eciv 520 a structural analysis ii lecture 4 – basic relationships
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ECIV 520 A Structural Analysis II
Lecture 4 – Basic Relationships
Process of Matrix/FEM Analysis
Accurate Approximations of Solutions
Reliability of Solution depends on choice of Mathematical Model
Process of Matrix/FEM Analysis
Reliability of Solution depends on choice of Mathematical Model
Process of Matrix/FEM Analysis
Accurate Approximations of Solutions
Process of Matrix/FEM Analysis
Process of Matrix/FEM Analysis
Accurate Approximations of Solutions
Reliability of Solution depends on choice of Mathematical Model
StressResultant Force and Moment represent the resultant effects of the actual distribution of force acting over sectioned area
Stress
AssumptionsMaterial is continuousMaterial is cohesive
Force can be replaced by the three componentsFx, Fy (tangent) Fz (normal)
Quotient of force and area is constantIndication of intensity of force
Consider a finite but very small area
Normal & Shear Stress
Normal StressIntensity of force acting normal to A
Shear StressIntensity of force acting tangent to A
dA
dF
A
F zz
Az
0
lim
dA
dF
A
F xx
Azx
0
limdA
dF
A
F yy
Azy
0
lim
General State of Stress
Set of stress components depend on orientation of cube
Basic Relationships of Elasticity Theory
Concentrated
Distributed on Surface
Distributed in Volume
Equilibrium
Equilibrium
Write Equations of Equilibrium
Fx=0
Fy=0
Fz=0
EquilibriumEquilibrium - X
X
dAdzz
dA
dAdzz
dA
dAdxx
dA
xzxz
xz
xyxy
xy
xx
x
EquilibriumEquilibrium
Write Equations of Equilibrium
Fx=0
Fy=0
Fz=0
Boundary Conditions
Prescribed Displacements
Boundary Conditions
Equilibrium at Surface
Deformation
Intensity of Internal Loads is specified using the concept of
Normal and Shear STRESS
Forces applied on bodies tend to change the body’s
SHAPE and SIZE
Body Deforms
Deformation
• Deformation of body is not uniform throughout volume
• To study deformational changes in a uniform manner consider very short line segments within the body (almost straight)
Deformation is described by changes in length of short line segments and the
changes in angles between them
Strain
Deformation is specified using the concept of
Normal and Shear STRAIN
Normal Strain - Definition
Normal Strain: Elongation or Contraction of a line segment per unit of length
s
ss
'
avgε
s
ssAB
'
n along limε
ss ε1'
Normal Strain - Units
Dimensionless Quantity: Ratio of Length Units
Common Practice
SI
m/mm/m (micrometer/meter)
US
in/in
Experimental Work: Percent
0.001 m/m = 0.1%
=480x10-6: 480x10-6 in/in 480 m/m 480 (micros)
Shear Strain – DefinitionShear Strain: Change in angle that occurs between two line
segments that were originally perpendicular to one another
(rad) 'lim2
talongACnalongAB
nt
negative 2
'
positive 2
'
nt
nt
Cartesian Strain Components
zz
yy
xx
ε1
ε1
ε1
zx
yz
xy
2
2
2
Normal Strains: Change VolumeShear Strains: Change Size
Small Strain Analysis
Most engineering design involves application for which only small deformations are allowed
DO NOT CONFUSE Small Deformations with Small Deflections
Small Deformations => <<1
Small Strain Analysis: First order approximations are made about size
Strain-Displacement Relations
AssumptionSmall Deformations
For each face of the cube
Stress-Strain (Constitutive)Relations
Isotropic Material: E,
Generalized Hooke’s Law
Stress-Strain (Constitutive)Relations
Note that:
Equations (a) can be solved for
Stress-Strain (Constitutive)Relations
Or in matrix form
Stress-Strain (Constitutive)Relations
xy
xz
yz
z
y
x
xy
xz
yz
z
y
x
Stress-Strain: Material Matrix
Material Matrix
Special Cases
One Dimensional: v=0No Poisson Effect
= E
Reduces to:
Special CasesTwo Dimensional – Plane StressThin Planar Bodies subjected to in plane loading
Special CasesTwo Dimensional – Plane Strain
Long Bodies Uniform Cross Section subjected to transverse loading
Special Cases
For other situations such as inostropy obtain the appropriate material matrix
Two Dimensional – Plane Stress Orthotropic Material
Dm=
Strain EnergyDuring material deformation energy is stored (strain energy)
yxAF zL
V
zyx
zFU
2
1
2
1
2
1
Strain Energy Density
2
1
V
Uu
e.g. Normal Stress
Strain Energy
In the general state of stress for conservative systems
dVUV
T εσ2
1
Principle of Virtual Work
PFdxWPx
2
1
0
Load Applied Gradually Due to another Force
'PWP
Apply Real LoadsApply Virtual Load
PVW Concept
Real
Deformns
udLP '
Internal
Virtual
Forces
Principle of Virtual Work
A body is in equilibrium if the internal virtual work equals the external virtual work for every
kinematically admissible displacement field
Principle of Virtual Work
V
zx
yz
xy
z
y
x
zxyzxyzyx
V
Tie
dV
dVUW
εσ