Engineering topics
Publications and source records attributed to Chern, S. S..
The initiation and propagation of quasi-fracture are discussed. The interrelations among applied stress, temperature, and initiation time in quasi-fracture are examined. The relationship between molecular deformation, microstructure orientation, and quasi-fracture propagation is addressed. Quasi-fracture behavior as a function of time is examined.
The way in which the craze growth behavior is affected by the craze density surrounding the craze is investigated. The craze density is defined as the total number of crazes per unit surface area. It is noted that heretofore the growth rate of a craze has been considered either constant or inversely proportional to time. By allowing for the effect of the surrounding population of crazes, it is found that the craze growth rate is influenced by the local effective stress acting in the vicinity of the craze. It is found that measured data of craze length as a function of time are greatly affected by local interactions. With regard to the craze number, it is found that at high stresses a large number of small crazes becomes visible after a relatively short time, whereas under small stresses a large number of large crazes becomes visible after a long loading time.
The laser speckle method has been found quite useful in obtaining in-plane displacement measurements. It is especially useful if the displacement field in a small region can be effectively determined. By obtaining directly the speckle patterns at higher magnifications, a better distinction of the displacements in the vicinity of a crack tip is possible. In this short report some results are obtained and compared with those calculated from linear fracture mechanics and the finite element method for an aluminum crack.
During the past thirty years, a tremendous amount of research was done on the development of crazing in polymers. The phenomenon of crazing was recognized as an unusual deformation behavior associated with a process of molecular orientation in a solid to resist failure. The craze absorbs a fairly large amount of energy during the crazing process. When a craze does occur the surrounding bulk material is usually stretched to several hundred percent of its original dimension and creates a new phase. The total energy absorbed by a craze during the crazing process in creep was calculated analytically with the help of some experimental measurements. A comparison of the energy absorption by the new phase and that by the original bulk uncrazed medium is made.
In response to a tensile stress glassy polymers exhibit crazing (surface cracks) which is a phenomenon of considerable practical and theoretical importance in the use of many plastics. An attempt is made to demonstrate how craze growth behavior is affected by the craze density (which is defined as the total number of crazes per unit surface area) surrounding the craze.
The development of crazing is not only a function of stress, but also a function of time. Under a simple state of tension, a craze opening displacement is closely associated with the viscoelastic behavior of the original bulk polymer medium in which individual crazes initiate and develop. Within each craze region, molecular orientation takes place when conditions permit, and a new phase of rearranged molecules governs its local behavior. Based upon a time-dependent viscoelastic two-dimensional model, using a computer program the craze opening displacement field has been calculated, time-dependent craze length was also computed by taking into consideration the molecular orientation mechanism and large deformations in the craze region. Examples are given for simple viscoelastic media with simplified stress distributions. It is interesting to find out that the occurrence of crazing may be interpreted in terms of the stability or instability of the constitutive behavior of the bulk polymer.
An induced junction solar cell is fabricated on a p-type silicon substrate by first diffusing a grid of criss-crossed current collecting n+ stripes and thermally growing a thin SiO2 film, and then, using silicon-rich chemical vapor deposition (CVD), producing a layer of SiO2 having inherent defects, such as silicon interstices, which function as deep traps for spontaneous positive charges. Ion implantation increases the stable positive charge distribution for a greater inversion layer in the p-type silicon near the surface. After etching through the oxide to parallel collecting stripes, a pattern of metal is produced consisting of a set of contact stripes over the exposed collecting stripes and a diamond shaped pattern which functions as a current collection bus. Then the reverse side is metallized.
An improved chemical vapor deposition reactor is characterized by a vapor deposition chamber configured to substantially eliminate non-uniformities in films deposited on substrates by control of gas flow and removing gas phase reaction materials from the chamber. Uniformity in the thickness of films is produced by having reactive gases injected through multiple jets which are placed at uniformally distributed locations. Gas phase reaction materials are removed through an exhaust chimney which is positioned above the centrally located, heated pad or platform on which substrates are placed. A baffle is situated above the heated platform below the mouth of the chimney to prevent downdraft dispersion and scattering of gas phase reactant materials.
Formation of large particles on substrate is eliminated by actively exhausting reacted gases. Effluent gas backflow is prevented by pumping in curtain of nitrogen above fresh reactive gases from several directions.