lecture sixteen - texas a&m...

Strain 1Lecture 16

Architectural Structures IENDS 231

S2008abn

ARCHITECTURAL STRUCTURES I:STATICS AND STRENGTH OF MATERIALS ENDS 231

DR. ANNE NICHOLS

SPRING 2008

sixteen

elasticity& strain

lecture

Strain 2Lecture 16


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Deformations • materials deform• axially loaded materials change

length• normal stress is load per unit

area• STRAIN:

– change in length over length– UNITLESS

Lδε =

Strain 3Lecture 16


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Shearing Strain• deformations

with shear• parallelogram• change in angles• stress:• strain:

– unitless (radians)

sδ

L

γ

φ

τφφδγ ≅== tan

Ls

Strain 4Lecture 16


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Shearing Strain• deformations

with torsion• twist • change in angle of line• stress:• strain:

– unitless (radians)

γτ

Lρφγ =

Strain 5Lecture 16


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Load and Deformation• for stress, need P & A• for strain, need δ & L

– how?– TEST with load and

measure– plot P/A vs. ε

Strain 6Lecture 16


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Material Behavior• every material has its own response

– 10,000 psi– L = 10 in– Douglas Fir vs.

steel?

Strain 7Lecture 16


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Behavior Types• ductile - “necking”• true stress

• engineering stress– (simplified)

APf =

oAPf =

Strain 8Lecture 16


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Behavior Types• brittle

• semi-brittle

Strain 9Lecture 16


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Stress to Strain• important to us in -ε diagrams:

– straight section– LINEAR-ELASTIC– recovers shape

(no permanentdeformation)

f

Strain 10Lecture 16


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Hooke’s Law• straight line has constant slope• Hooke’s Law

• E – Modulus of elasticity– Young’s modulus– units just like stress

f

ε

E

1 ε⋅= Ef

Strain 11Lecture 16


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Stiffness• ability to resist strain

• steels– same E– different

yield points– different

ultimate strength

uf

Strain 12Lecture 16


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Isotropy & Anisotropy• ISOTROPIC

– materials with E same at any direction of loading

– ex. steel

• ANISOTROPIC– materials with different E

at any direction of loading– ex. wood is orthotropic

Strain 13Lecture 16


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Elastic, Plastic, Fatigue• elastic springs back• plastic has permanent

deformation• fatigue caused by

reversed loading cycles

Strain 14Lecture 16


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Plastic Behavior• ductile

at yield stress

Strain 15Lecture 16


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Lateral Strain• or “what happens to the cross section

with axial stress”

• strain in lateral direction – negative– equal for isometric materials

Efx

x =ε

0== zy ff

zy εε =

Strain 16Lecture 16


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Poisson’s Ratio• constant relationship between

longitudinal strain and lateral strain

• sign!

x

z

x

y

strainaxialstrainlateral

εε

εε

μ −=−=−=

Efx

zyμεε −==

5.00 << μ

Strain 17Lecture 16


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Calculating Strain• from Hooke’s law

• substitute

• get ⇒

ε⋅= Ef

LE

AP δ

⋅=

AEPL

=δ

Strain 18Lecture 16


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Orthotropic Materials• non-isometric• directional values of

E and μ• ex:

– plywood– laminates– polymer

composites

Strain 19Lecture 16


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• why we use fave

• increase in stress atchanges in geometry– sharp notches– holes– corners

–

Stress Concentrations

Strain 20Lecture 16


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22max

maxf

APf

ov ==−

• if we need to knowwhere max f and fvhappen:

Maximum Stresses

F

oAPf =max1cos0 =→°= θθ

5.0sincos45 ==→°= θθθ

Strain 21Lecture 16


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Maximum Stresses

Strain 22Lecture 16


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Design of Members• beyond allowable stress...• materials aren’t uniform 100% of the

time– ultimate strength or capacity to failure may

be different and some strengths hard to test for

• RISK & UNCERTAINTY

APf u

u =

Strain 23Lecture 16


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Factor of Safety• accommodate uncertainty with a safety

factor:

• with linear relation between load and stress:

SFloadultimateloadallowable

.=

stressallowablestressultimate

loadallowableloadultimateSF ==.

Strain 24Lecture 16


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Load and Resistance Factor Design• loads on structures are

– not constant– can be more influential on failure– happen more or less often– UNCERTAINTY

φ - resistance factorγ - load factor for (D)ead & (L)ive load

nLLDDu RRRR φγγ ≤+=

lecture sixteen - texas a&m...

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