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The Effectiveness of Flaw Detection Caused by Cracking Using Acoustic Emission Technique

Several damage mechanisms act as sources of acoustic emission. In general, every mechanism that affects the integrity of the structure or equipment is a source of acoustic emission. This is due to the nature of the technique's principles, which is based on the fact that the material itself emits the signals by releasing energy during the degradation process. Particularly, damages originated from cracking mechanisms are primary sources of acoustic emission, having a high probability of detection (80% to 100%). This fact is actually discussed in various publications. The work presented herein discusses the principles of the technique, the origin of the phenomenon which allows monitoring the nucleation and the initial stages of diverse flaw mechanisms. A correlation with areas of stress concentration given by finite elements modeling is also discussed. Monitoring by Acoustic Emission (AE) is a non-destructive testing method (NDT). The basic principle of the technique is the detection of transient waves generated by the degradation process of the material, which in its turn is caused by the presence of anomalies in this material. These signals, or stress waves, are generated when the material is subject to thermal or mechanical stresses. The two major features of AE technique are the ability to monitor a structure in a non- intrusive global manner, and to locate specific regions in the structure where the anomalies are likely to be found. The advantage of a non-intrusive global monitoring lies in the fact that the equipment or structures are monitored as a whole without significant interference in their operation, thus avoiding unnecessary production interruptions. The advantage of locating specific regions that show evidence of discontinuities, or other anomalies, is that other NDT techniques complementary inspections can be performed in these regions, and not in larger areas of the equipment, reducing costs and stop time to carry out these extra inspections. II. Principle The complete theoretical fundamentals of AE is an arrangement of several areas such ultrasonic, digital signals processing, spectral analysis, materials engineering, fracture mechanics and others. However, the description here is only an explanation of the basic concept of discontinuities growth when the material is subject to mechanical stress. Results of inspection carried out in an equipment or structure using AE become more useful when the properties of the technique, with its function of global detection and without operational interference, are properly understood. The concept of the technique is shown in figure 1.