Acoustic emission inspection
Acoustic emission is an effective method of nondestructive testing and evaluation of materials, based on the detection of elastic waves that are generated by sudden deformation of stressed material. Waves propagate from the source directly to the sensors, where they are converted into electrical signals. Acoustic emission control instruments measure these signals, on the basis of which an assessment of the state and behavior of the structure of the object under study takes place.
Traditional methods of nondestructive testing (ultrasonic, radiation, eddy current) allow us to detect geometric heterogeneities (defects) by radiation into the structure of an object of some form of energy. In contrast to these methods, in acoustical emission control, a different approach is used: non-geometrical inhomogeneities are detected, but microscopic movements are detected. With this approach, even the smallest cracks, faults, inclusions, leaks of gases or liquids are detected - a large number of various processes producing acoustic emission.
From the point of view of the theory and practice of the method of acoustic emission, absolutely any defect can produce its own signal. At the same time, it can pass quite a long distance (up to tens of meters) until it reaches the sensors. Moreover, the defect can be detected not only remotely, but also by calculating the difference in arrival times of the waves to sensors located in different locations.
The main features of the acoustic control method, determining its capabilities and scope:
- Provides detection of defects according to the degree of their danger.
- Has a high sensitivity to growing defects and allows in working conditions to determine the crack growth up to fractions of millimeters.
- The limiting sensitivity of the instruments can theoretically be up to 1 * 10 (-6) mm2.
- Integrity of the method provides control of the whole object using one or several converters fixedly mounted on the surface of the object.
- The method allows you to monitor a variety of technological processes, as well as the processes of changing the properties and state of materials.
- The orientation and position of the object does not affect the detectability of defects.
- A feature of the method that limits its use is the possible, in some cases, difficulty in isolating the necessary signals from interference. If the signals are small in amplitude, then their isolation from the interference is a difficult task.
- With the development of a defect and the approach of its dimensions to a critical value, the amplitude of the signals and the rate of their generation increase sharply. This leads to a significant increase in the probability of finding a defect.
Instruments based on acoustic monitoring methods can be used to diagnose high-load and large-sized objects of increased danger, as well as objects where access to the control surface is limited (some types of pipelines, pressure vessels, boilers, tanks, units). The method is actively used to control a wide variety of objects in the process of their production, with acceptance tests and inspections.