Embed Size (px)
Citation preview
2
Design concept & rationale
metallurgySteel
fabrication
Member + connection
erection
structural system
Loads action
Strength, stability , rigidityForce, deformation
S ≤ RPerformance:Safety, serviceability, durability
Advantages:…………
3
Structural steelOutlines
requirement of properties for structural steel
factors affecting steel properties
state of stress and stress concentration
fatigue failure
steel grade, steel products & steel selection
recent advances of structural steel
4
Requirement of steel propertiesuni-axial tensile test: preparation
tensile-test specimenSample location: transverse or longitudinalSpecimen: shape and dimension
loading methodRate of stressing
temperatureroom temperature
5
/A σ→
Δ 0/ ε→
yf
yf
uf
ufN
0.2%
Proportional limitLinear elasticYield stressPerfectly plasticStrain-hardeningUltimate stressLateral contractionNeckingFractureModulus of elasticityProof stressTrue stress
, , , ,y u yf f Eε δ
, / , / , /p y u t cf f E d d Eσ σ ε σ ε= =
,σ ε
pσ
Requirement of steel propertiesuni-axial tensile test: stress-strain diagram
6
Yield strain is 10 to 15 times proportional strain for mild steel or low-alloy steel. How about the ultimate strain?
Nominal stress= conventional stress= engineering stressTrue stress
7
Requirement of steel propertiesstrengthProportional limit, yield point, tensile strengthYield-to-tensile strength ratio (0.6~0.7 for mild steel)
ductilityPercentage elongation at failure, reduction of area
toughnessstatic toughness, impact toughness
cold-formingcold bent test
weldabilityfor construction & usage
durabilitycorrosion resistance, fatigue resistance
8
Requirement of steel propertiesductility: elongation & reduction of area
ductility: occur remarkable residual strain (plastic deformation) without fracture after stress exceeding yield point
percentage elongation at failure
percentage reduction of areatrue assessment criteria but difficult to measurethrough-thickness property
9
Requirement of steel propertiestoughness: static & impact toughness
impact toughness: measure of impact resistance or the ability to absorb sudden increase in stress at notch
Charpy V-notch testtemperature-dependent
strength + plasticity
10
Requirement of steel propertiescold-forming ability
cold-forming property: ability to resist crack while producing plastic deformation under cold-forming work
Cold-bent testassess ductility and weldability
11
Requirement of steel propertiesweldability
weldabilityConstruction: no crack in welds and HAZ area under normal weld conditionUsage: mechanical properties of welds and HAZ area are not less than the base metal
Carbon Equivalent
12
Requirement of steel propertiesdurability
corrosion resistance30-40% demolishedCorrosion-resist steel: Cu-P-Ti-Re
fatigue resistanceNo statement here
13
Factors affecting steel properties
chemical compositionFe, C, Si, Mn, V, S, P, O, N, others
process of metallurgySmelt, cast, rolling, residual stress while cooling
time effecttime - hardening
cold workhistory of strain & stress, cold work
temperatureelevated temperature and low temperature
rate of stressingthe higher rate, the higher stress
14
Factors affecting steel propertieschemical composition
↓↓↓↓↑nitrogen↓↓↓↓↓oxygen↑↓↓↓↑phosphorus↓↓↓↓↓Sulfur↑---↑vanadium
- (↓)- (↓)--↑manganese----↑silicon↓↓↓↓↑Carbon
↑↓Iron
durabilityweldabilitytoughnessductilitystrength
15
Factors affecting steel properties process of steelmaking
16
Factors affecting steel properties process of steelmaking
17
Factors affecting steel propertiesprocess of steelmaking
produce - smeltBasic oxygen furnace, electric-arc furnace
casting (ladle treatment)Rimming steel (Mn)Semikilled steelKilled steel (Si)Ultra-killed steel
hot rolling1200-1300 degreetransverse & longitudinal
residual stress while cooling
18
Factors affecting steel propertiestime-effect and cold work hardening
time effectStrength increase, ductility decrease, crispyHeat treatment
cold work hardeningHistory of stressingCold work: cut, punch,
roll, press, fold, drill,plane, strike
rate of stressingthe higher rate, the higher stress
19
Factors affecting steel propertiestemperatureelevated temperature
250℃, brittle600℃, soft
20
Factors affecting steel propertiestemperaturelow temperature
toughness decrease suddenly
Brittle failure transition ductile
Ene
rgy
abso
rbed
Ak
Temperature
Transition temperatureat steepest slope
21
State of stress and stress concentrationcombined stress and criteria of yield
equivalent stress (von Mises stress)
)(3 222222zxyzxyxzzyyxzyxzs τττσσσσσσσσσσ +++−−−++=
( ) ( ) ( )[ ]213
232
2212
1 σσσσσσσ −+−+−=zs
Plane stress?Stress in beam?
22
criteria of elastic failure1st: The maximum-principal-stress theory2nd: The maximum-principal-strain theory3rd: The maximum-shear-stress theoryThe maximum-strain-energy theory4th: The maximum-distortion-energy theory
von Mises theory (4th):
Elasticity
Plasticity
(perfect elasto-plastic model)
State of stress and stress concentrationcombined stress and criteria of yield
yzs f<σ
yzs f=σ
σ
ε
yf
uf
23
State of stress and stress concentrationyielding under biaxial or triaxial stresses
1σ
1ε
yfUni-axial stress
Bi-/tri-axial stress with same sign
Bi-/tri-axial stress with different sign
Effects of biaxial or triaxial stresses on yielding
24
State of stress and stress concentrationyielding under biaxial or triaxial stresses
Yielding under biaxial stresses: any stress is smaller than yield strength?
Pure shear condition: yyy ff 58.03/ ≈=τ
25
nσ
State of stress and stress concentrationstress concentration: definition
The uniform stress pattern is disruption and the intensity of stress increases greatly within a very short distance. The condition is described as STRESS CONCENTRATIONS.It is due to the abrupt changes in geometry caused by imperfection of structural steel or manufacture. It includes holes, grooves, notches, keyways, threads, or abrupt changes in plate width or thickness.
Stress concentration factor:
oσmσn
mK σσ=
26
State of stress and stress concentrationstress concentration and brittle failure
Constant thickness
xx ,, εσA
nxnxn ,, εσA
Steel bar with abrupt change of width at middleAA <n
xnxxnx , εεσσ >>
xx ,, εσA
yxnxny εμεμεε =−<−=
0,0 nynx >> σσ
x
y
Subjected to axial tensile load in x-direction, then
Lateral strain is obtained by poisson’s ratio,
Lateral contraction at middle is larger than that at the ends, therefore, the lateral stress at middle must be tensile stress
Abrupt change in width at middle →stress concentration →biaxial tensile stress field → brittle failure
27
State of stress and stress concentrationstress concentration and change of geometry
10
10
10
10
100
18
0.425
σ
ε
The more abrupt the geometry change,the severer the stress concentration,the larger the increase of tensile strength,the worse the capacity of plastic deformation
28
State of stress and stress concentrationstress concentration and perfect plasticity of steel
For a material with perfect elastoplastic constitutive relation, once the peak stress reaches yield point, its stress will remain this value and the stress concentration factor will decrease.
1-1
2-2
σ
ε
1
2
1
2
o2σo1σ
m1σ ym2 f≈σ
1p2p
22
2
1
11 KK
n
m
n
m =>=σσ
σσ
Idealized stress-strain plot for an elastoplastic material
29
State of stress and stress concentrationstress beyond yield point: low-cycle fatigue
σ
ε
σ
ε
yf
yf
y2 f
yf
yfσ
ε
hysteretic curves of steel
stress/cycle hardening
Bauschinger Effect
plastic energy absorbed
30
Fatigue failurehigh-cycle fatigue: basic concepts
fatigue failure: steel material subjected to dynamic loads is likely to fail at a lower stress than when the same loads are applied statically, especially when the loads are repeated for alarge number of cycles.
σ
ε
σt
Number of cycles to failureNominal stress at failure
31
Fatigue failurehigh-cycle fatigue: failure mechanism
Progressive fracture:imperfection of material (local damage)
microscopic crack forms (crack initiation)
crack gradually enlarges (crack propagation)
crack becomes unstable andsudden fracture of steel occurs(unstable crack growth and fracture)
Characteristic of fracture
32
Fatigue failurehigh-cycle fatigue: basic concepts
fatigue life: number of stress cycles to failure under certain cycle symbol.
Factors governing fatigue failuretypes of stress (tensile, compressive, shear or combined stress)cycle symbolnumber of cycles to failurestress concentrationresidual stresssurface conditionrange of stress
33
Fatigue failurefatigue research I: max. stress method
pσ
research backgroundSmall steel specimen for fatigue tests
be applicableNon-welded elements
main conceptsfatigue life = cycle numbers to failure
fatigue strength: ( ) the max. stress (absolute value) →
fatigue life under specified cycle symbol
endurance limit = threshold stress
stress ratio:
σ tσ+
σ
1σt
σ+
σ
t
σ+σ
1σ
t
σ+
σ
maxσ
tσ+σ
minσ
maxσ
maxσminσ
minσ
1ρ =−
1 0ρ− < <
0ρ =
0 1ρ< <
1ρ =
max minσ σ=
Definition of maxσ
maxmin /σσρ =
34
maxσ
minσ
1ρ = − 1ρ =
1σ−
0σ
yfP
maxσ σ=
minσ0 1
1
K σ σσ
−
−
−=
Pmax 0 minKσ σ σ σ= = +
P 0max 1 K
σσ σρ
= =−
While design, fatigue life N is a definite value for a designated structural detailp
max σσ ≤
Fatigue failurefatigue research I: max. stress method
Engineering application: Goodman diagramfor a specified stress ratioand fatigue life:
N
Pσ
Endurance limit (threshold stress)
(fatigue strength)
.Constρ =
35
Fatigue failurefatigue research II: stress range method
research backgroundFull-scale structural specimen for fatigue tests
be applicablewelded elements
main conceptsfatigue failure is governed bystress range of the part of theelement, rather than stress ratio
stress range:
σ
t
σ+
σ
1σ
t
σ+
σ
t
σ+σ
1σ
t
σ+
σ
tσ+σ
σΔ1ρ = −
1 0ρ− < <
0ρ =
0 1ρ< <
1ρ =
σΔ
σΔ
σΔ
0σΔ =0σΔ ≥
minmax σσσ −=Δ
36
stress ratio
yield stress of steel
structural details
Fatigue failurefatigue research II: stress range method
Engineering application: βσσ /1)/(][ Nc=Δ≤Δ
σ
t
σ+
σ
1σ
t
σ+
σ
t
σ+σ
1σ
t
σ+
σ
tσ+σ
σΔ1ρ = −
1 0ρ− < <
0ρ =
0 1ρ< <
1ρ =
σΔ
σΔ
σΔ
0σΔ =
0σΔ ≥
Built-up I beam withsingle-layer flange plate
lgN
lg σΔlg B A lg NσΔ = −
1B = A lg c, Aβ
=
Built-up I beam withdouble-layer flange plates
37
Fatigue failurefatigue research II: stress range method
Mechanism of stress range methodStress range and residual stress
σ
ε
yf
y0.7fy0 .8 fσΔ =
σ
t
m ax y0.4 fσ =
y0.8 fσΔ =
cr,max y0.7fσ =
38
Fatigue failurefatigue failure under variable amplitude stress cycles
fatigue under constant and variableamplitude stress cycles
Miner criteria:
1N
=∑i
in
t
σ+iσΔ jσΔ
in iN jn N
σ tσ+
σj
39
Structural steelclassification & steel grade
carbon structural steelQ195, Q215, Q235, Q255, Q275
low alloy structural steelQ295, Q345, Q390, Q420, Q490
quality carbon structural steel31 types, 20, 45
quality structural steel wireshigh-strength
40
Structural steelsteel grade: carbon structural steel
common used: Q235product quality documentationMechanical properties: yield strength, tensile strength, elongationChemical composition: C, Mn, Si, S, P
expressed by:quality grade: A, B, C, Ddeoxidization: F, b, Z, TZ
Q235A[F,b,(z)] No Ce GuaranteeQ235B[F,b,(z)] 20℃, Ak≥27JQ235C [(z)] 0℃, Ak≥27JQ235D [(TZ)] -20℃, Ak≥27J
fy = 235MPafu = 400MPafvy = ?E = ?
41
Structural steelsteel grade: high-strength low alloy structural steel
common used: Q345, Q390, Q420 (alloy <5%)
product quality documentationMechanical properties: yield & tensile strength, elongation, cold workChemical composition: C, Mn, Si, S, P, V, Nb, Ti
expressed by:quality grade: A, B, C, D, Edeoxidization: Z, TZ, plus heat treatment
Q345/390/420A No toughness GuaranteeQ345/390/420B 20℃, Ak≥34JQ345/390/420C 0℃, Ak≥34JQ345/390/420D -20℃, Ak≥34JQ345/390/420E -40℃, Ak≥27J
Q345:fy = 345MPafu = ? 470~630MPa
fvy = ?E = ?
42
Structural steelsteel grade: quality carbon steel (wires)
quality carbon steelHeat treatment: thermal refining, temperingAdvantages: less impurity, less imperfectionClassification: 31 types, 20, 45 for high-strength bolts
quality structural steel wires (rope)wires: quality carbon steel → cold workhigh strength: 1570~1770MPaexpressed by: 6×7, 8×19
43
Structural steelsteel products (shapes)
steel plateSteel sheet: 0.35 ~ 1mm ~ 4mmThicker steel plate: rolled steel plate 4.5~20mm, thicker plate 20~60mmSuper-thick steel plate: >60mmFlat steel: 12~200mm width
(hot rolled) shaped steelI-section, channel, angle, H-section, T-section, tube
cold-formed thin-wall sectionsangle, channel, Z-section, hat-section, tubular
welded sections (built-up)I-section, box-section
44
Structural steeldesign value of steel strength (design index)
expression of design indexASD (Allowable Stress Design):
LRFD (limit states design - factor):
determination of design valueAs mentioned in Introduction Part
design value of structural steelsteel plate: steel grade, thicknesscast-steel: just authorizedweld: butt weld / fillet weldbolt: ultimate strength
Kf /][ y=σ
Ry /γff =
45
Structural steelprinciples for selection of structural steel
structural types and importanceImportant / normal / secondary
characteristic of loadsStatic load / dynamic loadPeriod of actions
connection methodsweld / bolt
temperature (steel located)north / south, indoor / outdoor
stress conditiontension / compression, through-thickness loading
46
Recent advances ofStructural steel
fire-resistant structural steel
corrosion-resistant structural steel
high-strength structural steelHigh-strength, weldability (Ceq)Through-thickness steel
ultra-low yield point steelstructural casting (steel)high friction factor steel plate0.3~0.5 → 0.9
47
Recent advances of Structural steelfire-resistant structural steel
traditional structural steel600℃:remain one-third strength
fire-resistant steel600℃:remain two-third strength
48
Recent advances of Structural steelcorrosion-resistant structural steel
additional chemical constituentCu, Cr, Mn, RE
USA Cro-Ten steel2~8 times corrosion resistanceBaosteel 09CuPTiRE2~3 times corrosion resistance
stainless steelStrength, ductility, weldability
49
Recent advances of Structural steelthrough-thickness structural steel
thickness >100mmexcellent toughness
column beam
50
Recent advances of Structural steelultra-low yield point structural steel
low yield point: 100~120MPatensile strength: 250~260MPaelongation: approximate 60%aim: keep key members out of plasticity
51
Recent advances of Structural steelstructural casting steel
