Creep behavior experimental studies

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. 

V. Gaffard, Experimental Study and Modelhng of High Temperature Creep Flow and Damage Behavior of 9CrlMo-NhV Steels and Weldments (These), Ecole des Mines de Paris, 2005. 

Diffusional creep is of great practical importance since it controls the long-term creep deformation at very low deformation rates, which correspond to service conditions. Nevertheless, experimental studies of diffusional creep are rare even for conventional disordered alloys. As to ordered intermetallic alloys, diffusional creep has been studied in the case of an advanced NijAl alloy (Schneibel et al., 1986). Much more work is necessary with respect to the various intermetallic alloys being developed. The diffusion data that are needed to analyze the creep behavior are rare or not available at all for the intermetallic phases of interest (see Chapter 32 by Larikov in this volume). 

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. 

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... 

A complete description of the viscoelastic properties of a material requires information over very long times. To supplement creep and stress relaxation measurements which are limited by experimental limitations, experiments are therefore performed in which an oscillating stress or strain is applied to the specimen. These constitute an important class of experiments for studying the viscoelastic behavior of polymeric solids. In addition to elastic modulus, it is possible to measure by these methods the viscous behavior of the material in terms of characteristic damping parameters. 

Measurement of C requires more sophisticated and expensive rheometers and more involved experimental procedures. It must be remembered that experiments have to he carried out below the critical strain value (see Sec II), or in [he region of linear viscoelastic behavior. This region is determined by measuring the complex modulus G as a function of the applied strain at a constant oscillation frequency (usually 1 Hz). Up to 7, G does not vary with the strain above Yr, G tends to drop. The evaluation of oscillatory parameters is more often restricted to product formulation studies and research. However, a controlled-fall penetrometer may be used to compare the degree of elasticity between different samples. Creep compliance and creep relaxation experiments may be obtained by means of this type of device. In fact, a penetrometer may be the only way to assess viscoeIa.sticity when the sample does not adhere to solid surfaces, or adheres too well, or cures to become a solid or semisolid. This is the case of many dental products such as fillings, impression putties, sealants, and cements. 

During the 1940s and early 1950s, Leaderman [L4,L5],Tobolsky [A2,A3, Dll, T5], Ferry [F3], and others made extensive studies of the small-strain behavior of elastomers and related polymers. These studies involved creep (deformation under applied stress), stress relaxation following applied stresses, and imposed oscillatory strains. These and other experimental techniques used have been described in special detail in the monograph of Ferry [F3]. These studies showed that all of these deformations could be represented in terms of the superposition principle of Boltzmann [B26] [see Eq.(43)]. 

The present study concurs witli the acceptance of 5,0% bending as a maximum upper limit for tensile strength tests However, the Issue is more clouded for tensile stress rupture or creep tests whore the effects of temperature, stress state, and creep rate may severely affect not only the material behavior but also the experimentally measured displacements or tlmes-to-failure. Thus, at this time no upper limit for percent bending for tensile creep or stress rupture can be recommended. 

As discussed briefly in the next section, polymers have a unique response to mechanical loads and are properly treated as materials which in some instances behave as elastic solids and in some instances as viscous fluids. As such their properties (mechanical, electrical, optical, etc.) are time dependent and cannot be treated mathematically by the laws of either solids or fluids. The study of such materials began long before the macromolecu-lar nature of polymers was understood. Indeed, as will be evident in later chapters on viscoelasticity, James Clerk Maxwell (1831-79), a Scottish physicist and the first professor of experimental physics at Cambridge, developed one of the very first mathematical models to explain such peculiar behavior. Lord Kelvin (Sir William Thomson, (1824-1907)), another Scottish physicist, also developed a similar mathematical model. Undoubtedly, each had observed the creep and/or relaxation behavior of natural materials such as pitch, tar, bread dough, etc. and was intrigued to explain such behavior. Of course, these observations were only a minor portion of their overall contributions to the physics of matter. 

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