Material property test after curing time

These tests were: the tensile tearing test, compression test and the torsion test. In all of the tests volume change in the material during the process had occured. It is interesting, how the same material responds to the increasing of loads in each experiment. Important conclusions may be derived from these tests for the valid region of the used theories in the calculations 3.2.1 Quality control tests in the factory

Firstly the quality control test of the factory is described. The aim of this test is to define the comprehensive strength of the polyurethane soft foam. This test is carried out, on randomly chosen finished seat foams that are taken out of the batch. The test method is based on the MSZ 10193/4-78 Hungarian national standard that was adapted for the test of SUZUKI car seat foams and for IKARUS bus seats. The comprehensive tensile strength test machine ESR-02 was developed by IMAG. A 200 mm diameter flat disc is pushed into the foam to a speci-fied depth and the spring back force is recorded in Newtons after 20 second has lapsed.

The measuring procedure in detail for softer car seats are:

− The machine touches the foam by a 5N compressing force, this position of the pressing disc is the zero pressing point,

− the foam is pressed with constant velocity until 75% of its initial thickness and then de-pressed with the same velocity,

− new zero pressing point setting (same procedure as previously),

− The foam is pressed with constant velocity until 20% of its initial thickness,

− after 20 seconds the pressing force is registered.

This compression force is considered as the hardness of the foam by the factory. A foam qual-ity is considered acceptable, if the hardness measured by this procedure is within the pre-scribed range. Some seat foams have different hardness e.g. bus seats. The test procedure is almost the same as for car seats with the following differences; the foam is pressed until 70%

of its initial thickness and the second pressing is until 60% of its initial thickness, the pressing force is registered after 30 seconds. The ESZ-2 testing machine is computer controlled and the

pressing forces, waiting times are stored for the foam part types as default values. This ma-chine can be seen on Fig. 18 during a testing of a car seat.

Fig. 18: Measuring the hardness of a seat foam for quality control at IMAG Ltd.

The hardness of the foams is measured according to ISO 2439: 1980 standard and always given in Newtons. At IMAG Ltd. the hardness is measured only for quality assurance. Be-cause these measurements are done on special equipment on finished parts, the hardness of different products cannot be compared and the results are not independent of the geometry of the test pieces, hence these data cannot serve as an input for a general mechanical analysis.

3.2.2 Tensile test

The most important and fundamental test of material testing is the tensile test. Important data such as initial Young’s modulus, tearing strength, tensile strength, percentage of stretch, tearing stretch can be acquired together with the tensile curve. From this curve, the properties of the material under tensile load can be obtained in the function of strain.

The test was carried out at the Department of Polymer Engineering of the Technical Univer-sity of Budapest (with the kind help of the department staff) according to ISO EN 8067:1989 standard. The test pieces were cut out from every tested foam material by the use of a cutting stamp on a screw press, to ensure the precise shape for every piece. All test pieces were torn on a Zwick-WS4 tensile test machine with the tearing speed of 100 mm/min. The environ-mental conditions were 21°C and 85% humidity. For the test a special stretch measuring adapter was used to measure the displacement precisely for the determination of the moduli.

Three specimens of every foam with the same parameters were tested (altogether 45), the val-ues presented in Table 17 are the nominal valval-ues of the three tests. There was no significant difference between the tearing strength and the shape of the curves in each kind of foam, so there was no need for further specimens. The definition of tensile strength is:





= ₂

0 max

mm N A

R_m F (⁴⁹)

Where: Fmax [N] is the maximum tension force registered at disruption, A0 [mm²] is the initial cross section of the tensile test specimen

Fig. 19.: The tensile curve in the function of stress and strain, of foams 6, 13 and 15

Tensile strength, ultimate strain, Young’s modulus shear modulus and Mooney-Rivlin con-stants values can be seen in Table 17 of Appendix A.7.3. Some tensile curves obtained from the experiments can be seen on Fig. 19.These curves are almost linear with a little progressiv-ity, close to parabolic. At this speed the deformation may be taken as infinitesimally slow, so no flow disturbance can be observed. The break of the test pieces always happened at a plane surface perpendicular to the axis of tearing. The curve is always strictly monotone rising till the point of tear. The values may be approximated by taking the curve as a straight line from the origin to the end. This characteristic is valid for every foam that was tested. The correla-tion between the tensile strength and the ultimate stress is that the slope of the tensile curve is increasing by the increase of the hardness of the foam material.

3.2.3 Compression test

One of the most important type of test for polyurethane materials is the indentation or com-pression test. The shape of the curve obtained from this experiment best characterises the ma-terial with its distinctive shape. It is also important, because almost all polyurethane mama-terial is used for compression loads, for seats and springs (See: introduction), so the response of the compression has to be known very precisely. From the test the shape of the compression curve with the indentation modulus and the a,b,c parameters for the proposed analytical func-tion for the compression curve can be obtained. The maximum compression force is of less importance, that is the reason they are not stated here. The shape and characteristic of the curve is more important. All the test specimens were cut out of each foam by a band saw with the dimension of 100x100x100 mms (three from each foam). The compression test was car-ried out at the Department of Polymer Engineering of the Technical University of Budapest according to ISO 2439 : 1980 Polymeric materials-Determination of Hardness standard with the environmental conditions of 21°C and 85% humidity. The used machine was the Zwick-WS4 set to compression measuring. The test was conducted with the help of the staff of the department. The foams were fixed on the bottom part of the machine and a plate was attached to the force measuring cell. The compression speed was set to 100mm/min, which can be said to be small. All specimens were compressed to 25% of its initial height (75% of compres-sion). Every measurement was recorded on registering paper. There was no significant differ-ence between the shapes of the curves in each kind of foam, so there was no need for further test specimens. A characteristic curve of the tests of foam 1 can be seen on Fig. 20. The char-acteristics of the compression curves of the tested foams can be seen in Appendix A.7.1, to-gether with a table of the calculated a, b, c parameters.

Compression curve for foam 1

0 2 4 6 8 10 12 14 16 18

0 0,2 0,4 0,6 0,8 1

compression strain ε [-]

σ comp [kPa]

loading unloading

Fig. 20: The loading and unloading of compression of foam 1

3.2.4 Torsion test

Torsion is a non-homogeneous deformation. If the deformation is infinitesimal, the defor-mation does not require volume change. This is not true however for compressible materials.

During the deformation the planes perpendicular to the axis of torsion, stay parallel to each other. By the torsion, the middle part of the specimen contracts and thus reduces its volume (Fig. 26). The length of the specimen kept constant by the machine. Since there were no data concerning the torsional behaviour of these materials test had to be conducted to acquire the behaviour and parameters for the theories. There were also problems for the procedure of the test, because there were no standards found for the experiment. The task was to measure the torsion shear stress τt in the function of the shear strain γ. The equipment has to measure very small forces and torques. The test specimens must have a precise shape for the required preci-sion of the test. The tests were carried out at the WELMAT Ltd. in Budapest, Hungary with the kind help of the staff of the Ltd). The test specimens were designed after consultations with the chief engineer (Fehérváry Attila) of the company. The test equipment was assembled at the company for this experiment from a torsional tester connected to a tensile test machine that could measure very small forces. The equipment can be seen on Fig. 21.

Fig. 21: a, b. Test equipment for the torsion test

1, Thread transmitting the force 2,Test piece, 3, Thread making the rotation of the registering cylinder connected to the torsion machine, 4, Torsion machine, 5, Manual lever, 6, registering cylinder, 7, tensile machine, 8, rod transmitting the force from the torque of the torsion machine to the tensile machine, 9 force measuring scale.

All the test pieces were cut out from each foam with a hole saw specially designed and manufactured for this test (Fig. 22). The test pieces are all cylinders of Ø100 mms in diameter and 100 mms of height. These dimensions had to be chosen carefully to be in the measuring range of the test equipment. If they were too small the torque would not had been measurable.

They all have a Ø20 mm hole through their axis for construction reasons and they had to be cut, because all standards required the pieces to be cut.

8 3

1

5 2

6

4 7

9

Fig. 22:Hole saw Fig. 23: Cut out test pieces

It was also a difficult task to exert the torque evenly on the end faces of the cylinders and to manage that this torque is parallel to the axis of the cylinder. The end faces of the cylinder had to be parallel to each other and care had to be taken that the axis of the cylinder is always per-pendicular to them. To achieve these requirements a device for each test pieces were designed and manufactured. It is made of two pieces. It consist of a tube of an outer diameter of Ø19 mm which has a cylinder inserted in it (both are made of steel). The inside diameter of the tube and the outside of the cylinder is matched carefully to allow proper movement. Both pieces have a large disc welded to them. The two end faces of the foam pieces are glued to these discs with a special bond. The end of the tube and the cylinder has two parallel surfaces made on them. This has the shape for the torsional tester for gripping. The device and the test pieces can be seen on Fig. 24 and Fig. 25.

Fig. 24:The lining device Fig. 25: Test pieces for the experiment

Each test pieces were put on the equipment one by one. One end face of the test piece was fixed to the torsion end of the machine and the other to a tensile test machine. By turning manually the machine, it exerted a torque on the foam which was measure by the tensile ma-chine. The tensile tester measures force, but it was transformed to torque via a 134 mm long axel that was inserted between the tensile and the torsion machine. The torque/twist curve and its values were registered on a roll-graph. The feed of the paper was controlled by the twist of the cylinder and the displacement movement of the registering pen was controlled by the force from the tensile tester. This force was then calculated to torque by the following expres-sion Every torexpres-sion experiment was made to 90° angle of twist.

ax ten

t F k

T = (50)

where: Tt is the torsional torque [Nm], Ften is the force measured by the tensile test machine [N] and kax

is the length of the axel between the two machines, the length is 0.134 m

Fig. 26: The necking of the specimen at large torsion angle (90˚)

The curves gained by the experiments can be seen on Fig. 27. All curves show a degressive characteristic in the function of the increased twist angle (Appendix A.7.2). It is due to the necking of the specimen and the compressibility of the material (Fig. 26). It is monotone, but not strictly monotone, because the curve has a number of steps in them, due to the structure of the material. The curve of test foam 7 can be seen on Fig. 27

0,00 0,20 0,40 0,60 0,80 1,00 1,20 1,40 1,60

0,00 0,20 0,40 0,60 0,80 1,00 1,20 1,40 1,60

φ torsion angle [rad]

Torque [Nm]

Torsion curve Polinom. (Torsion curve)

Fig. 27: Torsion curve, torsional torque v.s. torsion angle of test foam 7.