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Basic Science > Biomechanics > Flashcards

Biomechanics Flashcards

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1
Q

What is an isotropic material?

A
  • The material behaves similarly in all directions of force
  • e.g. metals / alloys
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2
Q

What is an anisotropic material?

A
  • A material shows directionally dependent behaviour e.g most living tissue. -cortical bone
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3
Q

Define stress ?

A
  • Force per unit area
  • In Newton / metre 2
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4
Q

What is strain?

A
  • Change in length of a material/ original length
  • it has NO UNITS
  • doesn’t take into account x sectional area of material
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5
Q

What is young’s modulus?

A
  • Stress /strain
  • N/m2
  • Gradient of stress/ strain graph
  • idea of stiffness I material
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6
Q

What is stiffness?

A
  • Deflection under a given load.
  • The steeper the stress- strain curve the stiffer the material .
  • The less steep the curve the more flexible the material
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7
Q

What is the Yongs modulus of

cartilage

Tendon

Cancellous bone

UMWPE

Pmma

bone cement

Cortical bone

Ti alloy

Stainless steel

Colballt chrome

A

cartilage 0.02

0.5 Cancellous bone

1 UMWPE

Pmma bone cement 2

Cortical bone 20

Ti alloy 100

Stainless steel 200

Cobalt chrome 200

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8
Q

What is hook’s law

A
  • Where stress is proportional to strain such that deformation is recoverable- elastic portion of graph
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9
Q

What is yield stress ?

A
  • The stress necessary to produce a specific amount of permanent deformation
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10
Q

What is yield point?

A
  • Point in the graph where plastic deformation starts-
  • the point at which further deformation is no longer recoverable
  • In ortho this is close to the yield stress
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11
Q

What’s is strain hardening?

A
  • Where plastic deformation actually increases Materials resistance to further deformation
  • e.g. cold working of metal alloys
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12
Q

What is fracture stress?

A
  • A stress level at which a material’s integrity is breeched and is fractured
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13
Q

What is the ultimate tensile stress?

A
  • The max amount of stress a material can with stand before fracture is imminent
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14
Q

What is brittleness ?

A
  • Brittle materials do not deform plastically but display elastic behaviour right up to failure- e.g ceramic
  • yield stress almost = to fracture stress
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15
Q

What is ductility ?

A
  • A ductile material undergoes a large amount of plastic deformation before failure- e.g.metals
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16
Q

What is strain energy?

A
  • The area under a stress- strain curve.
  • Combines recoverable strain energy - elastic region of curve and absorbed strain energy ( plastic region of curve)
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17
Q

What is toughness?

A
  • The energy per unit volume that a material can absorb before failure.
  • The area under a stress/ strain graph
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18
Q

What is hardness?

A
  • Ability of the material to resist stratching and indentation on the surface
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19
Q

What is fatigue failure ? How is it demonstrated?

A
  • Failure of a material with repetitive loading at stress levels below the ultimate tensile strength
  • In a SN CURVE LOG STRESS VS LOG OF NUMBER OF CYCLES ( millions)
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20
Q

What is the endurance limit ?

A
  • Stress at which a material can withstand 10 million cycles without experiencing Fatigue failure typically hip operate above endurance limit, TKR operate at limit esp polyethylene -> fatigue failure
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21
Q

In regard to materials shape what is stiffness and rigidity ?

A
  • Stiffness-the materials Ability to resist deformation
  • Rigidity- the structures ability to resist deformation
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22
Q

What is notch sensitivity ?

A
  • **Is the extent at which the sensitivity of a material to fracture is increased by the presence of a surface inhomogeneity **
  • eg ductile materials (SS) have low notch sensitivity cf brittle materials such as ceramic/titanium have HIGH notch sensitivity
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23
Q

What is Viscoelasticity?

A
  • Time dependent behaviour which is characterised by
  • CREEP
  • STRESS RELAXATION
  • TIME DEPENDENT STRAIN BEHAVIOUR
  • HYSTERESIS
  • ie in CARTILAGE, LIGAMENTS AND INTERVERTEBRAL DISCS
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24
Q

What is creep?

A
  • Time DEPENDENT DEFORMATION IN RESPONSE TO A CONSTANT LOAD
25
Q

What is stress relaxation ?

A
  • TIME DEPENDENT DECREASE IN LOAD REQUIRED TO MAINTAIN A MATERIAL AT A CONSTANT STRAIN
26
Q

What is hysteresis?

A
  • The difference in a stress strain curve between loading and unloading is due to strain energy lost in heat from internal friction forces
27
Q

What is bending rigidity?

A

Second moment of area x young’s modulus Incorporates the materials shape, size and structure Ie choosing a material that is stiff x2 chrome cf Ti makes a rectangular plate have a increased sma and. So rigidity increased by a third power ie 2 to the power 3 = 8

28
Q

Describe time dependent strain behaviour?

A
  • Behaviour of plastine
  • Gradually pull apart a blob of plastine -> a long thin thread of plastine before it eventually breaks into two
  • however if we pull the plastine apart quickly, then the plastine breaks quickly
  • the strength required to break the plasticine is higher when pulling the plastine apart
  • the rate of change of length of plasticine ( the strain rate) affects the behaviour
  • the faster the strain rate m the higher the stress at a given level of strain
  • conversely a low strain rate requires more time but less stress to fx the material
29
Q

When do tensile stresses occur?

A

When 2 forces pull away from each other along the same line

30
Q

When do compressive stresses occur?

A

When 2 forces push towards each other along the same libe

31
Q

What is Hooke’s Law?

A

The force needed to extend or compress a spring by some distance is proportional to that distance. That is, F= Kx where k is a constant factor characteristic of the spring, its stiffness ie stress proportional to strain- the linear portion of the stress/strain graph where Young’s modulus is constant

32
Q

Draw a stress -strain curve and label the regions?

A

Diagram

33
Q

What is the elastic region?

What can you determine here in this part of the graph?

A

The stress- strain relationship is linear.

Hooke’s law is obeyed- stress proprtional to strain Young’s Modulus- the stiffness increases as the gadient of the line increases ALL the deformation in this part of the graph is ELASTIC ie RECOVERABLE

34
Q

What is the toe region? name an example?

A
  • The initial portion of the curve where there is a no linear relationship uncrimping of collagen fibres in tendons- as the tendon is stretches the collagen unfurl until they are straight and then stiffness increases quickly
35
Q

What is the plastic region of the graph? Where does this commence from?

A

AT this point further deformity is NO LONGER RECOVERABLE ie PLASTIC At the YIELD POINT

36
Q

What is the YIELD POINT?

A

Where there is A DRAMATIC INCREASE IN STRAIN WITH LITTLE INCREASE IN STRESS

37
Q

What follows the yield point?

A

A PEFECT PLASTIC REGION then a region where STRAIN HARDENING occurs

38
Q

What is the STRAIN HARDENING ?

A

The phenomon where PLASTIC DEFORMATION ACTUALLY INCREASES THE MATERIALS RESISTANCE TO FURTHER DEFORMATION e.g.- COLD WORKING OF ALLOYS

39
Q

What is YIELD STRESS?

A

The STRESS NECESSARY TO PRODUCE A SPECIFIC AMOUNT OF PERMANENT DEFORMATION i.e 0.002 - 2%- very close in ortho to the yield point

40
Q

What is strength of a material?

A
  • It represents the DEGREE OF RESISTANCE TO DEFORMATION OF A MATERIAL
  • a material is strong if it has a high ULTIMATE TENSILE STRENGTH
41
Q

What is Hardness of a material?

A

The SURFACE PROPERTY OF A MATERIAL; THE ABILITY OF A MATERIAL TO RESIST STRATCHING AND INDENTATION ON THE SURFACE not determined by the stress strain curve

42
Q

What are the underlying mechanisms responsible for disco-elastic behaviour?

A

FRICTION internally between micro-elements in the structure. Movement of interstitial fluid through the material creates a drag that produced the viscoleastic behaviour

43
Q

What is a shearing force?

A
  • IS A force applied PARALLEL to or in line with the SURFACE OF AN OBJECT
44
Q

When do SHEAR FORCES occur?

A
  • 2 FORCES are directed to parallel to each other but not along the same line or the same direction
45
Q

When do tensile stresses occur?

A
  • When 2 forces pull away from each other along the same line
46
Q

What is shear modulus?

A
  • Shear modulus = shear stress/ Shear strain
  • shear modulus is between 30-50% of the elastic modulus for most materials
  • bone is weakest against shear forces ( also tensile), while strong in compression
  • Bone tends to fail in shear
47
Q

What is the neutral axis of a beam?

A
  • A cross section area of a beam, there is graduation of stresses from extreme edges of the beam, where the compressive ( posterior) and tensile forces ( anterior) respectively towards the centre of the beam.
  • The midpoint where there is no resulting force is the neutral axis
48
Q

How do we calculate the bending stress?

A
  • Applied force x distance from neutral axis

​ second moment area of material

49
Q

what is the second moment area of material?

What affects it?

A
  • Is a variable that decribes the spatial distribution of a material within a structure
  • the type of material does not affect the SMA
  • Organisation and shape of the material affects SMA
50
Q

What is the second moment area of material for a rectangle?

A
  • The perpendicular distance away from the neutral axis h has a third power effect on SMA
51
Q

what is the second moment area of material for a solid circular cross section?

A
  • The radius ( which the distance from the neutral axis) has a 4th power effect on the SMA
52
Q

What is the effect on SMA with a hollow circular cross section?

A
  • The total SMA = the SMA of the Solid portion - SMA of the missing inner hollow portion
53
Q

What is the larger SMA for a solid or hollow pipe?

A
  • Comparing the SMA of a solid rod to a hollow rod, the SMA is larger for the hollow pipe
  • As a result the hollow pipe deforms less ( ie stronger) than the solid pipe of the same cross sectional area
  • e.g as bone ages the outward appostional growth enlarges the medullary canal and increases the overall girth of the whole bone.
  • this increase the SMA and with it the strength and resistance to fatigue to fx
54
Q

What is bending rigidity?

A
  • SMA x Youngs modulus
  • rigidity then incorporates both the nature of the material and its shape, size and structure
  • e.g doubing the thickness of a rectangular plate -> increase in SMA and increase rigidity by a third power of the multiplication
  • ie doubling plate 23 = 8
55
Q

What is a torsional force?

A
  • Equal and opposite shear forces as applied to a cyclinder constrained in space-> twist= torsional force
56
Q

How do you calculate the shear stress generated by the torque forces at any given point ?

A
  • Shear stress= Applied torque x distance from axis of twist/ polar moment of intertia
57
Q

What is the polar moment of internia?

A
  • Similar to SMA
  • is a variable parameter related to size and shape of a structure but not the material from which it is constructed
58
Q

What is torsional rigidity?

A
  • = PMA x Shear modulus
  • a measure of the resistance of a material in a particular size nad shape to torsional forces
  • so a cylinder the PMA varies to the power 4 radius
  • so IM rod that is tiwce at thick has 24=16 x the rigidity
  • when cf nails of the same diameter & length , cannulation or slotting of the nail decreases the PMA according to the earlier formula
  • so torsional rigidity of slotted nails is lessened although bending ridigity is affected only minimally

Basic Science (31 decks)

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