A study of the fracture behaviour of Kevlar 29 reinforced dental cement is undertaken using both linear elastic and nonlinear elastic fracture mechanics techniques. Results from both approaches--of which the nonlinear elastic is believed to be more appropriate--indicate that a reinforcing effect is obtained for the fracture toughness even at very low fibre content. The flexural strength and modulus are apparently not improved, however, by the incorporation of Kevlar 29 fibres in the PMMA cement, probably because of the presence of voids, the poor fibre/matrix interfacial bonding and unsatisfying cement mixing practice. When compared to other PMMA composite cements, the present system appears to be probably more effective than carbon/PMMA, for example, in terms of fracture toughness. More experimental and analytical work is needed so as to optimize the mechanical properties with respect to structural parameters and cement preparation technique.
In this paper a new circuit design for automatic electrostatic vibroscopes is presented. Previously, the component which stabilized the self-oscillatory motion of the filament in the feedback loop of electrostatic vibroscopes was a negative temperature coefficient thermistor. In the present system this device is replaced with a programmable analog compander. This modification represents an improvement because response time is faster, the steady-state conditions are more stable, and the overall performance of the vibroscope is less susceptible to errors caused by changes in environmental conditions.
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