Actual crack characteristic

The actual crack characteristic of the pipe section with a crack length / L = 100 mm, a crack depth 3 mm, and a wall thickness 5 = 8 mm is... 

Fig. 33. The crack trajectory predicted by the finite element analysis using Franc2dl for the adhesive C system studied. The result reflects the overall characteristics of the actual crack trajectory such as the characteristic length of the crack as shown in the SEM micrograph.

Since supercritical fluids were chosen for their ability to penetrate small cracks and crevices, additional tests were performed to evaluate this characteristic. A test cube modeled after a similar fixture fiibricated by Ferranti Aerospace, was developed and manufactured to md in this study. The cube had a number of blind holes, tapped holes, dtannels and crevices to simulate actual hardware. Beryllium, 300 Series stainless steel and aluminum cubes were constructed to simulate the conunon metals found in the instrument. In addition, the sides of the cube were removable to facilitate deposition of the contaminants into these blind holes and crevices and later analysis of cleaning effectiveness. The base of the cube was equipped vrith a scanning electron microscope (SEM) mount so that the cube could be examined directly in the SEM. Figure 3 is a photograph of a test cube. Extensive evaluations with these test cubes indicated that supercritical fluids were indeed effective at removing contaminants from cracks and crewces. 

Equilibrium tests at the local deformation stage of a sample provide adequate changes of external forces to internal efforts of a material to resist with corresponding static development of the main crack. These tests are most appropriate when using bending or tension of the samples, because the fracture process will be defined by development of a unique breakaway type of crack, which allows determination of the actual surface area of the fracture. This means that the tests correctly provide the real physical processes of fracture of a concrete and the principles of nonlinear fracture mechanics with traditional mechanical characteristics of concrete and allows determination of a set of power and energy parameters of the material fracture. 

First, a linear analysis (for holes and cracks, for example) shows much higher stresses (infinite for a crack) than are actually present. This is due to the fact that a linear analysis does not account for the damage or process zone created at the edge of the flaw, which redistributes the stresses to some extent. For this reason, analysis in the presence of flaws using a linear model requires the use of some characteristic or averaging distance [17,18] at which, or over which, stresses are evaluated and compared to the unnotched strength values. 

The dimensions of the RVE are not constant for a material structure, i.e. they may vary with the loading and with development of the crack pattern if the RVE is selected to represent the cracked material in its actual state. Also the RVE may be different for different material characteristics if the focus of material representation is reduced to only one group of its characteristics, then the RVE does not take into account the others. In that sense two different groups of material properties are distinguished structure-sensitive and structure-insensitive ones. 

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