This paper presents the results of investigations of the effect of graphene oxide and surface shot peening on the mechanical properties and fatigue life of bolts made of austenitic 304 steel. An innovative method for the uniform deposition of graphene oxide on screws is presented. The process involved activating the surface using plasma and then performing graphene oxide deposition using centrifugal force and vacuum drying. The screw specimens prepared in this way were subjected to a surface peening process. Comparative studies have shown that the combination of graphene oxide deposition and shot peening processes results in an increase in fatigue life of approximately 42 ÷ 275% (depending on the stress amplitude level) compared to the as-delivered samples. The results presented are promising and may provide a basis for further research on the application of graphene and its derivatives to increase fatigue life and improve the mechanical properties of machine components.

Abstract Laboratory for Materials Strength Testing (LMST) has been conducting accredited mechanical research for aviation from 2003. Among accredited procedures are e.g. low and high cycle fatigue tests, fracture toughness tests and fatigue crack growth rate tests. The main goal of them is obtaining materials constants and characteristics. However knowledge how to conduct these tests could be used also in other applications, for instance in the work on development of Structural Health Monitoring systems (SHM). When cracks propagate in a controlled way in laboratory conditions, it allows verifying the operation of a single sensor or a network of sensors. In this paper, an overview of mechanical tests carried out at the Laboratory for Materials Strength Testing within Air Force Institute of Technology (AFIT) work on research and development of SHM systems is presented. Specimens prepared from materials such as aluminum alloys (among other withdrawn PZL-130 Orlik TC-II aircraft) and CFRP composite were tested under different mechanical loads, i.e., cycle and impact loads. In the presented research, both constant amplitude and spectrum loads were applied.

Abstract In this work, the compressive residual strength tests results, Compression After Impact (CAI), are presented. The specimens were made of carbon-epoxy prepreg E722-02 UHS 130-14. Two variants of specimens were tested: samples undamaged and samples with damage that was centrally introduced by a drop-weight impact, as per the ASTM D7136/7136M standard. An impactor with potential energy equal to 15J and the type of support required by the standard were used. The size of impacted damages, defined as an area of damage on a plane perpendicular to the impact direction, and the equivalent diameter were specified using the flash thermography method. The tests were performed using the fixtures manufactured according to the ASTM D7137/7137M standard. The specimens were compressed to determine the residual strength. This value was afterwards used to specify the force levels for the fatigue tests. The fatigue tests were carried out under force control – with a sinusoidal shape, stress ratio R equal to 0.1 and frequency f 1Hz. Maximum force in a loading cycle Pmax was being increased after each thousand of cycles N until its value was close to the residual strength determined in the previously mentioned tests. In this work, the following relationships were presented: force-displacement P-δ for both static and fatigue tests and displacement-loading cycles δ-N for fatigue tests. A method of conducting the fatigue tests of CFRP composite was proposed, in which both the CAI specimens and CAI fixture were used. This allowed researchers to accelerate making initial comparisons between the two groups of specimens with damages – grouped relative to the way of conditioning.

Abstract The objective of the research presented in this paper was to determine the honeycomb core compliance of a sandwich structure of the horizontal stabilizer of the MiG-29 fighter jet in the static compression test. The study of the specimen was conducted based on the ASTM C365/C365M standard. The article presents the results of experimentally determined dependencies and strength parameters, i.e. the force-displacement dependence, the compressive modulus and the honeycomb core deformations.

Abstract The article presents a method for measuring Poisson’s ratio of the polyurethane foam, including shapes of the auxetic foam. The impact of technology applied in Air Force Institute of Technology, that is a method for processing the selected polyurethane foams, on the value of Poisson’s ratio of the auxetic foam shape was presented. In order to manufacture the auxetic foam shapes, two methods; mechanical-thermal and mechanical-chemical-thermal, were applied.

AbstractThe Composite Patch Bonded Repair (CPBR) is one of the most cost-efficient types of aircraft structure repair. In the CPBR, the damaged structure is reinforced by applying a composite patch. The boron-epoxy composite (BFRP) is a popular choice for these types of repair. The BFRP was utilized to repair the cracked resistance welding joints in the upper flap of the MiG-29’s RD-33 engine intake. In the present paper, the numerical results are shown, along with the comparative tests of the undamaged inlet flap, the damaged inlet flap and of the CPBR repaired inlet flap.

The issue of energy absorption during impact is present in various aspects of life. The possibility of dissipating unwanted energy gives huge opportunities for a variety applications such as helmets, car bumpers, smart body armours and protective pads. Nevertheless, there are numerous technical problems with achieving a compromise between good energy absorption efficiency and other important properties such as flexibility, weight and thickness. The article describes a study of composite structures based on shear thickening fluids (STF) and auxetic foams. The composites are developed as a potential component of products with high energy absorbing efficiency. The study reports on the rheological behavior of STF and force absorbing properties of the manufactured composites. In the experiment, two types of STF and eleven types of auxetic foams were used. Force absorbing tests for the produced samples were performed by dropping an impactor with the energy of 5 J. It was proved that the addition of STF to the auxetic foams increases the force absorbing efficiency.

Abstract The article described selected methods for determining a softening point of plastics (polymers). The way of determining a softening point of polyurethane foams was suggested. Moreover, the influence of annealing temperature of an auxetic polyurethane foam on its volume was presented.

One of the basic methods for joining composites are adhesive joints. In contrast to traditional methods used in connecting metal structures (bolting, riveting), bonding ensures uniform stress distribution. Bonding requires appropriate technological conditions and proper surface preparation. In the case of bonding composites, surface preparation methods are based on mechanical processing. The authors conducted a study to compare the different methods of surface preparation used in the adhesive bonding of composites. As a parameter defining the efficiency of the method, stress failure in a tensile test was used. Specimens were made based on the ASTM-D1002 standard using a carbon-epoxy prepreg and DP-490 3M adhesive. The article contains the results of tensile tests for three types of surface treatment.

Polyurethane foams processed according to a hybrid mechanical-chemical-thermal (M-Ch-T) process developed at Air Force Institute of Technology (AFIT) have been tested to verify the inner structure deformation after the process. The X-ray microtomography of both the original EUROFOAM S28280 foam and the processed specimens allowed the creation of three-dimensional virtual models of tested specimens and enabled evaluation of homogeneity of foam structures. In order to examine variations in the processed material's inner structure, samples from different locations of the specimen were scanned. Moreover, a foam specimen was divided into nine subspecimens to perform a set of static tests on each of them separately and to check the gradient of properties throughout the sample volume. Presented methods allowed to visualize the inner structure of the processed foams as well as to estimate inhomogeneity. Obtained results will be used for optimization of the manufacturing process. (C) 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Abstract Seat cushion inserts in military helicopters crew seats, as suggested by the helicopters manufacturers, are made of traditional polyurethane foams. Elastic polyurethane auxetic foams are materials that exhibit different utility properties compared to traditionally used polyurethane foams, such as polyether or polystyrene foams. All the differences result from the primary physical property of elastic polyurethane auxetic foams which is a negative Poisson’s ratio. Auxetic materials are characterized by better utility properties than conventional foam materials – they can potentially increase safety in the event of a crash and offer higher comfort during regular use. Application of auxetic materials as seat cushion inserts would also decrease harmful health effects of vibrations. This paper presents the results of the fatigue tests carried out on different foam samples by pressing an indenter into the foams' surface that was much larger than the indenter’s surface. A maximum value of the load used during the test was within a defined range in every fatigue cycle. In order to test 150×150×50 mm foam samples a special indenter was designed and manufactured according to the PN-EN ISO 3385 and PN-EN ISO 2439 standards. The indenter’s dimensions were consistent with the standards in relation to the tested foams' size. The fatigue tests of both conventional and auxetic foams were carried out according to the above given standards by applying 80,000 load cycles at 70 cycle/min frequency. Tests of viscoelastic foam and multilayer foam specimens, for which the upper layer was made of viscoelastic foam, were carried out according to the ASTM D 3574 standard applying 12,000 load cycles at 10 cycle/min frequency. All the tests were carried out using the MTS 370.10 strength testing machine. Changes in thickness and density were determined throughout the tests. Moreover, the influence of the volumetric compression ratio on the fatigue properties of auxetic foam samples and the dependence of foam deflection on the number of cycles were examined. Finally, the test results obtained for conventional and auxetic foams were compared and discussed.

The paper has been intended to present some modification of Least Squares Method (LSM) as used for describing of experimentally gained fatigue crack propagation data by means of the NASGRO equation. In particular, the specific nature of the NASGRO equation and consequent difficulties with theoretical description of test data have been shown. An algorithm has been presented of how to find coefficients of the NASGRO equation for the modified LSM criterion. Computations have been performed for the aluminum alloy 2024 taken from the rotor blades of Mi-8 helicopter.

Auxetic materials exhibit counterintuitive and interesting properties, which can be utilized to develop new families of products, which can meet sophisticated needs. Study presented within this paper is focused on manufacturing and testing auxetic polyurethane foams for potential introduction for seats in military helicopters. Firstly, a manufacturing process was developed which benefits from the well‐known methods of processing conventional foams and allows preparing necessary test specimens for further tests. Secondly, a series of static and dynamic tests was performed during which the market available conventional foams used in transport are compared with processed auxetic foams.

Modification of the least squares method criteria is presented. It is intended to enhance the approximation of test data for fatigue crack growth rate. In particular, changes are introduced to improve approximation in the cases when the range of values for at least one of the variables is within the range of several orders of magnitude or differs from the range of values for other variables. Differences between particular criteria and their influence on the approximation of test data is illustrated further on. Calculations are conducted for aluminum alloy 2024 taken from Mi-2 helicopter rotor blades.

Abstract Air Force Institute of Technology participates in the service life assessment programme SEWST. The aim of this programme, funded by the Polish Ministry of Defense, is to modify the operation system of PZL-130 "Orlik" TC-II turbo propelled trainer aircraft. The structural part of the programme is focused on the Full Scale Fatigue Test of the whole airframe to be conducted at the VZLU in the Czech Republic. The load spectrum for the test was developed by the AFIT based on the flight test results. The basic load block represents 200 simulated flight hours and consists of 194 flights showing different levels of severity. At the end of the Full Scale Fatigue Test a teardown inspection is planned during which it would be most beneficial to be able to determine crack propagation rate by means of a crack surface inspection. Markers are usually visible on most fatigue crack surfaces, however they occur randomly therefore it is almost impossible to conclude anything about the crack history. Since the preliminary load block consisted of separate flights (flight loads together with landing and taxing loads) showing significantly different levels of severity, the easiest way to modify the load block was to change the order of flights within the block. Hence a pilot programme was started at the AFIT which was focused on the determination of the influence of flight sequence on crack appearance. Several load blocks were determined using various techniques of rearranging the order of flights within the preliminary load spectrum. This approach ensured the preservation of the initial severity of the load block and simultaneously enabled a significant increase in the probability of the markers occurrence introducing neither artificial underloads nor overloads that would most probably affect the crack propagation rate. Fatigue crack surfaces were inspected using Scanning Electron Microscope. As a result of the investigations a series of images were obtained showing the specimen microstructure with visible markers arranged in the desired sequences. Based on the obtained pictures the most promising load block arrangements were chosen for the Full Scale Fatigue Test.

ZL-130TCII Fracture Markers Solution for Full-Scale Fatigue TestIn the context of PZL-130TC-II full-scale fatigue test, several strategies of fatigue loadings that create fracture surface markings were considered. One block of spectrum is made of 200 flights. By reordering those flights, a block which should create a fracture marker, was developed. It was very important that reordering the load spectrum or adding overloads or underloads did not change spectrum severity. Pilot tests of aluminium alloys specimens were carried out to finalise appropriate marker intervals before commencing full-scale tests. The experiment was conducted with the MTS machine with 810.23 system. The results and conclusions are presented within this paper.