Integration of optical fibre sensors by material extrusion 3-D printing – The effect of bottom interlayer thickness

•A novel 3-D printing based solution for integrating optical fiber sensors into thermoplastic composite structures is proposed.•A critical variable in this fiber attachment process (and for surface bonding in general) is the bottom interlayer thickness, which was in-situ investigated for the firsttime.•Experimentally measured strain transfer coefficients along the optical fiber sensors remained constant under creep loading.•A simple calculation method that enables to predict the shear lag effect for optical fibre sensors was derived and validated.