卷 55, 编号 2 (2019)

Results of applying a Lamb-wave-based ultrasonic technique to detect impact damages to a CFRP honeycomb panel are presented. The technique is based on analyzing changes in wave propagation due to defects and makes use of a network of piezoelectric transducers adhesively bonded to the panel surface. Preliminary experiments and tests have been carried out in which defect simulators (metal discs attached at various points of the CFRP panel) and low energy impact damages inflicted by drop-weight operation were detected. Results of defect location (calculated coordinates) are considered, as well as a damage index. A detailed analysis of the results has made it possible to identify the specific features and shortcomings of the technique. Possible ways are proposed for eliminating these shortcomings and upgrading the methodology of ultrasonic data processing in general.

Methods for assessing scattered damage to materials are considered. Ultrasonic methods based on recording backscattered ultrasonic signals are among the most practically feasible methods for assessing scattered damage to materials at the mesolevel (the size of the probing wavelength). We describe methods for determining scattered damage in the volume of a material based on scanning the object surface with a normal double-crystal piezoelectric transducer, recording and statistically processing the backscattered signal in the form of an A-scan, constructing the spatial distribution of backscattering cross section in the form of a B-scan or tomographic image, and assessing damage in the material volume based on the relative change in the backscattering cross section or the “disorder” of its spatial image.

Acoustic emission signals from concrete compression damage process are non-stationary and their arrival time is unknown. Therefore, the definition on the acoustic emission event in the time series is not clear. In this paper, the acoustic emission signals associated with concrete specimen under uniaxial compression are acquired by full-digital acoustic emission system. The overlapping phenomenon of acoustic emission signal is presented and discussed by the analysis of time interval between adjacent hits. Acoustic emission signals are classified into three categories according to their waveform characteristics. And the result shows that as the loading process continues, more and more amount of continuous type of acoustic emission signals appeared, indicating the acoustic emission signal does not carry the features of burst type, which will introduce obvious errors in the calculation of acoustic emission event. In contrast, the average signal level used in the analysis of continuous type of acoustic emission signal shows a good regularity with the damage process of concrete.

A mathematical model has been developed for digital radiographic imaging of large-sized objects. The model takes account of scanning time, the parameters of source and recorder of bremsstrahlung due to the test object, and geometrical scanning scheme. A high-performance algorithm is proposed for the numerical modeling of digital radiographic images. The algorithm has allowed producing realistic images of large-sized stepped and wedge-shaped test objects and test objects typical of pipeline transport. The rationale for selecting the scanning time and the digit capacity of an analog-to-digital converter is demonstrated. Numerical simulation of radiographic images is shown to provide a basis for the correct choice of the parameters of digital radiography systems as applied to testing large objects.

This paper deals with the problem of the X-ray image segmentation used to detect weld defects for a non-destructive testing task. In this work we have implemented a multiphase method using the Mumford and Shah piecewise smooth model to extract the size and the texture information of defect and its region. Using this model which is more robust to the noise and less sensitive to the position of the initial curve, we have obtained a maximum of information for several regions in the X-ray image (the geometry of weld beads and defects). Our results show that the model developed can do at the same time the image segmentation and restoration.

A method of nondestructive testing of the yield strength of steels is considered that is based on processing the diagram of dynamic spherical-tip indentation into the material being tested. The distinctive feature of the method is that it uses a part of the impact indentation diagram that covers two stages of deformation: purely elastic and elastoplastic. The range of strains in which the plastic flow of material starts developing under impact microindentation is theoretically justified. The possibility is shown for recording processes occurring at the initial stage of microplastic deformation with the equipment already available. Experiments have been carried out on deforming low- and medium-carbon steels, as well as pipe steels. As a result of comparing the average contact pressure under dynamic indentation with the static yield strength determined in sample stretching, a linear relationship has been revealed between the two. It is shown that the developed method allows one to determine the yield strength of steels with sufficient accuracy, viz., the measurement error does not exceed 60 MPa, or 13%, in the considered range from 230 to 1400 MPa.