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A repeated impact testing machine
Wear, 41(197?) 373 - 381 0 Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands
A REPEATED
IMPACT
TESTING
373
MACHINE
C. J. STUD&IAN* and 3. E. FIELD Cauendish Laboratory,
Mudingley
Road, Cambridge
(Gt. Britain)
(Received June 1, 1976)
Summary An apparatus to study repeated low velocity impact damage to the surface of a specimen is described. It consists of a spring-loaded projectile weighing up to 1.1 kg, with a tungsten carbide sphere attached to one end. The sphere strikes a fixed flat specimen at velocities up to 10 m s-l. The specimen face may be perpendicular to, or inclined at an angle to, the line of motion of the projectile. The apparatus has been used for studies of the impact damage to soil-working surfaces in agriculture, but it also has wider application as illustrated by damage studies on water-quenched carbon steel and polymethylmethac~late (PMMA).
Introduction In the past, impact testing of materials has been largely limited to the Charpy or Izod method, where a specimen is struck by a heavy pendulum and fractured in bending, the energy absorbed in this process being recorded. No attention is paid to the magnitude of the load or to the behaviour of the material in the area in which it is struck. The tests therefore have little theoretical basis and are of limited use except in quality control. More recently, impact tests have been developed to measure the dynamic fracture toughness of materials. These have theoretical implications relating to crack propagation within the material [l] but again only the behaviour of the specimen as a whole is considered. However, if a component is subject to noncatastrophic repeated impacts in a localised area, a wear process may occur; it is this type of impact damage with which this paper is concerned. There are a number of practical situations in which impact wear damage occurs, For example, Richardson 12, 31 has indicated the importance of impact resistance as a parameter in determining the choice of materials for soilworking surfaces in agriculture, and Studman [ 4 - 6] has studied impact loads occurring during tillage. These observations form the motivation for the present research. *Present address: Agricultural Engineering Department, Massey University, New Zealand.
374
In an impact, four factors determine the severity of the collision. These are the impact velocity, the masses of the colliding objects, the shape of the objects in the contact area and the mechanical properties of the materials involved. This paper describes an apparatus which is able repeatedly to impact a fixed flat surface using a projectile whose mass may be up to 1.1 kg at velocities not exceeding 10 m s- ‘. The specimen is struck by a tungsten carbide sphere. The projectile may strike the surface along the normal or at an angle. Previous work on repeated impact testing Several researchers have produced repeated impact rigs, the design varying according to the particular requirements of the research project. Although some are in effect merely conventional impact tests modified to allow repetitive impacting until failure occurs [7 - lo], others have considered repeated impact damage to surfaces [ 11 - 171. Of these, Sorokin and Matyushima [ 11,121 repeatedly dropped specimens onto abrasive paper, investigating wear resistance of a high carbon steel as a function of hardness. It was found that at low impact energies the wear resistance increased with hardness but at high impact energies (above 600 DPN) the wear resistance began to fall. When steels with different carbon contents were studied, the wear resistance was a maximum at 0.8% carbon content. These results are presented in the review by Krushchov [ 131. Wellinger and Breckel [ 141 used three rigs to study impact damage on copper, ~uminium and steel alloys and investigated the relationship between impact force and velocity. They observed oxidation, flaking and severe deformation in the soft workhardening materials used. Bayer et al. [ 15, 16 ] described an impact rig able to work at 50 impacts per second, However the projectile weighed only 1 g in order to enable it to be brought to rest before the next impact. They described the preliminary results on the wear of a mild steel projectile striking a hard surface and found that after lo6 cycles wear began abruptly; this was indicated by the “appearance of a fine granular texture” on the surface. It appears that little research has been done on harder materials impacted by heavy masses, and the apparatus described here was designed to investigate this area.
Description
of apparatus
Specimen mounting The apparatus is shown in Figs. 1 and 2. The specimen was clamped into a dodecagonal holder using positioning screws. When necessary, lead or steel strips were used for packing. The holder was positioned in the support using two parallel blocks which located against any two parallel sides of the dodecagon. The specimen could therefore be rotated in steps of 30” in a horizontal plane, and in addition the holder could be moved parallel to the blocks on a screw thread. Thus different parts of the specimen could be
375
Fig. 1. Impact apparatus: S specimen, D dodecagonal specimen holder, J spring-loaded bolts, L lever to release specimen, A swivel arm, B and 0 locking screw for swivel arm, B bullet, I indenter and holder, V linear bearing, N supporting pillars for bullet linear bearing, Q soft springs to hold bullet against rubber pad before impact, 2 rubber pad, R spring holder rod, M variable speed motor, H hammer, Sp main spring, C cam and freewheel gear, U adjustable spring-loading bar, HB buffer to prevent excessive hammer movement, W supporting pillars for hammer and motor, X shock-absorbent material, Y safety cage (opened), l? photocells for velocity measurement.
T
D
E
Fig. 2. Specimen mounting arrangement with specimen and holder removed: D dodecagonal specimen holder, E specimen positioning screws, F positioning blocks, G screw thread to move holder parallel to blocks, J spring-loaded bolts, T Tufnol, K electrical contact plate with piezoelectric crystal directly underneath.
376
tested and if required the specimen and holder could be removed and replaced during an experiment. The holder was clamped with spring-loaded bolts and was released by pulling a lever. The specimen was mounted onto a PZT-5A piezoelectric crystal used for load measurement. The crystal was embedded in Tufnol to absorb stress waves. The entire mount could be rotated about a horizontal axis by aligning the correct hole on the swivel arm. The projectile
The projectile consisted of a cylinder roughly 130 mm long with a sphere attached to the striking end. The cylinder ran in a linear bearing. Six cylinders weighing from 37 to 1150 g were available, using an insert for the lighter cylinders. The sphere was from 1 to 15 mm in diameter, thus providing a wide choice for impact mass and radius of curvature. Normally the bullet rested against a rubber pad at the foot of the linear bearing and was supported by two springs attached to a small rod projecting from the upper end of the bullet. Hammer
After some consideration of alternative methods, a spring-loaded hammer mechanism was chosen. A variable speed motor rotated the cam to load the spring. The energy stored in the spring could be adjusted by altering the length of the stroke. When the cam passed top dead centre a freewheel gear allowed the hammer to be released. After striking the bullet, the hammer was stopped by a rubber buffer. The hammer and motor were mounted on a second pair of pillars, these pillars being isolated from the bullet and specimen holder by shock-absorbent material in order to minimise vibrations. The hammer was enclosed in a wire-mesh cage as a safety precaution. Velocity
measurement
The velocity just before impact was measured with two photocells. A thin brass plate was brazed onto the small horizontal rod attached to the bullet. As the bullet travelled downwards, the plate interrupted the beams from two light-emitting diodes shining on the photocells. The output from the photocells was fed into a timer via a Schmidt trigger, and thus the time taken for the bullet to traverse a known distance could be obtained. Although downward motion of the bullet was continually retarded by the springs, the decrease in velocity could be made small by a suitable choice of springs. The velocity was measured over the last millimetre of travel before impact by adjusting the position of the photocells. Load measurement
The load was measured by recording the output from the piezoelectric crystal on a storage oscilloscope. This is a useful technique providing that the crystal is properly mounted and that stress wave effects are considered. Its general validity is discussed in ref.6. The crystal was ~~ibra~d using the method described by Crook 1181 and Hancox [ 191.
377
Fig. 3. Damage to hardened steel after 50 impacts with a 3 mm tungsten carbide sphere at 1.08 m s-l (bullet mass 1.24 kg). Flakes of material have detached from the central region.
Results and discussion Examples of the damage produced by single and repeated impacts on a water-quenched carbon steel hardened to 850 DPN and on PMMA are shown in Figs. 3 and 4. The steel was carefully ground and then polished using a 6 pm diamond paste wheel [ 61. Glass- and plasma-sprayed hardfacings have also been examined [6]. The steel is shown in Fig. 3 after 50 impacts at 1.08 m s-l with a 3 mm tungsten carbide indenter. The total bullet weight was 1.24 kg and the maximum load was approximately 5 kN. A permanent indentation was made on the first cycle and this increased slowly in size with the number of cycles. Radial cracks were also produced outside the contact area. These features have been observed in quasi-static indentations under the same maximum load [20] and can be explained with reference to the stress distributions during loading and unloading [ 211. However, inside the contact area, flakes of material were removed from the surface (the dark region near the centre of the depression in Fig. 3). This damage also increased with the number of cycles and appears to be due to workhardening of the surface layers combined with the residual stresses left after unloading.
Fig. 4. Angled impacts on PMMA using tungsten carbide indenters: (a) 10” to normal, 3 mm indenter, mass 1.24 kg, at 1.03 m s-l; (b) 10” to normal, 10 mm indenter, mass 1.27 kg, at 1.00 ms-l; (c) 20” to normal, 3 mm indenter, mass 1.24 kg, at 1.04 mm s-l; (d) 50 impacts at 10” to normal, 10 mm indenter, mass 1.27 kg at 1.03 m Cl.
Microhardness measurements showed that after 200 impacts the hardness near the centre was 1080 + 30 DPN compared with a value of 880 ? 30 DPN just outside the contact area (six measurements of each). Evidence for residual stresses causing crack propagation after unloading in hardened steels has recently been presented [ 211. Angled impacts Examples of damage produced in polymethylmethacrylate (PMMA) when the surface was inclined and impacted are shown in Fig. 4. Circumferential striations (crazes) were formed on the higher side of the area of contact (Figs. 4(a) and 4(b)). They develop preferentially in this position because the tensile Hertzian contact stresses are reinforced by friction (see ref. 22 for an analysis of a similar situation). If the angle of impact was increased, or if the specimen was repeatedly impacted, the crazes developed into cracks and material was removed from the surface (the dark regions on the left in Figs. 4(c) and 4(d)). It should be emphasized that cracking of this
379
type does not occur under slow indentation conditions and its appearance here is a consequence of the more brittle behaviour that PMMA exhibits at higher rates of loading. On the lower side of the area of contact, radial crazes can develop (Fig. 4(a)). Similar craze patterns have been reported by Puttick [23] during quasi-static indentation experiments on PMMA where the load was applied normal to the surface. Puttick explained this damage with reference to Hill’s and Nadai’s models for the expansion of a cylindrical hole in an infinite flat plate of an elastoplastic material [24, 251. This stress system applies when there has been penetration of the indenter into the solid and material is being forced outwards. The radial crazes have not developed in Fig. 4(b) because, although the mass and impact velocity were similar to those in Fig. 4(a), the indenter radius was much greater thus giving less penetration. The radial crazes form less readily in the upper region of the crater, partly because, although the mass and impact velocity were similar to those in area is reduced, resulting in less pile-up of material. During unloading the radial crazes should extend further. This is because tensile residual stresses develop in this region in specimens which have deformed plastically during loading [21, 261. With inclined impacts on steels the crack patterns were essentially the same as with PMMA (though of course the initial step of craze formation was absent). Cracking and wear were most pronounced near the upper part of the crater. The wear damage developed more quickly than for normal impacts, showing the great importance of the frictional stress component.
Conclusions An impact apparatus has been developed which is able to impact specimens repeatedly at low velocities and at controlled angles of impact. Single impacts produced radial and circumferential cracks in a hardened carbon steel, and radial and circumferential crazes in PMMA. Flakes of material were removed in steel after repeated impacts at both normal and angled incidence. It was found that workhardening of the surface layers took place. In addition, previous work has shown that, with this class of material, residual stresses after loading can contribute to tensile failure. Both these factors are thought to contribute to wear. In PMMA, material was removed by angled impacts when the circumferential crazes developed into full cracks. The apparatus is proving a useful tool for the study of impact damage to soil-working surfaces in agriculture, since it simulates impact with stones and other objects in the soil. Examples have been given which show that the approach has general interest since it can provide data on the behaviour of materials under low velocity large mass impact conditions.
380
Acknowledgments This work was completed partly at the Cavendish Laboratory, Cambridge, and partly at the National Institute of Agricultural Engineering, Silsoe, Bedfordshire. We wish to thank the S.R.C. for a grant to the laboratory. C. J. Studman thanks the A.R.C. for support during the period of research and the N.I.A.E. for permission to publish these results. We would also like to thank B.I.S.R.A. for supplying the steel specimens.
References 1 A.S.T.M. Impact testing of metals, ASTM Spec. Tech. Publ. STP466, 1970. 2 R. C. D. Richardson, Maximum hardness of strained surfaces and the abrasive wear of metals and alloys, Wear, 10 (1967) 353 - 382. 3 R. C. D. Richardson, The wear of metal shares in agricultural soil, Ph. D. Thesis, University of London, 1967. 4 C. J. Studman, The recording of impact loads during tillage, J. Agric. Eng. Res., 20 (1975) 405 ” 411. 5 C. J. Studman, Impact loads on soil working surfaces, J. Agric. Eng. Res., 20 (1975) 413 - 422. 6 C. J. Studman, Impact damage to brittle materials during the tillage of stony soils, Ph. D. Thesis, University of Cambridge, 1974. 7 R. E. Schramm, R. L. Durchotz and R. P. Reed, Apparatus for impact fatigue testing, J. Res. Nat. Bur. Stand., Sect. C, 75 (1971) 95 - 98. 8 J. D. Butler and H. Brown, Izod machine for repeated impact tests, Strain, 6 (1970)67. 9 E. L. Layland, How materials perform under repeated impact, Mater. Methods, 44 (1956) 104 - 105. 10 I. F. Panshin, Increase in resistance of steel to repeated impact, Tr. Kurgan. Mashinostroit. Inst., 12 (1969) 56 - 63. 11 G. M. Sorokin and I. I. Matyushima, Wear tests carried out by impact of an abrasive on a massive sample, Zavod. Lab., 37 (1971) 218 - 220. 12 G. M. Sorokin, Wear resistance criteria for steel under impact on abrasive, Machinoved., 3 (1973) 111 - 115. 13 M. M. Kruschov, Principles of abrasive wear, Wear, 28 (1974) 69 - 88. 14 K. Wellinger and H. Breckel, Kenngrossen und Verschleiss beim stoss Metallischer Werkstoffe, Wear, 13 (1969) 257 - 281. 15 R. G. Bayer, P. A. Engel and J. L. Sirico, Impact wear testing, Wear, 19 (1972) 343 - 354. 16 P. A. Engel, T. H. Lyons and J. L. Sirico, Impact wear model for steel specimens, Wear, 23 (1973) 185 - 201. 17 A. I. Kalimov and E. K. Pochtennyi, Apparatus for dynamic testing of materials, Ind. Lab., 35 (1969) 136 - 138. 18 A. W. Crook, A study of some impacts between metal bodies by a piezoelectric method, Proc. R. Sot. London, Ser. A, 212 (1972) 377. 19 N. L. Hancox, The deformation of solids under repeated liquid impact, Ph. D. Thesis, University of Cambridge, 1962. 20 C. J. Studman and J. E. Field, Indentation behaviour of hard metals, J. Phys. D, Q (1976) 857 - 867. 21 C. J. Studman and J. E. Field, Indentation of hard metals: the role of residual stresses, J. Mater. Sci., in the press. 22 G. M. Hamilton and L. E. Goodman, The stress field created by a circular sliding contact, J. Appl. Mech., 33 (1966) 371 - 376.
381 23 K. E. Futtick, The indentation of Ferspex, J. Fhys. E., 6 (1973) 116 - 119. 24 R. Hill, The Mathematical Theory of Plasticity, Oxford Univ. Press: Clarendon Press, London, 1950. 25 A. Nadai, Theory of Fracture and Flow of Solids, McGraw-Hill, New York, 1950. 26 K. L. Johnson, in Engineering Plasticity, Cambridge Univ. Press, London, 1968, p. 341.
A REPEATED
IMPACT
TESTING
373
MACHINE
C. J. STUD&IAN* and 3. E. FIELD Cauendish Laboratory,
Mudingley
Road, Cambridge
(Gt. Britain)
(Received June 1, 1976)
Summary An apparatus to study repeated low velocity impact damage to the surface of a specimen is described. It consists of a spring-loaded projectile weighing up to 1.1 kg, with a tungsten carbide sphere attached to one end. The sphere strikes a fixed flat specimen at velocities up to 10 m s-l. The specimen face may be perpendicular to, or inclined at an angle to, the line of motion of the projectile. The apparatus has been used for studies of the impact damage to soil-working surfaces in agriculture, but it also has wider application as illustrated by damage studies on water-quenched carbon steel and polymethylmethac~late (PMMA).
Introduction In the past, impact testing of materials has been largely limited to the Charpy or Izod method, where a specimen is struck by a heavy pendulum and fractured in bending, the energy absorbed in this process being recorded. No attention is paid to the magnitude of the load or to the behaviour of the material in the area in which it is struck. The tests therefore have little theoretical basis and are of limited use except in quality control. More recently, impact tests have been developed to measure the dynamic fracture toughness of materials. These have theoretical implications relating to crack propagation within the material [l] but again only the behaviour of the specimen as a whole is considered. However, if a component is subject to noncatastrophic repeated impacts in a localised area, a wear process may occur; it is this type of impact damage with which this paper is concerned. There are a number of practical situations in which impact wear damage occurs, For example, Richardson 12, 31 has indicated the importance of impact resistance as a parameter in determining the choice of materials for soilworking surfaces in agriculture, and Studman [ 4 - 6] has studied impact loads occurring during tillage. These observations form the motivation for the present research. *Present address: Agricultural Engineering Department, Massey University, New Zealand.
374
In an impact, four factors determine the severity of the collision. These are the impact velocity, the masses of the colliding objects, the shape of the objects in the contact area and the mechanical properties of the materials involved. This paper describes an apparatus which is able repeatedly to impact a fixed flat surface using a projectile whose mass may be up to 1.1 kg at velocities not exceeding 10 m s- ‘. The specimen is struck by a tungsten carbide sphere. The projectile may strike the surface along the normal or at an angle. Previous work on repeated impact testing Several researchers have produced repeated impact rigs, the design varying according to the particular requirements of the research project. Although some are in effect merely conventional impact tests modified to allow repetitive impacting until failure occurs [7 - lo], others have considered repeated impact damage to surfaces [ 11 - 171. Of these, Sorokin and Matyushima [ 11,121 repeatedly dropped specimens onto abrasive paper, investigating wear resistance of a high carbon steel as a function of hardness. It was found that at low impact energies the wear resistance increased with hardness but at high impact energies (above 600 DPN) the wear resistance began to fall. When steels with different carbon contents were studied, the wear resistance was a maximum at 0.8% carbon content. These results are presented in the review by Krushchov [ 131. Wellinger and Breckel [ 141 used three rigs to study impact damage on copper, ~uminium and steel alloys and investigated the relationship between impact force and velocity. They observed oxidation, flaking and severe deformation in the soft workhardening materials used. Bayer et al. [ 15, 16 ] described an impact rig able to work at 50 impacts per second, However the projectile weighed only 1 g in order to enable it to be brought to rest before the next impact. They described the preliminary results on the wear of a mild steel projectile striking a hard surface and found that after lo6 cycles wear began abruptly; this was indicated by the “appearance of a fine granular texture” on the surface. It appears that little research has been done on harder materials impacted by heavy masses, and the apparatus described here was designed to investigate this area.
Description
of apparatus
Specimen mounting The apparatus is shown in Figs. 1 and 2. The specimen was clamped into a dodecagonal holder using positioning screws. When necessary, lead or steel strips were used for packing. The holder was positioned in the support using two parallel blocks which located against any two parallel sides of the dodecagon. The specimen could therefore be rotated in steps of 30” in a horizontal plane, and in addition the holder could be moved parallel to the blocks on a screw thread. Thus different parts of the specimen could be
375
Fig. 1. Impact apparatus: S specimen, D dodecagonal specimen holder, J spring-loaded bolts, L lever to release specimen, A swivel arm, B and 0 locking screw for swivel arm, B bullet, I indenter and holder, V linear bearing, N supporting pillars for bullet linear bearing, Q soft springs to hold bullet against rubber pad before impact, 2 rubber pad, R spring holder rod, M variable speed motor, H hammer, Sp main spring, C cam and freewheel gear, U adjustable spring-loading bar, HB buffer to prevent excessive hammer movement, W supporting pillars for hammer and motor, X shock-absorbent material, Y safety cage (opened), l? photocells for velocity measurement.
T
D
E
Fig. 2. Specimen mounting arrangement with specimen and holder removed: D dodecagonal specimen holder, E specimen positioning screws, F positioning blocks, G screw thread to move holder parallel to blocks, J spring-loaded bolts, T Tufnol, K electrical contact plate with piezoelectric crystal directly underneath.
376
tested and if required the specimen and holder could be removed and replaced during an experiment. The holder was clamped with spring-loaded bolts and was released by pulling a lever. The specimen was mounted onto a PZT-5A piezoelectric crystal used for load measurement. The crystal was embedded in Tufnol to absorb stress waves. The entire mount could be rotated about a horizontal axis by aligning the correct hole on the swivel arm. The projectile
The projectile consisted of a cylinder roughly 130 mm long with a sphere attached to the striking end. The cylinder ran in a linear bearing. Six cylinders weighing from 37 to 1150 g were available, using an insert for the lighter cylinders. The sphere was from 1 to 15 mm in diameter, thus providing a wide choice for impact mass and radius of curvature. Normally the bullet rested against a rubber pad at the foot of the linear bearing and was supported by two springs attached to a small rod projecting from the upper end of the bullet. Hammer
After some consideration of alternative methods, a spring-loaded hammer mechanism was chosen. A variable speed motor rotated the cam to load the spring. The energy stored in the spring could be adjusted by altering the length of the stroke. When the cam passed top dead centre a freewheel gear allowed the hammer to be released. After striking the bullet, the hammer was stopped by a rubber buffer. The hammer and motor were mounted on a second pair of pillars, these pillars being isolated from the bullet and specimen holder by shock-absorbent material in order to minimise vibrations. The hammer was enclosed in a wire-mesh cage as a safety precaution. Velocity
measurement
The velocity just before impact was measured with two photocells. A thin brass plate was brazed onto the small horizontal rod attached to the bullet. As the bullet travelled downwards, the plate interrupted the beams from two light-emitting diodes shining on the photocells. The output from the photocells was fed into a timer via a Schmidt trigger, and thus the time taken for the bullet to traverse a known distance could be obtained. Although downward motion of the bullet was continually retarded by the springs, the decrease in velocity could be made small by a suitable choice of springs. The velocity was measured over the last millimetre of travel before impact by adjusting the position of the photocells. Load measurement
The load was measured by recording the output from the piezoelectric crystal on a storage oscilloscope. This is a useful technique providing that the crystal is properly mounted and that stress wave effects are considered. Its general validity is discussed in ref.6. The crystal was ~~ibra~d using the method described by Crook 1181 and Hancox [ 191.
377
Fig. 3. Damage to hardened steel after 50 impacts with a 3 mm tungsten carbide sphere at 1.08 m s-l (bullet mass 1.24 kg). Flakes of material have detached from the central region.
Results and discussion Examples of the damage produced by single and repeated impacts on a water-quenched carbon steel hardened to 850 DPN and on PMMA are shown in Figs. 3 and 4. The steel was carefully ground and then polished using a 6 pm diamond paste wheel [ 61. Glass- and plasma-sprayed hardfacings have also been examined [6]. The steel is shown in Fig. 3 after 50 impacts at 1.08 m s-l with a 3 mm tungsten carbide indenter. The total bullet weight was 1.24 kg and the maximum load was approximately 5 kN. A permanent indentation was made on the first cycle and this increased slowly in size with the number of cycles. Radial cracks were also produced outside the contact area. These features have been observed in quasi-static indentations under the same maximum load [20] and can be explained with reference to the stress distributions during loading and unloading [ 211. However, inside the contact area, flakes of material were removed from the surface (the dark region near the centre of the depression in Fig. 3). This damage also increased with the number of cycles and appears to be due to workhardening of the surface layers combined with the residual stresses left after unloading.
Fig. 4. Angled impacts on PMMA using tungsten carbide indenters: (a) 10” to normal, 3 mm indenter, mass 1.24 kg, at 1.03 m s-l; (b) 10” to normal, 10 mm indenter, mass 1.27 kg, at 1.00 ms-l; (c) 20” to normal, 3 mm indenter, mass 1.24 kg, at 1.04 mm s-l; (d) 50 impacts at 10” to normal, 10 mm indenter, mass 1.27 kg at 1.03 m Cl.
Microhardness measurements showed that after 200 impacts the hardness near the centre was 1080 + 30 DPN compared with a value of 880 ? 30 DPN just outside the contact area (six measurements of each). Evidence for residual stresses causing crack propagation after unloading in hardened steels has recently been presented [ 211. Angled impacts Examples of damage produced in polymethylmethacrylate (PMMA) when the surface was inclined and impacted are shown in Fig. 4. Circumferential striations (crazes) were formed on the higher side of the area of contact (Figs. 4(a) and 4(b)). They develop preferentially in this position because the tensile Hertzian contact stresses are reinforced by friction (see ref. 22 for an analysis of a similar situation). If the angle of impact was increased, or if the specimen was repeatedly impacted, the crazes developed into cracks and material was removed from the surface (the dark regions on the left in Figs. 4(c) and 4(d)). It should be emphasized that cracking of this
379
type does not occur under slow indentation conditions and its appearance here is a consequence of the more brittle behaviour that PMMA exhibits at higher rates of loading. On the lower side of the area of contact, radial crazes can develop (Fig. 4(a)). Similar craze patterns have been reported by Puttick [23] during quasi-static indentation experiments on PMMA where the load was applied normal to the surface. Puttick explained this damage with reference to Hill’s and Nadai’s models for the expansion of a cylindrical hole in an infinite flat plate of an elastoplastic material [24, 251. This stress system applies when there has been penetration of the indenter into the solid and material is being forced outwards. The radial crazes have not developed in Fig. 4(b) because, although the mass and impact velocity were similar to those in Fig. 4(a), the indenter radius was much greater thus giving less penetration. The radial crazes form less readily in the upper region of the crater, partly because, although the mass and impact velocity were similar to those in area is reduced, resulting in less pile-up of material. During unloading the radial crazes should extend further. This is because tensile residual stresses develop in this region in specimens which have deformed plastically during loading [21, 261. With inclined impacts on steels the crack patterns were essentially the same as with PMMA (though of course the initial step of craze formation was absent). Cracking and wear were most pronounced near the upper part of the crater. The wear damage developed more quickly than for normal impacts, showing the great importance of the frictional stress component.
Conclusions An impact apparatus has been developed which is able to impact specimens repeatedly at low velocities and at controlled angles of impact. Single impacts produced radial and circumferential cracks in a hardened carbon steel, and radial and circumferential crazes in PMMA. Flakes of material were removed in steel after repeated impacts at both normal and angled incidence. It was found that workhardening of the surface layers took place. In addition, previous work has shown that, with this class of material, residual stresses after loading can contribute to tensile failure. Both these factors are thought to contribute to wear. In PMMA, material was removed by angled impacts when the circumferential crazes developed into full cracks. The apparatus is proving a useful tool for the study of impact damage to soil-working surfaces in agriculture, since it simulates impact with stones and other objects in the soil. Examples have been given which show that the approach has general interest since it can provide data on the behaviour of materials under low velocity large mass impact conditions.
380
Acknowledgments This work was completed partly at the Cavendish Laboratory, Cambridge, and partly at the National Institute of Agricultural Engineering, Silsoe, Bedfordshire. We wish to thank the S.R.C. for a grant to the laboratory. C. J. Studman thanks the A.R.C. for support during the period of research and the N.I.A.E. for permission to publish these results. We would also like to thank B.I.S.R.A. for supplying the steel specimens.
References 1 A.S.T.M. Impact testing of metals, ASTM Spec. Tech. Publ. STP466, 1970. 2 R. C. D. Richardson, Maximum hardness of strained surfaces and the abrasive wear of metals and alloys, Wear, 10 (1967) 353 - 382. 3 R. C. D. Richardson, The wear of metal shares in agricultural soil, Ph. D. Thesis, University of London, 1967. 4 C. J. Studman, The recording of impact loads during tillage, J. Agric. Eng. Res., 20 (1975) 405 ” 411. 5 C. J. Studman, Impact loads on soil working surfaces, J. Agric. Eng. Res., 20 (1975) 413 - 422. 6 C. J. Studman, Impact damage to brittle materials during the tillage of stony soils, Ph. D. Thesis, University of Cambridge, 1974. 7 R. E. Schramm, R. L. Durchotz and R. P. Reed, Apparatus for impact fatigue testing, J. Res. Nat. Bur. Stand., Sect. C, 75 (1971) 95 - 98. 8 J. D. Butler and H. Brown, Izod machine for repeated impact tests, Strain, 6 (1970)67. 9 E. L. Layland, How materials perform under repeated impact, Mater. Methods, 44 (1956) 104 - 105. 10 I. F. Panshin, Increase in resistance of steel to repeated impact, Tr. Kurgan. Mashinostroit. Inst., 12 (1969) 56 - 63. 11 G. M. Sorokin and I. I. Matyushima, Wear tests carried out by impact of an abrasive on a massive sample, Zavod. Lab., 37 (1971) 218 - 220. 12 G. M. Sorokin, Wear resistance criteria for steel under impact on abrasive, Machinoved., 3 (1973) 111 - 115. 13 M. M. Kruschov, Principles of abrasive wear, Wear, 28 (1974) 69 - 88. 14 K. Wellinger and H. Breckel, Kenngrossen und Verschleiss beim stoss Metallischer Werkstoffe, Wear, 13 (1969) 257 - 281. 15 R. G. Bayer, P. A. Engel and J. L. Sirico, Impact wear testing, Wear, 19 (1972) 343 - 354. 16 P. A. Engel, T. H. Lyons and J. L. Sirico, Impact wear model for steel specimens, Wear, 23 (1973) 185 - 201. 17 A. I. Kalimov and E. K. Pochtennyi, Apparatus for dynamic testing of materials, Ind. Lab., 35 (1969) 136 - 138. 18 A. W. Crook, A study of some impacts between metal bodies by a piezoelectric method, Proc. R. Sot. London, Ser. A, 212 (1972) 377. 19 N. L. Hancox, The deformation of solids under repeated liquid impact, Ph. D. Thesis, University of Cambridge, 1962. 20 C. J. Studman and J. E. Field, Indentation behaviour of hard metals, J. Phys. D, Q (1976) 857 - 867. 21 C. J. Studman and J. E. Field, Indentation of hard metals: the role of residual stresses, J. Mater. Sci., in the press. 22 G. M. Hamilton and L. E. Goodman, The stress field created by a circular sliding contact, J. Appl. Mech., 33 (1966) 371 - 376.
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