Uniaxial tensile experiments

Note that /3 and /4 are stress components in the plane of isotropy and, therefore, have the same Weibull parameters. The parameters i and /3i would be obtained from uniaxial tensile experiments along the material orientation direction, dt. The parameters a2 and /Efe would be obtained from torsional experiments of thin-walled tubular specimens where the shear stress is applied across the material orientation direction. The final two parameters, a3 and /33, would be obtained from uniaxial tensile experiments transverse to the material orientation direction. 

Fig. 5 Volume dilatation for a rubber specimen undergoing a uniaxial tensile experiment [62], The volume change remainslimited over a wide strain range...

Uniaxial tensile experiments were conducted on a block copolymer and the stress was measured as a function of strain ... 

Hordijk, D.A. and Reinhardt, H.W., Fracture of concrete in uniaxial tensile experiments as influenced by curing conditions. Int. Conf. on Fracture and Damage of Concrete and Rock, Vienna, July 1988. 

Studies carried out in uniaxial tensile experiments mirror some of the processing conditions in industrial production. Uniaxial tensile tests were conducted at temperatures of 85°C-105°C and stretch rate of 7.5 to 12.5 mm/s. Several tests were conducted at each testing condition to verify repeatability. The results of these tests are plotted in terms of true stress vs. strain. The strain is defined as extension ratio of the current length, 1, to the initial length, lo, given as follows ... 

The shear component of the applied stress appears to be the major factor in causing yielding. The uniaxial tensile stress in a conventional stress-strain experiment can be resolved into a shear stress and a dilational (negative compressive) stress normal to the parallel sides of test specimens ofthe type shown in Fig. 11-20. Yielding occurs when the shear strain energy reaches a critical value that depends on the material, according to the von Mises yield criterion, which applies fairly well to polymers. 

For uniaxial tensile testing, dog bone-shaped samples are placed between two clamps and stretched at constant extension rates. Similarly, for unconfined compression tests, cylindrical specimens are compressed between two parallel plates. From these experiments, three important quantities can be determined (Fig. 4.16) ... 

There are no convenient data from low-temperature shear or uniaxial tensile or compressive large-strain deformation experiments available because of the onset of a brittle-like response as discussed in Section 7.5.5 for metallic glasses. Therefore, we evaluate the model above in the context of the simulations by Demkowicz and Argon of the plasticity of amorphous Si. This provides some direct comparisons of the model with the results presented in Fig. 7.20(a) and formally separates the brittle-like response from the plastic response. For the detailed comparison we note that, for Si, v = 0.42, giving By = 0.282 and = 0.544, and that // = 39.7 GPa at 300 K. Since the simulations presented in Fig. 7.20(a) pertain to the most slowly quenched structure, we take = 0.22 and complete the parameter selection by choosing A in eq. (7.31) as 1.38 for 0 K and 1.667 for 300 K, resulting in corresponding steady-state concentrations of of 0.45 for 0 K and 0.375 for 300 K. The best choice for the characteristic relaxation shear strain y for both cases is taken as 0.1. 

Correspondingly, eq. (13.2) representing the strain-rate dependence of the plastic resistance is taken to be given by a standard uniaxial reference experiment at a reference strain rate e gf, typically of magnitude 10 s that evokes a reference tensile uniaxial plastic resistance o-j-gf, which in this case would be the tensile yield stress o-q. The form of the idealized power law relating eg to Ug is given by the exponent m of the equivalent stress, which must be temperature-dependent in a form given in Chapter 8 as... 

Triple-shape polymers can change on demand from a first shape (A) to a second shape (B) and from there to a third shape (C), when stimulated by two subsequent temperature increases [10, 26, 27]. Specific cyclic, thermomechanical tensile experiments were developed to characterize the triple-shape effect (Chapter Shape-Memory Polymers and Shape-Changing Polymers [101] and Sect. 2.2) quantitatively. Analogous to the experiments for dual-shape materials, each cycle of these tests consisted of a programming and a recovery module. A cycle started with creating the two temporary shapes (B and A) by a two-step uniaxial deformation, followed by the recovery module, where shape (B) and finally shape (C) were recovered. 

An important issue is the influence of an electrochemical environment on the cyclic deformation behavior of metals [74,33-35]. As illustrated by the data in Fig. 1 for a carbon-manganese steel in high-temperature water, environment does not typically affect the relationship between stresses and strains derived from the maximum tensile (or compressive) points of steady-state (saturation) hysteresis loops [36]. Such loops should relate to elastic and plastic deformation prior to substantial CF microcracking. CF data of the sort shown in Fig, 1 are produced by either stress or total strain controlled uniaxial fatigue experiments, identical to the methods... 

A direct simple method to study the viscoelastic properties of a given sample is the creep experiment It is carried out by instantaneously applying a constant force, which is then followed by a measurement of the resulting deformation as a function of time. Figure 5.1 indicates schematically a possible result, referring to the case where an uniaxial tensile load is applied, which then leads to an elongation AL. In general, it will be found that the creep curve represents a superposition of three contributions... 

To facilitate understanding the low tensile force behavior of CKA and calmodulin upon forced uniaxial extension from their two ends, a steered molecular dynamics simulation (SMD) was carried out by the same group. The SMD tries to simulate the uniaxial pulling experiment by inserting a virtual harmonic spring between one end of the sample polymer and a hard surface representing cantilever or substrate surfaces. 

In order to determine which is the most appropriate yield criterion for a particular polymer it is necessary to follow the yield behaviour under a variety of states of stress. This is most conveniently done by working in plane stress = 0) and making measurements in pure shear (o- = -0-2) and biaxial tension (o-i, 02 > 0) as well as in the simple uniaxial cases. The results of such experiments on glassy polystyrene are shown in Fig. 5.28. The modified von Mises and Tresca envelopes are also plotted. In both cases they have been fitted to the measured uniaxial tensile and compressive yield stresses, oy, and oy. It can be seen that the von Mises... 

Even though the uniaxial tensile test is one of the simplest experimental methods, it provides essential information about the mechanical behavior and properties of materials. In this experiment, a nominally smooth dogbone specimen is subjected to uniaxial deformation at a constant strain rate... 

It can be concluded that the model for a uniaxial tensile test, as described in this paper, is suitable for studying several phenomena that occur in uniaxial tensile tests on softening materials like concrete. For simulating an experiment as good as possible, FE-calculations should be performed. With the model and a small computer, however, it is possible to get a better understanding of the behaviour of a specimen in a uniaxial tensile test and of the influence of several parameters on this behaviour, rather quickly. 

As far as the typical shape of the descending branch, a plateau followed by a steep drop, in a uniaxial tensile test on concrete is concerned, it is demonstrated that this shape can be the result of the macro-structural effect as discussed in this paper. The results do not necessarily support the explanation for the typical shape of the descending branch, as proposed by Van Mier and Nooru-Mohamed [8]. The influence of specimen dimensions and boundary conditions on this behaviour, as observed in experiments, was confirmed by calculations with the model. 

Example 14.7 Obtain an expression for the net tensile stress (negleeting the solvent contribution) in a uniaxial extension experiment aceording to the upper-convected Maxwell equation. 

Uniaxial tensile tests were conducted at different strain rates on neat 1 wt%, 2 wt% and 3 wt% vapor-grown carbon nanofiber-reinforced epoxy. Based on the experiment results, the following conclusions were made ... 

Table 1. Summary of bulk mechanical properties arising from uniaxial tensile tests and fracture mechanics experiments.

An exact derivation and implementation of the constitutive equations needed for this multimode model is not given here the interested reader is refeued to References 13 and 67. What is important to note is that the method just desctihed allows the determination of the relaxation spectrum from a simple set of uniaxial tensile or compression experiments at different strain rates and does not require lengthy relaxation experiments. The nonlinearity in stress is the same as used in the flow stress itself... 

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