Constant deformation tests tensile specimen

Fig. 9.5 Tensile specimen used in constant-deformation test [17]. Reprinted, with permission, from ASTM G 49-85 Standard Practice for Preparation and Use of Direct Tension Stress-Corrosion Test Specimens, copyright ASTM international, West Conshohocken, PA.

Plastic deformation is commonly measured by measuring the strain as a function of time at a constant load and temperature. The data is usually plotted as strain versus time. Deformation strain can be measured under many possible loading configurations. Because of problems associated with the preparation and gripping of tensile specimens, plastic deformation data are often collected using bend and compression tests. 

ISO 4600 details a ball or pin impression method for determining the ESCR. In this procedure, a hole of specified diameter is drilled in the plastic. An oversized ball or pin is inserted into the hole, and the polymer is exposed to a stress cracking agent. The applied deformation, given by the diameter of the ball or pin, is constant. The test is multiaxial, relatively easy to perform, and with not very well-defined specimens, and the influence of the surface is limited. Drawbacks are the small testing surface and the undefined stress state. After exposure, tensile or flexural tests may be performed on the specimens. This leads to the determination of either the residual tensile strength or the residual deformation at break. 

There exists a related but different German Standard DIN 53 442 which uses dumb-bell-shaped specimens differing from those used for tensile testing by a rounded middle section. Another difference in comparison with the above ASTM method is the use of constant deformation amplitude of the vibrations. This results in a stress amplitude decreasing with time due to stress relaxation. Apart from this, the stress amplitude diminishes also due to the heating of the specimen. The results are reported in a similar manner as required by the ASTM standard with the stress amplitude relating to the first cycle. 

Characterization of the viscoelastic properties of polymers are classified into two categories static and dynamic measurements. The static mechanical tests involve creep, stress relaxation, and stress-strain measurements. In a creep test, a constant stress is applied to the specimen, and its deformation is measured as a function of time. In a stress relaxation test, the specimen is deformed a fixed amount, and the change in the stress is measured as a function of time. The stress-strain measurement is carried out by stretching the sample at constant tensile speed and then recording the load and deformation simultaneously. 

In the tensile creep test according to ISO 22088, Part 2 [264], specimens are exposed to various loads immersed in a medium, and the times to fracture are determined as functions of load. The time-to-fracture curves thus obtained provide indications as to service life reduction due to stress cracking relative to a reference medium (most often air). Typical results of such time-to-fracture curves are shown in Figure 2.30. This testing method is particularly sensitive, because due to the constant loads present, stress relaxation is not possible. Thus, this method is also suitable for thermoplastics in which the stress generated under constant deformation is strongly reduced by relaxation over time. 

Strain rate This is determined from the specimen gauge length during tensile test. In evaluation of SCO, a constant strain rate of lo-6sec l is applied to a tensile specimen in a given environment It represents the rate of deformation of a metal. 

German Standards DIN 53442 Constant amplitude of deformation Dumbbell shaped (tensile dumbbell) test specimens Variable frequency... 

Creep can be defined as deformation that occurs over time when the material is subjected to constant stress at constant temperature. In this investigation, the creep was measured in tension. The molded tensile bars with 0.5-in. taper were placed in an Instron 1331 servohydraulic testing machine, in load control, using a fixed mean level of 120 kg, and an amplitude of zero. The elevated test temperature of 80°C is achieved using a Thermotron environmental chamber. Testing is controlled by an IBM-compatible PC running Instron MAX software. Failure times (hours to creep rupture) were averaged for three specimens. 

Figure 5.17a represents tensile test specimens (compositions and designations in Table 5.1), before and after testing, obtained by optical micrographs, shown in Fig. 5.17b. As may be seen in 5.6a, grades B, C and D exhibit quite large strain, but specimens A, E, F and G fracture at elongations of less than 15 %. The effect of the strain rate on the tensile deformation is illustrated for specimen D (see Table) at 1600 and 1650 °C in Fig. 5.18a and that of the temperature at a constant strain rate is seen in Fig. 5.19b. The generally known fact that the temperature has an opposite effect on the flow curves and on strain hardening may also be seen in Fig. 5.18.

The simplest mechanical test method is tensile testing, where a rectangular or dumbbellshaped specimen is placed between two clamps and then uniaxially drawn with constant speed (64,65). In the case of pure elastic deformation, the stress a and the resulting deformation are proportional to each other. The original dimensions of the test specimen are completely and immediately restored after removal of the stress. The proportionality constant E is called the modulus. It is given by Hooke s law (Eq. 19), where a is the tensile stress (N m ), y the strain, and E Young s modulus (N m ) ... 

There are several modes of operation of DMAs. The most common is the rotational/torsional type of instrument, although a number of linear tensile-compressive type are now available. These may operate in either a constant-strain or a constant-stress mode. In the former, the specimen is always deflected to a defined strain while the stress is measured. Constant-stress machines are the converse. These instruments are preferred for creepmode type experiments while constant-strain instruments lend themselves better to stress-relaxation studies. The decision of what type of DMA to use for a test then depends not only on the type of behavior under study but also on the mode of deformation. Torsional DMAs provide data in a shear mode, while tensile-compressive machines yield a tensile or compressive mode. With the application of proper fixtures, the linear DMAs are also able to perform flexural and cantilever-type measurements. 

Stress-strain behavior represents the response of a material to loading. Tests are performed on a universal testing machine (UTM), sometimes referred to as a tensile tester because of the primary mode of deformation used to characterize this form of behavior. Specimens are typically deformed at a constant speed, for reasons explained later. Since the properties vary significantly with temperature, tests may be conducted within an environmental chamber to obtain data at elevated and subambient temperatures. The most common information obtained from these tests are the modulus and tensile strength. 

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