Tensile testing importance

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. 

The measurement of mechanical properties is a major part of the domain of characterisation. The tensile test is the key procedure, and this in turn is linked with the various tests to measure fracture toughness... crudely speaking, the capacity to withstand the weakening effects of defects. Elaborate test procedures have been developed to examine resistance to high-speed impact of projectiles, a property of civil (birdstrike on aircraft) as well as military importance. Another kind of lest is needed to measure the elastic moduli in different directions of an anisotropic crystal this is, for instance, vital for the proper exploitation of quartz crystal slices in quartz watches. 

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. 

An important implication of the presence of the shear-extension coupling coefficient is that off-axis (non-principal material direction) tensile loadings for composite materials result in shear deformation in addition to the usual axial extension. This subject is investigated further in Section 2.8. At this point, recognize that Equation (2.97) is a quantification of the foregoing implication for tensile tests and of the qualitative observations made in Section 1.2. 

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. 

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. 

We have written Eq. (5.4) with variables grouped as they are in order to define two very important quantities. The first quantity in parentheses is called the modulus—or in this case, the tensile modulus, E, since a tensile force is being applied. The tensile modulus is sometimes called Young s modulus, elastic modulus, or modulus of elasticity, since it describes the elastic, or recoverable, response to the applied force, as represented by the springs. The second set of parentheses in Eq. (5.4) represents the tensile strain, which is indicated by the Greek lowercase epsilon, e. The strain is defined as the displacement, r — rp, relative to the initial position, rp, so that it is an indication of relative displacement and not absolute displacement. This allows comparisons to be made between tensile test performed at a variety of length scales. Equation (5.4) thus becomes... 

The measurement of dimensions is covered in chapter 7. It is, however, necessary to stress the importance of accurate measurement of the small cross-sectional area of tensile test pieces. 

Three important properties can be inferred from the tensile test i.e. elastic limit (i.e. the point of maximum elastic elongation), tensile strength and E-modulus. In many cases the transition point between the elastic and plastic deformation is not visible in the graph. For that reason it has been determined that this point is situated at an value of 0.002 and the accompanying tensile stress is determined as represented in figure 10.9. 

Another important factor for tensile testing is cyclic testing or applying force and releasing (or relaxing) force on a test specimen to evaluate its ability to endure cyclic fatigue. A typical stress vs. strain relationship is shown in Fig. 3.16 and a cyclic stress relationship in Fig. 3.17. 

Slow strain test. The strain rate chosen frequently for the tests, based on several studies, indicates important susceptibility to cracking at about 2 x 10 6 s 1 for steels, aluminum and magnesium alloys. However, the tests refer to open-circuit conditions and the strain rate sensitivity of cracking is dependent upon potential as well as solution composition. Where necessary the potential of the specimens can be controlled using a potentiostat during slow-strain-rate tensile testing.171 The reduction of area is a simple and appropriate way to quantify the susceptibility to SCC. 

Besides crystallinity itself, the kind and amount of side chains is of importance for crazing. A schematic investigation of the influence of chain branches was performed in PE that were introduced via copolymerization [84], A long-chain branched low-density polyethylene (PE-LD) was compared to a linear low-density polyethylene (PE-LLD) with different short chains [85]. Type and concentration of the copolymers were chosen in order to attain the same density of 0.920 g/cm3 and melt flow rate of 25 g/10 min for all polymers. The ESC resistance was measured in a long-term tensile test of notched specimens at 50 °C in 10% Igepal solution. 

A tensile test on the peel arm is used to obtain the parameters of elastic modulus, plastic modulus and yield strain. In this test, it has been necessary to use an extensometer for measuring strain at small magnitudes (i.e. up to about 2%) in order to obtain sufficient accuracy in the determination of Ei. It is also important to continue the tensile test to fracture, in order to define enough of the plastic region for an accurate... 

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