Fatigue structural observations

In the case of structural joints, cohesive failure of the adhesive is often observed. Indeed, in many instances, the requirement will be to engineer failure within the bulk of the adhesive layer. Modelling of the adhesive response as a function of water uptake with temperature is carried out to represent the influence of environmental exposure. With fatigued structural joints, by combining the use of fracture mechanics and appropriate modelling tools, the resultant crack propagation rates and consequently durability levels can be predicted as a function of various environmental and mechanical test parameters, such as frequency. Fatigue threshold values can be determined, which are used to predict durability performance. " ... 

Metallic filaments, however, are not always full of such defects. Whiskers with diameters of a few micrometers even approach the picture we have from an ideal crystal. The absence of even intrinsic crystalline defects gives rise to the well-known size effect in the rupture stress. Interestingly, also in the case of a completely disordered crystal structure such as amorphous metals, which apart from a few alloys of complex composition can only be produced in filamentary form, an extremely high resistance to fracture and fatigue is observed. 

At the turn of the century it was still widely believed that, while a metal in its nalural slate is crystalline, after bending backwards and forwards (i.e., the process of fatigue damage), metal loctilly becomes amorphous (devoid of crystalline structure). Isolated observations (e.g., Percy 1864) showed that evidence of... 

The first work reported in this area was an investigation of isotactic polypropylene which was fatigued at a rate of 10 hz with an elongation of one to five percent. The spectra showed no shifts in frequency but reversible intensity changes were observed. The data were interpreted in terms of the presence of a smectic-like structure as a result of the cyclic stress 304i 305>. 

Liquid reactive rubbers were also used for UP and vinyl ester formulations (Suspene et al., 1993 Siebert et al., 1996). Increases in fracture energy and fatigue- crack resistance were reported for some systems, although no significant improvements were observed for some other systems. These different behaviors are probably related to the heterogeneous structure of the matrix (Chapter 7). Toughening mechanisms in three-phase systems are not yet well established. 

To find out possible reasons for the mechanical fatigue, measurements inside the pipe (noise) as well as in pipework and steel structure have verified the vibrations to be very low. Finally to avoid any risk of fatigue, grids supporting the candles have been installed. However, since the installation of this grid a tendency of bridging has been observed, which we did not experience earlier. 

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