Characteristics of acoustic emission temporal sequences during the failure of a concrete structure under loading

The characteristics of acoustic emission (AE) signals given off in the course of the failure of a concrete structure is explored based on the laboratory experiments with concrete specimens. It is observed that the failure of a concrete structure experiences three stages divided by two inflexion points on the AE event curve, which are sequentially no damage, damage initiation and propagation, and major failure stages. In the first stage, existing micro cracks and defects are compacted by loading, but no damage propagated, hence few AE signals are detected, and it appears that there exists a nearly linear relationship between the relative stress and relative strain. In the second stage, the AE event frequency increases, implying that micro cracks begin to emerge inside the concrete structure, which is consistent with the damage mechanics. When the load is over 80 % of that breaks the structure, i.e. the maximum load, there is a vertical jump on the AE event count curve, which suggests that the failure propagation speeds up. After the second inflexion point, the AE event density increases faster than before, and there is another jump just before breaking, which indicates a quick propagation stage. These findings are valuable for evaluating the damage situations, and for studying and monitoring the dynamic process of the failure behaviors of a concrete structure.

Vibration parameter measurement using the temporal digital hologram sequence and windowed Fourier transform

Real time digital recording and numerical reconstruction of a temporal digital hologram sequence have become feasible in recent years.They provide a new measurement method which enjoys the valuable advantages of being full-field,noncontact and high precision.In this paper,a combined method of temporal digital hologram sequence and windowed Fourier transform is proposed to measure the kinematic parameters of random vibration.A series of holograms are recorded by CCD camera and the original phase can be reconstructed by Fresnel reconstruction algorithm.The three-dimensional windowed Fourier transform is used to filter noise in phase and extract the instantaneous kinematic parameters of the specimen,such as the displacement,velocity and acceleration.An experiment is conducted on a chloroprene rubber latex membrane.Results demonstrate that the proposed method determines the vibration parameters precisely and enjoys many merits.

Rice Grain Damage by Combination and Sequence Infestations by the Rice Leaffolder, Cnaphalocrocis medinalis Guenee(Lepidoptera: Pyralidae), and the White-Backed Rice Planthopper, Sogatella furcifera Horváth(Hemiptera: Delphacidae)

The rice leaffolder （RLF）, Cnaphalocrocis medinalis Guenée （Lepidoptera：Pyralidae）, and the white-backed rice planthopper （WBPH）, Sogatella furcifera Horváth （Hemiptera： Delphacidae）, are major insect pests in China and several other Asian countries. These two pests commonly occur simultaneously or in a temporal sequence. Thus, the investigation of the effect of complex infestations or temporal sequence infestations by these pests on rice yield has a practical signiifcance for the control of these pests. The present study comprised experiments with the following four different variables in potted rice at the tillering stage：single pest species infestation, complex infestation, complete combination infestation and temporal sequence infestation （C. medinalis infestation prior to S. furcifera and S. furcifera infestation prior to C. medinalis）. The results showed that the four infestations resulted in a signiifcant decrease in 1 000-grain weight （1 000GW） and rate of yield loss （RYL） but an increase in blighted grain rate （BGR）, with a signiifcant positive correlation with the infestation density. However, the inlfuences of the complex infestation, complete combination infestation or sequence infestation on the 1 000GW, BGR and RYL were greater than those of the single pest species infestations but did not have addition effects, i.e., the effects of the complex infestation and combination infestation or sequence infestation on the 1 000GW, BGR and RYL were less than the additive effects of the two single pest species infestations at the same densities. In the condition of the same total infestation pressure, no signiifcant differences in the 1 000GW, BGR and RYL were found between C. medinalis infestation prior to S. furcifera and S. furcifera infestation prior to C. medinalis as well as between the sequence infestation and the complex infestation.