In-situ local strain measurement in textile composites with embedded optical fibre sensors

To understand the local strains inside a textile composite, numerical simulations are typically done on the scale of one repetitive unit cell of the weaving pattern. Periodic boundary conditions are applied to the edges of the unit cell and different load cases can then be applied to the unit cell of the textile composite. Most often, the periodic boundary conditions are applied on all faces of the unit cell, which implies the assumption that the material is repeating itself over an infinite distance in all three orthogonal directions. This assumption is more or less valid for the textile composite material in the midplane of thick laminates, where it is constrained by neighbouring material in all three directions. It is very difficult to validate such simulations, because local strain measurements inside a textile composite have rarely been done, and the interpretation is not straightforward. This paper shows the successful use of embedded optical fibre sensors to measure the local strains inside a satin weave carbon/PPS composite (typically used in aerospace applications). The length of the Bragg grating inside the optical fibre sensor has been chosen such that it is longer than the length of one unit cell of the satin weave architecture (7.4 mm). The read-outs of the optical fibre sensor give the minimum and maximum local strains that occur along the length of the Bragg grating

Damage signature of fatigued fabric reinforced plastics in the pulsed ultrasonic polar scan

This study investigates the use of both the amplitude and time-of-flight based pulsed ultrasonic polar scan (P-UPS) for the nondestructive detection and evaluation of fatigue damage in fiber reinforced composites. Several thermoplastic carbon fabric reinforced PPS specimens (CETEX), loaded under various fatigue conditions, have been scanned at multiple material spots according to the P-UPS technique in order to extract material degradation in a quantitative way. The P-UPS results indicate that shear dominated fatigued carbon/PPS goes with a reduction of shear properties combined with large fiber distortions. The P-UPS results of the tension-tension fatigued carbon/PPS samples on the other hand reveal a directional degradation of the stiffness properties, reaching a maximum reduction of -12.8% along the loading direction. The P-UPS extracted damage characteristics are fully supported by simulations, conventional destructive tests as well as visual inspection. The results demonstrate the excellent capability of the P-UPS method for nondestructively assessing and quantifying both shear-dominated and tension-tension fatigue damage in fabric reinforced plastics

Damage of woven composite under tensile and shear stress using infrared thermography and micrographic cuts

Infrared thermography was used to study damage developing in woven fabrics. Two different experiments were performed, a ±45° tensile test and a rail shear test. These two different types of tests show different damage scenarios, even if the shear stress/strain curves are similar. The ±45° tension test shows matrix hardening and matrix cracking whereas the rail shear test shows only matrix hardening. The infrared thermography was used to perform an energy balance, which enabled the visualization of the portion of dissipated energy caused by matrix cracking. The results showed that when the resin is subjected to pure shear, a larger amount of energy is stored by the material, whereas when the resin is subjected to hydrostatic pressure, the main part of mechanical energy is dissipated as heat

On the prediction of the bi-axial failure envelope of a UD CFRP composite lamina using computational micromechanics : effect of microscale parameters on macroscale stress–strain behavior

A computational micromechanics based Finite Element (FE) analysis methodology is presented to predict the bi-axial failure envelope of a unidirectional (UD) carbon-epoxy composite ply. In order to estimate the effect of various microscale parameters that are influencing the macroscopic stress-strain behavior, under individual load cases, detailed numerical studies are conducted using a 3D RVE (Representative Volume Element) model. The constituent epoxy matrix plastic deformation in the RVE is captured using the linear Drucker-Prager plasticity model. The effect of the fiber-matrix interface damage, followed by frictional sliding of the constituent materials on the computed interface tractions is captured using the cohesive zone damage model combined with the Coulomb friction law, which is implemented into Abaqus using VUMAT. From the detailed FE analysis of the RVE under individual load cases, it is observed that the predicted macroscopic stress-strain behavior is sensitive to the fiber-matrix interface properties as well as the in-situ epoxy stress-strain behavior. Hence, using a coupled experimental-computational micromechanics approach the interface and the in-situ epoxy material properties are calibrated and validated. Using the calibrated interface and in-situ epoxy material properties, the bi-axial (transverse tension/transverse compression - in-plane shear) failure envelope of a UD composite ply is estimated. Comparing the predicted damage profiles and the failure envelope with the experimental results leads to good agreement and validates the proposed numerical methodology

Nanoporous-Gold-Based Electrode Morphology Libraries for Investigating Structure-Property Relationships in Nucleic Acid Based Electrochemical Biosensors.

Nanoporous gold (np-Au) electrode coatings significantly enhance the performance of electrochemical nucleic acid biosensors because of their three-dimensional nanoscale network, high electrical conductivity, facile surface functionalization, and biocompatibility. Contrary to planar electrodes, the np-Au electrodes also exhibit sensitive detection in the presence of common biofouling media due to their porous structure. However, the pore size of the nanomatrix plays a critical role in dictating the extent of biomolecular capture and transport. Small pores perform better in the case of target detection in complex samples by filtering out the large nonspecific proteins. On the other hand, larger pores increase the accessibility of target nucleic acids in the nanoporous structure, enhancing the detection limits of the sensor at the expense of more interference from biofouling molecules. Here, we report a microfabricated np-Au multiple electrode array that displays a range of electrode morphologies on the same chip for identifying feature sizes that reduce the nonspecific adsorption of proteins but facilitate the permeation of target DNA molecules into the pores. We demonstrate the utility of the electrode morphology library in studying DNA functionalization and target detection in complex biological media with a special emphasis on revealing ranges of electrode morphologies that mutually enhance the limit of detection and biofouling resilience. We expect this technique to assist in the development of high-performance biosensors for point-of-care diagnostics and facilitate studies on the electrode structure-property relationships in potential applications ranging from neural electrodes to catalysts