EP0758427B1

EP0758427B1 - Profiled fence post made of polymer material

Info

Publication number: EP0758427B1
Application number: EP95917247A
Authority: EP
Inventors: Marnix Allegaert
Original assignee: Bekaert NV SA
Current assignee: Bekaert NV SA
Priority date: 1994-05-04
Filing date: 1995-05-03
Publication date: 2001-07-11
Anticipated expiration: 2015-05-03
Also published as: WO1995030815A1; MX9605310A; EP0758427A1; CA2189228A1; ES2161287T3; ATE203084T1; BE1008449A3; DE69521695D1; DE69521695T2; PT758427E

Abstract

The invention relates to a profiled fence post (1) made of polymer material reinforced with a number of continuous steel reinforcement wires (3) running in the longitudinal direction from one end to the other, which have a diameter ranging between 1.5 and 5 mm and are embedded in adherent relationship in said polymer material, and in which said reinforcement wires have an elastic limit of at least 1200 N/mm<2>, and preferably more than 1400 N/mm<2>.

Description

The invention relates to a profiled fence post, made of polymer material, reinforced with a number of continuous steel reinforcement wires running in the longitudinal direction from one end to the other, which have a diameter ranging between 1.5 and 5 mm and which are embedded in adherent relationship in said polymer material.

Such posts are implanted in the ground, and when a transverse force is exerted on the post, specifically the location where the post emerges from the ground is subject to a large bending moment. On order to better resist hereto, the posts are usually tubular, or, in general: profiled. This means that the surface area of the useful cross-section (cross-section of the material) is maximally 30% of the surface area of the smallest possible circumscribed convex figure, as is the case with most tubular shapes, L-profiles, T-profiles and I-profiles, oriented in accordance with the anticipated direction of the transverse forces. And to be useful as a post, such a convex figure must have an equivalent diameter - i.e. the diameter of the circle having the same surface area -ranging between 2 and 25 cm, and usually between 3 and 12 cm. Although from this point on, the invention will be further explained specifically in relation to tubular fence posts, it should be clear that the invention extends to include any profiled post of any specially adapted dimensions whatsoever.

Such tubular fence posts are already known, for example, from Dutch patent application no. 8600065. In this case the post is reinforced with a number of continuous steel reinforcement wires running in the longitudinal direction from one end to the other, these wires having a diameter of around 3 mm and may be embedded in adherent relationship in said polymer material. In general, the invention will be used for posts in which the reinforcement wires have a thickness ranging between 1.5 and 5 mm, and preferably between 2 and 4 mm.

Furthermore, the polymer material for such a post will in general be a hard PVC or a copolymer of PVC with another monomer, such as vinyl acetate, or a polypropylene or HD-polyethylene. Such tubes are produced by means of an extrusion process in which the reinforcement wires are fed in the direction of the extrusion towards the entrance of the extrusion machine. In the case where, due to chemical and/or mechanical adhesion, the reinforcement wires of themselves already adhere firmly enough to the polymer material, and/or the wires exhibit sufficient surface irregularities or roughness for the adhesion, then, upon being fed into the extrusion machine, the wires do not need to be coated with a special adhesive layer, a so-called "primer". In other cases, such as with hard PVC on smooth reinforcement wire, it will be necessary first to coat the latter with a primer, a well-known manufacturing technique for plasticized wires.

As already mentioned, such posts are implanted in the ground, often cast in a concrete block, and when the post is subjected to a transverse force, the place on the post where it emerges from the ground is subjected to a particularly great bending moment. On increasing this transverse force, a conventional post will give way relatively quickly under the increasing bending moment and exhibit a permanent crease, thus having to be replaced in the fence. It is an object of the invention to provide a post of the type described above, i.e. made of polymer material and reinforced with steel wires, by means of which a higher resistance to such a bending moment is obtained, without having to complicate the reinforcement and/or to jeopardize the extrudability, of the polymer material together with the reinforcement.

According to the invention, said reinforcement wires have an oil-hardened and tempered martensitic structure and an elastic limit of at least 1200 N/mm²

The idea underlying the invention relates to the fact that, up till now, efforts to improve the bending strength have always focused on making the reinforcement wires adhere better in the polymer material, so that these wires would be better able to take over the tensile forces from the polymer material. But herein the fact was overlooked, that, when bending such a post, the conventional force distribution pattern does not apply, in which over the cross-section of the post there would only be a tensile zone, a pressure zone and a neutral line between the two. Upon bending indeed, the polymer material between the wires is subject to great shearing stresses in the longitudinal direction. And since, in order to keep the extrusion process simple and easy, there is no transverse reinforcement provided, the polymer material between the wires begins to flow relatively easily in the direction of longitudinal shearing. And because of this, the force distribution pattern begins to deviate from the conventional pattern and begins to tend towards a superposition thereof, on a configuration in which each separate reinforcement wire with the polymer material around it exhibits its own pressure zone, tensile zone and neutral line. This entails that the reinforcement wires, and more especially those located far from the conventional neutral line, in fact are much less than normally subject to tensile and pressure forces, but are greatly subject to bending forces. For these wires, therefore, it is not a question of better taking up the tensile forces, but rather of being well able to resist permanent bending. Therefore, stiff wires are required: thick and made of steel with a high elastic limit. Up till now, no attention was paid to this fact and it was proposed, among other things, that the reinforcement wires could be replaced by bundles of wires.

There is yet another important side effect when such a reinforcement with stiff wires is utilized: these posts can be bent surprisingly far without leaving a permanent crease. They can in fact be bent far beyond the elastic limit of the relatively soft polymer material, so that the latter indeed shows a plastic deformation. But at this point, the reinforcement wires have not yet reached their elastic limit: neither in flexure, nor, for the wires that are located far from the conventional neutral line, in tension or pressure. When the post is released, all the wires seek to recover both their original straightness and their original length. And thanks to their great stiffness, combined with the good adhesion, they will draw the polymer material along in this movement and hence deform it in the opposite sense back into the original straight condition of the wires and post.

When a post is reinforced with a number of longitudinal steel reinforcement elements, one must ensure with respect to each individual reinforcing element, that each element, in addition to having a normal resistance to tension or pressure, would also be able to offer a large resistance against bending in the expected direction. Thus, in the first place, it is better for such an element to be in the form of one thick wire instead of a bundle of several wires having the same total steel cross-section. The conventional measure taken with reinforcement wires, wherein thick wires (which normally speaking have not undergone a large cross-sectional reduction during drawing and thus have no appreciable tensile strength) are replaced by a bundle of finer wires, which then have undergone a higher cross-sectional reduction and thus possess a higher tensile strength, is therefore not recommended here. It can however be useful to use profiled wires which have a larger bending resistance in the expected direction of bending. For a round post, flattened wires with the largest cross-sectional dimension running towards the central axis of the post, can so advantageously be used.

However, even when it is advantageous for each individual reinforcement element to consist of one single stiff wire, this still does not mean that the entire reinforcement for the entire useful cross-section of the post must be concentrated in one single very thick reinforcement element, or in a very small number of such elements. A certain even distribution of the reinforcement steel is necessary, i.e. over different elements spread over the cross-section. In bending indeed, as has already been mentioned, each element develops its own pressure and tensile zone (including a neutral line) around itself in the polymer material. And the lesser the number of elements over which this reinforcement steel is distributed, the further these zones will extend. Upon bending, then, the parts that lie too far from such a neutral line can start to flow too soon and exhibit an excessive and irreversible plastic deformation. It can thus be put as a norm that the distribution is not, or insufficiently uniform, when, when exerting a transverse force on the top of the post to produce a 5° deviation from vertical (straight line from the top to the foot of the post) and then removing this transverse force, the post no longer returns to its original vertical position.

The distribution to be utilized will depend on the geometry. Thus, it will be aimed at achieving a greater density in the parts of the cross-section of the material which are located further from the conventional neutral line than those parts in the center. These parts lying further out will participate to a greater extent in the development of the bending strength due to the fact that their tensile or pressure resistance is better utilized. Thus a tubular form in which the reinforcement wires are uniformly distributed over the perimeter of the tube wall is a preferred embodiment of the invention.

In general, it will thus be appropriate, for ghe conventional reinforcement percentages of 4 to 20%, to maintain the usual order of magnitude of diameters, in the range between 1.5 and 5 mm, with a limited number of single reinforcement wires being fed into the extrusion machine (by preference from 2 to 4 mm for a percentage of 6 to 14%). "Reinforcement percentage" here means: the percentage of steel surface area compared to the total surface area of the transected material in the cross-section of the post.

Where, as to geometry, it seems to be appropriate to maintain the usual dimensions and geometries, then, for steel material, this is not the case. The second point that should be given attention to is the stiffness of the reinforcement wire, and the fact that this is not defined by the tensile strength of the steel, but by its elastic limit. Under a gradually increasing tensile force on the wire, as is known, this is the tension per unit of cross-sectional surface area (and therefore in N/mm²) under which the material begins to flow, i.e. under which it begins to show plastic deformation in a permanent manner. In this respect, then, the reinforcement wires can be improved, in accordance with the invention, by ensuring that the elastic limit of the steel is at least 1200 N/mm², and preferably more than 1400 N/mm². By this elastic limit, the so-called "0.2 limit" is meant. Since the "beginning" of the flowing is difficult to observe, in most standard tests this beginning is defined as the situation in which an elongation of 0.2% is reached. In order to obtain such a high elastic limit, it is necessary to harden the wire.

It is preferable to achieve the required hardness by means of oil hardening. Here, as is known, wire with the final diameter is continuously led through a continuous furnace where it is heated to austenitization temperature, and then upon exit from the continuous furnace it runs through an oil bath where it is quenched and thus obtains a hard martensitic structure, and thereupon is fed through a heating element where said structure is softened to a certain extent because the pure quenched structure is too brittle. When these thermal treatments are carried out on straight wire, then a good straight wire is thus obtained for use in the extrusion of the post.

The invention is further explained here on the basis of a number of figures representing a cross-section of the post.

Figure 1: is a round tubular post with an even wall thickness and uniform distribution of the reinforcement wires;
Figure 2: is an essentially round, tubular post with a substantially uniform distribution of the reinforcement wires;
Figure 3: is a round tubular post with a substantially even wall thickness and with thickenings around the reinforcement wires;
Figure 4: is a round tubular post with a less uniform distribution of the reinforcement wires;
Figure 5: is a post with an I-shaped cross-sectional profile.

Figure 1 shows a post in the shape of a round tube 1 with an even wall thickness. Here 3 is a reinforcement wire. The broken line 2 represents a fence wire mesh which is attached to the post. The shape of the tube, however, may however only be essentially round, which means that at certain locations, it deviates from the round shape, as for example the locations 4 where a fence wire mesh will be attached with a snap connection (Figure 2). The wall thickness does not necessarily need to be the same everywhere. Thus it can vary, and the wall can display local thickenings for the purpose of providing space for the reinforcement wires when they are relatively thick in relation to the average wall thickness (Figure 3). The tube can also be square or rectangular. The average wall thickness over the circumference will generally lie in the range between 3 and 15% of the diameter or equivalent diameter of the post, and preferably between 5 and 10%.

When it is not known from which direction the possible forces will be applied to the tubular post, or when one does not wish to take this into account, then the selected number of wires will preferably be distributed as uniformly as possible around the circumference of the tube wall. However, at locations where the shape of the post deviates from roundness, for example for attaching the fence mesh, it is also possible to deviate from the strictly uniform distribution (Figure 2, the reinforcement wires 5 and 6), in which case the distribution is only substantially uniform. The wires shall however not be distributed over too small a number of reinforcement wires, for reasons which have already been presented above, nor over too large a number of reinforcement wires, because then the wires will be thinner and less stiff. A distribution over a minimum of 4 and a maximum of 30 wires is therefore most suitable.

Deviations from a strictly uniform distribution are also possible when one wants the post to offer more resistance against transverse forces in the plane perpendicular to the fence. it is then taken into account that part of the force distribution pattern is still the standard pattern over the entire cross-section, with a pressure zone, a tensile zone and a neutral line running parallel with the fence. A distribution is therefore used which involves a greater concentration of wires far from the neutral line. Reference is made here to Figure 4, in which the broken line 6 represents the neutral line and the broken line 2 once again represents a fence mesh. In this case, a different post profile can also be chosen, for example an I-profile, with the reinforcement wires more concentrated far from the neutral line (Figure 5).

Claims

Profiled fence post (1), made of polymer material reinforced with a number of continuous steel reinforcement wires (3) running in the longitudinal direction from one end to the other, which have a diameter ranging between 1.5 and 5 mm and which are embedded in adherent relationship in said polymer material, characterized in that said reinforcement wires (3) have an oil-hardened and tempered martensitic structure and an elastic limit of at least 1200 N/mm².
Fence post (1) according to claim 1, characterized in that said reinforcement wires (3) have an elastic limit of more than 1400 N/mm².
Fence post (1) according to one of the foregoing claims, characterized in that the post has a reinforcement percentage ranging between 4 and 20% .
Fence post (1) according to one of the foregoing claims, characterized in that the post (1) is tubular, in which a limited number of reinforcement wires (3), between 4 and 30 and with a diameter ranging between 2 and 4 mm, are substantially uniformly distributed over the pipe wall.