One of the most relevant geometrical factors defining an adhesive joint is the thickness of the adhesive layer. The influence of the adhesive layer thickness on the joint strength has not been precisely understood so far. This article presents simplified analytical formulas for adhesive joint strength and adhesive joint coefficient for different joint loading, assuming, inter alia: linear-elastic strain of adhesive layer, elastic strain of adherends and only one kind of stress in adhesive. On the basis of the presented adhesive joint coefficient, the butt joint was selected for the tests of the influence of adhesive thickness on the adhesive failure stress. The tests showed clearly that with an increase in the thickness of the tested adhesive layers (up to about 0.17 mm), the value of their failure stress decreased quasi linearly. Furthermore, some adhesive joints (inter alia subjected to shearing) may display the optimum value of the thickness of the adhesive layer in terms of the strength of the joint. Thus, the aim of this work was to explain the phenomenon of optimal adhesive layer thickness in some types of adhesive joints. The verifying test was conducted with use of single simple lap joints. Finally, with the use of the FE method, the authors were able to obtain stresses in the adhesive layers of lap joints for loads that destroyed that joints in the experiment, and the FEM-calculated failure stresses for lap joints were compared with the adhesive failure stresses determined experimentally using the butt specimens. Numerical calculations were conducted with the use of the continuum mechanics approach (stress-based), and the non-linear behavior of the adhesive and plastic strain of the adherends was taken into account.

The background of this work is the search for the most effective ways of joining composites, inter alia in aeronautical applications. The purpose of this study was to analyze the impact of mechanical fastener types on the static strength of lap joints of composite elements and the impact of fasteners on the mechanism of failure of such joints under fatigue load. The second objective was to check to what extent the hybridization of such joints, consisting of supplementing them with an adhesive joint, affects their strength and the mechanism of failure of such joints loaded with fatigue. Damage to composite joints was observed using computed tomography technology. The fasteners used in this study (aluminum rivets, Hi-lok and Jo-Bolt) differed not only in terms of the materials they were made of, but also in terms of the pressure forces they exerted on the joined parts. Finally, in order to check how a partially cracked adhesive joint affects the load on the fasteners, numerical calculations were carried out. Analyzing the results of the research, it was found that partial damage to the adhesive joint of the hybrid joint does not increase the load on the rivets and does not impair the fatigue life of the joint. An important advantage of hybrid joint is the two-stage destruction of the connection, which significantly increases the safety of aircraft structures and facilitates the process of supervising their technical condition.

The focus of this paper is to identify a method for defining the needs of logistical operational support based on the mean time to failure (MTTF) factor. The research was based on a helicopter intended for flight training. The MTTF indicator for selected equipment was determined based on failure data from previous flight operations. As the basic operational data for the developed method, the time from the beginning of the operation or the flight time from the last damage and the method of restoring airworthiness were selected. The MTTF and replacement index for the device were determined. The next step was to determine the index, based on selected probability distributions. The results were analyzed and presented in graphical form, and conclusions were drawn. Based on the MTTF index and replacement index, the logistics needs of selected devices were determined. The obtained results were compared with the actual exchanges of devices made in the year in question. The research proved that the MTTF reliability factor and the analysis of trends in value changes could be used to determine the needs for the logistical security of the operation process, particularly in relation to the equipment subject to accidental failures. This is important for maintaining high availability of an aircraft or other technical objects.

This paper presents the results of experimental research and numerical calculations regarding the static strength and fatigue life of hybrid joints. In the experiments, specimens built as single-lap adhesive–mechanical joints (hybrid joints) were tested. In a two-stage process of the failure of the hybrid joints, the adhesive joint was damaged first. Therefore, it was assumed that the assembly of fasteners closer to the edge of the overlap (beyond the ranges recommended for mechanical joints) limits the negative impact of the peeling phenomenon on the strength and performance properties of hybrid joints. The specimens used in the experiments were prepared from composite elements (i.e., carbon fiber-reinforced polymer (CFRP)), as well as from the aluminum alloy 2024T4. Because the detection of fatigue damage in composite materials is a complex problem, computed tomography was used to evaluate the degradation of the composite material. Experimental and numerical comparative analyses of the static strength and fatigue life of hybrid joints with adhesive and mechanical joints confirmed the assumptions made.

Predicting Fatigue Crack Growth and Fatigue Life Under Variable Amplitude Loading: A probabilistic approach to the description of fatigue crack growth and fatigue life estimation of a component subjected to variable amplitude loading is presented in the paper. The core of the model is a differential equation originated from the Paris formula. In order to consider the influence of overload-underload cycles existing in an exploitive load spectrum on crack growth rate for an aeronautical aluminum alloy sheet, the modified Willenborg retardation model was applied.