However, the sensors mentioned above are commonly used to monitor the damage in position with no compress loading or evidence of fretting phenomenon. The primary structure of an aircraft is prone to fatigue damage. When a crack-sensitive sensor is mounted in a critical bolt-jointed metallic structure of aircraft to monitor cracking, the sensor is subjected to huge loading transferred by the bolt, which may significantly increase the risk of sensor failure. Although the array sensor like MWM and MWM-array can detect 50-μm long cracks, the available output signal is so tiny that it can hardly be detected. It is a hot topic of research to enhance the available output signal.
To monitor cracks in the metal structure of an aircraft, a kind of change-prone embedded grating eddy current array sensor is proposed, and a crack extension experiment aiming at verifying the performance of the sensor is carried out. The sensor is capable of measuring the crack with the accuracy of 1 mm, and the average error is 4.6 % compared with fracture analysis. A corrosive environment may affect the distribution of the magnetic field, but the purpose of the sensor serves detection successfully. With the help of the washer, the sensor is easy to integrate with the structure to be monitored and the load is transferred by the washer to protect the sensor.
Variations in the stress ratios can significantly affect fatigue life. The presence of a mean stress component has a substantial effect on fatigue failure. When a tensile mean stress is added to the alternating stresses, a component will fail at lower alternating stress than it does under a fully reversed stress.
Because most engineering materials contain discontinuities most metal fatigue cracks initiate from discontinuities in highly stressed regions of the component. The failure may be due the discontinuity, design, improper maintenance or other causes. A failure analysis can determine the cause of the failure.
There are many sources and occurrences of metal fatigue in the chemical and refining industries. They range from low-cycle thermal stresses in an FCCU, to the relentless pressure cycling of a PSA, to the ultra-high cycles of a rotating pump.
A great proportion of metal structures and facilities is subjected to repeated or cyclic loading. From bridges that bear the load of vehicles to airplanes that are impacted by air pressure differentials. Material fatigue is a common failure pattern that is hazardous when not detected.
Scientists also focus on another aspect to emphasize the importance of thoroughly understanding microscopic crack initiation and propagation mechanism. In industry, companies tend to replace many metal parts in fear of fatigue without being certain about the condition of the material. Common tests do not focus on the initial stages of crack development and are conducted in larger samples than those actually used.
Laminations are unwanted discontinuities lying parallel to the pipe surface that are usually marked by a concentration of non-metallic material. The rolling-out of inclusions, blowholes or pipes in the parent material causes them. Typically, laminations are not significant, but they may mask cracks. Surface-breaking laminations can be initiation points for fatigue cracks and hydrogen cracking in the pipe body. Laminations may cause detrimental planar features and cracking in welds when the two coincide, such as SAW weld hot tears due to laminations.
The toes of fusion welds are particularly susceptible to cold cracking due to the microstructures present in the HAZ, as well as the change in shape. This discontinuity in profile acts as a stress raiser to locally increase the applied stress during formation or, later during service, as a stress concentrator for fatigue cracking and the like. From the toe location, and depending on the shape of the underlying weld, the crack may remain in the HAZ, or it can propagate into the parent or weld metal. Toe cracking is of course limited to fusion welds, as only these have the geometry where toes are present. Hence, toe cracking is not present in EFW or ERW pipe. 153554b96e