Michel Barbezat

Material characterization of the pig kidney in relation with the biomechanical analysis of renal trauma.

The objective of this study was an investigation of the material properties of the fresh pig kidney and parametric characterization of its elastic and inelastic material behavior. The material investigation included density measurements, uniaxial as well as three-dimensional compression tests, tensile tests. and shear tests on the samples extracted from the fresh pig kidney. For comparison, density measurements on a number of soft synthetic materials were also performed. Compression tests on the radial and the tangential specimens from the cortex tissue were performed at various loading rates. Three-axial compression tests were performed on the cortex tissues placed in a compression chamber. Shear tests were performed by punching a cylinder into a slice of the cortex. Tensile tests were carried out on the outer capsule. For characterization of the material behavior, a non-linear theoretical simulation based on a two parameter Blatz model was used. For characterization of the time-dependent behavior of the pig kidney cortex, a four-parameter linear viscoelastic model was employed. From the present experimental and theoretical studies, a number of conclusions were derived: (1) The general behavior of the pig kidney cortex samples under compression showed the general non-linear features typical of the soft tissues; the stress strain diagram was composed of a very flat part at very low stress level to about 30% relative deformation which was followed by a steeply rising stiffening leading to the radial rupture of samples marked by a maximum nominal rupture strain of about 50%. (2) The uniaxial compression tests on the radial and the tangential samples from the cortex tissue showed an increase of the rupture stress with the increase in the loading rate, but a decrease in the related rupture strain. (3) The long-term uniaxial compression tests on the cortex specimens under sustained constant load showed an instantaneous deformation followed by a creep response which eventually approached an asymptote. (4) Simulation of the non-linear material behavior of the cortex tissue under uniaxial compression by the Blatz model gave two pairs of material parameters for the cortex in the radial and the tangential directions. Furthermore, fitting of the assumed four-parameter linear viscoelastic model with the experimental data resulted in the viscoelastic material parameters.

Performance of integrated active fiber composites in fiber reinforced epoxy laminates

Active fiber composite (AFC) composed of lead zirconate titanate (PZT) fibers with interdigitated electrodes (IDEs) has been integrated into orthotropic glass fiber reinforced plastic (GFRP) laminates to characterize the performance of AFC as a smart material component in laminated materials. Monotonic cyclic tensile loading was performed on integrated specimens at different strain levels. The AFC output was monitored to determine the effect of applied strain level on the AFC performance. It was found that the AFC sensitivity degraded beyond strains of 0.20% and approached a minimum at 0.50% strain. The degradation in the AFC performance appears to be attributed to the dominating effect of PZT fiber fragmentation during testing, as opposed to depolarization. Acoustic emission (AE) monitoring was used to detect damage in laminates during testing and was correlated with crack evidence from microscopy observations during testing to characterize damage evolution in response to strain levels.

Piezoelectric Fiber Composites as Sensor Elements for Structural Health Monitoring and Adaptive Material Systems

Piezoelectric Active Fiber Composites (AFC) and Macro Fiber Composites (MFC) have the potential to provide various sensor functions for nondestructive test methods. AFC have been integrated into fiber-reinforced laminates as a first step towards structures with sensing capability. These developments constitute initial stages for developing adaptive composite structures or structures with integrated health monitoring system. So far, the use of AFC and MFC has been explored in selected nondestructive tests for defect detection in model composite systems on laboratory scale with e.g., Acoustic Emission, Acousto-Ultrasonics, and Electromechanical Impedance testing. The present article will focus on limitations and current prospects for structural health monitoring with AFC or MFC and discuss selected concepts and approaches.

Fire Behavior of Thin CFRP Pretensioned High-Strength Concrete Slabs

More sustainable precast concrete structural elements are emerging from the research community utilizing high-strength, self-consolidating fiber-reinforced concrete (HPSCC) reinforced with noncorroding prestressed carbon-fiber-reinforced polymer (CFRP). An example of this is a new type of precast CFRP pretensioned HPSCC panel intended as load-bearing beams or columns for use in building envelopes. Such elements have recently been applied to architectural facade elements in Europe. A key issue in the implementation of these elements as load-carrying members in buildings is demonstrating satisfactory performance in fire. It is well known that the bond between FRP reinforcing bars and concrete deteriorates at elevated temperatures. It is also known that high-strength concrete is susceptible to explosive spalling when subjected to fire. Reductions in FRP reinforcement tensile and bond strength during fire, effects on the load-bearing capacity of prestressed concrete structures, and the explosive spalling response of HPSCC during fire all remain largely unknown. This paper provides insights into the fire behavior of CFRP prestressed HPSCC slabs through an experimental study on thin slabs exposed to a standard fire while subjected to sustained service loads. It is shown that the fire resistance of these elements is governed by fire-induced spalling or, if spalling is prevented by the use of high dosages of polypropylene microfibers in the concrete, by thermal splitting-crack-induced bond failure of the CFRP tendons in their prestress transfer zone. Neither reductions in tensile strength of the tendons nor reductions in bond strength due to resin softening at high temperature appeared to play critical roles for the tests described in this paper. Key areas for future research are highlighted.

Numerical Optimization of a Compact and Reusable Pretensioning Anchorage System for CFRP Tendons

Efficient use of pultruded carbon fiber–reinforced plastic profiles (CFRP tendons) to prestress high-performance concrete (HPC) highly depends on the performance of the anchorage system and on material choice. For current applications, a prestressing degree of approximately 40% of the CFRP material strength is utilized in pretensioned concrete elements. A higher prestress implicates lower costs of fully prestressed concrete elements. The present project aimed to optimize the design of a removable and reusable pretensioning anchorage system for sand-coated CFRP rods. The optimized design was achieved by means of finite-element calculations in which parametric studies were complemented with extensive experimental work for validation. Analytical results demonstrated a reduction up to 25% for the relevant stress peaks in the tendons. The static rupture load under laboratory conditions increased by 25%, and the pretensioning level on-site could be increased by 50%. This improvement in production efficiency can be explained by easier applicability of the new system, i.e., failure tolerant assembly and prestressing process.

Acoustic Emission Monitoring of Leaks in Pipes for Transport of Liquid and Gaseous Media: A Model Experiment

In order to explore potential applications for Active Fiber Composite (AFC) elements made from piezoelectric fibers for structural integrity monitoring, a model experiment for leak testing on pipe segments has been designed. A pipe segment made of aluminum with a diameter of 60 mm has been operated with gaseous (compressed air) and liquid media (water) for a range of operating pressures (between about 5 and 8 bar). Artificial leaks of various sizes (diameter) have been introduced. In the preliminary experiments presented here, commercial Acoustic Emission (AE) sensors have been used instead of the AFC elements. AE sensors mounted on waveguides in three different locations have monitored the flow of the media with and without leaks. AE signals and AE waveforms have been recorded and analysed for media flow with pressures ranging from about 5 to about 8 bar. The experiments to date show distinct differences in the FFT spectra depending on whether a leak is present or not.

High yield synthesis of amine functionalized graphene oxide and its surface properties

Graphene and its derivatives have attracted great research interest due to their many exciting properties leading to a number of potential applications. However, for many practical applications including reinforcement in epoxy resin matrices, chemical functionalization of graphene is often a necessary requirement. Herein we report a simple temperature-assisted reflux method to synthesize graphene oxide (GO) functionalized with n-butylamine in high yield without the use of toxic chemicals. X-ray photoelectron spectroscopy and energy dispersive analysis by X-rays showed successful attachment of amine groups onto GO to form (GO-ButA). The functionalized GO was further characterized using Raman, nuclear magnetic resonance and Fourier transform infrared spectroscopies. The surface morphology and particle size, etc. were characterized using scanning electron microscopy. The solution properties of the GO-ButA were investigated by dispersing in water and common organic solvents which indicated an increased hydrophobic nature of the product which was also confirmed by contact angle measurements. Further, the interaction of GO-ButA with epoxy resin was tested by dispersing it in an epoxy resin which indicated an improved and more stable dispersion as compared to that of GO which shows the potential application of GO-ButA as a reinforcement (filler) in epoxy based composites.

Fracture Behavior of GFRP Laminates with Nanocomposite Epoxy Resin Matrix

Nano-sized, functionalized organo-silicate fillers dispersed in the epoxy matrix are one approach that is investigated for improving the delamination resistance of fiber-reinforced composites. The variety of nano-silicate fillers available on the market, of processing conditions and the related characterization effort make it desirable to have a simple, easily analyzed screening method for both, nano-modified resins and laminates. An impact test using hail simulation equipment and visual assessment on glass-fiber laminates with nano-modified epoxy matrix yields rough indications of the effect of nano-modification on the fracture behavior of the specimens that correlate with more sophisticated macroscopic and microscopic characterization.

Fire Behaviour of CFRP Prestressed High Strength Concrete Slabs

More sustainable precast concrete elements are emerging utilizing high-performance, self-consolidating, fibre-reinforced concrete (HPSCC) reinforced with high-strength, lightweight, and non-corroding prestressed carbon fibre reinforced polymer reinforcement. One example of this is a new type of precast carbon FRP pretensioned HPSCC member intended as load-bearing elements for building envelopes. Their performance in fire must be understood before they can be widely used with confidence. It is known that the bond strength between both steel and FRP reinforcing bars and concrete deteriorates at elevated temperature and that high strength concrete tends to an explosive spalling failure when subjected to a fire. The bond strength reductions in fire, their impacts on the load-bearing capacity of prestressed concrete elements, and the spalling behaviour of HPSCC remain largely unknown. This paper gives insight in the fire behaviour of CFRP prestressed HPSCC slabs and presents selected results of an experimental fire test study on thin-walled slabs.

Comparison of quasistatic to impact mechanical properties of multiwall carbon nanotube/polycarbonate composites

We report the quasistatic tensile and impact penetration properties (falling dart test) of injection-molded polycarbonate samples, as a function of multiwall carbon nanotube (MWNT) concentration (0.0-2.5%). The MWNT were incorporated by dilution of a commercial MWNT/polycarbonate masterbatch. The stiffness and quasistatic yield strength of the composites increased approximately linearly with MWNT concentration in all measurements. The energy absorbed in fracture was, however, a negative function of the MWNT concentration, and exhibited different dependencies in quasistatic and impact tests. Small-angle x-ray scattering (SAXS) showed that the dispersion of the MWNT was similar at all concentrations. The negative effects on energy absorption are attributed to agglomerates remaining in the samples, which were observed in optical microscopy and SAXS. Overall, there was a good correspondence between static and dynamic energy absorption.