Mats lecture 13 & 14 Flashcards
Mechanical properties of Materials, and Beyond Elasticity
What is the modulus of resilience (Er) graphically?
-The area contained under the elastic portion of a stress strain curve
What is the definition of Ductility?
-Is the ability of a material to be permanently deformed “without breaking when a force is applied”
List the common measures of ductility
-The percent elongtion
-The percent reduction in area
What is the difference between engineering stress and true stress?
Engineering stress: force acting on initial area
True stress: Force acting on current area
What is the difference between engineering strain and true strain?
engineering strain: displacement over initial strength
True strain: displacement over current length
In other words, what is engineering strain?
The amount that a material deforms per unit length in tensile test
what is engineering strain also known as?
Nominal strain
In other words, what is true strain?
-It equals the natural log of the quotient of current length over the original length
What happens when engineering strain is small?
Engineering stress is around true stress
In what materials is engineering strain normally small during elastic deformation?
-in most of the metals
In what region do most engineering designs operate?
-in the elastic region
At what point are the equations that relate true and engineering stress and strains valid till?
-Valid up to the necking point
Is the yield strength apparent in metals?
NO
-The transition is very gradual
how much is the offset yield strength for metals?
0.2%
What happens to metals undergoing plastic deformation?
-Their strength increases
-Their strength increases to a maximum (the tensile strength), after which necking leads to fracture
What is the definition of plastic strain
-the permanent strain that results from plastic deformation
What is spring back?
-When the load is removed from a plastically deformed material, there is a recovery called Spring Back
What does ductility mean in the context of plastic deformation?
-Ductility is a measure of how much plastic deformation a material can withstand
-It is simply the strain to failure
Describe the mechanical responses of polymers and what they depend on
-Can range from brittle to Viscous
(depends on both structure and temperature)
Are ceramics suitable for standard tensile tests?
No
-because they are brittle
-They are tested in compression or in bending
List the drawbacks to the tensile test
-It requires machining to prepare specimens
-It is destructive
What does the hardness test measure?
-measures the resistance to penetration of the surface of a material by a hard object
List the main material groups from least hard to most hard
-Polymer materials are exceptionally soft
-metals and alloys have intermediate hardness
-Ceramics are exceptionally hard
What are the most common hardness tests?
-Rockwell test
-Brinell test
Describe what happens in a Brinell hardness test
-A hard steel sphere (10 mm in diameter) is forced into the surface of the material
-The diameter of the impression is used to calculate the HB number
Describe the Rockwell hardness test
-Uses a small diameter steel ball for soft materials and a diamond cone, for harder materials
-The depth of penetration of the indenter is automatically measured by the testing machine and converted to a Rockwell hardness number (HR)
Describe what the Nanoindentation is
-method used to measure local properties of materials
-The imposed load and displacement are continuously measured with micronewton and sub-nanometer resolution
