- Email: [email protected]
The impact toughness characteristics of steel wire-reinforced polymer composites
June 1999
Materials Letters 39 Ž1999. 324–328 www.elsevier.comrlocatermatlet
The impact toughness characteristics of steel wire-reinforced polymer composites T.S. Srivatsan ) , P.C. Lam, J. Krause
1
Department of Mechanical Engineering, The UniÕersity of Akron, Akron, OH 44325-3903, USA Received 11 March 1998; accepted 23 November 1998
Abstract In this paper, the impact toughness properties of fine steel wire-reinforced polymer matrix composites is presented and discussed. Strips of the polymer composite is attached on to a standard Charpy V-notch impact specimen. Temperature influences, in the range from cryogenic Žy1908C. to ambient Ž308C. on the impact toughness properties of the polymer composites are highlighted in light of energy absorbed and macroscopic fracture behavior of the dynamically deformed samples. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Metal–polymer composites; Steel; Impact toughness
1. Introduction Layers of high strength, high modulus steel cords, embedded in a soft, ductile and plastically deforming matrix is an attractive and viable technique for achieving significant enhancements in both stiffness and strength of the soft composite matrix, while concurrently making their mechanical response macroscopically anisotropic w1x. Of the several competing reinforcement choices, fine steel cord is often an important reinforcement for a wide spectrum of polymeric materials including rubber and its current use exceeds 1 million tons per year throughout the world w2x. For example, the growing need for reducing the weight of a radial tire culminated in a need
) 1
Corresponding author Presently with LUK, Wooster, OH, USA.
for developing steel cords having attractive combinations of strength, stiffness and damage tolerance. A comprehensive study of the ability of the polymer matrix composite to withstand stresses in the presence of a defect or flaw is the fundamental basis of fracture mechanics. Under dynamic loading conditions the resistance of the polymer–metal wire composite to fracture in the presence of a flaw is its impact toughness. In this study, the fracture toughness of an impact specimen having a 45 degree V-notch of 2 mm depth and a root radius of 0.2 mm is quantified in terms of the energy absorbed by the material upon dynamic loading, achieved from impact by a falling pendulum, and to catastrophic failure. In the presence of a well-defined notch, the crack growth toughness of the material is dependent on the conjoint and mutually interactive influences of the following: Ža. modifications in material composition to minimize segregation, and innovative and
00167-577Xr99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 9 9 . 0 0 0 2 8 - 2
T.S. SriÕatsan et al.r Materials Letters 39 (1999) 324–328
novel processing Žprimary and secondary. techniques to minimize processing-related defects; Žb. the deformation characteristics of the microstructure. For ductile solids the occurrence of localized deformation at and around the vicinity of the crack tip coupled with a microstructure which promotes crack tip blunting and bifurcation is beneficial to improving impact toughness; Žc. thick sections conforming to plane strain condition have lower critical fracture toughness than thin sections conforming to requirements of plane stress; Žd. increasing the rate of loading or strain rate, has a detrimental influence on toughness; and Že. decreasing the test temperature reduces both strength and toughness.
2. Test specimen, material and procedures The Charpy impact test uses a 50 = 10 = 10 mm3 sample with a 458 V-notch and a 0.125 mm notch root radius has proven itself to be an attractive, viable and a cost-effective technique that finds wide application for the purposes of determining the impact toughness properties of monolithic materials and even their composite counterparts. The toughness of the material being equivalent to the amount of energy it can absorb prior to catastrophic failure. Steel wire-reinforced polymer composite specimens of appropriate size w50 = 10 = 2 mm3 Žthickness.x were glued on to a standard Charpy V-notch sample. The difference in energy absorbed-to-failure between the plain CVN sample and a composite sample Žpolymer composite sheet glued on to a metal sample. was used as a measure of the relative impact resistance of the steel wire-reinforced polymer composite. The adhesive chosen was a two-part epoxy Žtype: J.B. Weld. which is widely used for purposes of gluing metal-onto-metal. The steel wire-reinforced polymer composite strips were adhered to the base metal on the side containing the notch. Problems associated with pull-out of the steel wires from the soft polymer matrix were obviated by curing the polymerrsteel wire composite specimen in an oven so as to facilitate proper bonding of the reinforcement to the soft polymer matrix. A notch was carefully machined in the composite to maintain continuity with the notch in the base material. For this
325
purpose a bench grinder was used to grind the notch at an angle of 458 with the edge of the grinding disc. The base material chosen was a low strength steel Žtype: carbon steel 1018 – cold finished.. Two different polymers Žreferred to in this manuscript as: Type A and Type B. each having steel wire reinforcement were chosen. Each polymer had both the eight fine steel wire reinforcements embedded in the matrix, and 16 steel wire reinforcements embedded in the polymer matrix. In all four polymer matrix composites were deformed under conditions of impact loading. Strips of each polymer composite, of appropriate size Ž50 = 10 = 2 mm3 ., were carefully glued, using an epoxy-based adhesive, on to the base material wcarbon steel 1018-CF Žcold finished.x ŽFig. 1.. Impact tests were done at temperatures of 308C Žroom temperature., 08C Ženvironment of ordinary ice., y708C Ženvironment of dry-ice., and
Fig. 1. The Charpy impact composite specimen: Ža. with eight steel wire reinforcements, Žb. with 16 steel wire reinforcements.
T.S. SriÕatsan et al.r Materials Letters 39 (1999) 324–328
326
Table 1 Impact test results on fine steel wire-reinforced polymer composite Material
Number of wires
Impact toughness ŽN m. Temperatures 08C
308C
1018 ŽCF. 1018 ŽCF.q Type A
Nil 8
3 4
4 5
8 15
10 14
16 8
4 4
11 8
12 15
11 14
16
4
7
14
14
y1908C
1018 ŽCF.q Type B
y708
Results are the mean values based on duplicate tests.
y1908C Žimmersion in an environment of liquid nitrogen.. The lower test temperatures simulating extreme conditions of winter and Alaskan-type of environment. At each temperature the test samples were exposed Žsoaked. for 60 min so as to achieve stability with the temperature.
3. Results and discussion The impact test data is summarized in Table 1. The results reported are the mean values based on duplicate tests. The test data is used to present variations of temperature influences on impact toughness ŽFigs. 2–4.. These figures reveal that decreasing the test temperature results in a concurrent degradation in the impact toughness of both the base material Ž1018-CF. and the fine steel wires-rein-
Fig. 2. Influence of test temperature on impact toughness of the base material: carbon steel 1018 – cold finished.
Fig. 3. Influence of test temperature on impact toughness of the composite sample wbase materialqpolymer–steel wire composite ŽType A.x.
forced polymer matrix. In general, at each test temperature the impact toughness Žquantified in terms of energy absorbed ŽN m. to fracture or failure. of the polymer matrix reinforced with eight fine steel wires ŽFig. 3. and 16 steel wires ŽFig. 4. was greater than that of the base material Ž1018-CF. ŽFig. 2., for both polymer Type A and Type B. The difference in the impact toughness of the polymer matrix reinforced with eight and 16 steel wires was at best marginal at all test temperatures. Essentially, little difference in the impact toughness value was evident between the two polymers. A comparison of the dynamic toughness of the chosen polymer composites with the base material is shown in Fig. 5. To rationalize temperature influences on behavior of the polymer-steel wire composite specimens it is
Fig. 4. Influence of test temperature on impact toughness of the composite sample wbase materialqpolymer–steel wire composite ŽType B.x.
T.S. SriÕatsan et al.r Materials Letters 39 (1999) 324–328
Fig. 5. A two-dimensional representation comparing the impact fracture toughness of the base material with the steel wire reinforced composite strips at the different temperatures.
essential to establish the underlying mechanisms controlling fracture under conditions of impact loading. At the ambient test temperature Ž308C. fracture of the base metal Ž1018-CF. was predominantly ductile with evidence of plastic deformation and microscopic features comprising a population of voids of varying size and shallow near-equiaxed shaped dimples, reminiscent of fully ductile failure ŽFig. 6.. A considerable amount of deformation, through constriction at the root of the notch was evident. However, at the lower Žcryogenic. test temperatures Žy708C and y1908C. fracture of the base material
327
revealed features reminiscent of limited ductility mechanisms, namely: numerous microscopic cracks and tear ridges with little evidence of the formation and presence of voids and dimples ŽFig. 6.. The eight steel wires-reinforced polymer composite specimen exhibited limited plastic deformation at the ambient temperature. However, at the cryogenic temperatures Žy708C and y1908C. ductility was limited at both the macroscopic and microscopic levels and aided by failure of the glue and decohesion of the steel wire-reinforced polymer composite strip from the base 1018-CF material. For the 16 steel wire composite plastic deformation was evident at ambient temperature Ž308C.. At the lower test temperatures Žy708C and y1908C. failure of the 16 steel wire reinforced matrix occurred primarily through failure of the glue and separation of the polymer composite strip from the base material. A visual comparison of the fracture morphology, at the different temperatures, of the eight wire and 16 steel wire-reinforced polymer matrix revealed a distinct change in macroscopic fracture mode from: Ža. ductile with total separation and failure of both the polymer–steel wire composite and the base material, to Žb. predominantly brittle with failure of the base material and decohesion of the polymer matrix composite strip from the base material at the cryogenic temperatures.
Fig. 6. Scanning electron micrographs of the fracture surface of the base material Ž1018-CF. showing features reminiscent of locally ductile failure.
328
T.S. SriÕatsan et al.r Materials Letters 39 (1999) 324–328
4. Conclusions The results of this study on an experimental evaluation of the impact toughness properties of steel wire-reinforced polymer matrix provides the following observations. Ža. The lower bound in ductile-to-brittle transition temperature ŽDBTT. was for the base material Žsteel 1018-CF. and the composite samples Žbase material q steel wires-reinforced polymer strip. determined to be around 258C. This implies that below this temperature failure of both the base material Žcarbon steel 1018-CF. and that of the composite samples occurs in a brittle mode. At temperatures above the transition temperature the composite samples shows macroscopic features reminiscent of locally ductile failure. Žb. An overall transition temperature could not be established for the composite sample. This required the upper boundary of the impact toughness curve to be known, that is the impact properties of both the base material and the composite samples at temperatures higher than the maximum temperature used in this study Ž308C.. Žc. The impact toughness of the polymer composites and the base material degraded with a decrease
in test temperature. For polymer Type A, the toughness of the 16 steel wire-reinforced polymer matrix was inferior to the impact toughness of the eight steel wire-reinforced polymer matrix at 08C and 308C with no difference at the lowest temperature Žy1908C.. No appreciable difference was detected for polymer Type B over the range of temperatures tested.
Acknowledgements The authors gratefully acknowledge the financial support of The University of Akron ŽAkron, OH, USA. and State of Ohio, Board of Regents ŽColumbus, OH, USA.. The polymer composite samples used in this investigation were provided by Goodyear Tire and Rubber ŽProgram Monitor: Dr. A. Prakash..
References w1x L. Jozef, Rod requirements for steel cord filaments, Wire Industry 61 Ž1994. 471–472. w2x T. Takahashi, I. Ochiai, H. Tashiro, S. Ohashi, S. Nishida T. Turai, Strengthening of steel wire for tire cord, Nippon Steel Technical Report Number 64, January 1995, pp. 45–50.
Materials Letters 39 Ž1999. 324–328 www.elsevier.comrlocatermatlet
The impact toughness characteristics of steel wire-reinforced polymer composites T.S. Srivatsan ) , P.C. Lam, J. Krause
1
Department of Mechanical Engineering, The UniÕersity of Akron, Akron, OH 44325-3903, USA Received 11 March 1998; accepted 23 November 1998
Abstract In this paper, the impact toughness properties of fine steel wire-reinforced polymer matrix composites is presented and discussed. Strips of the polymer composite is attached on to a standard Charpy V-notch impact specimen. Temperature influences, in the range from cryogenic Žy1908C. to ambient Ž308C. on the impact toughness properties of the polymer composites are highlighted in light of energy absorbed and macroscopic fracture behavior of the dynamically deformed samples. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Metal–polymer composites; Steel; Impact toughness
1. Introduction Layers of high strength, high modulus steel cords, embedded in a soft, ductile and plastically deforming matrix is an attractive and viable technique for achieving significant enhancements in both stiffness and strength of the soft composite matrix, while concurrently making their mechanical response macroscopically anisotropic w1x. Of the several competing reinforcement choices, fine steel cord is often an important reinforcement for a wide spectrum of polymeric materials including rubber and its current use exceeds 1 million tons per year throughout the world w2x. For example, the growing need for reducing the weight of a radial tire culminated in a need
) 1
Corresponding author Presently with LUK, Wooster, OH, USA.
for developing steel cords having attractive combinations of strength, stiffness and damage tolerance. A comprehensive study of the ability of the polymer matrix composite to withstand stresses in the presence of a defect or flaw is the fundamental basis of fracture mechanics. Under dynamic loading conditions the resistance of the polymer–metal wire composite to fracture in the presence of a flaw is its impact toughness. In this study, the fracture toughness of an impact specimen having a 45 degree V-notch of 2 mm depth and a root radius of 0.2 mm is quantified in terms of the energy absorbed by the material upon dynamic loading, achieved from impact by a falling pendulum, and to catastrophic failure. In the presence of a well-defined notch, the crack growth toughness of the material is dependent on the conjoint and mutually interactive influences of the following: Ža. modifications in material composition to minimize segregation, and innovative and
00167-577Xr99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 9 9 . 0 0 0 2 8 - 2
T.S. SriÕatsan et al.r Materials Letters 39 (1999) 324–328
novel processing Žprimary and secondary. techniques to minimize processing-related defects; Žb. the deformation characteristics of the microstructure. For ductile solids the occurrence of localized deformation at and around the vicinity of the crack tip coupled with a microstructure which promotes crack tip blunting and bifurcation is beneficial to improving impact toughness; Žc. thick sections conforming to plane strain condition have lower critical fracture toughness than thin sections conforming to requirements of plane stress; Žd. increasing the rate of loading or strain rate, has a detrimental influence on toughness; and Že. decreasing the test temperature reduces both strength and toughness.
2. Test specimen, material and procedures The Charpy impact test uses a 50 = 10 = 10 mm3 sample with a 458 V-notch and a 0.125 mm notch root radius has proven itself to be an attractive, viable and a cost-effective technique that finds wide application for the purposes of determining the impact toughness properties of monolithic materials and even their composite counterparts. The toughness of the material being equivalent to the amount of energy it can absorb prior to catastrophic failure. Steel wire-reinforced polymer composite specimens of appropriate size w50 = 10 = 2 mm3 Žthickness.x were glued on to a standard Charpy V-notch sample. The difference in energy absorbed-to-failure between the plain CVN sample and a composite sample Žpolymer composite sheet glued on to a metal sample. was used as a measure of the relative impact resistance of the steel wire-reinforced polymer composite. The adhesive chosen was a two-part epoxy Žtype: J.B. Weld. which is widely used for purposes of gluing metal-onto-metal. The steel wire-reinforced polymer composite strips were adhered to the base metal on the side containing the notch. Problems associated with pull-out of the steel wires from the soft polymer matrix were obviated by curing the polymerrsteel wire composite specimen in an oven so as to facilitate proper bonding of the reinforcement to the soft polymer matrix. A notch was carefully machined in the composite to maintain continuity with the notch in the base material. For this
325
purpose a bench grinder was used to grind the notch at an angle of 458 with the edge of the grinding disc. The base material chosen was a low strength steel Žtype: carbon steel 1018 – cold finished.. Two different polymers Žreferred to in this manuscript as: Type A and Type B. each having steel wire reinforcement were chosen. Each polymer had both the eight fine steel wire reinforcements embedded in the matrix, and 16 steel wire reinforcements embedded in the polymer matrix. In all four polymer matrix composites were deformed under conditions of impact loading. Strips of each polymer composite, of appropriate size Ž50 = 10 = 2 mm3 ., were carefully glued, using an epoxy-based adhesive, on to the base material wcarbon steel 1018-CF Žcold finished.x ŽFig. 1.. Impact tests were done at temperatures of 308C Žroom temperature., 08C Ženvironment of ordinary ice., y708C Ženvironment of dry-ice., and
Fig. 1. The Charpy impact composite specimen: Ža. with eight steel wire reinforcements, Žb. with 16 steel wire reinforcements.
T.S. SriÕatsan et al.r Materials Letters 39 (1999) 324–328
326
Table 1 Impact test results on fine steel wire-reinforced polymer composite Material
Number of wires
Impact toughness ŽN m. Temperatures 08C
308C
1018 ŽCF. 1018 ŽCF.q Type A
Nil 8
3 4
4 5
8 15
10 14
16 8
4 4
11 8
12 15
11 14
16
4
7
14
14
y1908C
1018 ŽCF.q Type B
y708
Results are the mean values based on duplicate tests.
y1908C Žimmersion in an environment of liquid nitrogen.. The lower test temperatures simulating extreme conditions of winter and Alaskan-type of environment. At each temperature the test samples were exposed Žsoaked. for 60 min so as to achieve stability with the temperature.
3. Results and discussion The impact test data is summarized in Table 1. The results reported are the mean values based on duplicate tests. The test data is used to present variations of temperature influences on impact toughness ŽFigs. 2–4.. These figures reveal that decreasing the test temperature results in a concurrent degradation in the impact toughness of both the base material Ž1018-CF. and the fine steel wires-rein-
Fig. 2. Influence of test temperature on impact toughness of the base material: carbon steel 1018 – cold finished.
Fig. 3. Influence of test temperature on impact toughness of the composite sample wbase materialqpolymer–steel wire composite ŽType A.x.
forced polymer matrix. In general, at each test temperature the impact toughness Žquantified in terms of energy absorbed ŽN m. to fracture or failure. of the polymer matrix reinforced with eight fine steel wires ŽFig. 3. and 16 steel wires ŽFig. 4. was greater than that of the base material Ž1018-CF. ŽFig. 2., for both polymer Type A and Type B. The difference in the impact toughness of the polymer matrix reinforced with eight and 16 steel wires was at best marginal at all test temperatures. Essentially, little difference in the impact toughness value was evident between the two polymers. A comparison of the dynamic toughness of the chosen polymer composites with the base material is shown in Fig. 5. To rationalize temperature influences on behavior of the polymer-steel wire composite specimens it is
Fig. 4. Influence of test temperature on impact toughness of the composite sample wbase materialqpolymer–steel wire composite ŽType B.x.
T.S. SriÕatsan et al.r Materials Letters 39 (1999) 324–328
Fig. 5. A two-dimensional representation comparing the impact fracture toughness of the base material with the steel wire reinforced composite strips at the different temperatures.
essential to establish the underlying mechanisms controlling fracture under conditions of impact loading. At the ambient test temperature Ž308C. fracture of the base metal Ž1018-CF. was predominantly ductile with evidence of plastic deformation and microscopic features comprising a population of voids of varying size and shallow near-equiaxed shaped dimples, reminiscent of fully ductile failure ŽFig. 6.. A considerable amount of deformation, through constriction at the root of the notch was evident. However, at the lower Žcryogenic. test temperatures Žy708C and y1908C. fracture of the base material
327
revealed features reminiscent of limited ductility mechanisms, namely: numerous microscopic cracks and tear ridges with little evidence of the formation and presence of voids and dimples ŽFig. 6.. The eight steel wires-reinforced polymer composite specimen exhibited limited plastic deformation at the ambient temperature. However, at the cryogenic temperatures Žy708C and y1908C. ductility was limited at both the macroscopic and microscopic levels and aided by failure of the glue and decohesion of the steel wire-reinforced polymer composite strip from the base 1018-CF material. For the 16 steel wire composite plastic deformation was evident at ambient temperature Ž308C.. At the lower test temperatures Žy708C and y1908C. failure of the 16 steel wire reinforced matrix occurred primarily through failure of the glue and separation of the polymer composite strip from the base material. A visual comparison of the fracture morphology, at the different temperatures, of the eight wire and 16 steel wire-reinforced polymer matrix revealed a distinct change in macroscopic fracture mode from: Ža. ductile with total separation and failure of both the polymer–steel wire composite and the base material, to Žb. predominantly brittle with failure of the base material and decohesion of the polymer matrix composite strip from the base material at the cryogenic temperatures.
Fig. 6. Scanning electron micrographs of the fracture surface of the base material Ž1018-CF. showing features reminiscent of locally ductile failure.
328
T.S. SriÕatsan et al.r Materials Letters 39 (1999) 324–328
4. Conclusions The results of this study on an experimental evaluation of the impact toughness properties of steel wire-reinforced polymer matrix provides the following observations. Ža. The lower bound in ductile-to-brittle transition temperature ŽDBTT. was for the base material Žsteel 1018-CF. and the composite samples Žbase material q steel wires-reinforced polymer strip. determined to be around 258C. This implies that below this temperature failure of both the base material Žcarbon steel 1018-CF. and that of the composite samples occurs in a brittle mode. At temperatures above the transition temperature the composite samples shows macroscopic features reminiscent of locally ductile failure. Žb. An overall transition temperature could not be established for the composite sample. This required the upper boundary of the impact toughness curve to be known, that is the impact properties of both the base material and the composite samples at temperatures higher than the maximum temperature used in this study Ž308C.. Žc. The impact toughness of the polymer composites and the base material degraded with a decrease
in test temperature. For polymer Type A, the toughness of the 16 steel wire-reinforced polymer matrix was inferior to the impact toughness of the eight steel wire-reinforced polymer matrix at 08C and 308C with no difference at the lowest temperature Žy1908C.. No appreciable difference was detected for polymer Type B over the range of temperatures tested.
Acknowledgements The authors gratefully acknowledge the financial support of The University of Akron ŽAkron, OH, USA. and State of Ohio, Board of Regents ŽColumbus, OH, USA.. The polymer composite samples used in this investigation were provided by Goodyear Tire and Rubber ŽProgram Monitor: Dr. A. Prakash..
References w1x L. Jozef, Rod requirements for steel cord filaments, Wire Industry 61 Ž1994. 471–472. w2x T. Takahashi, I. Ochiai, H. Tashiro, S. Ohashi, S. Nishida T. Turai, Strengthening of steel wire for tire cord, Nippon Steel Technical Report Number 64, January 1995, pp. 45–50.