Mechanical Test B

Question 1

Loading Conditions of Mechanical tests (4)

Answer 1

1. Monotonically Increasing (Static)
2. High Strain Rate (i.e. Impact)
3. Repeated (Fatigue)
4. Sustained (Creep)

Question 2

When and where do fatigue failures occur

Answer 2

Fatigue failures generally occur suddenly, without significant prior
deformation.

Most common in parts subjected to high cyclic loading:
Axles
Drive Shafts
Propeller Shafts & Blades
Crank Shafts

Can also occur in structural elements:

Aircraft wings & fuselage
Structures carrying high live loads (bridges)
Bracing, struts, ties

Question 3

Two basic stages involved in fatigue failure:

Answer 3

1. Crack initiation
2. Crack propagation

Question 4

Where do cracks typically initiate at a free surface at a point of high stress
concentration: (2)

Answer 4

Preexisting flaw
Discontinuity (thread root, hole, geometric change)

Question 5

what are the two basic types of machines

Answer 5

Constant Load
Load cycle remains constant
Strain gradually increases as specimen sustains damage

Constant Displacement
Displacement cycle remains constant
Stresses change as specimen sustains damage

Question 6

Drawbacks of Constant Moment Rotating Bending Machine

Answer 6

• Not suitable for nonzero mean
  stress.
• Specimen must be circular in
  cross-section.

Question 7

where does fatigue failure occur in concrete

Answer 7

paste aggregate interface

Question 8

common types of fatigue loading

Answer 8

alternating- looks like a sin wave
partly reversed- sin wave moved up a little
pulsating- sin wave with base of wave just touching the x axis
pulsating- all above x axis
random

Question 9

what causes random fatigue loadings

Answer 9

seismic
wind
waves

Question 10

Variability in Fatigue Testing Results:

Answer 10

1. For real materials, containing imperfections of various kinds, no two samples are truly identical.
2. It is virtually impossible to reproduce precisely the same test conditions over a large number of tests.

Question 11

problems with the palmgren-miner hypothesis (linear damage rule)

Answer 11

1. assumes that the damage accumulating in each cycle of loading is independent of stress history - not true
2. assumes that there is no effect due to the order in which direct stress levels are applied - not true

Question 12

Stress concentration severity increases with:

Answer 12

• increasing flaw size
• decreasing radius of curvature of flaw tip

Question 13

material properties and fatigue

Answer 13

Grain Size:
Finer grain size improves fatigue resistance.

Surface Finish:
Smooth surfaces less susceptible to crack initiation.

Surface Finish:
Grinding, plating, milling etc. all affect crack initiation potential.

Residual Stresses in Surface:
Tensile - reduce fatigue life
Compressive - improve fatigue life

Question 14

environmental conditions and fatigue

Answer 14

increase temp decreases fatigue life
fatigue limits may disappear at high temperatures
thermal cycling can induce stresses that lead to thermal fatigue if the material is restrained from expanding/contracting.

Question 15

corrosion

Answer 15

Surface pitting provides crack initiation points.

Fatigue limits disappear in corrosive environments.

Question 16

define creep

Answer 16

Creep is known as the plastic deformation of a material which occurs as a function of time when that material is subjected to a constant stress or load under the yield stress.

Question 17

concrete undergoes creep at what temp

Answer 17

Concrete undergoes creep at virtually any temperature
unless fully frozen.

Question 18

A typical creep curve shows four stages of elongation:

Answer 18

(1) instantaneous elongation as the load is applied,
(2) primary or transient creep,
(3) secondary or steady-state creep, and
(4) tertiary creep.

Question 19

Most common in parts subjected to high cyclic loading:

Answer 19

Axles
Drive Shafts
Propeller Shafts & Blades Crank Shafts