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Abstract The paper presents the results on application of Lamb waves based technique for impact damage detection and severity identification. The PZT network operates in the round-robin mode changing the actuator and sensor roles of the transducers in order to detect the response of the system in the presence of damage. The monitoring is performed via the analysis of three parameters: change of the amplitude (dA), change of the energy (dP) and cross-correlation (NCC) of the signals in baseline and damaged state. Testing of laminate CFRPs shows that the damage location is estimated with an error of 5–15 mm, while the computed Damage index is linearly dependent on the applied impact energy. For honeycomb CFRPs the NCC parameter do not provide accurate results, however, other parameters allow identification within the 5–20 mm error and reflect accurate data on the severity of the damage.

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.

Possibilities offered by an ultrasonic inspection technique that is based on using Lamb waves for monitoring the state of metal materials under static and cyclic loading have been studied. Dogbone V96ts3T12-aluminum-alloy specimens of the following two types have been used for research: initial material and a sample with a weld at the working zone center. The digital-image correlation method has been used as an additional source of information. Based on the tests, dependences of various informative parameters of acoustic vibrations on the applied load (for static tests) and running time (for cyclic tests) have been obtained. These dependences have been further supplemented with the results of calculation of deformation fields. The data gathered indicate sensitivity of the proposed ultrasonic inspection technique to various (elastic and plastic) deformation processes and allow one to monitor the growth of cracks under cyclic tests. Further development of the method assumes its examination from the viewpoint of embedded testing systems (the Structural Health Monitoring concept).

According to the data of the automated analysis of images of the surfaces with thermal-fatigue cracks, we study the specific features of the deformation and fracture processes in 25Kh1М1F heat-resistant steel. We reveal and quantitatively describe the regularities and characteristic stages of its deformation with transformations of the block structure of cracking on the surface as a result of merging of separate thermal-fatigue cracks into a network, which is determined by the ability of the metal to be deformed within the framework of islets of the intact material surrounded with cracks. We also estimate the measure of interaction of each crack with the set of cracks by the intensity of shear and rotation processes inside the blocks of the material that are not damaged by thermal-fatigue cracks and by the changes in their orientation relative to the direction of the applied load.

We develop a method for the digital-optical analysis of defects in materials with simultaneous measuring of several diagnostic parameters and estimation of the degree of damage to the material in the vicinity of a stress concentrator by analyzing the displacements and distortions of the preliminarily made surface marks. By using the results of mathematical description of the field of displacements on the metal surface, we propose to use informative signs for the quantitative analysis of the parameters of damage.

Cyclic tests of specimens of a carbon-fiber-based composite material were performed and their operation time was evaluated using an integral-type deformation pickup (ITDP). The pickup includes a polished aluminum foil (sensitive element), which is pasted on the surface of a specimen and deformed together with it. Its images were recorded with an МЬС-9 microscope additionally equipped with a digital camera. To analyze the deformation relief using ITDPs, a complex of informative parameters was calculated and the character and typical stages of their changes were discussed and interpreted. It was proposed to use this data when developing built-in sensors for monitoring the state of parts of critical equipment.

The deformation and fracture of specimens of a carbon-carbon composite material with different dimensions of a stress concentrator in the form of a central hole with diameters of 7, 10, and 13 mm were studied using a developed combined method. The results of a numerical analysis of experimental data are presented in the form of diagrams (dependences) of the shear-deformation intensity and the acoustic-emission activity as functions of the loading time. The factors that cause similarity and differences of the investigation results are discussed and interpreted. It is proposed that the obtained data be used for the nondestructive testing of composite materials via the selection of characteristic stages of the deformation development and the moment that precedes fracturing.

Studies of the development of deformations and damage in Д16AT alloy specimens with side notches, which modeled cracks in objects, were performed by combining the acoustic emission, surface deformation mapping, and strain gauging methods. It was shown that a significant portion of the loading time is associated with crack propagation, which is accompanied by the appearance of a segment with a negative value of the strain strengthening coefficient in loading diagrams. According to the data from the correlation of digital images at the plastic deformation stage (within stage 2), two substages were distinguished: the first is related to the deformation localization directly at the notch apex, while the second is associated with an increase in the area of this region to dimensions that are comparable with the specimen cross section. It was revealed that, in the case of a significantly localized deformation, the staging-based approach to the analysis of deformation and fracturing processes is characterized by the better correlation between data that are recorded by different in situ methods.

The development of deformations and fracturing in D16AT alloy specimens with central holes was investigated using a combination of the acoustic emission, surface-deformation mapping, and strain-gauging methods. Three characteristic deformation-development stages were selected. Changes in the durations of the characteristic stages obtained from the data of the analysis of all informative parameters indicate that the specific duration gradually increases for the second stage and remains constant for the third stage. It was shown that the staging approach in studies and testing of deformations and fractures is quite informative and mutually complementary in the case of low concentrations of stresses. However, the durations of stages that were revealed on the basis of data from recording of informative parameters may be appreciably different. This is associated with the sensitivity of each method to deformation flaws at each scale level and the sensitivity/resolution of the recording equipment.

A combined method to investigation of localized deformation processes in notched aluminum alloy as well as carbon fiber reinforced composite specimens is applied in order to reveal characteristic stages of strain and fracture. Stress concentrators have the shape of a circular hole and edge crack of various sizes. Carbon fiber reinforced composites (CFRC) with notches of different size were also tested in order to illustrate the difference in deformation behavior between metal and reinforced polymeric composites specimens. Use of simultaneous strain data registration has allowed us to register and compare parameters under analysis during entire time of the experiments. The reasons of similarity and difference of the results are shown and discussed. It is offered to apply the obtained results for the aims of non-destructive testing of structural materials being based on revealing characteristic stages of strain development and a particularly prefracture stage.