Finite element modeling creep parameters

No attempt has been made to discuss, in a comprehensive manner, models which are based on finite element calculations or other numerical analyses. Only some results of Schmauder and McMeeking10 for transverse creep of power-law materials were discussed. The main reason that such analyses were, in general, omitted, is that they tend to be in the literature for a small number of specific problems and little has been done to provide comprehensive results for the range of parameters which would be technologically interesting, i.e., volume fractions of reinforcements from zero to 60%, reinforcement aspect ratios from 1 to 106, etc. Attention in this chapter was restricted to cases where comprehensive results could be stated. In almost all cases, this means that only approximate models were available for use. 

Two test cases are used to validate the linear viscoelastic analysis capability implemented in the present finite-element program named NOVA. In the first case, the tensile creep strain in a single eight-noded quadrilateral element was computed for both the plane-stress and plane-strain cases using the program NOVA. The results were then compared to the analytical solution for the plane-strain case presented in Reference 49. A uniform uniaxial tensile load of 13.79 MPa was applied on the test specimen. A three-parameter solid model was used to represent the tensile compliance of the adhesive. The Poisson s ratio was assumed to remain constant with time. The following time-dependent functions were used in Reference 49 to represent the tensile compliance for FM-73M at 72 °C ... 

Next, a model joint (or thick adherend specimen) problem presented in Reference 49 is analyzed using the present program, NOVA. In this case, a linear viscoelastic finite-element analysis was carried out on the model joint under a constant applied load of 4448 N giving an average adhesive shear stress of 13.79 MPa. The specimen geometry, descretization, and boundary conditions are shown in Figure 5. The thickness of the adhesive layer is taken to be 0.254 mm. A nine-parameter solid model was used to represent the tensile creep compliance of FM-73 at 72 °C and is given by... 

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. 

The description of anisotropic creep via an equivalent stress according to Hill was tested for compression creep of WHIPOX . It allows a fast numerical estimation of the CMCs deformation behavior. The Hill model consists of few parameters which can easily be determined with a couple of measurements. One further advantage is the built-in implementation in commercial finite element software, e.g. ANSYS. However, the reliability of the results is limited. For WHIPOX , the most important limitations are the assumed isochoric behavior of the material, tension and compression symmetry as well as the fact that only one stress exponent and creep equation can be specified. 

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