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Talk for ANSYS USERS GROUP

June 1999

Stress Categories for Design by Analysis of Pressure

Vessels

BACKGROUND

Developed in the early 1960s under Bernie Langer at theBettis Atomic Power Laboratory

Basis for SDB-63, Section III of the ASME Boiler and PressureVessel Code, and Section VIII, Division 2 of the ASME Boilerand Pressure Vessel Code (Appendix 4)

FAILURE THEORY

ASME "Design by Analysis" approach is based on Tresca(shear stress) failure theory with an elastic-perfectly plasticstress-strain curve

Tresca failure theory states that "yielding will occur when themaximum shear stress equals the shear stress at yielding in anuniaxial-tension test"

Maximum Shear Stress/2 = (Maximum Principal Stress Minimum Principal Stress)/2

Max Shear Stress = Max Principal Stress Min PrincipalStress => Stress Intensity

Tresca failure theory is less accurate than Von Mises

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(Distortion-Energy) failure theory, but slightly moreconservative and simpler

COMPARISON OF FAILURE THEORIES

Definitions

Normal Stress: Stress component normal to the planereference

Shear Stress: Stress component tangent to plane of reference

Membrane Stress: Uniform normal stress

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Bending Stress: Linear normal stress

General Stress: Stress away from any discontinuities

Local Stress: Stress near a discontinuity

Primary Stress: Stress developed by imposed loading that isnecessary to satisfy the simple laws of equilibrium of externaland internal forces and moments

Secondary Stress: Stress developed by constraint of adjacentparts or self-constraint of structure Secondary stresses areself-limiting They can be relieved by local yielding minor

distortions

Peak Stress: Localized stress that does not cause anynoticeable distortion and is objectionable only as a possiblesource of fatigue crack or brittle fracture

Stress Intensity:Difference between maximum and minimumprincipal stress

Design Stress Intensity : Allowable Stress Intensity typicallythe lesser of 2/3 the Yield Stress (YS) or 1/3 the UltimateTensile Stress (UTS)

Stress Categories

Five Basic Stress Categories

1. General Primary Membrane Stress Intensity (Pm)

2. Local Membrane Stress Intensity (PL) Primary Stress

3. Primary Membrane Plus Primary Bending Stress (PL + Pb)

either General or Local Membrane Stress

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LIMIT STRESS FOR COMBINED TENSION AND BENDING

ON A RECTANGULAR CROSS SECTION

STRAIN HISTORY BEYOND YIELD

Primary Membrane PlusPrimary Bending Stress (PL +

Pb)

1.5*Sm Sy 0.5*Su

Primary Plus SecondaryStress Intensity (PL + Pb + Q)

3.0*Sm 2*Sy Su

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HOPPER DIAGRAGM

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