Download - CH01 Basic Concepts
Materials for Civil and Construction Engineers
CHAPTER 1 Materials Engineering Concepts
Mamlouk/Zaniewski, Materials for Civil and Construction Engineers, Third Edition. Copyright © 2011 Pearson Education, Inc.
Mamlouk/Zaniewski, Materials for Civil and Construction Engineers, Third Edition. Copyright © 2011 Pearson Education, Inc.
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INTRODUCTION Ø Common civil
engineering materials: § steel § mineral aggregates § concrete § masonry § asphalt § wood § soil for geotechnical
engineers
Ø Less common materials § aluminum § glass § plastic § Fiber-reinforced
composites
Mamlouk/Zaniewski, Materials for Civil and Construction Engineers, Third Edition. Copyright © 2011 Pearson Education, Inc.
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New Materials Ø Advances in
§ polymers § adhesives § composites § geotextiles § coatings § synthetics
Ø High performance materials § higher strength to
weight ratio § improved durability § lower costs
Mamlouk/Zaniewski, Materials for Civil and Construction Engineers, Third Edition. Copyright © 2011 Pearson Education, Inc.
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Material Selection Considerations Ø Economic factors Ø Mechanical properties
Ø Non-mechanical properties
Ø Production/construction
Ø Aesthetic properties
Ø Sustainable considerations
Ø Emphasis § client’s needs § facility’s function
Mamlouk/Zaniewski, Materials for Civil and Construction Engineers, Third Edition. Copyright © 2011 Pearson Education, Inc.
1.1 Economic Factors
Factors to be considered § availability and cost of raw materials § manufacturing costs § transportation § placing § maintenance
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Mamlouk/Zaniewski, Materials for Civil and Construction Engineers, Third Edition. Copyright © 2011 Pearson Education, Inc.
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1.2 Mechanical Properties Ø Response of material to external loads Ø All materials deform under load depending on:
§ material properties § magnitude and type of load § geometry of the material element
Mamlouk/Zaniewski, Materials for Civil and Construction Engineers, Third Edition. Copyright © 2011 Pearson Education, Inc.
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Loading Conditions Ø Static (Dead) Loads – long term
§ applied and removed slowly so no vibrations § usually due to gravity
Ø Dynamic (Live) Loads – short term shock or vibration § periodic – repeating wave form (rotating
equipment) § transient – quick impulse that decays back to
resting (vehicles) § random – never repeats (earthquake)
Mamlouk/Zaniewski, Materials for Civil and Construction Engineers, Third Edition. Copyright © 2011 Pearson Education, Inc.
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Stress-Strain Relations All solid materials deform under load Ø stress is like force (or load) with the size factored out
so that we can directly compare different sizes § stress = force / area
σ = F / A (psi, ksi, kPa, MPa, GPa)
Ø strain is like deformation with the size factored out § strain = deformation / original length
ε = ΔL / L0 (%, in/in, mm/mm)
Mamlouk/Zaniewski, Materials for Civil and Construction Engineers, Third Edition. Copyright © 2011 Pearson Education, Inc.
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Typical Stress-Strain Diagrams Ø σ – ε is usually linear in the low stress range but
transforms into non-linear
Glass and chalk
Steel Aluminum alloys
Concrete Soft rubber
Mamlouk/Zaniewski, Materials for Civil and Construction Engineers, Third Edition. Copyright © 2011 Pearson Education, Inc.
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Elastic Behavior Ø Instantaneous response to load Ø Returns to its original shape upon unloading
§ stretches bonds between atoms without rearranging them
Mamlouk/Zaniewski, Materials for Civil and Construction Engineers, Third Edition. Copyright © 2011 Pearson Education, Inc.
Linear & Non-Linear Behavior Ø A linear material has a straight line stress-strain graph Ø An elastic material returns to its original shape
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Linear elastic
Non-linear elastic
Non-linear inelastic
Mamlouk/Zaniewski, Materials for Civil and Construction Engineers, Third Edition. Copyright © 2011 Pearson Education, Inc.
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Properties of an Elastic Material Ø Modulus of Elasticity or Young’s Modulus
E = Δσ / Δε § slope (rise over run) of the linear portion of stress-strain
curve
Ø Poisson’s Ratio ν = -εl / εa
§ relates lateral strain, εl, to axial strain, εa
§ as material is stretched the cross section shrinks and vice versa for compression
§ Range = 0 to 0.5 (practically 0.1 to 0.45)
Mamlouk/Zaniewski, Materials for Civil and Construction Engineers, Third Edition. Copyright © 2011 Pearson Education, Inc.
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Generalized Hooke's Law Ø For three directions (3D = triaxial)
( )E
zyxx
σσνσε
+−=
( )E
xzyy
σσνσε
+−=
( )E
yxzz
σσνσε
+−=
y x
z
( )
( )EE
E
AF
zzy
zz
yx
z
νσσνε
νσε
σσ
σ
−=
+−=
+−=
==
=
00
00
0
For axially loaded members, no stresses in the x and y directions
Mamlouk/Zaniewski, Materials for Civil and Construction Engineers, Third Edition. Copyright © 2011 Pearson Education, Inc.
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Ø What if response is not linear? Ø How do we find the slope (Modulus of Elasticity)?
Strain
Stre
ss
Initial Tangent Modulus
Secant Modulus
Chord Modulus
Tangent Modulus
Mamlouk/Zaniewski, Materials for Civil and Construction Engineers, Third Edition. Copyright © 2011 Pearson Education, Inc.
Typical Moduli and Poisson’s Ratios
Material Modulus (psi x 106)
Poisson’s Ratio
Aluminum 10-11 0.33 Brick 1.5-2.5 0.23-0.40 Concrete 2-6 0.11-0.21 Limestone 8.4 Steel 29 0.27 Wood 0.9-2.2
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Mamlouk/Zaniewski, Materials for Civil and Construction Engineers, Third Edition. Copyright © 2011 Pearson Education, Inc.
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Elastoplastic Behavior Most materials are linear elastic in small stress range
and then plastic § the transition point is elastic limit
Ø Elastic § stretches bonds between atoms without rearranging
them § recoverable deformations (springs back)
Ø Plastic § atomic bonds slip past each other and rearrange § permanent deformations (doesn’t spring all the way
back)
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Ø when unloaded, rebound parallel to the linear portion with some remaining plastic deformation § stretched bonds return, rearranged ones don’t
Ø when reloaded, follows the rebound line and then original curve
Ø strain hardening § stress increases during plastic deformation § reloading returns to previous peak stress
Elastic-perfectly plastic Strain hardening
Mamlouk/Zaniewski, Materials for Civil and Construction Engineers, Third Edition. Copyright © 2011 Pearson Education, Inc.
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Stre
ss
Total Strain
Elastic Strain
Plastic Strain
Force is applied resulting in stress and strain
Strain
When force is removed, stress returns to zero. Path is parallel to the initial slope of the curve. Part of the strain is “recovered,” this is elastic behavior. Part of the strain is permanent, this is plastic behavior.
Elastic Limit
New elastic limit
Reloading will resume to the highest previous stress level. Elastic limit is “reset to the previous highest stress level.”
Response to further loading follows original stress-strain behavior
Mamlouk/Zaniewski, Materials for Civil and Construction Engineers, Third Edition. Copyright © 2011 Pearson Education, Inc.
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What if there’s no clear transition point?
Extension method Offset method
Mamlouk/Zaniewski, Materials for Civil and Construction Engineers, Third Edition. Copyright © 2011 Pearson Education, Inc.
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Elements of Stress-Strain Diagram
Mamlouk/Zaniewski, Materials for Civil and Construction Engineers, Third Edition. Copyright © 2011 Pearson Education, Inc.
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Definitions Ø Proportional Limit
§ transition between linear and non-linear behavior Ø Elastic Limit (Yield Point)
§ transition between elastic and plastic behavior – maximum stress with full recovery
Ø Yielding § strain continues with little or no increase in stress
(after elastic limit) Ø Ultimate Stress
§ maximum stress on the curve (tensile or compressive strength)
Mamlouk/Zaniewski, Materials for Civil and Construction Engineers, Third Edition. Copyright © 2011 Pearson Education, Inc.
Definitions (Cont.) Ø Rupture Stress
§ point where specimen fractures or ruptures Ø Brittle Material
§ has little plastic deformation before failure (glass, concrete)
Ø Ductile Material § has lots of plastic deformation before failure
(structural steel, rubber)
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Mamlouk/Zaniewski, Materials for Civil and Construction Engineers, Third Edition. Copyright © 2011 Pearson Education, Inc.
Viscoelastic Behavior Ø Viscosity: Resistance to flow
(i.e., to shear force) § for linear materials: µ = shear stress/rate of shear strain, unit Pa.s or cP
Ø Viscoelastic materials § have both elastic and
viscous response § have delayed response
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Mamlouk/Zaniewski, Materials for Civil and Construction Engineers, Third Edition. Copyright © 2011 Pearson Education, Inc.
Ø Viscoelastic materials § Deformation depends on o Duration of load o Rate of loading
ü A quick shock or pulse may cause little deformation, while a sustained load can cause much deformation
o Temperature Ø Creep: Long-term deformation under constant load
§ Asphalt concrete creeps § Portland cement concrete creeps over decades
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Mamlouk/Zaniewski, Materials for Civil and Construction Engineers, Third Edition. Copyright © 2011 Pearson Education, Inc.
Ø Rheological models § used to model mechanically the time-dependent behavior of
materials § basic rheological elements
Ø Rheological models are combinations of elements
Maxwell Kelvin
Prandtl
Burgers
Spring St. Venant Dashpot
25 Mamlouk/Zaniewski, Materials for Civil and Construction Engineers, Third Edition. Copyright © 2011 Pearson Education, Inc.
Mamlouk/Zaniewski, Materials for Civil and Construction Engineers, Third Edition. Copyright © 2011 Pearson Education, Inc.
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Temperature & Time Effects Ø Temperature affects mechanical
behavior of all materials § high temp = ductile § low temp = brittle
Ø Impact fracture test measures toughness at different temperatures
Ø Viscoelastic materials like asphalt and polymers are greatly influenced by a change of only a few degrees
Ø Metals require a much greater temperature change but are similarly affected
Mamlouk/Zaniewski, Materials for Civil and Construction Engineers, Third Edition. Copyright © 2011 Pearson Education, Inc.
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Work & Energy Work (or Energy) = force x distance Ø Modulus of Resilience: energy required to reach yield
point
Ø Toughness: energy required to fracture
Mamlouk/Zaniewski, Materials for Civil and Construction Engineers, Third Edition. Copyright © 2011 Pearson Education, Inc.
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Failure and Safety Ø Several ways to fail –
§ fracture or breakage § fatigue (repeated stress) § general yielding § buckling § excessive deformation
Ø For safety, structures are designed to carry loads greater than anticipated
Mamlouk/Zaniewski, Materials for Civil and Construction Engineers, Third Edition. Copyright © 2011 Pearson Education, Inc.
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Endurance Limit
Mamlouk/Zaniewski, Materials for Civil and Construction Engineers, Third Edition. Copyright © 2011 Pearson Education, Inc.
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Factor of Safety Ø FS = (allowable stress / actual stress)
Ø FS is proportional to cost and is chosen by: o cost o material variability o accuracy in considering all loads o possible misuse o accuracy in measuring material response
(good testing?)
FS = allowable
failure
σ
σ> 1
Mamlouk/Zaniewski, Materials for Civil and Construction Engineers, Third Edition. Copyright © 2011 Pearson Education, Inc.
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1.3 Non-Mechanical Properties Other than load responses: Ø Density
Ø Thermal Expansion
Ø Surface Properties
Ø Abrasion & Wear Resistance
Ø Surface Texture
Mamlouk/Zaniewski, Materials for Civil and Construction Engineers, Third Edition. Copyright © 2011 Pearson Education, Inc.
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Density and Unit Weight Ø density = ρ = m / V
Ø unit weight = γ = W / V
Ø specific gravity
wG ρ
ρ=
Mamlouk/Zaniewski, Materials for Civil and Construction Engineers, Third Edition. Copyright © 2011 Pearson Education, Inc.
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Thermal Expansion Ø All materials expand and contract with temperature Ø Linear Coeff. of Thermal Expansion
αL = (ΔL / L0) ΔT
Ø Volumetric Coeff. of Thermal Expansion αV = ΔV / V0 ΔT
§ for isotropic materials αV = 3αL
Ø Stresses develop because of different rates of thermal expansion and contraction for different materials that are connected together § use expansion joints
Mamlouk/Zaniewski, Materials for Civil and Construction Engineers, Third Edition. Copyright © 2011 Pearson Education, Inc.
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Surface Characteristics Ø Corrosion and Degradation Ø Abrasion and Wear Resistance
Ø Surface Texture
Mamlouk/Zaniewski, Materials for Civil and Construction Engineers, Third Edition. Copyright © 2011 Pearson Education, Inc.
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1.4 Production and Construction
Production
§ availability and ability to fabricate material into desired shapes
Ø Construction
§ ability to build the structure on site (trained work force) o High early strength concrete used for early traffic
opening in pavement
Mamlouk/Zaniewski, Materials for Civil and Construction Engineers, Third Edition. Copyright © 2011 Pearson Education, Inc.
1.5 Aesthetic Characteristics Ø The civil engineer is responsible for working with the
architect
Ø The mix of artistic and technical design skills makes the project acceptable to the community
Ø Engineers should understand that there are many factors beyond the technical needs that must be considered
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Mamlouk/Zaniewski, Materials for Civil and Construction Engineers, Third Edition. Copyright © 2011 Pearson Education, Inc.
1.6 Sustainable Design Ø Sustainable design in the philosophy of designing
physical objects, the built environment and services to comply with the principles of economic, social, and ecological sustainability.
Ø The materials used for CE projects are important to the sustainability of the project.
Ø The Green Building Council developed the Leadership in Environment and Energy Design, LEED, building rating system to evaluate the sustainability of the project.
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Mamlouk/Zaniewski, Materials for Civil and Construction Engineers, Third Edition. Copyright © 2011 Pearson Education, Inc.
Sustainable Design (Cont.) Ø For new construction and major renovations the rating
areas include: § Sustainable sites § Water efficiency § Energy and atmosphere § Materials and resources § Indoor environmental quality § Innovation in design § Regional priority
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Mamlouk/Zaniewski, Materials for Civil and Construction Engineers, Third Edition. Copyright © 2011 Pearson Education, Inc.
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1.7 Material Variability Ø All materials have variability
§ Some materials are more uniform than others o Steel vs. concrete vs. wood
Ø Error vs. blunder
Ø Three sources of variance: § Material § Sampling § Testing
Ø Use good sampling and testing techniques to minimize those variabilities
Mamlouk/Zaniewski, Materials for Civil and Construction Engineers, Third Edition. Copyright © 2011 Pearson Education, Inc.
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Ø Precision: measure many times and get same result Ø Bias: tendency to deviate in one direction from true
value
Ø Accuracy: close to true value; absence of bias
Mamlouk/Zaniewski, Materials for Civil and Construction Engineers, Third Edition. Copyright © 2011 Pearson Education, Inc.
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Sampling Proper sampling must ensure that a random and
representative sample is taken from the population (e.g., stockpile, lot, etc.) § Random: have an equal chance of being selected § Representative: perfect average of the entire stockpile
Ø Sample size: § depends on materials variability & tolerance level of
results § more variability dictates a larger sample
Ø Rigorous statistical evaluations required for special applications: § high quality asphalt and Portland cement concrete
Mamlouk/Zaniewski, Materials for Civil and Construction Engineers, Third Edition. Copyright © 2011 Pearson Education, Inc.
Ø Describes many populations that occur in nature, including material properties
Ø Area under the curve between any two values represents the probability of occurrence
Normal Distribution
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Ø Decrease inspection frequency Ø Early detection of troubles
Ø Provide a record of quality
Ø Basis of acceptance
Control Charts
Mamlouk/Zaniewski, Materials for Civil and Construction Engineers, Third Edition. Copyright © 2011 Pearson Education, Inc.
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Experimental Error Caused by 3 factors: Ø Procedural errors
§ Are often undiscovered
Ø Machine errors (bias) § If known and constant can be easily corrected
Ø Human errors § Minimize by repetition, double-checking, etc.
o Always do more than one test
Mamlouk/Zaniewski, Materials for Civil and Construction Engineers, Third Edition. Copyright © 2011 Pearson Education, Inc.
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1.8 Laboratory Measuring Devices Ø Direct
§ Ruler, dial gauge, calipers § Physical & material properties are usually measured
(time, deformation, force, etc.) Ø Indirect
§ LVDT, strain gauge, load cell § measuring changes in electric voltage and relating to
deformation, stress, or strain § must be calibrated
Ø Electronic sensors can be easily connected to digital devices or computers: § CDAS (computerized data acquisition system)
Mamlouk/Zaniewski, Materials for Civil and Construction Engineers, Third Edition. Copyright © 2011 Pearson Education, Inc.
Dial Gauge
LVDT
Strain Gauge
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Proving Ring
Load Cell
Extensometer
Non-Contact Extensometer
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Ø Important considerations: § Sensitivity § Accuracy § Calibration
Ø Sensitivity of measuring devices: § the smallest value that can be read on the device’s
scale § sensitivity is not accuracy or precision § accuracy cannot be better than the sensitivity § When choosing a device, sensitivity depends on the
required accuracy, which depends on the type of test.