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Effect of monotonic and cyclic prestrain on the fatigue threshold in medium-carbon steels
Int J Fatigue 14 No 1 (1992) pp 41-44
Effect of monotonic and cyclic prestrain on the fatigue threshold in medium-carbon steels Li Nian and Du Bai-Ping
The effect of monotonic tensile and cyclic prestrain on fatigue threshold was studied. The fatigue threshold values of medium-carbon steels are found to decrease in the initial stage, then increase with an increasing amount of prestrain. A mechanism for the observed change in fatigue threshold is also presented. Key words: prestrain; fatigue threshold; microvoids; microcracks
Instant overloading is often to be found in the service conditions of machine components or structures. Plastic flow occurs around the stress concentration region, and results in changes in microstructure. In applications, strain hardening is generally employed for improving the fatigue behaviour as well as the resistance to fracture of steels. Strain hardening is also associated with changes of microstructure. In particular, the fatigue threshold Agth and stage-I crack propagation are closely related to the microstructure of steels. Then, how does prestrain exert an influence on AKth? Blacktop et al 1 showed the effect of prestrain on ~LKthin a 0.1% C-1.5% Mn steel. With an increase of prestrain by up to 10%, Agth decreases for a stress ratio of less than 0.3. The plastic deformation process at the crack tip results in this change in AK=h. However, high prestrains larger than 10% have not been studied and their effect on fatigue threshold was not elucidated. Tai Shankai et al 2, Schijve3 and Schulte and Nowack 4 pointed out that prestrain increases the crack propagation rate in aluminium alloys; a conclusion based on the plastic exhaustion of the material, since the prestrain reduces the plastic deformation capability of the material. 2 Schijve3 showed that the yield stress of prestrained material is increased owing to the reduction of the plastic zone size ahead of the crack tip, so that the crack opening force decreases. Schulte and Nowack 4 pointed out that, in addition to the reasons mentioned above, prestrain induces a high dislocation density with a large stored energy. During fatigue the stored energy will help in rearranging the tangled dislocations and accelerating the crack growth process. Arora et al s investigated a mild steel with stringer inclusions; they pointed out that, in prestrained materials, the crack propagation in stage II is found to have two distinct phases. Phase I indicates a crack retardation type of behaviour, while phase II results in stable crack growth. A tensile prestrain will result in a residual compressive stress field at the tips of the stringer inclusions; it would retard the crack growth rate to a great extent. The degree of retardation increases with an increase in the amount of prestrain. It is inferred that ~r~th increases with an increase in prestrain. In this paper, the effect of monotonic and cyclic prestrain on AKth is studied in two medium-carbon steels.
Material, specimens and e x p e r i m e n t Materials 40 Cr: C, 0.37%; Cr, 1.38% ;Si, 1.08% ;Mn, 0.34% ;P, 0.036%; S,0.026%. Group I. Normalizing: 860 °C salt bath, air cooling. Group II. Quenching and tempering: 860 °C salt bath, water quenching and oil cooling, tempering at 600 °C for 2h. 40 CrNiMo: C,0.42%; Cr, 1.04%; Ni, 1.40%; Mo, 0.21%; Si, 0.25% ;S,0.006% ;P,0.010%. Quenching and tempering: 860 °C salt bath, oil quenching and tempering at 600 °C for 2 h.
Prestrain of specimens
Monotonic tensile prestrain Monotonic tensile prestraining of steel bars was preformed for different cross sections reduced to c = 2 In D0/D; here Do and D are the diameters of the specimens before and after the prestrains, The plate specimens were cut from the bars. If necking occurs in the prestrain, the plate specimens cut from the bars should be centred at the maximum degree of prestrain where a discharging slot would be prepared. Details are given in Table 1.
Cyclic prestrain An [nstron servohydraulic testing machine of type 1342 was used for the cyclic prestraining. During cyclic loading at a stress ratio of R = -1, the strain amplitudes were controlled to within limits 0.5%, 1% and 2%. The cyclic loads were decreased stepwise (Fig. 1), until a stationary state was reached. Plate specimens were cut from the centre of the prestrain plate. Details are given in Table 2.
Experiment An Amsler high-frequency fatigue testing machine was used for the measurement of AKth. The experiments were carried out at f = 80 Hz, ambient temperature and atmospheric pressure. The crack length was measured by a travelling light microscope, of which the resolution power is 0.01 ram.
0142-1123/92/010041-04 © 1992 Butterworth-Heinemann Ltd Int J Fatigue January 1992
41
Table 1. Monotonic tensile prestrain (a) In 40 Cr steel (group I) Initial diameter, Do (mm) Diameter after prestrain, D (mm) e% (= 2 In Do~D) A/(th (MPa ~/-m)
AKop(MPa
0 8.93 8.46 8.74
26.04 25.20 6.6
26.04 24.30 13.8
26.08 23.18 23.6
26.02 21.76 35.8
26.04 20.06 52.2
26.04 19.94 53.4
3.88 4.09
4.71 4.90
9.52 10.19
14.11
14.42
15.22
5.86
8.43
8.22
~-m)
(b) In 40 Cr steel (group II)
Initial diameter, Do (mm) Diameter after prestrain, D (mm)
e(%)
0 7.93 8.13 7.35
A~th (MPa ~/-m)
26.42 25.40 7.9
26.42 24.30 16.7
26.40 21.88 37.6
26.40 20.90 46.7
7.7 7.6
14.05 13.33
14.11
16.43 18.29
21.76 20.79 9.1
21.75 18.13 36.4
21.76 16.78 52.7
21.73 16.10 60
7.55
14.5
14.5
15.7
(c) In 40 CrNiMo steel Initial diameter, Do (mm) Diameter after prestrain, D (mm) e
(%)
0 8.46 8.1 7.8
AKth (MPa V m )
Table 2. Cyclic prestrain e (%)
0
0.5
1
2
(rmax (first cycle) (MPa) ~max (35th cycle) (MPa) A/(th (MPa V'-m)
819 807 857 590 644 714 7.93 8.53 9.46 7.35 8.74 8.71 11.04 8.13 8.56
The fatigue threshold AK,h was measured for a reduction of stress intensity factor range AK at a stress ratio of R = 0.1. If no crack growth could be detected after 106 cycles, this value of AK was defined to be AK~h, which corresponded to a growth rate of 10 -s mm/cycle.
an increase of prestrain from 13.8 up to 35.8%. When the prestrain is larger than 35.8% the growth of AK,h becomes slow. In the region where the strains are larger than 23.6%, the change in the crack opening stress intensity factor is similar to that of AKth. The two other groups of specimens (Fig. 3 and Tables l(a) and (b)) exhibit the same shape of the AK~h-e% curve, except that the turning point at the initial stage in Fig. 3 is 7.9%. After cyclic prestrain, AK~hincreases monotonically with the increase of prestrain as shown in Fig. 4 and Table 2. For monotonic prestrain the yield strength increases in the form of an exponential hardening law; this results in a reduction of the plastic zone size ahead of the crack tip and a reduction of the roughness of the fracture surface. Consequently, the small magnitude of the residual elongation in the crack wake causes the opening force to decrease.
Dislocation configuration with prestrain Test results and discussion The effect of monotonic tensile prestrain on
AK~h Figure 2 and Table l(a) show the effect of prestrain on AK~h for 40 Cr steel (group I). With the small prestrain values of 6.6 and 13.8% an appreciable decrease in AK~h appears, but the AKth value goes through a minimum and increases with
42
Dislocations generate tangles or forest dislocations; their density increases rapidly, see Fig. 5. For high prestrain values a wall structure starts to form. Based on these dislocation configurations, fatigue damage is easily accumulated and a typical fatigue wall structure is established, leading to crack growth. This results in the decrease of AKth in the lowprestrain region.
Int J Fatigue January 1992
0
16
L~
12
=_
8!
o tl0Cr x q0CrNiMo
<3 q
.0
2 o
I
~
I 10
0
1 20
I 30
I
I 50
I 60
,E C~s)
I I I I I
,
I qo
Fig. 3 The effects of monotonic prestrain on A/~h in 40 Cr (group II) and 40 CrNiMo steels
I 12
o/
Fig. 1 The actual hysteresis loops in the load-strain plots for a specimen subjected to a prestrain of 2% at a stress ratio of - 1 in 40Cr (group II) steel
/ /
10
/
/
#_
o
16
/
<~
12
o.
8
o
/
AKth
L•
/
K
op I
I
I
I
I
0.5
1.0
1.5
2.0
2.5
3.0
,E C~) I 10
I 20
I 30
I 40
I 50
60
Fig. 4 The effect of cyclic prestrain at K = - 1 on ~Kth, in 40 Cr (group II) steel
~: (~)
Fig. 2 The effect of monotonic prestrain on ~Kth and Kop, in 40 Cr (group I) steel
With a further increase of prestrain, the fatigue threshold turns out to increase. SEM observations reveal that in the fractured surface at the threshold region the roughness is greatly increased and that microvoids and microcracks become predominant. The number and the size of these microdefects increase along with the increase in the value of prestrain, see Fig. 6. The roughening of the crack wake increases the crack opening force. The compressive residual stress is concentrated at the microvoids and microcracks owing to the unloading of the tensile stress in the prestrain treatment. Therefore, the effective stress intensity factor ahead of the crack tip decreases and the magnitude of the fatigue threshold increases.
Int J Fatigue January 1992
Fig. 5 Dislocation configuration of a monotonic prestrain of 7.9% in 40 Cr (group II) steel, × 73000
43
Fig. 6 Microvoids and microcracks of a monotonic prestrain of 52.2% in 40 Cr (group II) steel, x 2000
In addition, the dispersed microvoids and microcracks would reduce the load stress concentration at the crack tip, the effective loading stress intensity factor decreases when the crack grows, and the microvoids and microcracks would make for a zigzag crack path. The deflection of the crack results in a reduction of mode-I stress intensity factor and the crack path would be elongated owing to its non-linear growth. The experimental results of Ref. 5 for stringer inclusions explain the effects of load scattering and crack deflection. Few microvoids and microcracks appeared in the smallprestrain region, where the fatigue threshold decreases; this is associated with the combined effect of two opposing factors, the dislocation configuration and the microvoids. Only if the effect of the microvoids becomes predominant does AKthstart to increase again. The original dislocation configurations of normalized and high-temperature tempered specimens are different. The lower normalized dislocation density one tolerates the larger the range of dislocation generation; thus a remarkable fall in AK,h is exhibited. With cyclic prestrain the wall structure is easily established. A number of cyclic loadings in prestrain induces fatigue damage and produces a large number of microvoids and microcracks, which prevail over the role of the dislocation density; thus AKth increases monotonically, as in Fig. 7. However, even AKthis high in these microvoid materials; its crack growth rate is extremely rapid. If the loading stress intensity factor is a little higher than the AKth value, the microvoids and microcracks are easily linked together and the crack increases suddenly.
Fig. 7 Microvoids and microcracks of a cyclic prestrain of 2% in 40 Cr (group II) steel, x 5000
2) 3)
fatigue threshold first decreases, then increases with the increase of prestrain. Cyclic prestrain makes the fatigue threshold increase monotonically with the increase of cyclic prestrain. Two opposing factors, dislocation density and microvoids and microcracks, affect the relation between the prestrain and the fatigue threshold. The increase in the dislocation density lowers the AKth value. The large number of microvoids and microcracks produced in monotonic prestrain and large-magnitude cyclic prestrain play a predominant role; the fatigue threshold increases.
References 1.
Blacktop, J., Nicholson, C.E., Brook, R. and Towers, R.T. Proc of Int Conf on Fatigue Threshold, Stockholm, 1981 pp 629-638
2.
Tel Shankei end Liu, H.W. Eng Fract Mech 6 4 (1974) pp 631-638
3.
Schijve, J. Eng Fract Mech 8 4 (1976) pp 575-581
4.
Schulte, K. and Nowack, H. Eng Fract Mech 13 4 (1980) pp 1009-1021
5.
Arora, P.R., Raghavan, M.R. and Praeed, Y.V.R.K. Eng Fract Mech 29 1 (1988) pp 67-69
Authors
Conclusions 1)
44
Monotonic tensile prestrain results in a change of the fatigue threshold in two medium-carbon steels. The
The authors are with the Research Institute for the Strength of Metals, Xi'an Jiaotong University, Xi'an, China. Received 15 May 1991; accepted in revised form 3 October 1991.
Int J F a t i g u e J a n u a r y
1992
Effect of monotonic and cyclic prestrain on the fatigue threshold in medium-carbon steels Li Nian and Du Bai-Ping
The effect of monotonic tensile and cyclic prestrain on fatigue threshold was studied. The fatigue threshold values of medium-carbon steels are found to decrease in the initial stage, then increase with an increasing amount of prestrain. A mechanism for the observed change in fatigue threshold is also presented. Key words: prestrain; fatigue threshold; microvoids; microcracks
Instant overloading is often to be found in the service conditions of machine components or structures. Plastic flow occurs around the stress concentration region, and results in changes in microstructure. In applications, strain hardening is generally employed for improving the fatigue behaviour as well as the resistance to fracture of steels. Strain hardening is also associated with changes of microstructure. In particular, the fatigue threshold Agth and stage-I crack propagation are closely related to the microstructure of steels. Then, how does prestrain exert an influence on AKth? Blacktop et al 1 showed the effect of prestrain on ~LKthin a 0.1% C-1.5% Mn steel. With an increase of prestrain by up to 10%, Agth decreases for a stress ratio of less than 0.3. The plastic deformation process at the crack tip results in this change in AK=h. However, high prestrains larger than 10% have not been studied and their effect on fatigue threshold was not elucidated. Tai Shankai et al 2, Schijve3 and Schulte and Nowack 4 pointed out that prestrain increases the crack propagation rate in aluminium alloys; a conclusion based on the plastic exhaustion of the material, since the prestrain reduces the plastic deformation capability of the material. 2 Schijve3 showed that the yield stress of prestrained material is increased owing to the reduction of the plastic zone size ahead of the crack tip, so that the crack opening force decreases. Schulte and Nowack 4 pointed out that, in addition to the reasons mentioned above, prestrain induces a high dislocation density with a large stored energy. During fatigue the stored energy will help in rearranging the tangled dislocations and accelerating the crack growth process. Arora et al s investigated a mild steel with stringer inclusions; they pointed out that, in prestrained materials, the crack propagation in stage II is found to have two distinct phases. Phase I indicates a crack retardation type of behaviour, while phase II results in stable crack growth. A tensile prestrain will result in a residual compressive stress field at the tips of the stringer inclusions; it would retard the crack growth rate to a great extent. The degree of retardation increases with an increase in the amount of prestrain. It is inferred that ~r~th increases with an increase in prestrain. In this paper, the effect of monotonic and cyclic prestrain on AKth is studied in two medium-carbon steels.
Material, specimens and e x p e r i m e n t Materials 40 Cr: C, 0.37%; Cr, 1.38% ;Si, 1.08% ;Mn, 0.34% ;P, 0.036%; S,0.026%. Group I. Normalizing: 860 °C salt bath, air cooling. Group II. Quenching and tempering: 860 °C salt bath, water quenching and oil cooling, tempering at 600 °C for 2h. 40 CrNiMo: C,0.42%; Cr, 1.04%; Ni, 1.40%; Mo, 0.21%; Si, 0.25% ;S,0.006% ;P,0.010%. Quenching and tempering: 860 °C salt bath, oil quenching and tempering at 600 °C for 2 h.
Prestrain of specimens
Monotonic tensile prestrain Monotonic tensile prestraining of steel bars was preformed for different cross sections reduced to c = 2 In D0/D; here Do and D are the diameters of the specimens before and after the prestrains, The plate specimens were cut from the bars. If necking occurs in the prestrain, the plate specimens cut from the bars should be centred at the maximum degree of prestrain where a discharging slot would be prepared. Details are given in Table 1.
Cyclic prestrain An [nstron servohydraulic testing machine of type 1342 was used for the cyclic prestraining. During cyclic loading at a stress ratio of R = -1, the strain amplitudes were controlled to within limits 0.5%, 1% and 2%. The cyclic loads were decreased stepwise (Fig. 1), until a stationary state was reached. Plate specimens were cut from the centre of the prestrain plate. Details are given in Table 2.
Experiment An Amsler high-frequency fatigue testing machine was used for the measurement of AKth. The experiments were carried out at f = 80 Hz, ambient temperature and atmospheric pressure. The crack length was measured by a travelling light microscope, of which the resolution power is 0.01 ram.
0142-1123/92/010041-04 © 1992 Butterworth-Heinemann Ltd Int J Fatigue January 1992
41
Table 1. Monotonic tensile prestrain (a) In 40 Cr steel (group I) Initial diameter, Do (mm) Diameter after prestrain, D (mm) e% (= 2 In Do~D) A/(th (MPa ~/-m)
AKop(MPa
0 8.93 8.46 8.74
26.04 25.20 6.6
26.04 24.30 13.8
26.08 23.18 23.6
26.02 21.76 35.8
26.04 20.06 52.2
26.04 19.94 53.4
3.88 4.09
4.71 4.90
9.52 10.19
14.11
14.42
15.22
5.86
8.43
8.22
~-m)
(b) In 40 Cr steel (group II)
Initial diameter, Do (mm) Diameter after prestrain, D (mm)
e(%)
0 7.93 8.13 7.35
A~th (MPa ~/-m)
26.42 25.40 7.9
26.42 24.30 16.7
26.40 21.88 37.6
26.40 20.90 46.7
7.7 7.6
14.05 13.33
14.11
16.43 18.29
21.76 20.79 9.1
21.75 18.13 36.4
21.76 16.78 52.7
21.73 16.10 60
7.55
14.5
14.5
15.7
(c) In 40 CrNiMo steel Initial diameter, Do (mm) Diameter after prestrain, D (mm) e
(%)
0 8.46 8.1 7.8
AKth (MPa V m )
Table 2. Cyclic prestrain e (%)
0
0.5
1
2
(rmax (first cycle) (MPa) ~max (35th cycle) (MPa) A/(th (MPa V'-m)
819 807 857 590 644 714 7.93 8.53 9.46 7.35 8.74 8.71 11.04 8.13 8.56
The fatigue threshold AK,h was measured for a reduction of stress intensity factor range AK at a stress ratio of R = 0.1. If no crack growth could be detected after 106 cycles, this value of AK was defined to be AK~h, which corresponded to a growth rate of 10 -s mm/cycle.
an increase of prestrain from 13.8 up to 35.8%. When the prestrain is larger than 35.8% the growth of AK,h becomes slow. In the region where the strains are larger than 23.6%, the change in the crack opening stress intensity factor is similar to that of AKth. The two other groups of specimens (Fig. 3 and Tables l(a) and (b)) exhibit the same shape of the AK~h-e% curve, except that the turning point at the initial stage in Fig. 3 is 7.9%. After cyclic prestrain, AK~hincreases monotonically with the increase of prestrain as shown in Fig. 4 and Table 2. For monotonic prestrain the yield strength increases in the form of an exponential hardening law; this results in a reduction of the plastic zone size ahead of the crack tip and a reduction of the roughness of the fracture surface. Consequently, the small magnitude of the residual elongation in the crack wake causes the opening force to decrease.
Dislocation configuration with prestrain Test results and discussion The effect of monotonic tensile prestrain on
AK~h Figure 2 and Table l(a) show the effect of prestrain on AK~h for 40 Cr steel (group I). With the small prestrain values of 6.6 and 13.8% an appreciable decrease in AK~h appears, but the AKth value goes through a minimum and increases with
42
Dislocations generate tangles or forest dislocations; their density increases rapidly, see Fig. 5. For high prestrain values a wall structure starts to form. Based on these dislocation configurations, fatigue damage is easily accumulated and a typical fatigue wall structure is established, leading to crack growth. This results in the decrease of AKth in the lowprestrain region.
Int J Fatigue January 1992
0
16
L~
12
=_
8!
o tl0Cr x q0CrNiMo
<3 q
.0
2 o
I
~
I 10
0
1 20
I 30
I
I 50
I 60
,E C~s)
I I I I I
,
I qo
Fig. 3 The effects of monotonic prestrain on A/~h in 40 Cr (group II) and 40 CrNiMo steels
I 12
o/
Fig. 1 The actual hysteresis loops in the load-strain plots for a specimen subjected to a prestrain of 2% at a stress ratio of - 1 in 40Cr (group II) steel
/ /
10
/
/
#_
o
16
/
<~
12
o.
8
o
/
AKth
L•
/
K
op I
I
I
I
I
0.5
1.0
1.5
2.0
2.5
3.0
,E C~) I 10
I 20
I 30
I 40
I 50
60
Fig. 4 The effect of cyclic prestrain at K = - 1 on ~Kth, in 40 Cr (group II) steel
~: (~)
Fig. 2 The effect of monotonic prestrain on ~Kth and Kop, in 40 Cr (group I) steel
With a further increase of prestrain, the fatigue threshold turns out to increase. SEM observations reveal that in the fractured surface at the threshold region the roughness is greatly increased and that microvoids and microcracks become predominant. The number and the size of these microdefects increase along with the increase in the value of prestrain, see Fig. 6. The roughening of the crack wake increases the crack opening force. The compressive residual stress is concentrated at the microvoids and microcracks owing to the unloading of the tensile stress in the prestrain treatment. Therefore, the effective stress intensity factor ahead of the crack tip decreases and the magnitude of the fatigue threshold increases.
Int J Fatigue January 1992
Fig. 5 Dislocation configuration of a monotonic prestrain of 7.9% in 40 Cr (group II) steel, × 73000
43
Fig. 6 Microvoids and microcracks of a monotonic prestrain of 52.2% in 40 Cr (group II) steel, x 2000
In addition, the dispersed microvoids and microcracks would reduce the load stress concentration at the crack tip, the effective loading stress intensity factor decreases when the crack grows, and the microvoids and microcracks would make for a zigzag crack path. The deflection of the crack results in a reduction of mode-I stress intensity factor and the crack path would be elongated owing to its non-linear growth. The experimental results of Ref. 5 for stringer inclusions explain the effects of load scattering and crack deflection. Few microvoids and microcracks appeared in the smallprestrain region, where the fatigue threshold decreases; this is associated with the combined effect of two opposing factors, the dislocation configuration and the microvoids. Only if the effect of the microvoids becomes predominant does AKthstart to increase again. The original dislocation configurations of normalized and high-temperature tempered specimens are different. The lower normalized dislocation density one tolerates the larger the range of dislocation generation; thus a remarkable fall in AK,h is exhibited. With cyclic prestrain the wall structure is easily established. A number of cyclic loadings in prestrain induces fatigue damage and produces a large number of microvoids and microcracks, which prevail over the role of the dislocation density; thus AKth increases monotonically, as in Fig. 7. However, even AKthis high in these microvoid materials; its crack growth rate is extremely rapid. If the loading stress intensity factor is a little higher than the AKth value, the microvoids and microcracks are easily linked together and the crack increases suddenly.
Fig. 7 Microvoids and microcracks of a cyclic prestrain of 2% in 40 Cr (group II) steel, x 5000
2) 3)
fatigue threshold first decreases, then increases with the increase of prestrain. Cyclic prestrain makes the fatigue threshold increase monotonically with the increase of cyclic prestrain. Two opposing factors, dislocation density and microvoids and microcracks, affect the relation between the prestrain and the fatigue threshold. The increase in the dislocation density lowers the AKth value. The large number of microvoids and microcracks produced in monotonic prestrain and large-magnitude cyclic prestrain play a predominant role; the fatigue threshold increases.
References 1.
Blacktop, J., Nicholson, C.E., Brook, R. and Towers, R.T. Proc of Int Conf on Fatigue Threshold, Stockholm, 1981 pp 629-638
2.
Tel Shankei end Liu, H.W. Eng Fract Mech 6 4 (1974) pp 631-638
3.
Schijve, J. Eng Fract Mech 8 4 (1976) pp 575-581
4.
Schulte, K. and Nowack, H. Eng Fract Mech 13 4 (1980) pp 1009-1021
5.
Arora, P.R., Raghavan, M.R. and Praeed, Y.V.R.K. Eng Fract Mech 29 1 (1988) pp 67-69
Authors
Conclusions 1)
44
Monotonic tensile prestrain results in a change of the fatigue threshold in two medium-carbon steels. The
The authors are with the Research Institute for the Strength of Metals, Xi'an Jiaotong University, Xi'an, China. Received 15 May 1991; accepted in revised form 3 October 1991.
Int J F a t i g u e J a n u a r y
1992