Experimental determination of the multi-axial strain transfer from CFRP-laminates to embedded Bragg sensor

When embedded, optical fibre Bragg gratings are considered to be very valuable in terms of strain measurements of large composite structures for a number of reasons (safety rules, design criteria…). However, the strain field measured by the embedded optical fibre Bragg grating is not necessarily the one present in the composite material. Especially the measurement of transverse strain components is not that straight forward! In a previous paper, the multi-axial strain transfer from host material to sensor was determined by using a finite element method. In this paper, a method is defined to experimentally determine the multi-axial strain transfer. As an example, the strain transfer of a cross-ply laminate to a non-coated 80μm diameter Bragg sensor was determined. The different experiments (tensile tests and transverse compression tests) needed to obtain this transfer matrix are discussed. Good similarity was found between the numerically and experimentally determined transfer matrices

SMARTFIBER: enabling cost-effective, miniaturized structural health-monitoring of composite structures

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On the orthotropic elasto-plastic material response of additively manufactured polyamide 12

The mechanical response of polymers such as polyamide 12 (PA-12) manufactured through additive manufac-turing, is significantly affected by the layered manufacturing approach and the printer settings used during the creation of the parts. As a result, the mechanical performance can differ significantly from PA-12 parts creat-ed through conventional techniques such as injection molding, and a detailed study of the material mechanical behavior is necessary. This work presents an in-depth study of the response of PA-12 to tensile loading and the challenges involved in obtaining qualitative and repeatable results. The full elasto-plastic curves are meas-ured during tensile testing and the effect of printing direction is taken into account in order to investigate whether orthotropic material behavior can be observed. All parts were manufactured using commercially avail-able selective laser sintering (SLS) printers. Digital image correlation was used extensively to obtain high-accuracy strain measurement over the entire elasto-plastic range up to failure. The results show an isotropic elastic response of PA-12, with orthotropic failure properties and the presence of significant viscous contribu-tions in the material response

A finite element model capable of predicting resin pockets for arbitrary inclusions in composite laminates

This work presents the progress in the development of a finite element model capable of predicting resin pockets occurring in composite structures with embedded sensors. The F.E.- model is built using standard tools in ABAQUS software, avoiding the need of specialized coding. Both progresses in material characterization as well as finite element modeling are shown. The model will eventually be used to optimize the shape of an embedded optical fibre interrogator used within the FP7 ‘SmartFiber’ projec

Improved accuracy in the determination of flexural rigidity of textile fabrics by the Peirce cantilever test (ASTM D1388)

Within the field of composite manufacturing simulations, it is well known that the bending behavior of fabrics and prepregs has a significant influence on the drapeability and final geometry of a composite part. Due to sliding between reinforcements within a fabric, the bending properties cannot be determined from in-plane properties and a separate test is required. The Peirce cantilever test represents a popular way of determining the flexural rigidity for these materials, and is the preferred method in the ASTM D1388 standard. This work illustrates the severe inaccuracies (up to 72% error) in the current ASTM D1388 standard as well as the original formulation by Peirce, caused by ignoring higher-order effects. A modified approach accounting for higher-order effects and yielding significantly improved accuracy is presented. The method is validated using finite element simulations and experimental testing. Since no independent tests other than the ASTM D1388 standard are available to determine the bending stiffness of fabric materials, experimental validation is performed on an isotropic, homogeneous Upilex-50S foil for which the flexural rigidity and tensile stiffness are related. The flexural rigidity and elastic modulus are determined through both the cantilever test (ASTM D1388) and tensile testing. The results show that the proposed method measures an elastic modulus close to that determined through tensile testing (within 1%), while both the Peirce formulation (+18%) and ASTM standard (+72%) over-estimate the elastic modulus. The proposed methodology allows for a more accurate determination of flexural rigidity, and enables the more accurate simulation of composite forming processes

Optical fiber coating optimization tool for composite embedded health monitoring purposes through a novel transfer matrix method

This work presents a new methodology based on finite element analysis, allowing the user to quickly optimize the coating thickness for any type of load case within any type of lay-up (given certain boundary conditions on minimum layer thickness). The method finds the same optimal values as Dasgupta for axial loads and Hadjiprocopiou for transverse loads