Observations were performed with a Tesla BS 301

scanning electron microscope.

3. RESULTS OF INVESTIGATIONS

3.1. Critical points of the dependence σ = f (ε)

The typical stress-strain curves illustrating the dependence σ = f (ε), where σ is the stress and ε is the elongation of the examined reinforcing steel bar, are shown in Fig. 3. Since the quasi-elastic zone of the material with a compositestructure has been studied only theoretically, it is not considered here. We analyze the stress-strain curve starting from theyield stress σy. In this case, the yield stress is sharp. The samples of reinforcing steel bar 12 and 14 mm in diameters weregradually strained and hence σ0.2 was determined. The yield plateau for larger bar diameters becomes more clearly

pronounced, and its length also increases. Figure 3 shows the main points of the dependenceσ

= f (ε). Once the yield stress

has been reached, the active plastic deformation starts. For most examined bar diameters, the strain is first localized asLüder’s band. The local strain corresponding to it is depicted as a segment of the curve with a constant stress – the yield

plateau of length ε pl. The yield plateau is followed by the segment of strain hardening extending to the end of thehomogeneous strain zone with the total length being equal to εh. The stress at which the homogeneous deformation ends iscalled the ultimate stress σu.

To study the parameters of strain hardening, the dependence of the strain hardening coefficient θ = f (ε) was used,where θ = ∂σ/∂ε. In this case, the stage of linear hardening (stage II) and the parabolic stage (stage III) can be identified in

Strain, %

0

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εeng

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σu

εtrue.loc

σeng

εpl

εh

εloc

εtrue

S t r e s s ,

M P a

200

400

600

800

10 20 30 40

Fig. 3. Stress-strain diagram for uniaxial tension. The main points are the yield stress σy (when the

tension diagram has no yield plateau, the engineering yield stress is σ0.2 = σy), ultimate stress συ,engineering stress σeng, true stress at fracture in the neck σtrue, length of the yield plateau ε pl, quasi-homogeneous strain εh, engineering overall elongation εeng, engineering localized strain in theneck εloc, true localized strain at fracture in the neck εtrue.loc, and true elongation εtrue.



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the segment of the curve located between the end of the yield plateau and the end of the homogeneous strain region in Fig. 3[8]. Not to overburden Fig. 3, these stages are not shown. The stress σu characterizes the transition to the localized strainstage. It is commonly associated with the Considère condition θ = σu [9]. When the stress exceeds σu, the strain is localizedand a neck is formed. The engineering localized strain is observed in the segment with the decreasing stress (the segmentεloc in Fig. 3). Since the neck cross sectional area decreases fast, the true strain and stress εtrue and σtrue differ from theengineering ones. Measuring the geometric parameters of the neck, the true curve σ = f (ε) can be reconstructed and thevalues of true elongation εtrue.loc and εtrue can be determined. It should be emphasized that the typical dependence σ = f (ε) isobserved for macro- and mesosamples cut from different structural zones of quasi-composite steel. The shape of thedependence σ = f (ε) is independent of the bar diameter, but their individual segments and their characteristics depend on thereinforcing steel bar diameter. Some of these dependences are considered below.

3.2. Size effect

Figure 4 shows the dependence σ = f (ε) for different bar diameters. From the figure it can be seen that withincreasing reinforcing bar diameter, the yield stress decreases, whereas the length of the yield plateau and the total plasticityof the material increase. Engineering stress-strain curves σ = f (ε) are shown in Fig. 4. Their true parameters can be obtained

based on the study of the geometric parameters of the neck. Figure 4b – d shows the localized strain patterns. With the helpof the scribe marks, the localized elongation was measured in the neck and in its vicinity. An example of its distribution isillustrated by Fig. 5 for the bar 40 mm in diameter.

The data shown in Fig. 4 are indicative of the size effect for the examined bar diameters. Due to the complexity of the structure of the reinforcing steel bar attendant to changes in its cross-sectional area (diameter), the size effect studied inthe present paper is also complex in character. First, it has some specific features of the conventional size effect caused bythe dependence of the probability of defect occurrence on the size of a specimen [10]. Second, the quasi-composite structureof the TMH bar cross sections affects the characteristics of the size effect.

b

d

c

Strain, %

0

a

3

2 1

S t r e s s ,

M P a

200

400

600

800

10 20 30

Fig. 4. Stress-strain curves (a) (the dependence σ = f (ε), where σ is the stress and ε is the elongation) for reinforcingsteel bars 12 (1), 18 (2), and 40 mm in diameters (3) and photographs of the localized strain zones (b – d ).



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The yield and ultimate stresses σy and σu decrease with increasing bar diameter. The dependences σy(1/d ) and σu

(1/d ) (where d is the bar diameter) are linear. These data are shown in Fig. 5b. They confirm the existence of the size effect.It turns out that all the mechanical characteristics depend on the reinforcing steel bar diameter. The length of the yield

plateau ε pl and the degree of homogeneous deformation εh increase with the bar diameter, whereas the true localized strainεtrue.loc and the reduction of area at fracture Ψ decrease. The larger the cross sectional area, the stronger the effect of self-

tempering caused by accumulated internal heat. Undoubtedly, elucidation of a role of the structural characteristics and sizeeffect calls for the study of the composition and defect structure distributions over the bar cross section in more detail.

3.3. Quasi-composite structure

The examined material of the bar after etching of its end cross section reveals concentric zones distinguished by thedegree of etching and color. Figure 6a shows the optical image of the end cross section of the bar 18 mm in diameter with

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Fig. 5. Mechanical characteristics: a) distributions of the strain (1) and reduction of area at fracture (2)measured in the strain localization zone – the neck – along the reinforcing steel bar 40 mm in diameter; b)dependences of the ultimate (1) and yield stresses (2) on the reciprocal bar diameter; c) dependences of thehomogeneous strain (1) and the length of the yield plateau (2) on the bar diameter; d ) dependences of thetrue localized strain (1) and reduction of area at fracture (2) on the bar diameter.



267

the clearly pronounced annular structure. The number of annular zones is 5 (see Fig. 6b). The number of macrostructuralzones and their parameters depend on the reinforcing bar diameter (cf . Figs. 6a and 6c). These zones have different areas.As a rule, the intermediate zone has the largest area (Fig. 6). The central and near-surface zones are comparatively small andhave close areas.

Metallographic analysis of the structure of reinforcing steel bars after etching of the grain boundaries andintragranular structural elements reveals qualitative and quantitative differences in the structure of zones. As follows from

a b c

Fig. 6. Metallographic image of the end cross section of bars and the scheme of layers: a) concentric zones for the bar 18 mm in diameter, b) scheme of the detected zones, c) concentric zones for the bar 14 mm in diameter.

b

d

a

c

50 µm

50 µm 50 µm

50 µm

Fig. 7. Metallographic image of the structures formed in the reinforcing bar after thermomechanical hardening:a) the martensite structure of the near-surface zone of the bar 12 mm in diameter, b) the lower bainite structureof the bar 18 mm in diameter, c) the upper bainite structure of the bar 18 mm in diameter, and d ) the structureof the central zone of the bar 40 mm in diameter.



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the images shown in Fig. 7, the etching pattern of the near-surface zone is typical of the martensite structure. Regions withthe ferrite-pearlite structure are observed in the central zone, but its structure is characterized by the fine intragranular contrast caused by etching of the carbide phase particles located in the grain body. The intermediate zone contains elementsof both zones considered above.

Analysis of the grain structure revealed coarse grains with sizes of 30–40 µm having the equilibrium configurationof the boundaries and fine grains with sizes of 3–5 µm located along the boundaries of coarse grains. The sizes of finegrains are virtually independent of their location in the material. The sizes of coarse grains increase when going from the bar

surface to its center. As indicated above, the zone structure of the reinforcing steel bar depends on the method of thermaltreatment.

Analysis of the grain structure of the central zone of bars with large diameters demonstrates that after thermalhardening it has the ferrite-pearlite structure. The methods of metallography do not allow one to fully investigate the defectstructure of the material and its composition to explain the above processes. The method of TEM which allows the defectstructure of the material to be investigated both qualitatively and quantitatively is best suited for this purpose.

3.4. Transmission electron microscopy (the fine structure)

Our investigations demonstrate that the structure of reinforcing steel bars is formed by the ferrite-cementitemixture. Ferrite (the solid solution of carbon in the crystalline lattice of α-iron) has several morphological types caused by

different rates of bar cooling from the rolling temperature. Let us consider them.The structure formed by the martensitic mechanism is shown in Fig. 8a. Morphologically, it belongs to packet

martensite and represents α-iron crystals parallel to each other. The average transverse crystal sizes are ≈0.15–0.2 mm. Asingle stack includes up to 20 parallel lath crystals. Such stacks of martensite laths are called packets. Adjacent packets aredisoriented from each other and hence divide the initial grain into some regions. A single grain may contain up to severaltens of packets depending on the cooling rate. The average packet size is equal to a few micrometers.

Martensite crystals in the packet are separated by the high-angle boundaries analogous to the grain boundaries. Theexamined material was subjected to thermal treatment and hence had the tempered martensite structure. Additional heatingof martensite-hardened steel (tempering of steel) led to precipitation of the Fe3C iron carbide particles inside martensitecrystals, on their boundaries, and along the packet and grain boundaries. The particles located inside crystals have elongatedshapes and average sizes of ≈30 nm. The particles located on the boundaries form interlayers whose transverse sizes are 50– 60 nm. Tempering of martensite-hardened steel is accompanied by a certain decrease in the degree of imperfection of its

crystalline structure. For the examined material, the density of dislocations in tempered martensite was ≈6⋅1010 cm –2. Thestructure of tempered martensite in the external layer of the steel sample indicates that it has an increased strength. Figure8b shows the region between the martensite and bainite zones.

Morphologically, the bainite structure shown in Fig. 8c is close to the martensite structure; these structures haveanalogous mechanisms of formation. Bainite represents crystals of lenticular shapes whose transverse sizes are severalmicrons and longitudinal sizes are several tens of microns. This structure is formed at temperatures exceeding those of martensite formation. The mechanism of bainite formation is diffusion-shear one. This leads first, to the precipitation of ironcarbide (Fe3C) particles and second, to the formation of defect (dislocation) substructure. The dislocation density in bainitecrystals is lower than that in martensite crystals and is equal to 2–3⋅1010 cm –2. The strength of the bainite layer is lower thanthat of the martensite one.

One more structure type observed in examined steel is the pearlite structure. The pearlite colony (block) represents

the ferrite grain comprising ordered cementite particles (Fig. 5e). In the central zone there are regions of plate (Fig. 8e) andglobular pearlite (Fig. 8 f ). In some specific cases, ferrite grains without cementite precipitation were observed in examinedsteel. In these cases, the ferrite grain was less imperfect, and cementite particles in the form of spheres or interlayers werelocated in joints or along the grain boundaries. We note that in the initial state, this structure is rare in occurrence. Thedislocation density was in the range of (1.5–1.7)⋅1010 cm –2. The strength was higher than that of lower bainite.



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3.5. Variations of the structural parameters and mechanical characteristics over the bar cross section

Quasi-composite steel layers were qualitatively described in Section 3.4. Undoubtedly, the central part of thesample was treated softer than its surface. The sizes of cementite particles located on the α-phase crystal boundariesincrease, and the long-range stress fields attenuate (Fig. 9a) with increasing radial distance from the sample surface. Thegrain sizes, dislocation density, and a number of other structural and mechanical characteristics depend nonmonotonicallyon the radial distance from the sample surface. The point is that the martensitic phase has the highest strength, whereas

0,4 0,4

0,4 0,4

0,4

0,4

0.4 µm 0.4 µm

0.4µm0.4

µm

0.4 µm 0.4 µm

a b

c d

e f

Fig. 8. Electron micrograph of structures formed after thermomechanical hardening of reinforcing steelsamples of different diameters: a) tempered packet martensite structure in the sample 12 mm in diameter, b)the boundary between martensite and bainite structures in the sample 14 mm in diameter, c) lower bainiterepresented by large plates with quasi-chaotic distribution of cementite particles in the sample 18 mm indiameter, d ) upper bainite with well-defined rows of cementite in the sample 18 mm in diameter, e) ferrite-cementite mixture (quasi-pearlite) comprising ferrite grains and quasi-pearlite in the sample 18 mm indiameter, f ) ferrite-pearlite structure in the sample 40 mm in diameter



270

lower bainite has the least strength. The properties of quasi-pearlite and upper bainite are similar (Fig. 9b and c). Therefore,we determine the mechanical properties directly in the field tests of the samples cut from these quasi-composite layers.

Figure 9b shows the dependences σ = f (ε). It can be seen that the material of different layers has differentcharacteristics. The nonmonotonic dependence of the stress characteristics on the sample cross section can be seen fromFig. 9c. The data shown in this figure were obtained for the sample 18 mm in diameter. The plastic characteristics of thematerial of individual layers increase almost monotonically from the surface to the center of the sample cross section. Thus,the properties of the quasi-composite structure and its individual components are actually demonstrated on the example of the reinforcing steel bar 18 mm in diameter (Fig. 9). Next problems are the investigation of others TMH steel samples withdifferent diameters and the development of a theory of steel with a quasi-composite structure.

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εloc

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Fig. 9. Quantitative parameters, engineering stress-strain curves, and dependences of the mechanicalcharacteristics of layers on the radius of the reinforcing bar cross section for the bar 18 mm in diameter: a)density and excess density of dislocations; b) engineering stress-strain curves for tempered martensite (1),lower bainite (2), upper bainite (3), and quasi-pearlite (4); c) stress; d ) plasticity.



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3.6. Pattern of the fracture surface

Different structures of the examined samples over their cross sections lead to several fracture mechanisms and theformation of different structures at fracture.

The pattern of the fracture surface is considered on the example of the reinforcing bar 40 mm in diameter (Fig. 10).Zones with different fracture mechanisms can be clearly seen in the total bar cross section. The external martensite layer has

30

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30 µm30 µm

30 µm30 µm

a b

c d

Zone A Zone B

Zone C Zone D

Zone D

Zone A

Zone B

Zone C

Fig. 10. Photograph and fractograms (a – d ) for fracture of the reinforcing bar 40 mm in diameter:a) microstructure for fracture in zone A with the ductile fracture perpendicular to the bar axis and the quasi-shear parallel to it, b) zone B with the quasi-shear, c) the structure of zone C with the dimple fracture, d ) thestructure of zone D with dimples and ridges. SEM.



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a very interesting fracture surface – mixed dimple fracture with a quasi-shear (the fracture classification see in [11]). Thefracture in the external layer is anisotropic in character. The dimple fracture is perpendicular to the bar axis, and the quasi-

shear is parallel to it. The fractogram shown in Fig. 10a was specially rotated so that both components were seen. Zone Bhas mostly the lower bainite structure (Fig. 10b). The fracture in zone C is ductile in character (Fig. 10c). Ridges anddimples are more developed in the central zone (Fig. 10d ). From Fig. 10 we can conclude that the most ductile fracture isobserved in the central and near-surface layers. The surface fraction of quasi-shear is larger for the intermediate layer.

Samples cut from individual layers can be used to study the fracture pattern in more detail. The fracture surface isshown in Fig. 11. The fracture of the martensite structure (Fig. 11a) is mostly tough in character (dimples and ridges of different microgeometry). The mixed fracture is observed in the bainite structure in which surface fractions of the quasi-shear and branched fracture increase when going from lower bainite to upper one (Fig. 11 b and c). The quasi-pearlitestructure indicates that we revert to the dimple fracture with ridges (Fig. 11d ). Thus, from the viewpoint of the physics of steel fracture, the reinforcing bar as a whole and the samples cut from individual layers have analogous fracture patterns.

CONCLUSIONS

The mechanical properties of reinforcing steel with a quasi-composite structure have been studied in the present paper. Reinforcing bars of different diameters and microsamples cut from individual layers of the material with martensite, bainite, and ferrite-pearlite structure were tested in tension. The critical parameters of the stress-strain dependence wereanalyzed in detail. Five critical stresses and six values of elongation were identified for the dependence σ = f (ε). The stagesof homogeneous and localized strains were analyzed in detail. The size effect – the dependence of the mechanical propertieson the reinforcing bar diameter – was established. The lengths of the yield plateau and homogeneous strain zones increase

a b

c d

30 µm30 µm

30 µm 30 µm

Fig. 11. Fractograms for fracture of the samples cut from different layers of the reinforcing bar 18 mm indiameter: a) packet martensite, b) lower bainite, c) upper bainite, and d ) quasi-pearlite. SEM.



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with the reinforcing bar diameter, whereas the localized, yield, and ultimate stresses decrease. This effect is connected withthe dependence of the parameters of the quasi-composite structure on the reinforcing bar diameter. The structure of differentzones of the thermally hardened bar was analyzed in detail by the methods of optical and transmission electron microscopy.The mechanical properties of the material of different zones were determined along with their changes over the bar crosssection. The dislocation density and the strength characteristics are higher for the external layers of reinforcing bars,whereas the plasticity is higher for the central zone. The strength and plastic properties of the material of different bar zonesare determined by the morphology and dispersity of steel in different zones. Therefore, martensite and ferrite-pearlite

mixture are stronger than bainite. The fracture surface and the fracture parameters for macro- and microsamples werestudied in detail. It was found that the martensite structure yields dimple fracture, and the bainite structure yields the quasi-shear fracture.

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kozlov_plevkov_yur'ev_gromov_stress-strain curves fracture mechnisms and size effect for...

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