LIGHTWEIGHT TIRE FEATURING A LAYER OF CIRCUMFERENTIAL REINFORCING ELEMENTS

Abstract

The invention relates to a radial carcass tire (1), where two working crown layers (41, 42) and a layer of circumferential reinforcing elements (43) are present to constitute the crown reinforcement (4) over at least 75% of the width L5 of the tread (5), the absolute value of the difference between the absolute values ​​of angles α2 and α1 being greater than 7°, α2 being greater than α1 in absolute value, the mean angle α satisfying the relation 13+131*exp(-L / 100) < α < 28+110*exp(-L / 100). According to the invention, the secant modulus of elasticity at 10% elongation of at least one calendered layer of at least one working crown layer (41, 42) is less than 8.5 MPa. Figure for the abridged version: Fig 1

Description

Description Title of the invention: LIGHTWEIGHT TIRE COMPRISING A LAYER OF CIRCUMFERENCE REINFORCING ELEMENTS REFERENCE DOCUMENTS

[0001] — The present invention relates to a tire with a radial carcass reinforcement and more specifically a tire intended to equip vehicles carrying heavy loads and vehicles traveling at sustained speeds, such as, for example, trucks, tractors, trailers or road buses.

[0002] — Generally speaking, in heavy-duty tires, the reinforcement of The carcass is anchored on both sides in the area of ​​the bead and is surmounted by ra- dia- by a top reinforcement consisting of at least two layers, su- perpendicular and formed of parallel wires or cables in each layer and crossed by a layer to the next, following the circumferential direction of the angles included between 10° and 45°. These working layers, forming the working reinforcement, can still need to be covered with at least one so-called protective layer made up of elements of advantageously metallic and extensible reinforcements, called elastic. It can also include a layer of low-extensibility wires or metallic cables making an angle between 45° and 90° with the circumferential direction, this the layer, known as the triangulation layer, being radially located between the frame reinforcement and the The first top layer, known as the working layer, is made up of parallel wires or cables. exhibiting angles of no more than 45° in absolute value. The triangulation sheet form with at least said work surface a triangulated framework, which presents, under the various stresses it undergoes, few deformations, the triangulation sheet whose essential role is to absorb the transverse compression forces of which the object, the set of reinforcement elements in the area of ​​the apex of the pneumatic.

[0003] — Cables are said to be inextensible when said cables exhibit, under a force of a tensile strength equal to 10% of the breaking force exhibits a relative elongation of at most 0.2%.

[0004] — Cables are said to be elastic when said cables exhibit, under a force of tensile strength equal to the breaking load, a relative elongation of at least 3% with a maximum tangent modulus less than 150 GPa.

[0005] — Circumferential reinforcing elements are reinforcing elements that make with the circumferential direction of the angles included in the interval + 2.5°, 2.5° around 0°.

[0006] — The circumferential direction of the tire, or longitudinal direction, is the tangent direction to the periphery of the tire and defined by the direction of tire bearing. The transverse or axial direction of the tire is parallel to the axis of rotation of the tire. Radial direction is a direction that intersects the axis of rotation of the tire and is perpendicular to it. The axis of rotation of the tire is the axis around which it rotates in normal use. A radial or meridian plane is a plane that contains the axis of rotation of the tire. The circumferential median plane, or equatorial plane, is a plane perpendicular to the axis of rotation of the tire and which divides the tire into two halves. The "modulus of elasticity" of a rubbery mixture is understood to be a secant modulus of extension at 10% elongation and at room temperature. Regarding rubber compounds, the secant modulus of elasticity at 10% elongation is the elastic modulus of the compound measured during a uniaxial tensile test at an elongation value of 0.1 (i.e., 10% elongation, expressed as a percentage). A constant uniaxial tensile speed is applied to the specimen, and its elongation and the force exerted are measured. The measurement is performed using an INSTRON tensile testing machine at a temperature of 23°C and a relative humidity of 50% (ISO 23529 standard). The conditions for measuring and using the results to determine elongation and stress are as described in standard NF ISO 37: 2012-03. Stress is determined for an elongation of 0.1 and the secant modulus of elasticity at 10% elongation is calculated by taking the ratio of this stress value to the elongation value.A person skilled in the art will know how to choose and adapt the dimensions of the test specimen according to the quantity of mixture accessible and available, particularly in the case of specimen sampling from a finished product such as a tire. Some modern tires, known as "road" tires, are designed for high-speed driving and increasingly long journeys, due to improvements in the road network and the growth of highways worldwide. The overall conditions under which such a tire is expected to operate undoubtedly allow for an increase in the number of kilometers traveled, as tire wear is reduced; however, the tire's durability, and particularly that of the tread, is compromised. There are indeed constraints at the level of the top reinforcement and more particularly shear stresses between the top layers, combined with a non-negligible rise in operating temperature at the extremities of the axially shortest top layer, which result in the appearance and propagation of cracks in the rubber at the said extremities. In order to improve the endurance of the crown reinforcement of the type of tire studied, solutions relating to the structure and quality of the layers and / or profiles of rubber compounds which are arranged between and / or around the ends of plies and more particularly the ends of the axially shortest plies have already been provided. It is known to introduce a layer of rubbery compound between the ends of the working layers to create a decoupling between said ends and thus limit shear stresses. Such decoupling layers must, however, exhibit very good cohesion. Such layers of rubbery compounds are described, for example, in patent application WO 2004 / 076204. French patent FR 1 389 428, to improve the resistance to degradation of rubber compounds located in the vicinity of the edges of the top reinforcement, recommends the use, in combination with a low hysteresis tread, of a rubber profile covering at least the sides and marginal edges of the top reinforcement and made of a low hysteresis rubber compound. French patent FR 2 222 232, to avoid separations between top reinforcement layers, teaches to encase the ends of the reinforcement in a rubber mattress, the Shore A hardness of which is different from that of the tread surmounting said reinforcement, and greater than the Shore A hardness of the rubbery mixture profile disposed between the edges of the top reinforcement layers and the carcass reinforcement. The tires produced in this way effectively improve performance, particularly in terms of endurance. Furthermore, it is known for producing tires with very wide treads or for increasing the load-bearing capacity of tires of a given size by incorporating a layer of circumferential reinforcing elements. Patent application WO 99 / 24269, for example, describes the presence of such a layer of circumferential reinforcing elements. The layer of circumferential reinforcing elements is usually made up of at least one wire rope wound to form a spiral whose angle of laying with respect to the circumferential direction is less than 2.5°. Furthermore, the use of tires on heavy-duty vehicles designed for construction site access exposes the tires to impacts when driving on stony ground. These impacts are, of course, detrimental to performance in terms of durability. It is still known to those skilled in the art to increase the number of plies constituting the crown reinforcement to improve the tire's endurance against such shocks. Regardless of the solution considered among those presented previously, the presence of one or more additional layers of reinforcing elements leads to a greater mass of the tire and higher tire manufacturing costs. A tire with a lighter crown reinforcement, improving its impact resistance, is also known from document WO 2017 / 149223. However, the inventors have pointed out that when driving on surfaces extremely demanding on tires, the endurance performance of such tires can be degraded, for example, under driving conditions that combine vehicle speed, tire load, and a particularly aggressive surface. Indeed, a degradation in endurance performance can be observed, for example, when driving at relatively high speeds on construction site approaches, which are very aggressive to tires. The inventors have thus set themselves the mission of providing tires for "Heavy Goods Vehicles" for example of the "construction site approach" type, whose overall mass is limited and whose endurance performance with regard to the shocks suffered on the tread is improved regardless of the nature of the ground and the driving conditions. This objective is achieved according to the invention by a tire intended to be mounted on a hollow rim of the 15° drop center type, comprising a radial carcass reinforcement, said tire comprising a crown reinforcement comprising two working crown layers, each formed of reinforcing elements inserted between two calendered layers of rubber compound crossed from one layer to the other making angles (01, œ2) greater than 8° with the circumferential direction, said angles 01 and œ2 being oriented on either side of the circumferential direction, and at least one layer of circumferential reinforcing elements, the crown reinforcement being radially capped by a tread, said tread being joined to two beads by means of two sidewalls,said two top working layers and said at least one layer of circumferential reinforcing elements being the only ones present to constitute the top reinforcement over at least 75% of the axial width of the top reinforcement, the reinforcing elements of the radially outermost working layer forming an angle α2 with the circumferential direction greater in absolute value than the angle ε1 formed by the reinforcing elements of the radially innermost working layer with the circumferential direction, the absolute value of the difference between the absolute values ​​of angles α2 and α1 being greater than 7°, the mean angle α satisfying the relation: , 13+131*exp(-L / 100) < to < 28+110*exp(-L / 100), a being defined by the relation a = Arctan((tan(la1l)*tan(la2l))!?), L being the maximum width of the tire measured along the axial direction and expressed in mm, the secant modulus of elasticity at 10% elongation of at least one calendered layer of at least one working apex layer being less than 8.5 MPa. For the purposes of the invention, a hollow rim of the 15° drop center type or rim with a wedged seat is a one-piece rim, as defined in the ETRTO, whose seats for receiving the tire beads have a frustoconical shape, the angle formed with the axial direction being substantially equivalent to 15°. These seats are further extended by rim hooks of reduced height compared to rim hooks with flat bases whose rim seats have substantially cylindrical shapes. The width L is measured on a tire mounted on its nominal rim and inflated to its nominal pressure, according to ETRTO, and is expressed in millimeters. The angles a1 and a2, expressed in degrees, are measured on a cross-section of the tire. The angle measurements are, according to the invention, carried out at the level of the circumferential median plane. Advantageously according to the invention, the maximum value of tan(d), noted tan(8)nax of said at least one calendering layer, of at least one top working layer, whose secant modulus of elasticity at 10% elongation is less than 8.5, is less than 0.100. The loss factor tan(0) is a dynamic property of the rubber compound layer. It is measured on a viscoelastic analyzer (Metravib VA4000), according to ASTM D 5992-96. The response of a vulcanized composition sample (cylindrical specimen 2 mm thick and 78 mm² cross-section) is recorded under sinusoidal loading in alternating simple shear at a frequency of 10 Hz and a temperature of 100°C. A deformation amplitude sweep is performed from 0.1 to 50% (forward cycle), then from 590% to 1% (reverse cycle). For the return cycle, the maximum observed value of tan(8), denoted tan(φ)pux, is indicated. A person skilled in the art will be able to choose and adapt the dimensions of the test specimen according to the quantity of accessible and available mixture, particularly in the case of specimen sampling from a finished product such as a tire. Rolling resistance is the resistance that occurs when the tire rolls.It is represented by the hysteretic losses related to tire deformation during one revolution. The frequency values ​​related to the tire revolution correspond to tan(φ) values ​​measured between 30 and 100°C. The tan(φ) value at 100°C thus corresponds to an indicator of the tire's rolling resistance. According to a preferred embodiment of the invention, the secant modulus of elasticity to 10% elongation of the calendering layers of said two top working layers is less than 8.5 MPa and preferably, the tan(5),,x value of the calendering layers of said at least two top working layers is less than 0.100. According to a preferred embodiment of the invention, said at least one calendering layer, at least one top working layer, whose secant modulus of elasticity at 10% elongation is less than 8.5, is an elastomeric blend based on natural rubber or synthetic polyisoprene with a majority of cis-1,4 chains and optionally at least one other diene elastomer, the natural rubber or synthetic polyisoprene in the case of cutting being present at a major rate compared to the rate of the other diene elastomer(s) used, and a reinforcing filler consisting of: a) either by carbon black with a specific surface area BET greater than 60 m² / g, i) used at a rate between 20 and 40 parts per cent when the structure index of the black (COAN) is greater than 85, ii) employed at a rate between 20 and 60 parts per annum when the black structure index (COAN) is less than 85, b) either by carbon black with a specific surface area BET of less than 60 m² / g, regardless of its structure index, used at a rate between 20 and 80 parts per liter, and preferably between 30 and 50 parts per liter, c) either by a white filler of the silica and / or alumina type comprising SIOH and / or AIOH surface functions chosen from the group formed by precipitated or pyrogenated silicas, aluminas or aluminosilicates or even carbon blacks modified during or after synthesis with a specific surface area of ​​BET between 30 and 260 m³ / g used at a rate between 20 and 80 parts per cent, and preferably between 30 and 50 parts per cent, d) either by cutting carbon black as described in (a) and / or carbon black as described in (b) and / or a white filler as described in (c), wherein the overall filler content is between 20 and 80 parts per cent, and preferably between 40 and 60 parts per cent. The BET specific surface area measurement is carried out according to the method of BRUNAUER, EMMET and TELLER described in "The Journal of the American Chemical Society", vol. 60, page 309, February 1938, corresponding to the NFT 45007 standard of November 1987. The structure index of black COAN (Compressed Oil Absorption Number) is measured according to the ASTM D3493 standard. When using clear or white charge, it is necessary to use a coupling and / or coating agent chosen from those known to those skilled in the art. Examples of preferential coupling agents include: Sulfur-containing alkoxysilanes of the bis-(3-triethoxysilylpropyl) polysulfide type, including bis-(3-triethoxysilylpropyl) tetrasulfide, marketed by DEGUSSA under the names Si69 for the pure liquid product and X50S for the solid product (a 50 / 50 by weight mixture with N330 black). Examples of coating agents include fatty alcohols, alkylalkoxysilanes such as hexadecyltrimethoxy or triethoxysilane, respectively marketed by DEGUSSA under the names Si116 and Si216, diphenylguanidine, polyethylene glycol, and silicone oil, possibly modified with OH or alkoxy groups.The coating and / or coupling agent is used in a weight ratio to the filler > 1 / 100 and < 20 / 100, and preferably between 2 / 100 and 15 / 100 when the clear filler represents the entire reinforcing filler and between 1 / 100 and 20 / 100 when the reinforcing filler consists of a cut of carbon black and clear filler. Other examples of reinforcing fillers having the morphology and SiOH and / or AIOH surface functions of the silica and / or alumina-type materials described above, and which can be used according to the invention as a partial or total replacement thereof, include carbon blacks modified either during synthesis by adding a silicon and / or aluminum compound to the furnace feed oil, or after synthesis by adding an acid to an aqueous suspension of carbon black in a sodium silicate and / or aluminate solution, so as to at least partially coat the surface of the carbon black with SiOH and / or AIOH functions. Non-limiting examples of this type of carbon-based filler with surface SiOH and / or AIOH functions include CSDP-type fillers described in Conference No. 24 of the ACS Meeting, Rubber Division, Anaheim, California, May 6-9, 1997, as well as those of patent application EP-A-0 799 854. When a clear filler is used as the sole reinforcing filler, hysteresis and cohesion properties are achieved using precipitated or fumed silica, precipitated alumina, or an aluminosilicate with a specific surface area (BET) between 30 and 260 m° / g. Non-limiting examples of this type of filler include Akzo's KS404 silica, Degussa's Ultrasil VN2 or VN3 and BV3370GR, Huber's Zeopol 8745, Rhodia's Zeosil 175MP or Zeosil 1165MP, PPG's HI-SIL 2000, etc. Among the diene elastomers that can be used in blending with natural rubber or a synthetic polyisoprene with a majority of cis-1,4 chains, we can mention polybutadiene (BR), preferably with a majority of cis-1,4 chains, a styrene-butadiene (SBR) copolymer solution or emulsion, a butadiene-isoprene copolymer (BIR), or a styrene-butadiene-isoprene terpolymer (SBIR). These elastomers can be modified during or after polymerization. Polymerization is achieved using branching agents such as divinylbenzene or star-forming agents such as carbonates, tin halogens, silicon halogens, or functionalizing agents that graft oxygenated carbonyl or carboxyl groups onto the chain or its end, or an amine group, for example, by the action of dimethyl or diethylaminobenzophenone. In the case of natural rubber or synthetic polyisoprene blends with a majority of cis-1,4 chains and one or more of the diene elastomers mentioned above, natural rubber or synthetic polyisoprene is preferably used in a majority proportion, and more preferably in a proportion exceeding 70%. The results obtained with tires conforming to the invention have indeed demonstrated that performance in terms of endurance can be improved by lightening the tire's crown reinforcement, regardless of the type of ground and driving conditions. Furthermore, lightening the tire's crown reinforcement simplifies manufacturing and reduces production costs. Against all expectations, the results actually highlighted that the tires according to the invention can be lightened by reducing the number of constituent layers of the crown reinforcement while improving the endurance properties of the crown of the tire, particularly with regard to shocks appearing on the tread regardless of the nature of the ground and the driving conditions. It is indeed known to those skilled in the art that in order to improve the endurance performance of the crown reinforcement of a tire with regard to this type of shock, it is common to increase the number of layers of reinforcing elements as well as to increase the rigidity of the calendering compounds. The inventors believe these results can be attributed to the fact that the angle formed with the circumferential direction by the reinforcing elements of the innermost radially working crown layer is smaller in absolute value than that formed by the reinforcing elements of the outermost radially working crown layer. They observed that this smaller angle appears to delay the tensioning of the reinforcing elements during such an impact. Typically, during impacts comparable to those observed when driving on gravel, any failure of reinforcing elements is observed in the innermost radially working layer. These findings suggest that, in the face of this type of stress, the difference in angles between the reinforcing elements of the two working crown layers improves the tire's durability while reducing the number of crown reinforcement layers. The value of the mean angle, increased compared to more usual values known from conventional tires for this type of application, it further improves the tire crown's endurance properties with respect to impacts occurring on the tread. The presence of at least one layer of circumferential reinforcement elements allows for a mean angle between the reinforcement elements of the two working crown layers that is greater than the mean angle defined by the reinforcement elements of the two working crown layers in more conventional tires.Indeed, the circumferential rigidity provided by the presence of at least one layer of circumferential reinforcing elements increases the average angle formed by the reinforcing elements of each of the working apex layers with the circumferential direction. This average angle appears to improve the tire's performance, particularly its maneuverability, when driving under heavy loads or at a very large angle to the direction of travel. The inventors have thus demonstrated that the tire's dynamic properties, especially its drift rigidity, are maintained or even improved regardless of the application. However, the inventors also believe they have demonstrated that the angle formed with the circumferential direction by the reinforcement elements of the innermost radially working top layer, which is smaller in absolute value than that formed by the reinforcement elements of the outermost radially working top layer, leads, during extreme stresses, particularly during high-speed driving, to high stresses on the reinforcement elements of the innermost radially working layer, which can impair performance in terms of endurance. The use of low stiffness calendering compound for this radially innermost working top layer, according to the invention, is still an unfavorable factor for the endurance of this radially innermost working top layer under high stresses such as high-speed rolling. Typically, the secant moduli of elasticity at 10% elongation of the tread layers are greater than 8.5 MPa and most often greater than 10 MPa. Such moduli of elasticity are particularly necessary to limit the compression of the tread reinforcement elements, especially when the vehicle follows a winding path. Indeed, the axial shear forces acting on the tread in the contact patch area lead to compression of the tread reinforcement elements. The inventors were able to demonstrate that the layer of circumferential reinforcing elements allows for choices of elastic moduli for the rubber compounds. weaker calendering layers of the top working layers without harming the endurance properties of the tire due to the compression of the reinforcing elements of said top working layers as described previously. The inventors have also been able to demonstrate that the cohesion of the calendering layers of the top working layers, when it has a secant modulus of elasticity at 10% elongation of less than 8.5 MPa, remains satisfactory. For the purposes of this invention, a cohesive rubber compound is a rubber compound that is particularly resistant to cracking. The cohesion of a compound is thus evaluated by a fatigue cracking test performed on a "PS" (pure shear) specimen. This test consists of determining, after notching the specimen, the crack propagation rate "Vp" (nm / cycle) as a function of the energy release rate "E" (J / m²). The experimental range covered by the measurement is between -20°C and +150°C, with an atmosphere of air or nitrogen. The specimen is subjected to an imposed dynamic displacement with an amplitude between 0.1 mm and 10 mm in the form of an impulse-type load (tangent "haversine" signal) with a rest time equal to the duration of the impulse; the signal frequency is on the order of 10 Hz on average. The measure comprises 3 parts: - Accommodation of the "PS" test specimen, from 1000 cycles to 27% deformation. - an energy characterization to determine the law "E" = f (deformation). The energy release rate "E" is equal to WO*h0, where WO = energy supplied to the material per cycle and per unit volume and h0 = initial height of the specimen. Analysis of the "force / displacement" data thus provides the relationship between "E" and the magnitude of the stress. - Cracking measurement, after notching of the specimen "PS". The information collected leads to determining the crack propagation speed "Vp" as a function of the imposed stress level "E". The inventors have notably demonstrated that the presence of at least one layer of circumferential reinforcing elements contributes to a reduced evolution of the cohesion of the calendered layers of the working crown layers. Indeed, more common tire designs, which include calendered layers of the working crown layers with secant moduli of elasticity at 10% elongation and 8.5 MPa, lead to a weakening of the cohesion of these calendered layers of the working crown layers. The inventors observe that the presence of at least one layer of circumferential reinforcing elements, which helps to limit the compression stresses on the reinforcing elements of the working crown layers, leads to a lower cohesion. evolution of the cohesion of the calendered layers. The inventors thus consider that the cohesion of the calendered layers of the working top layers, lower than that which exists in more common tire designs, is satisfactory in the design of the tire according to the invention. In addition, the less rigid mixtures of the calenderings of the top working layers associated with a layer of circumferential reinforcing elements help to limit the temperature rises generated when subjected to shear stresses. Furthermore, the lighter crown reinforcement leads to a reduction in the overall thickness of the tire crown. The inventors have also demonstrated that, during sustained high-speed driving, compared to more conventionally designed tires, the temperature of the tire crown is lower. The combination of the lightweight crown reinforcement as described in the invention with low-stiffness calendering compounds, which might seem to compromise the tire's durability under extreme operating conditions, actually leads to the preservation, or even improvement, of the tire's durability performance. The inventors believe this result can be attributed to cumulative effects on the tire crown temperature, stemming from both the lightweight crown reinforcement design and the presence of less stiff calendering compounds alongside a layer of circumferential reinforcing elements. The combination of these elements appears to reverse the expected effects on durability. Advantageously, according to the invention, the utilization ratio of the F2 / FR2 failure potential of the outermost working layer is less than 1 / 6, and wherein: FR2 is the uniaxial tensile breaking strength of each of the cables in the radially outermost working layer. F2 = p, * Tc * [(tan(la1){tan(lœ1l)+tan(la21))) / cos“(la2l) + Cr], with Tec = 0.078 * P * Rs* (1-(Rs2-RL2) / (2*Rt*Rs)), P: the nominal inflation pressure of the tire according to ETRTO, Cr, = 0.00035*(min((L-80) / sin(lll), (L-80) / sin(la2l), 480)-480), "Pp": the spacing of the reinforcement elements of the outermost radially working top layer, measured perpendicular to the reinforcement elements at the level of the circumferential median plane, Rs = Re — Es, Re: outer radius of the tire measured at the outermost radial point on the surface of the tire tread, said surface being extrapolated to fill any gaps, Es: radial distance between the radially outermost point of the tire and its orthogonal projection onto the radially outer face of a reinforcement element of the radially innermost working crown layer, RL: average of the radii of the axially outermost points of the main part of the carcass reinforcement layer on each side of the tire, Rt: the radius of the circle passing through three points located on the outer surface of the tread outside the grooves, defined from a shoulder end at respective axial distances equal to YA, 42 and % of the axial width of the tread. The thickness Es and the pitch p are measured on a cross-section of the tire and are expressed in millimeters. The meridian section of the tire is defined according to the invention such that the barycenters of the beads form an axially oriented straight line, said barycenters being separated from each other by a distance equal to the nominal rim width J increased by 20 mm and decreased by twice the axially measured distance between a barycenter of a bead and a point on the outer surface of the tire. The inventors further observe that the choice of the absolute value of the difference between the absolute values ​​of the aforementioned angles θ1 and θ2, combined with the mean angle α and the utilization ratio of the breaking potential F2 / FR2 as defined according to this advantageous embodiment of the invention, can eliminate the protective layer usually placed radially outside the other layers of the top reinforcement. Such a layer is typically present to be sacrificed in the event of tire damage such as cuts that could compromise the integrity of metallic reinforcement elements through corrosion phenomena associated with fatigue of said reinforcement elements.The inventors have indeed observed that the reinforcing elements of the outermost radially working crown layer of a tire according to the invention are subjected to less stress during inflation or normal driving than the reinforcing elements of the outermost radially working crown layer of a more conventional tire. Such a more conventional tire exhibits significant differences in the absolute angles between the reinforcing elements of the various smaller working layers, an angle of the reinforcing elements of the innermost radially working layer that is greater than or equal in absolute value to that of the reinforcing elements of the outermost radially working layer, and a higher F2 / FR2 breaking potential utilization ratio. The reinforcing elements of the outermost radially working crown layer of a... The tires according to the invention thus exhibit endurance properties far superior to those of a more conventional tire; the inventors thus observe that the elimination of the protective layer is made possible and makes it possible to contribute to the weight reduction of the tire and the limitation of temperature increases in the area of ​​the top of the tire. According to a preferred embodiment of the invention, the absolute value of the difference between the absolute values ​​of angles a2 and a1 is greater than or equal to 10° and preferably greater than 14°. According to this embodiment, and in accordance with the interpretations given above, it will be possible to further improve the endurance performance of the reinforcement elements of the outermost radially working layer and / or further improve the performance of the tire with respect to shocks such as those experienced when driving on stony ground. Preferably, the absolute value of the difference between the absolute values ​​of angles a2 and al is less than 25° and preferably even less than 20°. Beyond these values, the tire would be likely to exhibit uneven wear under certain conditions of use. Advantageously, according to the invention, the utilization ratio of the breaking potential F2 / FR2 of the outermost radially working layer is less than 1 / 8. Such a utilization ratio of the breaking potential F2 / FR2 further contributes to improving the endurance performance of the reinforcement elements of the outermost radially working layer during tire use. Preferably, according to the invention, the utilization ratio of the breaking potential F1 / FR1 of the innermost radially working layer is less than 1 / 3, where: FRI is the uniaxial tensile breaking force of each of the cables in the innermost radially working layer, F1 = p, * Tc * [(tan(la21) / (tan(la1l)+tan(la21))) / cos*{la1l) + Cr], with Pi: the spacing of the reinforcement elements of the radially innermost working top layer, measured perpendicular to the reinforcement elements at the level of the circumferential median plane. Preferably, the utilization ratio of the F1 / FR1 failure potential of the innermost radially working layer is at least 30% greater than the utilization ratio of the F2 / FR2 failure potential of the outermost radially working layer. According to an advantageous embodiment of the invention, the axially widest working top layer is radially inside the other working top layers. According to a preferred embodiment of the invention, optimizing the thinning of the tire crown, the two working crown layers and said at least one only a layer of circumferential reinforcing elements are present to constitute the top reinforcement over the entire axial width of the top reinforcement. According to an advantageous embodiment of the invention, the layer of circumferential reinforcing elements has an axial width greater than 0.5xS. S is the maximum axial width of the tire, when it is mounted on its service rim and inflated to its recommended pressure. The axial widths of the layers of reinforcing elements are measured on a cross-section of a tire, the tire therefore being in an uninflated state. According to a preferred embodiment of the invention, the layer of circumferential reinforcement elements is radially arranged between two working top layers. According to this embodiment of the invention, the layer of circumferential reinforcing elements allows for greater limitation of the compressive stresses on the carcass reinforcement elements than a similar layer placed radially outside the working layers. It is preferably radially separated from the carcass reinforcement by at least one working layer in order to limit the stresses on said reinforcing elements and prevent excessive fatigue. Advantageously, according to the invention, the axial widths of the radially adjacent working top layers to said at least one layer of circumferential reinforcing elements are greater than the axial width of said at least one layer of circumferential reinforcing elements and preferably, said working top layers adjacent to said at least one layer of circumferential reinforcing elements are on either side of the equatorial plane and in the immediate axial extension of said at least one layer of circumferential reinforcing elements coupled over an axial width, to then be decoupled by a layer of rubber mixture at least over the remainder of the width common to said two working layers. For the purposes of the invention, top working layers are said to be coupled if the respective reinforcing elements of each of the layers are separated radially by a distance less than the average diameter of the circle circumscribed about the reinforcing elements, said thickness of rubber being measured radially between the respectively radially upper and lower generators of said reinforcing elements. The average diameter of the circle circumscribed about the reinforcing elements is defined as the average diameter of the circles circumscribed about the reinforcing elements. of each of the top working layers. The presence of such couplings between the working top layers adjacent to the layer of circumferential reinforcing elements allows the reduction of tensile stresses acting on the axially outermost circumferential elements located closest to the coupling. The thickness of the decoupling profiles between the top working layers, axially outside the coupling zone, measured at the ends of the narrowest working layer, shall be at least 2 mm, and preferably greater than 2.5 mm. According to an advantageous embodiment of the invention, the reinforcement elements of at least one layer of circumferential reinforcement elements are metallic reinforcement elements having a secant modulus at 0.7% elongation between 10 and 120 GPa and a maximum tangent modulus less than 150 GPa. According to a preferred embodiment, the secant modulus of the 0.7% elongation reinforcement elements is less than 100 GPa and greater than 20 GPa, preferably between 30 and 90 GPa and preferably still less than 80 GPa. Preferably also, the maximum tangent modulus of the reinforcing elements is less than 130 GPa and preferably even less than 120 GPa. The moduli expressed above are measured on a tensile stress-elongation curve determined with a preload of 20 MPa. The tensile stress corresponds to a measured tension referred to the metal cross-section of the reinforcing element. The measurements are taken on cables extracted from the tire along a portion of the circumferential reinforcing element layer extending from one axial end of said layer over an axial width of 50 mm towards the interior of said layer. The moduli of the same reinforcement elements can be measured on a tensile stress-elongation curve determined with a prestress of 10 MPa. The tensile stress corresponds to a measured tension referred to the overall cross-section of the reinforcement element. The overall cross-section of the reinforcement element is the cross-section of a composite element made of metal and rubber, the latter having penetrated the reinforcement element during the tire curing process. According to this formulation relating to the overall section of the reinforcement element, the reinforcement elements of the axially external parts and of the central part of at least one layer of circumferential reinforcement elements are metallic reinforcement elements having a secant modulus at 0.7% elongation between 5 and 60 GPa and a maximum tangent modulus less than 75 GPa. According to a preferred embodiment, the secant modulus of the 0.7% elongation reinforcement elements is less than 50 GPa and greater than 10 GPa, preferably between 15 and 45 GPa and preferably still less than 40 GPa. Preferably also, the maximum tangent modulus of the reinforcing elements is less than 65 GPa and preferably even less than 60 GPa. According to a preferred embodiment, the reinforcing elements of at least one layer of circumferential reinforcing elements are metallic reinforcing elements having a tensile stress-as-relative-elongation curve with low slopes for low elongations and a substantially constant and steep slope for higher elongations. The different characteristics of the reinforcement elements stated above are measured on reinforcement elements taken from tires. Reinforcing elements particularly suited to creating at least one layer of circumferential reinforcement elements according to the invention are, for example, assemblies of formula 21.23, whose construction is 3x(0.26+6x0.23) 4.8 / 7.5 SS; this stranded cable consists of 21 elementary wires of formula 3 x (1+6), with 3 strands twisted together, each consisting of 7 wires, one wire forming a central core with a diameter of 0.26 mm, and 6 wound wires with a diameter of 0.23 mm. Such a cable has a secant modulus at 0.7% of 45 GPa and a maximum tangent modulus of 98 GPa, measured on a tensile stress-elongation curve determined with a prestress of 20 MPa, the tensile stress corresponding to a measured tension referred to the metal cross-section of the reinforcing element.On a tensile stress versus elongation curve determined with a prestress of 10 MPa, the tensile stress corresponding to a measured tension referred to the overall section of the reinforcing element, this cable of formula 21.23 has a secant modulus at 0.7% equal to 23 GPa and a maximum tangent modulus equal to 49 GPa. Similarly, another example of a reinforcing element is a 21.28 assembly, whose construction is 3x(0.32+6x0.28) 5.6 / 9.3 SS. This cable exhibits a secant modulus at 0.7% of 56 GPa and a maximum tangent modulus of 102 GPa, measured on a tensile stress-strain curve determined with a prestress of 20 MPa, the tensile stress corresponding to a measured tension referred to the metal cross-section of the reinforcing element. On a tensile stress-strain curve determined with a prestress of 10 MPa, the tensile stress corresponding to a measured tension referred to the overall cross-section of the reinforcing element, this 21.28 cable exhibits a secant modulus at 0.7% of 27 GPa and a maximum tangent modulus of 49 GPa. The use of such reinforcing elements in at least one layer of circumferential reinforcing elements makes it possible in particular to maintain satisfactory layer stiffness even after the shaping and baking stages in conventional manufacturing processes. According to a second embodiment of the invention, the circumferential reinforcing elements can be formed from inextensible metallic elements and cut to form sections of length much shorter than the circumference of the shortest layer, but preferably greater than 0.1 times said circumference, the cuts between sections being axially offset from one another. Even more preferably, the tensile modulus of elasticity per unit width of the additional layer is less than the tensile modulus of elasticity, measured under the same conditions, of the most extensible top working layer.Such an embodiment makes it possible to give, in a simple way, to the layer of circumferential reinforcing elements a modulus that can easily be adjusted (by choosing the intervals between sections of the same row), but in all cases, lower than the modulus of the layer made up of the same metallic elements but continuous, the modulus of the additional layer being measured on a vulcanized layer of cut elements, taken from the tire. According to a third embodiment of the invention, the circumferential reinforcing elements are corrugated metallic elements, the ratio a / A of the wave amplitude to the wavelength being at most equal to 0.09. Preferably, the tensile modulus of elasticity per unit width of the additional layer is less than the tensile modulus of elasticity, measured under the same conditions, of the most extensible working top layer. The metallic components are preferably steel cables. According to a preferred embodiment of the invention, the reinforcement elements of the top working layers are inextensible metal cables. Other advantageous details and features of the invention will become apparent from the description of examples of embodiments of the invention with reference to [Fig.1] which represents a meridian view of a diagram of a tire according to one embodiment of the invention. Figure 1 is not shown to scale for ease of understanding. The figure represents only a half-view of a tire which extends symmetrically with respect to the axis XX", which represents the circumferential median plane, or equatorial plane, of a tire. In [Fig. 1], the tire | is size 315 / 70 R 22.5. Said tire 1 comprises a radial carcass reinforcement 2 anchored in two beads, not shown in the figure. The carcass reinforcement is formed of a single layer of cords metallic. This frame reinforcement 2 is confined by a top reinforcement 4, formed radially from the inside to the outside: - a first working layer 41 formed of metal cables oriented at an angle of 18°, - a layer of circumferential reinforcing elements 43 formed of 21x23 steel metal cables, - of a second working layer 42 formed of metal cables oriented at an angle equal to 30° and crossed with the metal cables of layer 41, the cables of each of the working layers 41, 42 being oriented on either side of the circumferential direction. The metal cables constituting the reinforcement elements of the two working layers are formula 9.35 cables. They are distributed in each of the working layers with a distance between the reinforcement elements, measured along the normal to the direction of the mean line of the cable equal to 2.2 mm. The top reinforcement is itself capped with a tread 5. The tire is inflated to a pressure of 9 bars. The axial width L4, of the first working layer 41 is equal to 252 mm. The axial width L4; of the second working layer 42 is equal to 232 mm. The axial width L43 of the layer of circumferential reinforcing elements 43 is equal to 194 mm. The axial width of the tread Ls is equal to 266 mm. The maximum axial width L is equal to 315.9 mm. The combined mass of the two working layers 41, 42 and the layer of circumferential reinforcing elements 43, including the mass of the metal cables and the calendering mixtures, thus amounts to 10.1 Kg. The difference between the angles formed by the cables of the first working top layer with the circumferential direction and those of the cables of the second working top layer is equal to 12°. The average angle is equal to 23.4° and is well within the range of 18.6° to 32.7°. The measured value of Re is equal to 508.5 mm. The measured value of Es is equal to 22.5 mm. The average RL value of the measured radii is equal to 401.3 mm. The Rt value determined on the tire is equal to 792 mm. The calculated value of Tc is equal to 308 N / mm. The calculated value of Cz is equal to -0.0029. The value of F1 is equal to 477.2 N. The value of F2 is equal to 323.3 N. The breaking forces of the reinforcement elements of the top working layers FR] and FR2 are equal to 2950 N. The F2 / FR2 breaking potential utilization ratio is equal to 11%. The F1 / FR1 breaking potential utilization ratio is equal to 16.2%. The F1 / FR1 rupture potential utilization ratio is 48% higher than the F2 / FR2 rupture potential utilization ratio. Various tires II, I2, I3, I4 and IS according to the invention are compared to various reference tires of the same size. Tires II, I2, I3, I4 and I5 have calendered layers made up respectively of the compounds 1, 2, 3, 4 and 5 described below. The first reference T1 tires differ from the tires according to the invention by the nature of the calendering compounds of the top working layers, these being made up of the R1 compound described below. The different mixtures used are listed below, expressing for each the secant modulus of elasticity at 10% elongation as well as the tan(8)max- values [Table 1] | Mixture R1 | png to | jraces this | Eat | in 100 100 160 15 10 bones | BlackN7 2" | ForN683 — 44 — 30 — 4 | Noë N326 of Silica 165G | Antioxidant (SPFDI 15 Res sa L65 Dé tés | Veteric acid — 53 5 T2 23 f 23 93 | Zinc oxide — in test st “F 11? in — | Cobait Salt (AcneCo) | Salt da = Cobalt 83 | Sas as sor — | sulfur &i C 42 if tos if if | DCBS Accelerator j 653 t8 0.3. 083 585 | 30t | TBBS Arcelor emo | 4 #4 | Retrider CTPPYI) | 625 ff! 025 0 025! 525 | MA Pa} 164 ses 558 225 êis 44 | task 2290 0.089 som 0063 3056 The values ​​of the constituents are expressed in pce (parts by weight per hundred parts of elastomers. Second reference tires T2 differ from reference tires T1 by a crown reinforcement formed radially from the inside out: - a first working layer formed of metal cables oriented at an angle of 22°, on the same side as the cables of the triangulation layer with respect to the circumferential direction, - a layer of circumferential reinforcing elements made of 21.23 steel metal cables, - a second working layer formed of metal cables oriented at an angle of 18° and crossed with the metal cables of the first working layer, the cables of each of the working layers being oriented on either side of the circumferential direction, - a protective layer formed of 6.35 elastic metal cables, the distance between the reinforcing elements of which, measured along the normal to the direction of the mean line of the cable, is equal to 2.5 mm, oriented at an angle equal to 18°, on the same side as the cables of the second working layer. The metal cables of the two working layers are formula 9.35 cables. They are distributed in each of the working layers with a distance between the reinforcing elements, measured along the normal to the direction of the mean line of the cable equal to 2.5 mm. The reference T2 tire is inflated to a pressure of 9 bars. The axial width of the first working layer is equal to 252 mm. The axial width of the second working layer is equal to 232 mm. The axial width of the layer of circumferential reinforcing elements 53 is equal to 194 mm. The axial width of the protective layer is equal to 188 mm. The combined mass of the working layers, the protective layer and the circumferential reinforcement layer of the T2 reference tires, including the mass of the wire ropes and calendering compounds, amounts to 12.2 Kg. The absolute value of the difference between the absolute values ​​of the angles formed by the cables of the first working top layer with the circumferential direction and those of the cables of the second working top layer is equal to 4°. The average angle is equal to 19.9°. The value of F1 is equal to 410 N. The value of F2 is equal to 424 N. The F1 and F2 values ​​are obtained by a finite element simulation, the high number of reinforcement layers in the top not allowing the use of a simple analytical model. The breaking forces of the reinforcement elements of the top working layers FR1 and FR2 are equal to 2600 N. The F2 / FR2 breaking potential utilization ratio is equal to 16.3%. The utilization ratio of the F1 / FR]1 breaking potential is equal to 15.8%. The F1 / FR]1 breaking potential utilization ratio is 3% lower than the ratio of use of the F2 / FR2 rupture potential. Initial endurance tests, particularly thermally demanding, were carried out on a test machine imposing on each of the tires a straight line run at a speed equal to the maximum speed index prescribed for said tire (speed index) under an initial load of 4000 Kg progressively increased to reduce the duration of the test. Other endurance tests, particularly demanding mechanically, were carried out on a test machine that cyclically applied a transverse force and a dynamic overload to the tires. The tests were performed on the tires according to the invention under conditions identical to those applied to the reference tires. The tests thus carried out showed that the distances covered during each of these tests are substantially identical for the tires according to the invention and the reference tires. Tests to characterize the impact resistance of a tire crown reinforcement were also performed. These tests consist of rolling a tire, inflated to a recommended pressure and subjected to a recommended load, over a cylindrical indenter with a diameter of 1.5 inches (38.1 mm), a hemispherical head, and a predetermined height, positioned at the center of the tread. The impact resistance is characterized by the critical height of the indenter, that is, the maximum height of the indenter that causes total rupture of the crown reinforcement, meaning the failure of all crown layers. The values ​​express the energy required to cause rupture of the crown block. The values ​​are expressed with a base of 100 corresponding to the measured value for the reference tire T2. [Tables 2] T1, 12, 13, 4, IS 130 Reference Ti {25 Reference T? 100 These results show that despite a reduction in the weight of the tire, in particular by a reduction in the mass of its crown reinforcement, the energy at rupture during an impact on the surface of the tread of the tires according to the invention is greater than that of the T2 tire and slightly greater than that of the "TI" tire. Final endurance tests designed to replicate driving conditions combining vehicle speed and particularly aggressive ground conditions have been carried out. These tests thus reproduce extreme conditions, notably those experienced by heavy goods vehicles of the "construction site approach" type. These latest tests consist of reproducing twenty-five times a 2-hour driving phase at 100 km / h on a circuit under the load and pressure conditions indicated on the tire followed by a 12-minute driving phase at 35 km / h on a gravel track. The low-speed driving phase on a gravel track aims to penalize endurance due to repeated impacts on the tread. The purpose of high-speed track driving is to increase tire temperature. This makes the tire more susceptible to the effects of repeated impacts on the tread and promotes the propagation of cracks initiated during driving on gravel tracks. At the end of the test, the tires are inspected using shearography and meticulously examined for any damage. This visual analysis allows for comparison of any cracks and their propagation. The tires are then scored and compared. A score above 100 indicates a less damaged tire, while a score of 100 is assigned to the most damaged tire. Only tires according to invention II and 14 were tested. [Tables 3] Note T4 120 It {115 Reference Ti 105 Reference T? 100 At the end of the test, the tires according to the invention II and 14 show less extensive damage than the reference tires T1 and T2. During these final endurance tests, the temperature of the tires at the ends of the crown block was measured following the first phase of 2 hours of driving at 100 km / h on a circuit. During these latest tests, reference T3 tires, which differ from reference T2 tires in the nature of the calendering compounds of the top working layers (these being made of compound 4), were also measured. [Tables 4] Temperature 14 90°C It has 1°C Reference Ti 94"C Reference T3 95°C — Reference T2 99°C These results show that the tires according to the invention do indeed exhibit lower temperatures than the reference tires.

Claims

Demands

1. Tire (1), intended to be mounted on a 15° type hollow rim drop center, comprising a radial frame reinforcement (2), said pneumatic including a top frame (4), comprising two top layers of working elements (41, 42) for reinforcement inserted between two layers of calendered rubber compound, crossed from one layer to the next, following the circumferential direction rential of angles (al, a2) greater than 8°, said angles al and a2 being oriented on either side of the circumferential direction, and at less a layer of circumferential reinforcing elements (43), the apex reinforcement (4) being radially capped with a band of bearing (5), said tread being joined to two beads (3) via two flanks, in which: - the said two top working layers (41, 42) and the said at least a layer of circumferential reinforcing elements (43) being only present to constitute the vertex armature (4) on at least 75% of the axial width of the apex reinforcement (5), - the reinforcement elements of the top working layer (42) radially the outermost forming an angle (a2) with the direction greater circumferential in absolute value than the angle (œl) formed by the reinforcement elements of the top working layer (41) ra- dialement the innermost one with the circumferential direction, - the absolute value of the difference between the absolute values ​​of the angles (a2) and (al) is greater than 7°, - the mean angle satisfies the relation: 13+131#exp(-L / 100) < a < 28+110*exp(-L / 100), a being defined by the relation a = Arctan((tan(lall)*#tan(la21))"), L being the maximum width of the tire measured along the axial direction and expressed in mm characterized in that the secant modulus of elasticity at 10% elongation of at least one calendered layer of at least one layer of peak working is less than 8.5 MPa.

2. Pneumatic according to claim 1, characterized in that the value maximum of tan(), denoted tan(6)max, of said at least one layer of calendering of at least one working top layer is less than 0.

100.

3. Pneumatic according to claim 1 or 2, characterized in that said at least one calendering layer of at least one vertex layer the working material is an elastomeric compound based on natural rubber or synthetic polyisoprene with a majority of cis-1,4 chains and possibly temporarily of at least one other diene elastomer, rubber natural or synthetic polyisoprene in case of cutting being present at a majority rate compared to the rate of the other or others diene elastomers used and a reinforcing filler consisting of: a) either by carbon black with a specific surface area BET greater than 60 m° / g, 1) employed at a rate between 20 and 40 percent when the index of black structure (COAN) is greater than 85, 11) employed at a rate between 20 and 60 percent when the index of black structure (COAN) is less than 85, b) either by carbon black with a specific surface area BET of less than 60 m° / g, whatever its structure index, used at a rate between between 20 and 80 pieces, and preferably between 30 and 50 pieces, c) either by a white filler of the silica and / or alumina type comprising SIOH and / or AIOH surface functions chosen from the group formed by precipitated or pyrogenated silicas, aluminas or alumino- silicates or even carbon blacks modified during or after the specific surface area synthesis BET between 30 and 260 m² / g employed at a rate between 20 and 80 percent, and preferably between 30 and 50 pieces, d) either by cutting carbon black as described in (a) and / or carbon black carbon described in (b) and / or a white charge described in (c), in which the overall load rate is between 20 and 80 percent, and preferably between 40 and 60 pieces. [Pneumatic claim (1) according to any one of claims 1 to 3, characterized in this that the ratio of use of the F2 / FR2 rupture potential of the layer of the outermost radial working apex (42) is less than 1 / 6, in which: FR2 is the uniaxial tensile breaking strength of each of the cables of the outermost radially working top layer, F2 = p, * Tc * [(tan(la1l) / ((tan(la1)+tan(la2l))) / cos?(la2l) + Cr], with Tc = 0.078 * P * Rs* ( 1-(Rs2-RL2) / (2*Rt*Rs) ), P: the nominal inflation pressure of the tire according to ETRTO, C; = 0.00035#(min(({L-80) / sin(la1!), (L-80) / sin(lo2l), 480)-480), Pp; : the spacing of the reinforcement elements of the layer of radially outermost working apex, measured perpendicularly lally to the reinforcing elements at the level of the circum- median ferrentiel, Rs = Re - Es, Re: Outer radius of the tire measured at the radial point more external on the surface of the tire tread, said surface being extrapolated to fill any potential gaps, Es: radial distance between the radially outermost point of the pneumatic and its orthogonal projection onto the radially ex- face exterior of a reinforcement element of the top layer of work radially, furthest inside, RL: average of the radii of the axially outermost points of each side of the tire, Rt: the radius of the circle passing through three points located on the surface ex- outer edge of the tread outside the grooves, defined from from one shoulder end to equal respective axial distances at LA, 2 and % of the tread width.

5. Pneumatic (1) according to claim 4, characterized in that the ratio of utilizing the F2 / FR2 rupture potential of the top layer of work (42) radially outermost is less than 1 / 8.

6. Pneumatic (1) according to claim 4 or 5, characterized in what is the ratio of the utilization of the rupture potential F1 / FR]1 of the layer the innermost radial working apex (41) is less than 1 / 3, in which: FR is the uniaxial tensile breaking strength of each of the cables of the innermost radially working layer, F1 =p, * Tc * [(tan(l:21) / ((tan(lœ11)+tan(la21))) / cos2(la11) + C;], with P1! The step for laying the reinforcement elements of the layer of radially innermost working apex, measured perpendicularly lularly to the reinforcement elements at the level of the median plane circum- conference.

7. Pneumatic (1) according to claim 6, characterized in that the ratio utilization of the F1 / FR1 rupture potential of the working layer (41) radially the innermost is at least 30% greater than the ratio utilization of the F2 / FR2 rupture potential of the working layer (42) radially the outermost.

8. Pneumatic (1) according to any one of the preceding claims, characterized in that the two top working layers (41, 42) and said at minus one layer of circumferential reinforcing elements (43) are only present to constitute the apex framework over the entirety of the axial width of the apex reinforcement (4).

9. Pneumatic (1) according to any one of the preceding claims, characterized in that the axial widths of the two top working layers (41, 42) are greater than the axial width of said at least one layer of circumferential reinforcing elements (43).

10. Pneumatic (1) according to claim 9, characterized in that the The top working layers (41, 42) are on either side of the plane equatorial and in the immediate axial extension of the said at least a layer of coupled circumferential reinforcing elements (43) on an axial width, to then be decoupled by profiles of rubber compound at least over the remaining common width to the said two working layers (41, 42).

11. Pneumatic (1) according to any one of the preceding claims, characterized in that the reinforcing elements of said at least one layer circumferential reinforcing elements (43) are elements of metallic reinforcements exhibiting a secant modulus of 0.7% elongation between 10 and 120 GPa and a tangent modulus maximum less than 150 GPa.

12. Pneumatic (1) according to any one of the preceding claims, characterized in that the reinforcement elements of the top layers of work (41, 42) are inextensible.