Clamp

This invention relates to a clamp for an elongate object comprising a split ring.

Clamps are needed for elongate objects for a number of applications, for example to secure a sheath to an elongate object (such as a braid to a cable) or to secure or couple hoses or pipes. Common clamps used for these applications are resilient metal clamps. These exert some force on the elongate object. Generally however they can only be deformed a small amount if they are to exert any securing force on the underlying object, so need to be positioned on the object via one end thereof. This is acceptable for many applications, but is not possible in other applications where sideways entry is required. For these applications it has been usual to employ mechanical clamps for example two half-annular rings which are drawn towards each other by the tightening of screws. These mechanical clamps are clumsy to employ and cannot accommodate any changes in dimensions which may occur, during service, to the underlying object.

We have now discovered that a beneficial new clamp can be made from a shape memory metal (SMA) which exhibits supere¬ lastic behaviour.

Materials, both organic and metallic, capable of possessing shape memory are well known. An article made of such materials can be deformed from an original, stable con¬ figuration to a second, unstable configuration. The article is said to have shape memory for the reason that it can be caused to revert, or to attempt to revert, from its unstable configuration to its original, stable configuration, i.e. it "remembers" its original shape. Such reversion may be automatic, or caused by the application of heat.

Among metallic alloys, the ability to possess shape memory is generally a result of the fact that the alloy undergoes a reversible transformation from an austenitic state to a mar¬ tensitic state with a change in temperature. This transfor¬ mation is sometimes referred to as a thermo-elastic martensitic transformation. An article made from such an alloy, for example a hollow sleeve, is easily deformed from its original configuration to a new configuration when cooled below the temperature at which the alloy is trans¬ formed from the austenitic state to the martensitic state.

The temperature at which this transformation begins is usually referred to as M _s and the temperature at which it finishes Mf. When an article thus deformed is warmed to the temperature at which the alloy starts to revert back to austenite, referred to as A _s (Af being the temperature at which the reversion is complete) the deformed object will begin to return to its original configuration.

However, as used herein, a shape memory alloy includes any alloy which can undergo a reversible deformation with or ' without a phase transformation, and with or without heat to effect the recovery.

Some memory metals exhibit superelastic behaviour. These materials are defined herein to be those which can undergo recoverable large deformations (greater than 4%) without the onset of plasticity.

Memory metals that exhibit superelastic behaviour may exhi¬ bit non-linear superelastic behaviour (also known as pseudoelastic behaviour) or linear superelastic behaviour. With non-linear superelastic (pseudoelastic) materials reco¬ verable strains up to about 8% can be obtained. With linear superelastic materials recoverable strains of about 4-5% can be obtained. These are now discussed in turn.

The non-linear superelastic behaviour results from the for¬ mation of stress-induced artensite. When an SMA sample

capable of exhibiting stress-induced martensite is stressed at a temperature above M _s (so that the austenitic state is initially stable) , but below M3 (the maximum temperature at which martensite formation can occur even under stress) it first deforms elastically and then, at a critical stress, begins to transform by the formation of stress-induced mar¬ tensite. Depending on whether the temperature is above or below A _s , the behaviour when the deforming stress is released differs. For temperatures below A _s , the alloy is initially heat treated so the alloy is in its austeniphase. In this case deformation below A _s forms stress-induced mar¬ tensite and this is stable. If the temperature is above A _s , the stress-induced martensite is unstable and transforms back to austenite, with the sample returning (or attempting to return) to its original shape.

In European Patent Application Publication No. 0140621 (MP873), the disclosure of which is incorporated herein by reference, a nickel/titanium/vanadium alloy having SIM over a wide temperature range is disclosed.

Figures 1 and 2 illustrate the stress-strain behaviour of an alloy which exhibits constant stress versus strain behaviour due to stress-induced martensite.

Figures 1 and 2 illustrate the phenomenon of stress-induced martensite by means of stress-strain curves. In both Figure 1 and Figure 2, the alloy is at a temperature between M _s and Md so that it is initially austenitic; and it will be assumed for the purposes of this discussion that M _s is equal to Mf, and A _s equal to Af. Figure 1 shows the case when the temperature is below A _s , so that any martensite formed by the applied stress is stable; while Figure 2 shows the case where the temperature is above As, so that austenite is the only stable phase at zero stress.

In Figure 1, when a stress is applied to the alloy, it deforms elastically along the line OA. At a critical applied stress. C , the austenitic alloy begins to transform

to (stress-induced) martensite. This transformation takes place at essentially constant stress until the alloy becomes fully martensitic at point B. From that point on, as further stress is applied, the martensite yields first elastically and then plastically (only elastic deformation is shown at point C) . When the stress is released, the mar¬ tensite recovers elastically to point D, at which there is zero residual stress, but a non-zero residual strain. Because the alloy is below As, the deformation is not reco¬ verable until heating above As results in a reversion to austenite. At that point, if the sample is unrestrained, the original shape will be essentially completely recovered: if not, it will be recovered to the extent permitted by the restraint. However, if the material is then allowed to re- cool to the original temperature at which it was deformed (or a temperature where SIM behaviour of this type is seen), the stress produced in the sample will be constant regardless of the strain provided that the strain lies within the "plateau" region of the stress-strain curve. That is, for a strain between £β and £^, the stress will be Cjj _j . This means that a known, constant force (calculable fro Otø can be applied over a wide (up to 5% or more for certain Ni/Ti alloys) strain range. Thus, though this resembles the conventional shape memory effect, because the alloy shows SIM and is below A _s a constant force can be achieved.

In Figure 2, when a stress is applied to the alloy, it deforms elastically along line OA, then by SIM along line AB, and by deformation of the martensite to point C, just as in Figure 1. However, the stress-strain behaviour on unloading is significantly different, since the alloy is above A _s and the stable phase is therefore austenite. As the stress is removed, the alloy recovers elastically from C to D; then, at a critical stress, <S^, the alloy reverts to austenite without requiring a change in temperature. Thus reversion occurs at essentially constant stress. Finally if the stress is removed from the reverted austenite, it reco¬ vers elastically along line EO.

The recoverable deformation asociated with the formaiton and reversion of stess-induced martensite has been referred to as pseudoelasticity.

Where the temperature is below A _s , and the material is not initially pretreated to provide it in the austenite phase, the material is always martensitic. In this case, for some materials, deformation causes cold working of the marten¬ site, and results in linear superelastic behaviour. Unlike the non-linear superelastic behaviour (pseudo elastic) the linear superelastic behaviour does not involve a phase transformation.

Figure 3 shows the stress-strain behaviour for a material exhibiting linear superelastic behaviour (loaded to 4% strain and unloaded compared to the stress strain behaviour for a material exhibiting non-linear superelastic behaviour (loaded to 8% strain and unloaded. The non-linear behaviour (corresponding to Figure 2) is shown in dotted lines.

The non-linear behaviour allows greater recoverable strain than the linear behaviour. Also the non-linear behaviour provides a constant force over a large strain range.

Linear superelastic behaviour is thought to result from the interaction between defects from the coldworking process and twins in the martensite. The interaction is thought to cause any migration of the twin boundaries during deformation to reverse during unloading.

An advantage of the use of an alloy that exhibits supere¬ lastic behaviour is that larger recoverable deformations can be introduced into articles than would be possible with alloys exhibiting traditional memory metal behaviour. Using the superelastic phenomenom recoverable strains of up to 4%, even 6% or even 8% can be introduced into memory metals.

The present invention uses the enhanced amount of reco¬ verable strain made possible by superelastic behaviour to

provide an article which can be positioned on, or made to contact another object in a certain manner by means of the greater deformation.

A first aspect of the present invention provides a clamp for an elongate object comprising a split ring made from a memory metal which exhibits superelasticity with at least 4% recoverable strain, the ends of the split ring having being moved relative to each other to render the ring recoverable.

A second aspect of the present invention provides a method of fastening a clamp onto an elongate object comprising:

(a) deforming a split ring made from a memory metal which exhibits superelastic behaviour with at least 4% recoverable strain by moving the ends of the split ring relative to each other,

(b) inducing or allowing recovery of the deformation thereby causing the ends of the split ring to move back towards their undeformed position thereby engaging the elongate object.

In one embodiment, the clamp is generally C-shaped, the ends of the C-clamp having been deformed away from each other to render the ring recoverable and to enable the clamp to be positioned sideways onto the elongate object.

The term "positioned sideways onto the elongate object" is used herein to distinguish from positioning the ring over one end of the elongate object. The ring can approach the object from any side and/or at any angle.

In order to be positioned sideways the ring must be suf¬ ficiently "open" to allow it to be passed onto the object. It is for this reason that the enhanced recoverable strain made possible by the superelastic behaviour is useful.

The split ring may itself comprise some resiliency.

In one embodiment the clamp is positioned around the elongate object and shrinks on recovery. In this case the distance between the ends of the split ring is preferably approximately the same as the diameter of the elongate object to be clamped (for cylindrical objects).

The clamp may exhibit linear superelastic or non-linear superelastic (pseudo elastic) behaviour.

In one embodiment where the clamp exhibits pseudo elastic behaviour, it is deformed for installation at a temperature below A _s , (having been pretreated so it is austenitic), so that martensite is the only stable form at zero stress, and the 'clamp exhibits the stress/strain behaviour shown in Figure 1. The ends of the clamp are deformed relative to each other by the formation of SIM. When the clamp has been placed adjacent the elongate object, it needs only to be heated above Af and allowed to cool to its original tem¬ perature, to cause the memory metal to recover and the ends thereof to move relative to each other towards their unde- formed position to tighten the clamp into contact with the object. Preferably the clamp contacts the object before complete recovery, i.e. there is some unresolved strain in the clamp. Provided this strain is within the plateau region of the stress/strain curve for the material (between A and £β in Figure 1) the clamp will exert a constant force on the elongate object, and this force will remain constant even if the strain is reduced. This is advantageous for certain applications, as explained later.

Where the clamp exhibits pseudo elastic behaviour and is deformed below the As temperature, A _s may be above or below room temperature. In one preferred embodiment As is below room temperature. In this case the clamp may be deformed and strained by SIM formation in liquid nitrogen. Removal of the clamp from the liquid nitrogen, and its consequent warming to room temperature therefore causes transformation to austenite and recovery of the article.

In another embodiment the clamp is deformed for installation at a temperature above A _s , so that austenite is the stable phase at zero stress, and the clamp exhibits the stress/strain behaviour of Figure 2. In this case after deformation of the ends of the clamp relative to each other, the clamp must be held in its deformed position by means of a tool (to prevent automatic reversion to austenite and recovery. After positioning of the clamp on the object the tool is then simply removed allowing the clamp to recover. Since the reversion to austenite takes place at a constant stress, a constant force is exerted by the recovering clamp. Preferably the clamp contacts the object before complete recovery, i.e. there is some unresolved strain in the clamp. Provided this strain is within the plateau region of the stress/strain curve for the material (between _A and _β in Figure 2) the clamp will exert a constant force on the object.

Where the clamp exhibits linear superelastic behaviour, the clamp (not initially pretreated, so it is and remains mar¬ tensitic) is defomed for installation at a temperature below A _s , and held in it deformed position by means of a tool. After positioning of the clamp adjacent the object, the tool is removed allowing the clamp to recover. The clamp exerts a continually decreasing force as it recovers in accordance with Figure 3.

The present invention also provides a clamp according to the invention in combination with an elongate object, before and after recovery of the clamp.

The ends of the clamp are deformed relative to each other to allow the clamp to be positioned. The ends may be moved in any direction. They may be moved towards or away from each other. For example, the ends may be moved sideways, generally parallel to the axis of the ring. Another option is for the ends to be moved generally tangentially to the ring. The clamp may vary in cross-section along its length.

Preferably there is more material towards the middle than the ends of the clamp: e.g. it is wider or thicker in the middle. This provides the optimum clamping force.

The ends of the clamp may initially and/or finally be separated from each other, or overlap.

The sideways positioning of the clamp provided by a prefered embodiment, and the clamp's subsequent recovery up to 4%, 6% or even 8% allows the clamp to be used for a number of applications. One typical application is to secure a braid (eg. a screening or earthing braid) to an underlying cable. The underlying cable may comprise a polymer sheath that is subject to in-service creep. Provided there is some unre¬ solved recovery in the clamp (ie it is in the plateau region of the stress/strain curve) the clamp will continue to exert a constant force on the underlying braid in spite of any cable polymer creep. For some applications where a braid is to be attached to a cable the end of the cable is not accessible, so side entry of the clamp is essential. such an application is, for example, where a cable passes through a bulk-head or wall to which it is to be earthed. Previous solutions for such an application have involving the use of mechanical clamps or crimps are more time consuming to apply, and do not automatically accommodate in-service creep of the underlying cable.

Another application for the invention is to repair a braid termination, that is to install a new earthing braid or a termination.

Another application is as a hose-clamp, for example in auto¬ mobile applications, to connect a host to a tubular outlet.

Embodiments of the invention are now described with reference to the drawings wherein:

Figures 1, 2 and 3 are graphs, previously discussed, illustrating the stress-strain behaviour of an alloy which is deformed by the formation of stress-induced martensite;

Figure 4 is a perspective view of a clamp according to the invention before and after recovery onto a braid and underlying cable; and

Figures 5a and 5b are perspective views of another embodiment of clamp, before and after recovery respectively.

Referring to Figure 4, a memory metal alloy clamp 2, which exhibits stress-induced martensite behaviour is shown in its pre-recovered position on the left of the figure positioned over a braid 4 on a cable 6. The function of the clamp is to secure the braid 4 to the cable 6. The clamp 2* is shown after recovery on the right of the figure. In the pre- recovery position the ends 8 of the clamp 2 are spaced apart a distance A which is substantially the same as the diameter of the cable 6. This enables the clamp to be positioned sideways on the cable, rather than over one end. The clamp may be resilient to enable the ends to be pulled apart, by hand or tool, slightly more, for easy installation.

The ends 8 of the clamp 2 have been deformed away from each other in a direction substantially tangentially to the clamp at a temperature between M _s and M^ and below A _s . "Thus the deformation is by the formation of stress-induced marten¬ site, and in the position shown on the left of Figure 3, where there is zero stress on the clamp the stable phase is martensite.

To recover the clamp it is heated above A _s causing the alloy to transform to austenite, recovering some of the defor¬ mation strain and exerting a constant force on the underlying braid and cable in accordance with the stress- strain curve of Figure 1. The ends 8' of the recovered clamp overlap to secure the braid 4 tightly onto cable 6 (see right of Figure 4). If, during service, polymeric layers of the cable creep, the clamp will tighten further ⁽ the ends 8' overlapping more) by recovering more of the original deformation strain, continuously exerting the

constant force, in accordance with the stress/strain curve of Figure 1.

Figures 5a and 5b shown an alternative form of C-clamp 10 10' before and after recovery respectively. In this case the ends 12 of the clamp are deformed away from each other laterally, in a direction substantially parallel to the axis of the clamp, and come towards each other 12' in the reco¬ vered position.