- Email: [email protected]
A technique for studying crack growth under repeated rolling contact
Wear, 9.5 (1984)
29 - 34
29
A TECHNIQUE FOR STUDYING CRACK GROWTH REPEATED ROLLING CONTACT H. YOSHIMURA,
UNDER
C. A. RUBIN and G. T. HAHN
Department of Mechanical TN 37235 (U.S.A.) (Received November 8,1983;
and Materials Engineering,
Vanderbilt
~nive~it~,
N~hville,
accepted February 13,1984)
summary A general technique for installing crack-like defects (with an initial length as small as about 0.3 mm) is described. Results of preliminary measurements of the cyclic crack growth of these defects are presented.
1. Introduction There is evidence that both wear and fracture under rolling contact can involve the growth of cracks below the rim under the action of the cyclic contact stresses [ 1 - 51. These are either pre-existing crack-like defects or cracks nucleated by the cyclic plastic deformation of the rim [4, 61. The lives of rolling components are foreshortened when such cracks grow and become large enough to cause the fr~entation of the rim, i.e. by spalling [ 1, 31, shelling 22, 51 etc. Fracture mechanics offers a methodology for characterizing the crack driving force which can be coupled with measurements of the crack growth rate to provide estimates of the lives of components subjected to rolling contact. This approach has been applied by Fleming and Suh [7] and Rosenfield [8] to the process of wear and by Keer and Bryant [9] and Chipperfield and Blicblau [lo] to rolling contact fatigue. However, relatively little progress has been made toward defining relations among the contact pressure, flaw geometry and rim life. This is partly because of the difficulty of producing flawed test pieces that can be subjected to rolling contact and are suitable for measuring the crack growth rate. A general technique for installing crack-like defects (with an initial length 2a as small as about 0.3 mm) below the rim of cylinders for rolling contact studies is described in this paper. Results of preliminary measurements of the cyclic crack growth of these defects in AISI-SAE 1018 steel, in 1100 aluminum and 7075-T6 aluminum alloy are described. 0043-1648/84/$3.00
@ Elsevier Sequoia/Printed
in The Netherlands
30
2. Experimental
procedure
The method for installing the defects is shown schematic~ly in Fig. 1. The “cracks” are produced by a four-step procedure including (1) drilling a small hole through the test cylinder with the hole axis parallel to the cylinder axis and the contact surface, (2) collapsing the hole by indenting the rim surface above the hole, (3) machining the surface of the test cylinder to remove the indent and (4) heat treating the cylinder to remove the residual cold work produced by the indenting step. ~icroh~dness measurements obtained in the region between the defect and the rim after heat treating a 7075-T6 aluminum alloy disk are essentially the same as those in the undeformed bulk. The size 2a of the defect is related to the drill diameter, e.g. 2u = D. The minimum size is limited by the buckling and vibration of the drill; the rn~~urn depth h is limited by the constraints that affect hole collapse, e.g. h < 6D. So far, defects as small as 2a = 0.3 mm have been produced using an ordinary shop drill press.
(a)
(bf
CC)
Fig. 1. Procedure for inserting a crack-like defect below the rim of a rolling contact specimen: (a) drill a small hole through the disk; (b) indent the rim, collapsing the hole; (c) remove the indent by machining and heat treating the specimen.
Prelimin~ crack growth experiments have been performed using cylindrical rolling contact specimens 6 mm wide and about 50 mm in diameter of AISI-SAE 1018 steel, 1100 grade aluminum and the 7075-T6 aluminum alloy. Each cylinder was fitted with four cracks with 2a = 0.6 mm by collapsing the holes produced by a drill bit of 0.5715 mm diameter about 1.3 2.0 mm from the original cylinder surface (57 mm diameter). The depths of the cracks below the rim after removing the indent (and before testing) were 0.5 mm < h < 1.8 mm. Final heat treatments and hardness values and estimates of the yield strengths of the materials are described in Table 1. Rolling contacts were produced with a machine that permits the test cylinder, keyed to a shaft supported by bearings mounted on a sliding frame, to roll against a hardened AISI-SAE 4150 steel “reaction” cylinder 101.6 mm in diameter on a parallel shaft supported by fixed bearings (Fig. 2). Contact loads of from about 7080 to about 10 600 N were applied to the sliding frame by a lever arrangement. The test cylinder was driven at a speed of about 300 rev min- ‘; the reaction cylinder was driven separately to produce
31 TABLE 1 Description of the test materials Material
Heat treatment
1018 steel
condition
Hardness
Tensile yield strengtha WPa)
Annealed; 880 “C for 1 h; furnace cooled
55 HRB
212
1100 Al
Annealed; 425 “C for 1 h; air cooled
36 HRH
707 5-T6 Al alloy
Aged (T6); 465 “C for 1 h; quenched in water; 120 “C for 24 h; quenched in water
90 HRB
69.7 572
-
Fig. 2. Schematic representation of the rolling contact testing machine showing the test cylinder (A) supported by bearings (B) mounted on a sliding frame (C) which rolls against a steel reaction cylinder (D) supported by fixed bearings (E). Contact loads are applied to the sliding frame by weights (P) and a lever arrangement (F). The sliding frame is guided by two supports, one in the front (not shown) and one in the rear (G).
matching surface speeds and essentially frictionless rolling without sliding. During the rolling operation, the contact surfaces were lubricated with a lithium-based molybdenum disulfide high-pressure-resistant lubricant. The test cylinders were subjected to between about lo4 and 3 X 10’ contacts at relative peak pressure ratios in the range 3.5 < p,,/k < 6 where the shear yield strength k = ~7s/3~” and u. is the yield strength given in Table 1. 3. Results and discussion The crack profile produced by this technique on the test cylinder midsection prior to rolling contact is illustrated in Fig. 3(a). With isolated excep-
(a)
(b)
Fig. 3. Crack profile on a 7075-T6 aluminum alloy test cylinder midsection: rolling (N = 0); (b) after rolling with N = 2 x 10” contacts at pa/k = 3.5.
Fig. 4. Crack profile on the surface of a 7075-T6 2 x lo4 rolling contacts at pa/k = 3.5.
(a) before
aluminum alloy test cylinder after N =
tions, the as-installed defects produced at relative depths h/D G 6 have sharp tips and are closed at their midpoints at a magnification of 40X, as shown in the example in Fig. 3(a). The as-installed defects do display some of the original curvature of the hole (Fig. 3(a)). In view of this, the mode II nature of the loading and the importance of the crack face friction f 81, the extent to which the installed defect is more or less severe (for a given size) than either an idealized planar crack or any one of the wide range of possible real defects is uncertain. More work is needed to characterize the relative severity of the crack-like defect produced by the hole collapse method. Cracks in the 7075-T6 cylinder grew under rolling contact in the center of the test cylinder (Fig. 3(b)) where the crack driving force is predominantly mode II (in-plane shear) in character. The average growth rate da/&N (assuming that growth initiated with the first contact) is 0.016 pm cycle-’ in this case. The cracks grew more rapidly near the cylinder surface (Fig. 4) where the crack driving force also displays a mode III (out-of-plane shear) component. Cracks in the interior of the 1100 grade aluminum and AISI-
33 TABLE 2 Summary of rolling contact experiments Maferial
Annealed AISI-SAE 1018 steel Annealed 1100 Al 7075-T6 Al alloy 7075-T6 Al alloy
Number N of contacts
Polk
lo5 + 10s 3.5x105 3.2 x 103 2 x 104
5 6 6 4 3.5
Crack growtha Aa (mm) Cylinder surface
Cylinder midsection
0
0
0.119 1.48 2.29
0 1.49 0.32
aGrowth displayed by the fastest growing of the four defects installed in the rim.
SAE 1018 steel test cylinders failed to grow even after larger numbers of contacts and higher pressures (Table 2). The cracks in the 1100 aluminum cylinder did grow near the cylinder surfaces. The results show that the crack-like defects produced by the hole collapse technique are subject to a mode II type of cyclic crack growth under repeated rolling contacts. The propensity for crack growth varies in different materials and is assisted by the mode III component of loading existing at the free surface. Systematic measurements of the crack growth rate in 7075T6 aluminum and AISI-SAE 4140 steel are under way.
Acknowledgments This paper is based on work supported by the National Science Foundation under Grant DMR-8108500. The authors wish to thank Professor W. Wright and Mr. W. Gentry for their contributions to the design and production of the rolling machine, Mr. J. Hightower for technical assistance and Ms. C. Wieger for her work on the manuscript.
References 1 W. D. Syniuta and C. J. Corrow, A scanning electron microscope fractographic study of rolling-contact fatigue, Wear, 15 (1970) 187 - 199. 2 G. C. Martin and W. W. Hay, The influence of wheel-rail contact forces on the formation of rail shells, ASME Paper 7%WA/RT-8, 1972 (American Society of Mechanical Engineers, New York). K. Sugino, K. Miyamoto and M. Nagumo, Failure process of bearing steel in rolling contact fatigue, Trans. Iron Steel Inst. Jpn., 11 (1971) 9 - 17. N. P. Suh, An overview of the delamination theory of wear, Wear, 44 (1977) 1 - 16. T. Kunikake, S. Nishimura and H. Tagashira, The metallographic observation of the tread of wheels subjected to various service conditions, Trans. Iron Steel Inst. Jpn., 10 (1970) 476 - 489. D. M. Fegredo and C. Pritchard, A metallographic examination of roflers subjected to wear under rolling-sliding conditions, Wear, 49 (1978) 67 - 78.
34 7 J. R. Fleming and N. P. Suh, Mechanics of crack propagation in delamination wear, Wear, 44 (1977) 39 - 56. 8 A. R. Rosenfield, A fracture mechanics approach to wear, Wear, 61 (1980) 125 - 132. 9 L. M. Keer and M. D. Bryant, A pitting model for rolling contact fatigue, ASME Paper 82-Lub-I 0, 1982 (American Society of Mechanical Engineers, New York). 10 C. G. Chipperfield and A. S. Blicblau, Modelling rolling contact fatigue in rails, personal communication.
29 - 34
29
A TECHNIQUE FOR STUDYING CRACK GROWTH REPEATED ROLLING CONTACT H. YOSHIMURA,
UNDER
C. A. RUBIN and G. T. HAHN
Department of Mechanical TN 37235 (U.S.A.) (Received November 8,1983;
and Materials Engineering,
Vanderbilt
~nive~it~,
N~hville,
accepted February 13,1984)
summary A general technique for installing crack-like defects (with an initial length as small as about 0.3 mm) is described. Results of preliminary measurements of the cyclic crack growth of these defects are presented.
1. Introduction There is evidence that both wear and fracture under rolling contact can involve the growth of cracks below the rim under the action of the cyclic contact stresses [ 1 - 51. These are either pre-existing crack-like defects or cracks nucleated by the cyclic plastic deformation of the rim [4, 61. The lives of rolling components are foreshortened when such cracks grow and become large enough to cause the fr~entation of the rim, i.e. by spalling [ 1, 31, shelling 22, 51 etc. Fracture mechanics offers a methodology for characterizing the crack driving force which can be coupled with measurements of the crack growth rate to provide estimates of the lives of components subjected to rolling contact. This approach has been applied by Fleming and Suh [7] and Rosenfield [8] to the process of wear and by Keer and Bryant [9] and Chipperfield and Blicblau [lo] to rolling contact fatigue. However, relatively little progress has been made toward defining relations among the contact pressure, flaw geometry and rim life. This is partly because of the difficulty of producing flawed test pieces that can be subjected to rolling contact and are suitable for measuring the crack growth rate. A general technique for installing crack-like defects (with an initial length 2a as small as about 0.3 mm) below the rim of cylinders for rolling contact studies is described in this paper. Results of preliminary measurements of the cyclic crack growth of these defects in AISI-SAE 1018 steel, in 1100 aluminum and 7075-T6 aluminum alloy are described. 0043-1648/84/$3.00
@ Elsevier Sequoia/Printed
in The Netherlands
30
2. Experimental
procedure
The method for installing the defects is shown schematic~ly in Fig. 1. The “cracks” are produced by a four-step procedure including (1) drilling a small hole through the test cylinder with the hole axis parallel to the cylinder axis and the contact surface, (2) collapsing the hole by indenting the rim surface above the hole, (3) machining the surface of the test cylinder to remove the indent and (4) heat treating the cylinder to remove the residual cold work produced by the indenting step. ~icroh~dness measurements obtained in the region between the defect and the rim after heat treating a 7075-T6 aluminum alloy disk are essentially the same as those in the undeformed bulk. The size 2a of the defect is related to the drill diameter, e.g. 2u = D. The minimum size is limited by the buckling and vibration of the drill; the rn~~urn depth h is limited by the constraints that affect hole collapse, e.g. h < 6D. So far, defects as small as 2a = 0.3 mm have been produced using an ordinary shop drill press.
(a)
(bf
CC)
Fig. 1. Procedure for inserting a crack-like defect below the rim of a rolling contact specimen: (a) drill a small hole through the disk; (b) indent the rim, collapsing the hole; (c) remove the indent by machining and heat treating the specimen.
Prelimin~ crack growth experiments have been performed using cylindrical rolling contact specimens 6 mm wide and about 50 mm in diameter of AISI-SAE 1018 steel, 1100 grade aluminum and the 7075-T6 aluminum alloy. Each cylinder was fitted with four cracks with 2a = 0.6 mm by collapsing the holes produced by a drill bit of 0.5715 mm diameter about 1.3 2.0 mm from the original cylinder surface (57 mm diameter). The depths of the cracks below the rim after removing the indent (and before testing) were 0.5 mm < h < 1.8 mm. Final heat treatments and hardness values and estimates of the yield strengths of the materials are described in Table 1. Rolling contacts were produced with a machine that permits the test cylinder, keyed to a shaft supported by bearings mounted on a sliding frame, to roll against a hardened AISI-SAE 4150 steel “reaction” cylinder 101.6 mm in diameter on a parallel shaft supported by fixed bearings (Fig. 2). Contact loads of from about 7080 to about 10 600 N were applied to the sliding frame by a lever arrangement. The test cylinder was driven at a speed of about 300 rev min- ‘; the reaction cylinder was driven separately to produce
31 TABLE 1 Description of the test materials Material
Heat treatment
1018 steel
condition
Hardness
Tensile yield strengtha WPa)
Annealed; 880 “C for 1 h; furnace cooled
55 HRB
212
1100 Al
Annealed; 425 “C for 1 h; air cooled
36 HRH
707 5-T6 Al alloy
Aged (T6); 465 “C for 1 h; quenched in water; 120 “C for 24 h; quenched in water
90 HRB
69.7 572
-
Fig. 2. Schematic representation of the rolling contact testing machine showing the test cylinder (A) supported by bearings (B) mounted on a sliding frame (C) which rolls against a steel reaction cylinder (D) supported by fixed bearings (E). Contact loads are applied to the sliding frame by weights (P) and a lever arrangement (F). The sliding frame is guided by two supports, one in the front (not shown) and one in the rear (G).
matching surface speeds and essentially frictionless rolling without sliding. During the rolling operation, the contact surfaces were lubricated with a lithium-based molybdenum disulfide high-pressure-resistant lubricant. The test cylinders were subjected to between about lo4 and 3 X 10’ contacts at relative peak pressure ratios in the range 3.5 < p,,/k < 6 where the shear yield strength k = ~7s/3~” and u. is the yield strength given in Table 1. 3. Results and discussion The crack profile produced by this technique on the test cylinder midsection prior to rolling contact is illustrated in Fig. 3(a). With isolated excep-
(a)
(b)
Fig. 3. Crack profile on a 7075-T6 aluminum alloy test cylinder midsection: rolling (N = 0); (b) after rolling with N = 2 x 10” contacts at pa/k = 3.5.
Fig. 4. Crack profile on the surface of a 7075-T6 2 x lo4 rolling contacts at pa/k = 3.5.
(a) before
aluminum alloy test cylinder after N =
tions, the as-installed defects produced at relative depths h/D G 6 have sharp tips and are closed at their midpoints at a magnification of 40X, as shown in the example in Fig. 3(a). The as-installed defects do display some of the original curvature of the hole (Fig. 3(a)). In view of this, the mode II nature of the loading and the importance of the crack face friction f 81, the extent to which the installed defect is more or less severe (for a given size) than either an idealized planar crack or any one of the wide range of possible real defects is uncertain. More work is needed to characterize the relative severity of the crack-like defect produced by the hole collapse method. Cracks in the 7075-T6 cylinder grew under rolling contact in the center of the test cylinder (Fig. 3(b)) where the crack driving force is predominantly mode II (in-plane shear) in character. The average growth rate da/&N (assuming that growth initiated with the first contact) is 0.016 pm cycle-’ in this case. The cracks grew more rapidly near the cylinder surface (Fig. 4) where the crack driving force also displays a mode III (out-of-plane shear) component. Cracks in the interior of the 1100 grade aluminum and AISI-
33 TABLE 2 Summary of rolling contact experiments Maferial
Annealed AISI-SAE 1018 steel Annealed 1100 Al 7075-T6 Al alloy 7075-T6 Al alloy
Number N of contacts
Polk
lo5 + 10s 3.5x105 3.2 x 103 2 x 104
5 6 6 4 3.5
Crack growtha Aa (mm) Cylinder surface
Cylinder midsection
0
0
0.119 1.48 2.29
0 1.49 0.32
aGrowth displayed by the fastest growing of the four defects installed in the rim.
SAE 1018 steel test cylinders failed to grow even after larger numbers of contacts and higher pressures (Table 2). The cracks in the 1100 aluminum cylinder did grow near the cylinder surfaces. The results show that the crack-like defects produced by the hole collapse technique are subject to a mode II type of cyclic crack growth under repeated rolling contacts. The propensity for crack growth varies in different materials and is assisted by the mode III component of loading existing at the free surface. Systematic measurements of the crack growth rate in 7075T6 aluminum and AISI-SAE 4140 steel are under way.
Acknowledgments This paper is based on work supported by the National Science Foundation under Grant DMR-8108500. The authors wish to thank Professor W. Wright and Mr. W. Gentry for their contributions to the design and production of the rolling machine, Mr. J. Hightower for technical assistance and Ms. C. Wieger for her work on the manuscript.
References 1 W. D. Syniuta and C. J. Corrow, A scanning electron microscope fractographic study of rolling-contact fatigue, Wear, 15 (1970) 187 - 199. 2 G. C. Martin and W. W. Hay, The influence of wheel-rail contact forces on the formation of rail shells, ASME Paper 7%WA/RT-8, 1972 (American Society of Mechanical Engineers, New York). K. Sugino, K. Miyamoto and M. Nagumo, Failure process of bearing steel in rolling contact fatigue, Trans. Iron Steel Inst. Jpn., 11 (1971) 9 - 17. N. P. Suh, An overview of the delamination theory of wear, Wear, 44 (1977) 1 - 16. T. Kunikake, S. Nishimura and H. Tagashira, The metallographic observation of the tread of wheels subjected to various service conditions, Trans. Iron Steel Inst. Jpn., 10 (1970) 476 - 489. D. M. Fegredo and C. Pritchard, A metallographic examination of roflers subjected to wear under rolling-sliding conditions, Wear, 49 (1978) 67 - 78.
34 7 J. R. Fleming and N. P. Suh, Mechanics of crack propagation in delamination wear, Wear, 44 (1977) 39 - 56. 8 A. R. Rosenfield, A fracture mechanics approach to wear, Wear, 61 (1980) 125 - 132. 9 L. M. Keer and M. D. Bryant, A pitting model for rolling contact fatigue, ASME Paper 82-Lub-I 0, 1982 (American Society of Mechanical Engineers, New York). 10 C. G. Chipperfield and A. S. Blicblau, Modelling rolling contact fatigue in rails, personal communication.