Prediction of Process-Induced Residual Stresses in Thermoplastic Composites

Abstract

A model to predict the macroscopic in-plane residual stress state of semicrystalline thermoplastic composite laminates induced by process cooling is presented. Heat transfer during processing is based upon an incremental transient formulation that consists of a finite difference heat transfer analysis coupled to the crystallization kinetics. Micromechanics is used to evaluate the instantaneous spatial variation of mechanical properties as a function of temperature and degree of crystallinity. Residual stresses are based upon an incremental laminate theory that includes temperature gradients, shrinkage due to crystallization and thermal contraction. Temperature dependent relaxation times are used to model first order viscoelastic effects. A parametric study is conducted to explore the sensitivity of residual stresses to process history. Input parameters varied include surface temperature history (cooling rate), the amount of shrinkage caused by crystallization and the relaxation time at the reference temperature. The model predictions were in good agreement with experimental residual stress measurements for unidirectional graphite (AS4) reinforced polyetheretherketone (PEEK) laminates.

Experimental and numerical study of distortion in flat, L‐shaped, and U‐shaped carbon fiber–epoxy composite parts

“…Studying changes of mechanical properties in composite materials in order to better understand the residual stresses and resulting distortion in composite parts is important [4,16,17,[25][26][27][28][29][30][31][32][33][34]. Knowledge of the mechanical properties variation in a composite part makes it possible to estimate the residual stresses developed during the curing of the part.…”

“…Chapman et al [27] considered three components, including stress analysis, mechanical properties, and thermal history to predict residual stresses during cooling of a composite part.…”

High-performance composites based on all-aromatic liquid crystal thermosets

“…Due to the semi-crystalline nature of these polymers the thermal history has a pronounced effect on the degree of crystallinity, which consequently has an effect on the overall mechanical properties of the final composite. [5] The microstructure, or the morphology of semi-crystalline thermoplastic composites, is highly dependent on the applied processing parameters. [6] In thick laminates, for example, the outer layers will cool at a much faster rate while the layers at the center of the composite will cool at a lower rate.…”

Thermal stresses in rings of thermoplastic composites made by automated fiber placement process

“…Residual stresses can lead to distortion of fi nished components, matrix cracking, and inter-ply delamination [1,2] . To ensure reliable and satisfactory performance, residual stresses within a composite part should be known before the part is put into use.…”