BACKGROUND OF THE INVENTION

Field of the Invention This invention relates to systems and methods for repairing structural members in which shape memory materials apply a corrective force to achieve a repair or strengthening objective. State of the Prior Art

Shape-memory is a physical phenomenon by which a plastically deformed metal is restored to its original shape by a solid state phase change caused by heating (Van Hambeck, I. J., "From a Seed to a Need: The Growth of Shape Memory Applications in Europe," Materials Research Society Symposium Proceedings, Vol. 246, 1992, pp. 377-387). This process of regaining the original shape is associated with a reverse transformation of the deformed martensitic phase to the higher temperature austenitic phase (Wu, R.H., "Structure and Properties of Rapidly Solidified Nitinol Materials," Materials Research Society Symposium Proceedings, Vol. 246, 1992, pp. 361-366). Typical materials which exhibit the shape-memory effect include nickel-titanium alloys (Nitinol) and some copper alloy systems. Nitinol can be plastically deformed in its low-temperature martensitic phase to strains as high as 6 to 8%, and then regain its original shape upon heating to the austenitic phase. The restraint of Nitinol from regaining the memory shape can result in stresses exceeding 700 Mpa (Gandhi, M.V. and Thompson, B.S., " Smart Materials and Structures," Chapman & Hall, 1993, pp. 192-215). Nitinol generally shows higher elastic modulus and yield stress at the high-temperature austemtic phase than at the low-temperature martensitic phase. The transformation temperature of Nitinol can be adjusted, and it typically ranges from -50 to 100 deg. C (Gandhi and Thompson, 1993).

Some typical applications of shape-memory alloys include switching and actuation in nuclear reactors, reaction vessels, chemical plants, robotics, etc. Shape-memory actuators can be moved back and forth and rotated by heating or employing other forms of control. Electrical energy is

typically employed for heating purposes; however, hot water, hot air, high- frequency induction heating, microwave heating, laser light, infrared rays, etc. can also be employed for heating. Shape-memory actuators have the distinct advantage of small size, because they are constructed as a single unit with a simple material-based mechanism for actuation. Other applications of shape- memory alloys include pipe couplings, blood clot filters, drive elements of artificial heating, etc. (Gandhi and Thompson, 1993).

U.S. Pat. No. 5,093,065 issued to R. L. Creedon in 1992 illustrates a prestressing arrangement with a shape-memory material member. In this approach, a formable material, such as concrete, is shaped around the shape- memory member. This shape-memory material is then shrunk by heating it above its characteristic threshold temperature, placing the formed material in compression (and the member itself in tension). U. S. Pat. No. 5,024,388 issued to K. Kaneko and M. Nishida in 1991 illustrates the construction of a stonework crusher utilizing thermal deformation shape-memory alloy. U. S. Pat. No.

5,040,283 issued to J. J. Pelgrom in 1991 illustrates a method for placing a body of shape-memory metal within a tube by a running sub which transports the body to a desired location within the tubing while the shape-memory metal is in its martensitic phase. Then the shape-memory metal is heated to regain its original shape which is tailored to its application inside the tubing.

SUMMARY OF INVENTION

The present invention uses the shape-memory phenomenon to transfer corrective forces to components or structures in order to accomplish repair or retrofit objectives. For this purpose, shape-memory plates or rods, or polymer matrix composite plates or rods incorporating shape-memory ribbons or wires, may be elongated and kept at a temperature below their transformation temperature, or may be used without prior elongation particularly if their transformation temperature is below ambient temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of plate and rod shaped repair members according to the present invention formed of a shape memory alloy;

FIG. 2 is a perspective view of plate and rod like repair members according to the invention formed of a polymer matrix containing fibers of shape memory alloy;

FIG. 3 is a perspective view of a structural component in need of repair to which is attached the plate of FIG. 1;

FIG. 4 is a perspective view of the beam of FIG. 3 after a repair has been effected by the plate;

FIG. 5 is a perspective view of a plate plastically elongated in more than one direction in order to transfer bi-directional forces to a structural member;

FIG. 6 is a perspective view of a column having a girdle formed with a shape memory alloy;

FIG. 7 is a perspective view of a beam having a shape memory plate anchored thereto by a shape memory girdle;

FIG. 8 is a perspective view of a structural member with rods formed of a shape memory alloy inserted into bore holes through the structural member;

FIG. 9 is an end view of a reinforcing rod formed of a shape memory alloy and anchored into a hole in a structural member by the shape memory phenomena;

FIG. 10 is a side view of an anchor formed of a shape memory alloy;

FIG. 11 is a side elevational view of a structural member incorporating shape memory alloy fibers having a transformation temperature below ambient;

FIG. 12 is a perspective view of the beam of FIG. 3 and further incorporating sensors and a processor;

FIG. 13 is a side elevational view in section of a method for applying an overlay to a substrate;

FIG. 14 is a perspective view of a beam employing a shape memory alloy plate having a transformation temperature below ambient;

FIG. 15 is a perspective view of the beam of FIG. 14 after a corrective force has been applied by the shape memory alloy plate; FIG. 16 is a side elevational view of a lap splice at a bridge column base; and

FIG. 17 is a cross-sectional view of the bridge column of FIG. 16 with a shape-memory alloy girdle according to the invention.

DETAILED DESCRIPTION OF THE INVENTION In repair and retrofit of concrete and also wood, steel and other components and structures, there are occasions where the closure of cracks, correction of deflections, reduction of stresses, enhancement of structural integrity and other repair or retrofit objectives can be effectively accomplished through the application of corrective forces to the component or the structure. Currently, such forces are transferred to the element or structure using steel or composite tendons and sheets, which are subjected to tension by hydraulic and mechanical means.

However, in a repair job, time and labor costs generally far exceed material costs. Turning to FIG. 1, a plate 10 or rod 12 formed of a shape memory alloy (SMA) can be employed, with its characteristic temperature controlled shape changing phenomena supplying the necessary corrective force in place of complicated mechanical or hydraulic jacks (not shown). The shape- memory plate 10 or rod 12 used in this invention may be made fully of a shape- memory alloy (SMA) as shown in FIG. 1. Alternatively, as shown in FIG. 2, plates 14 or rods 16 may be formed of a polymer matrix composite 18 incorporating ribbons or wires of SMA 20.

In one embodiment of the invention, FIG. 3, the shape-memory plate 10 is plastically elongated and then, while kept below its transformation temperature, is adhered or otherwise anchored to a component or structure, such as a concrete beam 22 to be repaired or retrofitted. Referring to FIG. 4, raising the temperature of the plate 10 (e.g. through electric heating) above the transformation temperature includes a solid state phase change causing the plate

10 to seek its original shape, i.e, shorten. The shortening tendency of plate 10 transfers corrective forces 24 to the beam 22 in order to accomplish repair or retrofit objective such as reducing crack widths, deformations and stresses. As shown in FIG. 4, the plate 10 closes cracks 26 previously formed in the beam 22. Referring to FIG. 5, a shape-memory plate 28 may be plastically elongated in two directions in order to transfer bi-directional forces 30 to a component or structure 32.

Referring to FIG. 6, it is often necessary to apply a compressive force to a column 34, as for instance to enhance its ability to withstand seismic or other loads. A compressive girdle 36 employing SMA can apply the desired compressive forces 38. The girdle is formed entirely of SMA, or alternatively, comprises a polymer matrix containing SMA elements, such as wires. In either event, the SMA is plastically elongated prior to the girdle 36 being applied to the column 34. Raising the temperature above the transformation temperature causes the SMA to contract and the girdle 36 to apply confining corrective pressure to the column 34. It will be evident that the girdle 36 may similarly be applied to other structured components such as beams.

Referring to FIG. 7, for example, this type of confining pressure applied by the girdle 36 is used for anchorage purposes. The girdle 36 aids in anchoring another shape-memory system 40 to a component or structure 42 for better transfer of corrective forces 44 to the component or structure 42 by the second SMA system 40.

Application of SMA's for repair is not limited to external surfaces of a component as disclosed thus far. Referring to FIG. 8, plastically elongated shape-memory rods 12 are placed inside holes 46 which are drilled into a component or structure 48. An epoxy or other suitable adhesive secures the rods inside the holes 46. Raising the temperature of the rods 12 above the transformation temperature induces the rods 12 to shorten, thus transferring corrective forces 50 to the component or structure 48 to accomplish such repair or retrofit objectives as closing cracks 52 or increasing the strength of the component or structure 48.

In FIG. 9, the shape-memory rod 12 is not only elongated but also, after its memory diameter or cross-sectional dimension has been established,

pressed laterally to reduce its diameter, and then placed inside hole 54. Upon heating, the reduced-diameter and elongated shape-memory rod 12 tends to expand and shorten inside the hole 54. The expansion tendency produces pressure inside the hole 54 which generates frictional resistance against movement and provides for the anchorage of the rod 12 inside the hole 54.

In another embodiment, referring to FIG. 10, the memory shape includes hooks 56 at the end of a shape-memory rod 58 for anchorage. During plastic elongation, the hook 56 is also straightened. After placing the rod 58 inside a hole 60 and upon heating to cause shortening tendencies, the hook 56 also re-emerges, providing for end anchorage of the rod 58 and transfer of forces to a component or structure 62. The anchorage methods of FIGS. 9 and 10 may be used in various applications; for example, referring to the method of FIG. 9, conventional reinforcing bars or pipes (not shown) can be coated with shape-memory alloys which are then reduced in thickness under pressure. Their expansion tendency upon heating can then be used for anchorage purposes.

In another embodiment of the invention, referring to FIG. 11, an elongated shape-memory rod, ribbon or wire is cut into discrete short fibers 64 which are mixed into formable materials 66 such as concrete, plastics or ceramics during mixing and forming. Later, during service life and upon failure marked by excess widths or strains of crack 68, the shape memory fibers 64 are heated, causing them to seek their shorter memory shape. Their shortening tendencies produce internal stresses 70 for closing the cracks 68 or reducing strains and deformations. The shape-memory fibers 64 can also be pressed laterally during elongation to provide for better anchorage upon heating and expansion using the mechanism introduced in FIG. 9.

Sensors can be used in conjunction with any of the previous shape- memory systems to automatically prompt the repair action based on the shape- memory phenomenon once failures marked by cracking, excess strains or the like are detected. For instance, the composite shape-memory system of FIG. 3 can incorporate fiber-optics or other sensors as shown in FIG. 12. Fiber optic strain sensors 72 detect failure signs such as excess widths of cracks 26 and other strains. In conjunction with a processor 74, a signal from the sensor will automatically actuate the shape-memory plate 10 when needed in order to

accomplish the necessary repair objectives. Such systems would thus have self- repair capabilities in case of future distress.

In yet another embodiment, the invention is used for providing better bonding of overlay systems to a substrate such as concrete, rock or other materials. Referring now to FIG. 13, holes 76 are drilled into a substrate 78. Elongated shape-memory rods 80 are then placed inside the holes 76 and anchored to the substrate 78 using adhesives and/or either of the concepts introduced in FIGS. 9 and 10. After placing an overlay 82 over the substrate 78, heating of the shape-memory rods 80 produces shortening tendencies which pull the overlay 82 against the substrate 78 and provide for a more integrated system of overlay and substrate. Preferably, the rods 80 are provided with enlarged heads 84 disposed within the overlay 82 to improve anchorage of the rods 80 within the overlay 82.

Referring now to FIG. 14, in a further embodiment of the invention, a super-elastic shape-memory plate 86, with transformation temperature below ambient temperature, is used without prior plastic elongation. After the plate 86 is adhered or anchored to a component or structure 88, any future cause of damage, say in the form of external loading 90, which would plastically elongate the shape-memory plate 86, would activate, as shown in FIG. 15, the shape-memory phenomenon (which now takes place at ambient temperature) and transfer corrective forces 92 to the component or structure to resist the load 90 and thereby providing a self-repair capability. This self-repair capability with super-elastic memory alloys which are not plastically elongated can be built into any of the systems of Figs. 5, 6, 8, 11 and 13. In the case of FIG. 11, this embodiment can use either discrete or continuous shape-memory systems which are incorporated into the deformable material without prior elongation.

Deficient seismic design procedures of the past have left major components of our concrete-based infrastructure with insufficient capability to withstand seismic excitations. Turning to FIG. 16, for example, a bridge column 84 designed in 1950' s and 1960 's commonly included a lap splice 86 of longitudinal reinforcement members 88, 98 at a base 90 of the column. At this location, inelastic flexural actions must develop to provide the ductile response

necessary to enable a structure supported by the column 84, such as a bridge, to survive intense seismic attack. Lap-splices 86 generally break down under the cyclic inelastic action, typical of seismic excitation, with a consequent reduction in flexural strength and energy absorption. Failure of lap-splices can be inhibited with adequate confinement pressure, such as that of the girdle of FIG. 6, developed across the potential splitting cracks. FIG. 17 illustrates another girdle embodiment where pre- elongated shape-memory Nitinol wires with a diameter of 3 mm located at 5- mm spacing (on-center) and embedded in a protective composite matrix 102 apply suggested confining pressure of approximately 1 Mpa for seismic retrofit of the column 84.

While the invention has been particularly described in connection with certain specific embodiments thereof, it is to be understood that this is by way of illustration and not of limitation, and the scope of the appended claims should be construed as broadly as the prior art will permit.