ce 380 lecture 1
TRANSCRIPT
-
8/8/2019 CE 380 Lecture 1
1/11
University of Alabama in Huntsville
Civil and Environmental Engineering
CE 380
Civil Engineering Materials
Spring 2010
Chapter 1Materials Engineering Concepts
-
8/8/2019 CE 380 Lecture 1
2/11
Materials Engineering Concepts
Civil and construction engineers are involved in theselection of construction materials with the
mechanical properties needed for each project.
Selection process must weigh the following factors
Materials ability to carry loads
Economic factors
Mechanical properties (strength and volume stability)
Non-mechanical properties (durability)
Production/construction considerations
Aesthetic properties
Materials used in ConstructionTraditional Materials (most frequent)
Steel
Aggregate
Concrete
Asphalt
Wood
Lesser extent MaterialsAluminum
Glass
Plastics
-
8/8/2019 CE 380 Lecture 1
3/11
Materials used in Construction
High-Performance Materials
Better quality, More economical, and Safer
Improved Materials by
Polymers
Adhesives
Composites
Coatings
Synthetic Products Changing their molecular structures
High-Performance MaterialsSuperplasticizers / Additive / Produces stronger concrete.
Advanced composite materials (High Strength-Weight Ratio)Fiber-reinforced concrete
Epoxy-coated steel reinforcement
Rapid-set concrete patching compounds
Light-weight aggregates
Polymer-modified asphalt
Fire-resistant building materials
-
8/8/2019 CE 380 Lecture 1
4/11
Selection Process Factors
1. Economic Factors
2. Mechanical Properties
3. Non-mechanical Properties
4. Production/Construction Considerations
The availability of the material
The ability to fabricate the material into the desired shapes
and specifications
5. Aesthetic Properties
The appearance of the material
1. Economic FactorsAvailability and cost of raw materials
Manufacturing costs
Transportation
Placing
Maintenance
-
8/8/2019 CE 380 Lecture 1
5/11
2. Mechanical Properties
Mechanical behavior and deformation of materials
depend on
Magnitude and type of load
Material properties
Geometry of the element
Loading ConditionsStatic (Sustained = Dead Loads)
Dynamic (vibration or shock)
Periodic (Rotating equipments)
Random (Never repeats) (Earthquakes)
Transient (Impulse load) (Truck on bridge)
-
8/8/2019 CE 380 Lecture 1
6/11
Stress-Strain RelationsElastic Behavior: Material deforms under loading and
returns to its original shape when the load is removedLinear and Nonlinear
Modulus of Elasticity (Youngs Modulus), E
For a homogenous, Isotropic, and linear elastic material
LLAFE//
L
D
F
F
Stress-Strain Relations (Cont.)
Poissons Ratio
L
D
F
F
F
F
CompressionTension
LL
DD
a
l
/
/
-
8/8/2019 CE 380 Lecture 1
7/11
Plastic Behavior: Permanent deformation of specimen
The atomic bonds stretch, then the atoms slip relative to eachother.
Elastoplastic material exhibits
linear elastic behavior followed
by plastic response.
If the load is removed after the plastic deformation , the stress-
strain will follow a straight line parallel to the elastic portionConsequently, some of the strain in the material will beremoved (elastic strain/recovery) and the remainder of thestrain will be permanent (plastic strain)
Elastoplastic Behavior
Elastoplastic Behavior (Cont.)Proportional limit is the transition point between
linear and nonlinear behavior
Elastic limit is the stress level (yield strength) at
which the behavior changes from elastic to plastic.
Offset method (0.2% strain )
Extension method (0.5% strain)
-
8/8/2019 CE 380 Lecture 1
8/11
Elastoplastic Behavior (Cont.)
Brittle Materials do not undergo plastic deformation
prior to failure (Concrete)
If a brittle material fails, the structure can collapse in a
catastrophic manner
Ductile Materials display appreciable plastic deformation
(Mild Steel)
Preferred for construction
Overloading a ductile material will result in distortions but thestructure will Not necessarily collapse.
Toughness
Energy per volumerequired to fracture aspecimen
Area under the total curve
Modulus of Resilience
Max. energy per volumethat can be elastically
stored by a specimen(absorbed energy thenrecovered upon unloading)
Area under the elasticcurve
Modulus of
Resilience
Toughness
Elastoplastic Behavior (Cont.)
-
8/8/2019 CE 380 Lecture 1
9/11
Failure ModesFracture
Brittle Material fractures when the static stress reaches the maximum
strength the material can carry.Ductile Material may fracture due to excessive plastic deformation
Fatiguesubjected to repeated loadings, creating stresses that are less than thestrength of the material
As the stress level decreases, the number of applications before failureincreases
Bridges and pavements
General Yielding in ductile materials and spreads in the wholestructure which results in a total collapse
Buckling
Excessive deformation
Factor of SafetyThe factor of safety is defined as the ratio of the
stress at failure to the allowable stress for design
(maximum anticipated stress):
failure = Failure stress of the material
allowable = Allowable stress for design
The larger FS, the larger is the required cross
section of the structure (higher costs)
allowable
failureSF
..
-
8/8/2019 CE 380 Lecture 1
10/11
Factor of Safety (Cont.)
The proper value of the factor of safety varies from one structure
to another and depends on several factors:
Cost of unpredictable failure in lives, dollars, and time
Variability in material properties
Degree of accuracy in considering all possible loads applied
to the structure, such as earthquakes
Possible misuse of the structure, such as improperly hanging
an object from a truss roof
Degree of accuracy of considering the proper response of
materials during design, such as assuming elastic response
although the material might not be perfectly elastic
3. Non-Mechanical PropertiesDensity and Unit Weight
Thermal Expansion
L= linear coefficient of thermal expansion
L = change in the length of the specimen
T = change in temperature
L
TL
L
-
8/8/2019 CE 380 Lecture 1
11/11