Static Mechanical Behavior

Mechanical tests measure the response or deformation of a material to periodic or varying forces. Generally an applied force and its resulting deformation both vary sinusoidally with time. From such tests it is possible to obtain simultaneously an elastic modulus and mechanical damping, the latter of which gives the amount of energy dissipated as heat during the deformation of the material. 

Description of material behavior is basic to all designing applications. Many of the problems that develop may be treated entirely within the framework of plastic s viscoelastic material response. While even these problems may become quite complex because of geometrical and loading conditions, linearity, reversibility, and rate independence generally applicable to elastic material description certainly eases the task of the analyst for dynamic and static loads that include conditions such as creep, fatigue, and impact. 

From past problems it became evident that the physical or mathematical description of the behavior of materials necessary to produce realistic solutions did not exist. Since at least the 1940s, there has been considerable effort expended toward the generation of both experimental data on the dynamic and static mechanical response of materials (steel, RP, URP, etc.) as well as the formulation of realistic constitutive theories. 

Interesting is that metals are unique under both dynamic and static loads that can be cited as outstanding cases. The mechanical engineer 

Summarization of all material behaviors can be by classifications. They include  

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Given the existence of interphases and the multiplicity of components and reactions that interact to form it, a predictive model for a priori prediction of composition, size, structure or behavior is not possible at this time except for the simplest of systems. An in-situ probe that can interogate the interphase and provide spatial chemical and morphological information does not exist. Interfacial static mechanical properties, fracture properties and environmental resistance have been shown to be grealy affected by the interphase. Careful analytical interfacial investigations will be required to quantify the interphase structure. With the proper amount of information, progress may be made to advance the ability to design composite materials in which the interphase can be considered as a material variable so that the proper relationship between composite components will be modified to include the interphase as well as the fiber and matrix (Fig. 26). 

While the static equilibrium behavior of polymer blends in thin film geometry thus is rather well understood, at least in principle, the kinetic behavior (Sects. 2.8,3.3) is much less well understood, since there is a delicate interplay between surface-directed spinodal decomposition, thickness-limited growth of wetting layers, and the hydrodynamic mechanisms of coarsening in this constrained geometry still needs investigation. 

Static mechanical measurements to evaluate the stress-strain relationship in cholesteric sidechain LCEs have been described [71, 72]. In [72] it has been found, for example, thatfor0.94nominal stress Cn is nearly zero as the poly domain structure must be converted first into a monodomain structure. For deformations A < 0.94, the nominal stress increases steeply. Similar results have also been reported elsewhere [71]. The nominal mechanical stress as a function of temperature for fixed compression has also been studied for cholesteric sidechain elastomers [71]. It turns out that the thermoelastic behavior is rather similar as that of the corresponding nematic LCE [2, 5]. 

S.M. Kurtz, M.L. Villarraga, K. Zhao, A.A. Edidin, Static and fatigue mechanical behavior of bone cement with elevated barium sulfate content for treatment of vertebral compression fractures, Biomateri-afr26(2005) 3699-3712. 

Thus, from the known density pb the measured wave speeds Cl and Cs, the elastic constants E and v can be determined. It is noted that the values are given at a high frequency associated with the ultrasound pulse used. The use of these properties at quasi-static loading conditions requires postexamination in order to evaluate whether the aerogel has frequency-or rate-dependent mechanical behavior. 

While TMA refers to a measurement of a static mechanical property, there are also techniques that employ dynamic measurement. In the torsional braid analysis (TEA), a sample is subjected to free torsional oscillation. The natural frequency and the decay of oscillations are measured. This provides information about the viscoelastic behavior of materials. However, these measurements are elaborate and time consuming. In dynamic mechanical analysis (DMA), a sample is exposed to forced oscillations. A large number of useful properties can be measured by this technique see also Section 6.2.6.5. 

Static and dynamic forces play key roles in the complex biochemical and biophysical processes that imderlie cell function. The mechanical behavior of individual cells is of interest for many different biologic processes. Single-ceU mechanics, including growth, cell division, active motion, and contractile mechanisms, can be quite dynamic and provide insight into mechanisms of stress and damage of structures. Cell mechanics can be involved in processes that lie at the root of many diseases and may provide opportunities as focal points for therapeutic interventions. 

At this point, it should be mentioned that for these two MFC systems, PP/PET and PE/PET, static mechanical test data were obtained only for the compositions 1 1 (by wt), but were compared to those of commercial PP and PE composites containing 30% (by wt) glass fibers. An independent study [50] on the effect of blend composition (30/70, 50/50, 70/30 by wt) of PP/PET and PE/PET blends showed that the respective values of the mechanical parameters (cr, E, and s) differ only slightly (around 20%) between blends containing 50 or 30% (by wt) reinforcing component (PET). In other words, the observed differences in the mechanical behavior of the short GF-reinforced composites (30% by wt) and the MFC systems (with 50 wt% reinforcing component) cannot be explained by the various contents of reinforcement, but are due to the specific characteristics of the MFCs. 

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