Deflection-compression tests

Mechanical Tests. Hardness measurements taken with a Shore A durometer are reported every time a pad specimen has been removed from its water bath for compression-deflection tests. Separate flat slabs, 0.12 and 0.50 in. thick, of polyurethane elastomer, which were cast from the same batch of material and given identical exposure to water as the pads, were employed for the hardness tests. 

The compressive strength of the foam is not affected by the evacuation process. Compression-deflection tests of a truly evacuated foam were essentially the same as those for an unevacuated foam. 

Figure 3a. Typical plot from the RS/1 program showing the linear relationship between Mean Curvature and the tested valnes for 40% Compression Deflection of the polynrethane foam samples.

Compression Force Deflection Test ASTM D 3574 - Test C. This test consists in measuring the force necessary to produce a 50% compression over the entire top area of a flexible foam specimen. The... 

Constant-Deflection-Compression-Set Test ASTM D 3574 -Test D. This test consists in deflecting the flexible foam specimen under specified conditions of time and temperature and noting the effect on the thickness of the specimen. Ordinarily the specimen is deflected to 50%, 75%, or 90% of its thickness. The entire assembly is then placed in a mechanically convected air oven at 70°C (158°F) and 5% RH for 22 hours. Following this exposure the specimen is then removed from the apparatus and the recovered thickness measured. The constant-deflection compression set, expressed as a percentage of the original thickness, is then calculated. 

FTC measurements were conducted 90 seconds after demoulding. The foam pad was removed from the mould, weighed and placed in the FTC apparatus (Instron 4502). The force detection device is equipped with a 5.0 kN capacity pressure transducer. The actual force is shown on a digital display. This device mimics the ASTM D3574, Indentation Force Deflection Test and provides a numerical value of freshly demoulded foams initial hardness or softness. The pad was compressed to 70 percent of its original thickness at a cross head velocity of 380 mm per minute with the force necessary to achieve the highest compression cycle recorded in Newtons. Values are reported as the FTC value for the foam based on the assumption that the lower the FTC values the better the dimensional stability of the foam. 

The compression force deflection test consists of measuring the load necessary to produce a 50 percent compression over the entire top area of the foam specimen. This test differs from the IFD test described previously in that the flat compression foot used in the test is larger than the specimen. The compression foot is brought into contact with the specimen. The specimen is compressed 50 percent of its original thickness and after 1 min the final load is observed. The compression force deflection is recorded as the load required for a 50 percent compression of specimen area. 

ASTM D5470 measures the thermal impedance of a specimen under load. The thermal impedence is a function of the thickness of the test specimen. In the case of elastomer gap filler materials, the height changes as the load is applied to the sample. The softer the sample, the thinner the sample is during the test. Thus, the effects of increased filler amount are masked by the compression/deflection characteristics of the sample. 

Creep-test specimens may be loaded in tension or flexure (to a lesser degree in compression) in a constant temperature environment. With the load kept constant, deflection or strain is recorded at regular intervals of hours, days, weeks, months, or years. Generally, results are obtained at three or more stress levels. 

Combined liquid crystalline polymers, 49 Combustion testing, 245 Composites, thermoplastic, 32 Compression force deflection (CPD), 244 Compression tests, 242 Condensation... 

When a plastic is subjected to an external load the observed stiffness changes with time. In a creep test the load is kept constant leading to an increase in strain. In a stress-relaxation test the deflection (frequently compression) is kept constant so that the stress is observed to relax. The changes will be primarily due to physical effects, and the strains may be reversible if sufficient time is allowed. At long durations the applied load can lead to failure, known as creep-rupture or stress-rupture. 

The Vicat softening temperature is the temperature at which a standard deflection occurs for defined test samples subjected to a given linear temperature increase and a compression loading from a defined indenter of a specified weight. The load used is often ION (Vicat A) or SON (Vicat B) and must be indicated with the results. In either case the polymer cannot be used under this compression load at this temperature. 

Two techniques allow determination of the compressibility of foam. The methods are related and the application dictates which method better defines the quality of the product. For example, for furniture cushions, indentation force deflection (IFD) is a more common test technique than compression force deflection. Both methods determine the amount of force required to compress a foam to a percentage of its thickness. In the IFD test, the plunger that compresses the foam is smaller than the foam sample. This presumably correlates to a person sitting in the center of a chair cushion. 

Deformation in compression, or a mixture of bend and compression, is quite often used very successfully in ad hoc tests on complete products, for example rubber bellows. Care must be taken when formulating product tests using simple and inexpensive apparatus that such details as the rate of application of the force or the dwell time before noting deflection are carefully standardised because these can have a large effect on the result obtained. 

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