Strain - A measure of the deformation or change in shape produced by stress.

1. Strain - A measure of the deformation or change in shape produced by stress. • Calculated as a change in length or dimension. • SYMBOL: e (The Greek letter epsilon) • FORMULA: • As unit length over unit length strain = change in length/original length • UNITS: metric – cm/cm English – in/in • As a percent strain = (change in length/original length)*100 • UNITS: metric and English - %strain

2. Calculating Strain • EXAMPLE: A tendon with an original length of 5 cm is stretched by 2 mm when a load is applied. Calculate the strain. As unit length over unit length: e = .2 cm/5 cm = .04 cm/cm (Remember that 1 mm = .1 cm) As a percent: e = (.2 cm/5 cm)*100 = 4 % strain

3. Stress-Strain Curve – Relates change in stress to the deformation it produces Contains the following regions: • elastic deformation – The material will return to its original shape when the stress is removed • yield point – The material starts to be permanently deformed. Starts at the elastic limit. • plastic deformation – Material shape is permanently changed. In many cases, less stress is required to produce a certain amount of strain. • failure – The material separates.

4. The Axes for the Stress-Strain Curve stress lb/in2 or N/cm2 strain in/in, cm/cm, or % strain

5. Elastic Deformation • The material retains its structural integrity stress lb/in2 or N/cm2 elastic deformation strain in/in, cm/cm, or % strain

6. Yield Point • The material starts to become permanently changed. Starts at the elastic limit. yield point stress lb/in2 or N/cm2 elastic limit elastic deformation strain in/in, cm/cm, or % strain

7. Plastic Deformation • The material is permanently altered. It won’t naturally return to its original shape. plastic deformation yield point stress lb/in2 or N/cm2 elastic deformation strain in/in, cm/cm, or % strain

8. Failure • The material fails structurally (tears or separates). plastic deformation yield point stress lb/in2 or N/cm2 failure elastic deformation strain in/in, cm/cm, or % strain

9. Stress-Strain Relationship A given change in stress (1) produces a given change in strain (A). 1 A

10. Stress-Strain Relationship In the elastic region, equal changes in stress (1 and 2) will yield the same changes in strain (A and B). 2 1 A B

11. Stress-Strain Relationship In the plastic region, a change in stress (3) equal to the prior changes (1 and 2) may produce a greater change in strain (C). 3 2 1 A B C

12. Terms Related to Stress and Strain • Stiffness – The resistance of a loaded material to deformation. The degree of slope in the stress-strain curve indicates the degree of stiffness. Bone is a stiff material. • Pliability – The ease of deforming a material. • Ductility – The ability of a material to undergo deformation or strain before failure. Skeletal muscle is fairly ductile. • Brittleness – The tendency of a material to break with little deformation.

13. Terms Related to Stress and Strain • Creep – The increase in strain over time with a constant loading. This principle is applied when using a series of casts to reshape limbs (clubfoot) or the spinal column (treatment of scoliosis). • Relaxation – A decrease in stress which occurs in a material when a constant deformation is present.

14. Terms Related to Stress and Strain • Resilient – An object which has a tendency to rebound from a surface or another object or to return to its original shape quickly when stress is suddenly applied and then removed. A tennis ball or golf ball would be resilient. Lead would be a non-resilient material. • Damped – A material or object which returns to its original shape slowly when stress is removed. The opposite of resilient.

15. Terms Related to Stress and Strain • Viscous – A thick material which resists flowing. • Elastic – A material which is resilient. • Viscoelastic – A material which exhibits viscous and elastic qualities. Muscle and many other tissues show viscoelastic characteristics. Viscoelastic materials undergo hysteresis. This means they lose energy when a load is removed.

16. Load and Injury • The body is designed to adapt to certain levels of mechanical stress. • This adaptation is necessary for normal development of strength and structural integrity. • A certain amount of loading beyond that which is usually encountered allows individuals to improve such qualities as strength and endurance.

17. Load and Injury • Most injuries occur as a result of mechanical stress on the body. • Injuries take place when the magnitude and frequency of the stress are too high to allow adaptation to take place.

18. Load and Injury Repetitive Loading – Usually a lower level load that occurs repeatedly over an extended period of time (as in running). • Produces microtrauma (small injuries that are usually inconsequential by themselves) • Microtrauma at any specific location can have a cumulative effect • The cause of fatigue injuries (such as stress fractures and many tendon ruptures)

19. Load and Injury Acute Loading – A large force which greatly exceeds the stress which the body is conditioned or designed to handle. • Produces macrotrauma (large injuries) • The cause of traumatic injuries as seen in falls, car accidents, etc.

20. Relationship Between Frequency and Magnitude of Loading It takes the right combination of load magnitude and load frequency to produce an injury. As load magnitude increases, less frequency is required to produce an injury. Load Magnitude Likelihood of Injury Text p 74 Frequency of Loading