Initiation cracking

The toughness of wood is important in design for exactly the same reasons that that of steel is it determines whether a structure (a frame building, a pit prop, the mast of a yacht) will fail suddenly and unexpectedly by the propagation of a fast crack. In a steel structure the initial crack is that of a defective weld, or is formed by corrosion or fatigue in a wooden structure the initial defect may be a knot, or a saw cut, or cell damage caused by severe mishandling. 

Another feature of an extruder is the presence of a gauze filter after the screw and before the die. This effectively filters out any inhomogeneous material which might otherwise clog the die. These screen packs as they are called, will normally filter the melt to 120-150 fim. However, there is conclusive evidence to show that even smaller particles than this can initiate cracks in plastics extrudates e.g. polyethylene pressure pipes. In such cases it has been found that fine melt filtration ( 45 p.m) can significantly improve the performance of the extrudate. 

Sometimes the failure occurs by propagation of a crack that starts at the top and travels downward until the interface is completely debonded. In this case, the fracture mechanics analysis using the energy balance approach has been applied [92] in which P, relates to specimen dimensions, elastic constants of fiber and matrix, initial crack length, and interfacial work of fracture (W,). 

The simultaneous action of cyclic stress alternating tensile and compressive and corrosive attack is known as corrosion fatigue. Corrosive attack can be in the form of pitting. These pits function as notches, acting as stress risers and initiate cracks. Once a crack is formed, the probability of pipe failure is enhanced by further corrosion as corrosion is accelerated by action of stress. The tip of the crack deep within the fracture, the area under the greatest stress, is anodic to the wider part of the crack. As corrosion progresses, the metal at the tip of the crack goes into the solution, the crack deepens and eventually penetrates the wall of the tube. 

The strains needed to initiate cracks in both the annealed and the sensitised materials were obtained using tapered slow-strain-rate specimens and the data are given in Fig. 8.36. As can be seen, there is little temperature dependence of the strain needed to initiate cracks in sensitised material whereas the annealed material was most susceptible to cracking at about 250°C. These results indicate the complicated response of Type 316 stainless steel to applied potential and demonstrate that, even though environmentally-assisted cracking may be generated by severe test methods, in this case the slow-strain-rate test, the results obtained must be used with care. For instance, the cracking of the annealed material at low potentials... 

The application of fracture mechanisms has shown the acceleration of the crack under fretting conditions in the first 100 fim of its growth . Corrosive conditions, e.g. seawater, further accelerate the initial crack growth rate. ... 

The most prevalent approach to achieve long-lasting and nonstaining ozone protection of rubber compounds is to use an inherently ozone-resistant, saturated backbone polymer in blends with a diene rubber. The ozone-resistant polymer must be used in sufficient concentration (minimum 25 phr) and must also be sufficiently dispersed to form domains that effectively block the continuous propagation of an ozone-initiated crack through the diene rubber phase within the compound. Elastomers such as ethylene-propylene-diene terpolymers, halogenated butyl mbbers, or brominated isobutylene-co-para-methylstyrene elastomers have been proposed in combination with NR and/or butadiene rubber. 

Film a pre-decorated film is laid out onto the surface of the part to be decorated. There are a myriad of insert shapes, which are often patented. The insert shape must favour its mechanical anchorage faces, flat sides, grooves, milling, barbs, perforations... that allow the flowing of the thermoplastic. However, sharp angles initiate cracks and the geometric anchors should be located as far as possible from the surface. 

Erosion/corrosion of the weld bead at the fracture location damaged the aluminum oxide film on the piping, thus allowing the mercury to wet and initiate cracking of the aluminium. 

Apparently two opposing effects play a part On the one hand the glass fibres take over a part of the stress, thus partially releasing the stress on the polymer. On the other hand, fibres cause, in particular at their ends, stress concentrations, which can initiate crack formation. 

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