Mechanical tests Flashcards
What are some mechanical properties?
- Strength
- Hardness
- Ductility
- Brittleness
- Toughness
What are some mechanical tests?
- Tensile tests
- Compressive
- Hardness
- Impact
- Fatigue
- Creep
What is the size effect?
- Bigger volume in a material
- Higher chance of having flaws in material
- Lower strength for same cross-sectional area when you have a bigger volume
What is the tensile test?
- Test that measures the amount of strain acquired depending on the amount of stress applied to a specimen when elongated
- A stress-strain graph is plotted from the test
What is Young’s modulus?
- The slope of the linear region of a stress-strain curve
- Acquired by the ratio of stress / strain
What is Hooke’s Law?
Uses Young’s modulus * strain = stress
What is elastic behaviour?
- Behaviour of a material to return to its original size when load is applied and removed
What is the difference between linear elastic behaviour and nonlinear elastic behaviour?
- Linear follows the same path when load is applied and removed and returns to its original form
- Nonlinear does not follow the same path when load is applied and removed but still returns to its original form
How do you increase the strength of a material?
- Increase stress above yielding stress
- Remove stress to 0
- Material has strain-hardened and its strength has been increased
How do you obtain yield stress (yield strength) for a nonlinear elastic material?
- Start from 0.2% offset of strain (0.002)
- Draw a line tangent to curve at 0
- Find point where your tangent line touches the stress-strain curve
- Find your yield strength
What are some examples of linear elastic materials?
- Steel
- Brittle materials (cast iron, concrete)
What are the strengths and weaknesses of brittle materials?
- They resist poorly to tensile stress
- They have high compressive strength
What are the 3 service conditions affecting behaviour of a failure mode?
- Temperature
- Strain rate
- Degree of triaxiality
How do the 3 service conditions affect a failure mode?
- High temperature = more ductile
- High strain rate = more brittle
- High degree of triaxiality = more brittle
What is necking?
Reduction of a cross-sectional area of a ductile material when subjected to tension
What is the failure mode of a ductile material under tension?
- 45 degrees to the axis of tensile stress
- Necking occurs before failure
What is the failure mode of brittle materials?
- Failure due to crack propagation perpendicular to axis of tensile stress
- No necking because brittle materials are not ductile
What are the 3 tension tests for concrete?
- Direct tension test
- Splitting tension test
- Pressure tension test
What is the barrel effect in a compression test?
- Concrete tries to expand perpendicularly to the compressive stress (Poisson effect)
- Friction forces between the concrete and the steel plates stops concrete expansion
- Concrete is in a barrel shape
What is 3-point bending in flexural testing?
- Bending of a material at 1 point of loading
- Maximum stress at that 1 point
What is 4-point bending in flexural testing?
- Bending of a material at 2 points of loading
- Maximum stress is in between these 2 points
What is the difference between 3-point bending and 4-point bending?
- Maximum stress of 3-point is higher than maximum stress of 4-point
- Only 1 point is under max stress whereas the whole section between the 2 points is under max stress
- Higher probability for failure at 1 point which means strength has to be higher
- 3-point bending better for steel whereas 4-point bending better for concrete since shear stress is negated in 4-point bending
What is hardness?
Ability of a material to resist damage at the surface level
What is the hardness test?
Testing a material’s resistance to an indenter or a cutting tool
What are the types of hardness tests?
- Superficial Rockwell (30kg)
- Common Rockwell (150kg)
- Brinell (10 mm ball at 3000kg)
- Knoop test
- Vickers diamond pyramid
- Nano-hardness
What is an impact test?
Testing a materials resistance to an impact
What is the impact test called and what does it do?
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)
What is fatigue?
Deformation caused by long term repeated stresses on a material
What are the stages of fatigue?
- Crack initiation
- Crack propagation
How does a crack initiate?
- Starts from a flaw in the material
- Starts from a discontinuity on the surface (anything that deviates from a flat surface)
What is the mechanical test for fatigue and what does it give?
- Fatigue test : material undergoes repeated cycles of compression and tension
- Graphs a stress-cycle graph (S-N diagram) that shows how much stress is loaded depending on how many cycles
What is the fatigue limit?
1.Maximum amount of stress without permanently damaging a material at a given number of cycles
What is creep?
Deformation caused by extremely long-term sustained stress on a material
What is a creep curve?
Graph of the tensile elongation of a material under constant load
What conditions affect creep?
- Higher temperatures = more creep
- Higher loads = more creep
