Search

The goal of the study is the fatigue process in polyimide-based composites reinforced with short carbon fibers. Parameters of mechanical hysteresis loops such as the loop area, secant and dynamic moduli were used in the study. Hysteresis loops were constructed using the developed hardware and software system based on the optical method of strain measurements using a digital image correlation (DIC) technique. Methods for calculating the moduli and the parameters of mechanical hysteresis loops is considered. The results of their evaluation and the experimental data on the fatigue behavior of polyimide-based composites reinforced with short carbon fibers are presented. It is shown that an important quantitative measure of the differences in the fatigue behavior of the studied composites is the hysteresis induced energy loss. For a composite with carbonized fibers, the energy loss level per cycle is 35 kJ/m3, whereas for a composite with graphitized fibers it is 34% lower (23 kJ/m3). At the same time, the fatigue durability of the latter is - 40 times lower. A decrease both in the secant modulus (up to 11%) and the dynamic modulus (up to 3.5%) was observed in cyclic tests. However, the reduction was twice as much in a composite with carbonized fibers possessing a longer durability. Thus, the DIC-based estimation of mechanical hysteresis loops by the parameters of the secant and dynamic moduli, as well as the loop area can be successfully used to interpret the difference in the fatigue characteristics at the stage of scattered damage accumulation, whereas an unambiguous prediction of the residual life appeared impossible. The problem requires further systematic studying using approaches of the fracture mechanics.

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.

The paper deals with the estimation of elastic modulus based on the digital image correlation (DIC) displacement fields. The proposed technique has been implemented in situ during the fatigue tests of hybrid composites combining carbon fiber fabric and dispersive micro/nanofillers reinforcements (hybrid composites). It has been shown the principals of elastic modulus assessment using stereoimages for 3D DIC synchronized with the loading process. The results demonstrate the fatigue behavior of hybrid composites under cyclic tensile loading and the degradation of their properties on the measurement of the stiffness.

The paper presents the recursive-search based algorithm f determining the displacements of surface patch (optical flow) to be applied on GPU utilizing the parallel computations principle. OpenCL computing language is used to implement the parallel computations algorithm which is focused on application on AMD Radeon GPUs. The results of testing of the proposed algorithm show the multiple decreases the time of displacement vector field construction while maintaining the accuracy and robustness.

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).

It is proposed to use weight coefficients of the bilateral filter to calculate the measure of similarity of image regions in optical flow calculation algorithms. The efficiency of using this measure is studied by an example of a three-dimensional recursive search algorithm. Weight coefficients are used for calculating the optical flow with subpixel accuracy by the Lucas–Kanade algorithm. A possibility of reducing the optical flow calculation error by means of using the proposed approaches is demonstrated by an example of processing of various types of images. A method of choosing the parameters of the weight functions of the bilateral filter is described and analyzed.

Experimental results on application of ultrasonic technique utilizing Lamb waves for evaluation mechanical state of AA2024T3 specimens under tensile testing are presented. PZTs used as actuators an...

The influence of the mesh size of the displacement vector field on the strain estimate in the digital image correlation method is considered. The reasons for the emergence and the magnitude of the strain estimate error in processing optical images of material surfaces with different textures are analyzed. The dependence of the mesh size for strain estimation on the magnitude of displacements and also on the presence and size of the discontinuity region in the strain field is studied. An adaptive algorithm for choosing the mesh size is proposed, which allows the strain calculation error to be reduced by a factor of 1.9 with simultaneous reduction of computational expenses by a factor of 2.1.

An algorithm of fatigue crack detection in optical images taken in fatigue tests of materials is proposed and tested. The algorithm is designed for automation of measurements of the crack propagation parameter and tracing the crack tip position in the course of cyclic loading for the purpose of shifting the optical system with respect to the examined sample surface to the “region of interest.” It is found that the coordinates of the image fragment containing the crack can be determined with a mean error of 1.93% of the total size of the raster. Testing of the algorithm on model images shows that the mean error of determining the crack tip position is smaller than 56 pixels.

A number of problems related to the digital image correlation method from the standpoint of both the hardware and software development and testing, as well as solving problems in the field of fatigue fracture mechanics are described. Testing of the developed computer stereo vision system was carried out using a series of stereo pairs reflecting the change in the location of the object in space as well as plane and out-of-plane deformations. It is shown that the error in determining spatial coordinates does not exceed 0.75 units, while the error in computing the strain tensor components in case of a system with a single camera is two orders of magnitude larger than that at using the stereo machine vision system. An algorithm for automatic crack detection on optical images and calculating its tip coordinates was proposed and tested. It is shown that when the frame size is 2000×1000 pixels the coordinates of the crack tip might be determined with an average error of about 56 pixels, while the average error of the crack area determination does not exceed 1.93%. A modified incremental algorithm for calculating displacements on a series of stereo pairs is proposed, which allows one to estimate large magnitude displacements during serial processing of images. An algorithm for measuring the J-integral using the digital image correlation method has been developed. It is shown that the deviation of the calculated J-integral values from the model ones is on average 1.75 %. A quantitative characterization of the fatigue crack growth process in metal alloys was carried out using the technique based on the digital image correlation (in terms of fracture mechanics) including the calculation of the fatigue crack growth rate da / dN , maximum strain (emax) and effective cycle asymmetry ( R eff).

The paper reports on a study of the structure and mechanical properties of 30CrMnSiNi2A high-strength steel irradiated with a Zr+ ion beam. The effect of irradiation on the steel was assessed by optical, transmission, and scanning electron microscopy as well as by X-ray di ITraction analysis of its irradiated and non-irradiated specimens 1 mm thick under static and cyclic tension, showing an increase in the fatigue life of the steel after irradiation. The deformation behavior and the mechanical properties of the specimens were compared, and the factors responsible for the increase in fatigue life were analyzed.

The paper is devoted to investigation of ultrasonic signal properties and their changes during cyclic loading of CFRP sandwich panels with aluminum honeycomb core. In order to estimate the mechanical state of the composite structures the Lamb wave based technique is proposed. Acoustic signals are generated and sensed by piezoelectric transducers adhesively bonded on the specimen surface. Sensed signals are analyzed by computing the set of informative parameters which characterize ultrasonic signals evolution during cyclic tests. The paper represents the results allowing identification of three stages of fatigue damaging and failure for two specimens subjected to tensile-compressive loading.

Experimental results of Lamb wave technique investigation for evaluation of fatigue state of AA2024T3 specimens under cyclic loading conditions are presented. Analysis of acoustic signal data obtained by the piezoelectric transducers network was done through the calculation of two parameters: maximum envelope and 2nd central moment. Understanding of relations between acoustic signals features and emergent damages will provide the ability to determine the state of the structure and evaluate the defects, therefore assume the preventative measures and avoid accidents.

The paper presents the results of application of different techniques for processing of digital image correlation data. The algorithms are described and applied for determination of elasticity modulus of carbon fiber reinforced polymer specimens tested by uniaxial tension. Two ways of evaluation of elongation along with two methods of elastic modulus calculation resulted in four techniques which have shown quite similar results with a scatter of 0.5-2%.

The structure and properties of 12Cr1MoV steel irradiated with a zirconium ion beam were studied by optical microscopy, scanning electron microscopy, and micro- and nanoindentation. It is shown that the modification covers the entire cross-section of the irradiated specimens to a depth of 1 mm. The data on irradiation-induced structural changes are used to interpret the changes in mechanical properties of the irradiated specimens under static and cyclic loading. Particular attention is given to analysis of strain estimation by the digital image correlation method.

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.

An approach to automatic determination of operation parameters of a hierarchical 3D recursive search (3DRS) algorithm is proposed and tested. A comparative study of the computational speed and noise immunity of the 3DRS algorithm used for constructing displacement vector fields, including the use of the Gaussian pyramids (hierarchical search), is performed. It is shown that application of the hierarchical 3DRS algorithm with operation parameters determined in this work can substantially increase its noise immunity and reduce the computational cost.

With the use of optical/scanning electron microscopy and microindentation some peculiarities of structure modification over cross-section as well as mechanical properties of 12Cr1MoV and 30CrMnSiNi2 steel specimens subjected to ion-arc treatment by Zr ion beam were investigated. It is shown that changing of the microstructure takes place over entire cross-section that is not the case of material softening due to annealing. Within the revealed results on structure modification the data on changing mechanical properties under cyclic and static tension of the irradiated specimens are interpreted. Of particular interest are results on strain measurement by digital image correlation technique.

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.

For the development of a technique for cyclic deformation assessment with the use of aluminum foil sensors (with thickness of 80 μm) glued onto the specimen surface the fatigue tests of carbon fiber reinforced composites were carried out. The DSLR camera mounted onto an optical microscope was used for capturing the images of sensors to reflect strain induced relief, which than was numerically estimated using various informative parameters (Shannon entropy, mean square error, fractal dimension and Fourier-spectrum energy) in order to obtain the cyclic deformation assessment of composite. The results are discussed in view of application of this method for the development of structural health monitoring (SHM) approach.

For the development of a technique for cyclic deformation assessment using aluminum foil sensors glued onto the specimen surface the fatigue tests of carbon fiber reinforced polymer (CFRP) specimens were carried out. The DSLR camera mounted onto an optical microscope was used for capturing the images of sensors to reflect strain induced relief, which than was numerically estimated using various informative parameters in order to obtain the cyclic deformation assessment of composite. The results are discussed in view of application of this method for the development of Structural Health Monitoring (SHM) approach.

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.

An algorithm of constructing displacement vector fields, based on the incremental approach to estimating displacements of image fragments, is proposed. Application of this algorithm for processing model and experimental optical images is analyzed. Noise immunity of the incremental algorithm for determining displacements is compared with that of the combined algorithm developed by the authors. It is shown that the proposed incremental algorithm ensures a more noise-immune estimate of displacements in processing images of a deformation relief essentially varying in time.

The use of the Bezier surface as a smoothing filter for vector fields is studied. Advantages of the Bezier surface over the moving average filter are determined. The algorithm of adaptation of the filter aperture size is modified. The efficiency of adaptation of the aperture size in vector field processing in the problem of estimating the material strain by the method of digital image correlation is considered.

Surface layer of a loaded solid is an individual structural level of deformation that was shown numerously within concept of physical mesomechanics. This gives rise to advance in its deformation development under loading as well as allows using this phenomenon to sense the strain induced structure changes. It is of specific importance for composite materials since they are highly heterogeneous while estimating their mechanical state is a topical applied problem. Fatigue tests of carbon fiber composite specimens were carried out for cyclic deformation estimation with the use of strain sensors made of thin (80 μm) aluminum foil glued to the specimen’s surface. The surface images were captured by DSLR camera mounted onto an optical microscope. Strain relief to form during cyclic loading was numerically estimated using different parameters: dispersion, mean square error, universal image quality index, fractal dimension and energy of Fourier spectrum. The results are discussed in view of deformation mismatch in thin foil and bulk specimen and are offered to be applied for the development of Structural Health Monitoring (SHM) approach.

The influence of adaptive bilateral filtration on the calculation of the fractal dimension of optical images is estimated. Filtration parameters are chosen on the basis of various criteria. The use of the maximum dispersion of the probability as the most effective parameter of estimating the image filtration quality is considered.

Various filtration methods are compared by an example of processing of model and experimental vector fields. A criterion for estimating the filtration efficiency is proposed. It is demonstrated that combined vector filters ensure more efficient filtration than other popular methods.

The influence of various methods of obtaining surface images on the calculated value of their fractal dimension as a quantitative characteristic of the surface state is studied. It is demonstrated that images obtained both by a scanning electron microscope and by a photocamera are characterized by a noticeable noise level, which alters the behavior of the fractal dimension. Various methods of correction of the observed effect are discussed.

Application of weight coefficients of the bilateral filter used to determine weighted similarity metrics of image ranges in optical flow computation algorithm that employs 3-dimension recursive search (3DRS) was investigated. By testing the algorithm applying images taken from the public test database Middlebury benchmark, the effectiveness of this weighted similarity metrics for solving the image processing problem was demonstrated. The necessity of matching the equation parameter values when calculating the weight coefficients aimed at taking into account image texture features was proved for reaching the higher noise resistance under the vector field construction. The adaptation technique which allows excluding manual determination of parameter values was proposed and its efficiency was demonstrated.

An optical flow determination algorithm is proposed, developed and tested in the article. The algorithm is aimed at improving the accuracy of displacement determination at the scene element boundaries (objects). The results show that the application of the proposed algorithm is rather promising for stereo vision applications. Variations in calculating parameters have allowed determining their rational values and reducing the average absolute error of the end point displacement determination (AEE). The peculiarity of the proposed algorithm is performing calculations within the local regions, which makes it possible to carry out such calculations simultaneously (to attract parallel calculations).

A Lamb wave based ultrasonic technique as well as optical image characterization was utilized to estimate a current mechanical state of glass fiber reinforced polymers (GFRP) under cyclic tension. The ultrasonic acoustic method was applied in a 'pitch-catch' mode using piezoelectric transducers adhesively bonded onto a specimen surface. Numerical evaluation of acoustic data was performed by calculating two informative parameters: maximum of amplitude of the received signal and variance of signal envelopes. Optical images were registered and then analysed by calculating Shannon entropy that makes it possible to characterize changing of GFRP specimen translucency. The obtained results were treated in order to find out the relation between the current mechanical state of a specimen and informative parameter values being computed from the acoustic and optical signals.

An algorithm of crack detection during fatigue testing of materials, designed to automate the process of cyclic loading and tracking the crack tip, was proposed and tested. The ultimate goal of the study is aimed at controlling the displacements of the optical system with regard to specimen under fatigue loading to ensure observation of the area of interest. It is shown that the image region that contains the crack may be detected and positioned with an average error of 1.17% for noisy images and 11.5% when blurred images are analyzed. In terms of determining the crack tip position. the algorithm provides the accuracy of its localization with the average error value of 9.23 pixels when processing noisy images, while it makes 42.7 pixels for blurry ones.

To analyze the deformation of solid bodies, it was proposed to use simultaneously three in situ methods of data registration, including loading diagram (macroscale level), correlation of digital images (mesoscale level), and acoustic emission (microscale level). The complex for multiscale in situ investigation of deformation and fracture processes, the operating principle of which is based on interconnection of duration of the characteristic stages of change in the informative parameters and their ratios with the leading scale level, was implemented. By the example of stretching of the samples of aluminum alloy D16 with cut of different depth, the characteristic stages of change in the derivative of the load applied, AE activity, and intensity of shear deformation on the degree of deformation were revealed. It was shown that the characteristic stages marked on graphs of changes in all informative parameters on time agree well between each other and refer to elastic deformation, parabolic strengthening, and crack propagation.

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.

The paper presents the results of preliminary testing of high-resolution shearographic device and software for signal processing to be applied for non-destructive evaluation (NDE) of composite materials. Shearography is an interferometric non-contact NDT/E method that allows obtaining strain data straightly at video frame rate to diagnose different composite structures to provide the safe operation of the structure and reduce the possibility of accidents. The optical part of the device based on the Canon 450D DSLR camera is described. The tests have been performed to ensure the proper functioning of the device and to establish the image processing technique.

The paper deals with the investigation of Lamb waves ultrasonic testing technique applied for evaluation of different stress-strain and damaged state of aluminum specimens at static and fatigue loading in order to develop a Structural Health Monitoring (SHM) approach. The experimental results of tensile testing of AA2024T3 specimens with welded joints are presented. Piezoelectric transducers used as actuators and sensors were adhesively bonded to the specimen's surface using two component epoxy. The set of static and cyclic tensile tests with two frequencies of acoustic testing (50 kHz and 335 kHz) were performed. The recorded signals were processed to calculate the maximum envelope in order to evaluate the changes of the stress-strain state of the specimen and its microstructure during static tension. The registered data are analyzed and discussed in terms of signal attenuation due to the formation of fatigue defects during cyclic loading. Understanding the relations between acoustic signal features and fatigue damages will provide us the ability to determine the damage state of the structure and its residual lifetime in order to design a robust SHM system.