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Origin of macroscopic wear grooves generated during sliding friction experiments
Int. J. Rock Mech. Min. Sci. & Geomech. Abstr. Vol. 12. pp. 367-371. Pergamon Press 1975. Printed in Great Britain
Origin of Macroscopic Wear Grooves Generated during Sliding Friction Experiments L E S T E R J. L A F O U N T A I N * M I C H A E L V. S W A I N t R O B E R T E. J A C K S O N ~
Surface features developed in sliding friction experiments closely resemble those features observed in the Hertzian fracture tests of materials science. Grooves from sliding friction tests on Henderson gneiss have the form of parabolas pointing in the direction of slip of the surface in which they are developed. In cross-section, the features deepen away from the direction of slip and are composed of highly fractured and comminuted material. We term these features macroscopic wear grooves. Hertzian fractures are similar in form and origin. They are arcuate to paraboloid resulting from the fracture of a substrate by a load-bearing indenter. We suggest that the process of Hertzian fracture is responsible for macroscopic wear grooves.
INTRODUCTION
Sliding friction tests c o n d u c t e d under conditions of low axial loads and confining pressures in dry quartzose rocks often show characteristic surface damage. A n u m b e r of workers[4,5, 14-17] have described these features as elongate paraboloids whose axes are oriented parallel to the direction of slip along the sliding surface. In quartz-rich sandstones the paraboloids tend to be both more elongate and narrower and have been termed microscopic wear grooves [5]. In tests of quartzofeldspathic schists the paraboloids become less numerous, but larger, more open, and deeper in cross-section. Accordingly, we here term them macroscopic wear grooves. In crosssection the surface features are actually pits floored by numerous crack networks. Coulson[3] cleaned the surfaces of their gouge material, examined the cross-sectional nature of the paraboloids, and described the fractures as Riedel shears [20]. Because the paraboloid grooves are characteristic of many stick slip tests and are produced during sliding by a deformation of the surface, the origin of the surface grooves seems directly related to the nature of sliding friction. Their areal and cross-sectional form * Dept. of Geology, University of North Carolina, Chapel Hill, NC 27514, U.S.A. Present address: E. D'Appolonia Consulting Engineers, Inc., 10 Duff Road, Pittsburgh, PA 15235, U.S.A. Martin Marietta Research Laboratories, 1450 S. Rolling Road, Baltimore, MD 21227, U.S.A. Present address: University of Cambridge, Dept. of Physics, Madingiey Rd., Cambridge CB30HE, England. ~U.S. Atomic Energy Commission, Licensing-Site Analysis Branch, Washington, DC 20545, U.S.A.
resembles features resulting from a process described by Hertzian-like fracturing as observed in various fields of materials science research. In addition, Brace fl] described Hertzian-like fractures in quartz indentation studies. Hertzian fractures are arcuate or paraboloid and result f r o m cracking under a load imposed by a surface indenter. The typical materials science test is that of a spherical button indenter consisting of some hard material, such as a sapphire ball, which loads a surface. This process is analogous to a sliding friction test where an asperity on the sliding surface impinges or indents the underlying sliding surface. This paper shows the similarity between both the process of Hertzian fracture and its surface results to the process and resulting surface features found in the sliding friction tests of rock mechanics. SURFACE M O R P H O L O G Y Surface wear grooves are most often seen in our sliding friction experiments involving H e n d e r s o n gneiss. The gneiss is a medium to dark gray, fine to medium-grained, quartzofeldspathic rock with a moderate to well developed foliation[14]. Tests were conducted on right circular cylinders (2-79 x 7.62 cm) possessing a 45 ° saw-cut made at a variety of orientations to the foliation. The sliding surfaces were polished with an 80 grit silicon carbide grinding wheel to produce a surface roughness in the range of 2.8-6-35 × 10-6 cm (central line average). The samples were tested dry at 140 and 500 bars confining pressure, 25°C, 10-' sec-' and 10-5 sec-' shortening rates. 367
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L . J . LaFountain, M. V. Swain and R. E. Jackson
Reflecting microscope examination of approximately 60 samples shows that the post-sliding surface is covered with a whitish-colored, finely crushed, saccharoidal gouge material (Fig. la) which is cohesive or welded. Welded gouge appears to be c o m p o s e d of extremely fine-grained quartz and feldspar surrounded by a matrix of isotropic material. This material appears glass-like and supports brittle fracture similar to the gouge reported by Friedman and others [8]. When the gouge material is r e m o v e d f r o m the surface using an air or w a t e r jet, m a c r o s c o p i c grooves are revealed which increase in width and depth opposite to the slip direction (Fig. lb). As such, the grooves have the shape of a parabola (Fig. l b, point 2) opening in the direction of travel of the overlying surface. The length of the grooves is difficult to determine because g r o o v e s run into and overlap other grooves. H o w e v e r , an average length ( 3 - 1 0 m m ) is apparent when the entire surface is examined. In many of the tests, when slip along the surface is calculated, the average length of the g r o o v e s is close to that of the amount of displacement along the surface. Surface grooves studied under high magnification show evidence of intense fracturing. U n d e r 60 power (Fig. 2), scanning electron microscope (SEM) studies indicate a very angular floor. At even higher magnification (Figs. 3 and 4) the floor of the surface grooves are seen to be c o m p o s e d of intensely fractured grains. Apparently, the process that produces the grooves is one of large scale fracturing which can only be seen when the surfaces are cleaned of the gouge material. The nature of the g r o o v e s was also studied in cross-section through the use of polished sections and a reflected light microscope. Tested cores were impregnated with e p o x y under a v a c u u m and subsequently sectioned parallel to their longitudinal axis but perpendicular to the sliding surface. T h e s e sections were polished with 400, 600, and 1200 grits on a lap wheel. Further polishing was done with 6 and 1 micron diamond paste on oiled cloth laps. The resultant
Fig. 2. SEM photograph ,howing a surface wear groove. Note the fractured nature of the groove floor. Area (AI is discussed in succeeding figures.
Fig. 3. A 1231× enlargement of Area (A) of Fig. 2. The fractured nature of the groove floor is clear.
sections reveal the surface grooves in cross-sections parallel to the geometric principal axis of the parabola. T h e s e cross-sections of the paraboloids confirm their previous description; they deepen in the direction opposite to slip and are c o m p o s e d of intensely fractured host rock or an admixture of host rock and gouge (Fig. 5). Fractures within the grooves are of complex orientation. Many a p p e a r to be parallel to fractures within the host rock a w a y f r o m the sliding surface. Others are subparallel to the sliding surface or at low angles to it. The latter fractures are similar to A B microscopic feather fractures[7]. The complex fracIllillliltllll[lllliliilllllllllllll[lllllllllllllllllllllllillllt!l!II ture networks are not as obvious when examined in transmitted light thin sections, but important characFig. 1. A post-test photograph of the halves of a specimen. (A) teristics of the gouge are apparent. Many of the grooves uncleaned surface with clumped gouge (I) in place, (B) cleaned are filled with extremely c o m m i n u t e d material (Fig. 6). surface showing macroscopic wear grooves (2). One division on the scale corresponds to I ram. To get the sense of slip, invert half (B) This gouge is opaque in transmitted light because of the e x t r e m e reduction in grain size. It often floors or fills the and move it in direction of arrow.
Origin of Macroscopic Wear Grooves
369
into two categories. First, from a surficial examination, the grooves are paraboloid in shape, opening in the direction the overlying surface has slid and they deepen away from the point of the parabola. SEM examination shows that the grooves are composed of highly fractured grains. Second, in cross-section, the grooves are seen to be floored by complex fracture patterns with highly comminuted and compacted glass-like gouge often found in the deeper floor of the groove. HERTZIAN FRACTURE Fig. 4. A 6183× enlargement of point (A) of Fig. 2. The highly fractured area is composed primarily of angular quartz particles and fine, comminuted gouge material.
Fig. 5. A reflected light photomicrograph (90x) showing a crosssectional view of the sliding surface (the N W - S E zone). That portion of the specimen above the sliding surface has moved to the right. A surface wear groove is seen in profile on the lower half of the sample. It is a zone of intense fracturing with comminuted grains flooring the groove.
On loading an elastic brittle material with a spherical indenter or a circular punch, high tensile stresses nucleate a shallow ring crack about the indenter from a suitably located surface flaw, which grows rapidly (unstably) into the shape of a truncated cone (mechanically stable). Frank & Lawn[6] have analyzed the Hertzian cone crack problem from the Griffith[12] energy balance criterion and have shown that the critical load ( P c ) to form a cone crack may be related to the indenter radius (r) and fracture surface energy (3,) by the relation P c / r oc 3,. Swain and others [21] have recently used this approach to monitor the influence of various environments on fracture surface energy for quartz and a number of silicate glasses. Alternatively, when a tangential force is applied to the specimen or indenter as is the case of sliding contact, the stress fields[13] and consequent fracture path [18] within the solid become increasingly dependent upon the coefficient of friction at the interface. Figure 7 indicates how the coefficient of friction influences the fracture path in the specimen. Both the fracture path and the load to fracture are sensitive functions of the coefficient of friction. Gilroy & Hirst [10] have suggested that the sliding load to cause fracture ( P s l ) is approximately of the form, Psi/Pc
oc
1 (1 + Kt~) 3
where (/z) is the coefficient of friction and (K) a
4af
Track Width (b)
/%/// Fig. 6. A transmitted light photomicrograph (253×) showing the sawcut (NW-SE) and a profile of a surface wear groove (outlined by ink) which deepens to the SW. The isotropic material in the groove is finely comminuted material.
groove and appears to be the source of the glass-like welded gouge. In ~ummarv. the nature of the surface grooves falls ROCK 12/12--C
/z~-t O. 1
/// y~0.2-0.3
I iI ~0.5
Fig. 7, Schematic variation of the surface (a) and subsurface (b) fracture paths due to a sliding indenter as the coefficient of friction (#,) changes. The shear tractions modify the shape of the cracks such that the cracks become more normal and even obtuse to the surface as the coefficient of friction increases. Arrow indicates direction of slider movement.
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L.J. LaFountain, M. V. Swain and R. E. Jackson
constant. The cubic dependence suggested[10] is found experimentally to be a slight overestimate. Experimental evidence also indicates that the nature of the substrate is important. Wilshaw [23] has pointed out that the surface fracture pattern is very much dependent upon the surface microcrack density. Unfortunately, a basic theoretical approach only applies to surfaces with very low densities of microcracks, although qualitatively, the arguments developed above apply to surfaces with heavy microcrack damage as would typically be the case for geologic specimens. Recently, Graham[ll] has suggested that the fracture behavior resulting from sliding contact is generally more complicted than the idealized situation portrayed in Fig. 7. In particular, Graham observes that in some situations normal Hertzian fractures as well as reversed (in terms of fracture dip angle) cracks develop. He also noted that the cracks produced by a sliding indenter on glass were characterized by three types of fine surface structure: (a) radial ridges or cleavage steps, (b) circumferential steps, and (c) smooth surfaces with no facets. The first two features are commonly observed on normal cone cracks (see Williams and others, 1970) whereas the smooth surfaces are typical of slow crack growth (< 10--4 m sec-') in glass. Since the smooth surfaces are always the reverse cracks, this suggests they are the result of release of residual stress caused by the sliding indenter. In fact, Wilshaw [23] has observed that cracks continue to propagate on a heavily microcracked surface after the indenter has been removed, presumably due to the relaxation of residual stresses set up by plastic deformation. Reverse cracking of this type is typically observed beneath pointed indenters in brittle materials on unloading, again due to release of residual stresses. Finally, it is interesting to point out the similarity between the fracture behavior observed by Graham[l l] associated with a heavily loaded small diamond sphere on quartz, and the damage observed by Engelder [5] on sliding surfaces of Westerly granite. The fracture phenomena in our study suggests that the damage observed by Engelder[5] resulted from initial fracturing via the Hertzian contact situation. On continued sliding, the crack network predicted by Graham [ 11] probably produces the comminuted material (gouge). DISCUSSION AND CONCLUSIONS There seems to be a striking similarity in form and process between the grooves observed on some sliding surfaces and those described in Hertzian fracture experiments. The grooves found on sliding surfaces are paraboloid similar to those produced by a sliding indenter in a Hertzian test. In cross-section, the Hertzian fracture dips back away from the direction of travel of the indenter while the grooves on sliding surfaces deepen toward the direction of indenter
travel. However, in the Hertzian case when the coefficient of friction increases into the range found in rock mechanics tests (/x >0.5), Hertzian fractures begin to dip in the same direction as the surface grooves deepen. In particular, the deep cracking networks reported by Graham[l l] approximate the form of the cross-section profiles of the surface grooves shown in Fig. 5. There is a direct analogy between the process of Hertzian fracture and the nature of loading in a rock mechanics sliding surface test. Both involve the movement of an indenter over an underlying surface. In the Hertzian test a ball is dragged over the surface while under a normal load. The rock mechanics analog is that of an asperity from the overlying sliding surface being moved across the underlying surface under a high normal load. Additional evidence for the existence of asperity loading may be seen in tests where anisotropic rocks are considered. Surface grooves are well developed only when anisotropy planes (planes of schistosity) lie in planes of high resolved shear stress. In these orientations material can slip, along foliation planes, out onto a sliding surface and serve as an indenter [ 17]. Perhaps a less esoteric example of Hertzian fracture can be found in the glacial groove marks in glacial scour terrains[2, 9]. The same paraboloid, crescentic grooves are reported which arise from an indenter, held within the base of the ice, passing over a rock substrate. It is not clear at this point whether or not surface wear features can be used as reliable indicators of paleoseismicity. The macroscopic wear grooves observed by us appear to be similar in morphology to the microscopic wear grooves present on slickensides as observed by Engelder[5]. While Engelder finds surface grooves only in stick slip tests, we find them in both stick slip and stable sliding. However, the two cases produce grooves which vary in dimension; the stable sliding tests showing the widest and longest surface grooves. An intensive effort still needs to be made to observe, categorize, and evaluate wear features on experimental and natural fault surfaces. Information of this sort could lead to a complete understanding of the condition at the time wear takes place. It is clear from many observations both in the fields of rock mechanics and materials science research that the wear process is exceedingly complex. No single hypothesis will account for all the features that have been observed. Instead, it seems likely that a combination of processes and interactions takes place, all of which are dependent upon material properties (minerals present, mineral slip systems, inherent fractures, anisotropy) and environmental conditions at the time the fault was active. Observations from the field of materials science research can and should be used in developing these concepts. In summary; we wish to emphasize the similarity between the plan and cross-sectional form of grooves found on rock mechanics sliding surfaces with the
Origin of Macroscopic Wear Grooves form of Hertzian fractures
s e e n in m a t e r i a l s s c i e n c e
t e s t i n g . I n a d d i t i o n , t h e r e is a d i r e c t C o m p a r i s o n between the process causing Hertzian fractures and the asperity loading hypothesis put forth to explain s u r f a c e d a m a g e in o u r f r i c t i o n e x p e r i m e n t s . I t s e e m s c l e a r t o u s t h a t t h e c o m p a r i s o n is t o o g r e a t t o b e f o r t u i t o u s a n d t h a t H e r t z i a n f r a c t u r e is a g o o d explanation for the paraboloid surface grooves we observe.
Acknowledgement--The authors wish to thank D. E. Dunn for his review of the manuscript. Received 26 March 1975.
REFERENCES 1. Brace W. F. Behavior of quartz during indentation. J. Geophys. Res. 71, 581 (1963). 2. Chamberlain T. C. The rock-scorings of the great ice invasions. U.S. Geol. Survey 7th Annual Rept. 147 (1886). 3. Coulson J. H. The effects of surface roughness on the shear strength of joints in rock. Technical Report MRD-2-70, Missouri River Division, Corps of Engineers, 282 p. (1970). 4. Engelder J. T. The influence on quartz fault-gouge on sliding mode and stickslip stress drops. Am. Geophys. Union Trans. 54, 465 (1973). 5. Engelder J. Microscopic wear grooves on slickensides: Indicators of paleoseismicity. 3". Geophys. Res. 79, 4387 (1974). 6. Frank F. C. & Lawn B. R. On the theory of Hertzian fracture. Proc. R. Soc., Series A, 291 (1967). 7. Friedman M. & Logan J. M. Microscopic feather fractures. Bull. geol. Soc. Am. 81, 3417 (1970). 8. Friedman M., Logan J. M. & Rigert J. A. Glass-indurated quartz
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gouge in sliding-friction experiments on sandstone. Bull. geol. Soc. Am. 85, 937 (1974). 9. Gilbert G. K. Crescentic gouges on glaciated surfaces. Bull. geol. Soc. Am. 17, 303 (1907). 10. Gilroy D. R. & Hirst W. Brittle fracture of glass under normal and sliding loads. Br. J. appl. Phys. (Z Phys. D) 2, 1784 (1969). 11. Graham J. Damage induced by a sliding diamond--an approach to hard rock drilling. Rock Mech. 4, 193 (1972). 12. Griffith A. A. The phenomenon of rupture and flow in solids. Phil. Trans. R. Soc., A. 221, 163 (1920). 13. Hamilton G. M. & Goodman L. E. The stress field created by a circular sliding contact. J. appl. Mech. 33, 371 (1966). 14. Jackson R. E. & Dunn D. E. Experimental sliding friction and cataclasis of foliated rocks. Int. J. Rock Mech. Min. Sci. & Geomech. Abstr. 11, 235 (1974). 15. Jackson R. E., LaFountain L. J. & Swain M. V. Sliding surface features, Hertzian fractures, and stick slip. Am. Geophys. Union Trans. 55, 428 (1974). 16. LaFountain L. J. & Dunn D. E. Anisotropy and the coefficient of sliding friction. Am. Geophys. Union Trans. 55, 428 (1974). 17. LaFountain, L. J. & Dunn D. E. Effect of anisotropy on the coefficient of sliding friction in schistose rocks. Int. J. Rock Mech. Min. Sci. & Geomech. Abstr. 11, 459 (1974). 18. Lawn B. R. Partial cone crack formation in a brittle material loaded with a sliding spherical indenter. Proc. R. Soc. Series A, 307 (1967). 19. Lawn B. R. & Swain M. V. Microfracture beneath point identations in brittle solids. J. Materials Sci. (in press). 20. Riedel W. Zur mechanik geologischer brucherscheinungen. Zentbl. Miner. Geol. Paliiont. pp. 354-368 (1929). 21. Swain M. V., Williams J. S., Lawn B. R. & Beek J. J. H. A comparative study of the fracture of various silica modifications using the Hertzian test. J. Materials Sci. 8, 1153 (1973). 22. Williams J. S., Lawn B. R. & Swain M. V. Cone crack closure in brittle solids. Physicus Status Solidi (A) 2, 7 (1970). 23. Wilshaw T. R. The response of strong solids to sliding contact loading. Unpublished internal report, Univ. of Sussex, School of Applied Sci., England.
Origin of Macroscopic Wear Grooves Generated during Sliding Friction Experiments L E S T E R J. L A F O U N T A I N * M I C H A E L V. S W A I N t R O B E R T E. J A C K S O N ~
Surface features developed in sliding friction experiments closely resemble those features observed in the Hertzian fracture tests of materials science. Grooves from sliding friction tests on Henderson gneiss have the form of parabolas pointing in the direction of slip of the surface in which they are developed. In cross-section, the features deepen away from the direction of slip and are composed of highly fractured and comminuted material. We term these features macroscopic wear grooves. Hertzian fractures are similar in form and origin. They are arcuate to paraboloid resulting from the fracture of a substrate by a load-bearing indenter. We suggest that the process of Hertzian fracture is responsible for macroscopic wear grooves.
INTRODUCTION
Sliding friction tests c o n d u c t e d under conditions of low axial loads and confining pressures in dry quartzose rocks often show characteristic surface damage. A n u m b e r of workers[4,5, 14-17] have described these features as elongate paraboloids whose axes are oriented parallel to the direction of slip along the sliding surface. In quartz-rich sandstones the paraboloids tend to be both more elongate and narrower and have been termed microscopic wear grooves [5]. In tests of quartzofeldspathic schists the paraboloids become less numerous, but larger, more open, and deeper in cross-section. Accordingly, we here term them macroscopic wear grooves. In crosssection the surface features are actually pits floored by numerous crack networks. Coulson[3] cleaned the surfaces of their gouge material, examined the cross-sectional nature of the paraboloids, and described the fractures as Riedel shears [20]. Because the paraboloid grooves are characteristic of many stick slip tests and are produced during sliding by a deformation of the surface, the origin of the surface grooves seems directly related to the nature of sliding friction. Their areal and cross-sectional form * Dept. of Geology, University of North Carolina, Chapel Hill, NC 27514, U.S.A. Present address: E. D'Appolonia Consulting Engineers, Inc., 10 Duff Road, Pittsburgh, PA 15235, U.S.A. Martin Marietta Research Laboratories, 1450 S. Rolling Road, Baltimore, MD 21227, U.S.A. Present address: University of Cambridge, Dept. of Physics, Madingiey Rd., Cambridge CB30HE, England. ~U.S. Atomic Energy Commission, Licensing-Site Analysis Branch, Washington, DC 20545, U.S.A.
resembles features resulting from a process described by Hertzian-like fracturing as observed in various fields of materials science research. In addition, Brace fl] described Hertzian-like fractures in quartz indentation studies. Hertzian fractures are arcuate or paraboloid and result f r o m cracking under a load imposed by a surface indenter. The typical materials science test is that of a spherical button indenter consisting of some hard material, such as a sapphire ball, which loads a surface. This process is analogous to a sliding friction test where an asperity on the sliding surface impinges or indents the underlying sliding surface. This paper shows the similarity between both the process of Hertzian fracture and its surface results to the process and resulting surface features found in the sliding friction tests of rock mechanics. SURFACE M O R P H O L O G Y Surface wear grooves are most often seen in our sliding friction experiments involving H e n d e r s o n gneiss. The gneiss is a medium to dark gray, fine to medium-grained, quartzofeldspathic rock with a moderate to well developed foliation[14]. Tests were conducted on right circular cylinders (2-79 x 7.62 cm) possessing a 45 ° saw-cut made at a variety of orientations to the foliation. The sliding surfaces were polished with an 80 grit silicon carbide grinding wheel to produce a surface roughness in the range of 2.8-6-35 × 10-6 cm (central line average). The samples were tested dry at 140 and 500 bars confining pressure, 25°C, 10-' sec-' and 10-5 sec-' shortening rates. 367
368
L . J . LaFountain, M. V. Swain and R. E. Jackson
Reflecting microscope examination of approximately 60 samples shows that the post-sliding surface is covered with a whitish-colored, finely crushed, saccharoidal gouge material (Fig. la) which is cohesive or welded. Welded gouge appears to be c o m p o s e d of extremely fine-grained quartz and feldspar surrounded by a matrix of isotropic material. This material appears glass-like and supports brittle fracture similar to the gouge reported by Friedman and others [8]. When the gouge material is r e m o v e d f r o m the surface using an air or w a t e r jet, m a c r o s c o p i c grooves are revealed which increase in width and depth opposite to the slip direction (Fig. lb). As such, the grooves have the shape of a parabola (Fig. l b, point 2) opening in the direction of travel of the overlying surface. The length of the grooves is difficult to determine because g r o o v e s run into and overlap other grooves. H o w e v e r , an average length ( 3 - 1 0 m m ) is apparent when the entire surface is examined. In many of the tests, when slip along the surface is calculated, the average length of the g r o o v e s is close to that of the amount of displacement along the surface. Surface grooves studied under high magnification show evidence of intense fracturing. U n d e r 60 power (Fig. 2), scanning electron microscope (SEM) studies indicate a very angular floor. At even higher magnification (Figs. 3 and 4) the floor of the surface grooves are seen to be c o m p o s e d of intensely fractured grains. Apparently, the process that produces the grooves is one of large scale fracturing which can only be seen when the surfaces are cleaned of the gouge material. The nature of the g r o o v e s was also studied in cross-section through the use of polished sections and a reflected light microscope. Tested cores were impregnated with e p o x y under a v a c u u m and subsequently sectioned parallel to their longitudinal axis but perpendicular to the sliding surface. T h e s e sections were polished with 400, 600, and 1200 grits on a lap wheel. Further polishing was done with 6 and 1 micron diamond paste on oiled cloth laps. The resultant
Fig. 2. SEM photograph ,howing a surface wear groove. Note the fractured nature of the groove floor. Area (AI is discussed in succeeding figures.
Fig. 3. A 1231× enlargement of Area (A) of Fig. 2. The fractured nature of the groove floor is clear.
sections reveal the surface grooves in cross-sections parallel to the geometric principal axis of the parabola. T h e s e cross-sections of the paraboloids confirm their previous description; they deepen in the direction opposite to slip and are c o m p o s e d of intensely fractured host rock or an admixture of host rock and gouge (Fig. 5). Fractures within the grooves are of complex orientation. Many a p p e a r to be parallel to fractures within the host rock a w a y f r o m the sliding surface. Others are subparallel to the sliding surface or at low angles to it. The latter fractures are similar to A B microscopic feather fractures[7]. The complex fracIllillliltllll[lllliliilllllllllllll[lllllllllllllllllllllllillllt!l!II ture networks are not as obvious when examined in transmitted light thin sections, but important characFig. 1. A post-test photograph of the halves of a specimen. (A) teristics of the gouge are apparent. Many of the grooves uncleaned surface with clumped gouge (I) in place, (B) cleaned are filled with extremely c o m m i n u t e d material (Fig. 6). surface showing macroscopic wear grooves (2). One division on the scale corresponds to I ram. To get the sense of slip, invert half (B) This gouge is opaque in transmitted light because of the e x t r e m e reduction in grain size. It often floors or fills the and move it in direction of arrow.
Origin of Macroscopic Wear Grooves
369
into two categories. First, from a surficial examination, the grooves are paraboloid in shape, opening in the direction the overlying surface has slid and they deepen away from the point of the parabola. SEM examination shows that the grooves are composed of highly fractured grains. Second, in cross-section, the grooves are seen to be floored by complex fracture patterns with highly comminuted and compacted glass-like gouge often found in the deeper floor of the groove. HERTZIAN FRACTURE Fig. 4. A 6183× enlargement of point (A) of Fig. 2. The highly fractured area is composed primarily of angular quartz particles and fine, comminuted gouge material.
Fig. 5. A reflected light photomicrograph (90x) showing a crosssectional view of the sliding surface (the N W - S E zone). That portion of the specimen above the sliding surface has moved to the right. A surface wear groove is seen in profile on the lower half of the sample. It is a zone of intense fracturing with comminuted grains flooring the groove.
On loading an elastic brittle material with a spherical indenter or a circular punch, high tensile stresses nucleate a shallow ring crack about the indenter from a suitably located surface flaw, which grows rapidly (unstably) into the shape of a truncated cone (mechanically stable). Frank & Lawn[6] have analyzed the Hertzian cone crack problem from the Griffith[12] energy balance criterion and have shown that the critical load ( P c ) to form a cone crack may be related to the indenter radius (r) and fracture surface energy (3,) by the relation P c / r oc 3,. Swain and others [21] have recently used this approach to monitor the influence of various environments on fracture surface energy for quartz and a number of silicate glasses. Alternatively, when a tangential force is applied to the specimen or indenter as is the case of sliding contact, the stress fields[13] and consequent fracture path [18] within the solid become increasingly dependent upon the coefficient of friction at the interface. Figure 7 indicates how the coefficient of friction influences the fracture path in the specimen. Both the fracture path and the load to fracture are sensitive functions of the coefficient of friction. Gilroy & Hirst [10] have suggested that the sliding load to cause fracture ( P s l ) is approximately of the form, Psi/Pc
oc
1 (1 + Kt~) 3
where (/z) is the coefficient of friction and (K) a
4af
Track Width (b)
/%/// Fig. 6. A transmitted light photomicrograph (253×) showing the sawcut (NW-SE) and a profile of a surface wear groove (outlined by ink) which deepens to the SW. The isotropic material in the groove is finely comminuted material.
groove and appears to be the source of the glass-like welded gouge. In ~ummarv. the nature of the surface grooves falls ROCK 12/12--C
/z~-t O. 1
/// y~0.2-0.3
I iI ~0.5
Fig. 7, Schematic variation of the surface (a) and subsurface (b) fracture paths due to a sliding indenter as the coefficient of friction (#,) changes. The shear tractions modify the shape of the cracks such that the cracks become more normal and even obtuse to the surface as the coefficient of friction increases. Arrow indicates direction of slider movement.
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L.J. LaFountain, M. V. Swain and R. E. Jackson
constant. The cubic dependence suggested[10] is found experimentally to be a slight overestimate. Experimental evidence also indicates that the nature of the substrate is important. Wilshaw [23] has pointed out that the surface fracture pattern is very much dependent upon the surface microcrack density. Unfortunately, a basic theoretical approach only applies to surfaces with very low densities of microcracks, although qualitatively, the arguments developed above apply to surfaces with heavy microcrack damage as would typically be the case for geologic specimens. Recently, Graham[ll] has suggested that the fracture behavior resulting from sliding contact is generally more complicted than the idealized situation portrayed in Fig. 7. In particular, Graham observes that in some situations normal Hertzian fractures as well as reversed (in terms of fracture dip angle) cracks develop. He also noted that the cracks produced by a sliding indenter on glass were characterized by three types of fine surface structure: (a) radial ridges or cleavage steps, (b) circumferential steps, and (c) smooth surfaces with no facets. The first two features are commonly observed on normal cone cracks (see Williams and others, 1970) whereas the smooth surfaces are typical of slow crack growth (< 10--4 m sec-') in glass. Since the smooth surfaces are always the reverse cracks, this suggests they are the result of release of residual stress caused by the sliding indenter. In fact, Wilshaw [23] has observed that cracks continue to propagate on a heavily microcracked surface after the indenter has been removed, presumably due to the relaxation of residual stresses set up by plastic deformation. Reverse cracking of this type is typically observed beneath pointed indenters in brittle materials on unloading, again due to release of residual stresses. Finally, it is interesting to point out the similarity between the fracture behavior observed by Graham[l l] associated with a heavily loaded small diamond sphere on quartz, and the damage observed by Engelder [5] on sliding surfaces of Westerly granite. The fracture phenomena in our study suggests that the damage observed by Engelder[5] resulted from initial fracturing via the Hertzian contact situation. On continued sliding, the crack network predicted by Graham [ 11] probably produces the comminuted material (gouge). DISCUSSION AND CONCLUSIONS There seems to be a striking similarity in form and process between the grooves observed on some sliding surfaces and those described in Hertzian fracture experiments. The grooves found on sliding surfaces are paraboloid similar to those produced by a sliding indenter in a Hertzian test. In cross-section, the Hertzian fracture dips back away from the direction of travel of the indenter while the grooves on sliding surfaces deepen toward the direction of indenter
travel. However, in the Hertzian case when the coefficient of friction increases into the range found in rock mechanics tests (/x >0.5), Hertzian fractures begin to dip in the same direction as the surface grooves deepen. In particular, the deep cracking networks reported by Graham[l l] approximate the form of the cross-section profiles of the surface grooves shown in Fig. 5. There is a direct analogy between the process of Hertzian fracture and the nature of loading in a rock mechanics sliding surface test. Both involve the movement of an indenter over an underlying surface. In the Hertzian test a ball is dragged over the surface while under a normal load. The rock mechanics analog is that of an asperity from the overlying sliding surface being moved across the underlying surface under a high normal load. Additional evidence for the existence of asperity loading may be seen in tests where anisotropic rocks are considered. Surface grooves are well developed only when anisotropy planes (planes of schistosity) lie in planes of high resolved shear stress. In these orientations material can slip, along foliation planes, out onto a sliding surface and serve as an indenter [ 17]. Perhaps a less esoteric example of Hertzian fracture can be found in the glacial groove marks in glacial scour terrains[2, 9]. The same paraboloid, crescentic grooves are reported which arise from an indenter, held within the base of the ice, passing over a rock substrate. It is not clear at this point whether or not surface wear features can be used as reliable indicators of paleoseismicity. The macroscopic wear grooves observed by us appear to be similar in morphology to the microscopic wear grooves present on slickensides as observed by Engelder[5]. While Engelder finds surface grooves only in stick slip tests, we find them in both stick slip and stable sliding. However, the two cases produce grooves which vary in dimension; the stable sliding tests showing the widest and longest surface grooves. An intensive effort still needs to be made to observe, categorize, and evaluate wear features on experimental and natural fault surfaces. Information of this sort could lead to a complete understanding of the condition at the time wear takes place. It is clear from many observations both in the fields of rock mechanics and materials science research that the wear process is exceedingly complex. No single hypothesis will account for all the features that have been observed. Instead, it seems likely that a combination of processes and interactions takes place, all of which are dependent upon material properties (minerals present, mineral slip systems, inherent fractures, anisotropy) and environmental conditions at the time the fault was active. Observations from the field of materials science research can and should be used in developing these concepts. In summary; we wish to emphasize the similarity between the plan and cross-sectional form of grooves found on rock mechanics sliding surfaces with the
Origin of Macroscopic Wear Grooves form of Hertzian fractures
s e e n in m a t e r i a l s s c i e n c e
t e s t i n g . I n a d d i t i o n , t h e r e is a d i r e c t C o m p a r i s o n between the process causing Hertzian fractures and the asperity loading hypothesis put forth to explain s u r f a c e d a m a g e in o u r f r i c t i o n e x p e r i m e n t s . I t s e e m s c l e a r t o u s t h a t t h e c o m p a r i s o n is t o o g r e a t t o b e f o r t u i t o u s a n d t h a t H e r t z i a n f r a c t u r e is a g o o d explanation for the paraboloid surface grooves we observe.
Acknowledgement--The authors wish to thank D. E. Dunn for his review of the manuscript. Received 26 March 1975.
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