Tribology Flashcards
What are the 2 types of fluids?
Newtonian and Non newtonian
What are the characteristics of a newtonian fluid?
Linear shear stress/strain relationship
Definition of friction?
Resistance to motion of surfaces gliding over one another
Arises from adherence of one surface to another
What is the equation for friction?
F= mu f x w
What influence does the apparent contact area have on friction?
NONE
Define roughness?
Ra = mean height of the asperities
What is viscosity?
Friction between 2 layers within a fluid - resistance to flow
What is shear with regards to fluids?
Rate of deformation of a fluid when subjected to shear stress
What is the characteristic of non-newtonian fluids ie synovium?
Apparent viscosity changes with shear rate
Name the different types of non-newtonian fluids? 5
Pseudo-plastic Dilatant Plastic Thixotropic Rheopexic
What is characteristic of Plastic fluids?
Rigid body at low stress - viscous at high stress (ALL GOLD TOMATO SAUCE)
What is characteristic of DILATANT fluids?
They exhibit shear thickening (Water flower mix)
What is the characteristic of pseudo-plastic fluids?
Shear thinning (Paint)
What are the 2 time dependent non newtonian fluid characteristics?
Thixotropic
Rheopexic
What is a Thixotropic fluid?
Pseudoplastic quality over time (Honey)
What is characteristic of Rheopexic fluid
Dilatant quality over time - rare (industrial materials)
What are the qualities of synovial fluid? 4
- Non- newtonian
- Pseudoplastic
- Thixotropic
- Viscoelastic
What are the 2 broad categories of lubrication
- Boundary
2. Fluid film
How does one distinguish the 2 categories of lubrication?
Lambda ratio
What is the lambda ratio
fluid film:roughness
From what lambda ratio is lubrication considered boundary?
<1
From what lambda ration is lubrication considered as fluid film?
3
What is boundary lubrication?
Monolayer of lubricin/phospholipid
Name the 3 dynamic fluid film lubrication types?
- Hyrdodynamic
- Elastohydrodynamic
- Micro-elastohydrodynamic
Name the 3 static fluid film lubrication types?
- Squeeze film
- Weeping
- Boosted
What is MEHD lubrication?
Asperities are ‘flattened’ out under high loads increasing the lambda ratio
Lubrication in metal on poly?
Boundary
Proposed lubrication in metal on metal?
Fluid film
What factors influence lubrication in prosthetic joints? 5
- Magnitude and direction of loading
- Geometry/ roughness
- Material properties
- Velocity at which the bearing operates
- Viscosity of lubricant
What is radial clearance?
Radius of cup - radius of head
What is the definition of wear?
Unwanted progressive loss of a bearing surface
What are the two broad types of wear?
Mechanical and chemical
Define wettability?
Affinity of a surface material to a lubricant
What is wettability measured with?
Theta contact angle
<45 = hydrophilic
>90 = hydrophobic
Define hardness/notch sensitivity?
Ability of a material to resist scratching
How many types of mechanical wear are there?
4 modes
3 types
What are the 3 types of mechanical wear?
- Adhesion
- Abrasion
- Fatigue
What are the 4 modes of wear
- Wear btw 2 articulating surfaces
- Wear between bearing and non-bearing surfaces (Head with metal cup - poly worn away)
- 3rd body wear
- Two non-bearing surfaces
What are the 2 quantifications of wear?
Volumetric (related to square radius of the head)
Linear (height)
Describe fatigue wear
also called delimitation - failure under ultimate tensile strength due to repeated loading cycles
What is the formula governing wear?
V = LX/H V - volume L - Load X - Sliding distance H - hardness
What is chemical wear/corrosion?
Gradual degradation of a metal by electrochemical attach in electrolytic environment in the body
What is the main feature leading to corrosive wear?
Disruption of the passivation layer
What are the types of local corrosive wear?
- Crevice
2. Pitting
What are the 5 types of general corrosive wear?
- Galvanic
- Fretting
- Stress (Fatigue)
- Inter/intragranular
- Inclusion
6 consequences of wear?
- Synovitis
- Aseptic Osteolysis and loosening
- Systemic distribution (Metallosis)
- Immune reaction (Pseudotumours)
- Increased joint friction
- Misalignment and catastrophic failure
Name the 4 steps of aseptic loosening?
- Particulate/debris formation
- Macrophage activation (TNF a, IL1, IL6)
- Prosthesis micro motion
- Debris dissemination (Increased hydrostatic pressure)
Metal on poly advantages?
- Longest track record
- Low cost
- Most modularity
Metal on poly disadvantages?
- Smaller head - impingement
2. Increased wear vs metal on metal
Best wear properties but worst mech properties of all bearing surfaces?
ceramic on ceramic
Also lowest coefficient of friction
