On abnormal thermal-expansion properties of more orthotropic M21E/IMA carbon-fiber-epoxy laminates

Abstract

The present paper reports on an anomaly of laminates’ coefficients of thermal expansion (CTE), which has been observed for more-orthotropic laminates of a series-quality M21E/IMA prepreg-made composite frame. It was found that measured CTEs increasingly deviate from theoretical CTE values, derived from unidirectional ply data, when the degree of laminate orthotropy increases. The paper comprises experimental CTE measurements for the unidirectional M21E/IMA ply and six specific laminates of a full-scale frame. Analyses suggest that the observed discrepancy correlates with the laminates' degree of orthotropy, which is quantified by the in-plane Young's modulus ratio Ex/Ey in this paper. The observed effect is considered critical as inhomogeneous CTEs lead to process-induced distortions of curved composite structures. Thus, they play a key role when carbon-fiber-epoxy composite frames shall be made in tight dimensional tolerances. Wrong CTE calculation leads to erroneous shape predictions, inaccurate tool compensation and finally to assembly problems.