Lecture 8: Material Properties – Solids Flashcards
What shape do gases, liquids, and solids assume?
gases: assumes the shape and volume of its container
liquids:assumes the shape of the part of the container it occupies
solids: retains a fixed volume
Are gases, liquids, and solids compressible?
gases: yes – free space between particles
liquids: no – little free space between particles
solids: no – little free space between particles
Do gases, liquids, and solids flow easily?
gases: yes
liquids: yes
solids: do not flow
Do gases, liquids, and solids resist compression, tension, and/or shear?
gases: compression
liquids: compression, tension
solids: compression, tension, shear
What is a fluid?
substances capable of flow – gas and liquid
What is compression?
stress generated when an inward force is applied to a material, perpendicular to the surface
What is tension?
stress generated when an outward force is applied to a material, perpendicular to the surface
What is shear?
stress generated when a force is applied to a material, parallel to the surface/object cross-section
What is the hierarchical scheme of solid classification?
(1) materials → (2) structures → (3) systems
What is the non-hierarchical scheme of solid classification (some dichotomous ways to classify materials )?
composition:
- simple: accumulations of only 1 material
- composite: combinations of 2 or more simple materials
directional dependence:
- isotropic: mechanical properties are not directionally dependent
- anisotropic: mechanical properties are directionally dependent
What are tensile mechanical behaviours?
capable of stretching – ie. tendons, ligaments
What are pilant mechanical behaviours?
capable of bending easily
What are rigid mechanical behaviours?
unable to be forced out of shape – ie. bones
What is a universal tester?
used to test the tensile compressive properties of material samples
- produces stress/strain curves
- performs many other tests
What is displacement of a Hookean material (ie. spring) directly proportional to?
the applied load
Why do many biological materials show a J-shaped stress/strain curve?
due to the progressive recruitment of stress-bearing members
What are on the x and y-axes of stress/strain curves?
x: strain
y: stress
Do stress/strain curves show a generalized property of the material?
NO – shows the properties of the particular sample being measured
- thin strip of material would not need as much force to change its length compared to a thick strip – can’t actually see if there’s a difference in properties of the materials because differences are being obscured by differences in size
- if you were to generalize the properties of the material, you need to remove the effect of the sample size from your analysis – then you can say this is a property of the material, not the chunk of the material (sample) you’re dealing with
Stress/Strain Curves
What does converting force to stress, and extension to strain do?
material specific (normalized) properties
- dividing force by cross-sectional area (to get stress) removes any effects of CS area
- converting extension to strain (dimensionless ratio (ie. division) of length due to stretching (delta L) to initial unsure the length (L0)) takes away any effect of length on the sample
- stress/strain curve graph tells you properties of the material itself, and has nothing to do with the piece of material
What material properties do stress/strain curves give? (5)
- stiffness or modulus
- strength
- extensibility
- toughness
- resilience
How is force related to stiffness?
more force needed = more stiff
Stress/Strain Curves – Stiffness (Modulus)
What is Young’s modulus of elasticity (E)?
‘stiffness’ or ‘elastic modulus’ of the material under tension/compression – force needed to change its length
Stress/Strain Curves – Stiffness (Modulus)
How do you determine stiffness from the graph?
slope = Young’s modulus
- steeper slope = stiffer material
Stress/Strain Curves – Strength and Extensibility
What is tensile strength?
stress at failure (breaking stress)
Stress/Strain Curves – Strength and Extensibility
What is extensibility?
strain at failure (breaking failure)
Stress/Strain Curves – Strength and Extensibility
How do you determine tensile strength from the graph?
y-value (stress) at the failure point of the curve
Stress/Strain Curves – Strength and Extensibility
How do you determine extensibility from the graph?
x-value (strain) at the failure point of the curve
Stress/Strain Curves – Compressive Strength
When does deformation occur?
at the point where the graph crosses the yield point
Stress/Strain Curves – Compressive Strength
How do you determine compressive strength and compressibility from the graph?
same as tensile strength and extensibility
Stress/Strain Curves – Toughness (Work of Extension)
What is toughness?
work required to stretch a unit volume of a material to failure
Stress/Strain Curves – Toughness (Work of Extension)
How do you determine toughness from the graph?
integrated area under stress/strain curve = work per volume absorbed as the material extends
Stress/Strain Curves – Resilience
How do you determine resilience from the graph?
work of contraction / work of extension
the larger the difference between the two areas (work of extension and work of contraction), the less resilient the material is – and more of the energy that is put in will be dissipated from the material as heat
What is stress?
force/cross-sectional area (pressure)
What is strain?
measure of energy recovered from elastic storage – dimensionless value expressed as % energy recovered
Stress/Strain Curves – Resilience
Is work of contraction or work of extension greater?
work of contraction is always going to be slightly less than the work of extension ∴ resilience only ever approaches 100%
Why are capture threads of spider silk (viscoelastic) low resilience?
because it needs to absorb the energy from the impact of a flying insect – thread is effective in absorbing energy of the insect’s impact, and not re-releasing it and catapulting the insect away
- insect collides with the thread and imparts a force on it, which causes thread to strain, and changes the length of the thread
- bug slows down, and work of contraction < work of extension
- this is useful for the thread because do NOT want thread that captures energy well and also releases it well (would catapult the insect out of the thread net)
Why are catgut (tennis racket strings) high resilience?
energy needs to be absorbed and released
What is shear stress?
occurs when a force is applied parallel to the surface of an object
not causing it to lengthen or compact, but tilting it over
What is shear strain?
the deformation – ratio of height to how far it is tilted
What is shear modulus?
stiffness of the object under shear
