Time-tensile tests

Creep curves or lines are determined in so-called time-tensile tests for extensive deformations, i.e., far beyond the range of linearity. Instead of maintaining a constant stress level, the simpler mode of load application is used, namely constant initial stress. Fig. 27. 

The paper discusses the application of dynamic indentation method and apparatus for the evaluation of viscoelastic properties of polymeric materials. The three-element model of viscoelastic material has been used to calculate the rigidity and the viscosity. Using a measurements of the indentation as a function of a current velocity change on impact with the material under test, the contact force and the displacement diagrams as a function of time are plotted. Experimental results of the testing of polyvinyl chloride cable coating by dynamic indentation method and data of the static tensile test are presented. 

Pressure Vessels. Refineries have many pressure vessels, e.g., hydrocracker reactors, cokers, and catalytic cracking regenerators, that operate within the creep range, i.e., above 650°F. However, the phenomenon of creep does not become an important factor until temperatures are over 800°F. Below this temperature, the design stresses are usually based on the short-time, elevated temperature, tensile test. 

Whether or not a polymer is rubbery or glass-like depends on the relative values of t and v. If t is much less than v, the orientation time, then in the time available little deformation occurs and the rubber behaves like a solid. This is the case in tests normally carried out with a material such as polystyrene at room temperature where the orientation time has a large value, much greater than the usual time scale of an experiment. On the other hand if t is much greater than there will be time for deformation and the material will be rubbery, as is normally the case with tests carried out on natural rubber at room temperature. It is, however, vital to note the dependence on the time scale of the experiment. Thus a material which shows rubbery behaviour in normal tensile tests could appear to be quite stiff if it were subjected to very high frequency vibrational stresses. 

A loop tack (Fig. 2c) test consists of allowing a tear-shaped loop of conditioned tape to drape into contact with a test surface of specified area (usually 25.4 x 25.4 mm), with the force of contact limited to the weight of the tape itself (ASTM Ref. D-6195). The ends of the loop are held in a tensile tester. After a momentary contact time the tester is engaged and the tape is removed at a specified speed. The maximum in the removal force is ordinarily observed just at the point where the two peel fronts Join. The value is reported in a force per area of tape width, or lb in. -. While this tack test has some popularity, it is perhaps more of a very short dwell time peel test, and it has variables more associated with that test, especially backing effects, since heavier backings lead to higher tack values. 

This equation is the basis of linear viscoelasticity and simply indicates that, in a tensile test for example, for a fixed value of elapsed time, the stress will be directly proportional to the strain. The different types of response described are shown schematically in Fig. 2.1. 

These latter curves are particularly important when they are obtained experimentally because they are less time consuming and require less specimen preparation than creep curves. Isochronous graphs at several time intervals can also be used to build up creep curves and indicate areas where the main experimental creep programme could be most profitably concentrated. They are also popular as evaluations of deformational behaviour because the data presentation is similar to the conventional tensile test data referred to in Section 2.3. It is interesting to note that the isochronous test method only differs from that of a conventional incremental loading tensile test in that (a) the presence of creep is recognised, and (b) the memory which the material has for its stress history is accounted for by the recovery periods. 

When testing finished wire-rope tensile test specimens to their breaking strength, suitable sockets shall be attached by the correct method. The length of test specimen shall not be less than 3 ft (0.91 m) between sockets for wire ropes up to 1-in. (25.4 mm) diameter and not less than 5 ft (1.52 m) between sockets for wire ropes 1 -J-in. (28.6 mm) to 3-in. (77 mm) diameter. On wire ropes larger than 3 in. (77 mm), the clear length of the test specimen shall be at least 20 times the rope diameter. The test shall be valid if failure occurs 2 in. (50.8 mm) from the sockets or holding mechanism. 

In an experiment similar to that referred to on p.4.100, tensile test bars were exposed at Clifton Junction, Manchester, for six months, during which time they were sprayed three times daily with sea-water. Whereas exposure to industrial atmosphere alone had little effect, bars of the same alloys were much more heavily attacked by sea-water spray. 

The concept of a ductile-to-brittle transition temperature in plastics is likewise well known in metals, notched metal products being more prone to brittle failure than unnotched specimens. Of course there are major differences, such as the short time moduli of many plastics compared with those in steel, that may be 30 x 106 psi (207 x 106 kPa). Although the ductile metals often undergo local necking during a tensile test, followed by failure in the neck, many ductile plastics exhibit the phenomenon called a propagating neck. Tliese different engineering characteristics also have important effects on certain aspects of impact resistance. 

For the product designer, however, a simple basic test, such as a tensile test, will help determine which plastic is best to meet the performance requirements of a product. At times, a complex test may be required. The test or tests to be used will depend on the product s performance requirements. 

Data analysis routines may change with time, and it is desirable to be able to reanalyze old data with new analysis software. Our tensile test analysis software creates plots of engineering stress as a function of engineering strain, as illustrated in Figure 3. Our flexure test software plots maximum fiber stress as a function of maximum fiber strain, with the option of including Poisson s ratio in the calculations. Both routines generate printed reports which present the test results in tabular form, as illustrated in Figure 4. 

Tensile testing is an important part of the physical characterization of free film coatings. The fundamental properties measured relate directly to performance properties of the coating. Because of the time required to obtain and analyze tensile data, a laboratory which routinely performs tensile tests may find that an automated system is needed. Although commercial packages are available, it is feasible to develop an in-house system with relatively little expense. This paper describes one such system as implemented at Glidden Coatings and Resins with very satisfactory results. 

An interesting feature of polarized IR spectroscopy is that rapid measurements can be performed while preserving molecular information (in contrast with birefringence) and without the need for a synchrotron source (X-ray diffraction). Time-resolved IRLD studies are almost exclusively realized in transmission because of its compatibility with various types of tensile testing devices. In the simplest implementation, p- and s-polarized spectra are sequentially acquired while the sample is deformed and/or relaxing. The time resolution is generally limited to several seconds per spectrum by the acquisition time of two spectra and by the speed at which the polarizer can be rotated. Siesler et al. have used such a rheo-optical technique to study the dynamics of multiple polymers and copolymers [40]. 

If the applied shear stress varies during the experiment, e.g. in a tensile test at a constant strain rate, the relaxation time of the activated transitions changes during the test. This is analogous to the concept of a reduced time, which has been introduced to model the acceleration of the relaxation processes due to the deformation. It is proposed that the reduced time is related to the transition rate of an Eyring process [58]. The differential Eq. 123 for the transition rate is rewritten as... 

The tensile strength of compacts [30] also provides useful information. Excellent specimens of square compacts are necessary to conduct the tensile testing. For this reason, a split die [31 ] (Fig. 2) is used to make compacts that are not flawed. The split die permits triaxial decompression, which relieves the stresses in the compact more uniformly in three dimensions and minimizes cracking. These specimens are then compressed with platens 0.4 times the width of the square compacts in the tensile testing apparatus. (Fig. 3). Occasionally nylon platens and side supports are used to reduce the tendency to fail in shear rather than tension. The force necessary to cause tensile failure (tensile forces are a maximum... 

What effect will an increase in the time of testing have on tensile strength ... 

It can be concluded that it is very difficult to predict the result from a polymer macrostructure, but it is relatively easy to measure the secondary species generated on irradiation by using known analytical techniques, such as measuring swelling, tensile tests, analysis using nuclear magnetic resonance (NMR), etc. The yield is then expressed by the G value, which represents the number of cross-links, scissions, double bonds, etc., produced for every 100 eV (1.6 X 10 J) dissipated in the material. For example, G (cross-links), abbreviated G(X), = 3.5 means that 3.5 cross-links are formed in the polymer per 100 eV under certain irradiation conditions. Similarly, the number of scissions formed is denoted by G(S). In order to determine the number of crosslinks or G(X), the number of scissions or G(S), etc., it is necessary to know the dose or dose rate and the time of exposure for these irradiation conditions. From the product yields it is possible to estimate what ratio of monomer units in a polymer is affected by irradiation. ... 

The TS of the compacted samples was determined by transverse compression with a custom-built tensile tester. Tensile failure was observed for all the rectangular compacts when compressed between flat-faced platens at a speed ranging between 0.006 and 0.016 mm/sec. Platen speed was adjusted between materials to maintain a time constant of 15 2 seconds to account for viscoelastic differences the constant is the time between the sample break point and when the measured force equals Fbreak/e in the force versus time profile, where the denominator is the mathematical e. Specially modified punch and die sets permitted the formation of square compacts with a centrally located hole (0.11 cm diameter) that acted as a stress concentrator during tensile testing. This capability permitted the determination of a compromised compact TS and thus facilitated an assessment of the defect sensitivity of each compacted material. At least two replicate determinations were performed for each mechanical testing procedure and mean values are reported. 

Tests for scorch and rate of cure should be distinguished from tests for degree of cure or optimum cure measured on the vulcanised material. The latter type of test estimates degree of cure by measuring the physical properties of test pieces vulcanised for various times, tensile properties, swelling and set measurements being the parameters most commonly used. 

In some ways modem tensile testing machines have reduced the need for a separate, particularly simple, routine control test. However, a test which is both simple in the sense of measuring one parameter and provides a relaxed modulus is intrinsically attractive. Such tests in various forms have existed for a long time but do not seem to have attained widespread popularity. A version in which a fixed stress is applied and the elongation after 1 min noted is given in ASTM D145689. A specific instrument developed for this... 

Whatever the selected method (static, monotonous, or dynamic), it gives access to a limited range of timescales. For example it is almost impossible to perform static experiments in times less than 1 s, or dynamic tests at frequencies lower than 10 1 Hz, or tensile tests at strain rates higher than 103 s-1. These timescales are, however, indirectly accessible because the polymers generally obey a time-temperature superposition principle ... 

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