Crack branching in cross-ply composites

Ph.D.George A. Kardomatea

Analysis of ultrasonic waveforms from smart sandwich composite structures under creep bending at elevated temperature

International audienceSandwich structures with polyurethane foam core and glass fiber‐reinforced polymer facesheets with three orientations were investigated experimentally and numerically under three‐point bending tests at 80 degrees C, at a relatively low load level associated with a linear viscoelastic response. Off‐the‐shelf piezoelectric transducers were inserted inside one of the facesheets and were interrogated in pitch‐catch at low ultrasonic frequencies during testing. The objective of this article is to investigate ability and sensitivity of the embedded transducers to detect creep deformation. The denoised received waveforms were analyzed in the time domain, where guided wave speeds were found to exhibit a drop due to temperature changes (most significant in the sandwich samples with off‐axis orientation), followed by an increase, eventually reaching an asymptotic value. The waveforms were also processed in the joint frequency‐time domain, with a novel signal processing technique built upon Gabor wavelet transforms and their contour lines. It is shown that this wavelet contour technique indirectly captures the trend of physically measured displacements and can differentiate among the three different fiber orientations in the facesheets, and among room temperature and 80 degrees C. This technique has the potential to effectively track creep time‐dependent response and life performance in smart sandwich composites

Analysis of ultrasonic waveforms from smart sandwich composite structures under creep bending at elevated temperature

International audienceSandwich structures with polyurethane foam core and glass fiber‐reinforced polymer facesheets with three orientations were investigated experimentally and numerically under three‐point bending tests at 80 degrees C, at a relatively low load level associated with a linear viscoelastic response. Off‐the‐shelf piezoelectric transducers were inserted inside one of the facesheets and were interrogated in pitch‐catch at low ultrasonic frequencies during testing. The objective of this article is to investigate ability and sensitivity of the embedded transducers to detect creep deformation. The denoised received waveforms were analyzed in the time domain, where guided wave speeds were found to exhibit a drop due to temperature changes (most significant in the sandwich samples with off‐axis orientation), followed by an increase, eventually reaching an asymptotic value. The waveforms were also processed in the joint frequency‐time domain, with a novel signal processing technique built upon Gabor wavelet transforms and their contour lines. It is shown that this wavelet contour technique indirectly captures the trend of physically measured displacements and can differentiate among the three different fiber orientations in the facesheets, and among room temperature and 80 degrees C. This technique has the potential to effectively track creep time‐dependent response and life performance in smart sandwich composites

A Testing Platform for Durability Studies of Polymers and Fiber-reinforced Polymer Composites under Concurrent Hygrothermo-mechanical Stimuli

The durability of polymers and fiber-reinforced polymer composites under service condition is a critical aspect to be addressed for their robust designs and condition-based maintenance. These materials are adopted in a wide range of engineering applications, from aircraft and ship structures, to bridges, wind turbine blades, biomaterials and biomedical implants. Polymers are viscoelastic materials, and their response may be highly nonlinear and thus make it challenging to predict and monitor their in-service performance. The laboratory-scale testing platform presented herein assists the investigation of the influence of concurrent mechanical loadings and environmental conditions on these materials. The platform was designed to be low-cost and user-friendly. Its chemically resistant materials make the platform adaptable to studies of chemical degradation due to in-service exposure to fluids. An example of experiment was conducted at RT on closed-cell polyurethane foam samples loaded with a weight corresponding to ~50% of their ultimate static and dry load. Results show that the testing apparatus is appropriate for these studies. Results also highlight the larger vulnerability of the polymer under concurrent loading, based on the higher mid-point displacements and lower residual failure loads. Recommendations are made for additional improvements to the testing apparatus

Surfactant-modified multiscale composites for improved tensile fatigue and impact damage sensing

This paper documents the mechanical and electrical performance of self-sensing conductive polymer composites prepared with a low-cost technique and small hardware, able to considerably improve the dispersion and the surface adhesion of multi-walled carbon nanotubes (CNTs) in epoxy resin with respect to amine-modified CNTs and as-received CNTs. Surface treatment of the CNTs is performed using hexamethylene diamine, or a mix of sulfuric and nitric acid, and one of two surfactants (for the diamine treatment only): Triton X-100 ( non-ionic) and cetyl pyridinium chloride, CPC (cationic). The effects of the treatments are shown in terms of the changes in mechanical properties and interpreted with the use of SEM, FTIR and XRD analyses. Moreover, a key and novel aspect of this work is that the improvements in dispersion and surface adhesion cause improvements of damage sensing capability under fatigue and impact. This was demonstrated on fiberglass-reinforced panels prepared with treated CNT/epoxy through hand lay-up. This study reveals that the diamine/CPC- based configuration is superior, due to improved mechanical performance, higher resistance to fatigue and impact damage (over 30 J) and increased damage sensitivity

Impact damage sensing of multiscale composites through epoxy matrix containing carbon nanotubes

Carbon nanotubes are used to provide increased electrical conductivity for polymer matrix materials, thus offering a method to monitor the structure's health. This work investigates the effect of impact damage on the electrical properties of multiscale composite samples, prepared with woven fiberglass reinforcement and epoxy resin modified with as-received multi-walled carbon nanotubes (MWCNTs). Moreover, this study addresses potential bias from manufacturing, and investigates the effectiveness of resistance measurements using two- and four-point probe methods. Transmission electron microscopy and static tensile tests results were used to evaluate, respectively, the dispersion of MWCNTs in the epoxy resin and the influence of the incorporation of these nanoparticles on the static tensile properties of the matrix, and interpret results from the resistance measurements on impacted specimens. In this study, the four-point probe method is shown to be much more repeatable and reliable than the two-point probe method. (c) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 201

Multifunctional sensors for UV and mechanical damage detection of aerospace structures

During space missions, on-board structural and electronic components are continuously exposed to radiations, which can degrade their performance or permanently damage them. The design of space-stable structures needs to take into account the combined effect of all the local environmental factors. Indeed, polymer-based materials in space exhibit larger degradation effects due to the combination of energetic UV radiations, vacuum, atomic oxygen, as well as large temperature gradients, which generate thermal stresses inside the structures. The health monitoring of composite structures in the aerospace and astronautic fields is a concept of great interest in view of the potential re-usability of such structures. Main applications include earth-deck panels and lateral panels of spacecrafts, in particular addressing shape and stress monitoring during ground testing and orbit operation. An open question in structural health monitoring (SHM) for composites remains the integration of sensors into the components, and the characterization of their micromechanical behavior. In this work, we investigate a novel approach to integrate both UV-sensing and piezoresistive capabilities into composite structures. Nanostructured films made of graphene and DNA strands were used to confer UV-sensitivity and piezoresistive response to the composites. These films were deposited on two types of composite laminates (carbon fiber/epoxy and glass fiber/epoxy) using spraying or spin coating processes. UV damage was assessed through changes of the sensor dielectric properties, while mechanical damage was investigated through analysis of the piezoresistive behavior of the film sensor under flexion. Scanning electron microscopy (SEM) was used to identify damage modes in the host structures