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Study of the formation of adiabatic shear bands in steels
Journal of Materials Processing Technology 119 (2001) 234±236
Study of the formation of adiabatic shear bands in steels M. Nabil Bassim* Faculty of Engineering, Department of Mechanical and Industrial Engineering, The University of Manitoba, Winnipeg, Manitoba, Canada R3T 5V6
Abstract The formation and evolution of adiabatic shear bands at high strain rate deformation of AISI 4340 steel was investigated using a torsional split Hopkinson bar system. The process was also modeled by ®nite element methods. Factors affecting the occurrence of the adiabatic shear bands such as specimen geometry were also examined. Depending on these factors, it was found that the adiabatic shear bands occur more likely when there is differences in properties and microstructure than in the rest of the sample and that a speci®c specimen geometry and dimensions favor the occurrence of high strain rates and hence the formation of the adiabatic shear bands. # 2001 Published by Elsevier Science B.V. Keywords: Adiabatic shear bands; Steels; Specimen geometry
1. Introduction Adiabatic shear bands (ASBs) are characteristic phenomena associated with deformation at very high strain rates and for large values of strain. In ASBs, the shear strains are localized within the band which exhibits very large amounts of strains relative to the neighboring material. ASBs are observed in many applications such as machine chips, dynamic deformation, ballistic impact and fracture [1±5]. Since ASBs are formed at high strain rates and in situations where large strains are achieved, testing for the occurrence of ASB requires the use of devices such as the torsional split Hopkinson bar or the direct impact compression split Hopkinson bar. The process of formation of HSB requires a complex mechanical and thermal event where the occurrence of workhardening due to increase in the yield strength of the material is compensated by thermal softening and signi®cant rise in temperature causing an adiabatic effect to be produced in a con®ned volume of the specimen. The combination of hardening and softening, occurring in a very short time interval, produces ASB [6±8]. Factors which affect the ASB formation include strain rate exponent, strain rate sensitivity, material hardness, microstructure and presence of imperfections such as inclusions. Also, the specimen geometry and dimensions was shown to have a signi®cant effect on the presence of ASB in deformed samples [9].
* Tel.: 1-2044748524; fax: 1-204-275-7507. E-mail address: [email protected] (M.N. Bassim).
0924-0136/01/$ ± see front matter # 2001 Published by Elsevier Science B.V. PII: S 0 9 2 4 - 0 1 3 6 ( 0 1 ) 0 0 9 5 2 - 9
Recently, a torsional split Hopkinson bar was used to study the formation of ASB in steels with different heat treatments corresponding to varied microstructures and hardness [10,11]. It was found that the ASB increase with the increase in the yield strength and hardness of the steels tested, e.g. 4140 and 4340 as well as a number of rail steels, some of them micro-alloyed with niobium and titanium [12]. The objective of this work is to further evaluate the role of some of the factors affecting the evolution of ASB such as specimen dimensions and examine the process of formation and evolution of ASBs during high strain rate deformation. 2. Experimental techniques Studies of the occurrence of ASB in steels were conducted initially using a direct impact split Hopkinson bar [12]. Strains of up to 2 were reached at strain rates of 104 s 1. Rail steels were tested in this program. The presence of ASB, with some containing micro-cracks, were observed in micro-alloyed steels as opposed to plain carbon steels. More recently, ASBs were studied using a torsional split Hopkinson bar shown in Fig. 1. The specimen shape and dimension is given in Fig. 2. It is interesting to note that, in this type of testing, the specimen dimensions are rather arbitrary. The test section consists of a thin-walled part between the ¯anges which are used for mounting the specimens onto the bars. The thin section would deform plastically while the ¯anges stay elastic. In this study, specimens of AISI 4340 steel were used which were annealed at 6808C and produced a hardness of HY 114-144.
M.N. Bassim / Journal of Materials Processing Technology 119 (2001) 234±236
235
Fig. 1. Torsional split Hopkinson bar.
To study the effect of specimen dimensions, specimens with several outside diameter (D), inside diameter (d) and length L of the their section were used. The polar moment of inertia J is given as P J (1) D4 d4 32 Once testing of the performance of the torsional split Hopkinson bar was established, the effect of microstructure was investigated by examining several microstructures, including as-received, annealed and quenched-and-tempered microstructures. 3. Theoretical considerations It is postulated that the adiabatic shear band would be formed and initialized by a local plastic deformation, causing strain hardening, at a local defect. The plastic strain
Fig. 2. Specimen geometry and dimensions.
energy is converted into thermal energy, leading to a rise of the temperature of the specimen and causing adiabatic heating. The material properties in the local band are changed, causing an ASB to be formed [13]. Thus, it is found that the ASB cannot be initialized without material or geometric defects such as inhomogenity of dimensions. This argument is applicable in steels due to inhomogeneous microstructures and hardness. Once the ASB is initialized at a local defect, the localized plastic strain increases with the elapsed time during deformation and the width of the ASB increases too. The simultaneous hardening and thermal softening proceeds till failure occurs. The stages of deformation during the formation and initialization of ASB were obtained by Feng and Bassim. Fig. 3 shows the variation of the stress resistance as a function of time. It can be seen that the evolution of ASB
Fig. 3. Change of stress resistance with time.
236
M.N. Bassim / Journal of Materials Processing Technology 119 (2001) 234±236
ASB, however, the temperature remains constant throughout the process. This would con®rm that without temperature changes, it is not possible for ASB to be formed. 4.2. Effect of specimen dimensions
Fig. 4. Temperature rise during test.
takes three stages. In the ®rst stage, there is no plastic deformation. In the second stage, strain hardening and thermal softening compete with each other. As deformation (time) increases, thermal softening outweighs strain hardening. This causes the temperature to rise (usually in the order of 2508C in steels). In the third stage, the stress resistance decreases signi®cantly and thermal softening dominates the deformation. 4. Results and discussion 4.1. Rise in temperature The rise of temperature due to formation of ASB is estimated from ®nite element modeling. Fig. 4 is a representative rise for a typical steel (in this case 4340 steel) in the middle of the ASB. Initially, the temperature is constant, and then as adiabatic heating causing thermal softening prevails, it increases signi®cantly by almost 2508C. Away from the
It was found that the occurrence of ASBs depends on a parameter JL which is de®ned as JL, where L is the length of the thin-walled section of the specimen. It was observed that specimens with JL up to 4000 mm5 show signi®cant plastic deformation in a very short period (20±40 ms) and high strain rates (up to 1100 s 1) and thus are more susceptible to formation of ASBs. Specimens which have a JL higher than 4000 mm5 develop plastic deformation after 200±300 ms and achieve strain rates of 400 s 1. This conclusion is shown in Fig. 5 for several specimens tested with values of JL ranging from 2400 to 6660. 5. Conclusions The following conclusions can be made about ASBs: (1) ASB initiate at local defects and unhomogenities in the material. (2) The process of formation and growth of ASB involves a number of steps where initially the ASB region exhibits workhardening. This is followed by a step of competition of workhardening and thermal softening. Finally, thermal softening predominates. The expected temperature rise is at least 2008C depending on the properties and microstructure of the metal. (3) Specimen geometry and dimensions are also contributing factors in the development of ASBs. Acknowledgements The support of the Natural Sciences and Engineering Research Council (NSERC) of Canada is appreciated. Input from Dr. H. Feng and from D. Mardis is also appreciated. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
Fig. 5. Effect of JL on strain rate: (1) JL 2404 mm5 ; (2) JL 3650 mm5 ; (3) JL 5344 mm5 ; (4) JL 5550 mm5 and (5) JL 6660 mm5 . Load: 60%.
[11] [12] [13]
J.C. Lemaire, W.A. Backofen, Metall. Trans. 3 (1972) 477. D.M. Turley, E.D. Doyle, S. Ramaligam, Mater. Sci. Eng. 55 (1982) 45. E. El-Magd, J. Phys. 4 (1994) C8±149. K.C. Dao, D.A. Schockey, J. Appl. Phys. 50 (1979) 8244. D.G. Brandon, L. Baum, D. Sherman, J. Phys. 4 (1994) C8±171. K. Cho, Y.C. Chi, J. Duffy, Metall. Trans. A 21 (1990) 1161. M.N. Bassim, N. Panic, in: M. Andrischky (Ed.), Advances in Materials and Processing Technologies (AMPT'97), p. 913. P. Perzyna, J. Phys. 4 (1994) C8±441. M.N. Bassim, in: I.M. Allison (Ed.), Proceedings of the 11th International Conference on Experimental Mechanics, 1998, p. 911. E. Cepus, M.Sc. Thesis, University of Manitoba, Winnipeg, Manitoba, 1995. E. Cepus, C.D. Lui, M.N. Bassim, J. Phys. 4 (1994) C8±553. R. Nakkalil, J.R. Hornaday, M.N. Bassim, Mater. Sci. Eng. A 141 (1992) 247±260. H. Feng, M.N. Bassim, Mater. Sci. Eng. (1999), in press.
Study of the formation of adiabatic shear bands in steels M. Nabil Bassim* Faculty of Engineering, Department of Mechanical and Industrial Engineering, The University of Manitoba, Winnipeg, Manitoba, Canada R3T 5V6
Abstract The formation and evolution of adiabatic shear bands at high strain rate deformation of AISI 4340 steel was investigated using a torsional split Hopkinson bar system. The process was also modeled by ®nite element methods. Factors affecting the occurrence of the adiabatic shear bands such as specimen geometry were also examined. Depending on these factors, it was found that the adiabatic shear bands occur more likely when there is differences in properties and microstructure than in the rest of the sample and that a speci®c specimen geometry and dimensions favor the occurrence of high strain rates and hence the formation of the adiabatic shear bands. # 2001 Published by Elsevier Science B.V. Keywords: Adiabatic shear bands; Steels; Specimen geometry
1. Introduction Adiabatic shear bands (ASBs) are characteristic phenomena associated with deformation at very high strain rates and for large values of strain. In ASBs, the shear strains are localized within the band which exhibits very large amounts of strains relative to the neighboring material. ASBs are observed in many applications such as machine chips, dynamic deformation, ballistic impact and fracture [1±5]. Since ASBs are formed at high strain rates and in situations where large strains are achieved, testing for the occurrence of ASB requires the use of devices such as the torsional split Hopkinson bar or the direct impact compression split Hopkinson bar. The process of formation of HSB requires a complex mechanical and thermal event where the occurrence of workhardening due to increase in the yield strength of the material is compensated by thermal softening and signi®cant rise in temperature causing an adiabatic effect to be produced in a con®ned volume of the specimen. The combination of hardening and softening, occurring in a very short time interval, produces ASB [6±8]. Factors which affect the ASB formation include strain rate exponent, strain rate sensitivity, material hardness, microstructure and presence of imperfections such as inclusions. Also, the specimen geometry and dimensions was shown to have a signi®cant effect on the presence of ASB in deformed samples [9].
* Tel.: 1-2044748524; fax: 1-204-275-7507. E-mail address: [email protected] (M.N. Bassim).
0924-0136/01/$ ± see front matter # 2001 Published by Elsevier Science B.V. PII: S 0 9 2 4 - 0 1 3 6 ( 0 1 ) 0 0 9 5 2 - 9
Recently, a torsional split Hopkinson bar was used to study the formation of ASB in steels with different heat treatments corresponding to varied microstructures and hardness [10,11]. It was found that the ASB increase with the increase in the yield strength and hardness of the steels tested, e.g. 4140 and 4340 as well as a number of rail steels, some of them micro-alloyed with niobium and titanium [12]. The objective of this work is to further evaluate the role of some of the factors affecting the evolution of ASB such as specimen dimensions and examine the process of formation and evolution of ASBs during high strain rate deformation. 2. Experimental techniques Studies of the occurrence of ASB in steels were conducted initially using a direct impact split Hopkinson bar [12]. Strains of up to 2 were reached at strain rates of 104 s 1. Rail steels were tested in this program. The presence of ASB, with some containing micro-cracks, were observed in micro-alloyed steels as opposed to plain carbon steels. More recently, ASBs were studied using a torsional split Hopkinson bar shown in Fig. 1. The specimen shape and dimension is given in Fig. 2. It is interesting to note that, in this type of testing, the specimen dimensions are rather arbitrary. The test section consists of a thin-walled part between the ¯anges which are used for mounting the specimens onto the bars. The thin section would deform plastically while the ¯anges stay elastic. In this study, specimens of AISI 4340 steel were used which were annealed at 6808C and produced a hardness of HY 114-144.
M.N. Bassim / Journal of Materials Processing Technology 119 (2001) 234±236
235
Fig. 1. Torsional split Hopkinson bar.
To study the effect of specimen dimensions, specimens with several outside diameter (D), inside diameter (d) and length L of the their section were used. The polar moment of inertia J is given as P J (1) D4 d4 32 Once testing of the performance of the torsional split Hopkinson bar was established, the effect of microstructure was investigated by examining several microstructures, including as-received, annealed and quenched-and-tempered microstructures. 3. Theoretical considerations It is postulated that the adiabatic shear band would be formed and initialized by a local plastic deformation, causing strain hardening, at a local defect. The plastic strain
Fig. 2. Specimen geometry and dimensions.
energy is converted into thermal energy, leading to a rise of the temperature of the specimen and causing adiabatic heating. The material properties in the local band are changed, causing an ASB to be formed [13]. Thus, it is found that the ASB cannot be initialized without material or geometric defects such as inhomogenity of dimensions. This argument is applicable in steels due to inhomogeneous microstructures and hardness. Once the ASB is initialized at a local defect, the localized plastic strain increases with the elapsed time during deformation and the width of the ASB increases too. The simultaneous hardening and thermal softening proceeds till failure occurs. The stages of deformation during the formation and initialization of ASB were obtained by Feng and Bassim. Fig. 3 shows the variation of the stress resistance as a function of time. It can be seen that the evolution of ASB
Fig. 3. Change of stress resistance with time.
236
M.N. Bassim / Journal of Materials Processing Technology 119 (2001) 234±236
ASB, however, the temperature remains constant throughout the process. This would con®rm that without temperature changes, it is not possible for ASB to be formed. 4.2. Effect of specimen dimensions
Fig. 4. Temperature rise during test.
takes three stages. In the ®rst stage, there is no plastic deformation. In the second stage, strain hardening and thermal softening compete with each other. As deformation (time) increases, thermal softening outweighs strain hardening. This causes the temperature to rise (usually in the order of 2508C in steels). In the third stage, the stress resistance decreases signi®cantly and thermal softening dominates the deformation. 4. Results and discussion 4.1. Rise in temperature The rise of temperature due to formation of ASB is estimated from ®nite element modeling. Fig. 4 is a representative rise for a typical steel (in this case 4340 steel) in the middle of the ASB. Initially, the temperature is constant, and then as adiabatic heating causing thermal softening prevails, it increases signi®cantly by almost 2508C. Away from the
It was found that the occurrence of ASBs depends on a parameter JL which is de®ned as JL, where L is the length of the thin-walled section of the specimen. It was observed that specimens with JL up to 4000 mm5 show signi®cant plastic deformation in a very short period (20±40 ms) and high strain rates (up to 1100 s 1) and thus are more susceptible to formation of ASBs. Specimens which have a JL higher than 4000 mm5 develop plastic deformation after 200±300 ms and achieve strain rates of 400 s 1. This conclusion is shown in Fig. 5 for several specimens tested with values of JL ranging from 2400 to 6660. 5. Conclusions The following conclusions can be made about ASBs: (1) ASB initiate at local defects and unhomogenities in the material. (2) The process of formation and growth of ASB involves a number of steps where initially the ASB region exhibits workhardening. This is followed by a step of competition of workhardening and thermal softening. Finally, thermal softening predominates. The expected temperature rise is at least 2008C depending on the properties and microstructure of the metal. (3) Specimen geometry and dimensions are also contributing factors in the development of ASBs. Acknowledgements The support of the Natural Sciences and Engineering Research Council (NSERC) of Canada is appreciated. Input from Dr. H. Feng and from D. Mardis is also appreciated. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
Fig. 5. Effect of JL on strain rate: (1) JL 2404 mm5 ; (2) JL 3650 mm5 ; (3) JL 5344 mm5 ; (4) JL 5550 mm5 and (5) JL 6660 mm5 . Load: 60%.
[11] [12] [13]
J.C. Lemaire, W.A. Backofen, Metall. Trans. 3 (1972) 477. D.M. Turley, E.D. Doyle, S. Ramaligam, Mater. Sci. Eng. 55 (1982) 45. E. El-Magd, J. Phys. 4 (1994) C8±149. K.C. Dao, D.A. Schockey, J. Appl. Phys. 50 (1979) 8244. D.G. Brandon, L. Baum, D. Sherman, J. Phys. 4 (1994) C8±171. K. Cho, Y.C. Chi, J. Duffy, Metall. Trans. A 21 (1990) 1161. M.N. Bassim, N. Panic, in: M. Andrischky (Ed.), Advances in Materials and Processing Technologies (AMPT'97), p. 913. P. Perzyna, J. Phys. 4 (1994) C8±441. M.N. Bassim, in: I.M. Allison (Ed.), Proceedings of the 11th International Conference on Experimental Mechanics, 1998, p. 911. E. Cepus, M.Sc. Thesis, University of Manitoba, Winnipeg, Manitoba, 1995. E. Cepus, C.D. Lui, M.N. Bassim, J. Phys. 4 (1994) C8±553. R. Nakkalil, J.R. Hornaday, M.N. Bassim, Mater. Sci. Eng. A 141 (1992) 247±260. H. Feng, M.N. Bassim, Mater. Sci. Eng. (1999), in press.