- Elastic instability
Elastic instability is a form of instability occurring in elastic systems, such as
buckling of beams and plates subject to large compressive loads.ingle degree of freedom-systems
Consider as a simple example a rigid beam of length "L", hinged in one end and free in the other, and having an angular spring attached to the hinged end. The beam is loaded in the free end by a force "F" acting in the compressive axial direction of the beam, see the figure to the right.
Moment equilibrium condition
Assuming a clockwise angular deflection heta, the clockwise moment exerted by the force becomes M_F = F L sin heta. The moment equilibrium equation is given by
F L sin heta = k_ heta heta
where k_ heta is the spring constant of the angular spring (Nm/radian). Assuming heta is small enough, implementing the
taylor expansion of thesine function and keeping the two first terms yieldsF L Bigg( heta - frac{1}{6} heta^3Bigg) approx k_ heta heta
which has three solutions, the trivial heta = 0, and
heta approx pm sqrt{6 Bigg( 1 - frac{k_ heta}{F L} Bigg)}
which is imaginary (i.e. not physical) for F L < k_ heta and real otherwise. This implies that for small compressive forces, the only equilibrium state is given by heta = 0, while if the force exceeds the value k_ heta/L there is suddenly another mode of deformation possible.
Energy method
The same result can be obtained by considering
energy relations. The energy stored in the angular spring isE_mathrm{spring} = int k_ heta heta mathrm{d} heta = frac{1}{2} k_ heta heta^2
and the work done by the force is simply the force multiplied by the distance, which is L (1 - cos heta). Thus,
E_mathrm{force} = int{F mathrm{d} x = F L (1 - cos heta )}
The energy equilibrium condition E_mathrm{spring} = E_mathrm{force} now yields F = k_ heta / L as before (besides from the trivial heta = 0).
tability of the solutions
Any solution heta is stable
iff a small change in the deformation angle Delta heta results in a reaction moment trying to restore the original angle of deformation. The net clockwise moment acting on the beam isM( heta) = F L sin heta - k_ heta heta
An
infinitesimal clockwise change of the deformation angle heta results in a momentM( heta + Delta heta) = M + Delta M = F L (sin heta + Delta heta cos heta ) - k_ heta ( heta + Delta heta)
which can be rewritten as
Delta M = Delta heta (F L cos heta - k_ heta)
since F L sin heta = k_ heta heta due to the moment equilibrium condition. Now, a solution heta is stable iff a clockwise change Delta heta > 0 results in a negative change of moment Delta M < 0 and vice versa. Thus, the condition for stability becomes
frac{Delta M}{Delta heta} = frac{mathrm{d} M}{mathrm{d} heta} = FL cos heta - k_ heta < 0
The solution heta = 0 is stable only for FL < k_ heta, which is expected. By expanding the
cosine term in the equation, we obtain the approximate stability condition
heta| > sqrt{2Bigg( 1 - frac{k_ heta}{F L} Bigg)}for FL > k_ heta, which the two other solutions satisfy. Hence, these solutions are stable.
Multiple degrees of freedom-systems
By attaching another rigid beam to the original system by means of an angular spring a two degrees of freedom-system is obtained. Assume for simplicity that the beam lengths and angular springs are equal. The equilibrium conditions become
F L ( sin heta_1 + sin heta_2 ) = k_ heta heta_1
F L sin heta_2 = k_ heta ( heta_2 - heta_1 )
where heta_1 and heta_2 are the angles of the two beams. Linearizing by assuming these angles are small yields
egin{pmatrix}F L - k_ heta & F L \k_ heta & F L - k_ hetaend{pmatrix}egin{pmatrix} heta_1 \ heta_2end{pmatrix} = egin{pmatrix}0 \0end{pmatrix}
The non-trivial solutions to the system is obtained by finding the roots of the
determinant of the systemmatrix , i.e. forfrac{F L}{k_ heta} = frac{3}{2} mp frac{sqrt{5{2} approx left{egin{matrix} 0.382\2.618 end{matrix} ight.
Thus, for the two degrees of freedom-system there are two critical values for the applied force "F". These correspond to two different modes of deformation which can be computed from the
nullspace of the system matrix. Dividing the equations by heta_1 yieldsfrac{ heta_2}{ heta_1} Big|_{ heta_1 e 0} = frac{k_ heta}{F L} - 1 approx left{egin{matrix} 1.618 & ext{for } F L/k_ heta approx 0.382\ -0.618 & ext{for } F L/k_ heta approx 2.618 end{matrix} ight.
For the lower critical force the ratio is positive and the two beams deflect in the same direction while for the higher force they form a "banana" shape. These two states of deformation represent the
buckling mode shapes of the system.ee also
*
Buckling Further reading
*"Theory of elastic stability", S. Timoshenko and J. Gere
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