ISO 16837:2019 pdf free download – Non-destructive testing – – Acoustic emission testing一Test method for damage qualification of reinforced concrete beams.
The frequency range shall be determined prior to the measurement, taking into account the performance of AE sensor and the amplifiers. A suggested range in concrete is from around 10 kl-Iz to 100 kHz, because such higher frequency components as over I MHz readily attenuate in concrete.
For signal analysis, AE parameters of AE count or AE hit shall be detected and processed. The measurement system shall be able to obtain time information along with AE data. In addition, such external parameters as load, strain and so forth are preferably recorded in the system, which can be equipped with enough memory to record the data measured. It is preferable that all the data recorded be analyzed digitally by computer.
5 Environmental noises
in advance of’the 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. Normally, to eliminate mechanical noises due to vibrations, traffics and so forth, a high-pass filter over several kHz is useful. Elimination ofelectrical noises is made by a low-pass filter over around 100 kHz.
6 Test procedure
6.1 Basics
The period ofthe measurement shall be prescribed, depending on the conditions ofa target structure or member, possibly in service. The sensitivity of the AE measurement system shall be checked routinely by employing the standard source. The variation within the channels shall be lower than ±3 dBAE. The threshold level for counting shall be set as low as possible, depending on the level ofthe noises.
AE activity is evaluated from the number ofAE counts or hits. The onset ofAE activity is identified at the time when AE hits of peak amplitude with 5 dBAE above the threshold are continuously observed under loading.
6.2 Failure modes ofreinforced concrete beams and AE activity
Two failure modes are known. One is the bending-mode failure. In the beginning of loading, tensile cracks are nucleated at the bottom of a moment span due to bending moment. When the beam is either under-reinforced for bending or fully reinforced against diagonal shear failure, steel reinforcement in the axial direction yields and tensile cracks are further nucleated and then propagate upward. AE counts or hits in a beam of the bending-mode failure increases acceleratedly with the increase in loading. The other is called the shear-mode failure. In the case that the beam is over-reinforced or little reinforced against diagonal shear failure, final failure results from the sudden generation of diagonal shear cracks in the shear span without yielding of reinforcement.
AE counts or hits are observed at a constant rate because the reinforcement withstands until the final failure. Right before the final stage, diagonal shear cracks are observed without any precursors along with the rapid increase in AE counts or hits. Since AE activity in a damaged beam is of main concern to apply the recommended practice, these two failure modes should be taken into consideration in the case of AE monitoring.
With respect to AE activity in the reinforced concrete beam, a relation between crack-mouth opening displacement (CMOD) and the presence of the Kaiser effect is known. The Kaiser effect disappears in the case that either the values of CMOD due to bending-mode failure become wider than 0,1 mm and 0,2 mm, or the shear-mode failure is observed. It is noted that the values of CMOD over 0,1 mm approximately correspond to the serviceability limit of the reinforced concrete beam.