Modern design data for short-fiber thermoplastic composites

Items fabricated from thermoplastics containing short glass fibers are anisotropic and the anisotropy varies from point to point. Guidance as to how such materials should be evaluated and what data are required for the design of end products molded from them is provided by a combination of the theories of viscoelasticity and anisotropic elasticity, whereby the stiffness and compliance coefficients of the latter are functions of time, temperature, and strain. A methodology then emerges, similar in overall nature to that pertaining to thermosetting resins containing long aligned fibers, but rather more complex because of the variations in fiber alignment. However, an adequately comprehensive evaluation in accordance with the formal rationale tends to be prohibitively expensive and, on the other hand, a curtailed evaluation can yield misleading data. Recent work aimed at resolving that impasse seems promising. The central principle is the use of special test specimens chosen because their flow geometries and possible flow irregularities are similar to those commonly found in commercial end products. The data that can be derived from such specimens can be related quantitatively to those derived from the conventional tests, and therefore the presentation and use of the new class of data entail no interpretation difficulties. Examples of what has been achieved and speculations about what might further be achieved are presented in the paper.