seminar - ultimate strength using fem

8/2/2019 Seminar - Ultimate Strength Using FEM

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What is FEA?

The finite element analysis (FEA) refers to thenumerical methods useful for solving problems ofengineering and mathematical physics.

Used in problems with complicated geometries,loadings, and material properties whereanalytical solutions cannot be obtained.

e.g. the Ship Structure.

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Ultimate Strength Analysis

As the applied hull girder load increases, themost highly stressed structural components of aships hull buckle in compression or yield intension.

The ship structure can withstand further loadingand fails only later.

Thus the maximum stress it can endure inbetween this is the ultimate strength and findingthis is Ultimate Strength Analysis(concepts inthe plastic theory of material behavior).

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Computation of the Ultimate Hull GirderStrength

methods

SimpleBeamTheory

PresumedStress

Distribution

Non-

Linear

FEM

IdealizedStructural

Unit

IntelligentSuper-size

FiniteElements

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Non-Linear Finite ElementMethods for Ultimate StrengthAnalysis

Other methods does not take into accountthe progressive failures of the structuralcomponents

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While the FEMs gives us a much morerefined and accurate computation.

The resulting computations can be totallywrong if the modeling technique applied isinadequate.

A ships hull is a very complicated structure

consisting of many components such as deck,bottom, side shell, bulkhead, frames,longitudinal.

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A Typical FEM Model for Non-Linear Analysis of the Cantilever

Box Girder

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1. GLOBAL HULL MODEL

The hull girder as such is taken for consideration.

Considered impractical because of the greatcomputational efforts required.

Consists of a coarse mesh that is used to representthe overall stiffness and the global stressdistribution of the primary stress distribution of theprimary members.

Structures not contributing to the global strengthof the vessel may be disregarded.

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GHM (contd)

Deformations and nominal stresses from thefollowing are described:

a) Vertical Hull Girder Bending

b) Vertical Shear Distribution between ships sideand bulkheads.

c) Horizontal hull girder bending, torsion of ship

girder.d) Transverse shear and bending.

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A Spherical

Tank LNGCarrier


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2.CARGO HOLD MODELS

Used to analyze the deformation response andnominal stresses of the primary members of themidship area.

a) One-Cargo Hold Model: used when shearingforces (H/V) are applied without V/H BMs,transverse frames can fail or at least deformbefore the stiffened panels between the adjacent

frames reach the ultimate limit state.

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CHM (contd)

b) Two-Cargo Hold Model

c) Three-Cargo Hold Model

b) and c) are used when the effects of rotationalrestraints at the transverse bulkheads need to betaken into consideration

These models generally cover + 1 + cargohold/tank length in the midship region.

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3. FRAME AND GIRDERMODELS

Used to analyze nominal stresses in the mainframing/girder system.

This can be included in the cargo hold model orrun separately with prescribed boundaryconditions.

One-Bay Sliced Hull Cross Section Model:

When H/V bending moment is predominant andthe transverse frames cannot fail before thestiffened panels between the transverse framesreach ultimate state.

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FGM (contd)

Two-Bay Sliced Hull Cross Section Model:

To take into account the effect of rotationalrestraints to the transverse frames.

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4. LOCAL STRUCTURE MODEL

Used to analyze stresses in local areas.

E.g. local plates, longitudinals, Brackets,

hatch cover openings, corrugations etcMagnitude of the stress results dependson the mesh divisions.

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Local models in D/B


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5. STRESS CONCENTRATIONMODELS

Used for the fatigue analysis of details were thegeometrical stress concentration is unknown.

Usually applied in conjunction with the Local FEmethods

Aim is to find the geometric stress concentrationdistribution rather than notch stress.

Results are used directly in fatigue analysis or inderiving the stress concentration factors.

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Inner Bottom


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Conclusion

Main advantage of non-linear FEM is that theinteraction between structural components can betaken into account.

The results serve as a benchmark for other analysisbased on simplified models.

The dimensions of the models are based on CommonStructural Rules (CSR) issued by IACS.

Purpose built softwares like ABAQUS, Simulia 2009are the popular choices.

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seminar - ultimate strength using fem

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