Modes of Failure
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Transcript of Modes of Failure
University of Sydney – BDes Design Studies 1A - Structures Modes of Failure
Mike Rosenman 2000
Modes of Failure
solids held together by bonds between their atoms
these bonds can be compressed
or extended
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University of Sydney – BDes Design Studies 1A - Structures Modes of Failure
Mike Rosenman 2000
tension
compression
shear
bending
stress pattens may be complex but consist of only 3 basic states of stress tension - compression - shear
Modes of Failure
buckling
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University of Sydney – BDes Design Studies 1A - Structures Modes of Failure
Mike Rosenman 2000
Tension
state of stress where material tends to be pulled apart
cable with weight becomes longer under pull
lengthening depends on X-section, length & load
larger the diameter - smaller the elongation
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University of Sydney – BDes Design Studies 1A - Structures Modes of Failure
Mike Rosenman 2000
Compression
state of stress in which particles pushed against the others
column under load shortens squashes
shortening of material
a steel column under compression shortens as much as a rod of same steel
lengthens in tension under same stress
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University of Sydney – BDes Design Studies 1A - Structures Modes of Failure
Mike Rosenman 2000
Compression (cont.)
can have no tension elements but must have compression elements
materials weak in tension often strong in compression
with modern materials of high compressive strength, e.g. steel can build columns much slimmer
but slenderness introduces new problem
compression elements very commonmust channel loads to ground
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University of Sydney – BDes Design Studies 1A - Structures Modes of Failure
Mike Rosenman 2000
Buckling
buckling is a basic design factor for slender elements in compression
buckling occurs even if load perfectly central
as compressive load increases, reach value where slender elements instead of shortening buckle & usually break
buckling load depends on:material, length, shape of X-section, restraints at ends
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University of Sydney – BDes Design Studies 1A - Structures Modes of Failure
Mike Rosenman 2000
Shear
state of stress in which particles of material slide relative to each other
• rivets tend to shear
• a hole puncher uses shear to punch out holes in paper
• load on short cantilever tends to shear beam from wall
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University of Sydney – BDes Design Studies 1A - Structures Modes of Failure
Mike Rosenman 2000
Bending
consider plank loaded as shown
upper fibres lengthen
plank ends move down
section between stones deflects up
curve is arc of circle
lower fibres shorten
middle fibres remain original length - Neutral Axis
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University of Sydney – BDes Design Studies 1A - Structures Modes of Failure
Mike Rosenman 2000
Bending (cont.)
concrete beam fails in tension due to bending
may fail in diagonal tension due to shear due to bending
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University of Sydney – BDes Design Studies 1A - Structures Modes of Failure
Mike Rosenman 2000
Behaviour of Materials
stress
strain
elasticity - plasticity - brittleness
safety factors
selecting appropriate materials
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University of Sydney – BDes Design Studies 1A - Structures Modes of Failure
Mike Rosenman 2000
Modes Of Failure - Under Stress
tension
compression
buckling
shear
bending
stress patterns complex but consistonly of three basic states of stresstension - compression - shear
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University of Sydney – BDes Design Studies 1A - Structures Modes of Failure
Mike Rosenman 2000
General Load-DeformationProperties Of Materials
application of load produces dimensional changes in a member
member undergoes change in size or shape or both
deformation may be reversible or irreversible
elastic or plastic
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University of Sydney – BDes Design Studies 1A - Structures Modes of Failure
Mike Rosenman 2000
Stress
internal forces developed within a structure due to action of external forces
stress is force intensity - force per unit area
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University of Sydney – BDes Design Studies 1A - Structures Modes of Failure
Mike Rosenman 2000
Fi = Fe
Fe
Fe
Stress (cont1.)
consider member in tension
stress is force intensity - force per unit area
Fe
Fe
X X
Fe
Stress = Force / Area f = F / A
Fe
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University of Sydney – BDes Design Studies 1A - Structures Modes of Failure
Mike Rosenman 2000
Stress (cont2.)
stress is force per unit area
1 pascal = 1 newton per square metre
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A load of 1 N on each square metre represents
an average stress of 1 N/m2, or 1 Pa
1m1m
1m
1m1m
1m1m
1N1N
1N1N
University of Sydney – BDes Design Studies 1A - Structures Modes of Failure
Mike Rosenman 2000
Stress (cont3.)
we use stress in megapascals (MPa) for most materials
1 MPa = 106 N/m2 = 1 N/mm2
we use stress in kilopascals (kPa) for floor loads and foundation pressures (loads distributed over an area)
( remember 1 m2 = 106 mm2)
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University of Sydney – BDes Design Studies 1A - Structures Modes of Failure
Mike Rosenman 2000
Stress (cont4.)
internal force not concentrated at single spot
stress developed DOES NOT DEPEND ON
MATERIAL OF MEMBER
distributed over entire cross-section
stress in a member depends only on force applied and cross-section f = F / A
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University of Sydney – BDes Design Studies 1A - Structures Modes of Failure
Mike Rosenman 2000
Strength of Members
strength depends on many factors
in tension, failure will occur by pulling apart at weakest location
weak spot (point of reduced X-section) determines capacity of whole member
f = F/A higher because of smaller A
if material can sustain stress member will carry load
as load increases stress increases eventually material fails (pulls apart)
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University of Sydney – BDes Design Studies 1A - Structures Modes of Failure
Mike Rosenman 2000
ratio ofchange in size or shape of element
to original size or shape
Strain
response to stress
have stress --> get strain
strain to do with change in size or shape
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University of Sydney – BDes Design Studies 1A - Structures Modes of Failure
Mike Rosenman 2000
STRAIN (cont.1)
for member subject to simple tensile force
dimensionless - millimetre / millimetre
strain =increase in length
original length e =LL
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University of Sydney – BDes Design Studies 1A - Structures Modes of Failure
Mike Rosenman 2000
STRAIN (cont2.)
determined by:
except for rubber bands, strains very small usually not visible
more a material strains under load - more the structure deflects
taking member of known length
subjecting it to a known load
measuring elongation
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