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Queen Mary University of LondonQueen Mary University of London
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School of Engineering and Materials Science
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PhD Thesis: Mechanical characteristics of fatigue failure in human tendons

Author: SCHECHTMAN, Helio

Year: 1995

Supervisor(s): Dan Bader

Tendons are fibre reinforced composite materials in which the collagen fibres largely determine their mechanical behaviour. Clinical data suggest fatigue failure as a mechanism of some tendon injuries.
An optimum test protocol was developed for two tendons of human lower limbs, the Extensor Digitorum Longus (EDL) and Extensor Hallucis Longus (EHL). The ultimate tensile strength (UTS), failure strain and tangent modulus were found to be consistent with reported values in the literature. For example, specimens of the EDL produced mean values for UTS of 100 MPa.

Characterisation of the fatigue life of human tendons was undertaken by imposing a square wave cyclic load and determining the number of cycles endured until macroscopic failure. A mathematical model for the fatigue life was derived and found to be of the form S = 101.3 - 14.87 log (N), where S is the maximum tensile stress, normalised with respect to the ultimate tensile strength, and N is the number of cycles to macroscopic failure. The probability of failure at any of the stress levels investigated was adequately described by a Log-Normal or Weibull distribution.

Dynamic characterisation of human tendons by the method of forced oscillations was carried out to establish three distinct modulii. It was observed that both the dynamic and storage modulii increased with imposed static stress in the form of a quadratic function, although the parameters were largely independent of frequency. These parameters were also found to be sensitive to changes due to fatigue.

A model of cumulative damage in fatigue in vivo was derived, which included both stress related and healing terms. This revealed the importance of the latter in the long term avoidance of tendon failure.