WO2019219694A1

WO2019219694A1 - Method for producing a composite spring, and composite spring

Info

Publication number: WO2019219694A1
Application number: PCT/EP2019/062361
Authority: WO
Inventors: Gregor Daun; Jan Wucherpfennig; Christian KORFF
Original assignee: Basf Polyurethanes Gmbh
Priority date: 2018-05-14
Filing date: 2019-05-14
Publication date: 2019-11-21

Abstract

The invention relates to a method for producing a composite spring (1), more particularly a composite helical compression spring, having a length (L). According to the invention, the method comprises the following steps: - producing or providing a composite spring (1) having an original length (Lo), wherein the composite spring (1) is made of a fibre-reinforced composite material having a plastic matrix material and reinforcement fibres embedded therein and - thermally preloading the composite spring (1) to the length (L) by means of compression at a predefined temperature for a predefined period, the length (L) being less than the original length (Lo). The invention further relates to a composite spring, more particularly a composite helical compression spring, which is made of a fibre-reinforced composite material having a plastic matrix material and reinforcement fibres embedded therein, wherein the composite spring is thermally preloaded under compression.

Description

Process for making a composite spring, and the like

The present invention relates to a method for producing a composite spring, in particular a composite helical compression spring. The invention further relates to such a composite spring, in particular a composite helical compression spring, which is formed from a fiber composite material, which has a plastic matrix material and embedded therein reinforcing fibers.

Composite springs of the type used in connection with the invention are generally available. Their properties are described, for example, in EP 0637700 B1, in Jang, D. and Jang, S., "Development of a Lightweight CFRP Coil Spring," SAE Technical Paper 2014-01-1057, 2014, doi: 10.4271 / 2014-01-1057 ; in Sardou, M. "Darmotte, E." and Zunino, C., "Light Weight, Low Cost, Composite Coil Springs are a Reality", SAE Technical Paper

2005-01-1698; and in Yahya Kara (2017) A Review: Fiber Reinforced Polymer Composite Helical Springs. J Mater Be Nanotechnol 5 (1): 101. doi: 10.15744 / 2348-9812.5.101.

Composite springs of the aforementioned type are used, among other things, for lightweight construction applications. Since composite springs are generally not susceptible to corrosion problems, have high fatigue strength, are nonmagnetic, and cause low noise emissions, composite springs are also increasingly being considered for the automotive industry. One of the challenges that composite springs have to face is their material behavior as the composite spring increases in service life. So far showed composite springs under

Load with increasing operating time and / or increasing number of experienced load cycles compared to steel springs a pronounced creep, resulting in constant force to a set as set shortening of the spring length and constant clamping length to a relaxation called reduction in the spring force. Setting at constant force and relaxation at constant length describe the same change process under different boundary conditions. Both have in common that the rate of change is high at the beginning and decreases with time and the resulting absolute change is often proportional to the logarithm of the operating time. While the normal, elastic suspension travel is designated as s, the change of the suspension travel by setting is called as ace. It is common in steel springs to describe creep over the ratio of creep deformation to spring travel As / s.

In automotive applications, this seating behavior can lead to an undesirable reduction in vehicle height or inclination or even alignment of the headlights. Several earlier attempts to introduce the composite coil spring in automobiles have also failed in this behavior.

In order for composite springs to become more competitive with respect to their fatigue behavior over the entire lifetime relative to the well-known steel springs, the aim was to improve the setting behavior of composite springs. It is generally known, static or dynamically pre-set steel springs in the context of the manufacturing process in order to limit a subsequent setting after delivery of the springs during operation as possible. In the case of steel springs, the process of presetting in the past has been carried out both cold and at elevated temperature by plasticizing beyond the elastic deformation of the steel springs. These mechanisms are always considered not to be composites. i.e. Composite materials has been considered applicable.

For thermoplastics, it is known that under mechanical stress at elevated temperatures plastic deformation of the material occur, which is the basis for various forming processes. In thermosetting plastics, such as epoxy resins, targeted influencing of the material structure under elevated temperatures is unusual. If anything, from the literature, a transformation of such thermoplastics above the

Glass transition temperature was taken. In this area, however, you work close to or above the load limit of the material, which can lead to breakage.

The invention was in the light of the object of the invention to provide a method for producing composite springs, in particular of composite helical compression springs, which leads to a composite spring with improved fatigue behavior, in particular to a composite spring with reduced settling behavior during operation.

The invention solves its underlying object in a method of the type described by proposing the following steps: producing or providing a composite spring with an original length Lo, wherein the composite spring is formed of a fiber composite material, which is a plastic Matrix material and reinforcing fibers embedded therein, and thermally setting the composite spring by compression at a predetermined temperature for a predetermined period of time to a designated length L, wherein the designated length L is less than the original length Lo, after the step of manufacturing or providing.

According to the invention, the term "composite" is understood to mean a composite material.

The invention is based on the surprising finding that, contrary to the opinion solidified in the prior art, it is possible to thermally set composite springs above room temperature at predetermined temperature and time conditions. As can be seen in particular from the descriptions of the preferred embodiments, it was also possible to identify process conditions which, despite significantly improved setting behavior, have no significant disadvantageous effect on the life of the composite spring. In that regard, reference is made to the following statements.

In a preferred embodiment of the invention, the predetermined temperature for the thermal presetting is below the glass transition temperature of the plastic matrix material. It has been recognized that inadmissibly strong plastic deformation of the material structure and fractures of the spring can be largely avoided during pre-setting, as long as the thermal Vorsetzen done at temperatures below the glass transition temperature of the plastic matrix material. If, in the context of the invention, the glass transition temperature is used, it is with regard to

quantitative determination of the glass transition temperature from the measurement by means of dynamic mechanical analysis (DMA) assumed. This measurement can be performed on dedicated DMA meters or in an analogous manner using a rheometer. The optimization consists essentially in the selection of the right temperature. At room temperature, the presetting process for a technical application is very or too slow to be considered for industrial production. The necessary deformation for reducing later settling is comparatively low, since presumably primarily local stress peaks in the material are broken down. At temperatures close to the glass transition temperature, the presetting process is very fast, but the deformation is extensive and quickly reaches an unacceptably high level. In general, it can also be observed that the lower the subsequent settling amount in a business, the larger the reserve amount must be, especially in a disproportionately high ratio. Preferably, the predetermined temperature is in a range between room temperature and below the glass transition temperature of the plastic matrix material.

The predetermined temperature is preferably in a range of above 30 ° C. More preferably, the predetermined temperature for the example springs in a range of 40 ° C to 1 10 ° C, more preferably from 40 ° C to 90 ° C or 40 ° C to 80 ° C.

The optimum choice of the predetermined temperature is a function of the plastic matrix material used. In a preferred embodiment, the predetermined temperature is 2K or more below the glass transition temperature of the respective intended plastic matrix material, preferably 5K or more below the glass transition temperature of the respectively provided plastic matrix material. More preferably, the predetermined temperature is in a range between 5K below the glass transition temperature and 90K below the glass transition temperature.

Still more preferably, the predetermined temperature is in a range of 30K to 90K below the glass transition temperature of the plastic matrix material, and more preferably 40 ° C or more or even 60K or more below the glass transition temperature of the plastic matrix material. They are preferably

Plastics selected for the plastic matrix material, which have a glass transition temperature of 120 ° C or higher.

The predetermined period for the presetting operation is preferably in a range of up to 24 hours, preferably in a range of 1 second to 8 hours, more preferably in a range of 1 second to 10 minutes, particularly preferably in a range of 8 hours to 24 hours, especially at temperatures in a range of 80 ° C to 90 ° C. In this time window finds the knowledge of the invention when presetting the composite spring, the most significant reduction of setting instead. In the context of the invention, the finding has emerged that the presetting time should be shorter the closer the presetting temperature approaches the glass transition temperature. Therefore, a particularly gentle pre-setting will preferably be carried out at temperatures which are in the lower range of the value ranges indicated above, and the presetting time will be oriented towards the upper end of the abovementioned ranges. On the other hand, it is particularly preferred to keep the shortest possible presetting periods at temperatures in the vicinity of the glass transition temperature.

For example, in an alternative embodiment, the presetting temperature is in a range between 105 ° C and 15 ° C, and the predetermined time for presetting is in a range between 3 minutes and 7 minutes. In a further preferred embodiment, the presetting temperature is 1 15 ° C to 120 ° C, and the Vorsetzdauer in a range between 30 seconds and 90 seconds. In these areas occurs in most composites used after Vorsetzen only low setting behavior, and there is equally no premature breakage of the spring to be feared.

By extending the period beyond the aforementioned range, further improvements can be made in terms of presetting. However, the latter area in particular represents a preferred compromise with a view to economic production. The composite spring preferably has a block dimension Lmin when fully compressed, and is compressed during presetting to a presetting length Lv which is greater

or equal to the block dimension and preferably less than the length of the composite spring to be produced.

In a preferred embodiment, the step of presetting comprises a static compression by applying a predetermined compression force, hereinafter also referred to Vorsetzkraft. The compression force is preferably kept constant during Vorsetzens. Depending on how long the period of the presetting operation is set, a change in length of the presetting length Lv during pre-setting is observed with constant compression force control.

Alternatively or as an additional step, the presetting length is preferably kept constant during presetting. When the Vorsetzlänge is controlled during the presetting step, a reduction in the compression force necessary to hold the presetting length is observed due to the onset of relaxation of the composite spring, depending on the selected pre-setting period.

In both variants, the spring can be heated to the presetting temperature before and / or during upsetting. In both variants, the spring can be relaxed under temperature after completion of Vorsetzens or cooled in the tensioned state. With both variants, a desired combination of spring force and spring length can be set. In addition, slight statistical variations in spring length can be compensated for in this step. This applies in particular to a correction / optimization of the force penetration point and for compliance with a length tolerance at delivery or the spring rate tolerance.

If, in the context of the invention, a plastic matrix material is used, then in the context of this is meant any material which is suitable for fixing the fibers in their position after being deposited. The higher the softening temperature, media resistance, fatigue strength, and processability of the material are formed, the more suitable the material is for use as a plastic matrix material in the present invention.

In preferred embodiments, the plastic matrix material is a thermosetting polymer, for example a polyester resin, vinyl ester resin, polyurethane resin or epoxy resin, polyurethane resin combinations (PU-PIR, PU acrylate).

In alternatively preferred embodiments, the plastic matrix material is a thermoplastic polymer, for example a polyamide, polypropylene, or polyethylene.

Depending on the selected plastic matrix material, the predetermined temperature to be provided for the presetting process is preferably set as a function of the glass transition temperature of the respectively selected plastic matrix material. The glass transition temperature of the respective material can either be taken from generally known data sheets intended for the materials, or alternatively determined experimentally.

The composite spring used in the method according to the invention preferably has a winding core around which at least one fiber roving is wound. In preferred embodiments, the winding core is preferably an arrangement of twisted fibers, a solid core, a sheathed solid core, a hollow core or a sheathed hollow core. In the context of the invention, a core is to be understood as meaning a component or element which is provided for depositing the fibers. In other words, a core may be any component or element having a deposition surface on which fibers can be deposited. The cross section of the core is preferably circular, but may be formed in principle, oval, elliptical, polygonal or mixed form of several of the above basic shapes. A hollow core would bring with it a weight reduction compared to a solid core due to an internal recess. If twisted fibers are used as the winding core, this is to be understood as meaning that the fibers are preferably twisted together.

Particularly preferably, the winding core is formed from a non-metallic material.

The at least one fiber roving of the composite is preferably formed of glass fibers, carbon fibers, aramid fibers, basalt fibers, sisal fibers, hemp fibers, cotton fibers or bamboo fibers, or a subcombination of two or more of these types of fibers.

Further preferably, the composite spring, and with it the finished composite spring, a multi-layered structure around the winding core around, wherein different rovings are arranged in several layers one above the other, or be. The rovings in the different layers preferably differ in their orientation relative to the rovings of other layers, each viewed relative to

Extension direction of the winding core, and / or in their fiber type, and / or in terms of their diameter.

In a further preferred embodiment, during the step of producing the composite spring, the embedding of the fiber material into the plastic matrix material takes place by means of impregnating the roving with an impregnating agent. An impregnating agent is to be understood in the context of the invention basically any plastic matrix material which leads by curing, such as by polymerization, for bonding the fibers and layers to one another and thus to a solidification of the fiber composite material. As impregnating agent in the context of the present invention, in particular a monomer or polymer-based liquid is used. In particular, the impregnating agent can be a liquid plastic matrix material during processing, such as, for example, a reactive, liquid thermoset system based on, for example, polyurethane, polyester, vinyl ester, epoxy resin, or a reactive thermoplastic system based on caprolactam, polyacrylic, or a thermoplastic Melt, for example based on polypropylene, polyethylene or polyamide. The impregnation takes place in preferred alternatives before, during or after the winding of the rovings onto the winding core.

The invention has been described above and according to a first aspect with reference to the inventive method. In a second aspect, the invention further relates to the initially described composite spring. The invention solves the task described in the composite spring in that the composite spring is pre-set thermally under compression. The composite spring according to the invention is preferably produced by the method according to the invention according to the first aspect and makes use of the same advantages and preferred embodiments as the method according to the invention.

Preferably, the plastic matrix material of the composite spring is a thermosetting polymer, for example a polyester resin, vinyl ester resin, polyurethane resin or epoxy resin. Alternatively, the plastic matrix material is a thermoplastic polymer, for example a polyamide, polypropylene, or polyethylene. The composite spring preferably has a winding core around which at least one fiber roving is wound.

The winding core of the composite spring is preferably formed from a non-metallic material, for example polyethylene.

The at least one fiber roving is preferably formed of glass fibers, carbon fibers, aramid fibers or a subcombination of two or more of these types of fibers.

Preferably, the composite spring has a multilayer structure around the winding core, with a plurality of (same or different) rovings in the plurality of layers being stacked.

To avoid repetition, reference is made to the above explanations regarding the method according to the first aspect with regard to the advantages of this preferred embodiment.

The invention will be described below with reference to the accompanying figures with reference to a preferred embodiment. Hereby show:

Figure 1 shows a schematic structure of a device for thermal

Pre-setting a composite spring,

FIG. 2 and FIG. 3 show alternative operating states of the device according to FIG. 1

Figure 4a is a detail view of a composite spring according to the invention, and

Figure 4b is a sectional view of a detail of the composite spring according to Figure 4a, and

Figure 5 is a graphical representation of test results.

FIG. 1 shows first a device 100 for the thermal presetting of composite springs. The device 100 has a receptacle 101, in which a composite spring 1 is taken with an original length Lo. The device 100 has a punch 103, which is movable by means of an actuator 105 in the direction of a longitudinal axis x for applying a Vorsetzkraft. The composite spring 1, which is formed in the present embodiment as a composite helical compression spring is coaxial with

Axis x aligned. In Figure 2, the process of thermal Vorsetzens is indicated. The composite spring 1 is compressed by the punch 103 to a Vorsetzlänge Lv, wherein on the composite spring 1 a Vorsetzkraft Fv acts. In addition, a predetermined temperature is set in the receptacle 101 by means of a heater 107, which assumes the composite spring 1. The state shown in Figure 2 is maintained in preferred embodiments for a period of between 1 minute and 24 hours. The length Lv to which the composite spring 1 is compressed is preferably greater than a block length D ™ (not shown) at which the turns of the composite spring 1 would abut directly adjacent turns. As well as the stamp can muster a Vorsetzkraft, alternatively, a Vorsetzweg can be applied, being driven to a clamped length.

FIG. 3 shows the device 100 of FIGS. 1 and 2 after the pre-setting process has been completed. The presetting force Fv is no longer applied to the stamp 103, and the stamp 103 has preferably been moved back to its original position by means of the actuator 105 as in FIG. The composite spring 1 in the interior of the receptacle 101 now assumes the intended for the final composite spring Lände L, which is less than or equal to the Vorsetzlänge Lv smaller than the original length Lo.

FIGS. 4a and 4b show, by way of example, a preferred structural design of the composite spring 1. The composite spring 1 has at least one first end turn 2, to which a plurality of turns 4 adjoin. The turns 4 may have identical turns or progressively and / or degressively changed slopes.

The composite spring 1 initially shown from the outside in FIG. 4a preferably has the structural structure shown in FIG. 4b. In its interior, the composite spring 1 on a winding core 3, which may be formed, for example, as a hollow core or solid core. Around the winding core 3, a first roving layer 5 with a first fiber orientation and layer thickness is arranged. At least one further roving layer is preferably arranged around the first roving layer 5, in the present exemplary embodiment a second roving layer 7 and a third roving layer 9. The roving layers 5, 7, 9 can be fiber thickness or orientation relative to the direction of extension of the winding core 3, fiber thickness or fiber thickness Fiber bundle thickness, layer thickness and other criteria differ, or even agree. Particularly preferably, the fiber rovings 5, 7, 9 are impregnated with the same impregnating agent and embedded in the same plastic matrix material. For this

the rovings 5, 7, 9 soaked, which can be done for example during, before or after the wrapping of the winding core 3.

To illustrate the invention, various embodiments have been carried out for the method to investigate the setting behavior of the thus treated composite springs 1.

The following examples examined dextrorotatory compression coil springs under compression with dimensions of spring diameter D = 108 mm, outside diameter of the composite strand d = 20 mm, inside diameter of the composite strand di = 10.0 mm. The slope was usually flatter at the end turns than at the middle turns. The pitch of the spring pads is listed in Table 1 with. The original length of the composite springs was Lo = 254 mm.

Table 1

As the plastic matrix material, an epoxy system composed of the resin bisphenol A disglycidyl ether (100 parts), the curing agent dicyandiamide (6.5 parts) and the accelerator Uron (3 parts) was used. The glass transition temperature of the system was measured by a dynamic mechanical analysis for the pure plastic at 137 ° C (polymer Tg, in the rheometer analog DMA). The material of the fiber rovings was glass, in particular 2400 tex rovings were used. The fiber mass fraction was determined at 65% + 1-2%. The springs had a winding core formed as a hollow core, namely a PE inliner of 10 mm outer diameter and 1 mm wall thickness, and therefore had no significant influence on the strength and rigidity of the springs.

The composite strands from which the composite springs were made were fabricated as continuous pull-twist tubes in a 50 m loop with 16 strands per ply. The springs were wound on an inner mold, enclosed with a two-part outer mold and cured after pulling the mechanically collapsing inner mold in the outer mold for 2 h at 120 ° C.

The composite springs had a multilayer structure. Several coats of fiber rovings were coarsely layered with a ratio of compressively loaded fibers to tensile fibers of 2/3: [-42 °, -48 °], [+ 42 °, + 48 °, + 42 °].

Test Series 1 - Load Level: Two composite springs were dynamically loaded without pretreatment at room temperature in a range from 18 ° C to 25 ° C at 5.3kN at a frequency of 3.5 1 / s and only broke after more than one million cycles. Tests for the static load of two further springs with 4 kN at 100 ° C and 1 10 ° C led to the break.

Series 2 - Settling Behavior: A single composite spring was loaded at a predetermined temperature five times as a result of up to 3.5 kN for initial evidence of deformation under temperature. Experiments were carried out at 40 ° C, 50 ° C, 60 ° C, 70 ° C, 80 ° C, 90 ° C, 100 ° C, 1 10 ° C, 120 ° C. The presetting length Lv of the composite springs decreased as a result of setting. For each temperature, the difference between the largest and the smallest deflection at 3.5 kN within the five strokes was determined as the settling amount at this temperature. The data showed that this difference increased with increasing temperature as a consequence of faster settling. This series of tests already proves the strong influence of the temperature on the setting process. During the last cycle at 120 ° C, the composite spring softened so much that it could only build up the 3.5 kN as a counterforce four times, and at the fifth stroke at 2.6 kN it was pushed together to block size.

The results are shown in Table 2:

Table 2; *: only 4 strokes possible

Trial 3 - Dynamic setting after pruning:

Three different composite springs were each loaded for 24 h in a static test unit with 3.5 kN Vorsetzkraft or compression force. A first composite spring was set at 60 ° C, a second composite spring was preset at 80 ° C and a third

Composite spring was preset at 100 ° C. Following this, they were dynamically pulsed over 500,000 cycles of 0.2 - 3.5 kN at room temperature (23 ° C) and a test frequency of 3.5 1 / s compared to a non-thermally superior fourth composite spring. None of the composite springs broke. The deposit amount was determined. The set at 80 ° C spring was then applied for more than 800,000 cycles with 4.6 kN without breaking, which proves that no significant damage was caused by the Vorsetzen.

The results are shown in Table 3:

Table 3

Series 4 - Static setting at elevated temperature after priming:

Two identical composite springs Fi and F2 were each loaded in a static test unit at a temperature of 100 ° C with 3.5 kN. The first composite spring Fi was loaded for 1 h, and the second composite spring F2 was loaded for a period of 24 h. Subsequently, both composite springs Fi, F2 were cooled under load to below 30 ° C. After that, they were statically loaded for 24 h and 96 h, respectively, compared to a non-preloaded third composite spring F3, whereby the first composite spring Fi and the third composite spring F3 were loaded with 3.5 kN and the over 24h supervisor second composite spring F2 was loaded with 2.5 kN. The load was carried out at a temperature of 80 ° C. The comparison between 2.5 and 3.5 kN tests is permissible since the plastic change in the path As is related to the elastic displacement s. The results are shown in FIG. As is apparent from Fig. 5, the setting behavior of the springs Fi and F2 was compared to the non-superior composite spring F3 significantly reduced. It should be noted that the loaded at 100 ° C for 1 h composite spring Fi was set by only about 17 mm, while the loaded at 100 ° C for 24 h composite spring F2 already been preset by about 84 mm was.

It has thus been shown that pre-setting composite springs at elevated temperatures significantly reduces settling under static or dynamic loading. The process of presetting can be significantly accelerated by increasing the temperature and the subsequent setting behavior can be significantly reduced. If the glass transition temperature is exceeded and the setting time is limited, large-scale deformations and potential damage to the composite springs can also be avoided.

Claims

Claims:

A method of making a composite spring (1), in particular a composite helical compression spring, of a designated length (L) comprising the steps of:

- Making or providing a composite spring (1) with an original length

(Lo), wherein the composite spring (1) is formed of a fiber composite material having a plastic matrix material and reinforcing fibers embedded therein, and

- Thermal Vorsetzen the composite spring (1) by compression at a predetermined temperature over a predetermined period of time to the length (L), wherein the length (L) is less than the original length (Lo), after the step of manufacturing or providing.

2. The method according to claim 1,

wherein the predetermined temperature is below the glass transition temperature of the plastic matrix material.

3. The method according to claim 2,

wherein the predetermined temperature is 5K or more below the glass transition temperature of the plastic matrix material, preferably in a range between 5K below the glass transition temperature of the plastic matrix material and 90K below the glass transition temperature of the plastic matrix material.

4. The method according to any one of the preceding claims,

wherein the predetermined period of time is in a range of up to 24 hours, preferably in a range of 8 hours to 24 hours.

5. The method according to any one of the preceding claims,

wherein the composite spring (1) has a block gauge (Lmin) and is compressed during presetting to a presetting length (Lv) larger than the block gauge (Lmin), and preferably smaller than the length (L).

6. The method according to any one of the preceding claims,

wherein the presetting step comprises static swaging by application of a predetermined crush force (Fv).

7. The method according to claim 6,

wherein during the presetting the compression force (Fv) is kept constant.

8. The method according to claim 6 or 7,

wherein during Vorsetzens the Vorsetzlänge (Lv) is kept constant.

9. The method according to any one of the preceding claims,

wherein the plastic matrix material is a thermosetting polymer, for example a polyester resin, vinyl ester resin, polyurethane resin or epoxy resin, or wherein the plastic matrix material is a thermoplastic polymer, for example a polyamide, polypropylene, or polyethylene.

10. The method according to any one of the preceding claims,

wherein the composite spring (1) has a winding core (3) around which at least one fiber roving (5, 7, 9) is wound around.

1 1. A method according to claim 10,

wherein the winding core (3) is formed of a non-metallic material.

12. The method according to claim 10 or 1 1,

the at least one fiber roving (5, 7, 9) made of glass fibers,

Carbon fibers, aramid fibers, basalt fibers, sisal fibers, hemp fibers, cotton fibers or bamboo fibers, or from a sub-combination of two or more of these types of fibers is formed.

13. The method according to any one of claims 10 to 12,

wherein the composite spring (1) has a multilayer structure around the winding core, wherein a plurality of fiber rovings (5, 7, 9) are arranged in two or more than two layers one above the other.

14. The method according to any one of claims 10 to 13,

wherein, during the step of producing the composite spring (1), embedding the fiber material into the plastic matrix material is accomplished by impregnating the fiber roving (s) with an impregnating agent.

15. Composite spring (1), in particular composite helical compression spring, which is formed from a fiber composite material which has a plastic matrix material and reinforcing fibers embedded therein,

characterized in that the composite spring (1) is set thermally under compression.