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 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.

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.