Assessment of cure residual strains through the thickness of carbon–epoxy laminates using FBGs, Part I: Elementary specimen

Variability of thermo-mechanical properties within a composite part is a well known issue. It is problematic at the early stage of structural design. This variability being generated during the processing phases, the characterisation of the initial state of a structure becomes a crucial operation. A first step to this objective is presented in this paper. It deals with the assessment of autoclave cure induced strains in carbon epoxy laminates. Distribution and amplitude are considered through the thickness of carbon/epoxy elementary and technological specimens thanks to a series of FBGs embedded at different levels of the ply stack. Part I of this work concerns the analysis of unidirectional reinforced specimens. It begins with an overview of the various phenomena that cause the presence of residual stresses after cure and how they can be measured in experiments. Their development being closely related to the rheology of the matrix, an analysis of the M21 resin is given and assumptions are made to justify the following assessment procedures. A discussion leads to select the onset of gelation as the key point to start measuring residual cure stresses. The autoclave cure monitoring of the [08] specimen is then carried out and analyzed. Depending on their location through the laminate thickness, the FBG sensors give different information highlighting the effects of interaction between the laminate and the tool/plate.