GB2329650A - Stressing of sectional elements - Google Patents

Abstract

Development of beneficial residual stresses, or removal of undesirable residual stresses in sectional elements. A stress is applied so as to generate a Poisson strain in the relevant area. The stress is then released and the Poisson strain leaves its own residual stress. The use of the Poisson approach results in the initial stress being particularly straightforward to apply, particularly to a rail section or I - beam, e.g. by the use as shown of conical rollers which contact head 60 and base 64 but not web 66, or by separate pairs of rollers or skewed rollers (Figs 6 and 8, not shown), which have the same effect. For an H - beam, wedges can be placed in the limbs of the H for cooperation with the conical rollers (Fig 7, not shown), or pressurised oil can be used (Fig 5, not shown).

Description

SECTIONAL ELEMENTS The present invention relates to sectional elements. In particular, it is concerned with the residual stress state of sectional elements, and the means by which a beneficial residual stress state may be achieved. The sectional elements to which the present invention finds particular (but not exclusive) application are rails and I-beams.

These types of sectional elements consist of a central portion, at either end of which is an element of greater lateral dimension. In an I-beam or H-beam these are the upper and lower flanges. In a rail section they are asymmetric and referred to as the foot and the head. In this Application, the terms web, head and foot will be used but it should be understood that these refer to the corresponding sections of an I-beam or any other sectional element to which the invention is applicable.

Such sections have a longitudinal axis, i.e. the axis running parallel to the length of the element, and a transverse axis, i.e. a direction running perpendicular to the longitudinal direction in the direction from the head to the foot or vice-versa. The remaining direction, the through-thickness axis, is rarely the subject of a residual stress, at least in the web, as the material is relatively thin.

Such sections normally need to be straightened after production.

Straightening commonly involves the application of compression to the section as a whole in its transverse direction. The result of this is a residual stress state in the final product generally as indicated in Figure 1. Thus, the head 10 and the foot 1 2 of the section (in this case a rail element) are in longitudinal tension whilst the web 14 is in longitudinal compression. At the free end 1 6 of the rail, the web 1 4 is in transverse tension.

This stress state is highly undesirable, as indicated in Figure 2. The combination of the tensile stresses at the head, foot, and free end of the web tend to open up any longitudinal cracks 1 8 present in the web. If the residual stresses are sufficient then such cracks, once opened, can propagate along the length of the section.

A common test for rail sections is to cut a "crack" of predetermined length into the web in a longitudinal direction. The degree of opening 20 of the crack is then measured to indicate the degree of residual stress. This measurement is referred to as the "Web Saw Opening" or WSO.

In use, both rails and other beams experience compressive transverse stresses in the web and tensile longitudinal stresses in the head and foot. In rail sections, these stresses arise as a locomotive travels over that section of rail. In an 1-beam, they occur as the beam is subjected to a bending moment around its through-thickness axis. These will clearly add to the existing residual stresses. The longitudinal tension in the head and foot will then contribute to fatigue damage and crack opening of the top and (particularly) bottom surface.

Existing methods of dealing with residual stress problems generally comprise annealing or soaking steps which aim to remove residual stresses entirely, or surface treatments such as shot-peening or surface rolling. The former tends to result in departures from straightness of the section and do not produce an inherently desirable residual stress1 whilst the latter produce somewhat severe stress gradients within the material and undesirable residual stresses below the surface.

In its first aspect, the present invention therefore proposes a method of introducing a desired longitudinal residual stress in to a section of the type defined which comprises the application of a force in the transverse or through-thickness direction to a selected sub-element of the section sufficient to produce a strain in that direction and thereby produce a plastic Poisson strain in the longitudinal direction in the selected sub-element, and the removal of the force thereby to leave a residual stress.

If there is a pre-existing residual stress, then the method will clearly introduce a further residual stress, and the final residual stress will be the algebraic sum of the original and the introduced stresses.

In another aspect, the present invention proposes a method of processing a section of the type defined that comprises the application of a force in the transverse or through-thickness direction thereby to produce a strain in that direction that results in a Poisson strain in the longitudinal direction of the opposite sign to a required change in longitudinal residual stress.

In a third aspect, the present invention proposes a method of processing a section of the type defined that comprises the application of a quasi-static force in the transverse or through-thickness direction to a selected sub-element of the section thereby to produce a strain in that direction in that sub-element giving rise to a simultaneous Poisson strain in the longitudinal direction having an opposite nature to that of a desired longitudinal residual stress in that selected sub-element of the section.

The longitudinal Poisson strain is in general approximately equal to the transverse strain multiplied by minus Poisson's ratio.

It is preferred if the selected sub-element is the web.

Once the applied force is released, residual stress is produced which will usually be opposite to an original undesirable residual stress or additive to a pre-existing desired residual stress. The final state will therefore be either desirable or at least less undesirable. Assuming that the sign of the original longitudinal residual stress in a sub-element was undesirable, application of transverse or through-thickness plastic strain of the opposite sign will produce a final residual stress that is therefore either desirable or less undesirable. Assuming that the magnitude of the original longitudinal residual stress in a sub-element was undesirably low, application of plastic transverse or through-thickness strain of the same sign will produce a final residual stress of that sign that is more desirable.

One way of achieving this is to place the web in transverse tension.

This can suitably be achieved by exerting an upward force to the lower faces of the head and a downward force to the upper faces of the foot. This is effectively an opening force in the gap between the head and foot. This imparts a transverse tensile stress in the web, which provokes a longitudinal compressive stress. A high enough longitudinal compressive stress, when released, leaves a longitudinal tensile residual stress.

A preferred method of achieving this is by the insertion of a wedging element in the region adjacent the web between the head and foot elements thereby to impart to the head and foot a transverse expansion force.

Preferably, this is carried out on both sides of the element. The wedging elements can be rollers or elongate static members arranged to form a vise.

It is preferred that the wedging elements do not contact the web.

An alternative or additional means of achieving this is to place the foot and/or head into a through-thickness compression. This will provoke a longitudinal tensile strain, if sufficient, which when allowed to relax will leave the desired longitudinal compressive stress.

As the forces in a rigid body must always balance, the longitudinal tensile stress in the web provokes longitudinal compressive stresses in the head and foot. These counteract the original residual stresses and are of themselves a desirable stress state in that end cracks in the web and surface cracks in the head and foot are all subject to a closing tendency.

An alternative or additional method is to place selectively the head and foot into transverse compression. This will provoke a longitudinal Poisson tension which, when released, leaves a residual compressive stress.

This, through the balancing of forces1 provokes a tensile longitudinal residual stress in the web. Thus, this example of mechanical deformation produces the same end result. However, the greater lateral thickness of the head and the foot as compared to the web implies that significantly greater forces will be called for.

Embodiments of the present invention will now be described by way of example, with reference to the accompanying Figures in which: Figure 1 is a side view of the end of a rail illustrating the usual residual stresses observed after straightening; Figure 2 shows the rail of Figure 1 incorporating a longitudinal crack within the web; Figure 3 shows a vertical cross-section through an embodiment of the invention; Figure 4 shows a top view of the embodiment of Figure 3; Figure 5 shows a vertical cross-section through a second embodiment of the present invention; Figure 6 shows a vertical cross-section through a third embodiment of the present invention; Figure 7 shows a vertical cross-section through a fourth embodiment of the present invention; Figure 8 shows a vertical cross-section through a fifth embodiment of the present invention; Figure 9 shows the relationship between applied force and derived stress achievable through the present invention; Figure 10 shows the stress states achievable by the present invention; Figure 11 shows comparative results of Web Saw Opening tests achievable via the present invention; and Figure 1 2 shows comparative values of measured residual stress in the head, web and foot of rails treated according to the invention and according to known methods.

Figures 1 and 2 have been referred to already, and no further description will be given here.

Figure 3 shows an embodiment of the present invention applied to a rail. A pair of rollers 50 are provided, which are generally disc-shaped and are rotatable about axis 52. The axial faces 54 of the rollers 50 are inclined at an angle of one in four such that the thickness of the roller 50 decreases with increasing distance from the axis 52.

As illustrated in Figures 3 and 4, the rollers 50 are applied either side of a rail 60, in this case of the dimensions set down in A.R.E.A. Standard 1 36RE. The rail 60 is then drawn in the direction of arrow 56 (fig. 4), and the rollers 50 are allowed to rotate about their axis 52. The rollers may be free to rotate or they may be driven.

For use with this specific rail dimensions, a suitable dimension for the roller involves a minimum width between the outside faces of 1 22mm, and a depth of at least 75mm. The result of these dimensions is that as the rail passes into the gap between the rollers, the head 62 and foot 64 are forced apart, placing the web 66 in transverse tension.

It will of course be a straightforward matter to adapt the dimensions for use with other standard and non-standard rail sizes.

Figure 5 shows schematically an alternative means for imposing a suitable mechanical deformation on the rail 60. A pair of sealing glands 100, 102 are placed either side of the rail 60 such as to contact the head 62 and foot 64 in a sealing manner. A high pressure hydraulic liquid such as oil is then injected into the longitudinal gaps 106 and 108. This exerts a transverse expansion force on the lower faces of the head 62 and upper faces of the foot 64, producing a desirable residual stress state as detailed above.

A further alternative embodiment is illustrated in Figure 6. In this case, the rail 60 is subjected to a stress state similar to that of Figures 3 and 4. Upper and lower braces 120 and 1 22 are provided, each of which is generally U-shaped and is adapted to envelop either the head 62 or foot 64 of the rail. Each carries an axle 124 disposed in a through-thickness direction, and carrying at its free end a roller 1 26 which contacts a surface of the head 62 or foot 64. The four rollers 1 26 in total contact the rail 60 on the same faces contacted by the roller 50 of Figures 3 and 4. The braces 1 20, 1 22 are then pulled in the transverse direction, exerting a stress state on the rail 60 identical to that of Figures 3 and 4.

Figure 7 shows how the arrangement of Figures 3 and 4 can be applied to an I-beam rather than a rail. Rollers 1 50 correspond generally to the rollers 50 of Figure 3, but differ in their exact dimensions so as to cater for dimensions of the l-beam 160. The I-beam consists of a central web 166, from either end of which extend a head 1 62 and a foot 164. Four formers 1 68 are placed in each of the four internal corners of the I-beam, i.e.

at the junctions of the web 166 and the head 162 and the foot 164. These formers 1 68 are generally in the form of a substantially right-angled prism which therefore fits neatly into the corner. They each present an oblique face 1 70 to the oblique surface 1 54 of the roller, and therefore allow a smooth wedging action without distorting the I-beam.

The formers 1 68 could of course be dispensed with and the rollers 1 50 shaped appropriately. However, it is believed that this arrangement provides superior results.

Figure 8 shows an alternative construction, applied to a rail 60, in which a pair of static formers 200 are provided alongside the web 66 and between the head 62 and foot 64. Each former 200 consists of a central axis bar 202, and a pair of wedges 204. The wedges 204 are initially at an oblique angle to each other, thereby allowing the former 200 to fit in the space otherwise occupied by the rollers 50. Each wedge 204 can rotate outwardly about the axis bar 202, thereby exerting an opening force to the rail as required. This rotation of the wedges 204 is achieved by compressing the combination of rail 60 and formers 200 on either side thereof, for example between a pair of rollers. Thus, the wedges 204 rotate outwardly and exert the necessary force, Figures 9 to 11 show the results of an experimental embodiment of the invention. In this experimental embodiment, planar wedges of the dimensions discussed in relation to the rollers 50 of Figure 3 were forced into the gap between the head and foot of a 136RE rail. The web thickness of the rail is 1 7.5mm and the web yield stress was determined as 450MPa.

This rail was known to have high residual stresses, with a WSO value of about 6mm.

The formers were 80mm thick and were laterally compressed into the gap between the head and the base so that a wedging action was produced.

As mentioned above, the slope of the bottom of the head and top of the base is 1:4 so that (neglecting friction) a given lateral compression force should produce a transverse tensile force four times higher. The forces were applied in line with a set of strain gauges used to measure the longitudinal (horizontal) and transverse (vertical) stresses. The results of the stresses are shown in Figure 9, in the vertical and horizontal directions. The elastic slope is also indicated, with a noted departure from elasticity in the horizontal direction at a vertical stress of less than 450MPa. It can be seen that as the force is increased (x axis) the apparent stress in the vertical direction increases markedly, resulting in a Poisson stress in the horizontal direction.

A maximum applied force of 1 030kN resulted in the mean apparent vertical stress of 1 443MPa. The maximum apparent horizontal stress was -642MPa.

This maximum 1 030kN force was then applied at a range of positions, starting 150mum away from a set of strain gauges. Measurements of the strain were taken whilst under load and after the load was removed, as shown in Figure 10. The vertical (transverse) stress of the first such loading, at position -150mm, was zero both during and after loading. The force was then applied at 50mm increments nearer to the gauge; at -1 00mm this resulted in a slight increase in vertical stress. This continued to increase to a maximum at position Omm, then declining to a minimum at +250mm.

It can be seen that by the time the force had been applied consistently along the length of the rail, there was a compressive longitudinal stress of about 100MPa in the rail which had not been present initially. At this position, therefore, the stress appears to be more compressive which is the opposite to that desired. However, the apparent stress registered by the gauge is due to plastic strain. The residual stress is actually related to the difference between the on load and the off load apparent stress at position 0 mm which is positive and therefore in tension. If this longitudinal tension had not been present the apparent stress would have been more in compression than it is and close to the difference of about -400MPa between the elastic slope and the horizontal stress at a force of 1030 kN in Figure 9. The change in residual stress is therefore an increase in tension of about 300 MPa which is a desirable result.

Figure 11 is a line drawing depiction of a 1 36RE rail which was left untreated in a first region X but subjected to the above experimental treatment over region Y. It can be seen visually that the Web Saw Opening value had decreased significantly, in fact from above 4mm to less than 1 mum.

Figure 1 2 shows comparative results for rails treated according to a variety of methods. The longitudinal residual stress is shown for the head, web and foot of each rail. The treatment methods were as follows: Rail treated according to the invention, above Rail subjected to 4200C in the web region x A.R.E.A standard 1 3 RE rail subjected to a mill heat treatment and roller straightened + UlC 60 rail subjected to a mill heat treatment and roller straightened * A.R.E.A standard 136 RE rail, low alloy head hardened and roller straightened A Unstraightened rail subjected to an off-line heat treatment A Unstraightened rail subjected to an on-line heat treatment.

It can be seen that the rail treated according to the invention is left in a state of residual compression in the base, an area particularly prone to fatigue damage. No other process achieved this.

An alternative process would be to place the foot and/or head into compression in the through-thickness direction. This could be done relatively straightforwardly, for example by passing the rail through appropriate rollers contacting the lateral faces of the head and/or foot. This would induce a longitudinal tensile poisson strain which, when released, would give rise to a longitudinal compressive vertical stress.

Claims (13)

1. A method of introducing a desired longitudinal residual stress in to a section of the type defined which comprises the application of a force in the transverse or through-thickness direction to a selected sub element of the section sufficient to produce a strain in that direction and thereby produce a plastic Poisson strain in the longitudinal direction in the selected sub-element, and the removal of the force thereby to leave a residual stress.

2. A method of processing a section of the type defined that comprises the application of a force in the transverse or through-thickness direction thereby to produce a strain in that direction that results in a Poisson strain in the longitudinal direction of the opposite sign to a required change in longitudinal residual stress.

3. A method of processing a section of the type defined that comprises the application of a quasi-static force in the transverse or through thickness direction to a selected sub-element of the section thereby to produce a strain in that direction in that sub-element giving rise to a simultaneous Poisson strain in the longitudinal direction having an opposite nature to that of a desired longitudinal residual stress in that selected sub-element of the section.

4. A method according to any preceding claim in which the selected sub element is the web.

5. A method according to any preceding claim in which the web is placed in transverse tension.

6. A method according to claim 5 in which an upward force is exerted to the lower faces of the head and a downward force to the upper faces of the foot.

7. A method according to claim 6 in which a wedging element is inserted in the region adjacent the web between the head and foot elements thereby to impart to the head and foot a transverse expansion force.

8. A method according to claim 7 in which a wedging element is inserted on both sides of the element.

9. A method according to claim 7 or claim 8 in which the wedging elements are rollers.

10. A method according to claim 7 or claim 8 in which the wedging elements are elongate static members arranged to form a vice.

11. A method according to any one of claims 7 to 10 in which the wedging elements do not contact the web.

12. A method according to any preceding claim in which the foot and/or the head are placed into a through-thickness compression.

13. A method according to any preceding claim in which the head and foot are placed selectively into transverse compression.

1 4. A method substantially as any one described herein with reference to and/or as illustrated in the accompanying drawings.