The effect of consecutive and gradually increasing strain on the mechanical behavior of rat medial collateral ligaments (MCLs) was studied to characterize and understand the damage evolution process. Displacement-controlled tensile tests were conducted on femur-MCL-tibia complexes (FMTCs) harvested from 9 Sprague Dawley rats. Each FMTC was loaded to a given displacement, unloaded, and re-loaded to another increasing displacement several times until complete failure occurred. From the recorded tensile stress-strain data, two different phenomena indicative of initiation and propagation of damage were observed: an elongation of the toe region and a decrease in stiffness of the linear region. The threshold strain at which the elongation of the toe region first appeared was found to be (2.59±1.37)% while the threshold strain at which the decrease in stiffness first occurred was determined to be (5.11 ± 1.25)%. These results suggest that there are two different mechanisms that control the damage process in ligaments and further investigation is needed to elucidate their micro-structural origin.

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