Uniaxial creep

Although nearly all creep and stress-relaxation tests are made in uniaxial tension, it is possible to make biaxial tests in which two stresses are applied at 90° to one another, as discussed in Section VI. In a uniaxial test there is a contraction in the transverse direction, but in a biaxial test the transverse contraction is reduced or even prevented. As a result, biaxial creep is less than uniaxial creep--in cquihiaxial loading it is roughly hall as much for equivalent loading conditions. In the linear region the biaxial strain 2 in each direction is (255.256)... 

Uniaxial creep experiments were carried out on two types of commercial polyethylene In the presence of various environments. One polymer, which will be designated hereafter as sample D, was a linear polyethylene having a weight-average molecular weight (My) of 99,000 and a nominal density of 0.966 g/cm. ... 

As mentioned already, the main consideration In this work will be the failure occurring as a result of necking during the uniaxial creep of polyethylene at relatively high loadings. Recently, Bernstein and Zapas [2] have extended the work of Erlcksen [3] on the instability of elastic bars to the case of viscoelastic materials, more specifically to the class of materials which behave according to the BKZ theory [4]. 

Equation 6.14 provides a formal connection between creep crack growth and the kinetics of creep deformation in that the steady-state crack growth rates can be predicted from the data on uniaxial creep deformation. Such a comparison was made by Yin et al. [3] and is reconstructed here to correct for the previously described discrepancies in the location of the crack-tip coordinates (from dr/2 to dr) with respect to the microstructural features, and in the fracture and crack growth models. Steady-state creep deformation and crack growth rate data on an AlSl 4340 steel (tempered at 477 K), obtained by Landes and Wei [2] at 297, 353, and 413 K, were used. (AU of these temperatures were below the homologous temperature of about 450 K.) The sensitivity of the model to ys, N, and cr is assessed. 

Integrating Equation 8.32 gives an equation for strain (e). This equation predicts uniaxial creep behavior for both drained and imdrained conditions. A minimum of two creep tests is required to determine the values of the parameters A, m, and a. 

Special specimen preparation as with tensile testing. However, the extraction of intrinsic mechanical parameters from creep indentation data is analytically complex [3, 4]. Confined compression or unconfined compression tests require preparation of cylindrical cored specimens of tissue and underlying bone. With unconfined compression, the free draining tissue edges and low aspect ratio, layered nature of the test specimen may introduce error. Compression of a laterally confined specimen by a porous plunger produces uniaxial deformation and fluid flow. Confined compression creep data has been analyzed to yield an aggregate equilibrium compressive modulus and permeability coefficient [5] and uniaxial creep compliance [6]. 

Q. Wei, et al., "High Temperature Uniaxial Creep Behavior of a Sintered In Situ Reinforced Silicon Nitride Ceramics", Ceramic Engineering Science Proceedings, ed. E. Usttlndag and G. Fischman, 20(3), 1999, The American Ceramic Society Westerville, OH, 463-470 G. Ziegler, "Thermo-Mechanical Properties of Silicon Nitride and Their Dependence on Microstructure", Mat. Sci. Forum, 47, 162-203 (1989)... 

More recently, in a series of papers Zapas and Crissman [5-7] reported the results of uniaxial creep experiments for different polyethylenes under varied... 

Thermal conductivity Water absorption Compressive modulus Uniaxial creep... 

Let us consider a simple uniaxial creep response. If a creep stress a is given by... 

The analytical solution of Eq. (31) can be obtained using the diagonalization technique or modal projection (Kreyszig, 2006), This is fully detailed in (Fafard et al., 2001). The specific solution for the uniaxial creep case ( =cr = constant) of a cylindrical sample is... 

Finally, for the uniaxial creep case (constant loading) of a cylindrical sample, Eq. (49) can be rewritten as... 

F. Lamouroux, M. Steen and J. L. Valles, Damage of a 2D AlzOs/SiC composite during uniaxial creep. Comp. Sci. Tech. 56, 825 830 (1996). 

Fig. 11.16 Uniaxial creep to yield of polycarbonate (Data from Brinson, (1973)).

Fig. 11.17 Uniaxial creep to failure of (a) Metlbond 1113-2 and (b) SMC-25 (Data from Gartner, (1978)).

In an early attempt by Pao and Marin [6] at a three-dimensional extension of their uniaxial creep law Equation (11.1), strains in the principal directions are given by terms such as... 

Fig. 4 Typical example of the uniaxial creep behaviour of cold-worked Zlrcaloy-2 under irradiation...

Figure 10.16 Uniaxial creep rupture strength of 9Cr and llCr-ODS steel bars at 700°C.

There appears to be no information on the uniaxial creep of polymers used as structural adhesives such as is available referring to the creep of adhesive joints in lap-shear or torsion. The latter is reserved for Chapter 7 where the few data that are available are given. An apparent exception is the careful study of a nylon-epoxy adhesive (FM 1000) by Shen and Rutherford (1972). These authors achieved something approaching uniaxial stress using a cylindrical butt joint in direct tension. However, what they called creep was simply the delayed elastic response. They likened the adhesive to a metal in its behaviour and used the classical but inappropriate concept of separating the creep behaviour into three stages as was done many years ago for metals. 

The finite element description of the nonlinear viscoelastic behavior of technical fabric was presented by Klosowski et al. [65]. The technical fabric called Panama used in this model was made of two polyester thread families woven perpendicularly to each other with the 2/2 weave. The long term uniaxial creep laboratory tests in directions were conducted at five different constant stress levels. The dense net model [66] together with the Schapery one-integral viscoelastic constitutive model [67] was assumed for the fabric behavior characterization and the least square method in the Levenberg-Marquardt variant was used for the parameters identification. 

This polymer in its commercial form exhibits sufficient photoconductivity to be used in imaging processes in spite of its stereochemical impurity and conformational disorder. Simple uniaxial creep experiments on thin films above the glass transition temperature were shown to produce only small orientations, i.e. order parameters < 2) when defined in the... 

Despite the rationale for the above general formulations, it has proven remarkably difficult to quantify them in terms of crack tip plasticity and to independently verify their accuracy. For instance, uniaxial creep deformation laws at low homologous temperatures are not necessarily applicable to the multiaxial stress conditions in the surface region adjacent to the crack tip, and the use of linear-elastic fracture mechanics must have limitations for the near-crack-tip, high-plastic-deformation conditions. 

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