Heat buildup test

An unfortunate confusion of terminology in rubber testing should be noted. The term flexometer has traditionally been generically applied to apparatus for heat buildup tests, whereas such apparatus generally work in compression or shear or a combination of the two. 

Shear, compression, or torsion cycles with constant stress or constant strain amplitude and various cycle shapes are all perfectly feasible but not commonly used except for the heat buildup tests below. A procedure for plastics taken from metals testing invoices rotating a cylindrical test piece with its ends constrained by bearings that are misaligned. The result is that each element of the test piece goes through a sinusoidal cycle from tension to compression. 

Flexometer tests are used to determine thermal stability under dynamic straining conditions. Measurements include temperature rise after a specified period of cycling, set and creep, and in some instances the time or number of cycles to failure in the form of thermal runaway or test piece destruction. In contrast to fatigue cracking tests, heat buildup tests... 

The Goodrich Flexometer is used for heat buildup testing (ASTM D-623). A cylindrical test specimen is alternately compressed and then released in rapid cycles for a specified period of time. The temperature rise within the test specimen is recorded. Additional data obtained are permanent set and static compression. The Goodrich Flexometer Test is sometimes called the Compression Fatigue Test, because the test specimen is fatigued, or weakened, by the rapidly cycling compression stress. 

The rate dependence of fatigue strength demands careful consideration of the potential for heat buildup in both the fatigue test and in service. Generally, since the buildup is a function of the viscous component of the material, the materials that tend toward... 

The important vulcanizate properties demanded by this application, low heat buildup, and low dynamic set have been determined in the Goodrich Flexometer test. The compound containing the antireversion agent exhibits a marked decrease in heat buildup and dynamic permanent set (Figures 14.21 and 14.22). 

Perkalink 900 is also active in compounds based on blends of NR with the synthetic elastomers SBR and BR. An evaluation in a tank pad formulation has provided evidence of reduced heat buildup on overcure overcure is a common problem in the manufacture of these relatively bulky components. The control and test formulations, in which two levels of the antireversion agent have been evaluated, are fisted in Table 14.52. Cure characteristics are given in Table 14.53. 

Incidentally, there are customers who come and ask, I know you have stated in your Abs Max table that I shouldn t apply more than 24V to the device. But what if I apply 28V for just lms The principled answer to that is, you can t apply even 24.01V, for even 10-12 seconds The company officially doesn t stand by it. Yes, internally they do test at higher stress levels than published, and have also got various guard-bands present (for their protection and reputation). But remember you don t know what these are. Also, keep in mind that voltage overstress leads to almost instantaneous death, whereas current ratings are related more to internal heat buildup, so you can always exceed them somewhat for a short time. 

An important difference between apparatus of type (a), (b), (e) or (f) and (c) or (d) is that in the former case the test piece is continuously and totally in contact with the abradant and there is no chance for the very considerable heat generated at the contact surface to be dissipated. The actual rate of slip will influence the rate of wear because, as the speed is increased, heat buildup will rise. Temperature rise during test is one of the important factors in obtaining correlation between laboratory and service. 

Dynamic heat buildup in applications such as wheels is important. Polyurethanes with a high resilience or a low tan 8 in the operating range are important. Polyurethane elastomers used in wheel applications can be evaluated using a test rig where the urethane can be run under load for a fixed period or until failure. 

The amount of testing and verification carried out depends largely on the importance of the part being produced and its future function. An end stop for a tube may only need the appearance, hardness, and the fact that it will correctly fit the tube checked. A flexible mounting of similar size also will need extensive testing to prove that it has the correct dynamic properties and can take the load and vibrations and not fail due to excessive heat buildup. 

A roller may be subjected to pressure in a hydraulic press to simulate actual service conditions. The deflection at various loads can be noted. Wheels can be run on a test rig at service speeds under a load and the life and heat buildup recorded. 

Antivibration mounts can be tested by measuring the reduction in the forces through the part under simulated working conditions. Any heat buildup also must be monitored. The tests may be carried out in an environmental chamber if nonambient temperatures are being used. 

In a so called heal buildup test the prime object is to induce a temperature rise in the lest piece, to measure its magnitude, and to study its effects. The heating is a result of the viscoelastic nature of polymers some of the work done in stressing the material is dissipated by viscous forces between molecules and converted to heat energy. Compression, shear, or some combination of them is normally used with relatively bulky test pieces. The tests are almost exclusively applied to rubbers and were generally intended to be relevant to tires. The geometries used are inevitably arbitrary and involve superimposing dynamic strain or stress cycles onto a prestress or strain. 

The prime factor in choosing an abradant is its relevance to service, but it also has to be available in a convenient form and, for anything but ad hoc tests, it is essential that it be reproducible. In consequence of these considerations, abrasive wheels and papers or cloths predominate where cutting by sharp asperities is to be simulated. The abrasive wheel is probably the most convenient, because of its low cost, its mechanical stability, and the ease with which it can be refaced to maintain a consistent surface. Abrasive papers and cloths are cheap and easy to use but are not so readily refaced and will deteriorate in cutting power more quickly. Although basically low in cost, both wheels and papers are a considerable c.xpense when bought as standard reference materials. Materials such as textiles or smooth metal plates are more relevant for some applications, but they abrade relatively slowly, and if conditions are accelerated they give rise to excessive heat buildup. 

As has been previously explained, rubber test methods differ in that, to avoid heat buildup, the velocity range is restricted. Many methods were described by James [18] in a review covering pedestrian friction and more recently by Mitsuhashi [19] in a review-covering both friction and abrasion. The two most versatile testers are those described by James and Newell [5] and more recently by Roberts [20]. Both avoid the misalignment problem spoken of in BS 903, part A6l [4] and both incorporate automatic control of the load arm. 

Use of empirically derived heat buildup data and optical properties measurements can significantly improve a producefs ability to predict maximum field service temperatures of vinyl materials. Use of empirical field methods described here in addition to laboratory tests can identify robust design criteria, enhance a product s service performance and ultimately contribute to customer satisfaction. 

The rate dependence of fatigue strength demands carefiil consideration of the potential for heat buildup in both the fatigue test and in service. Generally, since the buildup is a function of the viscous component of the material, the materials that tend toward viscous behavior will also display sensitivity to cyclic load frequency. Thus, TPs, particularly the crystalline polymers like polyethylene that are above their glass-transition temperatures, are expected to be more sensitive to the cyclic load rate, and highly crosslinked plastics or glass fiber reinforced TS plastics are much less sensitive to the frequency of load. 

B. Degradation Tests. Thickened solutions for mechanical degradation were prepared to 37.0-40.0 Pa s viscosities (t - 12s ), and exposed to mechanical stresses in a Waring blendor ( 4 speed). Water (lO C) was passed through an exterior jacket to minimize heat buildup in the liquid under deformation. 

Figure C.9 Effect of cure time on the physical properties of elastomers. AT is the heat buildup in a compression-flex test. (Ref Billmeyer, F.W., Textbook of Polymer Science, Interscience, New York, 1965, p. 539)...

Polypropylene Plastics Using a Biaxial Rotator D4803 Test Method for Predicting Heat Buildup in PVC... 

The core components of the apparatus consist of one cold plate and a guarded hot plate. A sample of the fabric or insulating wadding to be tested, 330 mm in diameter and disc-shaped, is placed over the heated hot metal plate the sample is heated by the hot plate, and the temperature on both sides of the sample is recorded using thermocouples. The apparatus is encased in a fan-assisted cabinet, and the fan ensures enough air movement to prevent heat buildup around the sample and also isolates the test... 

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