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Study of the variation of fracture toughness with loading rate using compact tension specimens
0013-7944/x7 53.00 + 00 Pergamon Journal\ Ltd.
~npnrerin~ f,o
TECHNICAL
STUDY TOUGHNESS
Metallurgical
NOTE
OF THE VARIATION OF FRACTURE WITH LOADING RATE USING COMPACT TENSION SPECIMENS
M. N. BASSIM, M. R. BAYOUMI and D. SHUM Sciences Laboratories, University of Manitoba, Winnipeg, R3T 2N2
Manitoba,
Canada
RECENTLY
Cl], an attempt to determine the fracture toughness of ductile materials at high rates of loading was undertaken. A modified Split Hopkinson pressure bar was developed which fractures compact tension specimens using tensile stress waves. The objective of this present study is to determine the variation of fracture toughness, expressed by the integral J, with loading rates ranging from slow, quasi-static, to dynamic loading rates. The dynamic test assembly proposed earlier was used. Because of the complications in interpreting the dynamic data using conventional approaches to calculation oi the J-integral at the onset of crack growth, a different approach which has recently met significant success was adopted, namely, to calculate the J-integral from measurement of the stretch zone width on the fractured specimen using scanning electron microscopy. In the present study, the fracture behavior of an annealed AISI 1045 steel, subject to tensile loading rates ranging from quasi-static to dynamic was investigated using compact tension specimens having three different a/H ratios. Experimental results based on stretch zone measurements indicate a significant decrease in fracture toughness for AISI 1045 steel at high loading rates as well as a slight dependence of the stretch zone (and hence J) with a/w. While for brittle materials, it is generally considered that the critical stress intensity factor K,< decreases with increasing the rate of loading, expressed as k,, several investigations [2-71 have noted that at sufficiently high loading rates K,, is seen to increase with loading rate. This is attributed to the fact that, as the rate of crack loading is increased, the deformation may become adiabatic with respect to the plastic zone and the subsequent gross relaxation at the crack tip would increase K,<. Other studies, by Costin et al [S] and more recently by Bayoumi and Bassim [9, lo] have reported significant decrease in fracture toughness with increasing loading rates and have related this observation to a change in fracture mode where, at low rates, void nucleation and coalescence prevail while at high rates, cleavage crack growth is dominant. In terms of testing at high loading rates, the wedge loaded compact tension (WLCT) specimen of Klepaczko [I l] is the most recent goemetry used in conjunction with the Split Hopkinson Pressure Bar. This WLCT fracture testing approach however, introduces compressive and frictional forces and therefore these WLCT systems are not strictly tensile test systems. In addition, the experimental results from these tests are extremely sensitive to a slight misalignment of the wedge striker. In this present investigation, a new modified Split Hopkinson bar system capable of handling compact tension specimens was used to determine the fracture toughness of AISI 1045 steel at high loading rates. This system was described in detail in ref. [l] and is shown in Fig. 1. The 1045 steel was supplied as a rectangular bar 12.7 mm thick and 76.2 mm wide. The bar was annealed at Xoo”C for 0.5 hour to obtain good ductility in the material before testing. Standard ASTM CTS specimens of dimensions 63 mm x 60 mm x 12.7 mm were prepared, with the notch in the L-T orientation, then fatigue precracked to obtain initial crack length of a/w = 0.5, 0.6 and 0.7. The test programs comprised: (1) A standard ASTM J-resistance curve at quasi-static speed on a specimen of a/w = 0.6 using a servo-hydraulic testing machine (Instron model 1320) to establish a baseline quasi-static fracture toughness value of J,,. (2) With the servo-hydraulic test frame, J,, was determined at quasi-static, slow and high loading speeds where the high speed represents the maximum loading speed obtainable from the servo-hydraulic machine, these are known as the Instrom speed tests. (3) With the dynamic tensile test assembly, tests were performed at two striker velocities, 12 m/s and 20 m/s respectively. The stretch zone width measurements was adopted to determine the fracture toughness J,sL after quasi-static and impact loading. The stretch zone being the transition between the end of the fatigue precrack and the stable crack growth. This zone is characterized by an extensive plastic deformation before crack initiation. Under plane-strain conditions, the critical value of the J-integral J,, is related to the stretch zone width by: J,,
= [4 o,d/(cos
B + sin p)] (l/G)
and d is the measured length of the stretch zone on the scanning electron microscope micrographs, of the beam (tilt angle), and G is the magnification. To calculate o,, the variation of the flow stress 9 is assumed to be similar to the variations strain rates [I21 namely a/ = a/$ @/&)?,s
(I) p is the incident
angle
of yield strength
with
(2)
where a,$ is the quasi-static flow stress taken as 450 MPa for AISI annealed 1045 steel at room temperature, T is the temperature. S is the structure, & is the quasi-static strain rate taken as 1O~V’s which corresponds approximately to a k, = 1 MPa m”% -‘, [12] is the strain rate at which a, is going to be evaluated, for example when h is the dynamic strain 619
620
Technical Note
Ntcolet transient recorder + disk drive
Fig I, Experimentaf setup drawing of dynamic tensile test using the split Hopkinson bar [If.
?n =
131
Thus, from eq. (2) the dynamic fiaw stress was found to be equal to 577 MPa. Halves of broken compact tension specimens were examined by a scanning electron microscope (JELL JSM - 840). All stretch zone measurements were conducted using stereoscopic pictures at fl= 37” and 45” at 180 magnification. The stretch zone width d was measured at different points along the crack front, and the results were averaged. The value af S,, under quasi-static and dynamic testing was calculated using eq. (I). A typical fracture surface of the slower fnstron speed tesfs is presented in Fig. 2, while typical mierographs for the impact loading tests are presented in Fig. 3, where Fig. 3a corresponds to u/w = 0.5 and Fig. 3b corresponds to an U/M;= 0.5. Examination of the fracture surfaces En Fig. 3 &arty shows that, for the case of dynamic luading, the a/iv = 0.5 specimen was fractured in a ductile manner white the a/w = 0.5 specimen was fractured in a brittle manner with unstable crack growth. The results of the stretch zone me~~su~ments (SZW = d) are graphically presented in Fig. 4, The fracture toughness parameter JS calculated from eq.
Technical Note
Fig. 2. Typical scanning electron micrograph for instron speed tests, x 27.5
621
622
Technical
Note
Fig. 3. Typical scanning electron micrographs for dynamic speed test: (a) exhibiting ductile fracture (a/w = 0.5), x 290 and (b) exhibiting brittle fracture surface (a/w = 0.6). x 290.
surface
Technical Note
623
_ 0
-
o/w =0.6 ?
0 a/w=O.6 x a/w=O.7 \-,
0
Impact loading
Instron loading
I
Fig. 5. Relationship between fracture toughness JszW and the strain rate 2.
(l), using the measured stretch zone width and the respective static and dynamic flow stress through eq. (2), is shown in Fig. 5 as a function of strain rate at different a/w ratios. It would appear that increasing the loading rate from Instrom speeds to impact loading using the Split Hopkinson Bar resulted in a transition of the fracture mode, namely from ductile fracture at low values of loading rates to brittle fracture at high loading rate. This means the ductility which plays an important part in crack blunting process, decreases as the loading rate increases and hence at high loading rate the associated plastic zone ahead of the crack tip diminishes. Also it shows no specimen geometry dependence at quasi-static speeds while at impact speeds J,, has a slight dependence on specimen geometry, specifically a/w ratio. In conclusion, a study of the fracture behavior of an annealed AI‘S1 1045 steel subjected to wide range of tensile loading was undertaken using ASTM compact tension specimens. The experimental results based on stretch zone width measurements indicate a si~ifi~ant decrease in fracture toughness for this steel at high loading rate. Fracture toughness based on stretch zone measurements shows a lesser dependence on initial crack length at quasi-static loading rates than those tested at high loading rates.
Acknowledgements-The authors acknowledge the support of the National Sciences and Engineering Research Council of Canada. One of the authors (DS) also is grateful to the same organization for a post-graduate scholarship.
REFERENCES [l] D. Shum, M. N. Bassim and M. R. Bayoumi, Int. J. Fracture 29, R3-R10 (1985). [2] F. Nilsson. Proc. Co& mechanical Properties of ~aier~als at High Rates of Strain, Oxford, pp. 185-204 (1984). 131 .I. R. Klepaczko, J. Engng Mater. Technol. 104, 29-35 (1982). [4] J. Eftis and J. M. Krafft, J. has. Engng 87, 257-263 (1965). [S] J. C. Radon and C. E. Turner, J. fran Steel Inst. 204, 842-845 (1966). [6] J. F. Kalthoff and D. A. Shockey, 1. appl. phys. 48, 986-993 (1977). [7] D. A. Shockey, J. F. Kalthoff and D. C. Ehrlich, In?. J. Fracture 22, 217-229 (1983). [8] L. S. Costin, J. Duffy and L. B. Freund, ASTM STP 627, 301-318 (1977). [9] M. R. Bayoumi, J. R. Klepaczko and M. N. Bassim, J. Test. Eval. 12, 316-323 (1984). [lo] M. R. Bayoumi and M. N. Bassim, Pm. Int. Co@ Strength of Materials and Alloys ICSMA7. Pergamon Press, New York (1985). [11] J. F. Knott, Fundamentals of Fracture Mechanics. Butterworths, London (1973). [12] G. E. Dieter, Mechanical Metallurgy, 2nd edn, McGraw-Hill, New York (1976). (Received 13 June 1986)
~npnrerin~ f,o
TECHNICAL
STUDY TOUGHNESS
Metallurgical
NOTE
OF THE VARIATION OF FRACTURE WITH LOADING RATE USING COMPACT TENSION SPECIMENS
M. N. BASSIM, M. R. BAYOUMI and D. SHUM Sciences Laboratories, University of Manitoba, Winnipeg, R3T 2N2
Manitoba,
Canada
RECENTLY
Cl], an attempt to determine the fracture toughness of ductile materials at high rates of loading was undertaken. A modified Split Hopkinson pressure bar was developed which fractures compact tension specimens using tensile stress waves. The objective of this present study is to determine the variation of fracture toughness, expressed by the integral J, with loading rates ranging from slow, quasi-static, to dynamic loading rates. The dynamic test assembly proposed earlier was used. Because of the complications in interpreting the dynamic data using conventional approaches to calculation oi the J-integral at the onset of crack growth, a different approach which has recently met significant success was adopted, namely, to calculate the J-integral from measurement of the stretch zone width on the fractured specimen using scanning electron microscopy. In the present study, the fracture behavior of an annealed AISI 1045 steel, subject to tensile loading rates ranging from quasi-static to dynamic was investigated using compact tension specimens having three different a/H ratios. Experimental results based on stretch zone measurements indicate a significant decrease in fracture toughness for AISI 1045 steel at high loading rates as well as a slight dependence of the stretch zone (and hence J) with a/w. While for brittle materials, it is generally considered that the critical stress intensity factor K,< decreases with increasing the rate of loading, expressed as k,, several investigations [2-71 have noted that at sufficiently high loading rates K,, is seen to increase with loading rate. This is attributed to the fact that, as the rate of crack loading is increased, the deformation may become adiabatic with respect to the plastic zone and the subsequent gross relaxation at the crack tip would increase K,<. Other studies, by Costin et al [S] and more recently by Bayoumi and Bassim [9, lo] have reported significant decrease in fracture toughness with increasing loading rates and have related this observation to a change in fracture mode where, at low rates, void nucleation and coalescence prevail while at high rates, cleavage crack growth is dominant. In terms of testing at high loading rates, the wedge loaded compact tension (WLCT) specimen of Klepaczko [I l] is the most recent goemetry used in conjunction with the Split Hopkinson Pressure Bar. This WLCT fracture testing approach however, introduces compressive and frictional forces and therefore these WLCT systems are not strictly tensile test systems. In addition, the experimental results from these tests are extremely sensitive to a slight misalignment of the wedge striker. In this present investigation, a new modified Split Hopkinson bar system capable of handling compact tension specimens was used to determine the fracture toughness of AISI 1045 steel at high loading rates. This system was described in detail in ref. [l] and is shown in Fig. 1. The 1045 steel was supplied as a rectangular bar 12.7 mm thick and 76.2 mm wide. The bar was annealed at Xoo”C for 0.5 hour to obtain good ductility in the material before testing. Standard ASTM CTS specimens of dimensions 63 mm x 60 mm x 12.7 mm were prepared, with the notch in the L-T orientation, then fatigue precracked to obtain initial crack length of a/w = 0.5, 0.6 and 0.7. The test programs comprised: (1) A standard ASTM J-resistance curve at quasi-static speed on a specimen of a/w = 0.6 using a servo-hydraulic testing machine (Instron model 1320) to establish a baseline quasi-static fracture toughness value of J,,. (2) With the servo-hydraulic test frame, J,, was determined at quasi-static, slow and high loading speeds where the high speed represents the maximum loading speed obtainable from the servo-hydraulic machine, these are known as the Instrom speed tests. (3) With the dynamic tensile test assembly, tests were performed at two striker velocities, 12 m/s and 20 m/s respectively. The stretch zone width measurements was adopted to determine the fracture toughness J,sL after quasi-static and impact loading. The stretch zone being the transition between the end of the fatigue precrack and the stable crack growth. This zone is characterized by an extensive plastic deformation before crack initiation. Under plane-strain conditions, the critical value of the J-integral J,, is related to the stretch zone width by: J,,
= [4 o,d/(cos
B + sin p)] (l/G)
and d is the measured length of the stretch zone on the scanning electron microscope micrographs, of the beam (tilt angle), and G is the magnification. To calculate o,, the variation of the flow stress 9 is assumed to be similar to the variations strain rates [I21 namely a/ = a/$ @/&)?,s
(I) p is the incident
angle
of yield strength
with
(2)
where a,$ is the quasi-static flow stress taken as 450 MPa for AISI annealed 1045 steel at room temperature, T is the temperature. S is the structure, & is the quasi-static strain rate taken as 1O~V’s which corresponds approximately to a k, = 1 MPa m”% -‘, [12] is the strain rate at which a, is going to be evaluated, for example when h is the dynamic strain 619
620
Technical Note
Ntcolet transient recorder + disk drive
Fig I, Experimentaf setup drawing of dynamic tensile test using the split Hopkinson bar [If.
?n =
131
Thus, from eq. (2) the dynamic fiaw stress was found to be equal to 577 MPa. Halves of broken compact tension specimens were examined by a scanning electron microscope (JELL JSM - 840). All stretch zone measurements were conducted using stereoscopic pictures at fl= 37” and 45” at 180 magnification. The stretch zone width d was measured at different points along the crack front, and the results were averaged. The value af S,, under quasi-static and dynamic testing was calculated using eq. (I). A typical fracture surface of the slower fnstron speed tesfs is presented in Fig. 2, while typical mierographs for the impact loading tests are presented in Fig. 3, where Fig. 3a corresponds to u/w = 0.5 and Fig. 3b corresponds to an U/M;= 0.5. Examination of the fracture surfaces En Fig. 3 &arty shows that, for the case of dynamic luading, the a/iv = 0.5 specimen was fractured in a ductile manner white the a/w = 0.5 specimen was fractured in a brittle manner with unstable crack growth. The results of the stretch zone me~~su~ments (SZW = d) are graphically presented in Fig. 4, The fracture toughness parameter JS calculated from eq.
Technical Note
Fig. 2. Typical scanning electron micrograph for instron speed tests, x 27.5
621
622
Technical
Note
Fig. 3. Typical scanning electron micrographs for dynamic speed test: (a) exhibiting ductile fracture (a/w = 0.5), x 290 and (b) exhibiting brittle fracture surface (a/w = 0.6). x 290.
surface
Technical Note
623
_ 0
-
o/w =0.6 ?
0 a/w=O.6 x a/w=O.7 \-,
0
Impact loading
Instron loading
I
Fig. 5. Relationship between fracture toughness JszW and the strain rate 2.
(l), using the measured stretch zone width and the respective static and dynamic flow stress through eq. (2), is shown in Fig. 5 as a function of strain rate at different a/w ratios. It would appear that increasing the loading rate from Instrom speeds to impact loading using the Split Hopkinson Bar resulted in a transition of the fracture mode, namely from ductile fracture at low values of loading rates to brittle fracture at high loading rate. This means the ductility which plays an important part in crack blunting process, decreases as the loading rate increases and hence at high loading rate the associated plastic zone ahead of the crack tip diminishes. Also it shows no specimen geometry dependence at quasi-static speeds while at impact speeds J,, has a slight dependence on specimen geometry, specifically a/w ratio. In conclusion, a study of the fracture behavior of an annealed AI‘S1 1045 steel subjected to wide range of tensile loading was undertaken using ASTM compact tension specimens. The experimental results based on stretch zone width measurements indicate a si~ifi~ant decrease in fracture toughness for this steel at high loading rate. Fracture toughness based on stretch zone measurements shows a lesser dependence on initial crack length at quasi-static loading rates than those tested at high loading rates.
Acknowledgements-The authors acknowledge the support of the National Sciences and Engineering Research Council of Canada. One of the authors (DS) also is grateful to the same organization for a post-graduate scholarship.
REFERENCES [l] D. Shum, M. N. Bassim and M. R. Bayoumi, Int. J. Fracture 29, R3-R10 (1985). [2] F. Nilsson. Proc. Co& mechanical Properties of ~aier~als at High Rates of Strain, Oxford, pp. 185-204 (1984). 131 .I. R. Klepaczko, J. Engng Mater. Technol. 104, 29-35 (1982). [4] J. Eftis and J. M. Krafft, J. has. Engng 87, 257-263 (1965). [S] J. C. Radon and C. E. Turner, J. fran Steel Inst. 204, 842-845 (1966). [6] J. F. Kalthoff and D. A. Shockey, 1. appl. phys. 48, 986-993 (1977). [7] D. A. Shockey, J. F. Kalthoff and D. C. Ehrlich, In?. J. Fracture 22, 217-229 (1983). [8] L. S. Costin, J. Duffy and L. B. Freund, ASTM STP 627, 301-318 (1977). [9] M. R. Bayoumi, J. R. Klepaczko and M. N. Bassim, J. Test. Eval. 12, 316-323 (1984). [lo] M. R. Bayoumi and M. N. Bassim, Pm. Int. Co@ Strength of Materials and Alloys ICSMA7. Pergamon Press, New York (1985). [11] J. F. Knott, Fundamentals of Fracture Mechanics. Butterworths, London (1973). [12] G. E. Dieter, Mechanical Metallurgy, 2nd edn, McGraw-Hill, New York (1976). (Received 13 June 1986)