Recommendation of RILEM TC 212-ACD: acoustic emission and related NDE techniques for crack detection and damage evaluation in concrete*
KeywordsAcoustic Emission Acoustic Emission Signal Acoustic Emission Event Acoustic Emission Sensor Acoustic Emission Energy
Acoustic emission (AE) is an inspection technique and one of nondestructive evaluation (NDE) methods by means of detecting elastic waves due to dynamic motions at AE sources, such as cracking, delamination, cleavage, and fretting in a material. The technique is widely applied to a variety of construction materials and basics for research are established . Concrete structures could deteriorate due to heavy traffic loads, fatigue, chemical reactions, unpredictable disasters, and poor workmanship, although concrete structures have long been referred to as maintenance-free. Eventually, retrofit and rehabilitation of the structures are in heavy demand all over the world. It results in the need for the development of advanced and effective inspection techniques prior to repair works. In this regard, AE techniques have been extensively studied in concrete engineering. These are inspection techniques by means of detecting elastic waves due to cracking and damage (micro-crack) accumulation, originally developed in materials science, seismology, and mechanical engineering.
Focussing on crack detection and damage evaluation, it is known that AE techniques are prospectively applicable to concrete and concrete structures. This is because AE events or phenomena are to be observed under in-service conditions. Here, a measurement method is standardized for detecting AE signals in concrete.
This recommendation is referred to several standards established or published in the references [2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18], of which the newest version shall be cited.
2 Definitions of technical terms
- (a)AE: transient elastic waves generated by the release of energy within a material. Microscopic fracture in concrete takes place with the release of stored strain energy as nucleating micro-cracks and generating elastic waves. These waves due to crack nucleation are referred to as AE waves, which propagate inside a material and are detected by an AE sensor as shown in Fig. 1.
AE signal: The electrical signal detected at a sensor, which is converted through the detection of AE wave (elastic wave).
Burst emission: A description of the signal that has a rapid rise to peak and slower decay to the noise level. Burst-type AE signals are typically detected in concrete. In contrast, under plastic deformation in metal, emissions are observed continuously without the decay. This is normally named “continuous emission.”
Channel: One line of AE signal detected by AE sensor and processed by the other devices.
Hit: A hit is the term to indicate that a given AE channel has detected and processed one AE transient signal. Counting methods of AE signals are later described in the Sect. 4 of “signal analysis and AE parameters.”
Event: AE wave can be detected in the form of hits on one or more channels. One event is a group of AE hits received from a single source by two or more channels, of which spatial coordinates could be located.
Array: Spatial arrangement of AE sensors for spatially locating AE sources.
Attenuation: The observed loss of a signal as it travels through a medium.
Noises: Signals produced by causes other than AE phenomena. Elimination of noises is essential for effective detection of AE signals.
3 Measuring system
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. So far, resonant frequencies of resonance-type sensors have been applied to concrete in the range of 50–250 kHz. AE sensor shall be robust enough against temperature change, moisture condition and mechanical vibrations in the environments.
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 range is closely related to elimination of noises. In concrete, a band-pass filter between around 10 kHz and several 100 kHz up to 3 MHz is employed.
4 Signal analysis and AE parameters
One waveform is to be counted as one AE hit, 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. The threshold level applied in AE measurement is 30–50 dB in concrete.
Arrival time differences in AE sensor array
External parameters: The measurement system shall be able to obtain time information along with AE parameters. In addition, such external parameters as load, strain and so forth are preferably recorded in the system.
5 Setup and measurement
5.1 Sensor setup
Propagation property of AE signals in the target structure
Stress distribution in the structure under inspection
5.2 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. In this respect, the use of filters is applicable after determining the proper frequency range.
In an existing structure, a measurement is normally conducted under loads which must 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 provides to locate AE sources, by applying the location routine available.
5.4 System inspection
Sensitivity of AE channels shall be conducted routinely by employing the standard source. Variation within the channels shall be less than 3% in the voltage.
5.5 Storage of data
The system shall be equipped with an enough memory to record the data measured. It is preferable that all the data recorded are analyzed digitally by computer.
Results of system inspection before and after the setup
Results of analytical data before and after the setup
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