In this thesis, we use IF Steel to be the matrix which composition of carbon is 50ppm approaches Iron and belongs to the part of BCC structure. After we use the load 350kgf precrack, while the propagation rate approaching 2x10-5~3x10-5mm/cycle, we immediately drop the stress intensity factor(£Gk) to a level which is determined by the propagation rate of 9x10-7mm/cycle. Then we record the cycle number as P1, and after the crack tip continues to propagate, we record the cycle number again as P2, and between the period of (P2 ¡V P1), we take 4 sample which marks are 0.25(P2 ¡VP1), 0.5(P2 ¡VP1), 0.75(P2 ¡VP1), (P2 ¡VP1). By this way, we can find out the difference of microstructure of the crack tip from a high load level to a low one. On this experiment, we observation the vein structure, dislocation walls, and so on. It proves that the same phenomenon could also be happened on BCC material, just like FCC material. As time goes by, after drop load, the stored energy is more and more strong. And the vein structure of reverse evolution would become the structure of dislocation cells of normal evolution. Then the crack would continue to propagate on the circumstances.

No / An extensive study of the fibre orientation structures developed in a transverse ribbed plate during injection moulding, and the use of these structures to investigate the effect of local fibre orientation state on crack initiation resistance, is reported. The fibre orientation results for the ribbed plate, measured using large area image analysis system developed at Leeds University, showed that after an initial settling down period, the central core region, where the fibres are aligned perpendicular to the flow direction, decreased in size monotonically, with an associated monotonic increase in the outer shell regions, where the fibres are aligned preferentially along the injection direction. Interestingly, the level of orientation in the two regions remained almost constant: only the proportions of the two regions were found to change with flow length. Across the plate, close to the gate, the central core region was found to have a lens-like shape, while at the other end of the plate the core was thinner and also consistent in thickness across the sample width. The transverse rib was found to cause little disturbance to the fibre orientation of the base plate. The different proportions of the shell and core regions at different locations over the ribbed plate provided an ideal case to test the proposition of Friedrich that the crack resistance of a short fibre reinforced material depends on the number of fibres that are perpendicular to the crack tip. The impact test results gathered in this way confirmed this hypothesis of Friedrich.