Cooling thermomechanical

Thermomechanical Analysis (TMA). Thermomechanical analysis (TMA) measures shape stability of a material at elevated temperatures by physically penetrating it with a metal rod. A schematic diagram of TMA equipment is shown in Fig. 2.23. In TMA, the test specimen s temperature is raised at a constant rate, the sample is placed inside the measuring device, and a rod with a specified weight is placed on top of it. To allow for measurements at low temperatures, the sample, oven, and rod can be cooled with liquid nitrogen. 

Other types of damage may be produced through thermomechanical effects. For example, when being annealed at 450°C a CVD aluminum film on a Si substrate is subjected to compressive thermoelastic stresses owing to the considerable difference between the thermal expansion coefficients of aluminum (a = 23 x 10 °C 0 and the silicon substrate (a. = 3.5 x 10 °C 0-When cooling, the film may therefore contract by as much as 1%. Due to the combined action... 

Fig. 5. Results for G c calculated from different thermomechanical loading experiments in comparison with the critical value of G c,mm = 0.17 N/mm and with an indication of the cooling rates...

Fig. 7. Critical stress intensity factors Kic obtained in isothermal loading (hollow boxes) compared to results obtained in thermomechanical loading (cooling down, filled boxes) with i speed range dK/dt of 0.006 -...

It is important in thermal analysis, as for any other analytical techniques, to obtain reproducible results. However, there are many experimental variables associated with thermal analysis and it is necessary to define at least five of these for each thermal analysis investigation. These five variables are the nature of the sample and its container (crucible), the heating (or cooling) rate, the sample-chamber atmosphere, and the sample mass. Some thermal analysis techniques require the recording of other variables, such as the load on the sample in thermomechanical analysis. In thermal analysis, the samples are usually in the solid state, but liquids can also be studied using special sample preparation techniques. Gases are not normally studied by thermal analysis techniques. 

Thermomechanical analysis allows the calculation of thermal expansivity from the same data set as used to calculate the Tg. Since many materials are used in contact with a dissimilar material in the final product, knowing the rate and amount of thermal expansion helps design around mismatches that can cause failure in the final product. These data are only available when the Tg is collected by thermal expansion, not by the flexure or penetration method. This is in many ways the simplest or most essential form of TMA measurement. A sample is prepared with parallel top and bottom surfaces and is allowed to expand under minimal load (normally 5mN or less, ideally OmN) as it is slowly heated and/or cooled. The CTE is calculated by ... 

Fig. 5.42 Microhardness data for actively cooled (AC) and heated (AH) conditions for friction stir welded 2195-T8 Al. HAZ, heat-affected zone TMAZ, thermomechanically affected zone DXZ, dynamically recrystallized zone. Source Ref 75...

Ti-6A1-4V is an alpha-beta alloy that can be modified extensively by both thermal and thermomechanical processing to produce a large variety of microstructures and hence a wide spectrum of mechanical properties. The beta-transus temperature is approximately 1000 °C (1830 °F) and is a function of interstitial content (Ref 1). Samples of Ti-6A1-4V cooled at relatively slow rates from elevated temperatures contain mainly the alpha and beta phases as a result of diffusional transformations, while those cooled rapidly may also contain martensitic phases such as the cc (hep structure) or the a" (orthorhombic structure) phases. 

Figure 7.43 Effect of the pretreatment of the network obtained by crosslinking styrene-0.57% DVB copolymer with monochlorodimethyl ether to 100% on the position and form of thermomechanical curves (1) control sample (2) the sample heated up to 136°C under a loading of 400 g and relaxed at 164°C for 2h without pressure (3) the sample heated up to 136°C and then cooled under a loading of 400g (12% residual deformations), then subjected to swelling and drying (4) the sample heated up to 136°C and then cooled under a loading of 400 g (10% residual deformations). (Reprinted from [202] with permission of Wiley Sons, Inc.)...

For engineering purposes, the most useful classification of polymers is based on their thermal (thermomechanical) response. Under this scheme, polymers are classified as thermoplastics or thermosets. As the name suggests, thermoplastic polymers soften and flow under the action of heat and pressure. Upon cooling, the polymer hardens and assumes the shape of the mold (container). Thermoplastics, when compounded with appropriate ingredients, can usually withstand several of these heating and cooling cycles without suffering any structural breakdown. This behavior is similar to that of candle wax. Examples of thermoplastic polymers are polyethylene, polystyrene, and nylon. 

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