Application of Acoustic Emission Technology in Crack Monitoring

Application of Acoustic Emission Technology in Crack Monitoring 

As a dynamic detection technology, acoustic emission technology can monitor the activity of internal defects of materials under stress, and can provide real-time or continuous information on the changes of defects with external variables such as load, time, and temperature. It is suitable for online monitoring of industrial processes or prediction of impending damage; it is also suitable for monitoring in environments that are difficult or impossible to access with other methods, such as high and low temperature, nuclear radiation, flammable, explosive and extremely toxic environments. The principle of acoustic emission detection is that the elastic wave of the acoustic emission source propagates to the surface of the material, causing surface displacement that can be detected by the acoustic emission sensor. The sensor converts the mechanical vibration of the material into an electrical signal, which is amplified, processed and recorded, and its waveform or characteristic parameters are displayed and recorded. After data analysis and identification, the mechanism of acoustic emission is evaluated.

In the static fatigue test, the logarithm of the acoustic emission count rate of the hot-pressed SiC sample has a good linear relationship with the logarithm of time. As the stress time increases, the detected acoustic emission count rate of the sample decreases exponentially. When the static load increases further, the sample breaks within the observed time. When the static load intensity sn=98.0 MPa (see Figure 1), the acoustic emission signal of the sample shows signs 30 s before the fracture, that is, the fracture occurs 30 s after the acoustic emission signal of subcritical crack growth appears. The larger σn is, the shorter the time between the appearance of the acoustic emission signal of microcrack growth and the occurrence of fracture. Therefore, the dynamic changes of internal cracks in ceramic materials can be predicted based on the change law of the acoustic emission signal of the sample.

Acoustic emission technology can be used to monitor the generation and expansion of cracks in corundum-mullite ceramic materials under thermal stress. The formation and growth of microcracks in ceramic materials under thermal stress mainly occur during the cooling process, and the peak value of the acoustic emission count rate during the cooling process is about 400 times that of the heating process. The higher the maximum heating temperature Tmax, the more intense the crack expansion during the cooling process, which is reflected in the increase of the acoustic emission signal amplitude as the crack of the ceramic material expands.

Acoustic emission technology is used to monitor the stress corrosion process of CT tensile specimens of hydrogen-permeated aluminum-magnesium stainless steel under the action of MgCl corrosive medium. According to the general law of changes in acoustic emission parameters, three stages of material damage development can be distinguished (see Figure 2). The first stage (AB) is the diffusion of hydrogen through the oxide film on the metal surface; its acoustic emission energy increases sharply, accompanied by continuous and partially separated acoustic emission signals; in the second stage (BC), the internal stress in the metal changes, the speed of acoustic emission signal generation decreases, and the corrosion signal is more uniform than the first stage; the third stage (CD) is a complete fracture and destruction process.