Exam 2 Flashcards
Compared to cross-linked polymers, linear polymer chains
have reduced strength and stiffness
A zirconia bar, loaded in tension, fractures at an applied stress of 300 MPa. The toughness of the material is 4.7 MPa*m^(1/2). What is the length of the critical interior flaw in the bar (assume geometry factor, Y, is 1.2)
109 microns
The negative ratio of transverse strain to longitudinal strain is
Poisson’s ratio
At the macroscale, under an applied normal stress, yield occurs when sigma=sigma_y. What is the condition for yield at the microscale in crystalline materials
????
Define endurance limit, what are its units?
Strain below which no fatigue fracture will occur
ksi, MPa
Define melting temperature. What materials have a melting temperature?
When a material changes from solid to liquid
Thermoplastics (viscous liquid to nonviscous liquid)
Metallics
Define glass transition temperature. What materials have a glass transition temperature?
Temperature when heating an amorphous material goes from glassy solid to viscous liquid
Amorphous and semi-crystal polymers
Deine Mode 1 plane strain fracture toughness (K_IC). What are its units?
K_IC = stress intensity factor
MPa*m^(1/2)
Identify 3 hardness scales discussed in class
Brinell
Vickers
Rockwell
Consider a tensile test on a thermoplastic material:
a) What is the effect on the measured stiffness of lowering the test temperature?
b) What is the effect of lowering the test rate?
a) Stiffness will greatly increase as the polymer drops in temperature
b) Lowering the test rate allows the possibility for the not continuous strands to slip past each other and break bonds. The polymer will stretch longer the slower E
What phenomenon begins at the ultimate tensile strength of a ductile material?
Necking
A titanium alloy has a fracture toughness of 15.2 MPa*m^(1/2). In a particular component, if the length of the longest internal flaw is 320 microns (10^-6 m), what is the fracture strength of the component (assume geometry factor, Y, is 1.1)
436 MPa
Compared to linear polymer chains, cross-linked polymers
have increased strength and stiffness
Define work hardening
Strengthening of a metal or polymer by plastic deformation.
What is the difference between a polymer blend and a copolymer?
Blend: secondary bonds between -mers
Copolymer: primary bonds between -mers
What is the temperature at which creep is a potential issue for crystalline materials?
?????
Define a slip system
the set of symmetrically identical slip planes and associated family of slip directions for which dislocation motion can easily occur and lead to plastic deformation.
Define isotropy
The ability of a metial when its’ properties are independent of the direction in which they are measured
Define endurance limit
the amplitude (or range) of cyclic stress that can be applied to the material without causing fatigue failure.
Define Poisson’s ratio (a labeled equation is acceptable). What are its units?
Equation: on exam
Unitless
The asphalt that is applied to some roadways is a polymer composite. During the summer, the raods develope a ‘wacy’ surface in heavy traffic areas or loctions where vehicles sit idle during traffic jams, etc. The asphalt does not deform in areas not diectly loaded by traffic. What is the physical basis for the deformation?
Creep
The ratio of stress to strain below the proportional limit is called the
Young’s modulus
Define yield strength. What are its units?
MPa. Multiple definitions
1) Proportional limit: point on stress-strain curve where sigma no longer proportional to E
2) 0.2% offset yield stress: draw a curve with slope E starting from E=.002; where it crosses, the sigma-E curve is equal to sigma_y
3) Upper/lower yield point on sigma/E curve ???
