EA040646B1 - METHOD FOR PROVIDING CONTROLLED DESTRUCTION OF ANCHOR BOLT ROD - Google Patents

Description

State of the art

The present invention relates to a method for providing controlled destruction of an anchor bolt shaft.

If, during installation, the anchor bolt is subjected to a load exceeding its tensile strength, it may break, resulting in loss of support for the load. The danger is that the torn off proximal section of the anchor bolt (hereinafter, the terms rod and bolt are used interchangeably to refer to the steel anchor bolt, its rod or rod-like body) is prone to being pushed out of the hole in the rock in which it was installed due to rapid release of potential energy.

It is important that when the anchor bolt breaks, this break occurs at a given location on the rod. Stem break point control is important to the safety solution proposed in South African Patent Application No. 2-18/02957, which is incorporated herein by reference.

Description 2018/02957 describes making a recess in the rod to initiate a rupture of the rod at a specific location remote from the safety element to ensure that the broken or flying section of the rod is retained as a result of the interaction of the safety element with the safety structure.

It is common to make a recess in the rod by hot or cold forging. This creates the effect of local hardening of the steel of the rod. A related problem is that the anchor bolt's support properties are based on the inherent ability of a steel bolt to stretch under load, absorbing energy and regulating the dynamic or quasi-static movement of the rock surrounding the bolt at its installation site. Cold forging or rolling of a bolt while creating a recess can result in localized work hardening that changes the properties of the steel material. This change in material properties, in turn, leads to a change in the performance of the anchor bolt.

The present invention at least partially solves the above problem.

The essence of the invention

The present invention proposes a method of adapting a metal rod intended for use as an anchor bolt to provide a predictable break of the specified rod; comprising the step of removing material from the rod in the form of a circumferential strip to reduce the diameter of the rod within said strip to a reduced diameter and thus provide a fracture zone within which the rod will break under a sufficient tensile load.

Said zone may have a length in the range of 10 to 45 mm.

Preferably said zone has a length in the range of 25 to 35 mm. More preferably in the range of 30 to 35 mm.

The fracture zone may have a reduced diameter, which is less than the diameter of the rod by 0.5 to 5%. Preferably, the reduced diameter is smaller than the rod diameter by 1 to 4%. More preferably, the reduced diameter is less than the rod diameter by 2 to 3%.

The material is removed in such a way as to ensure the stability of the reduced diameter within the specified band.

Preferably, the rod diameter is in the range of 16 to 25 mm.

The material may be removed by a subtractive machining process in any suitable manner, such as by cutting or grinding.

Brief description of the drawings

The present invention will be described with reference to the following drawings, in which: FIG. 1 is a vertical view of the leading end of an anchor bolt assembly with a fracture zone provided by the method of the present invention;

in fig. 2 shows a series of schematic views of a rod along the length illustrating the fracture of the rod with the formation of (a) a brittle fracture pattern and (b) and (c) a plastic fracture pattern;

in fig. 3 is a pair of photographs of broken ends of rod sections showing (a) a plastic fracture pattern and (b) a brittle fracture pattern; and in FIG. 4A and 4B are schematic representations of a bar with a fracture zone, respectively, in an unloaded and loaded condition.

Implementation of the invention

Fig. 1 illustrates an anchor bolt assembly 10 which is an example of applying a fracture zone or notch 12 to an anchor bolt shaft 14 to provide a solution to the problem described above. This example does not limit the present invention and it is assumed that there are other options for its application to an anchor bolt having such a recess.

With regard to the safety structure 16 for engaging with the crimp section of the sleeve 18 to prevent the proximal end 20 of the rod 14 from being pushed out of the hole in the rock (not shown) into which the assembly 10 is inserted, the rod must be broken in front of the said structure.

- 1 040646

If a break occurs between the specified structure and the proximal end section, then nothing will prevent the ejection.

A bar rupture is highly likely to occur when the breaking load applied to it in use rises to its ultimate strength. To ensure predictability of the rupture site of the rod 14, a notch 12 is provided at a predetermined location along the length of the rod. This recess provides predictability of the localization of the gap.

However, if the recess is to provide such predictability, then the recess must be made in such a way that the break of the rod occurs every time exactly within the boundaries of the recess. At the same time, the recess cannot have a significant negative impact on the ability of the rod to perform its support function.

The notch as a local weakening relative to the rest of the bar limits the inherent ability of the bar to support a particular load, which until then was due to the constituent steel material and diameter. If the recess is made by cold or hot forging, then strengthening the steel at the recess due to changes in material properties will lead to greater variability in tensile strength in relation to the level of limitation. The crystal structure of steel changes due to work hardening, becoming more brittle. This change in the ductility of the steel is not a reproducible or predictable change, and thus, in the vicinity of the recess made, the pin tends to break relatively unexpectedly under unpredictable loading, exhibiting the brittle fracture pattern shown in FIG. 2(a).

The Applicant has found that in order to ensure that the metal rod breaks in a predictable manner, i.e. predictably in terms of the location of the break and the load at which the break occurs, a recess 12 must be made in the stem 14 by applying a subtractive machining process. This process may be, for example, a cutting or grinding process. In other words, the recess must be made by removing the material, and not by moving the material during the forging or rolling process.

According to the present invention, the recess must be made by cutting to deepen below the surface of the rod to a stable reduced diameter (hereinafter, the terms reduced diameter and recess diameter are used interchangeably and mean the same thing). The reduced diameter of the recess can be in a relatively wide range, depending on the required technical characteristics of the anchor bolt for which the rod is used. Of course, the smaller the diameter of the rod at the recess, the lower the ultimate load capacity (UTS) of the rod. Applicant has found that removing material before reaching a reduced diameter of only 1% smaller than the rod diameter is sufficient to provide a reproducible fracture within the recess without compromising UTS.

However, it has been found that the shaft 14 not only needs to be machined into a recess, but the recess must be of a length that will allow a continuous ductile breakable neck 22 to form within the region of the recess when the rod is subjected to a load up to its ultimate strength. See fig. 3B which illustrates this ductile breakable neck, and FIG. 2(b) and (c) which illustrate the shape of the ductile rupture neck after rupture.

The two excavation parameters, which are the excavation depth and the excavation length, were investigated in two series of tests. A description of each of them is given below.

Quasi-static test

The test specimens were subjected to a quasi-static tensile test, each specimen being a 20 mm diameter rod. The samples included a control sample without a notch and three other samples, each with a notch length of 30 mm, with a notch diameter or reduced diameter of 19.8, 19.5 and 19 mm, respectively (error +/- 0.03). Samples were prepared in triplicate. Each recess was made by cutting in the center of the rod. Each rod was then prepared prior to tensile testing in a tensile testing machine.

During the tests, a total of twelve quasi-static tensile tests were carried out, and their results are presented in table. 1 and in FIG. 4. These results were compared with the results for control samples without notches. The tensile strength reduction values shown in the tables below the notch diameter values as column headings are the average of three copies of each set of specimens.

It is important to note that all test specimens achieved plastic failure within the notch zone as intended in this design (see FIG. 5) without significant reduction in ultimate tensile strength (UTS). The minimum guaranteed tensile strength standard for a 20 mm rod made of 500N steel is 210 kN. This value defines the minimum standard for a cutaway design. Quasi-static tests have confirmed that the minimum allowable reduced recess diameter is 19 mm, which provides an average rod UTS value of 213 kN (see Table 1). A notch diameter less than specified results in the bar UTS being below the minimum specification of 210 kN.

____________________________________________________________ Table 1

19.0 19.5 19.8 20.0 Sample 1 211.6 kN 222.1 kN 237.3 kN 226.4 kN Sample 2 213.0 kN 222.6 kN 228.3 kN 227.6 kN Sample 3 213.3 kN 220.3 kN 225.5 kN 226.4 kN Average 213 kN 222 kN 230 kN 227 kN decline UTS 94% 98% 100% 100% Result Compliant (>210 kN)

Fig. 3 illustrates the reduction in the diameter of the notch, resulting in a general deformation of the rod before failure and, consequently, in a decrease in the potential for dynamic absorption of energy. For this reason, and in order to maintain the UTS of the rod, the recess diameter should be as large as possible while guaranteeing stable fracture within the cutting zone.

Fig. 6 graphically illustrates the decrease in the UTS of a bar with a decrease in the diameter of the recess made by cutting. This enables the Applicant to predict the UTS of the bar for untested recess diameters ranging from 0.20.0 mm to 0.19.0 mm. This chart also compares UTS at each pocket diameter with the minimum UTS specification for a 20mm Vulcan shank.

Dynamic test

The test specimens were subjected to a dynamic impact test. Each sample measured 20 mm x 1100 mm x 10 mm and had an RD22 thread at each end of the rod, with samples including a control sample (no notch) and three other samples (notch length 30 mm) with a corresponding notch diameter or reduced diameter of 19.8. 19.6 and 19.4 mm (error +/- 0.03). The samples were made in triplicate. Each recess was made by cutting in the center of the rod.

Each sample was then prepared and inserted into the impact testing machine. Each rod was subjected to an energy pulse of 37.45 kJ at 5.45 m/s.

A total of fifteen dynamic shock tests were carried out in this series. Fig. 7 illustrates the change in the average energy absorption capacity as a function of the recess diameter. In table. Table 2 summarizes the area or mode of failure for each test piece (one 19.8 mm reduced diameter piece was excluded due to incomplete data collection during the test).

The average energy absorption capacity of a standard 20 mm x 1100 mm bar having a prior art cold rolled fracture assisting notch design was 22.5 kJ. This value defines the minimum standard for a cutaway design.

__________________________________________________________ Table 2

Sample 1 Sample 2 Sample 3 20.0 x 30 mm (without notch) Thread Thread Thread 19.8 x 30 mm Thread Thread No data 19.6 x 30mm excavation excavation excavation 19.5 x 30mm excavation excavation excavation 19.4 x 30mm excavation excavation excavation

As shown in FIG. 8, all of the test specimens, with the exception of one that was not tested, failed within the cut-out. The failure mode was the formation of a plastic neck within the cutting zone, which led to a local decrease in the diameter of the rod and the tensile strength of the rod. The tests also show that a pocket diameter of at least 19.6 mm (i.e., less than 2% less than a 20 mm bar diameter) is required to achieve stable dynamic failure within the cutting zone, and that to provide In order for the dynamic tensile strength of the rod to be equal to or greater than the minimum standard value, a minimum recess diameter of 19.4 mm is required.

As part of this experiment, the Applicant has found that a 20 mm diameter rod requires a minimum recess length of 30 mm to ensure that plastically destructible

- 3 040646 neck 22 will not correspond to changes in the diameter or geometric shape of the rod at the boundaries 24 and 26 of the recess. It was noted that if the plastically fractured neck does not correspond to changes in the diameter or geometric shape of the rod at the boundaries of the recess, then the instability of the rupture site and load returns. Surprisingly, this minimum length holds true for rod diameters from 16 to 22 mm.

Summarizing the above, the rod according to the present invention, having a recess made by cutting, is able to at least meet the performance characteristics achievable using the existing method of performing recess by cold rolling, while providing improved stability of the fracture zone, reliability design and ease of mass production.

Claims (8)

1. A method of adapting a metal rod intended for use as an anchor bolt to provide predictable rupture of said rod under load, comprising the step of removing material from the rod in the form of a circumferential strip to reduce the diameter of the rod within said strip to a reduced diameter, characterized in that the reduced diameter is less than the diameter of the rod by 0.5 to 5%, so as to provide a fracture zone having a length in the range of 10 to 45 mm, within which the rod will break under a sufficient tensile load. 2. The method according to claim 1, wherein said zone has a length in the range of 25 to 35 mm. 3. The method according to claim 2, wherein said zone has a length in the range of 30 to 35 mm. 4. A method according to any of claims 1 to 3, wherein the reduced diameter is stable within said band. 5. Method according to any one of claims 1 to 4, wherein the reduced diameter is less than the diameter of the rod by 1 to 4%. 6. The method according to claim 5, according to which the reduced diameter is less than the diameter of the rod by 2 to 3%. 7. A method according to any one of claims 1 to 6, wherein the rod diameter is in the range of 16 to 25 mm. 8. A method according to any one of claims 1 to 7, wherein the material is removed by a subtractive processing process.