Biomechanics unit 2 - biomech of skeletal muscle tissue deck 2 Flashcards
Is the stress and strain on a material independent ?
No - If a force is applied to a material then there will be a resulting deformation (i.e. stress and strain are related to eachother they are not independent)
Name a useful tool used to convey the mechanical properties and behaviour of a material
A stress-strain curve.
What are the X and Y axes of a stress-strain curve
- Strain = x-axis
- Stress = y-axis
What does a stress-strain curve specifically show?
It shows how a material deforms (strain) as it is loaded (stress)
When using stress-strain curves to compare different materials what does this show
It allows us to see which material is relatively more or less stiff, tough, ductile and/or brittle than the other material
What behaviours do all materials exhibit ?
- Stiff
- Tough
- Ductile (ability to deform) &/or brittle.
Although some materials may predominantly exhibit one of these behaviours, they will all exhibit all of these behaviours to a certain degree depending upon the magnitude of the load to which they are subjected. These behaviours can be demonstrated on a stress-strain curve
In a stress-strain curve describe the typical relationship between stress and strain
The stress increases with increasing strain
e.g. pic shows A typical stress-strain curve for cortical bone, as the bone is increasingly deformed it becomes increasingly harder to deform it further.
What are the two main regions of a stress-strain curve ?
The elastic region and the plastic region
What is the point which marks the division between the elastic and plastic regions called and what is the amount of stress and strain at this point called ?
- The division between the two regions is marked by the yield point (A in pic).
- The amount of strain at the yield point is called the yield strain and the amount of stress is called the yield stress.
In the elastic region describe the appearance of the curve and the relationship between stress and strain in this part of the stress-strain curve
- The curve is linear
- The stress is directly proportional to the strain - e.g. if the strain is doubled in the elastic region the stress will also double
Define what is meant by the term elastic behaviour
This is when a bone speciment will return to its original size and shape once the load is removed, this is provided that the bone specimen is not deformed beyond its yield point by the load
In the plastic region describe the appearance of the curve and the relationship between stress and strain in this part of the stress-strain curve
- The curve is not linear.
- In this region the bone yields to the applied load - for a small increase in stress the bone deforms by a large amount.
What happens when a bone speciment is deformed beyond the yield point and what is the term used to describe this
- It will not completely recover its original size and shape when the load is removed - it is permanently deformed
- This is termed plastic behaviour.
Eventually a bone will fracture due to increasing stress/strain, at this point what is the terms given to the stress and strain values here?
- The strain at this point is called the ultimate strain
- The stress is called the ultimate strength or ultimate stress.
State the equation for calculating youngs modulus
Youngs modulus = stress / strain
Youngs modulus = a constant formed by the ratio of stress and strain
What are the SI units of youngs modulus ?
N m-2 (since strain has no units)
What does youngs modulus describe ?
It describes how flexible or stiff a material is
What does a small and a large youngs modulus show ?
A material with a small Young’s modulus requires only a small amount of stress to produce a large strain; i.e. it is flexible.
e.g. rubber has a Young’s modulus of approximately 0.01 GN m-2
A material with a large Young’s modulus requires a large amount of stress to produce a small strain; i.e. the material is stiff.
e.g. diamond has a Young’s modulus of approximately 1200 GN m-2.
Think about them by e.g. 2000/ 1 will give you a large YM where a large amount of stress has been put in to give a small strain ==> stiff, and opposite around for a flexible material e.g. 10/5 small stress to produce a large strain. Can also rearrange the equation to calculate strain and talk about it that way too.
So far we have only considered bones under compressive and tensile loads, list the other types of loads
- Shear
- Bending
- Torsion loads
Define what shear loading is
This is when two forces are acting in opposite directions tend to cause layers within the material to slip or shear
Give an example of shear loading occurring in orthopaedics
- The screw is being sheared by the fracture fixation plate and bone,
- The bone cement is being sheared by the hip prosthesis and bone.
How does the strength of cortical bone compare when a compressive, tensile or shear force is applied ?
Cortical bone can withstand greater compressive loads, followed by tensile loads and then lastly shear loads before it fractures
- Bar chart in shows the ultimate strength of femoral cortical bone in adults for compressive, tensile and shear loadings. The ultimate strength of cortical bone is around 200 MN m-2 in compression, 130 MN m-2 in tension and 70 MN m-2 in shear.
- These values illustrate the relative strength of bone in compression and its weakness in shear.
Are fractures caused by shearing alone common and give an example of one ?
No they are rare
One of these rare examples is an intra-articular shearing fracture of the femoral condyles
Describe how In cantilever bending occurs and give an example of it
This is when one end of the object is fixed and a load is applied to the other end causing the object to bend
e.g. jumping on a diving board
Describe how three point bending occurs and give an example of it
In three point bending three forces are applied to the object
e.g. 2 people on a seesaw
Describe what happens to the two sides of a structure when it is bent
When a structure is bent, one side of it is elongated and the other is compressed.
What lies between the two sides of the structure which is being bent ?
A neutral axis
Does deformation occur along the neutral axis of a structure which is being bent ?
No - think of it as the half way point between one side being elongated and one side being compressed ==> both the elongation and compression balance themselves out ==> no deformation occurs
Describe the different in the location of the neutal axis between a symmetrical structure and an unsymmetrical one
In a symmetrical structure, e.g. the beam shown in Fig A, the neutral axis is along the structure’s geometric centre.
- However, in non-symmetrical structures the path of the neutral axis can be quite complex.
- The shape of the femur causes it to be bent (the femur itself) when it is loaded vertically, for example during standing, with the medial side being compressed and the lateral side elongated. In this case, the neutral axis runs approximately along the centre of the femur as shown in Fig B.
When a bone is subjected to a large bending load it will tend to fracture on what side?
The elongated surface of the bone which is under tension
Note; remember that bone is stronger under compression than under tension
Give an example of a typical bending fracture and state what type of bending this fracture results from
- A typical example of a bending fracture is the so called ‘boot top’ fracture sustained by skiers.
- The boot top fracture is a result of three point bending.
As the skier falls forward over the top of the ski boot a force is exerted on the proximal end of the tibia. As the distal end of the tibia is fixed in the boot, the tibia is bent over the top of the rigid ski boot as shown in Figure 14. If the bending load is large enough, the tibia will fracture.
If the torsional load is excessive then torsional fractures can result, what is the characteristic appearance of a fracture caused by a torsional load ?
Spiral appearance
Describe the distribution of stress and strain within a structure when a torsional load is applied
The stress and strain within the structure are not evenly distributed.
In Fig. B you will see that the centre of the solid bar is not distorted by the applied torsional load - there is a neutral axis extending through the centre of the bar - and the outer surface of the bar is the most distorted. In fact the stress and strain are greatest on the outer surface of the bar.
The material which forms the core of the bar is carrying only a small proportion of the torsional load whilst the outer portion of the bar is carrying the vast majority of the torsional load. This means that if the torsional load on the bar is increased, the resulting fracture will start at the outer surface where the stress and strain are the greatest.
What does the hollow structure of bones maximise ?
Their strength-to-weight ratio
(as the strong cortical bone is on the outer layer which is where the stress and strain on the bone is mainly distributed and therefore no bone in the centre where there isn’t really any stress and strain on the bone ==> decreasing weight of the bone but maxmising strength)
What part of the tibia do most torsional fractures occur and why?
The distal tibia - because the distal cross-sectional area of the tibia is smaller than the proximal cross-sectional area and although the amount of bone tissue is the same, the distal part is less able to resist torsional loads and therefore is most liable to fracture
Refer to stress = force / area
What does combined loading result from?
Irregular geometry of bones, and the combined actions of gravitational forces, muscle forces and ligament forces.
e.g. the irregular geometry of the femur means that when a compressive load is applied to the head of the femur a combined loading results, consisting of compressive and bending loadings
