tailieunhanh - The Self-Made Tapestry Phần 6

Đây là một hình ảnh rất đơn giản của gãy xương: cho một điều, nó khẳng định rằng một liên kết luôn luôn phải phá vỡ tại mỗi điểm dọc theo crack với từng stepbut thời gian trong thực tế không có lý do tại sao điều này đã được như vậy nếu căng thẳng là không lớnđủ. | This is a very simplistic picture of fracture for one thing it insists that one bond must always break at each point along the crack with each time stepbut in reality there is no reason why this has to be so if the stress isn t large enough. But all the same the model provides some indication of why cracks might have a fractal branching structure. A better model would make allowance for the fact that bonds can stretch a little without breaking they are not like rigid rods but more like springs. This means that each time a bond breaks it will release stress in the immediate vicinity and the surrounding bonds can relax somewhat. Fracture models that modify the dielectric breakdown picture to allow for bond stretching and relaxation have been developed by Paul Meakin Len Sander and others and they can generate a range of different fracture patterns depending on the assumptions made about bond elasticity and so forth an example is shown in Fig. . This crack has a much less dense network of branches than those generated by the pure dielectric-breakdown model and to my eye it looks much more like the kind of pattern you might finds creeping ominously across the ceiling. The fractal dimension is showing that the crack is less like a two-dimensional cluster and more like a two-dimensional cluster and more like a wiggly line. Fig. Crack formation can be modelled by a modified form of the dielectric breakdown model that allows bonds to stretch and relax. This can generate more tenuous almost one-dimensional branching patterns. Image Paul Meakin University of Oslo. Patterns in the dry season In all of these examples the crack starts at a single point and spreads from there as the material is stressed. But not all cracks are like that. Think of the fragmented hard mud of a dried-up pond during a drought Fig. . What has happened here is that as the wet mud at the pond bottom has become exposed and dried the tiny particles have all drawn closer together and .