the notch effects on the fatigue fracture behaviour of...

46

-

(2003. 8. 7. / 2003. 9. 8. )

The Notch Effects on the Fatigue Fracture Behaviour of Ferrite-

Martensite Dual Phase Steel

Young-Min Do

Department of Automobile Engineering, Doowon Technical College

(Received August 7, 2003 / Accepted September 8, 2003)

Abstract : For the tensile tests of the F.E.M., microvoids are created by the boundary separation process at the martensite

boundary or neighborhood and at inclusions within the ferrite, to grow to the ductile dimple fracture. For the case of the

M.E.F., microvoids created at the discontinuities of the martensite phase which exists at the grain boundary of the primary

ferrite are grown to coalescence with the cleavage cracks induced at the interior of the ferrite, which as a result show

the discontinuous brittle fracture behavior.

In spite of their similar tensile strengths, the fatigue limit and the notch sensitivity of the M.E.F. is superior to those of

the F.E.M.. The M.E.F. is much more insensitive to notch than F.E.M. from the stress concentration factor( ).

Key Words : Martensite Encapsulated islands of Ferrite(M.E.F.), Ferrite Encapsulated islands of Martensite(F.E.M.),

notch, stress concentration factor, fatigue strength, fatigue limit

1. 1)

(ferrite) 2

(martensite)

,

1)

. , .

,

. (Mn, Si, Cr)

2)

,

3), 2

4)

. ,

. ,

[email protected]

,

(fatigue damage)

. ,

(pin hole), , ,

(fillet)

.

(stress concentration effect)

,

.

56)

,

2

.

-

, 18 3, 2003 47

2

,

, 2

.

2.

25mm

SM20C, Table 1 .

Fig. 1

.

2 3

(M.E.F.

) 2

(F.E.M.) . ,

,

.

Fig. 2

, Table 2, 3

.

Table 1. Chemical composition of specimen.

material Chemical composition (wt. %)

C Si Mn P S

SS41 0.19 0.20 0.53 0.029 0.042

(a) F.E.M dual phase steel

(b) M.E.F. dual phase steelFi g. 1. Heat treatment process of F.E.M. and M.E.F. dual

phase steels.

(a) F.E.M. Microstructure

(b) M.E.F. MicrostructureFig. 2. Optical micrographs of dual phase steels.

Table 2. Metallurgical properties of specimen materials.

ferritegrain size

( m)

martensitegrain size

( m)

martensitevolumefraction

(%)

Hardness(Hv), 25gf

ferrite martensite ratio

F.E.M. 65 140 50.20 151 623 4.05

M.E.F. 65 - 51.60 151 623 4.05

Table 3. Mechanical properties of specimen materials.

Tensilestrength(MPa)

Yieldstrength(MPa)

Young'smodulus(MPa)

Elongation(%)

Reductionof area

(%)

Poisson'sratio( )

F.E.M. 726.77 406.60 1.98105

10.21 22.78 0.2704

M.E.F. 745.09 449.13 2.03105

7.19 10.26 0.2770

Fig. 3

Peterson

7) . =1.01

,

=1.42, =1.97, V

=3.2 .

(cantilever type

rotary bending fatigue test machine),

20Hz,, R = -1,

.

Journal of the KIIS, Vol. 18, No. 3, 200348

(a) Smooth specimen

(b) Arc groove specimen

(c) Hole specimen

(d) V groove specimen

Fi g. 3. Shape and dimensions of fatigue test specimens.

(Mitutoyo model MS) (Mitutoyo

model PJ-300) .

.

(Seiwa model SM-600) .

. , ,

,

.

3.

3.1.

Fig. 4 -

,

. F.E.M.

(microvoid)

,

(a) F.E.M. dual phase steel

(b) M.E.F. dual phase steel

Fig. 4. Micrographs showing the crack initiation site formed

during the tensile test (F : ferrite, M : martensite).

. ,

45o

. M.E.F.

2

-

, 18 3, 2003 49

. , , ,

, 2

. ,

,

. , F.E.M. M.E.F.

,

,

.

.

(a) F.E.M. dual phase steel

(b) M.E.F. dual phase steel

Fi g. 5. S.E.M. micrographs showing the fracture appearance with the corresponding microstructure formed during

the tensile test.

Fig. 5

SEM

. F.E.M. Fig. 4

, (shear rupture)8)

(dimple)

. M.E.F. Fig. 4

,

(cleavage crack)

.

.

56 (325390

m) .

(screw dislocation) (cleavage

step)9) (cleavage

rupture)8)

.

Fig. 6 5

105

,

,

. F.E.M.

,

. K

striation

. ,

F.E.M. M.E.F.

2

.

.

M.E.F.

K

striation

K


(a) F.E.M. (b) M.E.F.

Fi g. 6. Cracks of the dual phase steels propagated into the interior of the fatigue test specimen. (a : =306.52

Mpa, b : :328.26MPa) (F : ferrite, M : martensite)

, cleavage

,

.

M.E.F. ,

2

. 2

,

.

3.2.

Fig. 7, 8 ( ) parameter

F.E.M. M.E.F. (S-N

),

.

Fig. 7. S-N curves for F.E.M. dual phase steel

Fig. 8. S-N curves for M.E.F. dual phase steel

M.E.F. F.E.M.

,

M.E.F.

.

2

F.E.M. M.E.F.

,

.

Fig. 9 F.E.M., M.E.F.

,

HT80, HT50

10,11) .

-

, 18 3, 2003 51

,

HT80, HT50

. , HT80

M.E.F. =2.4

HT80

, F.E.M. =3.2

HT80

.

HT80

2

,

( 20 m) (Ni, Cr,

Mo)

HT80

. ,

HT80

M.E.F.

HT80

.

Fi g. 9. Relation between stress concentration factor and fatigue strength.

Fig. 10. Relation between stress concentration factor and

fatigue notch factor.

Fig. 11. Crack length vs. number of cycles for F.E.M. and M.E.F. dual phase steels loaded with the fatigue strength at 5105 cycles.


,

HT80 HT50 .

HT80, HT50

.

M.E.F.

0.43, 0.52, 0.34

F.E.M. 0.48, 0.52, 0.43

. M.E.F.

F.E.M. , =2.0

M.E.F. F.E.M.

.



. S-N 5

105 .

, F.E.M.

M.E.F.

. F.E.M.

. M.E.F.

,

Notch

.

F.E.M.

M.E.F. . ,

,

.

.

4.

2

F.E.M., M.E.F.

, ,

.

1)

F.E.M.

, . ,

M.E.F.

2

, .

2)

,

F.E.M. K

. , M.E.F.

K striation

K cleavage

.

3)

M.E.F. F.E.M.

, =2.0

M.E.F.

.

4) 5105 F.E.M., M.E.F.

2

F.E.M.

M.E.F.

,

.

1) ,

, , 20, 4, pp.

288295, 1980.

2) ,

, , 1984.

3) T. Kunio and H. Suzuki, Effect of Microduplex

Structure Size on Tensile Fracture Behavior of

Steels with Ferrite-Martensitic Structures, JSMS,

Vol. 28, No. 309, pp. 478484, 1979.

4) M. Saika and M. Shimizu, Effect of Micro-

structure on Ductility and Fracture Mechanisms of

Dual Phase Steels, JSME, Vol. 54, No. 507, pp.

20342038, 1988.

5) B.M. Wundt, Effect of Notches on Low-cycle

Fatigue, A Literature Surbey, ASTM STP 490,

pp. 1116, 1972.

6) F.W. Smith, A.F. Emery and A.S. Kobayashi,

Stress Intensity Factors for Semicircular Crack,

J. Appl. mech., Trans. of ASME, E, Vol. 89, pp.

950953, 1967.

7) R.E. Peterson, Stress Concentration Design Fac-

tors, John Willy & Sons, New York, pp. 48, 49,

50, 104, 1965.

8) A.S. Tetelman and A.J. McEvily, Fracture of

Structural Materials, John Willy & Sons, New

-

, 18 3, 2003 53

York, p. 40, 1967.

9) , , ,

, , pp. 6165,

1977.

10) T. Okada, S. Hattori, S. Yamagishi, The Notch

Effect on Corrosion-Fatigue Strength of High-

Strength Steels, JSME, Vol. 52, No. 473, pp. 124

130, 1986.

11) T. Okada, S. Hattori, S. Yamagishi, Corrosion

Fatigue of Notch Structural Steel Specimens in

Seawater, JSMS, Vol. 36, No. 409, pp. 1097

1103, 1987.

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