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factors responsible for micro-damage of equine tendons are cross-sectional area and collagen

content

(Riemersma and Schamhardt, 1985), composition of extra-cellular matrix (Jones and Boe, 1990),

longitudinal heterogenecity (Smith et al.,1994), inter fibre differences (Becker et al,. 1994) andelevation of core temperature ( Wilson and Goodship,1994 )

The factor of safety is the measure of design of safety margin of working load

The ultimate load when expressed in terms of unit area is known as tensile strength.

Theoretical value of TS should about 0.1 E. However, the observed TS is always several times

less (Table - 2 and 3). This discrepancy is due to the presence of micro-cracks which reduces the

strength (Epifanov, 1979).

According to Guy (1976), there is initially a tiny internal void, which grows into a micro

crack in tensile strain. Further formation of a crack relieves the elastic stress. As long as the

length of the micro-crack remains below a certain value, energy is required for it to develop.

Further extension of tendon results in a reduction of its energy. Thus, a rise in temperature will

enhance the progress of micro-crack. The cyclic tensile loading of equine tendon has been shown

to result in an elevation of core temperature (Wilson and Goodship, 1994). The cyclic

overloading creates cumulative micro-damage. When the transverse length of this micro-damage

reaches a critical value (say critical length of destructive process or simply critical length of

micro-damage), there is spontaneous rupture of the material. An

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increase in tensile strength decreases in the plastic work done in initiating fracture at the tip of

flaws.

The rate of growth of micro-crack is related with a measurement known as stress

concentration factor (K). It describes the distribution of stresses at the crack tip. When the value

of K achieves a critical value, it is known as fracture toughness, there is catastrophic failure of

the material (Pascoe, 1978). The extensor tendons of fore limb, the gastrocnemius tendon and

Achilles tendon had high tensile strength. Hence, these tendons appeared to be highly susceptible

to develop flaws.

A low but repeated (continuous or intermittent) stress can cause the rupture of the tendon.

This minimum stress is known as fatigue tensile stress. Which is obtained by stress-strain (SN)

curve. Since such study was not conducted, the FTS was assumed to be minimum working load

(= 4 CSA of muscle belly) per unit CSA of tendon. The calculated number of cycles gave an

understanding of tendon failure.

specimen.

From clinical point of view, the critical length of micro-damage and the number of cycles

of fatigue failure are important. The constant (C), appeared in eq (3), is a material property (Guy,

1976). Since the value of C was not known, the value of N was expressed in multiple of C-1

(Table4).

The critical length of micro-damage indicates the maximum transverse damage, which a

tendon can sustain under tensile strain. When the tendon attains the critical length, it ruptures

under the applied stress. The higher the stress the less is the durability of the tendon.