tailieunhanh - Dynamic Mechanical Analysis part 3

Tham khảo tài liệu 'dynamic mechanical analysis part 3', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Testing a Hookean material under different rates of loading shouldn t change the modulus. Yet both curvature in the stress-strain curves and rate dependence are common enough in polymers for commercial computer programs to be sold that address these issues. Adding the Newtonian element to the Hookean spring gives a method of introducing flow into how a polymer responds to an increasing load Figure . The curvature can be viewed as a function of the dashpot where the material slips irrecoverably. As the amount of curvature increases the increased curvature indicates the amount of liquid-like character in the material has increased. This is not to suggest that the Maxwell model the parallel arrangement of a spring and a dashpot seen in Figure is currently used to model a stress-strain curve. Better approaches exist. However the introduction of curvature to the stress-strain curve comes from the viscoelastic nature of real polymers. Several trends in polymer behavior14 are summarized in Figure . Molecular weight and molecular weight distribution have as expected significant effects on the stress-strain curve. Above a critical molecular weight Mc which is where the material begins exhibiting polymer-like properties mechanical properties increase with molecular weight. The dependence appears to correlate best with the weight average molecular in the Gel Permeation Chromatography GPC . For thermosets Tg here tracks with degree of cure. There is also a Tg value above which the corresponding increases in modulus are so small as to not be worth the cost of production. Distribution is important as the width of the distributions often has significant effects on the mechanical properties. In crystalline polymers the degree of crystallinity may be more important than the molecular weight above the Mc. As crystallinity increases both modulus and yield point increase and elongation at failure decreases. Increasing the degree of crystallinity generally increases the .