Temperature dependence, mechanical properties

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. 

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

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. 

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... 

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. 

Fatigue is an example of the influence of time on the mechanical properties of a material. Another example of a time-dependent mechanical property is creep. Creep, sometimes called viscoplasticity, is defined as time-dependent deformation nnder constant stress, usually at elevated temperatures. Elevated temperatures are necessary because creep is typically important only above Tmp % where T p is the absolute melting point of the material. 

Dynamic mechanical analysis (DMA) or dynamic mechanical thermal analysis (DMTA) provides a method for determining elastic and loss moduli of polymers as a function of temperature, frequency or time, or both [1-13]. Viscoelasticity describes the time-dependent mechanical properties of polymers, which in limiting cases can behave as either elastic solids or viscous liquids (Fig. 23.2). Knowledge of the viscoelastic behavior of polymers and its relation to molecular structure is essential in the understanding of both processing and end-use properties. 

In specifying rate constants in a reaction mechanism, it is common to give the forward rate constants parameterized as in Eq. 9.83 for every reaction, and temperature-dependent fits to the thermochemical properties of each species in the mechanism. Reverse rate constants are not given explicitly but are calculated from the equilibrium constant, as outlined above. This approach has at least two advantages. First, if the forward and reverse rate constants for reaction i were both explicitly specified, their ratio (via the expressions above) would implicitly imply the net thermochemistry of the reaction. Care would need to be taken to ensure that the net thermochemistry implied by all reactions in a complicated mechanism were internally self-consistent, which is necessary but by no means ensured. Second, for large reaction sets it is more concise to specify the rate coefficients for only the forward reactions and the temperature-dependent thermodynamic properties of each species, rather than listing rate coefficients for both the forward and reverse reactions. Nonetheless, both approaches to describing the reverse-reaction kinetics are used by practitioners. 

Temperature-dependant material property models were implemented into stmc-tural theory to establish a mechanical response model for FRP composites under elevated temperatures and fire in this chapter. On the basis of the finite difference method, the modeling mechanical responses were calculated and further vaUdated through experimental results obtained from the exposure of full-scale FRP beam and column elements to mechanical loading and fire for up to 2 h. Because of the revealed vulnerabihty of thermal exposed FRP components in compression, compact and slender specimens were further examined and their mechanical responses and time-to-failure were well predicted by the proposed models. 

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