Static Design Ekin

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ME 305 Machine Elements

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1

Chapter

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5

Static

Design

CriteriaDepartment of Mechanical EngineeringAtlm University

Dr. Ekin Bingl


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A structural member must be designed so that its ultimate

load is considereble larger than the load the member or

component will be allowed . The ratio of the ultimate load

to the alloweble load is defined as the factor of safety.

Stress

Strengthn

Factor of Safety

loadDesign

loadUltimaten


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Stress Concentration

In the development of the basic stress equations for

tension, compression, bending, and torsion, it was

assumed that no geometric irregularities occurred in the

member under consideration.

But in reality, machine elements have:

shoulders in shafts to fit bearings,

key slots in shafts for securing pulleys and gears.

A bolt has a head on one end and screw threads on the

other end

Other parts require holes, oil grooves, and notches of

various kinds.


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Any discontinuity in a machine part alters the stress distribution

in the neighborhood of the discontinuity so that the elementarystress equations no longer describe the state of stress in the part

at these locations.

Such discontinuities are called stress raisers, and the regions in

which they occur are calledareas ofstress concentration.


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A theoretical, or geometric, stress-concentration factor Ktor Kts

is used to relate the actual maximum stress at the discontinuity

to the nominal stress. The factors are defined by the equations

Kt is used for normal stresses and Kts for shear stresses.

It is a highly

localized effect.


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A static load is a stationary force or couple applied to a member.

To be stationary, the force or couple must be unchanging in

magnitude, point or points of application, and direction

Static load

Can mean a part has separated into two or more pieces; has become

permanently distorted, thus ruining its geometry; has had its

reliability downgraded; or has had its function compromised,whatever the reason.

Failure


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Static Strength

it is necessary to design using published values of yield

strength, ultimate strength, percentage reduction in area, and

percentage elongation.

Important thing is, to design against both static and dynamic

loads, 2-D and 3-D stress states, high and low temperatures,

and very large and very small parts.


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there is no universal theory of failure for the general case of material

properties and stress state. Structural metal behavior is typically

classified as: Ductile materials (Snek Malzemeler) andBrittle

materials(Gevrek Malzemeler ) .

Failure Theories

general falure crtera for steady loadng


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Brittle materials (fracture criteria)

f (true strain at fracture ) < 0.05, do not exhibit an identifiable

yieldstrength, and are typically classified by ultimate tensile andcompressive strengths, Sutand Suc,

Maximum normal stress (MNS)

Brittle Coulomb-Mohr (BCM),

Modified Mohr (MM)

Ductile materials (yield criteria)

f 0.05 and have an identifiable yield strength that is often the

same in compression as in tension (Syt (tension yield strength)=Syc(compressive yield strength ) = Sy )

Maximum shear stress (MSS),

Distortion energy (DE),

Ductile Coulomb-Mohr (DCM).


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fracture

Yield strength: the stress at which a material begins to deform

plastically


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Yielding begins whenever the maximum shear stress in apart becomes equal to the maximum shear stress in atension test specimen that begins to yield.

The shear yield strength is equal to one-half of the tension yield strength.

(maximum shear stress at yield is max = Sy/2)

2, max

A

P


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General Maximum-Shear-Stress Theory predicts yielding when :

For a general state of stress, three principal stresses can be determined

and ordered such that 1 2 3. The maximum shear stress is then

yield strength in shear

For design purposes , modifiy to incorporate a factor of safety, n.


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Static Design Ekin