SCHOOL OF MECHANICAL ENGINEERING SEMINAR Wednesday, July 11, 2012 at 15:00 Wolfson Building of Mechanical Engineering, Room 206

M.Sc. student of Prof. Rami Haj-Ali and Prof. Noam Eliaz

School of Mechanical Engineering, Tel-Aviv University

Silica aerogels have recently stimulated both scientific and engineering research interests because of their combined physical properties, e.g. extremely lightweight along with thermal, electrical and optical unique properties. They consist of an amorphous solid matrix surrounded by nano-scale sized pores, and therefore, they are transparent, highly porous, open cell, and have ultra-low density .

In general, aerogels are highly brittle materials, although they can have a high stiffness to density ratio. Due to their extreme porosity and low overall toughness, there is a major challenge to accurately test aerogels and perform mechanical characterization. The current research proposes and examines new mechanical testing setups for aerogel materials. Towards that goal, the non-contact full-field Digital Image Correlation (DIC) technique is proposed along with the different mechanical setups. One such mechanical characterization is carried out by static compression experiments, known as diametral compression test (Brazilian disk). An inverse mechanics computational scheme that employs both a finite-element (FE) model and a well-known elasticity solution is proposed to post-process the experimental data and calculate selected mechanical properties, such as Young's modulus .

The proposed new experimental procedures will allow investigating the mechanical properties and response (including failure) of different aerogel-based systems. These procedures have the potential to improve the manufacturing process by detecting material imperfections and non-uniform spatial variation in the material composition .

Preliminary results from limited number of coupons show the relation between the density and the Young's modulus to coincide with previously published trends. The proposed iterative inverse-mechanics solution is found to be an effective method for the determination of the Young's moduli. Additional testing setups are needed in order to fully characterize the mechanical properties of aerogels. Designs for additional setups will also be addressed.

M.Sc. student of Prof. Rami Haj-Ali

School of Mechanical Engineering, Tel-Aviv University

Calcific aortic valve disease is the most common heart valve disease in the Western countries, affecting approximately 25% of adults over 65 years. The disease is characterized by calcification of the aortic valve (AV) leaflets (cusps) and thickening the tissue to ultimately reaching a severe stenosis state affecting both the kinematics and blood flow of the AV. Recent research has shown that osteopontin protein responsible for bone growth is associated with the calcified cusps. Therefore, it is important to study the biomechanical behavior of calicified AVs in order to simulate the mechano-biology of this disease.

The current study proposes an innovative mechanical material model for calcification growth in prosthetic and native AVs. This model is inspired from the well-known Wolf's law for bone growth and adaptation specialized to the case of AV tissue calcification following our hypothesis that the calcification process in the cusps is controlled by the history of the mechanical response in the leaflet material. A proposed computational algorithm has been implemented as a material constitutive model in finite-element (FE) simulations of the entire aortic valve in order to predict the calcification progression and the changes in the mechanical response of the AV structure. The calcification model consists of two major parts; the first includes the integration for the evolution rate equation of the local density at each material point, while the second describes the changes in the elastic properties as a result of the evolving density. The model has been implemented in the form of composite layered material with several parameters responsible for initiation criteria, growth rate, and a length-scale of influence distance between the different spatial material points on the leaflet. Parametric results are shown for calcification growth in prosthetic and native AV subject to repeated cyclic loads. The computed growth patterns are compared to clinical observations and known patterns from both native and animal studies. The model is able to predict similar patterns of calcification. Future work is proposed to understand the role and influence of the model parameters and link those to the stages of the disease. In addition symmetric and asymmetric growth in tri and bi-leaflet AVs will be studied.

Mechanical Characterization of Light-Weight Silica Aerogel Materials

Chen Zur