Crack initiation limitations

Figure 4 shows results investigations by Sauer and Hara [73], where the crack initiation limit for PMMA is shown in addition to the failure line. Comparing the time period from the beginning of the experiment to crack initiation, it becomes clear that the lifetime is determined by the duration of the crack propagation phase. This is the fundamental assumption of the fracture mechanical description method. 

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-... 

There are two main approaches to avoidance of ozone damage. One is the addition of antiozonants. The more extensively used antiozonants are listed in Table 13-12. These antiozonants have limited solubility in elastomers and tend to bloom to the surface. The antiozonant action is not well understood chemically, but probably involves formation of a protective film. Crack initiation occurs at critical stress, which is strongly... 

Long-term exposure of composites to oxidative environments can have deleterious effects on short-term mechanical behavior, such as resistance to crack initiation. This is particularly true in the case where the composite oxidizes to form an oxide surface scale. Although such reactions can be beneficial in limiting oxidation reactions, when the composite is subsequently cooled to room temperature, the reaction product can be a source of flaws and increase the composite s susceptibility to crack initiation. The following example illustrates this point. 

A minimum roughness of the support surface is also required to produce defect-free membrane layers. In the present context, surface roughness is defined as the average perpendicular (to the surface) distance between peaks and dips in the support surface. As discussed in Chapter 6, several other definitions of roughness can be given and used. The roughness of the support may limit the minimum achievable layer thickness. From a fracture mechanics point of view, surface roughness determines the maximum size and sharpness of flaws which can act as crack initiators (via the stress intensity factor). 

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