Creep behavior experimental results

Although a reference stress and strain rate was used in the above discussion, practical composites may work in a regime far away from this equilibrium point this does not affect the results of the above discussion. As discussed in detail elsewhere,31 plots such as Fig. 5.4 are useful for estimating the constituent creep behavior from experimental studies of the initial and final creep behavior of a composite. 

Thus, the aim of the present paper is not to review the bulk of the results published to date relating to the creep response of various magnesium-based composites. Instead of such an approach this paper provides a comprehensive report on the extensive experimental results obtained by authors in an investigation of the high temperature creep behavior of the two magnesium alloys, AZ 91 and QE 22, and their discontinuous composites. The objective of the present research is a further attempt to clarify the direct and indirect strengthening effects of short-fiber and particulate reinforcements in creep of magnesium-matrix composites. 

Having described creep in single-crystal MgO, now it is appropriate to illustrate the behavior of its polycrystaUine form as well. But first, note that the experimental results (see Fig. 6.30) resemble the shape found in Fig. 6.1b. In addition to the transient curve seen in the Fig. 6.30, this experimental transient curve is joined with the steady-state creep curve. The solid line represents the fitting of both these parts of the creep along the experimental points. The transient creep, k, is expressed by an exponential decay relation, given as ... 

The comparison of calculated and experimental data of the creep curves showed a good correlation. After comparing the calculated results it can be concluded that the viscoelastic behavior of the technical fabric can be described by the one-integral model. In warp and weft directions the numerical curve fitting resulted in a difference of 3.5-9.9% and 0.3-2.6% respectively. Therefore, the Schapery model with the power function characterized more accurate creep behavior in weft direction than warp direction. Also the power function described the strain evaluation better than the exponential function. This research concluded that both the linear and nonlinear viscoelastic identifications based on different material models can be brought together and the results of linear characterization can be applied to the nonlinear description of the material. 

The reinforcement effect of sisal fiber content on the flexural creep performance and flexural modulus of cellulose derivatives/ starch composites was studied by A1 Verez et al. [15]. Fiber content and temperature effects were also considered, taking into account various methods and equations. At short times, a creep power law was employed. A master curve with the Arrhenius model was used to determine the creep resistance at longer times and different temperatures. Good fitting of the experimental results with the four-parameter model was reported, leading to a relationship between the observed creep behavior and the composite morphology. The addition of sisal fibers to the polymeric matrix promoted a significant improvement of the composite creep resistance. 

Quite likely the sliding process is mainly responsible for the high creep rates of the 30 , 45 or 60° unit cells. Yet, a Norton aeep law was assigned to both fibers and matrix. Assuming that the matrix dominates creep in the 90 samples, its creep behavior can be better described by a primary creep law with regard to the experimental results (90 curve. Figure 4 left) ... 

Experimental results of a quasi-unidirectional CMC revealed different creep behavior in tension and compression. Due to the porous matrix, the samples with 90° fiber orientation showed the largest creep deformation rates. In tension creep the fracture strain was 0.4-1% for 90° samples whereas in 0° fiber orientation creep strain above 8% was feasible. At the beginning of compression experiments, the absolute creep rate was highest and decreased continually. Creep parameters, i.e. temperature and stress dependencies, were deteimined in case of compression stress. Thereby, the activation energy averaged to 700 kJ/mol and the stress exponent varied with the fiber orientation from 3 to 1.9. The variation in the stress exponent was presumably caused by different stress exponents of fibers and matrix and quite likely due to the effect of matrix compaction. Latter one could be visualized by optical micrographs. 

In a subsequent series of experiments, Landes and Wei [2] demonstrated that the phenomenon is real, and modeled the crack growth response in terms of creep deformation rate within the crack-tip process zone. The effort has been further substantiated by the work of Yin et al. [3]. The results and model development from these studies are briefly summarized, and extension to probabihstic considerations is reviewed. It is hoped that this effort will be extended to understand the behavior of other systems, and affirm a mechanistic basis for understanding and design against creep-dominated failures. The author relies principally on the earher works of Li et aL [1], Landes and Wei [2], Yin et al. [3], Krafft [4] and Krafft and Mulherin [5]. The findings rely principally on the laborious experimental measurements by Landes and Wei [2], and the conceptual modeling framework by Kraftt... 

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