A5 Thermal Microcracking due to CTE Misalignment

Topic

During the post-cure cooling of this laminate, the mismatch between the fiber's coefficient of thermal expansion and that of the matrix—established in another model within this set describing effective thermal properties and the coefficient's anisotropy—generates residual stress within the matrix. As the temperature drops from the curing temperature to ambient temperature, the fiber and matrix attempt to contract at their respective rates; however, because they are bonded together, the component tending to contract more is constrained by the one tending to contract less. This mismatch between the two coefficients of thermal expansion causes the resulting mutual constraint to generate residual stress within the matrix—stress that would not exist if both materials contracted at the same rate.

This residual stress in the matrix produces a specific microcrack density, particularly in the laminate's transverse layers. In these transverse layers, the fibers are oriented such that they no longer reinforce the matrix in the direction of the residual stress; consequently, the unreinforced matrix directly absorbs that stress. Higher residual stress in the matrix leads to a higher microcrack density in these transverse layers, whereas lower residual stress results in a lower microcrack density.

In addition to producing this microcrack density, the residual stress triggers microcrack initiation the moment it exceeds a specific threshold: if the residual stress in the matrix surpasses this threshold, a localized microcrack initiates; if the stress remains below the threshold, the matrix does not initiate any new microcracks, even though it continues to accumulate residual stress. For this reason, the model—which began by accounting for the mismatch in the coefficient of thermal expansion between the fiber and the matrix—depends directly on another model within the same set: the one describing the effective thermal properties and the anisotropy of that coefficient. The fact that this mismatch (inherited from the other model) arises during post-curing cooling generates residual stress in the matrix; this residual stress, in turn, determines both the microcrack density in the laminate's transverse layers and whether or not the criterion for the initiation of a new microcrack is met.

ID:4421