The present invention concerns a method for the manufacture of a tension bar intended to form part of a rock bolt according to the introduction to claim 1. The invention concerns also a dynamic rock bolt according to the introduction to claim 8.

The insertion of rock bolts is the most common reinforcement of fracture-rich rock in mines and tunnels in order to achieve a reinforced ceiling. The rock bolts are introduced into boreholes and subsequently secured. Millions of rock bolts are consumed throughout the world every year. One fundamental requirement for rock bolts is that they are to be cost-effective, i.e. cheap to manufacture and able to support a heavy load. Rock bolts are usually dimensioned for rock reinforcement based on the calculated stationary static load. Rock, however, can seldom be regarded as ideally static: the rock will behave in a more or less dynamic manner due to motion and the formation of cracks. This means that rock bolts are designed and dimensioned based also on the dynamic effects of loading that arise. The term "rock bolt of dynamic type" is used to denote rock bolts with a particularly high capacity to absorb and support motions that arise in rock.

One cost-effective rock bolt of the type that is intended to be embedded in grout is the type known as a "ribbed" rock bolt, which comprises a hot-rolled rod of solid steel that, in the same manner as reinforcement rods used for concrete, is provided with interaction means or anchors in the form of radially protruding force-transfer cams (ribs/flanges/protrusions) that, in the form of a texture or surface unevenness that is formed, extend in the direction transverse to the rod out from the surface or periphery of the rod. The rock bolt is intended to be inserted into a borehole that has previously been filled with grout such that the ribbed rock bolt is surrounded by grout in the borehole. The ribbed rock bolt is provided at the opening of the borehole with an end fitting, normally in the form of a washer and nut, that is placed in contact with the area of rock that surrounds the opening of the borehole, with which end fitting the rock bolt can be given a certain degree of prestress. The yield strength of the steel of the said type of hot-rolled rock bolt is normally approximately 250-600 MPa.

In order to support dynamic load, i.e. load from rock material in motion, it has proved to be necessary to achieve a rock bolt with a higher strength and the ability to withstand a significant deformation and strain along its length. An improved resistance to deformation is particularly interesting for rock bolts that are used in fracture-rich rock, where a rock bolt is locally placed under load at locations at which it traverses wide cracks between blocks. In order to use the ability of the rock bolt to absorb strain efficiently, the parts of a rock bolt that traverses cracks between blocks in the rock must allow a significant degree of deformation and extension. The term "extension" is used in this part to denote not only deformation within the elastic region but also within the plastic region, giving permanent deformation. What is important, however, is that the rock bolt is firmly anchored by the grout in the relevant block between the cracks that appear. The insertion of rock bolts is nowadays essentially mechanised. The rock bolts are normally arranged in a straight line in a magazine at a vehicle, and can be displaced by means of a gripper to an outlet opening in the magazine. The rock bolts are gripped by means of pivoting arm, and are introduced into the line of a drilled axis in order to be inserted in a linear manner into the borehole. One further fundamental requirement for rock bolts is that they be adapted for mechanised insertion, whereby a rectilinear rock bolt is to be preferred.

Rock bolts are normally placed under the greatest load in the region of the wall surface of the borehole, whereby the load becomes less in the direction towards the bottom of the borehole. The fact that the rock bolts are placed under the greatest load at the region close to the wall surface means that breakages at the thread or nut are common. The rock bolt is, furthermore, in fracture-rich rock placed under local load at locations at which the rock bolt crosses fractures in the rock, whereby the rock bolt must demonstrate high load-bearing capacity and capacity for deformation along its complete length in order to function in the intended manner in fracture-rich rock.

A rock bolt for embedding in grout in rock is known from SE 532 203. This rock bolt comprises an extended circularly cylindrical solid shaft or rod with a threaded part at a principal end. The shaft comprises alternating shaft parts and anchor parts along its length. The said anchor parts are formed through compression strain or cold deformation, whereby the shaft has been reshaped in certain parts to broadened integrated anchors: through redistribution of the metallic material in the transverse or radial direction, the rod has been flattened in local planes that are oriented perpendicular to the longitudinal direction of the rod, and in this way the rod acquires an irregular form that makes it unsuitable for mechanical insertion. The shaft parts, which are significantly longer, according to the description at least 10 times longer than the anchor parts, are intended to glide relative to the grout in the borehole and to absorb local tensile strain between locally anchored anchors, which strain is caused by rock deformation.

The shaft demonstrates in this case extensive dimensional transitions with different cross- sectional profiles along the central section of its longitudinal axis, the cross-sectional forms of whose parts differ significantly from the general central section of the rod, and can in this way give rise to damage from metal fatigue. The term "central section" is here used to denote in principle the circularly cylindrical cross sections of the shaft parts or their general profile where these constitute the principal part of the shaft. The said compression-strained anchor parts demonstrate higher yield strength than the neighbouring circularly cylindrical shaft parts.

Due to the many anchor points of the rock bolt through the said anchors a considerably more secure rock bolt than the type of known rock bolt that is anchored only at an individual point along its length is obtained, where the known rock bolt has a shaft with a smooth surface, for which a fracture at the said anchor may lead to complete loss of the load-bearing capacity of the rock bolt.

It is, however, well-known that damage of rock bolts from metal fatigue is initiated and develops in regions where the strain is greatest, typically at the surface at dimensional transitions and sharp grooves, at joints, and at external and internal defects in the material. It is therefore important to eliminate by design, as far as is possible, each such peak of stress that arises. Metallic materials are sensitive for fatigue under dynamic loading and under cyclical variations of load. Damage can arise after repeated variations in load whose magnitudes may lie well below the static strength of the material. It is well-known that the selection of material is of less significance in the case of dynamic metal fatigue, and that the risk of damage is determined principally by the construction design. It is, therefore, important to minimise the number of initiation sites at the surface in order to achieve high fatigue strength. This is achieved by aspiring to produce as low a surface roughness, as few grooves, and as few dimensional transitions as possible. Cold-working in rolling mills makes it possible to achieve very good surface properties with a high surface finish.

One purpose of the present invention, therefore, is to achieve a method for the manufacture of a deformable and cost-effective rod or tensile element that is a component of a rock bolt, which rod has a particular ability to absorb dynamic loads and to resist a considerable degree of deformation, i.e. a large degree of bending along its length. A second purpose is to provide a compact and simple rock bolt that can absorb dynamic loads and is, in addition, suitable for mechanised handling and insertion into boreholes from a magazine mounted on a machine.

These purposes of the invention are achieved through a rod for a rock bolt manufactured according to the measures and actions that are specified in claim 1 , and a rock bolt that demonstrates the distinctive features and characteristics that are specified in claim 8.

Through a process known as "forging stretching", a linear rod that is a component of the rock bolt is cold-worked in its longitudinal direction with a degree of processing that differs locally along its length, such that the rod demonstrates, with respect to its central axis, a cross-sectional profile of constant form that forms along its length an alternating series of:

anchor parts with a texture that improves the adhesion of the part in the grout and whose degree of processing may be selected such that a higher yield strength is achieved, and

shaft parts with a surface configuration that improves the ability of the part to glide relative to the grout and whose degree of processing may be selected such that a lower yield strength is achieved, and

that the said locally processed parts are formed along partial pre-determined lengths of the rod.

The degree of forging stretching, which describes the amount by which the cross-sectional area of the rod blank is reduced while the blank becomes thinner and longer during the forging, may be changed, depending on the construction design and the quality of the steel. It may be appropriate in certain cases within the framework of the invention that the local degree of processing used to form the said shaft parts is, in practice zero, i.e. so low, or omitted, that the shaft parts remain unaffected during the cold-working steps. The yield strength specifies the highest stress that a steel can withstand without undergoing plastic deformation. The steel undergoes elastic deformation when subjected to stresses that are lower than the yield strength. The yield strength of steel is defined as the stress that gives a permanent deformation of 0.2% after the load has been removed.

The expression "texture" is here used to denote a formation on the surface periphery of the rod that has been produced in certain regions by plastic processing (cold-working), such as, for example, radially protruding force-transfer cams (ribs/flanges/protrusions) that extend in the transverse direction to the rod. In accordance with the invention, the rod demonstrates an essentially constant and regular cross-sectional profile along its longitudinal direction, i.e. when divided into parts, the shaft parts and anchor parts demonstrate essentially equivalent profile forms, with the difference that a regular embossed surface texture has been associated with the anchor parts for interaction with the grout, in the form of radially protruding force-transfer cams.

A rock bolt is in this way achieved that demonstrates essentially a lack of dimensional transitions with respect to diameter between anchor parts and shaft parts, whereby fractures that are a result of metal fatigue that arises from changes in load that arise can be minimised. The anchor parts constitute the relatively stronger elements of the rock bolt and are consequently less sensitive for deformation or fracture when under load, whereby the required anchoring effect is obtained. The shaft parts have a relatively lower degree of cold- working and a smooth surface configuration, and can glide relative to the grout. They can in this way be extended when under strain and their length can increase when varying loads arise.

As a consequence of the forging stretching, dimensional transitions and sharp grooves that arise can be reduced and in this way also the risk of damage of the rod as a result of metal fatigue from dynamic variations in load. In one execution, the threaded part of the rod at its principal end can be hardened through, for example, induction heating followed by rapid cooling, and the rock bolt may be equipped with a nut of higher steel quality.

The invention is described in more detail below with the guidance of a non-limiting embodiment that is shown in the attached drawings, in which:

Figure 1 shows a perspective view of a rock bolt according to the invention,

Figure 2 shows a longitudinal sectional view of a rock bolt according to the invention, inserted into a borehole in a rock wall with a fracture that is illustrated, and surrounded by grout in the borehole,

Figure 3 shows schematically a longitudinal view of a smooth circularly cylindrical solid blank in the form of a rod that, passing between two opposing rollers in a planetary roller mill, is provided with alternating shaft parts and anchor parts with pre- determined lengths along the length of the rod blank, Figure 3A shows an enlarged partial view of a texture produced by cold-working in the planetary roller mill, which texture is of local deeper areas, the task of which is to act as gripping means in the grout in the borehole,

Figure 3B shows an enlargement of a metallographic view of a longitudinal section of an area that has been surrounded by a ring with random grain orientation in a hot-rolled blank,

Figure 3C shows an enlargement of a metallographic view of a longitudinal section of an area that has been surrounded by a ring with preferred grain orientation in a cold-rolled condition, and grains that have been extended in the direction of rolling.

A rock bolt according to the present invention intended to be embedded in a borehole in rock with grout that may consist of a rapidly hardening artificial resin or concrete is shown in Figure 1. The rock bolt generally comprises a shaft in the form of an extended cylindrical solid rod 1 with a threaded part 2 with a nut 3 and a washer 4 at a principal end. The rod 1 comprises a series of shaft parts Is, each one of which has been given a length Ls that has been determined in advance. The said shaft parts 1 s are followed in an alternating manner by anchor parts 1 a, each one of which has been given a length La that has been determined in advance. As is made clear by the drawing, the shaft parts 1 s and the anchor parts la are distributed along the complete length of the rod 1 in an alternating manner. Each anchor part l a is intended to be anchored locally in the grout in order to absorb, in engagement with the rock, load that is caused by rock deformation, while the shaft parts 1 s are intended to glide relative to the grout in the borehole in order in this way to absorb dynamic load i.e. local tensile loads that arise between the said anchor parts la that are located consecutively and that are anchored locally in the grout and this way anchored locally in the rock. Each shaft part ls may demonstrate a length Ls that is equal to or greater than the length La of the anchor parts la, i.e. the length Ls of the shaft parts may be greater than the length La of the anchor parts, (Ls≥La).

The rock bolt, which is manufactured from carbon steel, is arranged for effective and secure anchoring at two of the most critical points of rock reinforcement, namely at the bottom 5 of borehole and in close association with the external wall surface 6 of the borehole in the rock. The anchor parts 1 a are in this case so distributed along the rod 1 that the resulting rock bolt demonstrates not only an anchor part 1 a at the part of the rod 1 that is terminated at the principal end, which end is provided with a threaded part 2, but demonstrates also an anchor part 1 a at the forward end of the rod 1 that is intended to be located at the farthest depth of the borehole in the direction towards the bottom 5 of the borehole. The term "carbon steel" is used to denote a steel whose principal component of the alloy, in addition to iron, is carbon. Silicon and manganese may be present in the alloy, in addition to carbon. The level of carbon is normally 0.01 %-0.8%: the level of silicon is below 0.3%: and the level of manganese below 0.8%.

The rock bolt is shown in Figure 2 inserted into a borehole in a rock wall, whereby a fracture

8 has been included in the drawing in order to illustrate the type of gliding and separation that, depending on the particular rock material, may occur along the fracture plane in the rock. Such fractures 8 mean that induced loads on the rock bolt will arise, with the appearance of more or less dynamic axial bolt forces, as a consequence of relative motion between neighbouring blocks. Series of rock bolts contribute to maintaining potentially unstable blocks in their location, whereby the task of the rock bolts is together to "sew" several blocks together.

In order for the rock bolt to resist dynamic loading, it is important that neighbouring blocks that are separated by a fracture are well anchored or, to be more precise, in engagement with the anchor parts 1 a of the shaft 1. A fracture 8 that is present in the rock, which fracture may in certain cases demonstrate a large width, must allow the shaft, in the event that variations in load arise, to be extended and to return to its original length, i.e. to undergo elastic deformation. Due to the fact that the shaft parts 1 s have been given a smooth surface configuration that can glide relative to the grout in the rock, motion between neighbouring blocks in a fracture can be absorbed by exploiting the ability of the rock bolt also to absorb varying loads. The shaft parts Is between the anchors la will in this way glide relative only to the grout in the borehole. This effect cannot be achieved by rock reinforcement with conventional rock bolts of the type known as "ribbed iron" type that are provided with transverse ribs along the complete length and in this way are fixed securely in the grout. As a consequence of this, the said known rock bolts of ribbed steel type fail at relatively low strains or extensions. As has been described above, the rod of the rock bolt is engaged with the grout, and in this way with the rock, through the said anchor parts 1 a, while the rod is allowed to be freely bent relative to the grout and the rock by means of the shaft parts 1 s that lie between them.

In order to maximise the load-absorbing properties, and in particular the dynamic load- absorbing properties, of the rock bolt, it is important that the rod 1 is designed in such a manner that it is allowed, by gliding freely relative to the grout, to bend elastically or to float or to be plastically deformed along the shaft parts Is and before any one of the anchor parts la fails. In order for this to be possible, there is proposed, according to the invention, a new method for the manufacture of the extended cylindrical rod 1 or shaft of the rock bolt that makes it possible to produce in a cost-effective manner a tension bar for such a rock bolt.

The term "cut surface at the reinforcement rod" is used to denote a part that has been cut off perpendicular to the longitudinal axis of the rod. It can be mentioned that the texture of cams and fins that are present on the surrounding periphery or outer surface of the rod is not normally considered to be a part of the cut surface in material of ribbed rod type.

With reference to Figure 3, there is schematically shown a smooth circularly cylindrical solid blank in the form of a rod that, passing between two opposing rollers in a planetary roller mill, is provided with alternating shaft parts 1 s and anchor parts 1 a with lengths along the length of the rod blank that have been determined in advance, in order to form a rod intended to be a component of a rock bolt. Planetary rolling technology for the manufacture of steel rods has long been known and will for this reason not be described in detail. It should be realised that the planetary roller mill shown in the drawing is only schematically depicted. Planetary roller mills of the type that allows the blank to be processed in a triangular manner from three opposing sides can be advantageously used for the present invention. Such a known planetary roller mill uses three conical rollers that are arranged at angles of 120° to each other.

The planetary roller mill in Figure 3 is operated as a cold roller mill and comprises a pair of roller stands with a pair of support rollers 10, 11 mounted in bearings in the pair of roller stands and groups of working rollers, generally denoted by the reference number 12, which working rollers surround the relevant support rollers. The blank is fed into a deformation zone in which plastic deformation takes place in order to reshape the blank to a rod intended to be a component of a rock bolt according to the invention. The blank is fed into the deformation zone in known manner by an introducer and possibly, in relevant cases, by a drawer, not shown in the drawings. The working rollers 12 are evenly distributed around the circumference of each support roller 10, 11 and are secured in place by means of holders, not shown in the drawings. Each working roller 12 has a peripheral contour in the form of the arc of a circle and is inserted into a cavity or opening in the relevant support roller 10, 11.

Thus two principal types of working roller 12 are used during the rolling operation: not only smooth rollers for the rolling of the shaft parts 1 s, but also grooved or profiled rollers for the rolling of the anchor parts la. Depending on the choice of form, the depth of the arranged grooves and the openings in the subsequent working rollers, the blank is gradually given not only the desired reduction in area but also an associated texture during its passage through the gap between the opposing working rollers. Figure 3A displays in an enlargement of detail the said texture stamped onto the steel blank after its passage through the rolling mill.

Each support roller 10, 11 is, in this embodiment, provided with four working rollers 12a distributed around the circumference, which working rollers are designed as segments of working roller with a texture that extends along an arc of a circle that corresponds to the linear length of an anchor part la produced on the blank. Each support roller 10, 11 is equipped in a corresponding manner with smooth working rollers 12s in the form of segments of working roller, the lengths of which correspond to the linear length of a shaft part 1 s produced on the blank. Through the choice of an appropriate number of working rollers 12a, 12s (segments of working roller) and the length of the arc of a circle arranged for the deformation, the resulting rod after its passage through the deformation zone obtains alternating shaft parts 1 s and anchor parts 1 a with lengths Ls and La, respectively, that have been determined in advance, and the desired texture and reduction in area.

Not only the anchor parts la but also the shaft parts ls can be rolled to a cylindrical configuration with the absence of dimensional transitions in the radial direction out from the principal axis of the rod 1 between the said parts, whereby the anchor parts on the surface may demonstrate a texture of parts that have been formed to transverse cams, the task of which is to interact with the grout, while the shaft parts demonstrate a texture with a smooth surface configuration intended to glide relative to the grout. The anchor parts 1 a and the shaft parts 1 s can be rolled to different lengths or to the same length.

The cold-working can be carried out without significant reduction in the cross-sectional area, or the cold-working can be carried out with a significant reduction in cross-sectional area. The anchor parts la can, in one design, be rolled with a significant reduction in cross-sectional area while the shaft parts 1 s are rolled without a significant reduction in cross-sectional area.

The anchor parts 1 a may demonstrate a texture with radially protruding force-transfer cam regions on the surface while the shaft parts Is demonstrate a smooth configuration on the surface, whereby the said parts with texture of depressions and the parts with a smooth surface configuration are formed along lengths La; Ls of rod with lengths that have been determined in advance.

Through choice of the local degree of cold- working in the deformation zone to form the said shaft parts, it is possible to control and monitor the degrees of processing of the different parts in such a manner that they differ between the parts. The degree of deformation of the shaft parts, for example, may be lower or close to zero, while the degree of deformation of the anchor parts la is higher, such that these parts acquire a higher yield strength.

It is generally known that hot-processed steel demonstrates a microstructure with grains whose orientations are random. This means that the crystal structure of the grain differs from grain to grain. Such a random orientation means that the mechanical properties of the steel are isotropic, i.e. equal in all directions. In contrast, during the cold-working of steel, the grains whose shape is changed orient themselves permanently in the manner in which they are deformed, i.e. they acquire an orientation that has been pre- determined and that is determined by the direction of rolling. This means that the deformed parts of the steel will be anisotropic, with mechanical properties that differ in different directions. This type of processing is known as "stretching" and means that the cross- sectional area of the rod blank is reduced, i.e. the blank becomes narrower and longer. Grains that are textured in this manner become stronger, i.e. they demonstrate a greater modulus of elasticity along the direction of the preferred orientation than grains with random orientations. Cold-rolling of a blank of steel rod material forms the grains in the steel rod such that they are extended and become reoriented with a preferred orientation that is parallel to the longitudinal axis of the rod blank. The material acquires what is known as a "flow line" that follows the form of the steel rod and results in the rod being strongest along its longitudinal axis, i.e. the direction of rolling extension of the rod. The term "cold-working" is used below to denote a process into which the material under treatment is introduced without preheating, and during which the temperature of the material remains below the recrystallisation temperature during the processing phase. The term "cold-working" is used below to denote a plastic reduction in area or the local forming of a material that takes place at room temperature or in any case below the recrystallisation temperature of the material, which is approximately 600 °C for carbon steel. Grains that have been textured are stronger in the direction of the flow lines, i.e. they demonstrate a higher modulus of elasticity in the longitudinal direction than in the transverse direction. Cold-drawn rods of the type described above can acquire yield strengths of up to 1300-1600 MPa, depending on the degree of deformation. Strengthening of the material occurs due to changes in dislocations in the crystal structure of the material.

Figure 3B shows an enlargement of a metallographic view of a longitudinal section of an area that has been surrounded by a ring with random grain orientation in a hot-rolled blank; while Figure 3C shows a metallographic view of a longitudinal section of an area that has been surrounded by a ring with preferred grain orientation in a cold-rolled condition, and grains that have been extended in the direction of rolling.

One advantage of cold-worked products is, furthermore, that the surface has a better surface finish than the surface of products that are manufactured by a hot- working process. There is, among other things, no oxide scale, which is generally found on hot-worked products. Microscopic investigation of cold-worked products shows a clear deformation of the crystals and a crystal orientation that is parallel to the direction of working. Cold-working of a steel material means that the austenite phase of the material is converted to martensite. The result is that the strength of the material can be significantly increased while it retains its high ductility.

It is known that the addition of alloy materials such as carbon, manganese and nickel can increase the area in which austenite forms in carbon steel. Given sufficiently high levels of addition of these substances in iron alloys, the area in which austenite forms can be increased to room temperature whereby what is known as "austenite steel" is obtained, which is steel that has austenitic properties at room temperature. What is characteristic for austenitic steels is the deformation hardening that occurs during cold-working, whereby they acquire considerably increased strength, even for relatively low degrees of processing, such as, for example, 1-10%. Through the formation of alloys that contain chromium, nickel, molybdenum and nitrogen, austenitic stainless steel is obtained, which means that the rock bolt will be able to resist corrosion. Stainless steels that form martensite during cold deformation are specified to be metastable, in which case the strength is the result of a changed microstructure.

The invention is not limited to what has been described above and shown in the drawings: it can be changed and modified in several different ways within the scope of the innovative concept defined by the attached patent claims.