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| {{Other uses}}
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| {{Mechanical failure modes}}
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| In science, '''buckling''' is a mathematical instability, leading to a [[structural failure|failure mode]]. Theoretically, buckling is caused by a [[Bifurcation theory|bifurcation]] in the solution to the equations of [[Mechanical equilibrium|static equilibrium]]. At a certain stage under an increasing load, further load is able to be sustained in one of two states of equilibrium: an undeformed state or a laterally-deformed state.
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| In practice, buckling is characterized by a sudden failure of a structural member subjected to high [[Stress (mechanics)|compressive stress]], where the actual compressive stress at the point of failure is less than the ultimate compressive stresses that the material is capable of withstanding. Mathematical analysis of buckling often makes use of an axial load eccentricity that introduces a secondary bending moment, which is not a part of the primary applied forces to which the member is subjected. As an applied load is increased on a member, such as column, it will ultimately become large enough to cause the member to become unstable and is said to have buckled. Further load will cause significant and somewhat unpredictable deformations, possibly leading to complete loss of the member's load-carrying capacity. If the deformations that follow buckling are not catastrophic the member will continue to carry the load that caused it to buckle. If the buckled member is part of a larger assemblage of components such as a building, any load applied to the structure beyond that which caused the member to buckle will be redistributed within the structure.
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| ==Columns==
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| [[File:Buckled column.svg|thumb|left|A column under a concentric axial load exhibiting the characteristic deformation of buckling]] [[File:Buckling beam element.svg|thumb|left|The eccentricity of the axial force results in a bending moment acting on the beam element.]] The ratio of the effective length of a [[column]] to the least [[radius of gyration]] of its cross section is called the ''slenderness ratio'' (sometimes expressed with the Greek letter lambda, λ). This ratio affords a means of classifying columns. Slenderness ratio is important for design considerations. All the following are approximate values used for convenience.
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| * A short [[steel]] column is one whose slenderness ratio does not exceed 50; an intermediate length steel column has a slenderness ratio ranging from about 50 to 200, and are dominated by the strength limit of the material, while a long steel column may be assumed to have a slenderness ratio greater than 200 and its behavior is dominated by the modulus of elasticity of the material.
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| * A short [[concrete]] column is one having a ratio of unsupported length to least dimension of the cross section equal to or less than 10. If the ratio is greater than 10, it is considered a long column (sometimes referred to as a slender column).
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| * [[Wood|Timber]] columns may be classified as short columns if the ratio of the length to least dimension of the cross section is equal to or less than 10. The dividing line between intermediate and long timber columns cannot be readily evaluated. One way of defining the lower limit of long timber columns would be to set it as the smallest value of the ratio of length to least cross sectional area that would just exceed a certain constant K of the material. Since K depends on the [[modulus of elasticity]] and the allowable compressive [[stress (physics)|stress]] parallel to the grain, it can be seen that this arbitrary limit would vary with the [[species]] of the timber. The value of K is given in most structural handbooks.
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| If the load on a column is applied through the [[center of gravity]] (centroid) of its cross section, it is called an [[axis of rotation|axial]] [[Structural load|load]]. A load at any other point in the cross section is known as an [[eccentric (mechanism)|eccentric]] load. A short column under the action of an axial load will fail by direct compression before it buckles, but a long column loaded in the same manner will fail by buckling ([[bending]]), the buckling effect being so large that the effect of the axial load may be neglected. The intermediate-length column will fail by a combination of direct compressive stress and bending.
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| In 1757, [[mathematician]] [[Leonhard Euler]] derived a formula that gives the maximum axial load that a long, slender, ideal column can carry without buckling. An ideal column is one that is perfectly straight, homogeneous, and free from initial stress. The maximum load, sometimes called the critical load, causes the column to be in a state of unstable [[mechanical equilibrium|equilibrium]]; that is, the introduction of the slightest lateral force will cause the column to fail by buckling. The formula derived by Euler for columns with no consideration for lateral forces is given below. However, if lateral forces are taken into consideration the value of critical load remains approximately the same.
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| :<math>F=\frac{\pi^2 EI}{(KL)^2}</math>
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| where
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| :<math>F</math> = maximum or critical [[force]] (vertical load on column),
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| :<math>E</math> = [[modulus of elasticity]],
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| :<math>I</math> = [[area moment of inertia]],
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| :<math>L</math> = unsupported length of column,
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| :<math>K</math> = column effective length factor, whose value depends on the conditions of end support of the column, as follows.
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| ::For both ends pinned (hinged, free to rotate), <math>K</math> = 1.0.
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| ::For both ends fixed, <math>K</math> = 0.50.
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| ::For one end fixed and the other end pinned, <math>K</math> = 0.699....
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| ::For one end fixed and the other end free to move laterally, <math>K</math> = 2.0.
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| :<math>K L</math> is the effective length of the column.
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| Examination of this formula reveals the following interesting facts with regard to the load-bearing ability of slender columns.
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| # [[Elasticity (physics)|Elasticity]] and not the compressive [[strength of materials|strength of the materials]] of the column determines the critical load.
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| # The critical load is directly [[proportionality (mathematics)|proportional]] to the [[second moment of area]] of the cross section.
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| # The boundary conditions have a considerable effect on the critical load of slender columns. The boundary conditions determine the mode of bending and the distance between inflection points on the deflected column. The inflection points in the deflection shape of the column are the points at which the curvature of the column change sign and are also the points at which the internal bending moments are zero. The closer together the inflection points are, the higher the resulting capacity of the column.
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| [[Image:Buckledmodel.JPG|thumb|left|A demonstration model illustrating the different "Euler" buckling modes. The model shows how the boundary conditions affect the critical load of a slender column. Notice that each of the columns are identical, apart from the boundary conditions.]]
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| The strength of a column may therefore be increased by distributing the material so as to increase the moment of inertia. This can be done without increasing the weight of the column by distributing the material as far from the principal axis of the cross section as possible, while keeping the material thick enough to prevent local buckling. This bears out the well-known fact that a tubular section is much more efficient than a solid section for column service.
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| Another bit of information that may be gleaned from this equation is the effect of length on critical load. For a given size column, doubling the unsupported length quarters the allowable load. The restraint offered by the end connections of a column also affects the critical load. If the connections are perfectly rigid, the critical load will be four times that for a similar column where there is no resistance to rotation (in which case the column is idealized as having hinges at the ends).
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| Since the radius of gyration is defined as the square root of the ratio of the column's cross section to its area, the above formula may be rearranged as follows. Using the Euler formula for hinged ends, and substituting A·r<sup>2</sup> for I, the following formula results.
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| :<math>\sigma = \frac{F}{A} = \frac{\pi^2 E}{(\ell/r)^2}</math>
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| where <math>F/A</math> is the allowable stress of the column, and <math>l/r</math> is the slenderness ratio.
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| Since structural columns are commonly of intermediate length, and it is impossible to obtain an ideal column, the Euler formula on its own has little practical application for ordinary design. Issues that cause deviation from the pure Euler column behaviour include imperfections in geometry in combination with plasticity/non-linear stress strain behaviour of the column's material. Consequently, a number of empirical column formulae have been developed to agree with test data, all of which embody the slenderness ratio. For design, appropriate [[factor of safety|safety factors]] are introduced into these formulae. One such formula is the [[Perry Robertson formula]] which estimates the critical buckling load based on an initial (small) curvature. The Rankine Gordon formula (Named for [[William John Macquorn Rankine]] and Perry Hugesworth Gordon (1899 – 1966)) is also based on experimental results and suggests that a column will buckle at a load F<sub>max</sub> given by:
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| :<math> \frac{1}{F_{max}} = \frac{1}{F_{e}} + \frac{1}{F_{c}}</math>
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| where F<sub>e</sub> is the Euler maximum load and F<sub>c</sub> is the maximum compressive load. This formula typically produces a conservative estimate of F<sub>max</sub>.
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| ===Self-buckling===
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| A free-standing, vertical column, with density <math>\rho</math>, Young's modulus <math>E</math>, and radius <math>r</math>, will buckle under its own weight if its height exceeds a certain critical height:<ref>{{cite journal|last=Kato|first=K.|title=Mathematical Investigation on the Mechanical Problems of Transmission Line|journal=Journal of the Japan Society of Mechanical Engineers|volume=19|pages=41|year=1915}}</ref><ref>{{cite book|last=Ratzersdorfer|first=Julius|title=Die Knickfestigkeit von Stäben und Stabwerken|publisher=J. Springer|location=Wein, Austria|year=1936|pages=107–109}}</ref><ref>{{cite journal|last=Cox|first=Steven J.|coauthors=C. Maeve McCarthy|title=The Shape of the Tallest Column|journal=Society for Industrial and Applied Mathematics|volume=29|pages=547–554|year=1998}}</ref>
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| :<math>h_{crit} = \left(\frac{9B^2}{4}\,\frac{EI }{\rho g\pi r^2}\right)^{1/3}</math>
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| where ''g'' is the acceleration due to gravity, ''I'' is the [[second moment of area]] of the beam cross section, and ''B'' is the first zero of the [[Bessel function]] of the first kind of order -1/3, which is equal to 1.86635086...
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| ==Buckling under tensile dead loading==
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| [[File:Continuous model tensile buckling.jpg|thumb|right|Fig. 2: Elastic beam system showing buckling under tensile dead loading.]]
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| Usually buckling and instability are associated to compression, but recently Zaccaria, Bigoni, Noselli and Misseroni (2011)<ref>[http://www.ing.unitn.it/~bigoni/tensile_buckling.html D. Zaccaria, D. Bigoni, G. Noselli and D. Misseroni, Structures buckling under tensile dead load. Proceedings of the Royal Society A, 2011, 467, 1686-1700.]</ref> have shown that buckling and instability can also occur in elastic structures subject to dead tensile load.
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| An example of a single-degree-of-freedom structure is shown in Fig. 1, where the critical load is also indicated.
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| Another example involving flexure of a structure made up of beam elements governed by the equation of the Euler's elastica is shown in Fig.2.
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| In both cases, there are no elements subject to compression. The instability and buckling in tension are related to the presence of the slider, the junction between the two rods, allowing only relative sliding between the connected pieces.
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| ==Constraints, curvature and multiple buckling==
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| [[Image:Sketch curvature 1.jpg|thumb|Fig. 3: A one-degree-of-freedom structure exhibiting a tensile (compressive) buckling load as related to the fact that the right end has to move along the circular profile labeled 'Ct' (labelled 'Cc').]]
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| Buckling of an elastic structure strongly depends on the curvature of the constraints against which the ends of the structure are prescribed to move (see Bigoni, Misseroni, Noselli and Zaccaria, 2012<ref>[http://www.ing.unitn.it/~bigoni/multiple_bifurcations.html D. Bigoni, D. Misseroni, G. Noselli and D. Zaccaria, Effects of the constraint's curvature on structural instability: tensile buckling and multiple bifurcations. Proceedings of the Royal Society A, 2012, doi:10.1098/rspa.2011.0732.]</ref>). In fact, even a single-degree-of-freedom system (see Fig.3) may exhibit a tensile (or a compressive) buckling load as related to the fact that one end has to move along the circular profile labeled 'Ct' (labelled 'Cc').
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| [[Image:buckling curvature 2.gif|thumb|Fig. 4: A one-degree-of-freedom structure with a 'S'-shaped bicircular profile exhibiting multiple bifurcations (both tensile and compressive).]]
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| The two circular profiles can be arranged in a 'S'-shaped profile, as shown in Fig.4; in that case a discontinuity of the constraint's curvature is introduced, leading to multiple bifurcations. Note that the single-degree-of-freedom structure shown in Fig.4 has two buckling loads (one tensile and one compressive). Watch a [http://www.ing.unitn.it/~bigoni/multiple_bifurcations.html movie] for more details.
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| ==Flutter instability==
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| Structures subject to a follower (nonconservative) load may suffer instabilities which are not of the buckling type and therefore are not detectable with a static approach.<ref>Bigoni, D. Nonlinear Solid Mechanics: Bifurcation Theory and Material Instability. Cambridge University Press, 2012 . ISBN 9781107025417.</ref> For instance, the so-called 'Ziegler column' is shown in Fig.5.
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| [[Image:Ziegler column.jpg|thumb|Fig. 5: A sketch of the 'Ziegler column', a two-degree-of-freedom system subject to a follower load (the force P remains always parallel to the rod BC), exhibiting flutter and divergence instability. The two rods, of linear mass density ρ, are rigid and connected through two rotational springs of stiffness k1 and k2.]]
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| This two-degree-of-freedom system does not display a quasi-static buckling, but becomes dynamically unstable. | |
| To see this, we note that the equations of motion are
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| :<math>\
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| \left\{
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| \begin{array}{l}
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| \frac{1}{3} \rho l_1^{2} \left(l_1 + 3 l_2\right)\ddot{\alpha}_1 + \frac{1}{2} \rho l_1 l_2^{2} \cos(\alpha_1 - \alpha_2)\ddot{\alpha}_2 + \frac{1}{2} \rho l_1 l_2^{2} \sin(\alpha_1 - \alpha_2)\dot{\alpha}_2^{2} + (k_1 + k_2)\alpha_1 - k_2\alpha_2\,+ \\ [5mm]
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| + (\beta_1 + \beta_2)\dot{\alpha}_1 - \beta_2 \dot{\alpha}_2 - l_1 P \sin(\alpha_1 - \alpha_2) = 0 , \\ [5mm]
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| \frac{1}{2} \rho l_1 l_2^{2} \cos(\alpha_1 - \alpha_2)\ddot{\alpha}_1 + \frac{1}{3} \rho l_2^{3}\ddot{\alpha}_2 - \frac{1}{2} \rho l_1 l_2^{2} \sin(\alpha_1 - \alpha_2)\dot{\alpha}_1^{2} - k_2(\alpha_1 - \alpha_2) - \beta_2(\dot{\alpha}_1 - \dot{\alpha}_2) = 0 ,
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| \end{array}
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| \right.
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| </math>
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| and their linearized version is
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| :<math>\
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| \left\{
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| \begin{array}{l}
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| \frac{1}{3} \rho l_1^{2} \left(l_1 + 3 l_2\right)\ddot{\alpha}_1 + \frac{1}{2} \rho l_1 l_2^{2} \ddot{\alpha}_2 + (k_1 + k_2)\alpha_1 - k_2\alpha_2 - l_1 P (\alpha_1 - \alpha_2) = 0 , \\ [5mm]
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| \frac{1}{2} \rho l_1 l_2^{2} \ddot{\alpha}_1 + \frac{1}{3} \rho l_2^{3}\ddot{\alpha}_2 - k_2(\alpha_1 - \alpha_2) = 0 .
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| \end{array}
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| \right.
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| </math>
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| Assuming a time-harmonic solution in the form
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| :<math>\ \alpha_j = A_j \, e^{-i \Omega \,t}, ~~~j=1,2 ,</math>
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| we find the critical loads for flutter (<math>\ P_f </math>) and divergence (<math>\ P_d </math>),
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| :<math>\ P_{f,d} = \frac{k_2}{l_1}\cdot \frac{k+(1+\lambda)^3 \mp \lambda\,\sqrt{k (3+4\lambda)}}{1 + 3\lambda/2} </math>
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| where <math>\ \lambda=l_1/l_2 </math> and <math>\ k=k_1/k_2 </math>.
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| [[Image:deformed shapes.jpg|thumb|Fig. 6: A sequence of deformed shapes at consecutive times intervals of the structure sketched in Fig.5 and exhibiting flutter (upper part) and divergence (lower part) instability.]]
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| Flutter instability corresponds to a vibrational motion of increasing amplitude and is shown in Fig.6 (upper part) together with the divergence instability (lower part) consisting in an exponential growth.
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| Recently, Bigoni and Noselli (2011)<ref>[http://www.ing.unitn.it/~bigoni D. Bigoni and G. Noselli, Experimental evidence of flutter and divergence instabilities induced by dry friction. Journal of the Mechanics and Physics of Solids, 2011, 59, 2208–2226.]</ref> have experimentally shown that flutter and divergence instabilities can be directly related to dry friction, watch the [http://www.ing.unitn.it/~bigoni/flutter.html movie] for more details.
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| == Various forms of buckling ==
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| Buckling is a state which defines a point where an equilibrium configuration becomes unstable and can manifest itself in several different phenomena. All can be classified as forms of [[bifurcation theory|bifurcation]].
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| There are four basic forms of bifurcation associated with loss of structural stability or buckling. These comprise two types of [[pitchfork bifurcation]], one [[saddle-node bifurcation]] (often referred to as a [[limit point]]) and one [[transcritical bifurcation]]. The pitchfork bifurcations are the most commonly studied forms and include the buckling of columns and struts, sometimes known as Euler buckling; the buckling of plates, sometimes known as local buckling, which is well known to be relatively safe and the buckling of shells, which is well-known to be a highly dangerous.<ref>"A general theory of elastic stability" By J. M. T. Thompson & G. W. Hunt, Wiley, 1973</ref>
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| In Euler buckling,<ref>"Buckling of Bars, Plates, and Shells" By Robert M. Jones</ref><ref>"Observations on eigenvalue buckling analysis within a finite element context" by Christopher J. Earls</ref> the applied load is increased by a small amount beyond the critical load, the structure deforms into a buckled configuration which is adjacent to the original configuration. For example, the Euler column pictured will start to bow when loaded slightly above its critical load, but will not suddenly collapse.
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| [[File:bifurcation buckling.png]]
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| In structures experiencing limit point instability, if the load is increased infinitesimally beyond the critical load, the structure undergoes a large deformation into a different stable configuration which is not adjacent to the original configuration. An example of this type of buckling is a toggle frame (pictured) which 'snaps' into its buckled configuration.
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| [[File:limit point instability.png]]
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| ==Bicycle wheels==
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| A conventional [[bicycle wheel]] consists of a thin rim kept under high compressive stress by the (roughly normal) inward pull of a large number of spokes. It can be considered as a loaded column that has been bent into a circle. If spoke tension is increased beyond a safe level, the wheel spontaneously fails into a characteristic saddle shape (sometimes called a "taco" or a "[[Pringles|pringle]]") like a three-dimensional Euler column. This is normally a purely elastic deformation and the rim will resume its proper plane shape if spoke tension is reduced slightly.
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| ==Surface materials==
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| [[Image:Spoorspatting Landgraaf.jpg|thumb|Sun kink in rail tracks]]
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| Buckling is also a failure mode in [[pavement (material)|pavement]] materials, primarily with concrete, since [[asphalt concrete|asphalt]] is more flexible. [[Radiant heat]] from the [[sun]] is absorbed in the road surface, causing it to [[thermal expansion|expand]], forcing adjacent pieces to push against each other. If the stress is great enough, the pavement can lift up and crack without warning. Going over a buckled section can be very jarring to [[automobile]] drivers, described as running over a [[speed hump]] at highway speeds.
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| Similarly, [[rail tracks]] also expand when heated, and can fail by buckling, a phenomenon called '''sun kink'''. It is more common for rails to move laterally, often pulling the underlain [[railroad tie]]s (sleepers) along.
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| :{| class="toccolours collapsible collapsed" width="60%" style="text-align:left" | |
| !More on sun kink
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| == Cause ==
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| The buckling [[force]] in the track due to warming up is a [[Function (mathematics)|function]] of the rise in temperature only and is independent of the track length:
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| <math>F = E A \alpha_L \Delta T</math>.
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| Derivation of buckling force function:
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| The linear [[thermal expansion]] due to heating of the track is found using
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| :<math>{\Delta L } = \alpha_L \Delta T \ L</math>
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| where
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| * ΔL = thermal expansion of the rail
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| * L = length of the rail/track
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| * α = [[coefficient of thermal expansion]]
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| * ΔT = increase in temperature
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| According to [[Hooke's law]] the extension due to a force (in the rail) is
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| :<math>\Delta L = \frac{F}{E A} L</math>
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| where
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| *ΔL = extension of the rail/track
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| * F = the force extending a rod, here the induced force in the rail
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| * E = [[modulus of elasticity]] of rail material (steel)
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| * A = cross section of rail
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| * L = length of rail
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| Putting these together gives
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| :<math>{\frac{F}{E A} L } = \alpha_L \Delta T \ L</math>
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| or
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| :<math>F = E A \alpha_L \Delta T</math>
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| ==Accidents==
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| These accidents were deemed to be sun kink related (''more information available at [[List of rail accidents (2000–2009)]]''):
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| * April 18, 2002 [[Amtrak]] ''[[Auto-Train]]'' derailment, off [[CSX]] tracks, near [[Crescent City, Florida]].
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| * July 29, 2002 [[Amtrak]] ''[[Capitol Limited (Amtrak train)|Capitol Limited]]'' derails, off [[CSX]] tracks, near [[Kensington, Maryland]].
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| * July 8, 2010 CSX train derails off tracks in [[Waxhaw, North Carolina]].
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| * July 6, 2012 WMATA [[Washington Metro|Metrorail]] train derails off tracks near Hyattsville, Maryland.<ref>http://dcist.com/2012/07/excessive_heat_probable_cause_in_gr.php</ref>
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| |}
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| ==Energy method==
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| Often it is very difficult to determine the exact buckling load in complex structures using the Euler formula, due to the difficulty in deciding the constant K. Therefore, maximum buckling load often is approximated using energy conservation. This way of calculating the maximum buckling load is often referred to as the energy method in structural analysis.
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| The first step in this method is to suggest a displacement function. This function must satisfy the most important boundary conditions, such as displacement and rotation. The more accurate the displacement function, the more accurate the result.
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| In this method, there are two equations used (for small deformations) to approximate the "inner" energy (the potential energy stored in elastic deformation of the structure) and "outer" energy (the work done on the system by external forces).
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| :<math>U_\text{inner} = \frac{E}{2} \int I(x)(w_{xx}(x))^2 \, dx</math>
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| :<math>U_\text{outer} = \frac{P_\text{Crit}}{2} \int (w_{x}(x))^2 \, dx</math>
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| where <math>w(x)</math> is the displacement function and the subscripts <math>x</math> and <math>xx</math> refer to the first and second derivatives of the displacement. Energy conservation yields:
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| :<math>U_\text{Inner}=U_\text{Outer} \, </math>
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| ==Flexural-torsional buckling==
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| Occurs in compression members only and it can be described as a combination of bending and twisting of a member. And it must be considered for design purposes, since the shape and cross sections are very critical. This mostly occurs in channels, structural tees, double-angle shapes, and equal-leg single angles.
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| ==Lateral-torsional buckling==
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| When a simply supported beam is loaded in [[flexure]], the top side is in [[compression (physical)|compression]], and the bottom side is in [[tension (mechanics)|tension]]. When a slender member is subjected to an axial force, failure takes place due to bending or torsion rather than direct compression of the material. If the beam is not supported in the lateral direction (i.e., perpendicular to the plane of bending), and the flexural load increases to a critical limit, the beam will fail due to lateral buckling of the compression flange. In wide-flange sections, if the compression flange buckles laterally, the cross section will also twist in torsion, resulting in a failure mode known as ''lateral-torsional buckling''.
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| ===The modification factor (''C''<sub>''b''</sub>)===
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| ''C''<sub>''b''</sub> is a modification factor used in the equation for nominal [[flexural strength]] when determining lateral-torsional buckling. The reason for this factor is to allow for non-uniform moment diagrams when the ends of a beam segment are braced. The conservative value for ''C''<sub>''b''</sub> can be taken as 1, regardless of beam configuration or loading, but in some cases it may be excessively conservative. ''C''<sub>''b''</sub> is always equal to or greater than 1, never less. For [[cantilever]]s or overhangs where the free end is unbraced, C<sub>b</sub> is equal to 1. A table of values of ''C''<sub>''b''</sub> for simply supported beams are shown in Figure 1.
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| If an appropriate value of ''C''<sub>''b''</sub> is not given in the table, it can be obtained via the following formula:
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| :<math>\ C_b = \frac{12.5M_\max}{2.5M_\max + 3M_A + 4M_B + 3M_C} </math>
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| where
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| :<math>M_\max</math> = absolute value of maximum moment in the unbraced segment, (kip-in)
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| :<math>M_A</math> = absolute value of maximum moment at quarter point of the unbraced segment, (kip-in)
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| :<math>M_B</math> = absolute value of maximum moment at centerline of the unbraced segment, (kip-in)
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| :<math>M_C</math> = absolute value of maximum moment at three-quarter point of the unbraced segment, (kip-in)
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| ==Plastic buckling==
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| Buckling will generally occur slightly before the calculated elastic buckling strength of a structure, due to non-linear behavior of the material. When the compressive load is near the buckling load, the structure will bow significantly and the material of the column will diverge from a linear stress-strain behavior. The stress-strain behavior of materials is not strictly linear even below yield, and the modulus of elasticity decreases as stress increases, and significantly so as the stresses approach the yield strength. This lower rigidity reduces the buckling strength of the structure and causes at a load less than that predicted by the assumption of lineal elastic behavior.
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| A more accurate approximation of the buckling load can be had by the use of the tangent modulus of elasticity, E<sub>t</sub>, in place of the elastic modulus of elasticity. The tangent modulus is a line drawn tangent to the stress-strain curve at a particular value of strain. Plots of the tangent modulus of elasticity for a variety of materials are available in standard references.
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| ==Dynamic buckling==
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| If a column is loaded suddenly and then the load released, the column can sustain a much higher load than its static (slowly applied) buckling load. This can happen in a long, unsupported column (rod) used as a drop hammer. The duration of compression at the impact end is the time required for a stress wave to travel up the rod to the other (free) end and back down as a relief wave. Maximum buckling occurs near the impact end at a wavelength much shorter than the length of the rod, and at a stress many times the buckling stress of a statically-loaded column. The critical condition for buckling amplitude to remain less than about 25 times the effective rod straightness imperfection at the buckle wavelength is | |
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| :<math>\sigma L = \rho c^2 h \, </math> | |
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| where <math>\sigma</math> is the impact stress, <math>L</math> is the length of the rod, <math>c</math> is the elastic wave speed, and <math>h</math> is the smaller lateral dimension of a rectangular rod. Because the buckle wavelength depends only on <math>\sigma</math> and <math>h</math>, this same formula holds for thin cylindrical shells of thickness <math>h</math>.<ref>Lindberg, H. E., and Florence, A. L., ''Dynamic Pulse Buckling'', [[Martinus Nijhoff Publishers]], 1987, pp. 11–56, 297–298.</ref>
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| ==Buckling of thin cylindrical shells subject to axial loads==
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| Solutions of Donnell's eight order differential equation gives the various buckling modes of a thin cylinder under compression. But this analysis, which is in accordance with the small deflection theory gives much higher values than shown from experiments. So it is customary to find the critical buckling load for various structures which are cylindrical in shape from pre-existing design curves where critical buckling load F<sub>cr</sub> is plotted against the ratio R/t, where R is the radius and t is the thickness of the cylinder for various values of L/R, L the length of the cylinder. If cut-outs are present in the cylinder, critical buckling loads as well as pre-buckling modes will be affected. Presence or absence of reinforcements of cut-outs will also affect the buckling load.
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| ==Buckling of pipes and pressure vessels subject to external overpressure==
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| Pipes and pressure vessels subject to external overpressure, caused for example by steam cooling within the pipe and condensing into water with subsequent massive pressure drop, risk buckling due to compressive [[hoop stress]]es. Design rules for calculation of the required wall thickness or reinforcement rings are given in various piping and pressure vessel codes.
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| ==See also==
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| * [[Perry Robertson formula]]
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| * [[Stiffening]]
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| * [[Wood method]]
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| ==References==
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| <references />
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| * [[Stephen Timoshenko|Timoshenko, S. P.]], and Gere, J. M., ''Theory of Elastic Stability'', 2 ed., McGraw-Hill, 1961.
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| * Nenezich, M., ''Thermoplastic Continuum Mechanics'', Journal of Aerospace Structures, Vol. 4, 2004.
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| * [http://contrails.iit.edu/DigitalCollection/1970/AFFDLTR70-025.pdf The Stability of Elastic Equilibrium] by W. T. Koiter, PhD Thesis, 1945.
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| * Dhakal Rajesh and Koichi Maekawa (October 2002). "Reinforcement Stability and Fracture of Cover Concrete in Reinforced Concrete Members”. [http://ascelibrary.aip.org.libdb.njit.edu:8888/getpdf/servlet/GetPDFServlet?filetype=pdf&id=JSENDH000128000010001253000001&idtype=cvips&ident=freesearch.]
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| * Willian T. Segui (2007). “Steel Design” Fourth Edition. United States. Chris Carson.
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| * Analysis and design of flight vehicle structures- E.F.Brune
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| ==External links==
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| *[http://www.amesweb.info/CompressionMemberDesign/CompressionMemberDesign.aspx Online Calculation Tool for Compression Member (Column) Design]
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| * The complete theory and example experimental results for long columns are available as a 39-page PDF document at http://lindberglce.com/tech/buklbook.htm
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| *[http://www.youtube.com/user/RoyalSociety#p/u/0/EKngs1vvcJU Video on Buckling under tensile dead loading]
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| *[http://ssmg.ing.unitn.it Laboratory for Physical Modeling of Structures and Photoelasticity (University of Trento, Italy)]
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| * http://www.midasuser.com.tw/t_support/tech_pds/files/Tech%20Note-Lateral%20Torsional%20Buckling.pdf
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| [[Category:Elasticity (physics)]]
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| [[Category:Materials science]]
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| [[Category:Mechanical failure modes]]
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| [[Category:Structural analysis]]
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| [[Category:Mechanics]]
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| [[ar:تحنيب (خواص المواد)]]
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| [[it:Instabilità delle strutture]]
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