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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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