Thermomechanical analysis dynamic force

Figure 11 Dynamic load thermomechanical analysis of a carbon fibre/epoxy resin beam measured in three point bending mode. The force steps between 0.5 and 1.5 N and hack every 12 s. Heating rate 5°C min in air...

The resins were tested dynamically by thermomechanical analysis (TMA) on a Mettler apparatus. Triplicate samples of beech wood alone, and of two beech wood plies each 0.6 mm thick bonded with each resin system, for sample dimensions of 21 X 6 X 1.2 mm were tested in non-isothermal mode between 40°C and 220°C at a heating rate of 10°C/min with a Mettler 40 TMA apparatus in three-point bending on a span of 18 mm. A force varying continuosly between 0.1 N, 0.5 N and back to 0.1 N was appUed on the specimens with each force cycle of 12 s (6 s/6 s). The classical mechanics relation between force and deflection E = [L /(4fc/i )][ AF/( A/)] (where L is the sample length, b and h are the sample width and thickness, AF is... 

Half way between conventional thermomechanical analysis and dynamic mechanical analysis is the technique of dynamic force (or load) TMA. This method uses a standard TMA instrument but the force is changed between two values in a stepwise (or sometimes sinusoidal) fashion. The dimensional changes of the specimen are monitored as a function of time (and temperature) but no attempt is made to determine the modulus and damping properties of the material. 

The description of thermomechanical and thermoelectrical measurements in a modest chapter such as this is an ambitious exercise. The author has attempted to cover a wide range of methods and applications with the intention of illustrating the diversity of this field whilst emphasising the relationships between the static techniques (such as TMA and TSCA) and the dynamic techniques (dynamic force TMA, DMA and DETA). With the exception of TMA, these methods are often promoted as some of the more advanced thermal analysis techniques. It is hoped that the preceding pages help to dispel this myth without belittling their ability to measure useful properties. 

Dilatometry under varying forces are the subject of thermomechanical analysis (TMA) and dynamic mechanical analysis (DMA), described in Sect. 4.5. The mass, needed for density measurement is always determined by weighing and is described in the discussion of thermogravimetry, TGA, in Sect. 4.6. 

A more common mechanical method is dynamic mechanical thermal analysis (DMTA). DMTA is also called dynamic mechanical analysis (DMA) or dynamic thermomechanical analysis. An oscillating force is applied to a sample of material and the resulting displacement of the sample is measured. From this the stiffness of the sample can be determined, and the sample modulus can be calculated. A plot of loss modulus as a function of temperature shows a maximum at Tg as shown in Figure 1.35. Figure 1.35 shows a series of blends of high-impact styrene (HIPS) and PPO. As the amount of PPO is increased, Tg increases. The single Tg indicates that these blends are miscible. 

Thermomechanical Analysis (TMA) measures unidirectional dimensional changes in materials as functions of time, temperature and applied force. The TMA measurements are coefficient of linear thermal expansion (CLTE), glass transition temperatures (Tg) and softening points (Ts). Newer applications of TMA include elasticity, melt viscosity, and heat deflection temperature. In addition to traditional TMA instruments, many modem dynamic mechanical thermal analysis (DMTA) instruments can operate in a TMA (static force) mode. The main differences between the two types of instruments are the size of the specimens and the materials used to fabricate the measurement fixtures (stage, probe, clamps, etc.). Most TMA instruments use quartz, while DMTA instruments use larger steel components. The specimens used in these experiments are... 

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