Fatigue detection

Figure 14-18, Potential components that should be integrated to provide drivers with a reliable and valid fatigue detection and alerting system (from Hermann, 2004, with permission from the author and lOS Press).

Load-cycling the materials below the yield exhibits macroscopic and nanoscopic fatigue detected from a decay of the peak stresses and peak nanostructure parameters, respectively. From the respective lifetimes we have for the first time assessed the reinforcement of the composite and the weakening of the PP by the MMT. It has been found that crystallite growth is obstructed even more when MMT is com-patibilized. It remains to be clarified, if this effect is caused by the compatibilizer itself or by improved exfoliation of the MMT. 

YOSHIOKA,T. Detection of Rolling Contact Sub-Surftice Fatigue Cracks Using Acoustic Emission Technique. Lubrication Engineering, Vol.49, No.4, 1993, p.303-308. 

Shear Horizontal (SH) waves generated by Electromagnetic Acoustic Transducer (EMAT) have been used for sizing fatigue cracks and machined notches in steels by Time-of-Flight Diffraction (TOED) method. The used EMATs have been Phased Array-Probes and have been operated by State-of-the-art PC based phased array systems. Test and system parameters have been optimised to maximise defect detection and signal processing methods have been applied to improve accuracy in the transit time measurements. 

For existing ships the only NDE method nominated by classification for the detection of fatigue cracks is close-up visual inspection - although all Surveyors have the option of requesting additional NDE when warranted. The sensitivity of visual inspection is influenced by the degree of surface preparation and the level of lighting at the inspected surface - which may not always meet the level of 500 lux nominated by some NDE specifications. 

Odors are characterized by quaUty and intensity. Descriptive quaUties such as sour, sweet, pungent, fishy, and spicy are commonly used. Intensity is deterrnined by how much the concentration of the odoriferous substance exceeds its detection threshold (the concentration at which most people can detect an odor). Odor intensity is approximately proportional to the logarithm of the concentration. However, several factors affect the abiUty of an individual to detect an odor the sensitivity of a subject s olfactory system, the presence of other masking odors, and olfactory fatigue (ie, reduced olfactory sensitivity during continued exposure to the odorous substance). In addition, the average person s sensitivity to odor decreases with age. 

Several nontechnical factors can significantly affect the results of a nondestmctive inspection. Many of these are classified as human factors (1,2,17). Operator experience affects the probabiUty of detection of most flaws. Typically, an inexperienced operator has more false rejects, known as Type II errors, than an experienced operator. A poor operator has few false rejects but is more likely to miss a defect in the inspection, known as a Type I error. Operator fatigue, boredom, or an unfavorable environment such as lighting, cold, or rain may further affect performance. Thus it usually is a good investment for the inspection company to assure that the operator environment is most amenable to inspection, that the equipment is suitable for the task, and that the operator is alert and well rested. 

Corrosion-fatigue cracks can be detected by nondestructive testing techniques such as magnetic particle inspection, radiography, ultrasonics, and dye penetrant. Corrosion-fatigue cracks may occur in numerous tubes simultaneously. Nondestructive testing of tubes at locations similar to those in which cracks are observed can be useftil. 

It is worrying that a vessel which is safe when it enters service may become unsafe by slow crack growth - either by fatigue or by stress corrosion. If the consequences of catastrophic failure are very serious, then additional safety can be gained by designing the vessel so that it will leak before it breaks (like the partly inflated balloon of Chapter 13). Leaks are easy to detect, and a leaking vessel can be taken out of service and repaired. How do we formulate this leak-before-break condition ... 

At 0.13 ppm by volume, H2S can be sensed by smell. At 4.6 ppm the smell is quite noticeable. As the concentration increases beyond 200 ppm, the sense of smell fatigues, and the gas can no longer be detected by odor. Thus, H2S cannot always be detected by smell. Even if H2S cannot be smelled, it is possible that there is sufficient H2S present to be life threat-... 

Low-frequency noise (in the range 3-50 Hz) may have other injurious effects on the body. Research has also indicated that a type of fatigue caused by low-frequency noise has a similar effect to that caused by alcohol. Infrasound (low-frequency sound) also has a synergistic effect with alcohol. Low-frequency noise is particularly important in the case of workers operating machinery (e.g. vehicles, cranes, etc.). It must also be remembered that very high power levels may be generated at low frequency and may not be readily detected by the ear. Attenuation of low-frequency noise is very difficult (see Section 42.7). 

D ye penetration inspection. This is a simple technique, requiring a minimum of operator training. In the hands of a skilled operator, it is capable of detecting fine cracks such as chloride stress corrosion cracks in austenitic stainless steels and fatigue cracks. 

---