Fatigue Properties of Polyurethane Foams, with Special Emphasis on Auxetic Foams, Used for Helicopter Pilot Seat Cushion Inserts

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.