Investigation of Microstructure and Stress Corrosion Cracking in Al-6061-T6 Alloy at Different Loads

Stress corrosion cracking (SCC) refers to the damage of mechanical components which are under the combined action of static load and corrosive environment. This phenomenon occurs in various applications including naval and aerospace industry where aluminum and steel alloys experience mechanical loadings in the presence of corrosive environments. In this research work, microstructural and environmental influence on corrosion behavior of Al-6061-T6 at different static loads was investigated. A new test fixture was developed for stress corrosion cracking. Dog-bone shaped tensile specimens of Al-6061-T6 were manufactured using CNC milling machine. Tests were conducted at constant loads of 200 N, 500 N and 800 N, in three different environments: dry ambient conditions, distilled water and 3.5% NaCl solution. Testing continued for different intervals of time i.e. 96 hours, 68 hours and 4.5 hours respectively. After each set of experiments, specimens were observed for cracks using metallurgical microscope. Detailed fractographic investigation of all the tested specimens was carried out using Scanning Electron Microscope (SEM). Excessive corrosion and material degradation was observed in specimens tested in distilled water and 3.5% NaCl environments. Microstructural analysis depicted pitting corrosion and crack deformation.  Some regions clearly showed that grain boundaries were attacked due to oxidation and chemical attack causing weakening of grain boundaries and resulted into intergranular corrosion. Precipitates and grain boundaries in Al-6061-T6 served as a reason of crack initiation due to hydrogen diffusion. Fractographic investigation provided the evidence of trans granular fracture as well as intergranular fracture which was observed as dimples and extensive ductile tearing.

Integrated Circuit Models of Bio-Cellular Networks

The core idea driving this PhD research project is that there is a finite set of bio-chemical processes that are recurrently found in bio-cellular networks; designing compact, efficient and robust electronic models for such basic reactions can lead to faster development of electronic mimetics for larger bioprocesses. In this thesis analog MOS transistor models for three of such fundamental bio-cellular processes have been proposed. It has been shown that natural analogies exist between bio-cellular reaction parameters and device parameters of an electronic transistor, and exploiting them simpler and faster electronic circuit simulators can be designed for bio-cellular processes. The proposed models use lesser count of transistors than the existing researches, hence leading to faster and cheaper execution of the hardware. To further strengthen the idea of modularization, these basic electronic modules have been cascaded with minor adjustments to form a complete bio-cellular pathway. The three non-linear bio-processes modeled on silicon substrate in this research are receptor-ligand binding reaction, Michaelis Menten reaction and Hill kinetics. The corresponding electronic implementations for the first two have been validated against the deterministic ordinary differential equation (ODE) models of the mentioned bio-processes. A new set of analogies between entities of electronic and biological domains have been established. It has been shown mathematically and through simulations that the gate voltage in an electronic transistor controls the saturation of device current the same way an enzyme or receptor contains the inundation of production rate in a bio-reaction; hence drain voltage stands analogous to substrate or ligand concentrations. Also, the characteristic relationship between device current and the two types of terminal voltages, drain and gate, allows a transistor to be used as a bio-concentration multiplier. Another set of equivalent parameters have been validated; the effect of dissociation constant in a receptor-ligand binding and Michaelis constant in a Michaelis Menten reaction is proportional to the effect of channel length in an electronic transistor therefore no external silicon circuitry is required to model these bio-constants, hence significantly reducing the size of the corresponding electronic models. The behavior of these electronic circuit models compares acceptably with that of respective bio-cellular reaction as reported in the standard literature of bio-chemistry and molecular biology. The circuit sizes have been compared with other existing efforts to model the same non-linear processes in electronic domain, and the proposed circuits have been found to use lesser number of transistors producing the same behaviors satisfactorily. For the third process, the Hill kinetics, a couple of electronics models have been proposed. Concrete mathematical and behavioral validation of Hill process is still under research. The proposed silicon models of the basic bio-processes, receptor-ligand binding reaction and Michaelis Menten reaction, have been cascaded with subtle alterations to implement the electronic design of a very vital pathway found in living cells, the cAMP-dependent pathway. For this pathway a deterministic ordinary differential equation model has been derived from the existing literature of bio-chemistry to show that the whole pathway cascade involves only a couple of bio-processes occurring repeatedly; hence can be designed rapidly and efficiently by combining the respective electronic sub-modules. Producing integrated circuit chip level electronic mimetics of bio-cellular processes can be beneficial in two ways. First, these chips can serve as faster bio-electronic simulators as compared to their computational counterparts. They can also be integrated with bio-systems as bio-sensors and bio-system regulators due to the compactness of size and power efficiency. Secondly, these cytomorphic designs, build on the lines of highly computational intensive yet power and space efficient bio-systems can help improve designs of computational mechanics, and even set new design direction in this domain.