Stress intensity, determination

Crack rates, da/dN, spread over the crack depths, increase in the range 10 to 1000 pm from 10 up to 10 and 10 pm/cyde and most pronounced in high oxygen containing water. Crack rates as a function of stress intensity, determined for steel A 533-B (c.f. 1.5403, 1.6310) for various oxygen contents lie higher than the values of the ASME Section XI reference curve. A test method used to determine the potential at the crack tip of steel (here pressure vessel steel A 533-(B) (UNS K12539, c.f. ... 

The computational process of analysis is hidden from the user, and visually the analysis is conducted in terms of M-02-91 or R6 [6] assessment procedure On the basis of data of stress state and defect configuration the necessary assessment parameters (limit load, stress intensity factor variation along the crack-like defect edge) are determined. Special attention is devoted to realization of sensitivity analysis. Effect of variations in calculated stress distribution and defect configuration are estimated by built-in way. 

In general, a stress intensity faetor, K, ean be determined for the stress eondition at the eraek tip from ... 

The stress intensity faetor ean then be eompared to the fraeture toughness for the material, K., whieh is a property of the material whieh measures its resistanee against eraek formation, where ean be determined direetly from tests or by the equation below (Ashby and Jones, 1989) ... 

As long as the stress intensity faetor is below the fraeture toughness for the material, the eraek ean be eonsidered to be in a stable mode (Norton, 1996), i.e. fast fraeture oeeurs when K = K. The development of a probabilistie model whieh satisfies the above ean be developed and referenee should be made to speeialized texts in this field sueh as Bloom (1983), but in general, the reliability is determined from the probabilistie requirement ... 

Also see Furman (1981) and Haugen (1980) for some elementary examples. For a eomprehensive referenee for the determination of stress intensity faetors for a variety of geometries and loading eonditions, see Murakami (1987). 

Fracture mechanics analysis requires the determination of the mode I stress intensity factor for a surface crack having a circular section profile. Here the circular section flaw will be approximated by a semi-elliptical flaw. 

Step 2. After a contact time t, the material is fractured or fatigued and the mechanical properties determined. The measured properties will be a function of the test configuration, rate of testing, temperature, etc., and include the critical strain energy release rate Gic, the critical stress intensity factor K[c, the critical... 

During fatigue the stress amplitude usually remains constant and brittle failure occurs as a result of crack growth from a sub-critical to a critical size. Clearly the rate at which these cracks grow is the determining factor in the life of the component. It has been shown quite conclusively for many polymeric materials that the rate at which cracks grow is related to the stress intensity factor by a relation of the form... 

The stress corrosion resistance of maraging steel has been evaluated both by the use of smooth specimens loaded to some fraction of the yield strength and taking the time to failure as an indication of resistance, and by the fracture mechanics approach which involves the use of specimens with a pre-existing crack. Using the latter approach it is possible to obtain crack propagation rates at known stress intensity factors (K) and to determine critical stress intensity factors (A iscc) below which a crack will not propagate (see Section 8.9). 

Pressure vessels are subjected to other loads in addition to pressure (see Section 13.4.7) and must be designed to withstand the worst combination of loading without failure It is not practical to give an explicit relationship for the vessel thickness to resist combined loads. A trial thickness must be assumed (based on that calculated for pressure alone) and the resultant stress from all loads determined to ensure that the maximum allowable stress intensity is not exceeded at any point. 

The reorientation of the B—H complex at 100 K complicates the analysis of the stress splitting data. The ratios of the intensities of the stress split components were extrapolated to zero stress to determine the site degeneracies for each stress orientation and hence to deduce the symmetry of the complex (Herrero and Stutzmann, 1988b). A unique configuration could not be found to fit the data for all stress directions it was suggested that the configuration of the complex must depend upon the applied stress. For the [110] stress direction it was proposed that the H is displaced from the trigonal axis in the direction away from the C site, while for [100] stress the H is supposed to be displaced toward the C site. 

Several additional, non-microstructural, inputs are required for the fracture model (i) Particle critical stress intensity factor, KIc. Here, the value determined in a previous study (Klc = 0.285 MPa in )[3] was adopted for all four graphites studied. This value is significantly less than the bulk Klc of graphites (typically -0.8-1.2 MPa rn). However, as discussed in the previous section, when considering fracture occurring in volumes commensurate in size with the process zone a reduced value of Klc is appropriate (ii) the specimen volume, taken to be the stressed volume of the ASTM tensile test specimens specimen used to determine the tensile strength distributions and (iii) the specimen breadth, b, of a square section specimen. For cylindrical specimens, such as those used here, an equivalent breadth is calculated such that the specimen cross sectional area is identical, i.e.,... 

In a first testing series, the fracture behavior of the neat, fully crosslinked epoxy network was studied. A fully unstable crack propagation behavior was observed and the critical stress intensity factor, Kj (0.82 MPaxm ), and the critical energy release rate, Gj (0.28 kj/m ), were determined [87]. These are typical values for highly crosslinked epoxy networks prepared with DGEBPA and aromatic or cycloaliphatic diamines. 

Various test geometries may be used to determine values of the fracture energy, G,c, and stress-intensity factor, KIc, at the onset of crack growth and the more common ones are illustrated in Fig. 1. 

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