A method for improving the properties of an elongate structural member

Abstract

 The longitudinally compressive and tensile proportionality limits and ultimate strengths of an elongated structural member are increased by applying to the member a longitudinally compressive load which is close to its maximum load. In order to prevent buckling of the member under such a load, the member is confined in an encasement which allows the member to yield longitudinally without bending. By applying to the member a longitudinally compressive load much greater than the member's original (i.e. untreated) ultimate compressive strength, both the ultimate strengths and the proportionality limits of the member are increased. In addition, application of a longitudinally compressive load close to or greater than the original ultimate compressive strength of the member also increases the straightness of the member and frees the member of residual stresses.

Description

[0001] THIS INVENTION relates generally to a method of improving the properties of an elongated structural member. Such a structural member may be solid, or hollow, i.e. a large tube.

[0002] Resistance to buckling is an important quality for many elongated structural members used under compressive loads. By improving resistance to buckling, better performance when subjected to compressive load can be achieved, In addition, increased resistance to buckling allows smaller, and therefore lighter and less expensive, members to be advantageously used in applications where these qualities are important. Consequently, extensive studies have been made of how to improve the buckling resistance of elongated structural members when under compressive loads.

[0003] Typically, the prior art has attempted to improve buckling resistance by applying radially compressive loads in order to increase the straightness of an elongated structural member (i.e., column). For example, U.S. Patent No. 2,178,141 to Frame discloses the cold-straightening of well casings by applying radially compressive loads incrementally and progressively along the casing length. As a result of the radial compression, residual stresses in the casing are reduced, giving the casing an increased resistance to buckling.

[0004] The present invention seeks to provide an improved method of improving the properties of an elongate structural member.

[0005] According to one aspect of this invention there is provided a method for increasing the elastic region of a column when under compression or tension, said method comprising the steps of: confining said column against movement in a direction transverse to the longitudinal axis of said column; applying to said confined column a longitudinally compressive load sufficient to cause the column to yield longitudinally; preventing said confined column from buckling while longitudinally yielding; and increasing said longitudinally compressive load on said column to a value close to the ultimate compressive strength of said column so as to increase the elastic region of said column.

[0006] Preferably the method includes the step of: further increasing said longitudinally compressive load on said column to a value approximately twice as great as the original ultimate compressive strength of said column so as to increase both the ultimate compressive and tensile strengths of said column.

[0007] The invention also provides a method for increasing the critical load of a column to a value substantially accurately predicted by Euler's equation,


wherein P equals the critical load of a column, E equals the Young's modulus of the column material, K is a constant representing end conditions of said column and equals the ratio between column length and its radius of gyration, said method comprising the steps of: longitudinally compressing a column whose critical load is less than that predicted by Euler's equation; preventing said column from buckling by restraining its ability to bend; and allowing said column to yield longitudinally in response to said longitudinal compression so as to increase the elasticity of the column such that said column's critical load is more accurately predicted by Euler's equation.

[0008] Preferably said longitudinal compression is much greater than the original ultimate compressive strength of said column so as to thereby increase both the ultimate compressive and tensile strengths of said column.

[0009] Advantageously said column is composed of mild steel and the critical load of the column after longitudinal compression is accurately predicted by Euler's equation for λ values as low as approximately 76.9.

[0010] According to another aspect the invention provides a method for increasing the elastic strength of an elongated column under compression or tension, said method comprising the steps of: applying a compressive load to the end of said column which is parallel to the longitudinal axis of said column and which is approximate to or greater than the ultimate compressive strength of said column; inhibiting the movement of said column in the direction transverse to said load; and allowing said column to yield axially without buckling.

[0011] Preferably the method includes the step of: controlling the movement of said column under compression in the direction transverse to said load such that residual stresses are relieved and the straightness of the column is improved.

[0012] According to another aspect of the invention there is provided a method for straightening an elongated column, said method comprising the steps of: applying a compressive load to the end of said column with said load being parallel to the longitudinal axis of said column and of substantially the same magnitude as the ultimate strength of said column; and controlling the yielding of said column in response to said load in directions orthogonal to the longitudinal axis of said column such that the uniformity of said column is improved.

[0013] Preferably the method includes the steps of: applying said compressive load to the end of said column at a magnitude substantially greater than the ultimate compressive strength of said column; and allowing said column to yield without buckling in response to said compressive load so as to increase the elasticity and the ultimate strength of said column in both compression and tension.

[0014] The invention also provides a method for increasing the ultimate compressive and tensile strengths of an elongated member utilising an encasement for the member and opposing pistons located at opposite ends of the encasement, said method comprising the steps of: inserting the member into the encasement such that the encasement inhibits bending of the member when it is placed under a longitudinally compressive load; moving said pistons against said opposite ends of said member so as to apply a longitudinally compressive load to said member which is approximately equal to the ultimate compressive strength of said member; and allowing said member to yield without buckling in response to the longitudinally compressive load to thereby increase the elastic region of the member.

[0015] Preferably the longitudinally compressive load applied to said member is increased to a magnitude greater than the original ultimate compressive strength of said member so as to increase both the ultimate compressive and tensile strengths of said member.

[0016] The present invention thus provides a method for increasing the longitudinally compressive and tensile proportionality limits and ultimate strengths of an elongated structural member by applying to the member a longitudinally compressive load. To increase the proportionality limits of the member, a longitudinally compressive load is applied to the member which is close to its maximum loαd. In order to prevent buckling of the member under such a load, the member is confined in an encasement which allows the member to yield longitudinally without bending. By applying to the member a longitudinally compressive load much greater than the member's original (i.e. untreated) ultimate compressive strength, both the ultimate strengths and the proportionality limits of the member are increased. In addition, application of a longitudinally compressive load close to or greater than the original ultimate compressive strength of the member also increases the straightness of the member and frees the member of residual stresses.

[0017] This invention thus provides a method of cold-working elongated structural members, which may be of diverse cross-sectional shapes, so as to increase their longitudinally compressive proportionality limit (as well as elastic limit and yield strength) and thereby reduce their susceptibility to buckling. In relation to this, this invention may also be used to straighten elongated structural members intended for use under longitudinal compression so as to increase the member's resistance to buckling.

[0018] This invention may increase the longitudinally tensile proportionality limit of an elongated structural member, and may provide a low cost method of increasing the ultimate compressive and tensile strengths of elongated structural members while at the same time increasing their straightness. Thus this invention may allow the use of lighter weight and lower cost elongated structural members in applications involving highly compressive and/or tensile loads. This invention may also relieve residual stresses of elongated structural members.

[0019] In order that the invention may be more readily understood, and so that further features thereof may be appreciated, the invention will now be described, by way of example, with reference to the accompanying drawings in which:

FIGURE I is a graph depicting the critical load curves for a typical elongated structural member as predicted by Euler's equation and as modified by Tetmajer's straight line;

FIGURE 2 is a stress-strain graph depicting curves for an elongated structural member before and after treatment by the method according to the invention;

FIGURES 3a, 3b and 3c are perspective views of rectangular encase- ments utilised to confine an elongated structural member positioned therein for treatment by a method in accordance with the invention;

FIGURE 4 is a cross-sectional view taken along the line 4-4 in Figure 3a showing an elongated structural member positioned in the rectangular encasement with two opposing compression pistons positioned against the opposite ends of the member at the open ends of the encasement;

FIGURE 5 is a transverse view of a first alternative embodiment of an apparatus for use when performing the method of the invention;

FIGURE 6 is a transverse view of a second alternative embodiment of an apparatus for use when performing the method of the invention; and

FIGURE 7 is a longitudinal sectional view of a third alternative embodiment of an apparatus for use when performing the method of the invention.

[0020] Referring first to Figure 1, a graph represented by the dashed curve A illustrates the critical load required to buckle a column under axial compression as predicted by Euler's equation. (A column as referred to herein is an elongated structural member). The ordinate of the graph in Figure I is the longitudinally compressive load P applied to the column in pounds per square inch. The abscissa of the graph is the ratio between the length of the column in centimetres or inches and its radius of gyration in centimetres or inches. This ratio is commonly referred to as the slenderness of a column and is symbolized by λ.

[0021] According to Euler's equation, the critical load P_c at which a column buckles is defined as:

where E is the Young's modulus for the material under compression, λ is the slenderness of the column (i.e. the ratio between the length and radius of gyration of the column) and K is a constant representing the column's end conditions.

[0022] In the stress-strain diagram of a mild steel element in Figure 2, the Young's modulus E for compression is equal in magnitude to the slope of the diagram's curve within the elastic limit (i.e. that portion of the curve which is linear). From the Young's modulus E for a material, Euler's equation predicts the critical compressive load curve shown by the dashed curve A in Figure I.

[0023] For short columns (.e.g. small values of λ), Euler's equation is found experimentally inaccurate and an alternative formula known as Tetmajer's formula is found to provide a better prediction of a short column's critical load. The prediction of Tetmajer's formula is shown by line B in Figure 1. As shown by the solid line B in Figure 1, Tetmajer's formula predicts that mild steel will have a critical load under compression that is less than predicted by Euler's equation for A values less than 120. For the case of h equal to zero, the critical-compressive load for a column, as shown by the Tetmajer line B in Figure I, is equivalent to the ultimate compressive strength P_u of the column.

[0024] Thus, from the point of view of the theoretical limit for a mild steel column predicted by Euler's equation, the elongated structural members need to be "over-designed" for values less than 120, if the said theoretical limit is to be approached in practise. As can be seen from the graph in Figure I, for a mild steel column of fixed length and a required critical compressive load strength of P_I, the column must have a cross-sectional area, or thickness, greater than that which Euler's equation indicates as necessary. In order to sustain the load P without buckling, its slenderness λ must have a value of À2 instead of the theoretically (as predicted by Euler's equation) possible λ₁ Typically, for members or columns made from a particular type of metal (e.g. mild steel), those which are more slender (i.e., a greater A value) are less expensive to manufacture than those which are less slender, mainly because less raw material is consumed. Moreoever, when used in a situation where the columns have to be transported from one location to another, a more slender column is lighter and less bulky and therefore may be transported more economically.

[0025] As the stress-strain diagram shown in Figure 2 illustrates, a column treated by the method according to the invention will exhibit both increased proportionality limits (i.e. the linear portion of the plotted curve) and ultimate strengths in compression and tension. The dashed curve V in Figure 2 illustrates the stress-strain relationship of a virgin column. After treatment by the method according to the invention, the same column exhibits a stress-strain relationship shown by the solid line T in Figure 2.

[0026] In performing one method in accordance with the present invention a longitudinally compressive load, which is greater than the compressive elastic limit of the column, is applied to the longitudinal ends of a column, whilst the column is restrained to prevent buckling thereof, such that the column yields longitudinally, without buckling. It has been found that this increases the proportionality limit (and also the elastic limit and yield strength) of the column both in compression and tension. An increase of the longitudinally compressive load to a level just in excess of the column's initial ultimate strength, increases both the column's compressive and tensile elastic limits and its compressive ultimate strength. A further increase in the longitudinally compressive load to a level much higher than the ultimate strength of the column material (for example, two times as great), results in an increase in both elastic limits and also an increase in the ultimate strength in both compression and tension.

[0027] If an infinitesimally small perpendicular deformation is produced on an axially loaded column, the external moment caused by a sufficiently great axial load will be greater than the internal moment of the column, thus causing the column to exceed its buckling capacity. As a result, the column bends and buckles. In order to avoid buckling during compressive loading when performing a method in accordance with the invention, an encasement as exemplified by the blocks shown in Figures 3a-3c are secured about the outer surface of the column so as to prevent the column from bending. Each block has an axial cavity which substantially corresponds to the shape and size of the column to be treated. The column is inserted in the cavity before it is compressively loaded. Because of the confined area of the cavity, bending is inhibited so that the external moment of the column is controlled and buckling is prevented. As a result, the column yields in a plastic-like flow which causes a smoothing of the column irregularities such that there is a general conformance of the column surface to the surface of the cavity. Any thin areas in the column cross-section become thicker, thus giving a more even distribution of material over the length of the column.

[0028] In the particular encasement shown in Figure 3a, an I-beam cavity is formed inside a block II. The I-beam cavity follows the axial length of the block I and is open at both the front end I la and the back end I lb so that an I-beam can be inserted into the cavity and compressed therein. The block I is formed of rigid material that has a greater critical compressive strength than that of the column inserted inside the block. Therefore, the walls of the cavity in the block I will not yield at a compressive load sufficient to cause the column to yield. In order to provide lateral support for a column under treatment, the shape and size of the block's cavity closely conform to that of the column. Many other possible column cross-sectional shapes could also benefit from the invention as shown by the L-cavity block II' and the cylindrical cavity block II" in Figures 3b and 3c, respectively.

[0029] In order to apply the compressive load to the column, pistons 15 and 17 (see Figure 4), which have a cross-sectional shape the same as the column 13, are inserted into the cavity of the block I at its opposing ends and are driven relatively toward each other by a piston drive 19. It will be appreciated that just as each differently shaped column requires its own block, it also requires its own set of pistons 15 and 17.

[0030] By applying a longitudinally compressive load of a magnitude approximately equal to or just in excess of the initial ultimate compressive strength of a mild steel column, the elastic region of the stress/strain diagram of the column is substantially increased. Line C and the crosshatched area in Figure I illustrate the typical properties of a column which has been treated by such a method. After the elastic region of the mild steel column has been increased it is found that Euler's equation accurately describes the behaviour of the treated column to a higher pressure than it describes an untreated column. Specifically, columns of mild steel treated in the manner described follow the hyperbola predicted by Euler's equation to as small a λ value as 76.9. However, as can be seen from line C in Figure I, the ultimate strength P of the treated mild steel columns is seen to be unaltered by the longitudinal compression since it is of the same magnitude as the ultimate strength P_u of a corresponding untreated column.

[0031] In Figure 2, line D illustrates the properties of a column which has been subjected to longitudinal compression, in accordance with a method of the invention with an applied load magnitude much greater than (for example two times) the initial ultimate strength P of an untreated column. This has been found to increase the ultimate strength of the column to a new value P' , which is much greater than P , as can be seen from line D. The area between lines C and D is shaded to indicate the increase in the elastic zone and ultimate strength gained by longitudinal compression at a load much greater than the ultimate strength P .

[0032] By way of example, for a mild steel column with an ultimate strength of 60,000 p.s.i., a longitudinally compressive load of 140,000 p.s.i. will increase the column's ultimate compressive strength by approximately 70% and its ultimate tensile strength by approximately 30%.

[0033] Placement of the column into the encasement elastically straightens the column by placing residual stresses on non-linear areas of the column. As the longitudinally compressive load on the mild steel column is steadily increased from no load to full load, the column first yields longitudinally at a load less than the ultimate compressive strength of the column, thus causing the mild steel to flow so as to relieve the residual stresses and redistribute material more evenly along the column's length (i.e. straighten the column). As the load approaches the ultimate compressive strength of the column, there is a significant increase in the elastic limit of the column and also a significant increase in the column's straightness. Further increases in the load, up to 140,000 p.s.i., cause the mild steel column to increase in ultimate strength both in tension and compression as stated. By so increasing the ultimate strengths of the column, its susceptibility to buckling and metal fatigue is reduced.

[0034] By treating the column in this manner, the non-linearities in the column are reduced in a single application of pressure. By increasing the linearity of the column (as well as increasing its elastic limit and ultimate strength), a significant increase in the column's resistance to buckling will result. Accordingly, columns can be made more slender (i.e., made out of less material), whilst being as strong as thicker untreated columns, thereby reducing the cost of a column needed to meet predetermined stress specifications. In order to allow for fast operation, it is desirable to provide an encasement which may be opened and closed for easy insertion and removal of a column to be treated. Also, because of dimensional tolerances in columns, it is desirable to provide an encasement which can be adjusted for small variations in the column size. One such encasement is shown in Figure 5 which illustrates an encasement 23 divided into two equal mating sections, 23a and 23b, and secured to a foundation 25. The cavity of the encasement 23 can be exposed by pivoting the encasement section 23b about a pivot 27 so as to allow a column 29 to be laid into the cavity. In order to open and close the encasement, a hydraulic piston 31 controls the movement of the pivoting encasement section 23b. A hydraulic pressure control circuit 33 controls the pressure within the hydraulic piston 31 so that the piston 31, working against a second foundation 35, pushes out a piston arm 37, thus causing the pivoting encasement section 23b to close with the fixed section 23a.

[0035] Because of size tolerances in the columns laid into the cavity, the precise position of the pivoting encasement section 23b, when it closes on the column and contacts it, may vary with individual columns. In order to position the pivoting encasement section 23b correctly when closed, a hydraulic pressure control circuit 33 is provided to sense the increased pressure when the pivoting encasement section 23b contacts the column 29. By positioning the pivoting encasement section 23b in this manner, small variations in size of the column can be accommodated by the encasement while still maintaining the encasement function of satisfactorily supporting the column in order to prevent buckling.

[0036] In order to prevent the opening of the pivoting encasement section 23b when longitudinal pressure is applied to the column 29, the hydraulic pressure in the hydraulic piston 31 must be increased as radial pressure mounts. To provide for this, a piston position transducer 39 is coupled to the piston arm 37 so as to provide the hydraulic pressure control circuit 33 with a signal indicative of the position of the pivoting encasement section 23b. In response to the position signal from the transducer 39, the hydraulic pressure control circuit 33 adjusts the pressure within the hydraulic piston 31 so as to hold the pivoting encasement section 23b in a steady position.

[0037] In some column applications, such as well casings, a slight deformation of a column's surface, which might be caused by a longitudinal compression that is not totally radially restrained, is not of great concern. For such columns, the encasement shown in Figure 6 can provide adequate confinement and support so as to prevent buckling of the column when it is subjected to the high longitudinal loads in accordance with the invention. At high longitudinally compression loads (e.g. close to the ultimate strength) radial bulges will occur on the surface of the column where the encasement does not restrain the radial yielding of the column. In the case of the cylindrical tubular column 41 in Figure 6, the treated column will have a slightly out-of-round cross-section.

[0038] Two pairs of opposing blocks, 43a, 43b and 43c, 43d, provide the encasement in Figure 6 which prevents buckling of the column when a high longitudinal load is applied to the column 41. Each block has a wedge cut surface so as to accommodate various sizes of cylindrical columns. Also to accommodate various column sizes, blocks 43a, 43b and 43c are mounted on hydraulic pistons 45a, 45b and 45c, respectively. Block 43d is secured to a foundation 47. Positioning of hydraulic pistons 45a, 45b and 45c is accomplished in a manner similar to that described in connection with Figure 5. It should be noted that although four blocks are illustrated in Figure 6, the specific number may be varied.

[0039] In order to reduce friction between the column and its encasement, an encasement 51 as illustrated in Figure 7 has two sections 51a and 51b connected by interlocking fingers 55a and 55b and mounted within an outer sheath 53. Compression of a column within the encasement 51 causes the encasement to move with the column within the sheath 53, thereby causing the interlocking fingers 55 to close as the column is compressed. By reducing the relative movement between the part of the encasement 51 actually touching and the column and the column itself, the friction between the column and the encasement is also reduced. As a result, there is more uniform yielding along the length of the column. To reduce the friction between the encasement 5l and the sheat 53, an anti-friction lining 57 can be added. For example, a lining of polytetrafluoroethylene, as sold under the Registered Trade Mark TEFLON could be included between the encasement 5 and sheath 53. Linear roller bearings or an oil film under pressure are also possible linings.

[0040] From the foregoing, it will be appreciated that the application of a longitudinal load to a column in accordance with the invention provides a simple and inexpensive method of (1) increasing the proportionality limits of a column, 2) increasing the ultimate strengths of a column and 3) increasing the straightness of a column while removing the residual stresses therein. By so providing, the invention allows columns to be utilised closer to their theoretical limits.

Claims

I. A method for increasing the elastic region of a column when under compression or tension, said method comprising the steps of: confining said column against movement in a direction transverse to the longitudinal axis of said column; applying to said confined column a longitudinally compressive load sufficient to cause the column to yield longitudinally; preventing said confined column from buckling while longitudinally yielding; and increasing said longitudinally compressive load on said column to a value close to the ultimate compressive strength of said column so as to increase the elastic region of said column.

2. A method according to claim I including the step of: further increasing said longitudinally compressive load on said column to a value approximately twice as great as the original ultimate compressive strength of said column so as to increase both the ultimate compressive and tensile strengths of said column.

3. A method for increasing the critical load of a column to a value substantially accurately predicted by Euler's equation,

wherein P equals the critical load of a column, E equals the Young's modulus of the column material, K is a constant representing end conditions of said column and X equals the ratio between column length and its radius of gyration, said method comprising the steps of: longitudinally compressing a column whose critical load is less than that predicted by Euler's equation; preventing said column from buckling by restraining its ability to bend; and allowing said column to yield longitudinally in response to said longitudinal compression so as to increase the elasticity of the column such that said column's critical load is more accurately predicted by Euler's equation.

4. A method according to claim 3 wherein said longitudinal compression is much greater than the original ultimate compressive strength of said column so as to thereby increase both the ultimate compressive and tensile strengths of said column.

5. A method according to claim 3 or 4 wherein said column is composed of mild steel and the critical load of the column after longitudinal compression is accurately predicted by Euler's equation for /\vatues as low as approximately 76.9.

6. A method for increasing the elastic strength of an elongated column under compression or tension, said method comprising the steps of: applying a compressive load to the end of said column which is parallel to the longitudinal axis of said column and which is approximate to or greater than the ultimate compressive strength of said column; inhibiting the movement of said column in the direction transverse to said load; and allowing said column to yield axially without buckling.

7. A method according to claim 6 including the step of: controlling the movement of said column under compression in the direction transverse to said load such that residual stresses are relieved and the straightness of the column is improved.

8. A method for straightening an elongated column, said method comprising the steps of: applying a compressive load to the end of said column with said load being parallel to the longitudinal axis of said column and of substantially the same magnitude as the ultimate strength of said column; and controlling the yielding of said column in response to said load in directions orthogonal to the longitudinal axis of said column such that the uniformity of said column is improved.

9. A method according to claim 8 including the steps of: applying said compressive load to the end of said column at a magnitude substantially greater than the ultimate compressive strength of said column; and allowing said column to yield without buckling in response to said compressive load so as to increase the elasticity and the ultimate strength of said column in both compression and tension.

10. A method for increasing the ultimate compressive and tensile strengths of an elongated member utilising an encasement for the member and opposing pistons located at opposite ends of the encasement, said method comprising the steps of: inserting the member into the encasement such that the encasement inhibits bending of the member when it is placed under a longitudinally compressive load; moving said pistons against said opposite ends of said member so as to apply a longitudinally compressive load to said member which is approximately equal to the ultimate compressive strength of said member; and allowing said member to yield without buckling in response to the longitudinally compressive load to thereby increase the elastic region of the member.

II. A method according to claim 10 wherein the longitudinally compressive load applied to said member is increased to a magnitude greater than the original ultimate compressive strength of said member so as to increase both the ultimate compressive and tensile strengths of said member.

Drawing