wb-mech 120 ch07 buckling

Workbench - Mechanical Introduction 12.0

Chapter 7

Li B kli A l iLinear Buckling Analysis

7-1ANSYS, Inc. Proprietary© 2009 ANSYS, Inc. All rights reserved.

May 5, 2009Inventory #002593

Linear Buckling

Training ManualChapter Overview• In this chapter, performing linear buckling analyses in Mechanical

will be covered.

• Contents:A. Background On BucklingB. Buckling Analysis Procedureg yC. Workshop 7-1

• The capabilities described in this section are generally applicable to• The capabilities described in this section are generally applicable to ANSYS DesignSpace Entra licenses and above.– Some options discussed in this chapter may require more advanced

licenses, but these are noted accordingly.licenses, but these are noted accordingly.



Linear Buckling

Training ManualA. Background on Buckling

• Many structures require an evaluation of their structural stability. Thin columns, compression members, and vacuum tanks are all examples of structures where stability considerations are important.examples of structures where stability considerations are important.

• At the onset of instability (buckling) a structure will have a very large change in displacement {Δx} under essentially no change in the load (beyond a small load perturbation).(beyond a small load perturbation).

F F

Stable Unstable



Linear Buckling

Training Manual… Background on Buckling• Eigenvalue or linear buckling analysis predicts the theoretical

buckling strength of an ideal linear elastic structure.

Thi th d d t th t tb k h f li l ti• This method corresponds to the textbook approach of linear elastic buckling analysis.

– The eigenvalue buckling solution of a Euler column will match theThe eigenvalue buckling solution of a Euler column will match the classical Euler solution.

• Imperfections and nonlinear behavior prevent most real world structures from achieving their theoretical elastic buckling strengthstructures from achieving their theoretical elastic buckling strength. Linear buckling generally yields unconservative results.

• Linear buckling will not account for:Material response that is inelastic– Material response that is inelastic.

– Nonlinear effects.– Imperfections in the structure which are not modeled (dents etc.).



Linear Buckling

Training Manual… Background on Buckling

• Although unconservative, linear buckling has various advantages:– It is computationally cheaper than a nonlinear buckling analysis, and

should be run as a first step to estimate the critical load (load at the onset p (of buckling).

• Relative comparisons can be made of the effect of differences in design to buckling

Li b kli b d d i t l t d t i h t th– Linear buckling can be used as a design tool to determine what the possible buckling mode shapes may be.

• The way in which a structure may buckle can be used as a possible guide in designg



Linear Buckling

Training Manual… Basics of Linear Buckling

• For a linear buckling analysis, the eigenvalue problem below is solved to get the buckling load multiplier λi and buckling modes ψi:

Assumptions:

[ ] [ ]( ){ } 0=+ ii SK ψλAssumptions:– [K] and [S] are constant:

• Linear elastic material behavior is assumed• Small deflection theory is used, and no nonlinearities includedSmall deflection theory is used, and no nonlinearities included

• It is important to remember these assumptions related to performing linear buckling analyses in Simulation.



Linear Buckling

Training ManualB. Buckling Analysis Procedure

• A Static Structural analysis will need to be performed prior to (or in conjunction with) a buckling analysis. The steps in italics are specific to buckling analysesto buckling analyses.– Attach Geometry– Assign Material Properties

D fi C t t R i (if li bl )– Define Contact Regions (if applicable)– Define Mesh Controls (optional)– Include Loads and Supports

S l St ti St t l A l i– Solve Static Structural Analysis– Link a Linear Buckling Analysis– Set Initial Conditions

R t R lt– Request Results– Solve the Model– Review Results



Linear Buckling

Training Manual… Geometry and Material Properties

• Any type of geometry supported by Simulation may be used in buckling analyses:– Solid bodiesSolid bodies– Surface bodies (with appropriate thickness defined)– Line bodies (with appropriate cross-sections defined)

• Only buckling modes and displacement results are available for line bodies.Only buckling modes and displacement results are available for line bodies.– Although Point Masses may be included in the model, only inertial loads

affect point masses, so the applicability of this feature may be limited in buckling analyses

• For material properties, Young’s Modulus and Poisson’s Ratio are required as a minimumq



Linear Buckling

Training Manual… Contact Regions

• Contact regions are available in free vibration analyses, however, contact behavior will differ for the nonlinear,contact types exactly as with modal analyses.

• Discussed earlier (see chapter 5).

Initially Touching Inside Pinball Region Outside Pinball RegionBonded Bonded Bonded Free

Contact Type Linear Buckling Analysis

Bonded Bonded Bonded FreeNo Separation No Separation No Separation FreeRough Bonded Free FreeFrictionless No Separation Free Free



Linear Buckling

Training Manual… Loads and Supports

• At least one structural load, which causes buckling, should be applied to the model:

(λ)– All structural loads will be multiplied by the load multiplier (λ) to determine the buckling load (see below).

– Compression-only supports are not recommended. – The structure should be fully constrained to prevent rigid-body motion.

F x λ = Buckling LoadF x λ = Buckling Load

In a buckling analysis all applied loads (F) are scaled by a

multiplication factor (λ) until the critical (buckling) load is reached



critical (buckling) load is reached

Linear Buckling

Training Manual… Loads and Supports

• Special considerations must be given if constant and proportional loads are present.– The user may iterate on the buckling solution adjusting the variableThe user may iterate on the buckling solution, adjusting the variable

loads until the load multiplier becomes 1.0 or nearly 1.0.– Consider the example of a column with self weight WO and an externally

applied force A.– A solution can be reached by iterating while adjusting the value of A until

λ = 1.0. This insures the self weight = actual weight or WO * λ = WO .



Linear Buckling

Training Manual… Buckling SetupBuckling analyses are always coupled to a structural analysis within the project schematic.– The “Pre-Stress” object in the tree contains the results from a structural

analysis.– The Details view of the “Analysis Settings” under the Linear Buckling

branch allows the user to specify the number of buckling modes to find.



Linear Buckling

Training Manual… Solving the Model

• After setting up the model the buckling analysis can be solved along with the static structural analysis.– A linear buckling analysis is more computationally expensive than a– A linear buckling analysis is more computationally expensive than a

static analysis on the same model. – The “Solution Information” branch provides detailed solution output.



Linear Buckling

Training Manual… Reviewing Results• After the solution is complete, the buckling modes can be reviewed:– The Load Multiplier for each buckling mode is shown in the Details view

as well as the graph and chart areas. The load multiplier times the li d l d t th di t d b kli l dapplied loads represent the predicted buckling load.

F = (F x λ)Fbuckle = (Fapplied x λ)



Linear Buckling

Training Manual… Reviewing Results• Interpreting the Load Multiplier (λ):– The tower model below has been solved twice. In the first case a unit

load is applied. In the second an expected load applied (see next page)



Linear Buckling

Training Manual… Reviewing Results

• Interpreting the Load Multiplier (λ):

LoadUnitadBucklingLo _*λ=λ=⇒ adBucklingLo λ=⇒ adBucklingLo

LoadActualadBucklingLo _*λ=

FactorSafetyLoadActualdBuclingLoa _

_==⇒ λ



Linear Buckling

Training Manual… Reviewing Results

• The buckling load multipliers can be reviewed in the “Timeline” section of the results under the “Linear Buckling” analysis branch– It is good practice to request more than one buckling mode to see if the g p q g

structure may be able to buckle in more than one way under a given applied load.



Linear Buckling

Training ManualC. Workshop 7.1 – Linear Buckling

• Workshop 7.1 – Linear Buckling• Goal:– Verify linear buckling results in Simulation for the pipe model

shown below. Results will be compared to closed form calculations from a handbook.



wb-mech 120 ch07 buckling

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