Fatigue detection by acoustic emissions

Acoustic emission refers to the generation of transient waves during the rapid release of energy from localized sources within a material. The source of these emissions is closely associated with the dislocation accompanying plastic deformation and the initiation and extension of fatigue cracks in material under stress. Other sources of acoustic emission are: melting, phase transformation, thermal stress, cool down cracking and the failure of bonds and fibres in composite materials.

Acoustic emissions are measured by piezoelectric transducers mounted on the surface of the structure under test and loading the structure. Sensors are coupled to the structure by means of a fluid couplant or by adhesive bonds. The output of each piezoelectric sensor is amplified through a low-noise preamplifier, filtered to remove any extraneous noise and furthered processed by suitable electronic equipment. Multiple sensors may be placed in the region of the source so that the source(s) may be spatially located.

Traditionally, acoustic emissions as a technique has be restricted to the monitoring of high cost structures due to the expense of the monitoring equipment. However, as equipment costs steadily fall, the range of viable applications expands rapidly.

Advantages of acoustic emissions monitoring

Compared to the alternative methods of structure monitoring (stress - rainflow analysis, peak stress monitoring, vibration analysis), acoustic emissions provide a direct measure of failure mechanisms in action. The method has:

• high sensitivity
• real-time
• localization of failure zone by time of arrival measurement
• non-invasive
• the science of acoustic emissions monitoring has yet to mature. The day when a monitoring device can be attached to a structure and useful information about the structure be obtained are a long way off.

Disadvantages of acoustic emissions monitoring

The disadvantage of acoustic emission monitoring is that commercial acoustic emissions systems can only estimate qualitatively how much damage is in the material and approximately how long the components will last. So, other methods are still needed to do more thorough examinations and provide quantitative results.

Operating environments are often very noisy, and the acoustic emission signals are usually very weak. Thus, signal discrimination can be very difficult, yet extremely important for successful applications.

Although acoustic emissions have been used in materials-related studies for about four decades, many problems still exist. The most important difficulty is associated with the reliability of acoustic emissions results.

Applications of acoustic monitoring

The applications of acoustic emissions are extensive. The complexity, material and size of a structure determines the number of sensors required. For example, a 15m diameter steel pressure sphere can be monitored with 30 sensors. A complex aircraft wing spare attachment point constructed in aluminum may require 50 sensors. Applications include:

• aircraft life estimation
• pressure vessel testing
• structural integrity testing
• production quality control
• materials testing
• concrete corrosion monitoring
• pipeline monitoring
• bridge monitoring
• wind turbine monitoring
• ship hull monitoring
• leak detection
• mine wall stability
• earthquake prediction

The last application is relatively new and involves seismic emissions from under ground. While not yet successful in prediction, the method shows promises.