Abstract
Stress relaxation of bovine femur was investigated as a function of temperature. All the relaxation curves measured were described by a linear combination of a Kohlrausch-Williams-Watts (KWW) function and a simple exponential type (Debye) function:
E(t)/Eo=A1 exp [-(t/τ1)β ]+A2 exp (?t/τ2),
A1+A2=1, 0<β≤1
where E0 is an initial value of the relaxation Young's modulus E(0), and A1 and A2 are portions of KWW and Debye relaxations, respectively and τ1 and τ2 are relaxation times of respective relaxations. β increases with temperature which was related to the increase in τ1 with temperature by a phenomenological discussion about a relaxation time distribution. This increase in β was discussed in terms of the pre-transition like phenomenon for the thermal denaturation of collagen molecule. The temperature dependence of τ2 was described by the Arrhenius-type equation. The apparent activation energy for the Debye type relaxation process was estimated to be 3.5 kcal/mol. The Debye type relaxation was attributed to the nucleating process of microcracks at the stress concentrated region. Though the elementary process of the nucleation was considered to be the molecular motion of collagen in bone matrix, the obtained activation energy value is smaller than those of collagen. In the case of stress relaxation by microcrack nucleation, the stress concentration reduces the apparent activation energy, which would be the case for the Debye type relaxation in bone.
