Macro-structural and mechanical properties of tortoise shells of Nigeria indigenous specie were studied to reveal its evolution as a natural load-bearing protective amour. The shell is revealed in this study to be a natural sandwich composite made of keratin, dorsal cortex, porous bone and ventral cortex. Indications of delamination which help the tortoise to absorb shocks were seen in the keratin. The dorsal and ventral cortexes were seen to be load bearing members with distribution of fibers in a manner that sustain dead load. The porosity seen in the middle porous layer ensures that the carapace is not too heavy for the tortoise to carry thus guarantees ease of locomotion, strength and damping. The average porosity of the studied shells is determined to be 31.0425% which as expected is smaller than the value 48.9% reported for porosity of shell of the turtle Terrapene Carolina in literature. The suture of the shell is revealed to house a zigzag interlocking design that allow slight deformation of the shell under light loads required for respiration, locomotion and metabolism but stiffen under excessive deformation. Stiffness of two specimens was determined to be and for the shells with masses 0.2362 kg and 0.2764Kg respectively. These stiffness values were as expected higher the typical values for those of turtles. The stiffness to mass ratio(k/m) for the two shells were respectively calculated to be and . The conditions established for equivalent synthetic shells to have less stiffness to mass ratio were and respectively. These conditions were shown not to be because of problems of stress concentration and weakness at the joints and delamination of the biological shells.
Tortoise Shell, metabolism, deformation, Stiffness
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