STRUCTURES COMPRISING POLYMERIC FIBERS

Abstract

The present invention relates a structure comprising rigid elements connected together by interconnecting elements in such a way to form a statically determined or statically over-determined structure, wherein said structure comprises at least one tension element comprising polymeric fibers having a stabilizing creep of at least 0.3% and at most 10% and a minimum creep rate lower than 110.sup.5% per second, said stabilizing creep and minimum creep being measured at a tension of 900 MPa and a temperature of 30 C. The present invention also relates to said structure being a framing structure, preferably a space frame; a suspended body; a platform, preferably a marine platform; or a wheel comprising spokes. Furthermore, the invention relates to the use of polymeric fibers having a stabilizing creep of at least 0.3% and at most 10% and a minimum creep rate lower than 110.sup.5% per second, said stabilizing creep and minimum creep being measured at a tension of 900 MPa and a temperature of 30 C. for a statically determined or statically over-determined structure, preferably for a framing structure, such as a space frame; for a suspended body; for a platform, preferably for a marine platform; or for a wheel comprising spokes.

Claims

1. A structure comprising rigid elements connected together by interconnecting elements in such a way to form a statically determined structure or a statically over-determined structure, wherein said structure comprises at least one tension element comprising polymeric fibers having a stabilizing creep of at least 0.3% and at most 10% and a minimum creep rate lower than 110.sup.5% per second, said stabilizing creep and minimum creep being measured at a tension of 900 MPa and a temperature of 30 C.

2. The structure according to claim 1, wherein said at least one tension element comprises polymeric fibers having a stabilizing creep of at least 0.5% and at most 5%, as measured at a tension of 900 MPa and a temperature of 30 C.

3. The structure according to claim 1, wherein said at least one tension element comprises polymeric fibers having a minimum creep rate lower than about 410.sup.6% per second, preferably lower than about 210.sup.6% per second as measured at a tension of 900 MPa and a temperature of 30 C.

4. The structure according to claim 1, wherein the structure is a 2D structure or a 3D structure.

5. The structure according to claim 1, wherein the structure is a framing structure, preferably a space frame structure; a suspended body; a platform, preferably a marine platform, or a wheel comprising spokes.

6. The structure according to claim 1, wherein the structure is a marine platform, preferably an offshore tension leg platform, comprising up to three tension elements comprising no creep stabilizing fibers and at least one tension element comprising creep stabilizing fibers.

7. The structure according to claim 1, wherein the structure is a marine platform comprising up to three tension elements comprising no creep stabilizing fibers and at least one tension element comprising creep stabilizing fibers, and wherein the total number of said tension elements is at least four.

8. The structure according to claim 1, wherein said at least one tension element comprises creep stabilizing fibers, wherein said fibers comprise preferably a polyolefin, more preferably a polyethylene and most preferably a ultrahigh molecular weight polyethylene.

9. The structure according to claim 1, wherein said at least one tension element comprises creep stabilizing fibers, and wherein said fibers comprise ultrahigh molecular weight polyethylene comprising olefinic branches or chlorinated ultrahigh molecular weight polyethylene.

10. The structure according to claim 1, wherein said at least one tension element comprises creep stabilizing fibers, and wherein said fibers comprise ultrahigh molecular weight polyethylene comprising alkyl branches, preferably ethyl or butyl branches.

11. Use of a polymeric fiber having a stabilizing creep of about at least 0.3% and at most about 10% and a minimum creep rate lower than about 110.sup.7 per second, said stabilizing creep and minimum creep being measured at a tension of 900 MPa and a temperature of 30 C. for making a statically determined structure or a statically over-determined structure according to claim 1, preferably for making a framing structure, more preferably a space frame; or a suspended body; or a platform, preferably a marine platform; or a wheel comprising spokes.

Description

EXAMPLES

Methods of Characterization

[0054] IV: the Intrinsic Viscosity for UHMWPE is determined according to ASTM D1601-99 (2004) at 135 C. in decalin, with a dissolution time of 16 hours, with BHT (Butylated Hydroxy Toluene) as anti-oxidant in an amount of 2 g/I solution. IV is obtained by extrapolating the viscosity as measured at different concentrations to zero concentration. [0055] dtex: fibers' titer (dtex) was measured by weighing 100 meters of fiber. The dtex of the fiber was calculated by dividing the weight in milligrams to 10. [0056] Tensile properties of fibers, particularly of the structures according to the Examples and Comparative Experiments herein comprising three multifilament yarns structures: tensile strength (or strength) and tensile modulus (or modulus) and elongation at break (or elongation at fracture), force at break were defined and determined on the three multifilament yarns as specified in ASTM D885M, using a nominal gauge length of the fiber of 500 mm, a crosshead speed of 5%/min (elongation rate of about 25 mm/min), at room temperature (about 23 C.) and about 50% relative humidity and using cylinders as end fixtures for the yarn. The tests were done using two cylinders having a diameter of 12 mm as end-fixtures for the yarn, the yarn being wound 12 times around each cylinder (in general, the yarn can be wound at least 12 times around each cylinder) and then fixated (i.e. by a knot) to a hook at the bottom of each cylinder. On the basis of the measured stress-strain curve, the modulus of the fibers may be determined as the gradient between 0.3 and 1% strain. For calculation of the modulus and strength, the tensile forces measured are divided by the titer, as determined by weighing 10 metres of fiber; values in GPa are calculated assuming a density of 0.97 g/cm.sup.3. [0057] Tensile properties of Dyneema DM20, Dyneema SK75 and Twaron single yarns: tensile strength (or strength) and tensile modulus (or modulus) and elongation at break (or elongation at fracture) were measured on multifilament yarns as specified in ASTM D885M, using a nominal gauge length of the fibre of 500 mm, a elongation rate of 250 mm/min and Instron 2714 clamps, of type Fibre Grip D5618C, at room temperature (about 23 C.) and about 50% relative humidity. On the basis of the measured stress-strain curve, the modulus of the fibers may be determined as the gradient between 0.3 and 1% strain. For calculation of the modulus and strength, the tensile forces measured are divided by the titer, as determined by weighing 10 metres of fiber; values in GPa are calculated assuming a density of 0.97 g/cm.sup.3. [0058] The theoretical maximum achievable strength is the sum of the individual yarn strength values. The tests at fracture used in the present application were designed for equal strength at theoretical maximum. This was obtained by using in the tests yarns of approximately equal length. In practice, this maximum theoretical value is typically not reached because length differences cannot be avoided and therefore it may be also referred to as the maximum practical initial strength. This situation is simulated in the test to fracture by reducing the length of the middle yarn with about 1.5% compared with the length of the other two yarns and then measuring the strength (examples B). In a next test (examples C), a similar set-up was used but now about 1.5% yarn length difference was loaded for 2 weeks at 60% of the load level measured in example B. After 2 weeks, the fracture load was measured and the results are presented herein in Table 1. [0059] Creep lifetime and elongation during the creep lifetime were determined as described in document WO2012139934. [0060] Stabilizing creep and minimum creep rate in the fibers [0061] The stabilizing creep was determined by plotting the creep behavior (Elongation [%] of the fiber versus Time [seconds] of said fiber as shown in FIG. 15), at a tension of 900 MPa and a temperature of 30 C. and as mentioned in the tensile properties of the fiber herein above. A tangent line is constructed on the creep curve in FIG. 15, at the location where the creep rate is minimum (i.e. where the slope of the tangent line is minimum). The intersection point of this tangent with the vertical axis (Elongation [%]) provides a first amount value of the stabilized creep in the fiber. The stabilizing creep is calculated as the value at this intersection point minus the value of the elastic strain (%). The elastic strain is typically the initial elongation distance (unit of length, e.g. mm) divided by the original length of the elongated fiber. The elastic strain may either be measured (e.g. from displacement of grips, preferably measuring the displacement between markings on the fiber) directly after reaching the creep load, e.g. after few seconds after reaching the creep load or alternatively it may be calculated by dividing the applied stress (measured as MPa) on the fiber, e.g. yarn by the tensile modulus (measured in same unit as the stress).

Example 1A

[0062] Three polymeric yarns comprising commercially available fibers from DSM under the trade name Dyneema DM20, having a titer of 1760 dtex, a twist rate of 40 turns per meter, a 32 cN/dtex initial specific yarn strength and a minimum creep rate of 1.310.sup.6% per second measured at a tension of 900 MPa and a temperature of 30 C. were used, the yarns being approximately equal in length (each of a nominal length of about 50 cm). All three yarns were situated in parallel position and fixated at the ends, forming a statically over-determined structure. Said yarns were tested to fracture (the force at break) according to the method as described herein. The results are shown in Table 1.

[0063] In FIG. 15, the elastic strain for the yarn sample of Example 1A is about 0.8%, which means that the stabilizing creep amount is about 1.6% minus about 0.8%, and resulting in about 0.8%. The intersection point of the tangent (i.e. at the location where the creep rate is minimum, i.e. where the slope of the tangent line is minimum) with the vertical axis (elongation [%]) provides a first amount value of the stabilized creep in the fiber, i.e. about 1.6% in FIG. 15.

[0064] The diagram of FIG. 15 can also be presented as a so called Sherby and Dorn plot. This is shown in FIG. 16 that illustrates the Sherby and Dorn plot of the results presented in FIG. 15. FIG. 16 shows that the creep rate of creep stabilized fibers of Example 1A may decrease over almost 5 decades, behavior that is typical for creep stabilized fibers. In FIG. 16, the minimum creep rate of the yarn sample of Example 1A is about 1.310.sup.8 per second (or 1.310.sup.6% per second); this is an average value.

[0065] The results are shown in Table 1.

Example 1B

[0066] Example 1B was performed by repeating Example 1A, with the difference that one of said three yarns (e.g. situated in between the two longer yarns) was 1.5% shorter than the other two yarns that were approximately equal in length. The results are shown in Table 1.

Example 1C

[0067] Example 10 was performed by repeating Example 1 B, with the difference that all yarns were first loaded for 2 weeks' time at a load of 60% of the initial load value (as applied in Example 1B). The results are shown in Table 1 and Table 2.

Comparative Experiment 1A

[0068] Comparative Experiment 1A was performed by repeating Example 1A, with the difference that the three polymeric yarns were commercially available under the trade name Dyneema SK75 having a titer of 1760 dtex, a twist rate of 40 turns per meter, a 35 cN/dtex initial specific yarn strength and a minimum creep rate of 2.410.sup.5% per second measured at a tension of 900 MPa and a temperature of 30 C. The results are shown in Table 1.

Comparative Experiment 1B

[0069] Comparative Experiment 1B was performed by repeating Comparative Experiment 1A, with the difference that one of said three yarns was 1.5% shorter than the other two yarns that were approximately equal in length. The results are shown in Table 1.

Comparative Experiment 1C

[0070] Comparative Experiment 10 was performed as intended by repeating Comparative Experiment 1B, with the difference that the yarns were loaded for 2 weeks' time at a load of 60% of the load value applied in Comparative Experiment 1 B. However, an excessive strain of 15% was already reached after 8.7 days. Such a large strain makes a structure useless in any application and therefore the experiment was stopped. No results were thus shown in Table 1 (not applicable).

Comparative Experiment 2A

[0071] Comparative Experiment 2A was performed by repeating Example 1A, with the difference that the three polymeric yarns were commercially available under the trade name Twaron, having a titer of 3220 dtex and a 22 cN/dtex initial specific yarn strength and having a stabilizing creep value very close to zero (not measurable anymore) at very low strain already, measured at a tension of 900 MPa and a temperature of 30 C. The results are shown in Table 1. Another type of Twaron (with different characteristics and/or composition) is expected to give comparable or worse results in terms of fracture at break and load distribution.

Comparative Experiment 2B

[0072] Comparative Experiment 2B was performed by repeating Comparative Experiment 2A, with the difference that one of said three yarns was 1.5% shorter than the other two yarns that were about equal in length. The results are shown in Table 1.

Comparative Experiment 2C

[0073] Comparative Experiment 2C was performed by repeating Comparative Experiment 2B, with the difference that the yarns were loaded for 2 weeks' time at a load of 60% of the load value applied in Comparative Experiment 2B. The results are shown in Table 1 and Table 2.

TABLE-US-00001 TABLE 1 Force at Sample break [N] Example 1A 1186 Example 1B 877 Example 1C 1060 Comparative Experiment 1A 1263 Comparative Experiment 1B 895 Comparative Experiment 1C not applicable Comparative Experiment 2A 1197 Comparative Experiment 2B 711 Comparative Experiment 2C 860

[0074] The results shown in Table 1 demonstrate that the lowest strength reduction occurred for the structures of Examples 1B in comparison with Comparative Experiments 1B and 2B. The data in Table 1 clearly shows that two weeks of loading causes a strength recovery. The strength recovery of the test according to the invention with creep stabilizing fibers (Examples 10) is larger than the strength recovery observed for the comparative experiments (Comparative Experiment 2C). In fact, the structure according to the invention almost reached the theoretical maximum strength value (Example 10), whereas the recovery of the Comparative Experiments 2C was lower and still 30% of the theoretical maximum strength was lost. Also, the creep of the structures of Comparative Experiments 1 A-C and 2A-C was not stabilized. 3% strain was measured for the structure of Example 1C (which is acceptable for structures), 15% strain was measured for the structure of Comparative Experiment 1C already after 8.7 days under load, which is non-acceptable for structures, thus the test was immediately stopped. The structure of Comparative Experiment 2C hardly showed hardly any creep behavior (0.25% strain), but a serious strength reduction due to length differences and only very limited recovery of the strength reduction. Consequently, it is demonstrated that the structures according to the present invention show improved safety during the majority of their life time.

TABLE-US-00002 TABLE 2 Short One of the Load yarn longer yarns difference Example 1C Initial load applied, N 242 141.5 100.5 Load measured after 187.8 168.6 19.2 2 weeks, N The load difference 19.1% after 2 weeks, % Comparative Experiment 2C Initial load applied, N 300 112.4 187.6 Load measured after 292.5 116.3 176.2 2 weeks, N The load difference 93.9% after 2 weeks, %

[0075] Table 2 shows that the load distribution of the fiber according to the present invention (structure of Example 1C) is almost equal after some time, whereas the load distribution of the Twaron fibers (structure of Comparative Experiment 3C) remains almost as unequal as it was at the start of the experiment. Table 2 shows that 19.1% load inequality remained after two weeks (81.9% of the load has been shared) weeks for the structures according to the present invention, compared to 93.9% load inequality remained after two weeks (only 6.1% of the load has been shared) for the reference structures comprising Twaron, at the same conditions. Accordingly, in contrast with the structures according to the present invention, the structures made according to the Comparative Experiment 3C lead to premature failure of the more heavily-loaded elements.