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Bending1 Bending1 Presentation Transcript

  • Euler-Bernoulli Beams:Bending, Buckling, and Vibration David M. Parks 2.002 Mechanics and Materials II Department of Mechanical Engineering MIT February 9, 2004
  • Linear Elastic Beam Theory• Basics of beams –Geometry of deformation –Equilibrium of “slices” –Constitutive equations•Applications: –Cantilever beam deflection –Buckling of beams under axial compression –Vibration of beams
  • Beam Theory: Slice Equilibrium Relations•q(x): distributed load/length Axial force balance:•N(x): axial force•V(x): shear force•M(x): bending moment Transverse force balance: Moment balance about ‘x+dx’:
  • Euler-Bernoulli Beam Theory: Displacement, strain, and stress distributionsBeam theory assumptions on spatial 1-D stress/strain relation:variation of displacement components: Stress distribution in terms of Displacement field: Axial strain distribution in beam: Axial strain varies linearly Through-thickness at section ‘x’ ε0- κ h/2 y ε0 εxx(y) ε0 + κ h/2
  • Slice Equilibrium: Section Axial Force N(x) and Bending Moment M(x) in terms of Displacement fieldsN(x): x-component of force equilibriumon slice at location ‘x’: σxx M(x): z-component of moment equilibrium on slice at location ‘x’:
  • Centroidal Coordinateschoice:
  • Tip-Loaded Cantilever Beam: Equilibrium •statically determinant: P support reactions R, M0 from equilibrium alone •reactions “present” because of x=0 geometrical boundary conditions v(0)=0; v’(0)=φ(0)=0Free body diagrams: •general equilibrium equations (CDL 3.11-12) satisfied How to determine lateral displacement v(x); especially at tip (x=L)?
  • Exercise: Cantilever Beam Under Self-Weight •Weight per unit lenth: q0 •q0 = ρAg=ρbhgFree body diagrams: Find: •Reactions: R and M0 •Shear force: V(x) •Bending moment: M(x)
  • Tip-Loaded Cantilever: Lateral Deflectionscurvature / moment relations: geometric boundary conditions tip deflection and rotation: stiffness and modulus:
  • Tip-Loaded Cantilever: Axial Strain Distribution strain field (no axial force): εxxTOPTOP εxxtop/bottom axial strain distribution: εxxBOTTOM εxxBOTTOM εxxTOP strain-gauged estimate of E:
  • Euler Column Buckling: Non-uniqueness of deformed configuration moment/curvature: ode for buckled shape:free body diagram(note: evaluated in deformedconfiguration): Note: linear 2nd order ode; Constant coefficients (but parametric: k2 = P/EI
  • Euler Column Buckling, Cont.ode for buckled shape:general solution to ode:boundary conditions: parametric consequences: non-trivial buckled shape only whenbuckling-based estimate of E:
  • Euler Column Buckling: General Observations•buckling load, Pcrit, is proportional to EI/L2•proportionality constant dependsstrongly on boundary conditions atboth ends:•the more kinematicallyrestrained the ends are, the largerthe constant and the higher thecritical buckling load (see Lab 1 handout)•safe design of long slender columnsrequires adequate margins with respectto buckling•buckling load may occur a a compressivestress value (σ=P/A) that is less than yield stress, σy
  • Euler-Bernoulli Beam Vibrationassume time-dependent lateral motion:lateral velocity of slice at ‘x’: lateral acceleration of slice at ‘x’: net lateral force (q(x,t)=0): mass of dx-thickness slice: moment balance: linear momentum balance (Newton):
  • Euler-Bernoulli Beam Vibration, Cont.(1)linear momentum balance: ode for mode shape, v(x), and vibration frequency, ω:moment/curvature: general solution to ode:
  • Euler-Bernoulli Beam Vibration, Cont(2) general solution to ode:pinned/pinned boundary conditions: Solution (n=1, first mode): A1: ‘arbitrary’ (but small) vibration amplitude pinned/pinned restricted solution: τ1: period of first mode: