Structural properties of tendon

Question 1

what is something we measure in material testing

Answer 1

force, displacement, CSA

Question 2

what do we compute in material testing

Answer 2

stress, strain

Question 3

what can we determine from material testing measurements

Answer 3

stiffness, elastic modulus, hysteresis, fatigue life

Question 4

what occurs during yield during a tendon failure test

Answer 4

microscopic damage at low levels of tendon

Question 5

what is the ultimate tensile stress of a tendon

Answer 5

80-120 MPa

Question 6

what is the ultimate tensile strain of a tendon

Answer 6

12-15%

Question 7

what is the approx youngs modulus of tendon

Answer 7

0.8-2 MPa

Question 8

what percent of the test to failure does the toe region take up (unfolding of collagen)

Answer 8

3%

Question 9

how does a tendon show hysteresis

Answer 9

loading and unloading profiles are not identical
difference in curves represents energy loss through heat and disruption of collagen cross links

Question 10

what does a high strain rate mean for a tendon

Answer 10

increased ultimate tensile stress and strain
increased youngs modulus

Question 11

when is biological tissue best able to carry load

Answer 11

when the loads are applied over very short time durations
- slower loading = more time under load to damage structures

Question 12

what is stiffness degradation in creep

Answer 12

repeated loading cycles applied (force remains the same)
deformation increases with each cycle
stiffness decreases with each cycle (slope of F-x curve decreases)
- reaches steady state after 10-20 cycles

Question 13

what is residual strain in creep

Answer 13

accumulated damage to tendon due to plastic deformation
- mechanical fatigue

Question 14

what is fatigue life

Answer 14

number of cycles of submaximal stress/strain a material can withstand before failure

Question 15

why would failure occur before a materials ultimate stress

Answer 15

reduction in modulus and increase in residual strain

Question 16

how does fatigue life change with higher initial peak strain

Answer 16

less possible cycles until failure

Question 17

how does fatigue life change with lower initial peak strain

Answer 17

more cycles can be withstood before failure

Question 18

what is the relationship between changes in strain and withstood cycles until failure

Answer 18

logarithmic
- small changes in strain = large changes in number of cycles withstood until failure

Question 19

what are the stages of loading leading to failure

Answer 19

• load = stretching of local tendon fires into plastic region
• leads to kinked fibres
• leads to reallocation of load to remaining fibres
• load exceeds these fibres and leads to failure