Mechanical tests

Question 1

What are some mechanical properties?

Answer 1

1. Strength
2. Hardness
3. Ductility
4. Brittleness
5. Toughness

Question 2

What are some mechanical tests?

Answer 2

1. Tensile tests
2. Compressive
3. Hardness
4. Impact
5. Fatigue
6. Creep

Question 3

What is the size effect?

Answer 3

1. Bigger volume in a material
2. Higher chance of having flaws in material
3. Lower strength for same cross-sectional area when you have a bigger volume

Question 4

What is the tensile test?

Answer 4

1. Test that measures the amount of strain acquired depending on the amount of stress applied to a specimen when elongated
2. A stress-strain graph is plotted from the test

Question 5

What is Young’s modulus?

Answer 5

1. The slope of the linear region of a stress-strain curve
2. Acquired by the ratio of stress / strain

Question 6

What is Hooke’s Law?

Answer 6

Uses Young’s modulus * strain = stress

Question 7

What is elastic behaviour?

Answer 7

1. Behaviour of a material to return to its original size when load is applied and removed

Question 8

What is the difference between linear elastic behaviour and nonlinear elastic behaviour?

Answer 8

1. Linear follows the same path when load is applied and removed and returns to its original form
2. Nonlinear does not follow the same path when load is applied and removed but still returns to its original form

Question 9

How do you increase the strength of a material?

Answer 9

1. Increase stress above yielding stress
2. Remove stress to 0
3. Material has strain-hardened and its strength has been increased

Question 10

How do you obtain yield stress (yield strength) for a nonlinear elastic material?

Answer 10

1. Start from 0.2% offset of strain (0.002)
2. Draw a line tangent to curve at 0
3. Find point where your tangent line touches the stress-strain curve
4. Find your yield strength

Question 11

What are some examples of linear elastic materials?

Answer 11

1. Steel
2. Brittle materials (cast iron, concrete)

Question 12

What are the strengths and weaknesses of brittle materials?

Answer 12

1. They resist poorly to tensile stress
2. They have high compressive strength

Question 13

What are the 3 service conditions affecting behaviour of a failure mode?

Answer 13

1. Temperature
2. Strain rate
3. Degree of triaxiality

Question 14

How do the 3 service conditions affect a failure mode?

Answer 14

1. High temperature = more ductile
2. High strain rate = more brittle
3. High degree of triaxiality = more brittle

Question 15

What is necking?

Answer 15

Reduction of a cross-sectional area of a ductile material when subjected to tension

Question 16

What is the failure mode of a ductile material under tension?

Answer 16

1. 45 degrees to the axis of tensile stress
2. Necking occurs before failure

Question 17

What is the failure mode of brittle materials?

Answer 17

1. Failure due to crack propagation perpendicular to axis of tensile stress
2. No necking because brittle materials are not ductile

Question 18

What are the 3 tension tests for concrete?

Answer 18

1. Direct tension test
2. Splitting tension test
3. Pressure tension test

Question 19

What is the barrel effect in a compression test?

Answer 19

1. Concrete tries to expand perpendicularly to the compressive stress (Poisson effect)
2. Friction forces between the concrete and the steel plates stops concrete expansion
3. Concrete is in a barrel shape

Question 20

What is 3-point bending in flexural testing?

Answer 20

1. Bending of a material at 1 point of loading
2. Maximum stress at that 1 point

Question 21

What is 4-point bending in flexural testing?

Answer 21

1. Bending of a material at 2 points of loading
2. Maximum stress is in between these 2 points

Question 22

What is the difference between 3-point bending and 4-point bending?

Answer 22

1. Maximum stress of 3-point is higher than maximum stress of 4-point
2. Only 1 point is under max stress whereas the whole section between the 2 points is under max stress
3. Higher probability for failure at 1 point which means strength has to be higher
4. 3-point bending better for steel whereas 4-point bending better for concrete since shear stress is negated in 4-point bending

Question 23

What is hardness?

Answer 23

Ability of a material to resist damage at the surface level

Question 24

What is the hardness test?

Answer 24

Testing a material’s resistance to an indenter or a cutting tool

Question 25

What are the types of hardness tests?

Answer 25

1. Superficial Rockwell (30kg) 2. Common Rockwell (150kg) 3. Brinell (10 mm ball at 3000kg) 4. Knoop test 5. Vickers diamond pyramid 6. Nano-hardness

Question 26

What is an impact test?

Answer 26

Testing a materials resistance to an impact

Question 27

What is the impact test called and what does it do?

Answer 27

1.Charpy test 2. Indent the material with a v-notch in the middle 3. Use a pendulum at a specific weight and a specific height to hit the material in the middle 4. Material is more ductile when there is a bigger shear plane (black region)

Question 28

What is fatigue?

Answer 28

Deformation caused by long term repeated stresses on a material

Question 29

What are the stages of fatigue?

Answer 29

1. Crack initiation 2. Crack propagation

Question 30

How does a crack initiate?

Answer 30

1. Starts from a flaw in the material 2. Starts from a discontinuity on the surface (anything that deviates from a flat surface)

Question 31

What is the mechanical test for fatigue and what does it give?

Answer 31

1. Fatigue test : material undergoes repeated cycles of compression and tension 2. Graphs a stress-cycle graph (S-N diagram) that shows how much stress is loaded depending on how many cycles

Question 32

What is the fatigue limit?

Answer 32

1.Maximum amount of stress without permanently damaging a material at a given number of cycles

Question 33

What is creep?

Answer 33

Deformation caused by extremely long-term sustained stress on a material

Question 34

What is a creep curve?

Answer 34

Graph of the tensile elongation of a material under constant load

Question 35

What conditions affect creep?

Answer 35

1. Higher temperatures = more creep 2. Higher loads = more creep