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Mapping of Changes Induced by Microwave Heating in a Common Photoelastic Plastic - Towards an Effective Control of Thermal RunawayAdekunle Oloyede, Ayodele Olofinjana and Paul Groombridge
School of Mechanical, Manufacturing and Medical Engineering
Queensland University of Technology, GPO. Box 2434, Brisbane
Thermal runaway phenomenon is a major hindrance to the use of microwave heating in manufacturing. The mechanical properties of a material can be modified when the internal temperatures induced during processing increase too rapidly. As a result, recent research has focused on developing suitable control systems utilizing temperature feedback to prevent damage caused by thermal runaway. Whilst this is effective the control parameters are found from time-consuming trial and error exercises in which the external temperature is used to deduce the internal temperature. A better understanding of the heat distribution in bulk materials during processing would provide useful insight into the impact of thermal runaway effects on manufacturing processes and enable process parameters to be predicted for any given situation.
The results of a study in which cubic photoelastic specimens were sliced into sections and put back together then heated using microwaves and later examined using a polariscope and an ultra micro indentation system(UMIS) are presented in this paper. A Hounsfield test was also used to determine the elastic modulus of the material. The stresses and hardness measured have enabled three-dimensional mapping of microwave-induced changes in this resin, and a concomitant temperature distribution evaluated from the theory of elasticity.
Keywords: thermal runaway, polymer joining, microwave joining, nano-indentation, temperature control
Microwaves heat volumetrically with a temperature distribution which decrease from the center of a body to its outer extremities. This unique feature, which contrast with conventional heating, is of interest in this study with respect to the exact nature of such temperature distribution . The establishment of the level of heat induced in different sections of a microwave-heated body would enable the development of control measures to counter thermal runaway that is capable of causing deleterious irreversible changes to the material or component being processed. For example, control software, which regulates the temperature at different stages of processing at high temperature, could be equipped with a temperature profile in time that would also keep the material being processed under safe conditions that avoid thermal runaway. Thermal runaway itself is the uncontrolled rapid heating, usually in a localized region of a body, which results in the damage to the component, e.g. in the drying of wood charring might occur, and in the joining of polymers undesirable melting may eventuate. Consequently, in order to weld polymers successfully it is necessary to control this effect , and hence the rationale for the present work.
This study will therefore determine the temperature gradient in a photo-sensitive polymer resin of a given volume using a unique methodology. It will also be established if the UMIS equipment could provide mechanical details which could be relied upon in the analysis of thermally induced conditions in a materials, thereby saving the time for extensive traditional destructive testing on a machine like the INSTRON or HOUNSFIELD. The UMIS yields hardness data from which both stiffness and stresses can be calculated.
Generalized Hooke's law would be used in the analysis and evaluation of the position-dependent temperatures in a heated volume. Thermal stresses would be induced in a body whenever its thermal expansion or contraction or that of any part of it associated with a change of temperature is restricted. In a body subjected to a uniform change in temperature, such restriction may be produced by external restraints or by the interaction of dissimilar materials. More commonly, thermal stresses result from temperature gradients within a body. In this case, the restriction on expansion (or contraction) results from the interaction of the elements of the body, the thermal expansions of which could be in accordance to the local temperature. A self-equilibrating system of stresses is thus set up corresponding to the amount of expansion prevented .This can be expressed as follows;
Where x, y and z refer to the three axial directions of deformation and e and s refer to strain and stress respectively along these directions. E is the Young's modulus of elasticity, and DT is the change in temperature and a is the coefficient of linear expansion of the material. Using the boundary conditions of our experimental arrangement in which the body is constrained on the x and y directions (fig.1); it can be shown that the temperature at any section, given the fracture stress and other properties is,
Equation (2) would be used to determine the temperature in each of the eleven slabs used in our experiments.
|Fig 1: Specimen arrangement in holder ready for microwave heating.|
The effect of microwave on temperature distribution in the photoelastic material was evaluated from the theory of elasticity. Total strain is equal to elastic strain plus thermal strain and since stress is proportional to strain, the thermal stress induced in the specimen can be determined by the fringe pattern using polariscope. In this experiment, a photoelastic resin has been used because of the intention to use the polariscope
Ultra Micro Indentation System (UMIS) test were carried out on the photoelastic specimens to obtain the initial hardness. This initial condition was then ascertained to be stress free using the polariscope, thereby establishing a reference state for each of the specimens used. A suitable heating time and power level was determined in a process of trial and error before the specimen started to melt. Eleven pieces of photoelastic specimen were placed in a PTFE holder and expose under microwave radiation at a single frequency (2.45GHz) for 1 min in a commercial microwave oven. Subsequently the specimens were cooled to room temperature and then viewed again using the polariscope. Eleven specimens were chosen so that we could obtain the temperature distribution in 5 specimens on either side of the central specimen.
Fig 1. presents the configuration of the specimen in which 11, 28mmx20mmx2mm specimen slabs were arranged within the confining walls of a PTFE holder such that the arrangement simulated a bulk or 3-D body. Due care was taken to ensure that there was practically no air gap between the slabs so that the experiment mimic more closely a solid body. Furthermore, this experimental method was chosen to obtain access into the different positions within the body as the slabs could be easily separated after heating and cooling have been completed. This is a cheap and efficient way of getting access to the type of data we want to obtain from the present study. Fig 2 is the data obtained from the Hounsfield testing equipment from which the stiffness and fracture stress for each slab were determined.
|Fig 2: Stress-Strain curve for microwave-heated and non-heated samples.|
Our results demonstrate that there is a variation with position in the mechanical properties of the specimens 1-11 used and that there is a distinct difference between the properties of the heated and unheated specimens shown in Figure 2 to 5. We observe that the use of slabs, which are re-assembled to simulate a 3-D body in our experiments, is indeed a viable concept yielding reliable results. The mechanical data obtained were used in the generalized Hooke's law relationship to evaluate the corresponding temperature in the eleven specimens. It can be seen that despite the fact that photoelastic results did not show any dramatic effects, the UMIS tests were able to demonstrate that a change occurred which was established around the central specimen (i.e specimen no. 6) with relatively lower hardness.
|Fig 3: Fracture stress obtained via destructive tensile loading of specimen slabs.|
|Fig 4: Temperature difference relative to room temperature for the model slabs 1 to 11 used to simulate the 3-D body. Temperatures are in deg. C.|
|Fig 5: Hardness values from the UMIS showing the inverse relationships between,Hardness, fracture stress and temperature distribution|
|Non-Microwave||Specimen 1||Specimen 2||Specimen 3||Specimen 4||
||Fig 6: The array of heated specimens showing heated regions. These unloaded state fringe patterns indicate the existence of residual thermal stresses which would modify the samples' mechanical responses.|
Our destructive tests also show this trend in which specimen 6 has the lowest fracture stress, thereby indicates, albeit inconclusively at this stage that the UMIS for practical purposes can be used as a non-destructive tool for investigating property differences in different position along a polymeric material especially after microwave heating. Finally, a comparison of figures 4 and 5 demonstrate that an inverse relationship exists between hardness measured by the UMIS both temperature, with a similar relationship between fracture stress and hardness.
The authors are grateful to Poh Kok Toh, Vernon Tan and Joe Devries for carrying out the experiments reported in this paper.
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