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Israel Marines-Garca, Damaris Galvn-Montiel, and Claude Bathias

April 2008 The Arabian Journal for Science and Engineering, Volume 33, Number 1B 237

FATIGUE LIFE ASSESSMENT OF HIGH-STRENGTH, LOW-

ALLOY STEEL AT HIGH FREQUENCY

Israel Marines-Garca*

Senior Structural Integrity Engineer, Tenaris-Tamsa R&D, Veracruz, Mexico

Damaris Galvn-Montiel

Associate Professor, CIICAp-UAEM, Cuernavaca, Morelos, 62210, Mexico

and Claude Bathias

Senior Professor, CNAM-ITMA, Paris, 75003, France

:

- - HSLA)D38MSV5S(.1010))VHCF

35Hz20 kHz1R=0.1 107. R= 1107R=0.1.)()VHCF( .

* Address for correspondence:

Dr. Israel Marines - Garcia

Tenaris Tamsa R & D

Carr. Mxico Veracruz via Xalapa, Km 433.7

91697 Veracruz, Ver.

MEXICO

Tel: +52 (229) 9894451

Fax: +52 (229) 9891114

E-mail: [email protected] E-mail: [email protected] E-mail: [email protected]

Paper Received 29 March 2006; Revised 8 June 2007; Accepted 18 September 2007

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The Arabian Journal for Science and Engineering, Volume 33, Number 1B April 2008238

ABSTRACT

A fatigue experimental assessment is described for an HSLA steel (high-strength,

low-alloy steel) D38MSV5S on very high cycle fatigue (VHCF) up to 1010cycles.

The fatigue testing has been conducted at load ratio, R= 0.1 and 1, under 20 kHz

and 35 Hz. Herein, it will be observed that the test frequency does not have any

effect on fatigue data. The fatigue failure can occur over 107 cycles. The fatigueendurance continues to decrease with an increasing number of cycles forR= 1, but,

for R = 0.1 no fatigue failure happened over 107 cycles. Finally, the same crack

initiation mechanism that has been observed by other researchers on VHCF (termed

fish-eye failure), has been found during our fractographic analysis.

Key words:gigacycle fatigue, high-strength, low-alloy steel, high frequency testing,

fish-eye failures.

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FATIGUE LIFE ASSESSMENT OF HIGH-STRENGTH, LOW-ALLOY STEEL AT HIGH

FREQUENCY

1. INTRODUCTIONHSLA D38MSV5S steel is widely used in structural parts at the automotive industry due to its high strength

characteristic. However, it is very well known that cyclic loading could cause structural failure at anytime. Therefore, it

is mandatory to carry out fatigue experimentation to perform safe design of automotive components, which have to

survive a relatively high number of load cycles in service. Bathias [1] and Baudry [2] mentioned that fatigue lifetime of

some car engine components could attain 109 cycles. Now, considering that some components manufactured in

D38MSV5S steel could reach in service the gigacycle fatigue regime, the present work is focused on obtaining

experimentally its fatigue behavior up to 1010 cycles. However, conventional fatigue machines (servo-hydraulic) are

limited in testing speed; this means that using those machines, it could be impossible to know the experimental fatigue

behavior at more than 107cycles (e.g.to obtain an SN curve up to 1010cycles, for a single specimen, it takes more than

9 years to attain such a number of cycles at 35 Hz ).

In general, SN curves are fixed at 107 cycles; after that lifetime, it is assumed that fatigue strength does not

decrease. (an assumption of asymptotic behavior is postulated) [3], but many materials do not exhibit this response;instead, they display a continuously decreasing stresslifetime response, even at a great number of cycles (10 8 to 1010

cycles) [410]. On the other hand, fatigue strength within the VHCF regime is occasionally estimated by average

strength f(106 and 107 cycles) and standard deviation (s), so, fatigue strength at 109cycles is given by f-3s[1, 11].

This consideration of fatigue strength estimation on VHCF range, using low cycle fatigue (LCF) data, is certainly not the

best way to decrease the risk of fatigue failure; for that reason, one must carry out experimental testing to plot the fatigue

behavior (SN curves) under VHCF, for better, safe, fatigue design. The way to do so, is to use high frequency fatigue

machines.

Ultrasonic fatigue devices have been adopted by several research groups around the world, in order to plot

experimentally the SN curves under VHCF regime. Nowadays, those devices achieved a high technical standard with

high accuracy and reliability [45, 1216]. Fatigue testing on HSLA steel was possible using a high frequency device

(natural resonance principle, 20 kHz). Fatigue characterization was under load ratio, R = 0.1 and 1. SEM analysis

shows that the crack initiation on VHCF switched location from surface to an internal crack initiation. Thus, a

hypothetical explanation related failures up to 109cycles of biphasic steel is presented, while, biphasic steel is loaded onits bulk elastic regime.

2. EXPERIMENTAL PROCEDURE

2.1. Testing Material and Specimens

The material used in this study was an HSLA steel (D38MSV5S). Its chemical composition percentage (weight) is

0.384 carbon, 5.67 silicon, 1.23 manganese, 0.012 phosphorus, 0.064 sulfur, 0.183 gallium, 0.018 molybdenum, 0.063

nickel, 0.025 aluminum, 0.063 copper, and 0.089 vanadium. The HSLA steel manufacture process was under NF EN

10267 standard. Figure 1(a) shows microstructure patterns, examined along two perpendicular planes, longitudinal and

transverse section. The micrographs clearly indicate that the HSLA steel has a fine texture. Ferrite (45%) and pearlite

(55%) distribution is uniform, however, there are some inclusions of oxide of aluminum (Al2O3) and inclusions

composed of aluminum, manganese, calcium, and magnesium (Al2O3MnOCaOMgO), Figure 1(b).

It was found that the inclusions as (Al2O3) and (Al2O3MnOCaOMgO) are similar to those located on both surface

and internal crack initiation. Fatigue experiments were performed with loading ratio,R= 1 and 0.1. The static strength

values of the investigated material are shown in Table 1.

Figure 2 shows the dimensions of the testing specimens. The specimens have an hourglass shape with minimum

diameter of 3 mm; their dimensions satisfy the natural resonance (axial mode) at 20 kHz. Specimens were designed by

finite element method. The relationship between displacements versus stress under natural resonance (vibration) was

calculated using commercial computational software (ANSYS) in order to determine the testing applied stress. The

surface of the specimens was polished before testing, in order to eliminate micro notches and to obtain a smooth surface.

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(a) (b)

Figure 1. HSLA steel (D38MSV5S) microstructure; (a) pearlite 55% ferrite 45% microstructure, 500 ; (b) inclusions

cleanness, 100 Table 1. Material properties of HSLA steel, D38MSV5S

Ed 10kHz

(Gpa)

Ed 20 kHz

(Gpa)y0.2% (MPa)

UTS

(MPa)

A

(%)(kgm3) HV30 HRc

208.3 211.5 608 878 20 7850 246 21.3

Figure 2. Shape and dimension of ultrasonic fatigue specimen

2.2. Test Method

Fatigue tests were performed in an open environment at room temperature using an ultrasonic fatigue test system [7,

11], namely, the piezoelectric (ceramic material which transforms electrical power to mechanical vibration) fatigue

technique that operates at a very high frequency. Using this method, specimens are excited to longitudinal resonance

R31 + 0.01

10 + 0.05

3 + 0.01

62.42 + 0.1

M5

12

L2

L1

8

L1 = 14.31 mm

L2 = 16.9 mm

Dimensions

en mm

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vibrations at ultrasonic frequency (20 kHz). This leads to a sinusoidal cyclic loading with maximum load amplitude in

the center of the specimen. Stress amplitudes are calculated using the measured cyclic strain (strain gages) or

displacement at the end of the specimen (optical fiber). The fatigue experiments were performed with constant cyclic

loads, with or without static preload, that is, load ratio R= 1 and 0.1 respectively. The preload of the tensiontension

fatigue tests (R= 0.1) was controlled on an INSTRON fatigue-testing machine. During testing, the specimen was cooled

by air to decrease temperature rise caused by atomic friction (vibration), keeping the superficial temperature of the

specimen between 20 and 30C. Failure of specimens may be detected by monitoring the resonance frequency, whichmakes possible the experiments automatic operation. Tests were not stopped until specimen failure occurred or cyclic

loading attained 1010cycles if failure did not happen.

3. RESULTS AND DISCUSSION

3.1. SN Curves

The results of fatigue experiments on D38MSV5S steel (R= 1) are shown in Figure 3, specimens that did not fail

are marked with an arrow; lines indicate a fracture probability of 50%.

Test results from cyclic loading conducted at 35 Hz are between 105and 107cycles. If the specimen did not fail at

107 cycles, the testing was stopped and the result on the SN curve was marked as run-out. International codes [3] define

107cycles as endurance limit (hypothesis of horizontal asymptote; it means that fatigue strength does not decrease any

more for a given number of cycles).

Comparing experimental results from 20 kHz and 35 Hz, no significant difference is observed below 107 cycles, and

the fatigue data coincide within the range of scatter, it means that the frequency has not an influence on results. On

VHCF (107109 cycles), it is still observed fatigue failures, moreover, the fatigue strength continues decreasing with the

number of cycles. The fatigue strength was calculated by staircase method at 10 6 and 1010 cycles, it is 417.5 and 315

MPa, respectively. If one estimates the safe fatigue strength value for 109cycles given by the average strength at 106

cycles f= 417.5 MPa and the standard deviation, s= 25, that is f3s, then the safe fatigue strength is 342.5 MPa

(109cycles). Now, compared to the lower experimental fatigue data, the difference is close to 25 MPa, Figures. 34.

Additional statistical analysis has been performed with the bastenaire methodology [17], Figure 5 and Table 2. The

bastenaire statistical method follows the best fit of experimental results (it is not a linear regression); however, the

estimated curve is always finished by a horizontal asymptote. Even on later observation, the fatigue strength (50%

failure probability) values are very close to those calculated by the staircase method.

250

300

350

400

450

1,E+03 1,E+04 1,E+05 1,E+06 1,E+07 1,E+08 1,E+09 1,E+10 1,E+11

N Cycles

max(MPa)

ITMA-CNAM 20 kHz R=-1

RENAULT 30 Hz R=-1

= -14,963Ln(N) + 609,82R2= 0,8795

Figure 3. Fatigue SN curve of high-strength, low-alloy steel D38MSV5S with R= 1, 20 kHz and 35 Hz

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Figure 3. Fatigue SN curve of high-strength, low-alloy steel D38MSV5S with R= 1, 20 kHz and 35 Hz

Figure 4. Fatigue stress limit comparison between estimation value and experimental results

(a) (b)

(c) (d)

Figure 5. Best fit SN curve and fatigue strength estimation by Bastenaire method [17] for different lifetime under load ratio

R= 1; (a) 1010

cycles, (b) 109cycles, (c) 10

8cycles and (d) 10

7cycles

290

315

340

365

390

415

440

465

490

1.E+03 1.E+04 1.E+05 1.E+06 1.E+07 1.E+08 1.E+09 1.E+10

NF(Cycles)

MAX

MPa

290

315

340

365

390

415

440

465

490

1.E+04 1.E+05 1.E+06 1.E+07 1.E+08 1.E+09

NF(Cycles)

MAX

MPa

290

315

340

365

390

415

440

465

490

1.E+04 1.E+05 1.E+06 1.E+07 1.E+08

NF(Cycles)

MAX

MPa

350

375

400

425

450

475

1.E+03 1.E+04 1.E+05 1.E+06 1.E+07

NF(Cycles)

MAXMPa

104 105 108 1010 1011109107106

N (cycles)

Fatigue stresslimit

(hypothesis of the horizontal asymptote

)

55 MPa

Experimental Fatiguestrength

104 105 108 1010 1011109107106104104 105105 108108 10101010 10111011109109107107106106

N (cycles)

)

MPa

Experimental Fatiguestrength

(MPa)

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Table 2. Fatigue Strength Estimation by Bastenaire Method [17] for Different Lifetime Under Load RatioR= 1

Fatigue strength (MPa)

Lifetime (cycles) 10% failure

probability

50% failure

probability

90% failure

probability

1010 310 325 340

109 311 326 341

108 311 325 339

107 378 395 413

The experimental results under cyclic loading ratioR= 0.1 are shown in Figure 6. In comparison with the tension

compression fatigue results, the fatigue strength decreases abruptly over a short lifetime period. It is noticed that the high

frequency results are a little higher on strength than those of low frequency, nevertheless, there is a good agreementbetween results obtained from both high and low test frequency.

There is no failure between 107and 1010 cycles for the cyclic stress close to plastic strain; in this case the hypothesis

of a horizontal asymptote is valid. The fatigue strength calculated at 107 cycles by the staircase method is 632 and 572.5

MPa at frequencies of 35 Hz and 20 kHz respectively. A high scatter in the experimental results is also observed, which

gives a great uncertainty. Therefore, those results will be treated carefully by statistics in order to carry out safe fatigue

design.

Figure 6. Fatigue SN curve of high-strength, low-alloy D38MSV5S steel with R = 0.1, 20 kHz and 35 Hz

430

480

530

580

630

680

730

780

1.E+04 1.E+05 1.E+06 1.E+07 1.E+08 1.E+09 1.E+10 1.E+11

N Cycles

max(MPa)

ITMA-CNAM 20 kHz

RENAULT 35 Hz= -33,761Ln(N) + 1102,9

R2= 0,4997

max

(MPa)

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3.2. Fractography

The analysis of fracture surface under SEM shows that failures initiated on different sites (surface or internal) and

from different kind of defects. Fatigue crack initiation (R= 1) is located on surface when less than 107cycles, then it

become internal (fish-eye). Figure 7 shows surface crack initiation under cyclic loading ratio, R= 1, under maximum

positive stress of 380 MPa. Figure 8 shows a typical example of fish-eye pattern (internal crack initiation), which failed

under maximal positive stress of 330 MPa at load ratioR= 1. Non-metallic inclusion was the internal crack initiator, thecircular area reveals fatigue crack growth until sudden final failure (unstable crack growing). However, it is noticed that

inclusions on most of the internal cracks were not the principal crack initiator. It was the microstructure with low

strength (ferrite); this can be explained as follows: the specimen from a macroscopicviewpoint is loaded under lower

cyclic stress amplitude (elastic zone), nevertheless from the microscopicviewpoint the ferrite grains are loaded under

higher cyclic stress amplitude (plastic zone), it means that fatigue failure may occur from ferrite grain when the applied

cyclic loading is higher than its strength. One could conclude that ferrite grains are tested plastically (preferential crack

initiation sites), even if the bulk material is tested elastically (Figure 9).

Figure 7. Surface crack initiation, HSLA D38MV5S steel, max =380 MPa, Nf=9.42106, R=1, 20KHz

Figure 8. Fatigue crack initiation from non-metallic inclusion (Al2O3+MnO+CaO+MgO), HSLA steel, max = 330 MPa, Nf=

7.03107cycles, R = 1, 20 kHz

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Figure 9. Deformation behavior of two different phases for a specific applied load

Fatigue testing conducted at load ratio,R= 0.1, does not present failure after 107cycles, crack initiation is always on

the surface. It means that fatigue crack initiation is strongly dependent on micro-defects and non-metallic inclusions

located on the surface. Also, it is observed that cyclic plastic deformation in the plane stress condition becomes

dangerously riskily high (max/y> 1, fully plastic testing). In Figure 10, a surface crack initiation is observed; its origin

is a non-metallic inclusion (Al2O3) under maximum cyclic stress of 625 MPa.

Figure 10. Surface crack initiation due to non-metallic inclusion

(Al2O3), HSLA steel, max=625 MPa, Nf=2.04106cycles, R = 0.1, 20 kHz

4. CONCLUSIONS

The fatigue properties of an HSLA steel (D38MSV5S) have been investigated at two load ratios, R= 1 and 0.1

under VHCF, using an ultrasonic fatigue testing device (natural resonance principle). In comparison with the results

obtained at a frequency of 35 Hz, the fatigue properties in the regime between 105and 1010cycles can be summarized as

follows: the fatigue data for the HSLA steel between 105and 109cycles may be approached by a sloping line and by the

Bastenaire method (best fit), between 104and 1010cycles in the SN curve with R= 1. Fatigue failure is still present

beyond 107 cycles. There is a difference of around 100 MPa between 106 and 109 cycles on experimental fatigue

strength. Ferrite grains and non-metallic inclusions initiated the cracks. In most cases, the failure initiation may switch

location from surface to an interior fish-eye after 107 cycles. Experimental results conducted at different frequencies

are in good agreement.

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[15] T. Wu, J. Ni, and C. Bathias, An Automatic Ultrasonic Fatigue Testing System for Studying Low Crack Growth at

Room and High Temperatures,ASTM STP,1231(1994), p. 598607.

[16] Israel Marines-Garcia, Jean-Pierre Doucet, and Claude Bathias, Development of a New Device to Perform Torsional

Ultrasonic Fatigue Testing,International Journal of Fatigue, 29(911)(2007), pp. 20942101.

[17] F. Bastenaires, Estimation et Prevision Statistiques de la Resistance et de la Dure des Matriaux en Fatigue, IRSID

report.

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