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 ($$\upsigma \hbox { vs }\updelta _\mathrm{n}$$) and tangential traction versus tangential separation $$(\tau \hbox { vs }\updelta _\mathrm{t})$$ 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 $$\hbox {T} = 40\,{^\circ }\hbox {C}$$, 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 $$100\hbox { mm }\times 75\hbox { mm}$$. Analysis of the data obtained for Mode I and Mode II loading shows that the Mode I energy release rate $${\varvec{\mathscr {{G}}}}_\mathrm{I }= 120\hbox { J}/\hbox {m}²$$ and Mode II energy release rate $${\varvec{\mathscr {{G}}}}_\mathrm{{II}} = 255\hbox { J}/\hbox {m}²$$, with the maximum normal traction $${\varvec{\upsigma }}_\mathrm{\mathrm{max}} = 0.50\hbox { MPa}$$ and the maximum shear traction $${\varvec{\tau }}_\mathrm{\mathrm{max}} = 0.35\hbox { MPa}$$.