David A. Darrow

Isolating Components of Processing Induced Warpage in Laminated Composites

This study considers laminate warpage (or spring-in) associated with the curing of reinforced thermoset composites. This processing induced spring-in shows up in flat parts as well as those with curved geometry, and is most prevalent with parts that do not have a closed cross section. Three contributions to spring-in were considered, namely: thickness cure shrinkage, mold expansion, and fiber volume fraction gradients. These effects were combined into a predictive finite element model (FEM). Mold stretching and thickness shrinkage were described through a modified in-and out-of-plane material thermal expansion, respectively, while fiber volume fraction gradients were accounted for by scaling the thermal and elastic properties of the composite through the thickness with the fiber volume fraction. For thin parts (< 2 mm) spring-in was dominated by fiber volume fraction gradient and mold stretching effects. For thick parts (> 2 mm) spring-in was dominated by thickness cure shrinkage. The FEM was able to account for 80% of the observed spring-in for parts ranging between 1 and 5 mm thick and having a 3 to 13 mm bend radius. This study shows that a relatively simple, linear-elastic analysis, can accurately describe competing contributions to spring-in over a range of material types and thicknesses.