Abstract:
The present invention is directed to a golf ball ( 1 ) having one or more nanostructured materials, i.e., materials with at least one dimension in the 1-100 nm range. The nanostructured materials are present within the hollow metal core ( 3 ), a surrounding polymer layer ( 4 ), or both. More particularly, the golf ball ( 1 ) may include a sphere comprising a nanostructured steel, a polymer comprising a nanoclay material, or both, which improve durability and performance. The present invention is also directed to a method for making the improved golf ball.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to the field of sporting equipment, more particularly to the fields of golf ball design and manufacturing. The invention also relates to the fields of polymer technology, nanotechnology, and metallurgy. 
       BACKGROUND OF THE INVENTION 
       [0002]    Golf balls are generally made of polymers, usually elastomeric materials. Except for one-piece balls that are used for range balls or practice balls, golf balls are typically two-piece or three-piece balls. Two-piece balls are comprised of a solid polymeric core and a cover. Three-piece balls are comprised of a central core, which may be solid or liquid filled, surrounded by a polymeric material and a cover. Three-piece balls also include wound balls. 
         [0003]    Whether one-piece, two-piece or three-piece, commercially available golf balls are made of nonmetallic materials such as elastomers, ionomer resins, polyurethanes, polyisoprenes, and nylons. Except for wound balls, these balls are made by injection molding and/or compression molding one layer around another. In order to obtain optimum playing characteristics, i.e. less hook and slice without sacrificing distances, golf ball designs are becoming increasingly complicated. Recent patents discuss four-piece balls with bands or regions having different densities. 
         [0004]    U.S. Pat. No. 6,846,248, for example, describes a golf ball having a continuous band or region formed of a material having a higher density than the remaining regions of the ball. The high-density band or region is positioned about the ball&#39;s spin axis in such a manner as to provide a gyroscopic center plane. This golf ball is not symmetrical and its performance characteristics are not symmetrical, i.e., it has a self-correcting flight pattern and would not comply with the rules of the United States Golf Association. 
         [0005]    A few very old patents discuss a ball having a hollow metal sphere of some type. See U.S. Pat. Nos. 697,816; 700,658; 713,772; 1,568,513; and 1,568,514, each of which is hereby incorporated herein by reference. However, these prior art designs suffer from several shortcomings including a hollow metal sphere design that is not durable enough to withstand impact forces from being struck by a club. For example the &#39;514 patent provides a golf ball having a hollow metal sphere comprising half shells. The half shells are only 0.005 inches in thickness with scalloped edges that are fit together but are not securely joined. The hollow metal sphere in these balls cannot withstand the impact of a golf club without permanent distortion, and the patents do not describe an efficient, cost effective method of manufacturing the golf balls. Not surprisingly, these early attempts at golf balls with hollow metal cores did not achieve commercial success. 
         [0006]    A more successful hollow-core golf ball has recently been described in U.S. Pat. Nos. 6,004,225 and 6,705,957, both of which are hereby incorporated by reference in their entireties. These patents describe a golf ball which uses a hollow metal sphere to minimize the density in the center of the ball while maximizing the density away from the center. The resulting high moment of inertia confers the characteristic of low spin for maximum distance and true flight; the ball is also less affected by green imperfections and topography for optimum rolling characteristics when putted. 
         [0007]    Hollow metal spheres suitable for use in golf balls also must have a unique set of properties for optimum performance. The sphere should have a high coefficient of restitution, i.e. it should be able to rebound from a golf club stroke with the imparted energy efficiently transformed back to kinetic energy. Other highly desirable properties are high mechanical strength, toughness, and resilience, so that the sphere can withstand repeated strikes with a golf club without fracturing, deforming, or suffering stress-induced changes in microstructure that may lead to mechanical failure or a loss in energy efficiency. The physical and mechanical properties of presently available hollow metal spheres are not optimum, and there is, accordingly, a need for metal spheres having improved performance as golf ball cores. 
         [0008]    The presence of a relatively incompressible metal core in a golf ball subjects the surrounding polymer layer to unusual conditions and stresses that are not encountered in golf balls having compressible polymer cores. When a metal core golf ball is struck by a club, the polymer layer is compressed against the metal core, and since the core does not yield to any significant degree the polymer tends to be displaced in a direction parallel to the surface of the core. An excessive amount of such displacement can break the bond between the core and the surrounding polymer layer, resulting in delamination. The polymer may also be fractured by the unusually large stresses put upon it when compressed between a club face and a metal core. There is, accordingly, a need for polymer compositions which are especially adapted for use with metal core golf balls. 
         [0009]    U.S. Pat. No. 6,710,114, which is incorporated herein by reference, discloses golf balls having a layer formed from a polymeric composite that includes at least two polymers with distinct microstructures. The polymer composite may include nanoparticles, which reportedly decrease the amount of crosslinking agent required while still providing excellent resilience. U.S. Pat. No. 6,653,382 discloses melt-processible, highly-neutralized ethylene acid copolymer thermoplastics with high resilience (high coefficient of restitution) and softness (low Atti compressions), and their use in golf ball components. 
         [0010]    Nanostructured steels, alloys, and carbides have been developed for their hardness and corrosion resistance. See for example U.S. Patent application publication Nos. 2004/0060620 and 2005/0081680, and references therein. Such materials have not previously been described as being suitable for golf ball cores. 
       SUMMARY OF THE INVENTION 
       [0011]    In broad terms, the present invention provides a golf ball comprising a one-piece metal first sphere, said metal sphere surrounded by a second sphere comprising a compressible, resilient material selected from the group consisting of natural rubber, synthetic polymer compositions, and combinations thereof, which is improved by or both of the spheres being formed from a nanostructured material. 
         [0012]    The present invention thereby provides an improved golf ball with a hollow metal spherical core which is durable, capable of maintaining structural integrity and symmetry, and has superior feel, rebound and flight trajectory. The improvements provided by the present invention are obtained by the incorporation of nanomaterials into the metal core, the surrounding polymer layer(s), or both. More specifically, the metal sphere may be made from a nanostructured metal, and one or more of the polymer layers may contain a nanostructured additive, such as a nanoclay or nanoparticulate silica. 
         [0013]    In certain embodiments of the present invention, the hollow metal sphere is made of titanium, a titanium alloy, or steel, and is surrounded by a polymer layer comprising a tough material such as ethylene (meth)acrylic acid ionomers (such as the resin marketed by DuPont under the trademark HPF™), polyether block amide (such as the resin marketed by the Arkema Group under the trademark Pebax™), and/or polybutadiene. Between about 0.1% and 10% and preferably between about 0.1% and 5% by weight of a nanoparticulate additive may be added to the polymer composition forming the polymer layer. Suitable nanoparticulate additives include, but are not limited to, nanoparticulate silica and nanoclay particles. The nanoclay material may be added in the form of a nanocomposite, e.g. nanoclay particles contained in a polymer carrier such as polypropylene (for example, the product marketed by Polyone Corporation under the trademark Nanoblend™ Concentrate  1001 ). The golf ball may optionally comprise one or more additional polymer layers, and an outer cover made from a hard, durable polymer such as that marketed by DuPont under the trademark Surlyn™. 
         [0014]    In certain other embodiments, the hollow metal sphere may comprise a nanostructured metal, such as for example the steel marketed under the trademark Nanoflex™ by Sandvik AB, Sandviken, Sweden). Nanoflex™ is a nanostructured steel that has can be heat-treated to attain especially high strength. In these embodiments, the metal sphere may be surrounded by a polymer layer as described above, with or without a nanoparticulate additive. 
         [0015]    Use of a steel material for the hollow metal sphere instead of titanium allows the use of a thinner-wall hollow metal shell and a larger diameter hollow metal core because the steel has a higher density and, for some types of steels, higher strength than titanium. A golf ball according to this embodiment may have a higher coefficient of restitution and higher moment of inertia that a golf ball having a hollow metal sphere comprising titanium or titanium alloy. 
         [0016]    Therefore, according to the present invention, a golf ball with improved performance characteristics is provided. In particular, the golf ball contains a hollow metal sphere of titanium, titanium alloy or steel, including a nanostructured steel material that is surrounded by a polymer layer. The second layer is preferably covered by a more durable material such as an ionomer. The cover material may also include a nanostructured material such as a nanoclay. Thus, the golf ball of the present invention is a durable ball that concentrates the weight and, therefore, the density away from the center of the ball to improve its performance characteristics while maintaining optimum rebound characteristics. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  is a partial cross-sectional perspective view of the golf ball of the present invention. 
           [0018]      FIG. 2  is a cross-sectional view of a three-piece golf ball of the present invention shown in  FIG. 1  wherein a second layer is placed between the sphere and the outer layer. 
           [0019]      FIG. 3  is a cross-sectional view of a two-piece golf ball of the present invention having a hard sphere surrounded by an outer layer. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0020]    U.S. Pat. Nos. 6,004,225 and 6,705,957, each of which is incorporated herein by reference, discuss a golf ball with a one-piece hollow metal sphere surrounded by a cover or a mantle layer and cover. These patents also discuss several methods for making the golf ball having a hollow metal sphere by hot forming or cold forming two halves of a sphere which are securely joined together by various welding techniques or other suitable means. This golf ball design concentrates the weight of the ball to the outer regions of the golf ball so that the ball exhibits less spin, and therefore less hook and slice when mis-hit. The one-piece hollow metal sphere, which is surrounded by at least one polymer layer, is capable of withstanding the hardest impacts from current titanium-faced golf clubs. The patents disclose at least one layer made of polymers typically used in golf balls between the hollow metal sphere and the cover. Due to the presence of the hollow metal sphere, the polymer layer and cover are subject to greater stresses than a conventional ball when struck by a club, which may result in failure of the polymeric layer. 
         [0021]    Nanostructured materials exhibit characteristics based on controlling the composition of the material at a sub-micrometer level, to vary the strength, ductility, hardness, formability, crack propagation resistance, or other physical and mechanical properties, or a combination thereof. For example, materials, including metals, such as carbon steel, stainless steel and, titanium with controlled grain sizes, may be used to make the hollow metal sphere of the golf ball of the present invention having beneficial characteristics based on grain size. Composite nanostructured materials may also be used to control the vibrational response or other characteristic of the golf ball containing a hollow metal sphere. For example, by varying the amount of second phase dispersions within a metal matrix composite, the strength and stiffness of the base material used for the sphere may be tailored. Additionally, nanostructured materials may be used to control dispersoid-dislocation interactions, such as Orowan bypassing, and Hall-Petch strengthening. Thus, nanostructured materials used as the second phase may not only carry a portion of the load on the hard sphere, but may also interact with the matrix material dislocations or grain boundaries to tailor the strength or stiffness of the sphere. 
         [0022]    The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which a preferred embodiment of the invention is shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiment set forth herein; rather, this embodiment is provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout. 
         [0023]    Referring now to  FIGS. 1 and 2 , an improved golf ball according to one embodiment of the present invention is illustrated. The golf ball  1  includes a hollow metal sphere  3  surrounded by a second polymer layer  4 . The sphere  3  may also be made of any other hard material as discussed in U.S. Pat. Nos. 6,004,225 and 6,705,957. The second layer may be surrounded by an outer cover  5 . 
         [0024]    The hollow metal sphere  3  may be made of titanium, titanium alloys, or steel, including carbon steel, stainless steel and steel alloys with an outside diameter ranging from about 1.0 to 1.50 inches (about 2.54 to 3.8 centimeters), and a thickness of about 0.02 to 0.16 inches (0.05 to 0.41 centimeters) and more preferably about 0.02 to 0.08 inches (0.05 to 0.20 centimeters). A preferred steel is an austenitic stainless steel which has very high tensile strength and is hardenable by heat treatment from the annealed condition with no softening after exposure to temperatures of up to 450° C. The steel material preferably has a density of about 7.9 g/cm 3  and a tensile strength at 20° C. of about 130 to 270 ksi after being cold rolled and about 200 to 370 ksi after being cold rolled and aged. More specifically, the steel may have a tensile strength of about 360 ksi when cold worked to 1650 MPa and aged at 475° C. for 4 hours. In an annealed condition, the steel material has an austenitic microstructure which is partially or completely transformed to a martensitic matrix after hardening by cold rolling the steel material. The steel material is then aged, preferably for between about 2 to 6 hours and more preferably about 4 hours at a temperature of between about 375° C. to 575° C. and more preferably about 475° C. An example of a nanostructured steel having these properties is the material manufactured by Sandivk AB of Sweden and sold under the trademark Nanoflex™. 
         [0025]    Because of the high strength of steel and the corresponding difficulty in forming such materials, it is preferred that the steel is in an annealed state prior to forming and welding, and then heat treated to maximize strength after the spheres are formed. Preferably, the materials are annealed by subjecting the material to an elevated temperature (preferably between about 300° C. and 1500° C. and more preferably between about 500° C. and 1300° C.) for a predetermined period of time, preferably about 2 minutes to 24 hours and more preferably from about 2 minutes to 6 hours, and then allowed to cool slowly for a predetermined period of time, preferably between about 5 minutes to 24 hours and more preferably between about 30 minutes to 6 hours. Typically steels, including carbon steel, alloy steel, such as 300M and stainless steels and nanostructured steels may be annealed at temperatures greater than about 500° C. and more preferably at temperatures greater than about 800° C. and held at temperature from about 2 minutes to 12 hours. After annealing the steel in the form or a thin plate or sheet having a thickness approximately equal to the predetermined thickness of the hollow metal sphere, the steel is stamped or formed into spherical portions, preferably symmetrical half spheres or shells. The half spheres or shells are then welded together to form a hollow metal sphere, preferably by electron beam welding. After welding, the hollow metal sphere is heat treated to increase hardness and strength. The heat treating step is performed by heating the hollow metal sphere to between about 300° C. to 1000° C. and more preferably between about 400° C. to 800° C. for a predetermined time period of preferably 30 minutes to 24 hours an more preferably between about 1 hour to 8 hours. The hollow metal spheres are preferably cooled quickly at a predetermine rate of about 100-1000° C. per second or more preferably from 200-500° C. per second until reaching about 25° C. The hollow metal spheres may be cooled in any suitable medium including air, inert gases such as nitrogen, oil, and other suitable coolants. The resulting hollow metal sphere will have a yield strength greater than about 450 MPa, 550 MPa, 650 MPa, and more preferably greater than about 750 MPa. In some instances the hollow metal sphere will have a yield strength that is greater than 950 MPa. In preferred embodiments, the Nanoflex material is annealed and heat treated as according to the manufacturer&#39;s recommendations. 
         [0026]    The second layer  4  is a polymer material, such as as ethylene (meth)acrylic acid ionomers (such as DuPont&#39;s HPF™ resin), a polyether block amide (such as Pebax™ resin made by the Arkema Group), and/or polybutadiene. The second layer  4  is preferably of molded construction. The second layer generally has an outside diameter of about 1.52 to 1.60 inches (3.86 to 4.06 centimeters) and a thickness of 0.05 to 0.65 inches (0.13 to 1.65 centimeters) and more preferably about 0.21 to 0.55 inches (0.53 to 1.4 centimeters). The outer cover  5  may be an ionomer, urethane, balata, or synthetic elastomer. The outer cover also features a dimple pattern as is well-known in the art. 
         [0027]    Yet another embodiment of the improved golf ball  1  according to one embodiment of the present invention is illustrated in  FIG. 3 . In this embodiment, the golf ball  1  includes a hard sphere  3 , as described above, surrounded by an outer cover  5  without an intermediate second layer. The sphere is preferably a nanostructured metal, more preferably titanium or stainless steel. This embodiment provides the greatest moment of inertia, less spin, greater rebound and, therefore, greater distance. 
         [0028]    In one embodiment of the invention as shown in  FIG. 2 , the hollow sphere  3  is not filled with any solid or liquid material. Thus, the hollow sphere contains a residual gas  2  such as air. The hollow sphere may also contain pressurized gas. In another embodiment, the hollow metal sphere contains only a small amount of residual gas because the hollow metal sphere is formed by electron beam welding under vacuum conditions. EBW is preferably carried out in a vacuum of less than about 0.01 mbar, but may be performed in lesser vacuums of 0.1 to about 1 mbar or even 10 mbar, to reduce the vacuum load and increase processing rates. After welding under a vacuum, the pressure exerted by the gas in the hollow metal sphere will be about the same as that of the vacuum under which the hollow metal sphere was welded, i.e., less than about 10 mbar, 1 mbar or even 0.01 mbar at a temperature of about 20° C. The hollow metal sphere may be further processed by drilling a very small hole or otherwise perforating the hollow metal sphere to produce a hole of between about 1 mm and 5 mm, or any other size sufficient to release the vacuum, thereby reducing the stress on the hollow metal sphere. This step may occur prior to heat treating the hollow metal spheres. 
         [0029]    In some embodiments, the sphere may be filled with any liquid, such as water, aqueous solutions of various solutes, vegetable and mineral oils, hydrocarbons and halocarbons. Liquid polymers may be introduced and polymerized in situ, or hot molten solids may be introduced and allowed to cool, to provide a solid core. Because a perforated sphere may release gas during the molding of the polymer sphere, thereby disrupting the bonding between the spheres, it is preferable that the sphere remain hermetically sealed if it is to be gas-filled. 
         [0030]    As described above, the improved golf ball of the present invention provides improved performance characteristics including low spin rate, long distance, and bite without adversely affecting rebound characteristics. The ball of the present invention minimizes hook and slice when improperly hit. The design of the golf ball allows variations in the material and the size of the sphere, second layer, and outer cover in order to optimize performance characteristics, and the incorporation of nanomaterials provides added strength and resilience to the metal and/or polymer components of the ball. 
         [0031]    The golf ball  1  may be made using conventional processes and techniques as are presently employed in the art such as injection molding and/or compression molding so that the ball will be spherical in shape, have equal aerodynamic properties, and have equal moments of inertia about any axis through its center. If nanostructured materials are incorporated into to a polymer that is injection molded, increasing the screw and back pressure during injection molding may improve dispersion of the material into the polymer. 
         [0032]    The hollow metal sphere  3  may be manufactured by forming two halves of a sphere by hot forming or cold forming which are then joined together by welding or other means sufficient to securely join the halves of the sphere together. Preferably, symmetrical halves of a metal sphere are formed by stamping. Other methods of forming the symmetrical halves of the sphere include hydroforming, metal spinning, and superplastic forming. The two halves are then preferably secured together by electron beam welding in a vacuum. If nanosteel is used to make the hollow metal sphere, the nanosteel should be annealed prior to stamping by increasing the material&#39;s temperature to about 1024° C. to 1052° C. After stamping, the material is heat-treated at temperatures of about 375° C. to 575° C. and preferably 475° C. for about 3 to 6 hours and preferably for about 4 hours to obtain a specified strength and then the two halves may be secured together preferably by electron beam welding. Alternatively, the hollow metal sphere can be formed by laser welding, electrical resistance welding or metal gluing two portions of a sphere together. The outer layer and, if desired, the second layer are injection or compression molded around the sphere using techniques that are well-known in the art. 
         [0033]    For example, Electron Beam Welding (EBW) uses a highly focused beam of electrons to locally heat the metal and cause a weld. In preferred embodiments, hemispheres are designed such that no weld filler is required, and material at the weld seam is fused via the EBW process to join the hemispheres. Typically, extra material is provided in the hemispheres to compensate for shrinkage that occurs when the material is subjected to the electron beam. Preferably, about 0.001 inches to 0.010 inches and more preferably 0.002 inches to 0.005 inches extra material is provided per hemisphere at the weld seam. The design in made such that after welding, the resulting shrinkage leads to a very spherical final part. Of course, weld filler may be used, but the preferred process is more cost effective as a feed system for the filler is not required. 
         [0034]    In EBW, electrons are extracted from a thermally activated cathode, accelerated by a significantly high voltage potential—preferably ranging from about 25,000 to 250,000 volts, and focused on the piece to be welded. EBW is typically performed in a vacuum, preferably a vacuum less than 0.01 mbar, but it may be performed in lower vacuums of less than 1 mbar or even 10 mbar, to reduce the vacuum load and increase processing rates. Hollow metal spheres that are formed by EBW do not have any undercut in the weld area. In other words, the weld area or seam has the identical or almost identical thickness as the adjacent sphere wall. After forming the hollow metal sphere, one or more polymer layers may be injection or compression molded around the hollow metal sphere. An adhesive coating may be applied to the outer surface of the hollow metal sphere prior to molding a polymer layer directly onto the hollow metal sphere. 
         [0035]    Examples of golf balls made according to the present invention are described in the following Examples. 
       EXAMPLE 1 
     Three Piece Ball 
     Titanium (Grade 2) Core, Second Polymer Layer Comprising a Dupont HPF 1000 or HPF 2000 Resin Blended with a Nanoclay Material (PolyOne™ Nanoblend™Concentrate), and a Surlyn Cover 
       [0036]    A hollow sphere comprising a titanium shell with an inside diameter of 1.108 inches (2.814 centimeters) and an outside diameter of 1.25 inches (3.175 centimeters), a specific gravity of 4.51 and a mass of 0.808 ounces (22.93 grams) is prepared by electron beam welding of two hemispheres. HPF resin (specific gravity=0.96) with a layer thickness of 0.153 inches (0.387 centimeters) and amass of 0.530 ounces (15.0 grams) containing between 0.1-10% by weight of nanoclay material in a polypropylene carrier (such as PolyOne® Nanoblend™ Concentrate  1001 —specific gravity of about 1.1) is then molded about the metal core. A cover of SURLYN™ (specific gravity of about 0.95) cover of thickness 0.0625 inches (0.159 centimeters), and a mass of 0.282 ounces (7.997 grams) is formed by routine methods. The pressure of the residual gas or air in the hollow metal sphere is less than about 1 mbar. The total mass of the ball is 1.620 ounces (45.93 grams). The moment of inertia of the ball of Example 1 is about 9.5% percent greater than a typical two-piece ball. 
       EXAMPLE 2 
     Three Piece Ball 
     Hollow Nanoflex™ Sphere, Second Layer of Polybutadiene, and a SURLYN™Cover 
       [0037]    A hollow sphere comprising a precipitation hardenable, austenitic stainless steel (Nanoflex™ manufactured by Sandvik AB) with an inside diameter of about 1.17 inches (2.98 centimeters) and an outside diameter of about 1.25 inches (3.18 centimeters), a density of about 7.9 and a mass of about 0.819 ounces (23.23 grams) is prepared by electron beam welding of two hemispheres. Polybutadiene (specific gravity of about 0.95, Shore D  25 ) with a layer thickness of about 0.15 inches (0.39 centimeters) and a mass of about 0.520 ounces (14.70 grams) is molded around the core. SURLYN™ (specific gravity of about 0.95) cover of thickness 0.0625 inches (0.159 centimeters), and a mass of about 0.28 ounces (8.0 grams) is then formed into an outer cover. The pressure of the residual gas or air in the hollow metal sphere is less than about 1 mbar. The total mass of the ball is 1.620 ounces (45.93 grams). The moment of inertia of the ball of Example 2 is about 11.4% percent greater than a typical two-piece ball. 
       EXAMPLE 3 
     Three Piece Ball 
     Titanium (Grade 2) Core, Second Layer of a Polyether Block Amide (e.g., Pebax™ by Arkema), and a Surlyn™ Cover 
       [0038]    A hollow sphere comprising a titanium shell with an inside diameter of about 1.2 inches (3.07 centimeters) and an outside diameter of about 1.35 inches (3.429 centimeters), a specific gravity of 4.5 and a mass of about 0.95 ounces (26.92 grams) is prepared by welding two hemispheres. Polyether block amide (specific gravity of about 1.0) with a layer thickness of about 0.1 inches (0.260 centimeters) and a mass of about 0.39 ounces (11.09 grams) is molded around the core. SURLYN™ (specific gravity of about 0.94) cover thickness of about 0.0625 inches (0.159 centimeters), and a mass of about 0.279 ounces (7.913 grams) is then formed into an outer cover. The pressure of the residual gas or air in the hollow metal sphere is less than about 1 mbar. The total mass of the ball is 1.620 ounces (45.93 grams). The moment of inertia of the ball of Example 2 is about 16.8% percent greater than a typical two-piece ball. 
         [0039]    Balls prepared by the methods and using the materials described above were found to be durable and not prone to deformation or delamination upon being struck repeatedly with a golf club, and their improved flight characteristics remained stable. The metal cores were resistant to fracture or deformation and retained their coefficient of restitution under repeated use.