Exam 2

Question 1

Compared to cross-linked polymers, linear polymer chains

Answer 1

have reduced strength and stiffness

Question 2

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)

Answer 2

109 microns

Question 3

The negative ratio of transverse strain to longitudinal strain is

Answer 3

Poisson’s ratio

Question 4

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

Answer 4

????

Question 5

Define endurance limit, what are its units?

Answer 5

Strain below which no fatigue fracture will occur

ksi, MPa

Question 6

Define melting temperature. What materials have a melting temperature?

Answer 6

When a material changes from solid to liquid
Thermoplastics (viscous liquid to nonviscous liquid)
Metallics

Question 7

Define glass transition temperature. What materials have a glass transition temperature?

Answer 7

Temperature when heating an amorphous material goes from glassy solid to viscous liquid

Amorphous and semi-crystal polymers

Question 8

Deine Mode 1 plane strain fracture toughness (K_IC). What are its units?

Answer 8

K_IC = stress intensity factor

MPa*m^(1/2)

Question 9

Identify 3 hardness scales discussed in class

Answer 9

Brinell
Vickers
Rockwell

Question 10

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?

Answer 10

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

Question 11

What phenomenon begins at the ultimate tensile strength of a ductile material?

Answer 11

Necking

Question 12

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)

Answer 12

436 MPa

Question 13

Compared to linear polymer chains, cross-linked polymers

Answer 13

have increased strength and stiffness

Question 14

Define work hardening

Answer 14

Strengthening of a metal or polymer by plastic deformation.

Question 15

What is the difference between a polymer blend and a copolymer?

Answer 15

Blend: secondary bonds between -mers
Copolymer: primary bonds between -mers

Question 16

What is the temperature at which creep is a potential issue for crystalline materials?

Answer 16

?????

Question 17

Define a slip system

Answer 17

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.

Question 18

Define isotropy

Answer 18

The ability of a metial when its’ properties are independent of the direction in which they are measured

Question 19

Define endurance limit

Answer 19

the amplitude (or range) of cyclic stress that can be applied to the material without causing fatigue failure.

Question 20

Define Poisson’s ratio (a labeled equation is acceptable). What are its units?

Answer 20

Equation: on exam

Unitless

Question 21

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?

Answer 21

Creep

Question 22

The ratio of stress to strain below the proportional limit is called the

Answer 22

Young’s modulus

Question 23

Define yield strength. What are its units?

Answer 23

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 ???