The loading of short carbon fibers (SCF) can significantly improve the fatigue properties of polymer matrix composites (PMC). PMCs are used in harsh conditions, where, in addition to mechanical properties, functional properties, for example, anti-friction, are of importance as well. For this purpose, special functional additives are additionally loaded into the composite, which, however, can significantly deteriorate the structure of the PMCs as well as their fatigue resistance. At the same time, assessment of damping is a common approach for studying the fatigue resistance of materials, including polymer matrix composites. However, the literature lacks relevant studies in which the relationship is established between the structure of PMCs with their fatigue resistance and damping properties. The paper studies polyetherimide (PEI)-based composites reinforced with SCF with a length of 200 μm and an aspect ratio of AR=20. Polytetrafluoroethylene (PTFE) with an average particle size of 3 μm was used as an antifriction additive. The mass fraction of components in the composition of the composites under study was as follows: in the two-component composite “PEI/SCF”: 90 % PEI + 10 % SCF; in three-component “PEI/SCF/PTFE”: 80 % PEI + 10 % SCF + 10 % PTFE. The purpose of the study was to establish the patterns of correlations “composition-structure-properties” under fatigue loading with the use of the parameters of mechanical hysteresis loops. Fatigue tests were carried out under load control mode at a maximum cycle stress of 35, 45 and 55 MPa for the three component PEI/SCF/PTFE composite, while it was 55, 65 and 75 MPa for the two-component PEI/SCF composite. The load pulse shape was sinusoidal with a cycle asymmetry coefficient of R=0, i.e. the minimum stress in all tests was taken to be zero. During the fatigue tests, two loading blocks were alternated: i) the main loading block with a frequency of 5 Hz and ii) measurement blocks (cycles) for recording the parameters of hysteresis loops with a loading frequency of 1 Hz. During the measuring cycle, load data was recorded and photographs of the sample surface were captured to determine strain and construct hysteresis loops. Based on the obtained mechanical hysteresis loops, the following parameters were determined: loop area, change of the dynamic modulus, and development of cyclic creep. The damping (loss) coefficient (factor) ψ was defined as the ratio of the hysteresis loop area to the area of elastic strain energy. Based on the results of the study, the fatigue properties of the PEI/SCF/PTFE antifriction composite were examined, while some recommendations on their improvement were given.

The study aims at describing characteristic features of acoustic emission (AE) signals during fatigue crack development in the residual stress field pre-created by laser shock peening (LSP). This method consists in the formation of the residual stress field by high-power short-pulse laser impact and generation of the elastic-plastic wave in the material. As a result of such impact, the material structure in the surface layer undergoes changes that affect not only the possibilities of fatigue crack initiation and development, but also the AE characteristics in the process of cyclic deformation. In the course of work, a series of samples from titanium alloy TC4 were subjected to LSP in the stress concentrator zone in order to retard fatigue crack initiation and development. Further, the treated specimens were tested under cyclic deformation conditions to demonstrate the effectiveness of the selected LSP mode. The crack length was measured by the electric potential drop method. AE signals were recorded on the investigated specimens during testing. On the basis of the experimental data obtained, cluster analysis of AE signals was carried out. The AE data were clearly divided into two clusters, which may indicate the presence of two dominant sources of AE signals, qualitatively reflecting the two predominant fracture mechanisms. It has been shown that the value of the cumulative energy of the AE signals in each of the clusters is significantly higher in the post-LSP samples compared to the base samples before the moment of crack growth. Thus, as a result of LSP, significant changes in the evolution of AE signals during cyclic deformation of titanium alloy TC4 specimens with a lateral semicircular notch are observed. This indirectly indicates that LSP affects the structural characteristics of the material, creating a residual stress field and extending the fatigue life of the specimens.

The study decribes the novel developed and verified non-destructive method for damage accumulation quantifying in stress concentration area under low-cycle fatigue conditions. The created approach employs the evolution of plastic print diameter caused by a spherical ball indentation over lifetime. The ball indentation is performed near the hole in plane rectangular specimens with different levels of damage accumulation. The main scientific novelty of the approach resides in involving current damage indicators, which can be reliably derived on a base of the simplest measurements of two in-plane displacement components by speckle-pattern interferometry. Comparison of the results following from the proposed method with analogous data obtained by known destructive approach provides a way to estimate the reliability of non-destructive technique. There is a unique way for comparing two approaches by using the same specimens because plastic prints are applied to the single external face of each specimen. The opposite face is successively used for registration of fringe patterns, which are attributed by narrow notch inserting. Fracture mechanics parameters, which are related to artificial notche and obtained on different stages of low-cycle fatigue, provide damage accumulation function in an explicit form. Experimental dependencies of the notch mouth opening displacement and stress intensity factor from the loading cycle number are constructed by data of fringe patterns interpretation for seven specimens. Damage accumulation functions constructed proceeding from the ball indentation technique and artificial notch inserting method are in good agreement. Thus, high efficiency of the ball indentation approach with respect to quantitative analysis of damage accumulation is clearly substantiated.

The paper presents the results of experimental studies of deformation, acoustic emission and accumulation of micro-damages in plain weave fabric carbon fiber reinforced plastic (CFRP) under cyclic tension along the warp threads. Strain measurements were carried out using an optical extensometer, which ensures accurate positioning of the specimen along the loading direction. The loading program included up to 100 stretching cycles with an asymmetry coefficient of R = 0.1 followed by failure. All specimens passed the pre-cyclic loading program. The maximum stress values in the cycles corresponded to 25...90% of the static tensile strength. It is revealed that in the process of cyclic tension, a one-sided accumulation of strain (cyclic creep) occurs. The accumulated strain depends nonmonotonically on the maximum stress in the cycle, increasing to 600 MPa and decreasing at high stresses. Analysis of the strain kinetics in cycles revealed an increase in the tangential modulus, which is obviously due to the straightening of the fibers. During rupture, an increase of 5-10% in residual strength was noted in comparison with the ultimate strength under single loading. Acoustic emission phenomena occur both in half-cycles of stretching and in half-cycles of unloading, the Kaiser effect is not observed. Cyclic tests at a temperature of 80 ° C showed a sharp decrease in the number of acoustic events and the disappearance of events in the medium frequency ranges. This is due to the fact that heating leads to a decrease in residual technological stresses in the mesostructure elements of the fabric composite, causing changes in the process of micro-damage across the fibers and to a shift in the places where the warp and weft threads intertwine. In all cases, the failure of the specimens occurs when the longitudinal strain reach a value of 1.50 ± 0.06%. This can serve as a basis for predicting strength in other cyclic loading programs.

The paper investigates the influence of preliminary low-velocity loading on the change in residual strength and fatigue characteristics of a fabric composite using modern testing and diagnostic equipment. A new testing method for layered composites is proposed, which consists of a preliminary low-velocity impact with a drop-weight load followed by cyclic compression. A series of tests were conducted on impact with a drop-weight in a wide energy range followed by a quasi-static and cyclic compression of damaged laminated fiberglass plates. Experimental dependences of residual static strength and fatigue life on the energy of preliminary low-velocity impact were obtained. We revealed the existence of a threshold value of the energy of a preliminary impact with a drop-weight load, below which the preliminary dynamic impact does not affect the residual mechanical characteristics. Based on previously developed models and their modifications, an approximation of the experimental data on the residual properties of the composite was carried out. The high descriptive ability of the proposed models was noted. The kinetics of the destruction process under quasi-static compression is analyzed based on data obtained using the VIC-3D contactless optical video system and the digital image correlation method. The non-uniform nature of the distribution of deformation fields on the surface of the damaged samples was noted. Ultrasonic scanning data made it possible to assess the development of the operational damage caused by preliminary low-speed transverse impacts. A relationship was established between the energy of the dynamic impact, residual mechanical characteristics and the characteristic size of the damage zone. The nonlinearity of the obtained dependencies was revealed. A conclusion is made about the need to study the mechanical behavior of polymer composites under conditions of combined effects to ensure the reliability and safety of critical structures.

This paper presents an overview of the accumulated experience in the development and implementation of simulation principles in predicting the residual fatigue life of structural elements during their design and operation in accordance with the concept of tolerance damage. The formulated principles assume the preservation of the accumulated damage under the completed operation of structural elements as heredity of the history of their loading. The proposed sequence of the developed approach implementation includes the analysis of the stress-strain state of the damaged structural element, the development and justification of the geometry and loading conditions in operation, as well as experimental verification and validation by testing simulation models. We analyzed blades and rotating disks of steam turbines and aircraft engines. Configurations and loading conditions of the proposed simulation models reproducing the state of the critical zones of the structural elements are presented. Combinations of conditions of a multi-axial stress state, defect shape, operating temperature and profile of the in-service deformation cycle are considered. Numerical solutions are obtained for the most complex situations of modeling the behavior of surface part-through defects with a combination of mixed modes by separation, longitudinal and transverse shear. The achieved results of residual life estimations are attributed to the composition of original testing devices and methods.