GB2128287A - Crack arrestor - Google Patents

Abstract

A crack is arrested by wrapping a band (10) of continuous non-metallic fibres in a resin matrix around the object (12) in a direction perpendicular and curing the resin matrix. The resin and fibres may be applied separately, and the fibres may be held in a unit. High tensile strength fibres are used, typically glass fibres in rovings suitably oriented in the band (10'). The arrestor may be formed in situ as above, or separately on a mandrel for example, and subsequently fitted to the object. The arrestor may be used to inhibit the formation of cracks, or to prevent the development of a crack already formed. A plurality of arrestors are normally fitted to the object, spaced along an object and/or at the ends of a formed crack. <IMAGE>

Description

SPECIFICATION Apparatus and method for arresting ductile fracture propagation Objects of metal and other materials which are subjected to high stresses tend to fail by developing one or more propagating ductile fractures, which are commonly called free-running cracks. The range of objets which exhibit such propagating ductile fractures is too large for all of the objects to be mentioned specifically in this application, but it includes compressed gas cylinders, blades of fans and windmills, vehicle axles, pipes and so one. One of the areas which presently appears to be facing the greatest potential problems due to propagating ductile fractures is that of pipes, and especially the relatively large diameter pipes which are used in pipelines.

Pipelines of a relatively large diameter that are formed by pipes of steel or other metal are playing increasingly important roles in the transportation of gases, such as natural gas, Cho2, and other volatile fluids. For example, with the increase in domestic gas reserves and a corresponding increase in the use of natural gas, there will be a corresponding increase in the need for pipelines for transporting the gas at gathering pipelines and through long-range transmission lines.

Especially if flawed or damaged, and if they contain fluid at relatively high pressures, these types of pipes are susceptible to a propagating ductile fracture, or free-running crack, that will travel at velocities of 400-900 feet per second.

The probability of this type of fracture initiating is increased if the product in the pipeline is corrosive, either because of the gas itself or because of moisture, which is often contained in the gas. The probability is also increased in the case of steel pipes carrying natural gas containing large quantities of hydrogen sulfide, which has been known to create hydrogen embrittlement.

Furthermore, any moisture in the gas and any CO, which may be present will generate carbonic acid, which can also damage steel pipes. In addition, the problem of failure is significantly compounded in a cold climate. Moreover, a propagating ductile fracture or free-running crack can be caused by impact from an external force such as a trenching machine accidentally striking the pipeline, earthquakes, and the like.

The tendency for objects to fail by a propagating ductile fracture is generally due to the nature of the material of which the object is made and the manner in which the material is worked or shaped to form the object. For example, many pipes for pipelines are made from a metal block which is elongated more in more direction than in another by being passed between pairs of rollers or similar means. Such a method produces an elongated plate which is then formed into a U.

configuration by bending it about its longitudinal axis. The U-shaped plate is further bent into a cylindrical configuration by bringing the sides of the U into abutment and joining the sides along the length thereof, such as by welding, to form a substantially straight longitudinal seam. In such a pipe, there is considerably greater strength in the longitudinal direction than there is in the circumferential or hoop direction, which defines an axis of inherent weakness, and, as a result, the pipe is able to withstand greater stress in the longitudinal direction than in the hoop direction.

Therefore, when such a pipe fails, it is the result of a hoop of indeterminate width being broken and the ends of the broken loop being separated. Then, adjacent hoops are broken as the result of the failure of the first hoop, and a line of separating hoop ends moves longitudinally along the pipe to define a propagating ductile fracture.

Although there have been numerous proposals to limit ductile fracture propagation in objects and especially in pipelines, including the use of heavy walled pipes, cables, concrete abutments, valves and metal sleeves, they have all been less than completely satisfactory.

For example, U.S. Patent No. 4 195 669 to Ives et al discloses arresting ductile fracture propagation by providing an encircling mass of material around the pipe at preselected intervals as a circumferential restraint. However, the steel collar, the steel cable windings and the reinforced concrete cast disclosed by Ives et al are all quite heavy and difficult to handle and install. In addition, the crevice between the encircling masses and the pipe, and the crevices between adjacent windings in the case of steel cables, are subject to the ingress of dirt and moisture and the resultant crevice corrosion, which weakens the pipes. Moreover, the corrosion attacks the encircling masses themselves, especially in the case of cables.

U.S. Patent Nos. 4 148 127 and 4 224 966 to Somerville disclose a method of applying a bandtype crack arrestor over the outer diameter of a pipe and applying radial force to the inner diameter of the pipe to engage the arrestor in a tight fit. The disclosed crack arrestors are preferably bands or rings of the same metal as the pipeline. Therefore, they suffer from the same great weight and handling and installation problems as the encircling masses of the Ives et al patent. In addition, the tight fit of the pipe with the edges of the crack arrestors tends to cause stress concentrations in the pipe where the crack arrestor edges engage it. Of course, stress concentrations can lead to premature failure of the pipe. The Somerville patents also disclose that the bands can be fiberglass, but state nothing more about fiberglass bands.If the band is a typical molded ring of randomly oriented glass fibers, the thickness required to stop a crack would be such as to make the band very bulky and unwieldy.

U.S. Patent No. 3 870 350 to Loncaric discloses a pipe having zones of increased crack resistance due to cylindrical steel members surrounding the pipe, to which they are at least partially welded. The steel members are heavy and, thus, difficult to handle and install, as well as being subject to corrosion.

Besides the need for crack arrestors, corrosion protection is desirable for pipes and other objects, even where no crack arrestors are involved, but to provide corrosion protection for a pipe in the region of a crack arrestor and to apply it in a manner which is compatible with the presence of crack arrestors presents additional problems.

It is therefore an object of the present invention to provide an improved apparatus and method for arresting a propagating ductile fracture in an object, such as a pipe.

It is a further object of the present invention to provide an apparatus and method of the above type in which the arrest of propagating ductile fracture is assured with a minimum of added labour and materials, and at a reasonably low cost.

It is another object of the present invention to provide an apparatus and method of the above type which is lightweight, corrosion-resistant and capable of protecting the object from impacts.

It is a still further object of the present invention to provide an apparatus and method of the above type which incorporates the provision of corrosion protection for the object.

Toward the fulfillment of these and other objects, a plurality of crack arrestors are placed around the object, encompassing and engaging the object at spaced intervals, each crack arrestor defined by a band of continuous, unidirectional, lightweight, high-strength nonmetallic fibers encapsulated in a resinous material.

Some nonmetallic fibers have very high tensile strengths, while being lightweight, corrosionresistant and very inexpensive. Glass fibers, for example, have a tensile strength which is approximately four times the strength of steel. As a result, continuous glass fibers which are aligned in one direction such as in a roving of a plurality of parallel flexible glass strands, have tremendous strength in the direction parallel to their length.

Therefore, a plurality of continuous unidirectional glass fibers in a plurality of parallel rovings wrapped around an object which tends to develop propagating ductile fractures or free-running cracks, in a direction perpendicular to the cracks, equalizes the hoop strength of the object with its longitudinal strength and greatly increases the stress that the object can endure without failing.

Moreover, glass fibers wrapped around an object in a band in the circumferential or hoop direction can stop a failure, in the form of a free-running crack, after it has begun.

Bands of such continuous, unidirectional glass fibers are well suited for application to pipes as crack arrestors, and can be wrapped at selected positions on pipe sections in the shop or can be wrapped around pipe already installed at a job site. The glass fibers can also be applied where there is concern for stress concentrations, such as at pipe joints and at corners or elbows. Moreover, such bands can be wrapped either by machine or by hand.

In the crack arrestors, the continuous unidirectional glass fibers are encapsulated in a matrix of resinous material which is later cured, such as asphalt enamel, coal tar enamel, polyethylene, or epoxy. The glass fibers are lightweight and not subject to corrosion and, once they are encased in the cured resin matrix, the resultant crack arrestor is immune to the corrosion and other environmental problems which plague the crack arresting devices of the prior art. In addition, the crack arrestor according to the present invention forms a bond with the object, providing corrosion protection for the pipe, eliminating the ingress of dirt and moisture, and preventing any crevice corrosion. Moreover, the crack arrestor is resilient and energy-absorbent, thereby protecting the pipe from external impacts and avoiding the induction of stress concentrations in the pipe.

The crack arrestors according to the present invention can be wrapped on bare pipe; they can be wrapped on pipe completely coated with a corrosion-resistant substance; and they can be wrapped on pipe having a wrap laden with a corrosion-resistant substance. The glass fibers of the crack arrestors can be saturated with the resinous material by being passed through a resin bath as they are wrapped on the pipe, or the fibers can be saturated with the resinous materials by coating rollers or spray heads after they are in place on the pipe. As another alternative, the fibers can be previously impregnated with the resin, wrapped on the pipe while dry, and then heated to allow the resin to flow.The glass fibers can be drawn from a plurality of spools and aligned in parallel as they are wound on the pipe, or they can be prealigned and held together by cross filaments to form mats which are wound around the pipe.

When the band of glass fibers in a resinous matrix is wound around the pipe to the appropriate thickness, the matrix is cured to form a hardened, fluid impervious, corrosion resistant, energy absorbing, lightweight mass. The curing is accomplished by such suitable means as the application of heat, ultraviolet radiation, or catalyst. Where the pipe is previously treated in a process which requires the application of heat, as in the case of coating the pipe with fusion bond epoxy for corrosion resistance, the glass fibers and the resinous matrix can be applied immediately after the aforementioned process, and the residual heat from the process can be utilized to cure the resinous matrix, thereby eliminating the need for any additional step to cure the matrix.

The present invention also contemplates forming the crack arrestor separate from the pipe by wrapping a band of glass fibers in a resin matrix on a mandrel, curing it, removing it from the mandrel, and then slipping it over the pipe when and where it is convenient. The crack arrestors of the present invention are very lightweight, so that a crack arrestor many feet in diameter can easily be lifted and installed by one man. No special means!of securing the crack arrestor to the pipe is required. All that is needed is some means of preventing the crack arrestor from moving along the pipe, such as filling the space between the crack arrestor and the pipe with foam material, such as a foam plastic, which eliminates the possibility of crevice corrosion.

Due to the strength of the glass fibers and the ability of the resin to absorb energy, the crack arrestors protect from external impacts the portions of the pipe which they cover and also prevent or reduce fragmentation of the pipe in the area of the crack arrestor upon failure of the pipe.

The above brief description, as well as further objects, features and advantages of the present invention will be more fully appreciated by reference to the following detailed description of a presently preferred but nonetheless illustrative embodiment in accordance with the present invention, when taken in conjunction with the accompanying drawings wherein:: Figure 1 is a perspective view of a crack arrestor according to the present invention being wound around a pipe; Figure 2 is an elevation of the crack arrestor of Figure 1 in place on a bare pipe; Figure 3 is an elevation of an alternate form of the crack arrestor being wound around a coated pipe; Figure 4 is an elevation of the crack arrestor of Figure 1 in place on a pipe wrapped for corrosion resistance; Figure 5 is an elevation of another embodiment of the crack arrestor according to the present invention; Figure 6 is an elevation of still another embodiment of the crack arrestor according to the present invention; Figure 7 is an elevation of yet another embodiment of the crack arrestor according to the present invention; Figure 8 is a cross section of a further embodiment of the crack arrestor according to the present invention;; Figure 9 is a cross section of a still further embodiment of the crack arrestor according to the present invention; Figure 10 is yet a further embodiment of the crack arrestor according to the present invention; and Figure 11 is a transverse cross section of a pipe having mounted thereon another, separately formed embodiment of the crack arrestor according to the present invention.

As can best be seen from Figure 1 , the crack arrestor according to the present invention, which is generally designated by the reference numeral 10, is wound around an object, such as a bare pipe 12, which is mounted on a rotating mandrel 14.

Although a pipe is illustrated in Figure lit is understood that the crack arrestor can similarly be wound around a compressed gas cylinder, a windmill blade or other object which might develop propagating ductile fractures, or freerunning cracks.

The crack arrestor comprises a band made up of a plurality of rovings 1 6, each of which includes a large number of continuous unidirectional, lightweight, high-strength, inorganic, nonconductive, nonmetallic fibers such as glass fibers. Although glass fibers will be referred to throughout the specification, similar fibers having a high tensile strength, such as Kevlar, may also be used. The rovings 16 are drawn from a plurality of spools 18 contained in a creel 20 and are fed through gathering and aligning devices 22 and 24 prior to being drawn through a bath 26 containing a resin 27 such as epoxy, asphalt enamel, coal tar enamel or polyethylene in which they are saturated with the resin.The rovings 16 continue on from the bath 26 to a roving applicator 28 which is mounted for movement on a bar 30 arranged parallel to the rotating mandrel 14, and some device is usually provided for removing excess resin. The roving applicator 28 can be moved back and forth along the bar 30 between predetermined limits to wind on the pipe 12 a band having the desired width. Moreover, additional layers of the continuous, resinencapsulated fibers can be wound onto the pipe 12 until a band or crack arrestor 10 having the appropriate thickness is achieved.

The apparatus shown for winding the crack arrestor 10 on the pipe 12 is merely exemplary, it being understood that the glass fibers contained in the rovings 16 can be wound on the pipe 12 without the resin, and the resin 27 can be applied to the glass fiber windings on the pipe 12 by coating rollers, spray nozzles or other suitable devices. However, applying the resin 27 to the rovings 16 prior to the winding of the rovings 1 6 on an object has the advantage that the tackiness of the resin 27 causes the rovings 1 6 to adhere to the object, thereby aiding in the starting of the windings.In any of the methods described, sufficient resin is applied so that the crack arrestor 10 comprises continuous, unidirectional glass fibers encapsulated in a matrix of the resin 27, and so that it forms a continuous bond with the object, leaving no space for the ingress of dirt or moisture which is apt to cause corrosion.

Besides the pipe 12 on the rotating mandrel 14 shown in Figure 1 , the crack arrestors 10 can be applied to stationary objects by other machines, and the crack arrestors can be applied to either rotating or stationary objects by hand. Some of the machinery is suitable for applying the crack arrestors 10 in the field as well as in the shop.

When the crack arrestors 10 are in place, the resin 27 is cured by the application of heat, ultraviolet radiation, catalysts or other suitable means to form a hardened, fluid impervious, corrosion resistant, energy absorbing, lightweight mass.

The glass fibers can be previously impregnated with the resin 27 and applied to the pipe 12 in a dry state. Once the pre-impregnated glass fibers are in place, the application of heat to the fibers causes the resin 27 to run, thereby forming a single mass of glass fibers encapsulated in a resin matrix. The further application of heat will cure the resin, as will any of the other curing processes previously described.

Preparatory to treating a pipe according to the method of the present invention, the pipe is preferably cleaned by either sand or grit blasting or by mechanical scraping and wire brushing to render the pipe surface free from oil, grease, dust, moisture and non-adhering mill scale. As is illustrated in Figure 2, the crack arrestor 10 is shown in place, bonded to the bare pipe 12.

Although the pipe 12 in Figure 2 is bare, the crack arrestor 10 can also be applied to coated pipe such as pipe having a coating for corrosion resistance. For example, a hot, relatively viscous substance, such as coal tar enamel, or an asphalt enamel can be applied to the primed outer surface of the pipe, as disclosed in detail in Applicant's copending Application, Number 82 29519, Serial Number 2 107 422.

Alternatively, the pipe can be precoated with a fluidized epoxy powder which is sprayed onto the preheated outer pipe surface, by an electrostatic process, or the like. Still further, a coating of extruded polyethylene can be extruded onto the outer pipe surface in a conventional manner.

As is illustrated in Figure 3, a crack arrestor 10' is partially wound around a coated pipe 12', the continuous glass fibres in the crack arrestor 10' being contained in rovings 16' held together by cross filaments 32 to define a mat 34. The mat 34 is saturated with resin 27 and wound around the pipe 12' to form another embodiment of crack arrestor 10' according to the present invention.

Mats of other configurations can also be employed.

Where a crack arrestor according to the present invention is applied to a coated pipe, as in the case illustrated in Figure 3 in which the crack arrestor 10' is wound around the coated pipe 12', the coating process involves the heating of the pipe. In such a case, the wrapping of the crack arrestor 12' can be done immediately after the coating so that the residual heat from the coating process cures the resin matrix of the crack arrestor, thereby eliminating the need of a separate step for curing the resin matrix.

As is illustrated in Figure 4, the crack arrestor 10 can be wound around pipe 12" already having a wrapping, such as a helical wrapping of paper, fabric or other material treated with asphalt or other anti-corrosion substance to provide corrosion protection for the pipe 12". The wrapping can also be an adhesive coated tape of polyethylene or polypropylene. The crack arrestor 10 of Figures 1 and 2 is shown in place over the wrapping 36, but the crack arrestor 10' of Figure 3, defined by the resin saturated mat 34, can also be used over the wrapping 36, as can other forms of the crack arrestor according to the present invention hereinafter to be described.

The crack arrestors are ordinarily wound to a width and thickness which will stop the propagation of a crack, and are placed at predetermined intervals along a pipeline. For example, one crack arrestor 10 can be provided for each section of pipe in the pipeline. It has been found that, in most instances, a crack arrestor according to the present invention, having a width less than the diameter of the pipe and a thickness less than the thickness of the pipe is sufficient, by itself, to stop a propagating fracture. Bands having a width equal to approximately two-thirds of the diameter of the pipe and having a thickness equal to 60% of the thickness of the pipe have successfully stopped a crack.

Crack arrestors can also be applied such that a band of glass fibers in a resin matrix is insufficient to stop the crack by itself, but a series of bands are provided so that, acting together, they decelerate and stop the propagation of the crack. For example, as is shown in Figure 5, a continuous helical band 36 of glass fibers in a resin matrix can be applied in a helix along the entire length of the pipe 12, being wound right across joints 38, usually in the form of welds, between pipe sections 1 2a, 1 2b and 12c. The helix has a pitch such that there is a space between adjacent revolutions of the helix. As an alternative, a plurality of closely spaced mini-bands 40 can be applied along the pipe 12 to decelerate and stop the propagation of a crack, as is illustrated in Figure 6.

In another arrarigement, alternate or periodic sections of the pipe 12, such as pipe sections 1 2b and 12d, can be wrapped with continuous glass fibers in a resin matrix to define a crack arrestor 42 extending the entire length of the pipe section, as is shown in Figure 7. Of course, with this arrangement, one crack arrestor 42 is sufficient to arrest a crack, and only half of the pipe sections or fewer need any crack arrest treatment.

Ordinarily, the windings of a crack arrestor according to the invention have rectangular crosssectional configurations, although configurations other than rectangular can be used. As is shown in Figure 8, the lateral edges 45 of the crack arrestor 44 are bevelled. As is shown in Figure 9, the cross-section of the crack arrestor 46 has an arcuate section such that the crack arrestor 46 is thickest at its centre and is tapered at its edges 47. The cross-sections of the crack arrestors 44 and 46 of Figures 8 and 9, respectively, are stippled to represent the cross-sections of the glass fibres. However, there are thousands of fibers in parallel, unidirectional relationship, each fiber preferably having a diameter less than .001 inch. The crack arrestors according to the present invention are wound essentially without tension, only sufficient tension being applied to permit the glass fibers to be wound neatly and orderly. As a result, the crack arrestors do not transmit significant stress to the pipes they wrap, and thus do not tend to produce stress concentrations in the pipes, especially along the edges of the crack arrestor. However, both of the embodiments of Figures 8 and 9 further reduce the stress between the crack arrestor and the pipe 12 along the edges of the crack arrestor, thereby eliminating any possibility of the crack arrestor causing stress concentrations in the pipe 12 at those places.

Since the crack arrestors according to the present invention can be wrapped onto the pipe in the field, they can be placed directly over the joints 38 in the pipe 12, between the pipe sections 12a, 12b and 12c, as shown by the crack arrestors 10 in Figure 10, thereby providing crack arrest capabilities and at the same time protecting the joints 38, which usually require some separate form of protection. Although Figure 10 shows the crack arrestor 10 of Figures 1 and 2 in place over the joints 38, the other embodiments of the crack arrestor according to the present invention, such as the crack arrestor 10' of Figure 3 employing the mat 34, could be placed over the joints 38.

Figure 11 is a transverse cross-section of the pipe 12 having mounted thereon a separable crack arrestor 48, which is formed of the same materials as the other embodiments of the crack arrestor described herein, but which is formed separate from the object to be treated, as by being wound onto a rotating mandrel, cured and removed. Since the glass fibers and the resin are very lightweight, a crack arrestor for a pipe many feet in diameter is light enough to be lifted and put into position by one man. The crack arrestor 48 is slipped over the pipe 12 and secured to the pipe to prevent it from sliding along the length of the pipe. One suitable mechanism for holding the separable crack arrestor 48 in place on the pipe 12 is the provision of a foam filling 50, such as a plastic foam in the space between the pipe 12 and the crack arrestor 48. The foam filling 50 not only bonds the crack arrestor 48 to the pipe 12, but also fills ir the space and prevents the ingress of dirt and moisture, thereby preventing the possibility of crevice corrosion.

Although it is apparent from the foregoing that the present invention provides significant advantages in the arrest of propagating ductile fractures or running cracks, it is understood that various changes and modifications may be made without departing from the spirit and scope of the present invention as recited in the appended claims and their legal equivalents.

Claims (26)

1. A crack arrestor for arresting a propagating ductile fracture in an object, the ductile fracture tending to propagate in a predetermined direction, the crack arrestor comprising: a band defined by a plurality of continuous, high tensile strength, nonmetallic fibers oriented perpendicular to said direction and encapsulated in a resin matrix, said band encompassing and engaging said object.

2. The crack arrestor of claim 1 wherein the fibers are glass fibers.

3. The crack arrestor of claim 1 wherein the resin is an epoxy.

4. The crack arrestor of claim 1 wherein the band includes a plurality of layers of the fibers.

5. The crack arrestor of claim 1 wherein the band is bonded to the object.

6. The crack arrestor of claim 1 wherein the fibers are held together by cross filaments to define a mat.

7. The crack arrestor of claim 1 wherein the band comprises a helix.

8. The crack arrestor óf claim 1 wherein the band comprises a plurality of spaced bands, each of which has a width and thickness insufficient to stop a crack, but together having sufficient strength to decelerate and stop a crack.

9. The crack arrestor of claim 1 wherein the band is bevelled along its edges.

10. The crack arrestor of claim 1 wherein the band has an arcuate cross section, being thickest at its center and tapered at its edges.

11. The crack arrestor of claim 1, further including a layer of foam material between the band and the object.

12. In combination, a crack arrestor and a pipe in which a ductile fracture tends to propagate in a direction parallel to the axis of the pipe, the crack arrestor comprising: a band defined by a plurality of continuous, high tensile strength, nonmetallic fibers oriented perpendicular to said direction and encapsulated in a resin matrix, said band encompassing and engaging the pipe.

13. The combination of claim 12 wherein the width of the band is less than the diameter of the pipe.

14. The combination of claim 12 wherein the width of the band is less than the diameter of the pipe.

15. The combination of claim 12 wherein the pipe is coated, and the crack arrestor is positioned over the coating.

16. The combination of claim 12 wherein the pipe is wrapped, and the crack arrestor is positioned over the wrapping.

17. The combination of claim 12 wherein the pipe includes a plurality of sections, a said band being positioned on some of the sections along substantially the entire length of the section, while the other sections have no band.

18. The combination of claim 12 wherein the pipe includes a plurality of sections and joints between said sections, and a said band is positioned over each joint.

19. A method of arresting a propagating ductile fracture in an object in which the ductile fracture tends to propagate in a predetermined direction comprising: wrapping a band of continuous, high tensile strength, nonmetallic fibers in a resin matrix around the article in a direction perpendicular to said direction, and curing the matrix.

20. The method of claim 19 wherein the fibers are coated with the resin prior to wrapping.

21. The method of claim 19 wherein the fibers are coated with resin subsequent to wrapping.

22. The method of claim 20 wherein the resin coating is dried prior to wrapping, and heat is applied subsequent to wrapping to cause the resin to flow.

23. The method of claim 19 or 22 wherein the steps or wrapping and curing are performed subsequent to treating the object for corrosion resistance in a process involving the application of heat, and the heat from the corrosion resistance process performs said step of curing the matrix.

24. A method of arresting a propagating ductile fracture in an object in which the ductile fracture tends to propagate in a predetermined direction comprising: wrapping a band of continuous, unidirectional, high tensile strength, nonmetallic fibers in a resin matrix around a mandrel, curing the resin matrix to form a hardened band, removing the hardened band from the mandrel, securing the hardened band in place on the object so that the fibers are perpendicular to said predetermined direction.

25. A crack arrestor substantially as described herein with reference to any of the accompanying drawings.

26. A method of arresting a propagating ductile fracture substantially as described herein with reference to the accompanying drawings.