Thermomechanical programming

Figure 8.3 Thermomechanical programming to obtain stress release and freeze in a...

Thermal analysis helps in measuring the various physical properties of the polymers. In this technique, a polymer sample is subjected to a controlled temperature program in a specific atmosphere and properties are measured as a function of temperature. The controlled temperature program may involve either isothermal or linear rise or fall of temperature. The most common thermoanalytical techniques are (1) differential scanning analysis (DSC), (2) thermomechanical analysis (TMA), and (3) thermogravimetry (TG). 

TMA s were run over a temperature range of -20°C to 210°C on a Perkin Elmer Thermomechanical Analyzer IMS-1. They were run with a 40 mil diameter penetration probe loaded with 200 grgms to give a pressure of 394 psi. The program rate was lOC/min. under a helium atmosphere and the Y-axis sensitivity... 

In the following sections, the thermomechanical cycles of the thermosetting polystyrene SMP and its syntactic foam wUl be discussed, which are programmed using the classical... 

Thermomechanical Behavior of Thermosetting SMP and SMP Based Syntactic Foam Programmed Using the Classical Method... 

Figure 3.15 Four-step thermomechanical cycles (step 1 high temperature loading—> step 2 cooling step 3 room temperature unloading step 4 free shape recovery) for the pure SMP and syntactic foam programmed under a stress-controlled condition with a pre-stress of 263 kPa at 79 °C followed by free recovery. Source [41] Reproduced with permission from Elsevier...

Since the programming is the same for the free recovery specimens and fully constrained specimens, the focus will be on step 4 of the thermomechanical cycles. The stress-temperature behavior under a fully constrained recovery condition is shown in Figure 3.19 for the two programming stresses (47 kPa and 263 kPa). The recovery stress-time behavior of the foam programmed at 47kPa pre-stress is also highlighted by the inset in Figure 3.19. The recovery stress comes from two parts thermal expansion stress and entropically stored stress or back stress. Since this is a 1-D fully constrained recovery, the thermal stress can be calculated as... 

In order to understand the thermomechanical cycle of the syntactic foam under different test conditions better, the test results are presented in both 3-D and 2-D format. Typical 2-D axial stress-time and temperature-time curves for the foam confined by the nylon liner, programmed at 79 °C, and under 60% pre-strain level, and fully confined shape recovery are shown in Figure 3.29. Typical 3-D axial stress-axial strain-ternperamre thermomechanical cycles for the syntactic foam at a programming ternperamre of 71 °C, pre-strain level of 30%, and fully confined shape recovery are shown in Figure 3.30. Typical 3-D axial stress-axial strain-time behaviors at a programming temperature of 79 °C, pre-strain level of 30%, and fully confined shape recovery are shown in Figure 3.31. 

Figure 3.30 Axial stress temperature-axial strain thermomechanical cycle at a programming temperature of 71 °C and pre-strain level of 30%. The subplot shows the three-step thermal mechanical cycle of a specimen confined by a steel liner (step 1 (pre-stressing) and step 2 (cooUng and unloading) represent programming and step 3 represents stress recovery). Source [42] Reproduced with permission from Elsevier...

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