Characterization of Mode I and Mode II traction–separation laws for cohesive separation of uncured thermoset tows.

As part of an effort to predict wrinkling of carbon-fiber tows during automated fiber placement, the cohesive zone traction–separation relations for two carbon fiber epoxy prepreg tows are quantified for Mode I and Mode II loading using a rigid double cantilever beam (RDCB) specimen. An explicit expression for normal traction versus normal separation ( σ vs δ n ) and tangential traction versus tangential separation (τ vs δ t) are derived using static equilibrium equations for an RDCB considering a compressive zone ahead of the process zone. The traction–separation relationships are in term of quantities that can be measured using a full field measurement technique (StereoDIC). The baseline traction–separation relationships in this work are obtained using conditions representative of those experienced by an uncured tow undergoing automated fiber placement (AFP) onto a substrate of a similar material with layup temperature T = 40 ∘ C , pressure p = 1 MPa and contact time t = 1 s. The RDCB specimen is loaded in displacement control at a constant load line displacement rate of 0.3 mm/min. Speckle images for StereoDIC are captured using stereo vision systems equipped for capturing images of the RDCB specimen with a field of view of 100 mm × 75 mm . Analysis of the data obtained for Mode I and Mode II loading shows that the Mode I energy release rate G I = 120 J / m 2 and Mode II energy release rate G II = 255 J / m 2 , with the maximum normal traction σ max = 0.50 MPa and the maximum shear traction τ max = 0.35 MPa . [ABSTRACT FROM AUTHOR]