Crack initiation testing

Crack initiation testing is used extensively for testing fatigue in terms of the number of cycles it takes for brittle material to fracture completely. The data are interpreted in the form of a cyclic stress amplitude vs. cycles to failure (S-N) curve. The same concept has been applied to study the crack initiation stress of gluteraldehyde fixed heart valve tissues (Vesely et al., 1995). First the crack initiation stress for a monofilament of a particular material is determined on the virgin suture before the barbs are inserted. Once the number of cycles it takes to initiate a crack in the virgin monofilament is known, we can then compare this result with the number of cycles it will take to initiate a crack at the cut point in the same barbed suture. This will provide evidence of the life expectancy of such a barb in vivo. This is crucially important, given that the loads exerted by different tissues are different, and therefore the number of cycles it takes to initiate a crack in a barbed suture will differ from tissue to tissue. Obviously, the in vivo performance will also depend on the amount... 

The crack initiation test was performed for silica, Tempax, and soda-lime glasses in water and in dehydrated heptane by using the scratch tester. Figure 5 shows an... 

Figure 6. Photos of a scratch groove after the crack initiation test for soda lime glass in heptane. Scacatch direction is from bottom to top in these photos.

Separated Anode/Cathode Realizing, as noted in the preceding, that locahzed corrosion is usually active to the surrounding metal surface, a stress specimen with a limited area exposed to the test solution (the anode) is elec trically connec ted to an unstressed specimen (the cathode). A potentiostat, used as a zero-resistance ammeter, is placed between the specimens for monitoring the galvanic current. It is possible to approximately correlate the galvanic current 7g and potential to crack initiation and propagation, and, eventually, catastrophic fail-... 

The surface finish of the specimen may also affect impact behaviour. Machined surfaces usually have tool marks which act as stress concentrations whereas moulded surfaces have a characteristic skin which can offer some protection against crack initiation. If the moulded surface is scratched, then this protection no longer exists. In addition, mouldings occasionally have an embossed surface for decorative effect and tests have shown that this can cause a considerable reduction in impact strength compared to a plain surface. 

Times to failure for various stainless steels tested in MgClj have been shown to increase with increasing proportions of martensite present Perhaps the role of martensite under anodic dissolution conditions is comparable to that of ferrite in duplex stainless steels where the enhanced dissolution of one phase prevents crack initiation in the other. There is, of course, another aspect of martensitic transformation that should be mentioned, i.e. the transformation of austenite to martensite either in the bulk material or at a growing crack tip that can give increased susceptibility to... 

Pre-cracked specimens are sometimes useful for other reasons than the analysis that they afiford in relation to stress-intensity factors. Such applications may be associated with the simulation of service situations, the relative ease with which stress-corrosion cracks can be initiated at pre-cracks or the advantages that sometimes accrue from the propagation of a single crack. The claim that has sometimes been made of pre-cracked specimen tests-that they circumvent the initiation stage of cracking in plain specimens, erroneously assumed invariably to be related to the creation of a corrosion pit that provides a measure of stress concentration approaching that... 

Fatigue crack initiation, in metal-matrix composites, 16 185 Fatigue crack initiation sites, 13 481 Fatigue crack propagation tests, 16 185-187... 

There is now one international standard for flex testing, ISO 1322 which covers both tests for crack initiation and for cut growth. Previously, these were separated into two standards. The different significance of the two... 

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