07 - Mechanical Tests

Question 1

Tests to determine the properties of different materials can be

Answer 1

• destructive
• nondestructive

Question 2

what is size effect

Answer 2

• the larger the test specimen, the more likely it is to have flaws of greater severity
• hence, the more likely it is to exhibit a lower fracture strength

(bigger will fail under a higher load, but not a higher stress)

Question 3

describe the tensile test

Answer 3

• sample is elongated at a constant rate and the load necessary to produce a certain elongation is measured as a dependent variable
• data plotted on a load-elongation or stress-strain curve

Question 4

engineering stress and strain

Answer 4

is the ratio of (load, change in length of speciment) to the (original cross-section, the the original length)

Question 5

The relation between stress and strain is linear for metals and ceramics in which region

Answer 5

elastic region - described by Hooke’s Law stress = Ee

Question 6

for brittle materials, the tensile strength is low or high and why

Answer 6

• brittle materials contain microscopic cracks which propagate relatively easily when they are under tensile stress
• so tensile strengths are low

Question 7

whether a material behaves in a brittle or ductile manner depends on

Answer 7

• atomic or molecular structure
• service condition

Question 8

what are the primary service conditions affecting behaviour

Answer 8

• temperature
• strain rate
• degree of triaxiality

Question 9

If you increase the temperature, decrease stain rate, decrease triaxiality

Answer 9

material is more ductile

Question 10

what is the most common speciment used for tension testing and why

Answer 10

• cylindrical specimen with threaded ends
• bc better stress distribution
• if test result is outside of gauge length then inconclusive

Question 11

during plastic deformation there is no change in what

Answer 11

• the volume of the specimen when measured in the unloaded state
• necking: length increase but A becomes smaller

Question 12

how do ductile materials fail in tension

Answer 12

• by shearing on planes at 45º to the direction of the applied tensile stress
• leads to local contraction of necking of the materials

Question 13

how do brittle materials fail in tension

Answer 13

• due to crack propagation perpendicular to the axis of loading
• because of their lack of ductility there is little to no necking

Question 14

due to the nature of standard tension testing technique, the resulting stresses calculated from these test results (eng stress) are _____ than stresses actually carried by specimen

Answer 14

lower

Question 15

is there necking in compressive testing

Answer 15

no because the A of the specimen increases

Question 16

extremely ductile materials are how often tested in compression?

Answer 16

• rarely
• bc the sample is constrained by friction at the points of contact with the platens of the apparatus

Question 17

what are the potential problems in bending tests

Answer 17

• high localized stresses at loading points (prevent with round support)
• frictional forces at loading points (prevent with round support and round force application piece)
• torsional bending (avoid twisting use square cross-section)

Question 18

what is hardness

Answer 18

• not a fundamental property
• ability to resist permanent deformation of its surface in the form of scratching, indentation, abrasion or cutting

Question 19

what do the hardness tests measure

Answer 19

• resistance of a material to an indenter or cutting tool

Question 20

what happens during hardness test

Answer 20

load is applied by slowly pressing the indenter at a right angle to the surface of the sample for a given period of time

Question 21

for more accurate results from the hardness test what should you do

Answer 21

never take the test near or at the edge of a test specimen

Question 22

what is an impact test

Answer 22

• measures a material’s resistance of damage during an impact event
• empirical in design bc impact events are random by nature
• not a fundamental property

Question 23

what do impact test measure

Answer 23

energy required to fracture a standard specimen under specified conditions

Question 24

what is the charpy test

Answer 24

• uses a notched rectangular bar, which is subject to an impulse load, giving a relative indication of notch toughness of a material under shock loading

Question 25

what is fracture energy

Answer 25

amount of energy that is irreversibily absorbed in the process of fracturing

Question 26

most metals absorb more energy when they fail in ductile or brittle

Answer 26

ductile

Question 27

reason the charpy impact test was developed to assess the transition from ductile to brittle behaviour. transition can be induced by three factors:

Answer 27

• temperature
• strain rate
• triaxiality