Biological metabolism in living cells dramatically diminishes at low temperatures, a fact that permits the long-term preservation of living cells and tissues for either scientific research or many medical and industrial applications (e.g., blood transfusion, bone marrow transplantation, artificial insemination, in vitro fertilization, food storage). However, there is an apparent contradiction between the concept of preservation and experimental findings that living cells can be damaged by the cryopreservation process itself. The challenge to cells during freezing is not their ability to endure storage at very low temperatures (less than -180 degrees C); rather, it is the lethality of an intermediate zone of temperature (-15 to -60 degrees C) that a cell must traverse twice--once during cooling and once during warming. Cryobiological research studies the underlying physical and biological factors affecting survival of cells at low temperatures (during the cooling and warming processes). These factors and mechanisms (or hypotheses) of cryoinjury and its prevention are reviewed and discussed, including the most famous two-factor hypothesis theory of Peter Mazur, concepts of cold shock, vitrification, cryoprotective agens (CPAs), lethal intracellular ice formation, osmotic injury during the addition/removal of CPAs and during the cooling/warming process, as well as modeling/methods in the cryobiological research.

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In this study, a damage size characterization algorithm has been developed to continuously obtain the extent of damage, which is vital for further investigations into the remaining life or residual strength of damaged structures. This technique uses an active PZT network with pulse-echo and pitch-catch configurations. In order to facilitate the identification of scattered wave components, a dual-PZT actuation scheme was applied to generate a comparatively pure A0 mode with an enhanced energy. The damage size characterization algorithm starts by identifying the damage location. To this end, relying on temporal information of the scattered signal, a diagnostic image was constructed to highlight the most probable location of damage. Then, as wave scattering occurs from the edges of damage sites, for each sensing path the most probable location of the wave scattering source was estimated and considered as one point on the damage boundary. As a result, the location of some points on the damage boundary are estimated. Since, in practice, the captured signals are usually polluted with noise, a data processing scheme was used to separate points correctly located on the damage boundary from those related to noise. Finally, a convex hull of selected points gives the approximate shape and size of the damage. The approach was validated by defining the location, size and shape of corrosion at its earliest stage of existence. Corrosion severity was also evaluated by obtaining reflection and transmission coefficients, subject to corrosion with different depths. The obtained experimental results demonstrated the potential of the algorithm in providing detailed information about the damage, such as its location, size, shape and severity.

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The detection capability of a given structural health monitoring (SHM) system strongly depends on its sensor network placement. In order to minimize the number of sensors while maximizing the detection capability, optimal design of the PZT sensor network placement is necessary for structural health monitoring (SHM) of a full-scale composite horizontal tail. In this study, the sensor network optimization was simplified as a problem of determining the sensor array placement between stiffeners to achieve the desired the coverage rate. First, an analysis of the structural layout and load distribution of a composite horizontal tail was performed. The constraint conditions of the optimal design were presented. Then, the SHM algorithm of the composite horizontal tail under static load was proposed. Based on the given SHM algorithm, a sensor network was designed for the full-scale composite horizontal tail structure. Effective profiles of cross-stiffener paths (CRPs) and uncross-stiffener paths (URPs) were estimated by a Lamb wave propagation experiment in a multi-stiffener composite specimen. Based on the coverage rate and the redundancy requirements, a seven-sensor array-network was chosen as the optimal sensor network for each airfoil. Finally, a preliminary SHM experiment was performed on a typical composite aircraft structure component. The reliability of the SHM result for a composite horizontal tail structure under static load was validated. In the result, the red zone represented the delamination damage. The detection capability of the optimized sensor network was verified by SHM of a full-scale composite horizontal tail; all the diagnosis results were obtained in two minutes. The result showed that all the damage in the monitoring region was covered by the sensor network.