Thermal softening

Bai [48] presents a linear stability analysis of plastic shear deformation. This involves the relationship between competing effects of work hardening, thermal softening, and thermal conduction. If the flow stress is given by Tq, and work hardening and thermal softening in the initial state are represented... 

Assuming that the cyclic waveform used in the previous section was sinusoidal then the effect of using a square wave is to reduce, at any frequency, the level of stress amplitude at which thermal softening failures start to occur. This is because there is a greater energy dissipation per cycle when a square wave is used. If a ramp waveform is applied, then there is less energy dissipation per cycle and so higher stresses are possible before thermal runaway occurs. 

The stress gradient also means that the occurrence of thermal softening failures is delayed. At any particular frequency of stressing, thermal softening failures will not occur until higher stresses if the stress system is bending rather than uniaxial. 

A uPVC rod of diameter 12 mm is subjected to an eccentric axial force at a distance of 3 ttun from the centre of the cross-section. If the force varies sinusoidally from — F to f at a frequency of 10 Hz, calculate the value of F so that fatigue failure will not occur in 10 cycles. Assume a safety factor of 2.5 and use the creep rupture and fatigue characteristics described in the previous question. Thermal softening effects may be ignored at the stress levels involved. 

Plastics are susceptible to brittle crack-growth fractures as a result of cyclic stresses in much the same way as metals. In addition, because of their high damping and low thermal conductivity, plastics are prone to thermal softening if the cyclic stress or cyclic rate is high. Examples of the TPs with the best fatigue resistance include PP and ethylene-propylene copolymers. 

Shiraishi, N., Matsunaga, T. and Yokota, T. (1979). Thermal softening and melting of esterified wood prepared in an N,0,-DMF cellulose solvent medium. Journal of Amlied Polymer Science, 24(12), 2361-2368. 

Alvarez, M. D., Canet, W. (2001a). Kinetics of thermal softening of potato tissue heated by different methods. Fur. Food Res. Technol., 212,454 64. 

Bourne, M. C. (1987). Effect of blanch temperature on kinetics of thermal softening of carrots and green beans. 

Bourne, M. C. (1989). Applications of chemical kinetic theory to the rate of thermal softening of vegetable tissue. In J. J. Jen (Ed.), Quality factors of fruits and vegetables. Chemistry and technology (pp. 98-110). American Chemical Society, Washington, Chap 9. 

Solomon, W. K., Jindal, V. K. (2003a). Comparison of mechanical tests for evaluating textural changes in potatoes during thermal softening. J Texture Stud, 33, 529-542. 

Mittal, G.S. 1994. Thermal softening of potatoes and carrots. Lebensm.-Wiss. Technol. 27 253-258. 

All of the chemical evidence that can be marshalled indicates that wood fiberboard manufacture exploits the thermoplastic properties of lignin. Defibering is effected by the thermal softening of lignin in the middle lamella at saturated steam pressures above 130C. Interfelted fiber mats are consolidated with or without densification pressure by the thermoplastic fusion of lignin-rich fiber surfaces at high board conversion temperatures. 

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