In this paper, a novel model is presented to describe the composite mechanical properties degradation during cyclic loading. The model is based on cumulative distribution functions using. Weibull probability distribution law and beta distribution are considered. The dependences of the fatigue sensitivity coefficient on the preliminary cyclic exposure are derived. The damage value function derivative using is proposed to define damage accumulation stages boundaries. Model parameters are obtained using experimental data. Determination coefficients are calculated. A high descriptive capability is noted. Rationality and expediency of using cumulative distribution functions as the approximation of experimental data on mechanical characteristics reduction after preliminary cyclic exposure is concluded.

Aging of composites is a pervasive problem that leads to mechanical properties degradation, reduced design life of a structure and premature accidental failure. The work is devoted to an experimental study of the preliminary temperature aging effect on the residual mechanical properties of structural CFRP. The joint use of test systems and systems for registration and analysis of acoustic emission signals was applied. The Short Beam Shear Test of CFRP specimens were carried out using the short beam method. The tests were carried out on universal electromechanical systems Instron 5882 and Instron 5965 in accordance with the recommendations of ASTM D2344. In the process of loading the samples were continuously recorded by using the acoustic emission signals system AMSY-6. A piezoelectric sensor with a frequency range of 300-800 kHz was used. The test and diagnostic systems were synchronized during the tests. In the course of the work the values of the interlayer shear strength were determined for the samples of CFRP. Typical types of the sample destruction are illustrated. When analyzing the change in the mechanical properties of the carbon fiber reinforced plastic from a temperature increase the critical values of temperatures were established in which a sharp decline in the strength and elastic characteristics of materials occurs due to an active destruction of the binder. The graphs of the energy parameter dependence and frequency characteristics of acoustic emission signals on time have been constructed and analyzed. The estimate of the processes of damage accumulation in composites is carried out. The change of the damage accumulation mechanisms was illustrated. The obtained results illustrate the effect of elevated temperatures and the duration of their impact on the mechanical behavior of structural CFRP specimens during the static tests for the interlayer shear.

2024 aluminum alloy is a common aeronautic material. During operations, construction elements made of aluminum alloys undertake complex cyclic loadings. Therefore, it is important to estimate the influence of these loadings on the durability of the material. Hereby, multiaxial fatigue tests with the same amplitude values of the second invariant of the stress deviator tensor are conducted, and test data are analyzed. The modified Sines method is utilized to predict fatigue experimental data. Results show that the model is accurate enough to fatigue behavior prediction of 2024 aluminum alloy.

In this paper, a previously developed modification of the Sainz model of multiaxial fatigue is reduced to an invariant form. Model constants were determined for different sets of setup experiments. It was supposed to introduce an additional summand to account for the phase shift between loading modes. The model is used to describe the fatigue behavior of the D16T aluminum alloy. Low-cycle fatigue tests under biaxial loading conditions are presented, with one mode changing cyclically and the other mode remaining constant in magnitude throughout the test. The results of cyclic durability prediction by the modified model provide good convergence.

The paper presents an experimental study on regularities of postcritical deformation of carbon-fiber-reinforced plastic (CFRP) under static ±45° tension. The employed test method is based on ASTM D3518. Displacement and strain fields were identified by a digital image correlation method (DIC) using a VIC-3D contactless optical video system. Acoustic emission signals were obtained using an AMSY-6 system. The surface analysis of samples was carried out using a CarlZeiss SteREO Discovery. V12 optical stereomicroscope and a DinoLite microscope. Three experimental test types were considered: active loading, deformation with unloadings, and tension under additional torsion vibrations with various amplitudes. Loading diagrams were constructed; they showed a number of stages in the damage accumulation process. It was analyzed how heterogeneous strain fields develop; a neck development during softening process was observed. It was noted that the loading system rigidity influences the failure moment. The research considered various shear strain calculation methods using a “virtual extensometer” instrument. Composite mechanical properties were obtained. A shear modulus reduction during a plastic strain increase was revealed. The acoustic emission signals were analyzed; three characteristic frequency bands were observed. Most of the contribution to cumulative energy was made by matrix cracking. A reduction of the number of AE signals associated with the violation of adhesion between the fibers and the matrix during postcritical deformation was observed. The research identified basic surface defects. An appearance of the defects corresponds with their identification by the AE system. It was revealed that the presence of additional torsion vibration leads to an increase in the softening stage length. It was concluded that due regard for the postcritical deformation stage and the loading system rigidity is reasonable during the structure strength analysis.