ISO 16836:2019 pdf free.Non-destructive testing – Acoustic emission testing- Measurement
method for acoustic emission signals in concrete.
AE sensors shall be sensitive enough to detect AE signals generated in the target structure, taking acoustic coupling into consideration. They convert elastic waves (motions) on the surface of a material into electric signals, preferably, without any distortions. A resonance-type sensor is most sensitive around the resonant frequency, while a broad-band sensor has approximately flat response in the range but is less sensitive than the resonance-type. AE sensor shall be robust enough against temperature change, moisture condition and mechanical vibrations in the environments.
Refer to Annex A for recommended types of sensors to be used in the concrete.
Sensitivity calibration of AE sensors shall be performed by employing the standard source, in addition to the calibration methods prescribed in ISO 12713 and ISO 12714. A simulated AE source due to pencillead break is defined in ASTM E976. This standard source is illustrated in Figure 3, where a guide ring is recommended to be employed.The frequency range shall be determined prior to the measurement, taking into account the performance of AE sensors and the amplifiers. Selection of the frequency rage is closely related to elimination of noises. In concrete, the use of a band-pass filter between several kHz and several 100 kHz is recommended.
One waveform is to be counted as one AE event, while the cycles over the threshold level are named as AE ring-down counts (or simply “counts”). Here, the threshold is a preset voltage level, which has to be exceeded before one AE signal is detected and processed. Methods for AE counting are illustrated in Figure 4. In the case of multi-channel observation, AE occurrence is monitored at each sensor location. At some sensors, AE signals may not be observed due to attenuation or undetectable propagation path. The number of the events which are counted at one of the channels corresponds to the number of AE hits defined above.
7 Setup and measurement
7.1 Sensor setup
AE sensors shall be calibrated properly in advance of the measurement. They are attached at proper locations to cover the target area. The number of AE sensors and the period of the measurement shall be prescribed, depending on the following conditions:
a) propagation property of AE signals in the target structure;
b) stress distribution in the structure under inspection.
7.2 Environmental noises
In advance to AE measurement, the noise level shall be estimated. Then, counteract against external noises, wind, rain, sunshine and so forth shall be conducted to decrease the noise level as low as possible. In the case that the noises have similar frequency contents, amplitudes to AE signals or sources of the noises are unknown, characteristics of the noises shall be estimated prior to the measurement. Based on this result, separation of AE signals from the noises shall be achieved.
7.3 Measurement
In an existing structure, a measurement is normally conducted under loads which shall not make a critical damage on functions of the structure to detect and locate active cracks. Based on the spatial area to be covered by AE sensors, those of proper frequency characteristics shall be selected. In advance to the test, attenuation properties of the target structure shall be estimated, by employing the standard source or the equivalent. Based on this information, sensor array shall be determined so as to keep the equivalent sensitivities in all the sensors. AE signals due to cracking shall be detected properly for the duration of the measurement. Concerning AE parameters detected, their trend, distribution, correlation and locations are monitored. In addition, multi-channel observation locates AE sources, by applying the location routine available.ISO 16836 pdf download.