Balancing Mechanisms of Distortion to Yield Distortion-free/Shape Stable Composites

Abstract

The underlying mechanisms of manufacturing distortion in composite components have been suggested to be anisotropy, material property gradients, and part-tool interactions. These mechanisms have been sub-divided into those that result in shape change with changing temperature (thermoelastic) and those that are one-time-only effects (non-thermoelastic), generally related to isothermal cure shrinkage. The responses related to anisotropy and material property gradient mechanisms are predicted to include both thermoelastic and non-thermoelastic contributions; however, stresses imposed on the laminate and the resulting distortion due to part—tool interaction is merely non-thermoelastic in nature. Previous research has suggested that this non-thermoelastic component is critical to generating a composite angle bracket on net-shape tooling that is simultaneously distortion-free and shape-stable with temperature. The current research investigates the degree of tailorability available in both the part—tool interaction and in the thermoelastic response to local corner asymmetry. Autoclave-cured angle brackets were produced with variations in the local stacking sequence in the corner to balance the thermoelastic contributions and with different mold releases to affect the stress transferred from the tool to the part. Two asymmetric stacking sequences are compared to the baseline symmetric laminate. The fraction of the corner radius that is made up of these regions of asymmetry is also adjusted to vary the thermoelastic effect. Mold release variations are investigated to evaluate the effect of changing levels of non-thermoelastic distortion. To measure the response, the curvatures of flat composite laminates are measured using an LVDT-based panel measurement technique, which enables measurement of changing curvature with temperature. The included angle of various composite angle bracket specimens is also measured as a function of temperature, using a laser reflection technique. The results of this testing indicate that tailoring of both non-thermoelastic and thermoelastic components of process-induced distortion can be accomplished. The balancing of the compensation of these effects is shown to result in an angle bracket specimen, which has very little distortion and is much more stable with temperature than the baseline laminate. Further, it is found that the variation in laminate stiffness with changing local laminate stacking sequence plays an important role in the tailoring of the distortion of this geometry. Overall, the results of the research point very clearly to a methodology for the composites engineer to tailor the laminate, and the process, to generate a distortion-free/shape stable angle bracket molded on net-shape tooling.