Static tests of AFM carbon fiber samples made by autoclave and vacuum molding methods were carried out. Acoustic (acoustic emission and ultrasonic) methods, strain gauging, and microanalysis of thin sections were used to test for defects. The location of acoustic emission signals in the area of stress raisers made it possible to establish that the number of defects in autoclave molding is ten times less than under vacuum molding. Ultrasonic and acoustic emission methods, strain gauging, and microanalysis allowed determining the structure of AFM carbon fiber, the coordinates of defects, and their type. During the testing of unloaded samples made by vacuum molding, manufacturing defects were found that grew in size during static stretching and led to the occurrence of new destructions. No manufacturing defects were found in the samples produced by autoclave molding. Microanalysis of samples produced by the vacuum method revealed defects associated with fiber destruction, matrix cracking, and delamination. Tests of samples prepared by autoclave molding have shown that there are practically no defects in them.
— Static break-down tests were carried out with samples made of carbon fiber plastics. Testing for defects was performed using acoustic emission and strain gaging methods. Four wire strain gages were glued to each sample in the aperture area, and four piezoelectric sensors were installed along aperture edges to form a working testing zone. Recording the signals associated with the failure of the composite material of the samples and their location were carried out by an acoustic emission system. During the tests, a strain gage system recorded loads and deformations that lead to the initiation of destruction process in carbon fiber plastics. The destruction type was determined based on analyzing the microsections made from the signal location zone in the sample. The main informative parameters of acoustic emission signals were associated with the destruction type of carbon fiber plastics.
This review article substantiates the need to take into account internal stresses when predicting mechanical parameters in polymer-composite materials (PCMs) subject to climatic aging. The patterns of formation and development of microcracks under cyclic mechanical loads are considered. It is shown that similar microcracks are formed in PCMs due to the action of internal stresses during thermal and thermal-moisture cycling, during the transformation of sorbed water into ice. The nature of microcracking depends on the properties and morphology of polymer matrices and fibers, the number of cycles, the type of stacking, temperature difference, bond strength, the ratio of strength and deformation at the micro- and macrolevels, and other factors. The processes of thermal oxidation and photo-oxidation activate microcracking of polymer matrices in PCMs, proving the need for a profound inquiry into the density of microcracks in open climatic conditions.
When investigating and diagnosing the technical condition of the hydraulic system or the aircraft, it is required to use the results of measurements of the flow and temperature of liquid in pipelines. On the basis of the specified requirements for flow and temperature transducers, it is most reasonable to use the thermoconvective method of constant temperature. Therefore, a thermoconvective transducer of flow and temperature of liquid has been developed. For parametric identification, an algorithm of composition of the following methods was used: steepest descent, quasi-Newton and coordinate methods. The effectiveness of the proposed algorithm in the conditions under consideration is proved by mathematical simulation of the model parameters vector optimal estimation procedure. Determination of the identified parameters estimates uncertainty was carried out according to the method based on the construction of joint confidence intervals of the required parameters estimates with a confidence probability α= 0.95. The calculated uncertainty of the parameter estimates was carried out at the flow measurement error δ= 3%.
Comparative tests related to locating acoustic emission (AE) signals due to shock impacts on a T700 carbon fiber sample were carried out. Piezoelectric acoustic emission transducers (AETs) and fiber-optic sensors (FOSs) were installed on the sample, forming rectangular location antennas measuring $$360 \times 280$$ mm. Strikes were delivered with balls weighing 10 and 18.5 g. Antennas consisting of four AET sensors and four FOS sensors and an antenna consisting of two AET sensors and two FOS sensors were organized. When using the antenna containing four FOS sensors, the impact on the sample was produced by a load weighing 530 g dropped from a height of 400 mm. AE signals were recorded by the SCAD-16.10 system with “floating” selection thresholds when the ball was dropped and during its repeated bounces. Then AE signal clusters were formed and recorded during the impact of loads. The arrival times of AE signals to the antenna sensors were calculated using the threshold method, the root mean square (RMS) deviation method and the two-interval method. It is shown that the maximum error in locating AE signals is observed when a steel ball with a diameter of 16 mm is dropped from a height of 300 mm and the minimum error is when using an electronic simulator.
