Temperature dependence, mechanical

The radiation and temperature dependent mechanical properties of viscoelastic materials (modulus and loss) are of great interest throughout the plastics, polymer, and rubber from initial design to routine production. There are a number of laboratory research instruments are available to determine these properties. All these hardness tests conducted on polymeric materials involve the penetration of the sample under consideration by loaded spheres or other geometric shapes [1]. Most of these tests are to some extent arbitrary because the penetration of an indenter into viscoelastic material increases with time. For example, standard durometer test (the "Shore A") is widely used to measure the static "hardness" or resistance to indentation. However, it does not measure basic material properties, and its results depend on the specimen geometry (it is difficult to make available the identity of the initial position of the devices on cylinder or spherical surfaces while measuring) and test conditions, and some arbitrary time must be selected to compare different materials. 

The WLF approach is a general extension of the VTF treatment to characterize relaxation processes in amorphous systems. Any temperature-dependent mechanical relaxation process, R, can be expressed in terms of a universal scaling law ... 

The PEG could stabilize proteins by two different temperature-dependent mechanisms. At lower temperatures, it is preferentially excluded from the protein surface but has been shown to interact with the unfolded form of the protein at higher temperatures, given its amphipathic nature (57). Thus, at lower temperatures, it may protect proteins via the mechanism of preferential exclusion, but at higher temperatures possibly by reducing the number of productive collisions between unfolded molecules. PEG is also a cryoprotectant and has been employed in Recombinate, a lyophilized formulation of recombinant Antihemophilic Factor, which utilizes PEG 3350 at a concentration of 1.5mg/mL. The low-molecular weight liquid PEGs (PEG 300-600) can be contaminated with peroxides and cause protein oxidation. If used, the peroxide content in the raw material must be minimized and controlled throughout its shelf life. The same holds true for polysorbates (discussed below). 

Table 1. Temperature Dependent Mechanical Properties of Ni and the 40% Al2O3-60% Ni...

The binding of ACh to synaptic vesicles has been studied in some detail. ACh was released from the vesicles by a temperature-dependent mechanism (Barker et al., 1967). It was not spontaneously released by the action of cations (Matsuda et al., 1968 Takeno et al., 1969). ACh was still bound to vesicles after gelfiltration in iso-osmotic sucrose, but was lost after gelfiltration in water (Marchbanks, 1968). These studies indicate that the vesicular ACh is protected behind a semipermeable membrane. 

Figure 5. Temperature dependent mechanical spectrum of completely cured adhesive.

Decomposition temperatures, whether sharp or vague, are seldom actually reached in service life. Reinforced composites are preeminently load-bearing materials, and it is their temperature-dependent mechanical properties, such as T, or the closely related heat distortion temperature, that usually determine the maximum use temperature, at least for short or intermediate term use. Strength, yield stress and modulus all decline with increasing temperature, reflecting the increasing mobility of the molecular structure, and unacceptable levels of physical property loss will often occur well before the onset of thermal or thermo-oxidative degradation. 

K Schulte, K Friedrich and G Hostenkamp, Temperature dependent mechanical behaviour of PEI and PES resins used as matrices for short fibre reinforced laminates , J Mater Sci 1986 21 3561-3570. 

Fig. 24 The temperature-dependent mechanical loss factor d [4] a crystalline k. .. crystalline...

The deformation behaviour of semi-crystalline materials is mainly determined by the behaviour of the two components - the crystalline and the amorphous phase with their characteristic temperature-dependent mechanical behaviour and sometimes their anisotropy. So the crystalline phase is elastically with a rather high modulus. Above a certain stress the crystallites break down into smaller fragments. Aligned chains enable recrystallisation. The mobility in the amorphous phase depends on the difference between the ambient temperature and the temperature characteristic of the glass transition, which is the dominant relaxation process in the temperature range under investigation. On the other side the amorphous phase is constrained within the crystalline one. So it shows to some extent stress relaxation or frozen stress. Both phases are connected via anchor molecules, bridging the phase boundaries. Those molecules are mainly responsible for stress transfer between the phases. 

Modeling of temperature-dependent mechanical properties for FRP materials was started in the 1980s. In many of the suggested models [1-5], -modulus were described as stepped functions of temperature achieved by coimecting experimentally gathered key points, such as the glass-transition temperature, Tg, and the decomposition temperature, T. -modulus values at different temperatures were obtained by DMA. 

The mechanical responses (stress, strain, displacement, and strength) of fiber-reinforced polymer (FRP) composites under elevated and high temperatures are affected significantly by their thermal exposure. On the other hand, mechanical responses have almost no influence on the thermal responses of these materials. As a result, the mechanical and thermal responses can be decoupled. This can be done by, in a first step, estimating the thermal responses (as introduced in Chapter 6) and then, based on the modeHng of temperature-dependent mechanical properties, predicting the mechanical responses of the FRP composites. 

In 1992, McManus and Springer [5, 6] presented a thermomechanical model that considered the interaction between mechanically induced stresses and pressures created by the decomposition of gases within the pyrolysis front. Again, temperature-dependent mechanical properties were determined at several specified temperature points as stepped functions. The issue of degradation of material properties at elevated temperatures was considered in Dao and Asaro s [7] thermo-mechanical model in 1999. The degradation curves used in the model were, once again, obtained by curve fitting of limited experimental data. 

In 2004, Gibson et al. [10] then presented an upgraded version by adding a new mechanical model. A function that assumes the relaxation intensity is normally distributed over the transition temperature was used to fit the temperature-dependent Young s modulus. Furthermore, in order to consider the resin decomposition, each mechanical property was modified by a power law factor. Predictions of mechanical responses based on the thermomechanical models were also performed by Bausano et al. [11] and Halverson et al. [12]. Mechanical properties were correlated to temperatures through dynamic mechanical analysis (DMA) but no special temperature-dependent mechanical property models were developed. 

The above-mentioned thermomechanical models only consider the elastic behavior of materials. Boyd et al. [13] reported on compression creep rapture tests performed on unidirectional laminates of E-glass/vinylester composites subjected to a combined compressive load and one-sided heating. Models were developed to describe the thermoviscoelasticity of the material as a function of time and temperature. In their work, the temperature-dependent mechanical properties were determined by fitting the Ramberg-Osgood equations and the temperature profiles were estimated by a transient 2D thermal analysis in ANSYS 9.0. 

A. Duckham, D.Z. Zhang, D. Liang, V. Luzin, R.C. Cammarata, R.L. Leheny, C.L. Chien, T.P. Weihs, Temperature dependent mechanical properties of ultra-fine grained FeCo-2 V. Acta Mater. 51(14), 4083 093 (2003)... 

Polymers where reversible shape memory is induced by a change in temperature are known as thermo-responsive ape memory polymers. For example, a hydrogel formed by acrylic acid and stearyl acrylate shows significant temperature dependent mechanical properties [128]. Below 50 °C, this hydrogel behaves like a tough polymer whereas above 50 °C it behaves like a soft material. This transition allows one to process the hydrogels above 50 °C, where they are easily malleable, into the desired shape, which can be... 

The temperature dependant mechanical properties and the cyclic stress-strain curve of the type 304 stainless steel material constituting the specimen are given in Table 3. The fatigue... 

FIGURE 2.6 Temperature-dependent mechanical plastic behavior... 

Commonly used DMA devices determine time and temperature dependent mechanical properties only at small strains and stresses. Therefore the following investigations were accomplished at a high load DMA (Gabo Eplexor 500), which allows dynamic loads up to 500 N. Thus, it is possible to cover a higher load range. 

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