High-Cycle Fatigue Properties of Automobile Cold-Rolled Steel Sheetwith Stress Variation

Chang-Yeol Jeong+

Department of Nuclear and Energy System Engineering, Dongguk University,707 Seokjang-Dong, Gyeongju 780-714, Republic of Korea

Since many components in the automotive body and chassis are produced by various manufacturing processes, have different chemicalcompositions, and are subjected to complex loading cycles, it is important to understand their loading mechanisms and susceptibility to damage.This research examined the mechanical properties of cold-rolled steel sheets and evaluated the effects of stress variations on fatigue behavior.Specifically, a series of load-controlled high-cycle fatigue tests were conducted by varying the stress levels of SPCC and SPRC340 sheetmaterials. The results showed that fatigue life and the fatigue limit increased with higher tensile and yield strengths. In addition, testing resultsindicated that the fatigue limit was higher than the monotonic yield strength due to cyclic hardening with plastic deformation during fatiguecycling. Regarding tensile properties upon pre-deformation, the yield strength increased with a higher amount of pre-deformation and wasgreater than the fatigue limit after deformation. Based on these experimental results, two types of fracture modes were observed under theapplied stress range. General fatigue fracture mode, which denotes failure by crack initiation, propagation and final rupture at low stressamplitude, was observed with fatigue lives larger than 4 © 105 cycles. On the other hand, constrained fracture mode occurred at stress levelshigher than 0.89 times the tensile strength and exhibited a fracture surface without fatigue crack initiation or propagation.[doi:10.2320/matertrans.M2013208]

(Received June 3, 2013; Accepted July 8, 2013; Published August 23, 2013)

Keywords: high-cycle fatigue, automobile, cold-rolled steel sheet, fracture mode

1. Introduction

Currently, the automotive industry has become subject torestrictive government regulations concerning fuel conserva-tion and safety along with environmental concerns. Theseregulations have prompted automakers to come up withinnovative solutions in order to design lighter cars forreduced fuel consumption while simultaneously improvingthe overall structure of vehicles for occupant safety. For thispurpose, weight reduction of the car body has been a mainfocus, and demand for increased vehicle safety has become amatter of considerable concern. Automotive structural sheetcomponents are succeptible to fatigue at certain local areaswhere high stresses occur due to geometry and loading. Sheetcomponents are often pressed in order to achieve the desiredpanel form as well as to improve panel stiffness. Especially,sharp press radii in combination with substantial sheetthickness reduction may exacerbate the stress applied tosheet components. Therefore, it is critical to assess the fatiguestrength of pressed sheet materials. This investigationcompared the high-cycle fatigue behavior of low yieldstrength steels (SPCC), a type of steel characterized byexcellent deep drawability, with that of high yield strengthsteels (SPRC340) of varying static load-bearing capability.Both steels were manufactured through two processes. Thefirst process used an electric furnace while the other involveda blast furnace, and the steels were tested as-received andafter pre-straining in order to simulate the effect of thesubsequent deformation process. Systematic study of themechanisms of fatigue damage in terms of initiation andpropagation of fatigue cracks to failure could provide greaterinsights into the alloy design of these steels and theirsubsequent engineering applications. Experimental resultshave shown remarkable differences in the mechanisms of

fatigue failure under the applied stress range regardless of theyield strength of steel sheets. Therefore, this investigationdetermined the fatigue strength and fracture mechanisms ofcold-rolled steels with different yield strengths made byseparate manufacturing processes.

2. Experimental

Two types of cold-rolled steel sheets for the automotivestructure were used in order to investigate mechanicalproperties. One type was low yield strength steel (SPCC)while the other was high yield strength steel (SPRC340), andtwo manufacturing processes (blast furnace and electricfurnace) were used. Table 1 lists the chemical compositionsof the steel alloys. High-cycle fatigue tests were conductedto determine the effects of composition and manufacturingprocess on fatigue properties. To avoid the effect of sharpedges on fatigue strength, the gauge areas of all specimenswere slightly smoothed out by hand polishing with fine(#2400) emery paper. Prior to the fatigue tests, to establishthe value of the maximum applied stress, ·max, tensile testsaccording to ASTM E8 standards1) were performed onindividual steel sheets. The initial ·max value for fatigueloading was selected to be about 0.9 times that of the ultimatetensile strength. The tensile properties were determined usinga Shimadzu AG-IS tensile testing machine in velocity-controlled mode. The fatigue tests were carried out on afully computerized servo-hydraulic MTS 810 fatigue testingsystem. The tests were conducted under load controlaccording to ASTM E466 standards.2) To avoid possiblebuckling, all samples were subjected to zero­tension fatiguecycles only, in a direction parallel to the rolling direction,using a load ratio (·min/·max) of R = 0 at room temperature.A sinusoidal waveform with a frequency of 40Hz was usedin all tests. The fatigue limit (maximum stress range or peakstress) was defined as the stress level below which no fatigue+Corresponding author, E-mail: jcy@dongguk.ac.kr

Materials Transactions, Vol. 54, No. 10 (2013) pp. 2037 to 2043©2013 The Japan Institute of Metals and Materials EXPRESS REGULAR ARTICLE

failure would occur at 107 cycles. The fracture surfaces wereexamined using a scanning electron microscope (JEOL JSM-7400F) to identify the sites of fatigue crack initiation and themechanisms of fatigue crack propagation.

3. Results and Discussion

3.1 MicrostructuresThe cold-rolled steels had a ferritic­pearlitic microstruc-

ture, as shown in Fig. 1. Both steels were fine-grain, withgrain sizes ranging from 19­22 µm (SPCC) and 14­20 µm(SPRC340), which is typical for cold-rolled microstructures.In both cases, there were no discernible differences betweenthe steel microstructures made using the electric and blastfurnaces. Carbon contents of the steels were low, promotinga mainly ferritic microstructure. A ferrite matrix with a lowamount of secondary phases and a few non-metallicinclusions can be characterized by high strain-hardeningcoefficients. This improves drawability due to prevention of

necking and maintenance of uniform elongation.3,4) Mn isoften added as a de-oxidizer and as a solution-hardeningelement. Together with undesirable contaminants such as Sor SiO2, Mn produces non-metallic inclusions such asMnO·MnS and 2MnO·SiO2 and also acts as a solutionhardener. The solubility of Mn in ferrite is high, about 10%,at room temperature. P is also a solution hardener. Lowcarbon steels normally contain micro-alloying elementssuch as niobium (Nb ³0.02%) and titanium (Ti ³0.01%).Addition of these micro-alloying elements induces increasesin strength and hardenability through microstructural refine-ment, solid-solution strengthening and precipitation harden-ing.5) Typical microstructures of the steels consisted of aferrite matrix with embedded precipitates composed oftitanium and niobium carbides (TiC and NbC), as shown inFig. 1. This ferrite has different morphologies based ondecomposition of austenite and has been identified aspolygonal ferrite with equiaxed grains and a low dislocationdensity, Widmanstatten ferrite having elongated grains and a

(a) (b)

(d)(c)

Fig. 1 Optical microscopy photographs showing the microstructures of (a) SPCC (electric), (b) SPCC (blast), (c) SPRC340 (electric) and(d) SPRC340 (blast).

Table 1 Chemical compositions of the cold-rolled steel sheets (mass%).

C Si Mn P S Cr Ni Cu Nb Ti Sn Fe

SPCC(electric)

0.029 0.018 0.163 0.004 0.006 0.025 0.041 0.064 0.001 0.007 0.013 Rem

SPCC(blast)

0.042 0.010 0.238 0.006 0.008 0.077 0.019 0.012 0.002 0.007 0.014 Rem

SPRC340(electric)

0.047 0.028 0.790 0.009 0.003 0.036 0.044 0.092 ® ® 0.005 Rem

SPRC340(blast)

0.037 0.059 0.465 0.021 0.009 0.009 0.010 0.009 ® ® 0.001 Rem

C.-Y. Jeong2038

dislocation substructure, granular ferrite containing islandsof micro-constituents and a high dislocation density, andbainitic ferrite consisting of parallel ferrite laths and a highdislocation density.5)

3.2 Mechanical properties3.2.1 Tensile properties

Tensile tests were conducted to measure the mechanicalproperties of the steel sheets with different chemicalcompositions made by separate manufacturing processes,and the results are shown in Fig. 2 and Table 2. The yieldstrengths of the electric furnace (thicker) and blast furnace(thinner) sheets were 179 and 169MPa for SPCC as well as234 and 213MPa for SPRC340, respectively. For both steelsheets, increased thickness was associated with improvedyield strength up to 10MPa for SPCC and 21MPa forSPRC340. A similar trend regarding yield strength withrespect to sheet thickness was also observed in a previouswork.6,7) Ductility characterized by percent elongation wassomewhat different, ranging from about 39 to 47%, amongdifferent materials (Table 2). The ultimate tensile strengths(UTSs) of the electric furnace (thicker) and blast furnace(thinner) steels were 315 and 309MPa for SPCC as well as366 and 342MPa for SPRC340, respectively, which werethen used to correlate the fatigue limits of the steel sheets.

Figure 3 shows a fracture surface of the tensile specimensof SPCC tested in RT. As shown, the fine-grained cold-rolledsteel exhibited ductile fracture tendency with a fine dimplestructure as a result of smaller second phase particles.Further, fine slip lines were observed on the gauge surfaces inwhich the slip direction was inclined to the loading, axis asshown in Fig. 3.

3.2.2 Fatigue properties(1) High-cycle fatigue test results

The S­N curves obtained for the four metal sheets areshown in Fig. 4. The fatigue limit (peak stress for R = 0condition, ·FL) for the blast furnace SPCC (about 255MPa)was lower than that for the electric furnace SPCC (265MPa).The results showed a difference in fatigue limit of 10MPabetween the electric furnace steel and blast furnace steel,most likely due to the difference strength according to sheetthickness. Further, the fatigue limit of the electric furnaceSPRC340 (320MPa) was 25MPa higher than that of the blastfurnace SPRC340 (295MPa). In this work, all fatigue limitsof the steel sheets were larger than the monotonic yieldstrengths due to cyclic hardening with plastic deformationduring fatigue cycling.8) This phenomenon has been reportedin the previous works9­11) and is discussed in the nextchapter.(2) Effects of pre-straining

Figure 5 shows the experimental results of the tensile testafter pre-straining. The main effect of pre-straining the testmaterials was to increase yield strengths from 179 to301MPa (SPCC) and from 234 to 371MPa (SPRC340).This was due to strain hardening of the matrix, and duringplastic deformation, many dislocations were generated. Thesedislocations influence each other through their stress fields,which leads to mutual obstruction and therefore reduction ofdislocation mobility.9) Regarding the tensile properties afterpre-deformation, the yield strength increased with a higheramount of pre-deformation and was greater than the fatiguelimit after deformation. Therefore, it can be assumed thatthe increase in yield strength due to strain hardening wasresponsible for the improved fatigue performance of thepre-strained material. It has been widely observed that thefatigue strength of materials increases roughly in proportionto the yield and tensile strengths.12,13) In Fig. 6, the fatiguelimit of the steels is displayed as a function of the tensilestrength. As seen in the results, higher yield strengths oftencorresponded to higher fatigue limits. The fatigue limits ofthe steel grades investigated in this study were in the samerange as previously reported values,9­11) and the ratio of thefatigue limit (peak stress, ·peak) to the UTS was 0.85, whichcan be converted into 0.425 in a general expression of theratio of the fatigue limit (stress amplitude, ·a) to the UTS. Ithas been reported that for R = ¹1 load condition, the ratio ofthe fatigue limit to the ultimate tensile strength (·FL/·UTS) isclose to 0.5 for the low and medium strength steels.14) Here·FL is usually taken as the stress amplitude (·a) which is halfof the peak stress for R = 0 condition. The ratio 0.425 issomewhat smaller than the value of previous data (0.5),which is caused by decrease of the fatigue limit due to

Fig. 2 Tensile test results of the cold-rolled steel sheets.

Table 2 Tensile Properties of the cold-rolled steel sheets.

Sheet thickness(mm)

Yield strength(MPa)

Tensile strength(MPa)

Uniform elongation(%)

Tensile elongation(%)

SPCC (electric) 0.8 179 315 24.3 45.7

SPCC (blast) 0.7 169 309 25.3 46.5

SPRC340 (electric) 1.2 234 366 20.3 39.3

SPRC340 (blast) 1.0 213 342 23.1 42.5

High-Cycle Fatigue Properties of Automobile Cold-Rolled Steel Sheet with Stress Variation 2039

positive mean stress effects (R = 0). These results are in linewith the requirements for durable steel caused by strainhardening during cyclic deformation.(3) Fracture mode

Figures 4 and 6 show the fracture ratio, defined as the ratioof applied stress to the UTS, which indicates a change infracture mode. The fracture ratio for the four steels was 0.89regardless of the steel type, which means that general fatiguefracture mode, which denotes failure by crack initiation,propagation and final rupture, occurred below 0.89. On theother hand, constrained fracture mode occurred at stress

levels higher than 0.89 times the tensile strength with fatiguelives smaller than 4 © 105 cycles and exhibited a fracturesurface without fatigue crack initiation or propagation. Thefracture morphology under the applied stress range is shownin Fig. 8. Although a similar fracture trend has been reportedfor low carbon and interstitial free steels,7,15) the failure wasmainly associated with fracture mode of no cracking in highstress range. In this study, slip bands played an important rolein fatigue cracking. Specifically, it can be seen that crackspropagated along the slip bands. Slip bands on the fatiguespecimen surface during fatigue cycling are shown in Fig. 7.Slip lines at 45°, which indicate the resolved shear stress inthe principal loading direction is at a maximum, wereobserved in the low stress range as shown in Fig. 7(a). On theother hand, complex surface morphology was shown in thehigh stress range due to the multiple slip mechanisms asshown in Fig. 7(b). Further, fatigue crack initiation, growthand failure mechanisms in specimens during the fatigue testare shown in Fig. 8. Specifically, Fig. 8(a) indicates thatcrack on the flat surface of the test sample occurred atabout 45°, and its growth tended to follow the directionperpendicular to the stress axis. Furthermore, the fatiguestriation lines progressed up to rupture, and plastic defor-mation occurred at the left side of the surface. Meanwhile,under high stress range conditions, concentration of stressoccurred at the center of the specimen gauge upon activationof multiple slips, resulting in constraint of uniformdeformation within the interior of the gauge center, as shownin Fig. 8(b). Therefore, increased brittle condition, which

Fig. 4 High cycle fatigue results of the cold-rolled steel sheets.

Fig. 3 SEM (Scanning electron microscope) photographs showing fracture morphologies after tensile test.

C.-Y. Jeong2040

implies the plain strain condition, was formed at the center ofthe gauge rather than at the edge of the specimen, inducingfatigue fractures from the center of the gauge and finalrupture at the edge. In this study, two types of fracture modesunder the applied stress range showed different roles for slipbands. These slip bands formed due to the interaction ofcyclic stress with dislocation movement or coalescence ofvoids. There were some micro-cracks resembling small rips,which lead to the formation of secondary cracks. Wanet al.16) proposed that fatigue initiation in low carbon steel isa possible mechanism of formation of intrusions andextrusions. Fractography of the low stress range specimenrevealed that fatigue cracks initiate on the surface edges ofsteel sheets, crack path changes from slanted to flat with

increasing crack length, and the fracture is predominantlytransgranular through striations.17,18) However, a transgranu-lar fracture is replaced by a non-crack fracture withincreasing stress range likely due to void coalescence causedby an increase in slip density in the gauge, resulting inconcentration of stress around the center of the gauge. Inthe present study, the fracture ratio of the transition fromtransgranular fatigue crack initiation and growth to non-crackfracture was 0.89 times the tensile strength of the steels.More research is needed to determine the fracture ratio valuesof other steels and materials as well as the physical meaning

(a)

(b)

(c)

Fig. 5 Effects of pre-strain on yield strength and fatigue limit, (a) SPCC,(b) SPRC340 and (c) yield strength and fatigue limit.

(a)

(b)

Fig. 7 Surface morphologies showing slip lines during fatigue cycling,(a) low stress range and (b) high stress range.

Fig. 6 Relationship between fatigue fracture mode and the ratio of appliedstress to tensile strength.

High-Cycle Fatigue Properties of Automobile Cold-Rolled Steel Sheet with Stress Variation 2041

of the ratio, which may be the critical transition boundarydepending on the slip systems and crystal structures ofmaterials. In conclusion, the micromechanisms of fatiguefailure in cold-rolled steel sheets are characteristicallydifferent. In a high stress range, catastrophic fatigue failureoccurs due to growth and coalescence of voids at the centerof the gauge, whereas in a low stress range, slip band crackinitiation is the primary mechanism of fatigue failure. Thisconforms to the above-mentioned findings that fracture modeis indeed one of the major factors affecting the fatigue livesof materials.

4. Conclusions

(1) For cold-rolled low carbon steels, the fatigue life andfatigue limit increased with increasing tensile and yieldstrengths. In a load ratio (·min/·max) of R = 0 condition,fatigue limit was higher than the monotonic yieldstrength due to cyclic hardening with plastic deforma-tion during fatigue cycling.

(2) Regarding the tensile properties after pre-deformation,the yield strength increased with a higher amount ofpre-strain and was greater than the fatigue limit afterdeformation.

(3) Two types of fracture modes were observed under theapplied stress range. General fatigue failure mode,

which denotes failure by crack initiation and propaga-tion, was the first mode, whereas constrained fracturemode occurred at stress levels higher than 0.89 timesthe tensile strength and exhibited a fracture surfacewithout fatigue crack initiation or propagation.

Acknowledgments

This work was supported by the HMC (Hyundai MotorCompany) Research Fund. Thanks are given for theirfinancial support.

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