WO2013039745A1

WO2013039745A1 - Creep-resistant high strength fiber-based assembly

Info

Publication number: WO2013039745A1
Application number: PCT/US2012/053736
Authority: WO
Inventors: David Phillip FERGENSON; Joel Del Eckels
Original assignee: Livermore Instruments, Inc.
Priority date: 2011-09-13
Filing date: 2012-09-05
Publication date: 2013-03-21

Abstract

The invention is a static dimensional material which comprises multiple submaterials interwoven or joined according to their lengths of characteristics such that, when one material is stretched to its elastic deformation limit, another will begin its elastic deformation regime. Ropes or cords made from the invention are useful in the securing of loads at a far greater strength-to-weight ratio than conventional materials that are currently used to secure loads. Furthermore, the tactile feedback inherent in the invention will alert the user that the load is secure or that the tie-down is losing its static quality and should be replaced, in either case long before the end user is exposed to hazard.

Description

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CREEP-RESISTANT HIGH STRENGTH FIBER-BASED ASSEMBLY

David Philip Fergenson and Joel Del Eckels

Relation to Other Applications

[0001] This application claims benefit of Provisional applications serial number 61/534,346, filed 09/13/2011.

Field of The Invention

[0002] A composite material made of multiple components in which as one component reaches its deformation limit under tension a second component is engages to prevent the first from overextension or creep. It is especially useful in rope, webbing, straps and bands for securing heavy loads for transport.

Background

[0003] Heavy loads are tied down for transportation by a variety of methods and there are advantages and disadvantages to each. One method is to use chains with a rotary tensioning device. The chains are heavy. A chain with a 70,000 lb. minimum breaking strength that is 4 feet long will weigh roughly 90 lbs. Nonetheless, the chains are static in nature. That is, they will not stretch during transit, which makes them well-suited to application aboard ships, flat bed trucks and railway cars.

[0004] Materials science has progressed significantly in the last half century and a variety of high strength synthetic fibers (HSSFs) including, for example, Kevlar™ and high molecular weight polyethylene is now available. These fibers are many times stronger than steel for a given mass. At least one commercial product made of high strength fiber that weighs just five lbs. is six feet long and has a minimum breaking strength of 154,000 lbs.(Mammut Tec A.G. Lifting Belt Sling, Article 7009120-020 specifications supplied by the manufacturer.)

[0005] Unfortunately, under tension, ropes or webbing made of these fibers tends to elongate permanently, a phenomenon known as "creep strain" or simply "creep". This elongation can be by as much as 3% of the length and can, in practice, lead to a failure of a tie down to secure a load even if the tie down itself does not rupture. A creep factor of only 3% over a distance of only four feet leads to the elongation of 36 mm, easily long LI-I81412 -1- enough to slip off of an attachment point of a tie-down for a heavy load. This has limited the use of advanced fibers' in tie downs and many other applications.

[0006] According to, among other sources, the document "Engineering with Aramid Fibers" by Cor Das and Nick O'Hear and distributed by Teijin Ltd., deformation of essentially any material occurs in three general domains. Upon the application of force, there is initially an elastic deformation. As the elastic deformation length is exceeded and load (or tension) continues to be applied to the fibers, the length passes through the linear domain, during which time the fibers are deformed plastically (the creep). Eventually, the creep length exceeds the linear domain and enters the logarithmic domain and rupture occurs thereafter. Essentially all materials exhibit creep at sufficiently high temperatures but that HSSFs are limited in application by the fact that they exhibit this phenomenon to a greater degree than competing materials starting at room temperature.

Summary

[0007] If a tie down assembly is composed of multiple subassemblies, whether they be individual fibers, yarns, ropes or straps, of staggered lengths, then the shortest loop will engage first upon tensioning the assembly and begin its elastic deformation regime with each loop engaging as the tension is increased and the shorter loops deform to their lengths This invention is an embodiment of this concept and presents several advantages that are detailed below. An alternative embodiment of the invention is to use loops composed of different materials whose different deformation characteristics provide what are, in effect, different lengths under stress. A combination of these concepts where different materials are present at different lengths to achieve the effect is also possible. HFFSs may be combined with more conventional materials in such assemblies.

Brief Description of the Drawings

[0008] Figure 1 is a stress versus strain curve for evlar ^~ reproduced from Army Research Laboratory document ARL-TR-3437, "Plain- Woven, 600-Denier Kevlar KM2 Fabric under Quasistatic, Uniaxial Tension", by Raftenberg, Schiedler, Moynihan and Smith, and annotated for this document.

[0009] Figure 2 is a notional creep curve for a high strength synthetic fiber.

[0010] Figure 3 is a diagram showing one aspect of the invention in its simplest embodiment, as an assembly composed of circular ropes of the same fiber with one rope

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being shorter than the others by its elastic deformation length at the intended working temperature.

[0011] Figure 4 shows an integrated system including a rope assembly with hooks at each end and a tensioning device, allowing loads to be secured.

[0012] Figure 5 shows the estimated increase in force required for an increase in deformation with various staggered lengths of aramid fibers, the preferred embodiment of the invention.

Detailed Description

[0013] High strength synthetic fibers (HSSFs), such as Kevlar™, have been commercially available for roughly half a century and have many uniquely advantages over other materials. (Kwoleck, S.L., U.S. Patent 3,819,587, "Wholly Aromatic Carboxylic Polycarbonamide Fiber Having an Orientation Angle of Less Than About 45 Degree," Issued June 25, 1974).

[0014] For example, HSSFs have strength to weight ratio that is many times that of steel and are less subject to corrosion. Some HSSFs have dramatically longer lifespans than steel under the same loading conditions. These characteristics have lead to the pervasive application of HSSFs in certain applications such as body armor, climbing ropes and hoists for sails. Unfortunately, however, all materials in general and HSSFs in particular exhibit longitudinal deformation under stress (or tension) over time that is known as "creep strain" or merely "creep." This creep can preclude the application of the HSSFs in applications requiring that the length of the material remain static over a prolonged period of time. An example of such an application is the securing of loads during maritime transportation, where the lengthening of the securing material combined with the motion of the ship in the ocean could lead to the lines slipping from their attachment points. Indeed, a search of commercially available maritime HSSF products readily turns up products where creep will not interfere with the utility of the product such as hoists and sails but does not reveal products where creep would interfere with the utility of the product such as lashings and tie downs. An approach to utilizing HSSF products where creep interferes with functionality is to select an HSSF with minimal creep. There are limitations to the minimum creep available for any HSSF material and selecting for

LI-I81412 -3- minimum creep places a constraint on the optimal selection of the HSSF for the application.

[0015] Actual stress versus deformation (£} measurements for a Kevlar weave were collected by the U.S. Army Research Center and are shown with annotations in Figure 1. Detail 1 shows the region of the curve where the weave deformed elastically in response to a constant load. Detail 2 shows the region of the curve where the weave began to creep. The slope of the curve at any point indicates the amount of marginal force required to produce a marginal amount of deformation. Note that the slope is steeper for the creep region, indicating that more force is required to deform the material in the creep regime than to deform it elastically. Note also that the curve, while steeper, is not so much steeper that multiple identical weaves in their elastic deformation region would not be bearing the majority of the force were they to be deformed elastically beginning where the elastic deformation region of the first fiber ended and the plastic creep region began. Critically, then, should a single shorter HSSF weave be incorporated along with several HSSF weaves that are longer by the elastic deformation length of the shorter weave, the material would tend to stay at the transition zone from elastic to plastic and creep would be significantly reduced for the shorter assembly. Similarly, if the fabrication tolerances of the two different lengths were not ideal and the shorter weave were slightly too short, then the shorter weave in the assembly would tend to creep until that condition was fulfilled. As used in this specification and claims "elastic deformation limit" means the point where creep would begin to occur.

[0016] A notional curve displaying typical creep conditions for an HSSF is shown in Figure 2. When load is initially applied to an HSSF weave, either rope or strap, the material is deformed elastically as in [10]. When the load is removed, the HSSF weave will shorten by roughly this elastic deformation length even if it has crept longer under strain and over time. As the force is applied over a longer time, the weave will lengthen permanently in a phenomenon that is roughly linear over enough time under a given force [20] . Ultimately, after sufficient time, the deformation will accelerate and the HSSF weave will elongate and rupture [30] , The time before this occurs may be in the years or decades, depending on the HSSF and the load.

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[0017] In practice, then, while a load must be secured with sufficient force to engage a tie down at an attachment point, applying too much stress at the time of attachment or during the natural motion of a load in transit could force the HSSF weave into the plastic deformation domain (beyond the "elastic deformation limit"), accelerating the failure of the assembly and possibly leading to a creep length that would allow the ends to slip off of their attachment points, at which point the load would not be secure. Thus, the tensioning of the fibers, within an appropriate range, is critical for static length applications.

[0018] Another consideration for any application of HSSFs is the concept of an engineering safety factor. It is relatively straightforward to assess the minimum breaking strength of a fiber under ideal conditions. It is then engineering practice to rate the HSSF assembly such that the minimum breaking strength is some multiple of the rated load. That multiple is known as the engineering safety factor. One way to achieve an engineering safety factor is to assemble a tie down from multiple subunits, each with the same break strength, where the number of subunits represents the engineering safety factor over the break strength of an individual subunit.

[0019] The present invention is a method and system of combining several submaterials (or subcomponets), preferably including one or more HSSFs, into a single composite material assembly that, by varying the lengths of the individual components and, inter alia, the materials out of which each component is composed, the deformation domains are aligned in such a way that creep is minimized, tensioning force is inherently optimized upon application, the assembly provides tactile feedback to the user that it is installed optimally, and the engineering safety factor is easily determined. Furthermore, as the assembly begins to wear out, a rigger (or other user) will feel it "going soft" and not seating as decisively as previously even though its strength is not appreciably compromised during this phase of its life cycle. This invention in some aspects uses loop- shaped HSSF weaves of staggered lengths such that, in one embodiment, the elastic length limit of the shortest weave is aligned with the nominal length of all other weaves.

[0020] In another embodiment, several components can lined up such that their elastic deformation limits each line up with the next longer weave's nominal length. In still another embodiment, a small amount of a more static, but lower strength material such as LI-I81412 -5- metal wire rope, may be incorporated in the assembly. And in yet another embodiment, the HSSFs may be selected such that, while all nominally the same length, one will reach its elastic deformation limit long before the others.

[0021] Figure 5 shows the estimated force versus deformation curves for a set of fibers in a 4: 1 ratio of longer to shorter fibers, staggered by different proportions of their lengths. In this figure, the longer fibers are referred to as the "protecting weaves" while the shorter fibers are referred to as the "limiting weaves". All fibers are assumed to have characteristics similar to those measured by Raftenberg et al whose data is reproduced in Figure 1, When the fibers in such an assembly are staggered by the correct proportion of their lengths, 2% in the simulation, when placed under tension as a by a rigger securing a load, the transition from only the limiting weaves being deformed elastically to the combined force of the protective weaves being deformed elastically and the limiting weave being deformed plastically combine as in [302] at which point the increased force will feed back to the user through the tensioning device and the user will feel the tie down "seat." In reality, it will reach a length at which the shortest HSSF weave would have to be deformed plastically in order to creep but that the elastic loads of the remaining fibers reduce the force on the loop, reducing the creep by roughly a factor of the number of HSSF weaves in the assembly. Should the manufacturing tolerance be inadequate to provide a perfect 2% shorter length, then the assembly may be manufactured with the protective weaves slightly too short where the seating will be less decisive [301] but where the assembly would appear to "wear in" over several uses until the proper proportions are achieved whereupon the optimal protective ratio would be achieved and where the assembly would spend the majority of its life.

[0022] In the embodiment using different material components, with one of the materials being longer than the others but with a shorter elastic deformation limit, the user would still feel the assembly seat (against the longer but less elastic fiber) while the more elastic fibers would prevent the less elastic fiber from creeping by reducing the load on that fiber. In a case where the least elastic fiber is a non-HSSF weave (such as a thin steel wire rope) the primary use of the rope would be to ensure proper tensioning and thus protect the remaining fibers from creep. The composite material assembly of this invention may be in the form of a rope, webbing, strap, band and the like as will be

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apparent to those skilled in the art. It may take the form of a fabric, such as one that would be useful to secure loads. While the invention has been described in general as a tie down, it will have many more applications where creep needs to be minimized, especially in a HSSF material.

[0023] In another aspect the invention is an apparatus to secure cargo that in one embodiment applies an attachment means to secure an end of the apparatus to surfaces, hooks, or another end of the apparatus; a tensioning means to provide force along the length of the apparatus by contracting the length of the apparatus disposed upon the cargo; and a composite material assembly comprising at least a first and second submaterial such that as tension is applied, the first submaterial reaches its elastic deformation limit at the same time as the second submaterial comes under tension.

"Cargo" as the term is used herein and in the claims means any movable object that needs to be secured from moving or shifting. Examples include, but are not limited to maritime cargo container, vehicles, military equipment, boxes tanks, truck trailers, mobile homes and the like. Cargo usually means an object to be transported but can also include any object subject to unwanted shifting or other movement caused by a movable surface on which it is placed (as a ship, truck, train etc.) but may also be objects that can t be moved by wind or water such as mobile homes recreation vehicles, other vehicles and the like. The composite material of the apparatus preferably has the first submaterial made of a high strength synthetic fiber and may have the second submaterial in the composite material of the same high strength synthetic fiber of a greater length such that it will begin to be deformed when the first submaterial reaches its own elastic deformation limit.

[0024] The composite material will generally be configured so that the differences in lengths of the first and second submaterials are achieved by weaving a cloth such that the path length of the first material is shorter than that of the second material. In another embodiment, that the second submaterial may have a far shorter elastic deformation regime than the first submaterial.

[0025] The composite material may be any suitable high strength fiber such as Kevlar™, or other aramid fibers.

[0026] The tensioning device, if used, may be any suitable device such as one that operates by winding the material around a cylinder.

LI-I81412 -7- [0027] Figure 3 shows an example of one embodiment where one HSSF component [101] is shorter than all other HSSF weaves [102] which are all of equal length and are longer than [101] by its elastic deformation limit or by slightly more or less than its elastic deformation limit.

[0028] A workable apparatus for securing cargo is illustrated in Figure 4. An assembly [202] such as shown in Figure 3 has connectors [201] and a tensioning device [203] that may have a torque regulator or measurement device on it if more precise tensioning than that contributed by the user given the tactile feedback described above is desired. The tensioning device may or may not be incarcerated on the HSSF component assembly [202] .

[0029] While the invention has been particularly shown and described in particular embodiments above, those skilled in the art will understand that changes in form and detail may be made without departing from the spirit and scope of the invention.

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Claims

1. Composite material assembly comprising at least a first and second submaterial such that the first submaterial reaches its elastic deformation limit below the point at which the second submaterial begins its deformation.

2. The composite material assembly of claim 1 wherein the first submaterial to reach its elastic deformation limit is made of a high strength synthetic fiber.

3. The composite material assembly of claim 2 wherein the second submaterial is the same high strength synthetic fiber as the first and is of a greater length such that it will begin to be deformed when the first submaterial reaches its own elastic deformation limit.

4. The composite material assembly in claim 3 where the differences in lengths are achieved by weaving a cloth such that the path length of the first submaterial is shorter than that of the second submaterial.

5. The composite material assembly of claim 4 wherein the ratio of the number of fibers of the second submaterial to those of the first submaterial constitute an engineering safety factor for the composite material.

6. The composite material assembly in claim 5 wherein the submaterials are aramid fibers.

7. The composite material assembly of claim 2 wherein the second material has a far shorter elastic deformation regime than the first material.

8. An apparatus to secure cargo comprising: an attachment means to secure an end of the apparatus to surfaces, hooks, or another end of the apparatus;

LI-I81412 -9- a tensioning means to provide force along the length of the apparatus by contracting the length of the apparatus disposed upon the cargo; and a composite material assembly comprising at least a first and second submaterials such that as tension is applied, the first submaterial reaches its elastic deformation limit at approximately the same length as places the second submaterial under tension.

9. The apparatus of claim 8 wherein the first submaterial in the composite material is made of a high strength synthetic fiber.

10. The apparatus of claim 9 wherein the second submaterial in the composite material is the same high strength synthetic fiber of a greater length such that it will begin to be deformed at the length at which the first submaterial reaches its own elastic deformation limit.

11. The apparatus of claim 10 wherein the differences in lengths are achieved by weaving a cloth such that the path length of the first material is shorter than that of the second material.

12. The apparatus of claim 11 wherein the ratio of the number of fibers of the second material to those of the first material constitute an engineering safety factor for the composite material.

13. The apparatus of claim 8 wherein both submaterials are aramid fibers.

14. The apparatus of claim 8 wherein the second submaterial has a far shorter elastic deformation regime than the first submaterial.

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15. The apparatus of claim 8 wherein the tensioning device operates by winding the material around a cylinder.

16. The apparatus of claim 8 wherein the composite material comprises a rope, webbing, strap, band or fabric.