Fracture roughness

The collapse of the cavity causes stress modification (known as readjustments) in the medium around the chimney, and can generate secondary fracturing. This fracturing can take the form of either vertical fractures, roughly parallel to the walls of the chimney, or radial fractures caused by the subsidence of the geological layers near the chimney. 

Tsang Y W. Witherspoon P A. 1983, The dependence of fracture mechanical and fluid flow properties on fracture roughness and sample size[J], J. of Geophys. Research, 88(B3) 2359-2366. 

Tsang Y W Witherspoon P A., 1982. Correlation between fracture roughness characteristics and fracture mechanical and fluid flow properties[C]. Proc. 23rd US Symp. on Rock Mech., 560-567. 

Melt fracture (Rough surface finish. Also called sharkskin. ) Melt temperature too low Die gaps too narrow Increase melt temperature Heat die lips Increase die gaps Use processing aids... 

For primary insulation or cable jackets, high production rates are achieved by extmding a tube of resin with a larger internal diameter than the base wke and a thicker wall than the final insulation. The tube is then drawn down to the desked size. An operating temperature of 315—400°C is preferred, depending on holdup time. The surface roughness caused by melt fracture determines the upper limit of production rates under specific extmsion conditions (76). Corrosion-resistant metals should be used for all parts of the extmsion equipment that come in contact with the molten polymer (77). 

Close examination revealed that the cracks originate on the external surface. Exposure of the fracture surface revealed a rough contour covered with dark copper oxide. Close examination of the internal surface revealed fewer, tighter fissures corresponding to the locations of cracks on the external surface. 

A relatively smooth region at the bottom of the J (top of photograph) is weld metal. The relatively rough surface that forms the trunk of the J is a fractured %-in. (1.9-cm) plate. 

Figure 15.27 Fracture face (right) contrasted with corresponding, intact weldment. Note that the transition from a rough to a smooth fracture surface occurs abruptly at the junction of the weld metal with the base metal.

There are two, and they are complementary. The tensile fracture strength (Chapter 17) is roughly... 

Fig. 26.7. The fracture toughness of wood, like its other properties, depends primarily on relative density. That across the grain is roughly ten times larger than that along the groin. Both vary as...

The first detailed book to describe the practice and theory of stereology was assembled by two Americans, DeHoff and Rhines (1968) both these men were famous practitioners in their day. There has been a steady stream of books since then a fine, concise and very clear overview is that by Exner (1996). In the last few years, a specialised form of microstructural analysis, entirely dependent on computerised image analysis, has emerged - fractal analysis, a form of measurement of roughness in two or three dimensions. Most of the voluminous literature of fractals, initiated by a mathematician, Benoit Mandelbrot at IBM, is irrelevant to materials science, but there is a sub-parepisteme of fractal analysis which relates the fractal dimension to fracture toughness one example of this has been analysed, together with an explanation of the meaning of fractal dimension , by Cahn (1989). 

Once it is recognized that particles adhere to a substrate so strongly that cohesive fracture often results upon application of a detachment force and that the contact region is better describable as an interphase [ 18J rather than a sharp demarcation or interface, the concept of treating a particle as an entity that is totally distinct from the substrate vanishes. Rather, one begins to see the substrate-particle structure somewhat as a composite material. To paraphrase this concept, one could, in many instances, treat surface roughness (a.k.a. asperities) as particles appended to the surface of a substrate. These asperities control the adhesion between two macroscopic bodies. 

If contact with a rough surface is poor, whether as a result of thermodynamic or kinetic factors, voids at the interface are likely to mean that practical adhesion is low. Voids can act as stress concentrators which, especially with a brittle adhesive, lead to low energy dissipation, i/f, and low fracture energy, F. However, it must be recognised that there are circumstances where the stress concentrations resulting from interfacial voids can lead to enhanced plastic deformation of a ductile adhesive and increase fracture energy by an increase in [44]. 

In recent years there has been a renewed appreciation of potential beneficial effects of roughness on a macroscale. For example Morris and Shanahan worked with sintered steel substrates bonded with a polyurethane adhesive [61]. They observed much higher fracture energy for joints with sintered steel compared with those with fully dense steel, and ascribed this to the mechanical interlocking of polymer within the pores. Extra energy was required to extend and break these polymer fibrils. 

---