Skeletal fragility is an important orthopedic concern including the prevention of osteoporosis, long-term stability of prosthetic implants and stress fractures. Damage in human cortical bone has been implicated as a cause of increased fragility and is thought to initiate bone remodeling. Therefore, characterization of the mechanisms of damage initiation and accumulation in bone is challenging not only from the engineering prospective but also has a potential of revealing new insights in its physiology. The main objectives of this dissertation work were to study the early stages of damage development in human cortical bone and develop a constitutive formulation describing damage behavior.

Laser Scanning Confocal Microscopy was utilized to study damage genesis. It was found that confocal microscopy allowed detection of the early stages of damage development within the lacunae-canalicular network of cortical bone. Based on those observations and a common knowledge about the operation of mechanosensing cells housed within the tissue it was proposed that damage could initiate bone remodeling much earlier than it is currently believed.

In the second portion of this work the evolutionary properties of chosen damage parameters were investigated under 3-point bending loading. The parameters of interest were the stiffness drop and the permanent strain. It was found that shear stresses play profound role in the failure behavior. It was also shown that damage parameter defined as the stiffness drop after successive cycles is a quadratic function of nonlinear strain. In addition a linear relationship was obtained between the permanent and total strain.

The third portion of this work was concerned with the development and validation of a constitutive model for cortical bone based on the continuum damage mechanics. It was demonstrated that bone is a linear viscoelastic material for stress levels below a threshold value. Beyond the threshold it behaves as a viscoelastic damaging material. Subsequently a coupled viscoelasticity-damage formulation was adopted and a model was derived based on thermodynamics of irreversible processes. The model was simplified for one-dimensional uniaxial case and experiments were performed for the model verification.