Microstructure, elastic properties and deformation mechanisms of horn keratin.

The effect of physical ageing on the properties of poly(ethylene terephthalate)

This chapter discusses the microstructure-related fracture behaviour of short-fibre reinforced thermoplastics (SFRP). The microstructure of injection-moulded SFRP was studied by microscopic methods and described by the reinforcing effectiveness parameter R. This R-term takes into account fibre orientation and fibre layering with respect to the load direction, fibre volume fraction, fibre aspect ratio and its distribution. The effects of matrix toughness, fibre volume fraction, specimen thickness, fibre aspect ratio, injection-moulding conditions and fibre/matrix interaction on fracture toughness of a composite are summarized and discussed. Failure properties of the matrices varied from highly ductile to highly brittle behaviour. Results of fracture toughness tests are discussed by the microstructural efficiency factor M approach which considers both fibre- and matrix-related energy dissipation mechanisms during breakdown of the composites (energy absorbing ratio, n) and reinforcement characteristics and its arrangement in the matrix (reinforcing effectiveness parameter, R) as well as stress condition factor (a) of the matrix. In addition, results on J-integral investigations carried out on selected SFRP are reported and discussed also by the microstructural efficiency approach. Fracture mechanical results for Kc, Gc and Jc values of different thermoplastics and related short-fibre reinforced composites taken from the literature are tabulated and reviewed in the appendix.

Microstructure and Fracture Mechanical Performance of Short-Fibre Reinforced Thermoplastics

Mechanical behaviour and morphology of toughened aliphatic polyamides

The effect of temperature on the impact behaviour of glass/polycarbonate laminates

It is shown that the mechanical properties of polycarbonate are controlled by the state of stress of the test specimen. Whereas the yield stress of the material is relatively insensitive to the state of stress, the tendency of the material to microcavitate is strongly dependent on the residual stresses in the surfaces of a test specimen. The effects of quenching, annealing, post notching, solvent swelling and rate of testing on the modulus of toughness are all related to the state of compression on the free surfaces. The mechanism of deformation is shown by cinematography to consist of the nucleation of a shear band followed by microcavitation along the propagating band. Maximum toughness is obtained by suppressing microcavitation so as to promote shear yielding. When microcavitation occurs, it is the competition between propagating the microcavitation throughout the material and the generation of microcracks in the advancing shear band that controls the ultimate elongation.

Effects of the state of stress on the toughness of polycarbonate

The effect of a number of different liquids on the rate of crazing of polycarbonate was studied. It was found that liquids either had no effect on the mechanical response, caused immediate cracking and fracture or caused a measurable rate of crazing, depending on the solubility parameter of the liquid. The rate of crazing in those liquids that caused crazing was found to vary inversely with the square root of time, indicating that diffusion through the end of the craze controlled the craze growth. Pre-immersion in one of the solvents, ethanol, had a strong effect on the subsequent rate of crazing under a constant load. The pre-immersion of pre-cracked samples initially caused an increase in rate of crazing. After a pre-immersion time of roughly 24 to 72 hours, the subsequent rate of crazing decreased with an increase in pre-immersion time. When the pre-immersion was conducted on an uncracked specimen and the crack was generated after the pre-immersion, the subsequent rate of crazing increased steadily with pre-immersion time. An explanation for the observed results is given.

The effect of the environment on the stress crazing of polycarbonate

The environmental crazing from a central hole in extruded polycarbonate sheets, using ethanol as a crazing liquid, was studied. A stress analysis was made to analyse the criteria for craze initiation and growth. The craze length was found to change linearly with the square root of time and the rate of crazing was found to vary exponentially with the applied stress (above a certain stress level). It was concluded that the principal strain is the controlling parameter for environmental craze initiation and growth.

The environmental stress crazing of polycarbonate

The growth of crazes from a sharp crack in extruded polycarbonate sheets immersed in ethanol was measured. Below a critical level of the stress intensity factor craze growth was controlled by solvent diffusion through the end of the notch and fracture was prevented by craze arrest. Above a critical level, growth was controlled by either end diffusion or a combination of end diffusion and diffusion through the faces of the extruded sheet, and in both cases the final result was brittle fracture. The effects of annealing and quenching was studied at various sheet thicknesses. In thin specimens annealing and/or quenching had a significant effect on crack growth rate, which was predictable in terms of the state of stress. As the specimen thickness increased, causing a transition from plane stress to plane strain conditions, the previous thermal history had a diminishing effect on craze growth rate. The effects of thermal history and thickness on the fracture toughness of polycarbonate was also investigated. It was found that thickness was the more important variable and that at a ½ in. thickness the effects of thermal history were statistically insignificant. The effect of ethanol exposure on fracture toughness was studied. It was found that exposure to solvent initially caused an increase in kIC with time to a maximum value, followed by a substantial decrease with time which eventually led to brittle fracture. This behavior was explained as a competition between plasticization of the crack tip and coalescence of crazes to form microcracks.

S. Petrie

Papers

Effects of the state of stress on the toughness of polycarbonate

The effect of the environment on the stress crazing of polycarbonate

The environmental stress crazing of polycarbonate

The effects of stress cracking on the fracture toughness of polycarbonate

Nucleation and growth of crazes in polycarbonate exposed to ethanol