Kaiser effect

It has been often quoted that AE history started in 1950 with publication of Kaiser s dissertation (Kaiser 1950). J. Kaiser in Fig. 2.5 tested tensile specimens of metallic materials, recording AE signals. He discovered a famous irreversibility, which is now called the Kaiser effect. B.H. Schofield in Fig. 2.6 found Kaiser s article in USA and reexamined it. He published his pioneering work as entitled Acoustic Emission (Schofielde 1961). This is the first use of the terminology of AE in history. 

The Kaiser effect (Kaiser 1953) has been applied as to know the initial stress condition of rock materials (Yoshikawa and Mogi 1989). Based on the Kaiser effect, T. Fowler proposed the Felicity ratio (Fowler 1986), which can show the damage quantitatively in tank stractures. The Felicity ratio was established because of the following feet. The Kaiser effect can only hold in the stable condition of the materials (i.e., intact condition). As to progress the internal instability, the Kaiser effect is gradually breaking down. As a result, AE activity starts to be observed even under lower stress than that of the maximum stress experienced. 

The Kaiser effect, which was first investigated by Wilhelm Kaiser in 1950, describes the phenomenon where a material under load emits acoustic waves only after a primary load level is exceeded. During reloading, these materials behave elastically before the previous maximum load level is reached. If the Kaiser effect is permanent for these materials, little or no AE will be recorded before the previous maximum stress level is achieved. 

Fig. 5.2. Example of the Kaiser effect occurred in a cyclically loaded concrete specimen. Thick black lines represents the AE activity, thin lines the load and the dashed lines indicate the Kaiser effect.

As stated in Chapter 4, the recommended practice for in situ monitoring of concrete structures by AE is currently established (NDIS-2421 2000). In order to assess the damage levels of reinforced concrete beams, one criterion to qualify the damage levels is proposed on the basis of two ratios associated with the Kaiser effect. A feasibility of this practice is experimentally examined by testing reinforced concrete beams, which have been damaged under incremental cyclic loading. 

In principle, the concrete structures undamaged are statically stable with high redundancy. AE activity is very low in a stable structure, because the Kaiser effect is closely associated with structural stability. Such ratios as the load ratio and the calm ratio are defined to estimate the Kaiser effect. 

The change of AE activities under cyclic loading is studied to qualify the damages. At each loading stage, two of the load ratio and the calm ratio were determined. Results are shown in Fig. 10.11 (Ohtsu, Uchida et al. 2002). Based on the maximum CMOD observed in the beams, classification limits are set as 0.9 for the load ratio and 0.05 for the calm ratio. This is because the serviceability limit of the CMOD is less than 0.1 mm in the standard specification, and the Kaiser effect was not observed in the case of the CMOD over 0.1 - 0.2 mm. 

A Event counting, inelastic deformation, statistical analysis, Kaiser effect... 

Kurita and Fujii [1979] Granite Uniaxial compression Kaiser effect... 

Event Counting, Kaiser Effect, b-Value, and Time Statistic... 

Hirata T (1987) Omori s Power Low Aftershock Sequences of Microfracturing in Rock Fracture Experiment. J Geophys Res 92 6215-6221 Holcomb DJ, Costin LS (1986) Detecting Damage Surfaces in Brittle Materials Using Acoustic Emission. J App Mech 108 536-544 Holcomb DJ (1993) General Theory of the Kaiser Effect. Int J Rock Mech Min Sci Geomech 30 929-935... 

As seen in Fig. 13.7, under a cyclic-incremental loading, the gradual increase of total number of AE hits was observed. From the both graphs, the Kaiser effect can be observed until the loading cycle of 150 kN, because a few AE hits are only observed and no increase in unloading cycles. Once the maximum repetitive load exceeded 200 kN, AE activity was observed even below the maximum previous load and tended to increase in unloading stages. 

Thus, it is demonstrated that the Kaiser effect is observed in a full-scale girder. For the comparison with AE results, residual deformations at the center of the girder are shown in Fig. 13.8. 

The deformations increase continuously with the increase in loading stages. Thus, no clear transition is observed at the load level of 200 kN, where the deformation exceeds 1.0 mm. Concerning the serviceability of the superstructure, deformations less than 2 mm are not critical, but AE observation suggests the begiiming of deterioration over 200 kN cycles as the breakdown of the Kaiser effect. 

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