Constant deformation tests specimens

Fig. 9.2 C rings used in constant deformation tests [11], Reprinted, with permission, from ASTM G 38-01 Standard Practice for Making and Using C-Ring Stress-Corrosion Test Specimens, copyright ASTM international West Conshohocken, PA.

Poison s ratio It is the proportion of lateral strain to longitudinal strain under conditions of uniform longitudinal stress within the proportional or elastic limit. When the material s deformation is within the elastic range it results in a lateral to longitudinal strain that will always be constant. In mathematical terms, Poisson s ratio is the diameter of the test specimen before and after elongation divided by the length of the specimen before and after elongation. Poisson s ratio will have more than one value if the material is not isotropic... 

In polymers the time dependence of an modulus plays a more important role than in metals. If polymers are loaded with a constant stress they undergo a deformation e, which increases with time. This process is named creep. Conversely, if a test specimen is elongated to a certain amount and kept under tension, the initial stress s decreases with time. This decay is called stress relaxation. 

A unique and considerably more elaborate multiaxial test employs a thick-walled hollow sphere test specimen which may be pressurized internally or externally with a nearly incompressible liquid. Figure 20 illustrates the essential features of the test device as described by Bennet and Anderson (5). The specimen is prepared by casting propellant in a mold fitted with a sand-poly (vinyl alcohol) mandrel inside the sphere which may be removed easily after curing. A constant displacement rate instrument drives the piston to pressurize the chamber and apply large deformations. The piston s total displacement volume is transferred to... 

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. 

PS, and in the range 142-155 °C for PC. The following mechanical test has been adopted the specimens are stretched at constant elongational strain rate (0.05 s l for PS and 0.025 s l for PC) up to a final extension ratio L/Lq close to 2.5 for all of the samples. The stress is then allowed to relax at constant deformation. 

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

The specimen in these tests is kept at constant deformation stress. The following shapes of... 

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. 

Generally, the corresponding creep strain versus time plots feature a sequence of three stages (1) of axial deformation for a test specimen under constant stress load ... 

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 ) ... 

Deformations and cracking due to shrinkage are tested on specimens in the form of small beams or plates, but most frequently the so-called ring tests are carried out (Figure 11.16). Specimens are cured in constant conditions and deformations are measured at determined intervals. Data on the first crack appearance are used to characterize the material from the shrinkage viewpoint (Grzybowski and Shah 1990 Shah and Weiss 2006). Determination of shrinkage deformations of specimens in variable ambient conditions provides new information on the time-dependent behaviour of concrete structures (Vandewalle 2000). 

The creep behaviour of Lumax GP-5200 (20 wt% of G/F) is good since the deformation (mm) of test specimen exposed to constant stress of 10 MPa at 70°C was very small. Since PBT is prone to thermal hydrolysis, the extended contact of Lumax with water at 90°C leads to the deterioration of properties. In Figures 2 and 3 changes of impact strength and tensile strength are shown as function of immersion time in water at 90°C. 

The test procedure is specified in IPC-TM-650, method 2.4.24.2. The test specimen should consist of a strip of laminate material compatible to the measuring equipment. For all samples with woven reinforcement, it is necessary to make siue that the specimens are cnt parallel or perpendicular to the woven structure. The analysis is based on an assumption of constant specimen geometry therefore, the test specimens must be stiff enough not to deform plastically during the experiment. AH copper needs to be etched off... 

Stress-strain tests were mentioned on page 24 and in Fig. 11-12. In such a tensile test a parallel-sided strip is held in two clamps that are separated at a constant speed, and the force needed to effect this is recorded as a function of clamp separation. The test specimens are usually dogbone shaped to promote deformation between the clamps and deter flow in the clamped portions of the material. The load-elongation data are converted to a stress-strain curve using the relations mentioned on p. 24. These are probably the most widely used of all mechanical tests on polymers. They provide useful information on the behavior of isotropic specimens, but their... 

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. 

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