Mechanical thermal response modeling

The time-to-failure of a structure or its components is an important issue for structural safety considerations in fire. On the basis of the strength degradation models for FRP materials under elevated and high temperatures developed in Chapter 5, the time-to-failure is predicted for GFRP tubes and laminates under both thermal and mechanical loading in compression. Temperature responses were again calculated using the thermal response model presented in Chapter 6. 

The temperature responses were described by the one-dimensional thermal response model in Chapter 6 as the inputs used for mechanical response modeling. 

As part of the DECOVALEX III project, several independent research teams have used various numerical models to analyze a full-scale engineered barrier experiment (FEBEX), currently conducted at the Grimsel Test Site in Switzerland. This paper presents the analysis conduced by the Berkeley Lab research team, using the fully coupled thermal-hydrological-mechanical (THM) numerical model ROCMAS. Specifically, the paper presents model predictions of coupled THM responses at FEBEX that are compared to field measurements. 

Thus, with respect to the initiation of reaction, early work demonstrated the usefulness of a macroscopic, thermal model of the process and enabled the response of the more sensitive azides to be rationalized in a qualitative and sometimes semiquantitative way. The more difficult task of understanding the phenomena on an electronic or a molecular basis began to bear fruit, and gross, quantitative predictions of slow decomposition by heat or light became possible. However, unless their sensitivities had been first empirically established by statistical experiments, it remained impossible to predict the response of samples to different stimuli or to induce reaction with finesse or precision. Spontaneous initiation and explosion, such as encountered with crystals growing in solution, could not be explained by any mechanism, thermal, photochemical, or tribochemical. 

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. 

On the basis of the thermal and mechanical response models presented in Chapters 6 and 7 and the information gained on F-modulus recovery from DMA, a new model for the prediction of post-fire stiffness is proposed in the following [12]. 

In this chapter, the post-fire behavior of FRP composites was evaluated and modeled on the stmctural level. Results from the models compared well with results from fuU-scale post-fire experiments on cellular GFRP beam and column specimens that had been subjected to mechanical and thermal loading up to 120 min with inclusion of different thermal boundary conditions. On the basis of the previously proposed thermal and mechanical response models, existing approaches for post-fire evaluation can be applied. Predicted temperature profiles and the conversion degrees of decomposition can be used to estimate the post-fire stiHhess from existing two- and three-layer models. The borders between different layers can be determined either by a temperature criterion or a RRC criterion. 

An internal liquid cooling system as an active fire protection was implemented in full-scale GFRP panels for beam and column applications, the resulting thermal responses have been introduced and modeled in Chapter 6 and the mechanical responses in Chapter 7. The fire endurance time of each scenario is summarized in Table 9.1 and more details can be found in the previous chapters. It can be concluded that combined mechanical loading and fire experiments on full-scale water-cooled cellular slabs and columns proved the feasibility of an effective fire protection. Fire endurance durations of up to 2 h could be reached at slow water... 

The above understanding forms the basis for the development of thermophysical and thermomechanical property sub-models for composite materials at elevated and high temperatures, and also for the description of the post-fire status of the material. By incorporating these thermophysical property sub-models into heat transfer theory, thermal responses can be calculated using finite difference method. By integrating the thermomechanical property sub-models within structural theory, the mechanical responses can be described using finite element method and the time-to-failure can also be predicted if a failure criterion is defined. 

Based on this understanding, a mechanism based constitutive model incorporating the nonlinear structural relaxation model into the continuum finite-deformation thermoviscoelastic framework was developed as follows. The aim of this effort was to estabUsh a quantitative understanding of the shape memory behavior of the thermally responsive thermoset SMP programmed at temperamres below Tg. To simplify the formulation, several basic assumptions were made in this study ... 

Thus it can be seen that while there are a range of substantial challenges which must be addressed in order to advance thermo-mechanical modeling, both the diversity of ideas generated and the rate of development in this area are perhaps sufficient to indicate that a fully validated, hybrid thermo-mechanical model of composite thermal response, decomposition, combustion and mechanical failure may soon be achieved. 

The above problems of fabrication and performance present a challenging task of identification of the governing material mechanisms. Use of nonlinear finite element analysis enables close simulation of actual thermal and mechanical loading conditions when combined with measurable geometrical and material parameters. As we continue to investigate real phenomena, we need to incorporate non-linearities in behavior into carefully refined models in order to achieve useful descriptions of structural responses. 

The incorporation of a third element, e.g. Cu, in electroless Ni-P coatings has been shown to improve thermal stability and other properties of these coatings [99]. Chassaing et al. [100] carried out an electrochemical study of electroless deposition of Ni-Cu-P alloys (55-65 wt% Ni, 25-35 wt% Cu, 7-10 wt% P). As mentioned earlier, pure Cu surfaces do not catalyze the oxidation of hypophosphite. They observed interactions between the anodic and cathodic processes both reactions exhibited faster kinetics in the full electroless solutions than their respective half cell environments (mixed potential theory model is apparently inapplicable). The mechanism responsible for this enhancement has not been established, however. It is possible that an adsorbed species related to hypophosphite mediates electron transfer between the surface and Ni2+ and Cu2+, rather in the manner that halide ions facilitate electron transfer in other systems, e.g., as has been recently demonstrated in the case of In electrodeposition from solutions containing Cl [101]. 

---